How long does it take for logging roads to recover from clearance?

Roads are generally terrible for biodiversity. They fragment habitat, can increase habitat loss and hunting as a result of increased access, and cause direct loss of biodiversity as a result of collisions. However, not all roads are the same. Some are massive, permanent structures, while others are temporary, dirt tracks that may seemingly disappear once they fall into disuse.

One example of ephemeral roads is those that logging companies construct in tropical forests to provide access and transport of logs. There has long been concern that these roads can increase the risk of fires occurring, as well as increasing access for hunting, and other forms of forest exploitation. However, in a recent(ish) paper* has shown that some of the negative effects of logging roads are relatively transient.

In the paper, Fritz Kleinschroth and colleagues showed that in Central African forests, after 30 years of recovery logging roads had similar canopy cover, species diversity, and leaf litter to logged forests nearby* . However, the amount of carbon stored in the form of biomass lagged behind and was only 6% of that seen for logged forests after 30 years of recovery. At this rate, biomass recovery would take more than 300 years.This incredibly slow recovery at first appears puzzling, given that secondary forests, which have had almost all their trees cut down in the past and turned into agricultural fields, tend to take between 60-100 years to recover biomass to pre-disturbance levels (see here for a blog post and here for a recent paper on this). However, the probable cause of this delayed recovery is the compaction of soils on the roads by heavy vehicles which reduces seed germination and root growth***. Taken together the authors suggest these results indicate that logging roads have the potential to act as areas in which timber species could regenerate and that they may become inaccessible to hunters within 10 years.

So how does this study compare to similar ones carried out previously? Firstly, this study is a little different because it is one of the few that used chronosequences to assess recovery, and so the only study I know of which can assess longer-term dynamics on logging roads. However, other studies present a similar picture for the recovery of biomass and forest structure – forest canopy cover recovers relatively quickly (see here and here), but biomass and basal area lag behind (see here and here). The major difference between this study and previous ones is that it presents a more optimistic outlook of biodiversity. Previous studies have estimated that species richness may be 50-95% lower on abandoned logging roads when compared to logged forests (see here, here, and here). As such, the relatively fast recovery of species richness shown by Kleinschroth and colleagues appears to be outside of the norm, and further similar studies will be needed to see whether the pattern of recovery shown in this paper is an outlier. So we can’t really give a solid answer to the question posed in the title of this post – sorry about that.

While results vary from study to study it is obvious that more efforts need to be made to reduce the number of logging roads, their initial impacts to forests, and to help them recover once they are abandoned. In order to reduce the number of roads, reduced-impact logging could be used. This method, which I have been accused of disliking in the past, seems to be very successful in reducing the number of roads in logging concessions where it has been used (see here for an example of this). This is done by producing a plan for road construction prior to any trees being cut, rather than the ad-hoc approach often taken in conventional logging. Reducing the impact of roads could be done by limiting their width. Finally, improving recovery could be done by planting seedlings/saplings on former logging roads, as well as reducing access to roads.

One suggestion that the authors made in their paper that I really like is to re-use logging roads when forests are re-logged. Given that logging typically occurs every 30 years, this would allow some time for recovery of biodiversity on the roads but clearing them would reduce the damage caused by their construction spreading to other areas of the forest.

*I admit it, I’ve been terrible at keeping up with my reading recently.

**John Healy and Fritz have written a nice summary of their paper on the website “The Conversation” which is well worth a read.

***Anecdotal, I know, but I have seen similar things on restored salt marshes in the UK where diggers have been used to breach sea walls. At least for the ones I remember, this resulted in reduced vegetation cover.

 

Are we in danger of underestimating biodiversity loss?

Almost all ecological research of human impacts on biodiversity looks at changes after they have happened. To do this, researchers usually compare a site where some kind of disturbance has happened to a nearby undisturbed site. This method is called space-for-time substitution. The assumption of this approach is that the only thing that differs between sites is this disturbance. However, this assumption is often incorrect. Sites may have had very different biodiversity before any disturbances, which can lead to under- or over-estimations of biodiversity changes as a result of human impacts. One result of this is that we aren’t really sure how tropical logging alters the composition of ecological communities. These problems are likely to be particularly acute when habitat fragmentation limits dispersal to some sites.

Up until recently there had been little work comparing how the results from space-for-time methods compare to methods that compare sites before and after disturbances. However, last week an elegantly designed study was published in the Journal of Applied Ecology which aimed to examine just this in the context of logging in Brazil. The paper aimed to compare space-for-time methods to before-after-control-impact (BACI) methods. Critically BACI studies measure biodiversity at sites at least once before the disturbance of interest takes place. Researchers then return to sites and remeasure them after the disturbance. Importantly both sites impacted by the disturbance and control sites are surveyed on both occasions. Using this method allows researchers to disentangle the effects of disturbances and any differences between sites prior to disturbance – a key advantage over space-for-time methods.

The paper by Filipe França and colleagues examined the differences in results obtained for space-for-time and BACI methods when looking at changes in dung beetle biodiversity in tropical logged forests in Brazil. To do this they surveyed 34 locations in a logging concession, 29 of which were subsequently logged at a variety of intensities. The intensity of logging (the number of trees/volume of wood removed per hectare) is a very important determinant of the impact of logging on biodiversity and carbon (see previous posts on this here and here). They then went back and re-surveyed these locations one year later. From the data collected, they calculated changes in species richness, community composition and total biomass of dung beetles.

Figure 1 – Differences between before-after-control-impact (BACI) approach and space-for-time substitution (SFT) for changes in dung beetle species richness, community composition, and biomass. For more details see Franca et al. (2016).

When comparing space-for-time and BACI the paper found that BACI characterised changes in biodiversity significantly better than space-for-time methods. Critically, space-for-time methods underestimated the relationship between logging intensity and biodiversity losses, with changes in species richness twice as severe as estimated by space-for-time (see Figure 1). BACI methods also consistently provided higher explanatory power and steeper slopes between logging intensity and biodiversity loss.

So what does this mean for how we do applied ecology? I think it is clear that we need to employ BACI methods more often in the future. However, BACI comes with logistical and financial constraints. Firstly, it is virtually impossible to predict where disturbances are going to happen before they occur. As a result, Franca and colleagues think that if we want to carry out more BACI research in the future, we need to develop closer ties with practitioners. This will involve building relationships with logging and oil palm companies, as well as agricultural businesses and property developers. This may make some researchers uncomfortable, but we need to do this if we are to provide robust evidence for decision makers. Secondly, BACI studies take longer to carry out, so we need to convince those that hold the purse strings that they are worth investing in.

BACI is clearly something we should be using more often but does this mean that space-for-time approach is useless? Should we even be using space-for-time methods at all? I’m not being hyperbolic just to get some attention- some have argued that we should stop using chronosequences altogether because ecological succession is unpredictable. After momentarily going into a bit a crisis about this when I read some papers on succession last year, I have come to a slightly different conclusion. Space-for-time substitution sometimes predicts temporal changes well, but sometimes it doesn’t. What we need is to work out when the use of space-for-time approaches are acceptable, and when it would be better to use temporal methods. Reviews have highlighted that as ecosystems increase in complexity space-for-time methods become less useful for monitoring changes in biodiversity. For example, large local species pools mean that post-disturbance colonisation may be very variable between sites. This problem is  compounded in fragmented landscapes where there are barriers to dispersal of seeds and animals. Every additional layer of complexity makes post-disturbance dynamics more and more difficult to predict. Ultimately, the best way to address this problem is through some kind of synthesis.

Working out when space-for-time approaches are useful and when they are not is not something we are going solve overnight. Before we can review the evidence, we need some evidence in the first place.  This is part of the reason why papers like the one by França and colleagues that I’ve discussed here are vitally important. So next time you think about designing a study see if you can assess how the results from temporal methods compare those from  space-for-time methods. The results might just take you by surprise.

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Filipe França & Hannah Griffiths have written a great post on the Journal of Applied Ecology blog going into more detail about the implications of their study. I strongly recommend you give it a look.

Second growth:The promise of tropical forest regeneration in an age of deforestation

Anyone who knows anything about secondary forests will have come across the work of Robin Chazdon. She has inspired at least one forest ecologist, me, that forests recovering from major disturbances are a subject worthy of study. I’m sure she has done the same for many others out there. So, coming towards the end of my PhD I was excited to see a book that she had written summarising the topic was due to be released and using the last of my NERC funds I bought it. And then I moved house to Spain, where the book sat untouched and unloved in a box for the next year. After I came back to the UK last year, I found the book again and decided I should stop putting off reading it. I read it on trains, buses, on my sofa and occasionally in bed. I once fell asleep reading this book, though admittedly that was on the way back from the BES annual meeting  in Edinburgh, and the gin from the previous night was probably the cause of my sleepiness rather than any bad writing.

The first thing to say is that this book is extremely comprehensive. Though it is not particularly lengthy, running to 316 pages of text, it covers a huge range of topics relating to forest regeneration from traditional knowledge and prehistoric forest transformations by humans to recovery pathways from fire, landslides, volcanic eruptions, logging, and agricultural use. There are also numerous sections on community assembly, functional traits, ecosystem function, and animal and plant interactions. The last section concentrates on reforestation and restoration of degraded forests, making a passionate plea for degraded forests to not be considered as wasteland.

For me the most fascinating parts of the book were those that covered traditional knowledge of forest regeneration and the history of human cultures in tropical forests – both subjects I knew practically nothing about before this book. I was captivated to read that the dayak people of Borneo have five words to define different stages of forest recovery – kurat uraq (1-3 year old scrub that forms after abandonment), kurat tuha (trees > 5 cm in diameter and 5-6 metres in height), kurat batang muda (trees 10-15 cm in diameter), kurat batang tuha (closed canopy secondary forest) and hutan bengkar (primary forest). As Chazdon points out this knowledge shows a striking resemblance to that of forest ecologists. Similarly, Mayan cultures in Central America and Soliga people in the Western Ghats have developed a subtle knowledge of the stages of forest succession. I have always been a bit skeptical of integrating traditional knowledge into ecological science, but this book convinced me that there could be some value to it.

Chazdon masterfully weaves together anthropology, archaeology and ecology in the discussion of prehistorical impacts of humans on tropical forests. She cites evidence of earthworks called geoglyphs similar to the Nazca lines found in the state of Acre in Brazil, swidden agriculture 20,000 years ago in Papua New Guinea and human populations in Central America to dispel the view that any forest is truly untouched. There are probably legacies of human use in most forests, we just can’t identify them. Based on this she, perhaps controversially, critiques recent work suggesting that mature tropical forest biomass density is increasing as a result of atmospheric carbon dioxide. Chazdon’s view is that this increase could well be as a result of recovery from unseen disturbances that happened generations ago.

The section on community turnover during succession is also excellent, with a detailed analysis of the characteristics of short- and long-lived pioneer and shade tolerant, late successional species. At points Chazdon playfully conjures up text resembling Shakespeare’s  “All the world’s a stage” monologue: “The term successional stage is apt. Successional pathways can be viewed as an improvisational drama in several acts, with each act featuring a different set of actors. Some actors perform throughout the drama, but others have cameo appearances of only one act. Although each act sets the stage for the next, forest regeneration has no director and only a roughly sketched script creating a high degree of spontaneity, randomness and uncertainty.” These are amongst my favourite parts of the book, with metaphor mixing with a solid science to help things stick in your mind that might otherwise be easily forgotten.

If I have any criticism of the book, it is that it’s a bit repetitive. This is probably because Chazdon sees succession as ‘an improvisational drama in several acts’ and so the book relies on case studies, rather than synthesising current knowledge to form generalities. However, I think that the repetition helps if you just want to dip in and out of chapters – I don’t think it is necessarily written to be read cover to cover like I did.

That aside if you are interested in the dynamics of forests in any way this book is essential reading. There is no better summary of current thinking on tropical forest succession out there.

“Like walking through an open cemetery”

“I have been working in human-modified tropical forests for the past 14 years, but seeing these fires first hand was devastating,” wrote Erika responding to one of my questions “The smell of wet soil was gone and I could only smell smoke…even the usual cacophony of forest sounds disappeared…it was like walking through an open cemetery.”

Erika de Berengeur Cesar, an up and coming Brazilian forest researcher, works at Lancaster University. For last two months, she has been slogging away in the field collecting data for her team’s project on human-modified forests. But this year hasn’t gone to plan. This isn’t a case of bad planning though, as with so may projects – 7 of her 20 sites had burned in some of the most widespread fires in recent times. After seeing her tweeting about this, I thought “I need to write something about this. It feels important.” So we fired a few tweets and emails back and forth, with Erika fitting answering my questions between her days in the field. After I had waited impatiently for a couple of days, Erika messaged me:

“Sorry, trying not to work weekends…not going very well though…Today I just learned that 9 of my 20 plots have burned.” 2 more plots. Aside from the wider situation, this was the stuff of researcher’s nightmares.

Erika de Berengeur Cesar in one of her logged plots before it burned (top) and the same plot after recent fires (bottom). Photo courtesy of Erika.

Fires in Brazil reached record levels in 2015, with more than a quarter of a million separate fires recorded. However, these fires are not generally ‘natural’ – “Fires in the region always have a human ignition source.” Erika told me “They are used in slash-and-burn agriculture, to clear pastures of weeds and also to burn downed timber in newly deforested areas.” This year’s strong El Niño has caused drier conditions than normal making it “easier for agricultural fires to escape the targeted area and sweep through the forests.” Indonesia is facing a similar problem, where forests have been burned to clear space for new oil plantations, in what the Guardian’s George Monbiot  has described as the ‘greatest environmental disaster of the 21st century – so far.’

When I queried why it matters that the forest is burning, Erika was clear what the major issue is – the loss of unique biodiversity. “Every year over 100 new species are found in Amazonian forests. To see all this going up in smoke is a crime against humanity. It is a tragedy.”

“How are these fires likely to affect biodiversity?” I asked.

“The Amazon has not co-evolved with periodic fires…This means that Amazonian forests are not used to these events and…do not cope very well with it. In terms of plant communities, there is a sharp increase in the abundance of pioneer species, while high-wood density climax species disappear….Fires negatively affect…rare bird species, and the habitat specialists, such as the ant-following insectivores and the terrestrial gleaners. Overall, burned forests are significantly less diverse than their unburned counterparts.”

Amazonian forests that have burned repeatedly may eventually come to resemble more open savannahs and contain  very different species to relatively undisturbed old-growth forest.

But it’s not just biodiversity that is affected by these fires, but humans as well. In Indonesia there were evacuations of children by the navy, although some of the children, according to reports, still died from breathing difficulties . In Brazil the fires have “affected many of the local people…who reported a number of respiratory problems, such as dry cough, difficulties in breathing, and sore throats,” according to Erika. “People had to spend days building fire breaks to protect their land, instead of directly working on their crops.” People working on these farms already have a tough life as it is, without having to worry if their source of income will go up in smoke.

So what will happen to these forests in the future? Given time and, vitally, protection they can recover but Erika thinks this is unlikely “These burned forests may never recover. After the fire, several large trees die, creating a number of gaps in the forest canopy, through which more light and wind can reach the forest floor, making it drier and, as a consequence, more vulnerable to further fire events.”

The research Erika and her team are carrying out will help to answer the question of how burned forests recover but it is obvious that degraded forests, such as these, need to be seen as a greater conservation priority. More than 50% of the globe’s forests are degraded in one way or another. We cannot afford to only protect primary forests anymore.

Edit: I got an email from Erika a bit ago after I asked her what the best solution would be. I thought I should include it here:

“Funnily enough there are already quite a few good policies in place. The problem is that none is followed. For example, every year there is a ‘burning calendar’ establishing when farmers can use fire to burn their pastures or their croplands. During the peak of the dry season, the use of fire is forbidden. In 2015, given the extreme drought, some states even extended the prohibitive period. So all quite reasonable and good, right? The problem is that no one follows this rules and there is no law enforcement in place. So people carry business as usual and the forests carry on burning. To put in practice the existing laws would be the best solution.”

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If you want to read more about the situation in Brazil take a look at the excellent article Erika has written  for ‘The Conversation.’

There are also a pair of videos that Erika’s team have made documenting the fires that you can see here and here.

New paper: Stand dieback and collapse in a temperate forest and its impact on forest structure and biodiversity

We recently published the first paper from my post-doc in Forest Ecology and Management, so I thought I’d share it here. It marks a bit of shift away from the tropical forests I have previously published about (see posts on that here and here), but it allowed me to continue my work on post-disturbance recovery.

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Scientists and policymakers around the world are concerned about the potential effects of forest dieback. Drought and the spread of new pathogens and pests have resulted in increased tree mortality in both the USA and Canada, and these threats are likely to increase in Europe as well. The IPCC recently highlighted forest dieback as a potential major threat, but one about which we know relatively little.  Changes in forest biodiversity and ecosystem services are likely to be particularly severe in ecosystems that show poor resilience. Failure to withstand or recover from drought or pest attack may lead to ‘regime shifts’ resulting in a very different type of system, with many fewer trees.

Die back in the New Forest has caused some areas to go from this…

…to this

Luckily for our group my boss, Adrian Newton, found out about a permanent transect that had been set up in the 1950s in a woodland in the New Forest that now appears to be suffering from dieback. The site had been surveyed 4 times between 1964 and 1999, and our team collected more data from the site in 2014. In our recent paper, published in Forest Ecology and Management we used this data to investigate dynamics of the woodland. In particular, we addressed the potential impacts of dieback on forest structure, the causes of these changes and their impact on biodiversity.

Basal area loss in Denny wood from 1964-2014

To cut a long story short, the forest lost about a third of its basal area (as you can see above) and over two-thirds of its juvenile trees over 50 years. over 90% of the loss of basal area was due to the death of large beech (Fagus sylvatica) and oak (Quercus rubor) trees.

Climate records from 1964-2014 showed that (a) mean temperature during April-September increased from 1960s to present day; and (b) there were numerous drought years post-1976 a year which was previously identified as a cause of current mortality.

The external factors causing these changes are not entirely clear, but there have been a number of significant droughts between 1964-2014 as well as increased temperatures (see figures above). In addition, the presence of a number of novel fungal pathogens has been noted in the forest, which may have interacted with drought to further weaken large trees. Recovery in the forest has been very limited, with almost no recruitment of saplings of the canopy dominants (beech & oak) in 50 years. This low recruitment is probably a result of the high density of ponies and deer in the woodland.

Relationship between percentage loss in subplot basal area and (a) percentage grass cover and (b) ground flora species richness.

The result of the changes in forest structure is that areas with little tree cover have seen large increases in grass cover and increased ground flora species richness (see figure above). Both of these results indicate that there may be a tipping point at which changes in structure result in rapid increases in grass cover and species richness of ground flora.

Many of the papers on resilience talk about alternative stable states, in which transitions from one type of system to another are difficult to reverse. Though, from the outside, it may appear that our field site shows evidence of a shift to a relatively treeless stable state, we think that this is incorrect. The theory underlying multiple stable states suggests that disturbances causing the regime shift should be a ‘pulse’, when disturbance occurs over a relatively short period and then does not occur again, rather than a ‘press’ disturbance, where the disturbance is present over long periods of time.  However, these conditions are not met by our site where both pulse (i.e. drought) and ongoing press (i.e. overgrazing)  disturbances are present. We think that both of these processes are needed to cause the forest to lose tree cover.

Even if the transition we  have observed is not strictly a ‘regime shift’ it’s still important. Dieback is apparently widespread in the New Forest and is on-going, so the potential impacts could be very significant. As with other cases of dieback it’s difficult to identify appropriate management responses. However, in the case of the New Forest the easiest way to restore resilience would be to protect tree regeneration from the high herbivore pressure in the area.

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If you want to read more about our study you can find the paper here and details of our project on forest resilience can be found here. Oh, and here’s a post I wrote about my project a while back. Also, feel free to comment below!

Tropical deforestation causes dramatic biotic homogenisation

Although species richness is most ecologists go-to metric to ‘take the temperature’ of an ecosystem, it is not always the most useful. Even when species richness doesn’t change much over time many species may be being added to or lost from a community. Changes in human land use can cause loss of a particular taxonomic or functional groups, which can have important implications for ecosystem processes such as pollination or seed dispersal. This non-random loss of species as a result of human impacts can result in biotic homogenisation – where the communities in different location become more similar to each other. Biotic homogenisation has been seen all over the world in response to drivers like urbanisation, agricultural land-use change, and eutrophication.

However, up until recently, there had been little work on how biotic homogenisation impacted multiple taxonomic groups across landscapes. Work has also been almost entirely carried out at a single spatial scale. Given that taxonomic groups are likely to differ in their response to disturbances and that landscape scale processes may play a critical role in species persistence. Fortunately last week a paper was published by Ricardo (aka Bob) Solar and colleagues in Ecology Letters that attempted to fill these knowledge gaps.

Specifically the paper attempted to determine how much of the change in community composition as a result of changes in tropical forest land-use change were attributable to replacement of species (termed turnover) and loss of species (termed nestedness). Bob and his colleagues did this for birds, dung beetles, plants, orchid bees and ants at 335 sites (!) in 36 different landscapes in 2 regions of Brazil. The sites used were either primary forest experiencing varying degrees of human disturbance, secondary forests, cattle pasture or arable farmland.

In short the paper shows that:

• Species richness decreases as land-use intensity increases
• Differences in community composition between deforested sites were much lower than for forested areas
• Species turnover caused the majority of changes in community composition, but loss of species became more important as the intensity of disturbance increased

The importance of loss of species (nestedness) in biotic homogenisation increased as intensity of disturbance increased at both (a) local and (b) landscape scales. Taken from Solar et al. 2015.

For me, the most interesting message of the paper the changes in community composition were largely attributable to replacement of species. This suggests that as species are lost following disturbance, colonisation of generalist species initially causes relatively little change in species richness. However, as land-use intensity increases the contribution of species loss to alteration in community composition became more important suggesting that communities in these locations tend to be made up of generalist species that are tolerant to human disturbances.

Conversion of forest to agricultural use led to much greater biotic homogenisation than degradation. Photo courtesy of Bob Solar.

Interestingly, the paper also shows that provided that forest cover is maintained there was relatively little biotic homogenisation. So while it is obvious from previous work that the maintenance of undisturbed forests is vital to conserve tropical forest biodiversity, it is also obvious that degraded forest can play an important role in conservation.  This is especially true where few undisturbed forests still exist or degraded forest is widespread such as in SE Asia and Central America.

This work effectively shows that taxonomic homogenisation is occurring at multiple scales as a result of human land-use change. The next step is to see what types of species are being lost/retained. This means looking at the interaction between species traits and the land-use gradient (see more on that here). Previous work has suggested that body size and feeding preferences may play an important role in determining whether bird species can persist in degraded forests. Looking at this will allow us to gain a greater understanding of how biodiversity change may alter ecosystem processes and ultimately the ecosystem services on which we all depend.

Does reduced impact logging in tropical forests benefit carbon storage and species richness?

After a bit of a traumatic review process* we have just had a paper published in Forest Ecology and Management on the impacts of tropical selective logging on carbon storage and tree species richness. I’m really pleased that we finally got this work out there. If you want to give it a look you can get it here.

Selective logging is one of the most widespread drivers of tropical forest degradation. As I have said before around 400 million hectares of tropical forest are now used for logging – an area twice the size of Russia. Or one hundred and ninety two and a half times the size of Wales – if that’s your thing**.

High intensity logging can result in loss of animal species richness, but on the whole logging is seen as one of the least damaging human uses of tropical forests. That said, there are still concerns about its sustainability in the long-term. Poorly managed concessions commonly remove high timber volumes and do not leave enough time between logging cycles to allow forests to recover.

To improve the sustainability of the practice, reduced impact logging has been proposed. This method aims to reduce negative environmental impacts by cutting lianas and vines before logging, identifying which trees to cut and mapping them before logging starts, planning the roads to be built, and training staff in methods to reduce damage to the forest.  The first papers testing this method showed promising results, appearing to indicate that reduced impact logging causes lower carbon emissions when compared to conventional methods.

However, many papers that have  looked at the impacts of reduced impact logging failed to account for the volume of wood taken out of forests. Crucially, if this differs between reduced impact and conventionally logged sites this represents a hidden treatment, which if not accounted for can lead to faulty conclusions. Given that there are calls to pay people who use reduced impact logging as a means to reduce carbon emissions, we need good, solid science to support this policy.

So, we tried to solve the question of whether reduced impact logging still reduces negative effects on residual tree damage, aboveground biomass, and tree species richness using meta-analysis. We compiled data from all over the globe, all from previously published papers.

Locations of study sites where data we used was collected

Cutting to the chase, the results for reduced impact logging were mixed.

It seemed to reduce the damage to residual trees once logging volume was accounted for…

Reduced impact logging (blue) tended to cause less residual damage than conventional logging (red) once logging intensity was accounted for

… however, this did not obviously result in reduced biomass losses, and evidence of an effect on tree species richness was poor as well.

Effects of logging intensity on (a) aboveground biomass and (b) tree species richness. Reduced impact logging sites are blue points, and conventional sites are red. Note the relatively low intensity for most reduced impact logging sites.

Though residual damage to trees was reduced, this didn’t cause a  reduction in overall biomass loss. This may be the result of a few different factors. Firstly, residual damage is often to smaller trees so it is not necessarily that surprising that this had little effect on biomass. Secondly, we are really lacking enough data to be sure of the relationship between biomass changes and reduced impact logging. Nearly all of the data is from forests logged at low intensity so we cannot say if the slope of the relationship differs from that of conventional logging.

In the case of tree  species richness, the relative lack of change over a gradient of logging intensity is not too surprising. Newly logged areas richness is probably enhanced by fast growing pioneer species. However, richness is not a fantastically useful measure of biodiversity, in the future it would be much more useful to be able to say what type of species are being lost/gained not just the total number of species in a site (see the recent paper by Zuzana Burivalova and colleagues that tries to do this with bird species).

So what does this all mean? Does our study mean that reduced impact logging doesn’t work? The short answer is no. The long answer is a bit more complicated than that.

First we need to decide whether reduced impact logging is synonymous with low yield logging. If it is then that is fine, but we need to be upfront about this. Logging is after all mainly about timber production. However,some people have previously argued that reduced impact logging can reduce damage whilst maintaining yields. If this is true, it would represent a win-win situation.

If we decide that reduced impact logging isn’t synonymous with low yields then our research question needs to change from “Does reduced impact logging cause less damage than conventional logging?” to “How do the impacts of reduced impact and conventional logging vary over a gradient of timber yields?” Generally in ecology we focus too much on using categorical x variables in statistical tests, and this case is a great example of why this approach can hold back our science (see the fantastic post by Brian McGill on this subject here).

Previous studies show that animal species richness declines with increasing logging intensity and reduced impact logging causes lower losses of animal populations. As a result, a combination of reduced impact logging and reduced logging intensity may appear the best way to reduce carbon emissions and biodiversity loss from logging. However, reducing local yields may cause expansion of logging into previously unlogged areas. This mirrors the current land sharing/sparing debate on how to balance agricultural yields and food production. This debate is taking off regarding logging, and I am keen to see more work on tropical logging that acknowledges the importance of yields. As I said to someone at a conference recently, if we ignore the importance of logging yields why study logged forests?

However, to inform this debate we need more powerful tests of different logging methods than we could do in our paper. One possible source of data for this are studies where logging intensity has been calculated for each sample plot used. For most of the studies I used logging intensity was only available at the site level. Getting this detail would give more statistical power to our tests and provide a more solid evidence base for management of tropical forests. Large-collaborative projects such as the tropical managed forests observatory represent a great chance to answer this question in a more satisfactory manner.

*I will write more about this next week.None of the journals were to blame, just some very biased reviewers.

**If any US citizens want this calculating as relative to Rhode island, I did it. It’s 1273.8 Rhode islands.

Is ecological succession predictable?

Over the last few years I have written quite a lot about forest succession. I have published a paper on the topic, have a paper in review about recovery of a forest under multiple stressors and will be starting more work on the it over the next few weeks. All in all, I think I have a reasonable idea what I’m talking about when it comes to succession, at least in forests. However, I’ve just read a paper on tropical forest succession that caught me a bit unawares*.

The paper in question is Natalia Norden and colleagues’ work that was recently published in PNAS. The authors collected data from 72 secondary forest plots monitored for 7-24 years at 7 different sites across tropical South and Central America. They then used this data to look whether we can predict trajectories plot stem density, basal area and species density during forest succession after total clearance. On the whole the paper found that trajectories were poorly predicted by models that looked at change as a function of forest age. From the figure below, you can pick some general trends in the direction of change with age – stem density might have a humped relationship with age for example. However, it is also clear that there is a huge amount of variation and some trajectories bounce around all over the place.

Observed successional trajectories of stem density, basal area and species density for the sites used by Norden et al.

It’s obvious from looking at the figure above that the age of a secondary forest doesn’t really act as a proxy for its successional stage. In fact Norden and colleagues found that on average age only explains 20% of within site variation. Even if that is better than the average ecology paper, it’s still not very good. To explain the rates of change of different variables, Norden et al. fitted a set of different non-linear models for each site. Again, their findings emphasised the large amount of variation between different sites. Due to these idiosyncrasies, the authors of the paper see space-for-time substitution as a flawed method for predicting the dynamics of forests. They also suggest that such approaches should not be used for studies of succession of any sort of vegetation, arguing that previous work these methods has made succession appear as if it is deterministic, and it is not.

Now I’m not sure how the numbers of studies that use chronosequences vs monitoring over time to study succession stack up, but I’d be willing to be bet >80% of these papers use chronosequences, at least in forests. There are good reasons for using them: they take much less time than monitoring (especially in systems containing long-lived organisms), they are much less expensive, the logistics are less complex and as a result of all of these things, they are easier to get funded than a 10-20 year research programme. Norden et al.’s warning against using chronosequences based on their results, begs the question “Do we have other evidence of how well chronosequences perform?” The answer is that we do, and it doesn’t look too good for chronosequences. For example, Ted Feldpausch and colleagues found that space-for-time substitution resulted in overestimates of biomass accumulation for young secondary forests in the Amazon. Recently Mora and colleagues similarly suggested that chronosequences were poor predictors of forest characteristics.

So, is the chronosequence dead? Well, maybe not just yet. However, I think as researchers we need to be more circumspect about their use. In particular I think there are 4 questions that we need to answer to get a more well rounded view of the usefulness of chronosequences:

1. How much variation in future dynamics do they actually predict? – Chronosequences are far from perfect, but it still offers us some insight into future dynamics. Mora et al. showed that chronosequences can still account for 32-57% of variance in future forest characteristics. There must be a reasonably large number of chronosequences that have been sampled more than once that could be used to test their predictive ability. We need more studies that address this head on. If it turns out that they are very poor at explaining future dynamics, then maybe it is time to switch to better methods.
2. What variables do they predict most effectively? – Structural components of a system (biomass, stem density etc) should be easier to predict than community composition, since changes in structure are less likely to depend on idiosyncrasies such as the identity of initial colonising species. However, again, this has been tested relatively rarely.
3. Do chronosequences have more predictive power in some systems than others? – Predictive power should be greatest when abiotic conditions are relatively constant across a landscape, disturbance history at all sites is relatively similar and in regions with relatively small species pools. Under all of these conditions there should be less chance of wildly different successional trajectories occurring.
4. Where do animals fit into all this? – Predicting animal abundance and community composition is rarely studied in chronosequences, probably because their response to succession is that much less predictable than plant communities. Even though they are likely to perform relatively poorly, a comparison of the predictive ability of chronosequences for animal compared to plant communities would be interesting.

What do you think? Are there any other questions we need to answer to determine the value of chronosequences? Or do you have any views on the use of chronosequences in non-forest systems?

*To be fair, this probably shouldn’t have been that much of a surprise, review papers have been suggesting that chronosequences are far from the best way to do things for a while. Although, there are also papers that suggest that careful use of chronosequences is perfectly ok.

Guest post: Responses of functional groups to forest recovery suggests irreplaceability of old-growth forests

Today we have a guest post from Becks Spake who is doing her PhD at Southampton University (as an aside, Becks in currently on the lookout for a post-doc, so feel free to get in touch with her). Last week a paper she and I (and others) have been working on together on recovery of temperate and boreal forests was published in Conservation Biology. I think it’s a really neat bit of work that emphasises the importance of old-growth forests for biodiversity. I’ll leave Becks to tell you the rest.

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Forest restoration measures are being increasingly implemented worldwide to enhance biodiversity and ecosystem service provision in degraded landscapes. These measures range from active restoration, such as planting, to passive restoration, where natural recovery is promoted following the removal of some environmental stressor such as grazing. Forest restoration is also used as a biodiversity offsetting mechanism to mitigate the loss of habitat incurred by development; such ‘restoration offsets’ generate new habitat at an offset site to compensate for the loss of habitat to development at the impact site.

Concerns about the value of replacing old-growth forest with plantations and young regenerating forest has motivated research on biodiversity recovery as forest stands age. Several reviews have quantified the recovery times required for biodiversity, including measures of species diversity and composition, to reach equivalence to some reference state, typically undisturbed old-growth forest. Such reviews have been produced for relatively charismatic taxonomic groups recovering in secondary tropical forests (e.g. Dunn 2004; Chazdon et al. 2009; Martin et al. 2013*). These syntheses have made important contributions to restoration science, showing that different taxonomic groups exhibit different patterns and rates of recovery with stand age and that these must be acknowledged by forest management strategies.

In our study, we attempted to assess the recovery of species richness in restored forests outside of the tropics. We synthesised data from empirical studies measuring species richness differences between old-growth and secondary forest in temperate, boreal and Mediterranean regions. We focussed on studies that investigated species-rich functional groupings of fungi, beetles and lichens (Fig.1), due to their relative unsexyness, and consequent under-representation in existing reviews, their importance to ecosystem function, and sensitivity to stand-level processes.

Fig.1 – Species from the functional groupings investigated. The functional groups were: saproxylic and non-saproxylic beetles, epiphytic lichens and deadwood, litter, and ectomycorrhizal fungi. Photos: Simon Currie.

Key findings

1. Functional group-specific responses to forest recovery

We found that functional groups responded differently to forest recovery (Fig. 2). Ectomycorrhizal fungi averaged 90 years for recovery to old-growth values, and epiphytic lichens took 180 years to reach 90% of old-growth values. Non-saproxylic beetle richness, in contrast, decreased as stand age of broadleaved forests increased.

Fig. 2 – Figure 1. Influence of stand age on percent change in species richness for 7 functional groups in planted and secondary forest relative to old-growth forest stands (horizontal dashed line, no difference between undisturbed old-growth forest and treatment [planted and secondary] forest stands; gray, 95% prediction intervals based on uncertainty in fixed effects only.

2. Pseudo-replication is widespread amongst empirical research investigating forest management impacts on biodiversity

Our systematic review yielded just 33 publications (90 individual studies) in which old-growth was compared with planted or secondary forests in a statistically robust way. For some functional groups, this led to small sample sizes and low precision in lnR values (Fig. 2). This low number of studies was due to the fact that a high proportion of studies were pseudo-replicated, with a lack of independence across replicates. Many forms of pseudo-replication exist (Fig. 3), the most common being simple segregation sensu Hurlbert (1984), in which multiple samples from a single contiguous treatment unit are analysed as if they were independent replicates that were interspersed with control replicates (Fig. 3 B-1). Differences between control and treatment replicates that are simply segregated cannot be unambiguously distinguished from other sources of spatial variation.

Figure 3. Schematic representation of various acceptable modes (A) of interspersing replicates (boxes) of two treatments (shaded, unshaded) and various ways (B) in which the principle of interspersion can be violated. From Hurlbert (1984).

Despite the widespread recognition of the problems associated with pseudo-replication, it still features prominently in peer-reviewed studies. The situation is the same in the tropics; Ramage et al. (2013) reviewed recent studies of the effects of logging on biodiversity in tropical forests (n = 77) and found that 68% of the studies were definitively pseudoreplicated, and only 7% were definitively free of pseudoreplication. Whilst data from pseudoreplicated studies can inform management in very local contexts, conclusions from these studies must not be generalised and data must not be included in meta-analyses.
Conservation implications

The primary goal of biodiversity offsetting is to achieve no net loss of biodiversity. Our results show that through restoration offsetting, this goal is unachievable within a reasonable time frame. The slow recovery of species richness for some functional groups essential to ecosystem functioning makes old-growth forest an effectively irreplaceable biodiversity resource that should never be compromised by development.

Our findings support the value of protecting old-growth forest through reserve creation, but also recognise the potential for planted and secondary forests to support biodiversity, if given enough time to recover. Our findings therefore also support the setting aside of overmature planted forest for biodiversity conservation, and the implementation of schemes that extend rotation-length of planted forests within production forest landscapes.

* For my blog post about this paper see here.

Land-sparing/sharing in tropical logged forests

The dichotomy between land-sharing and land-sparing has been used a lot in studies on the impacts of agriculture on biodiversity to compare between relatively intense, highly productive agriculture that spares natural ecosystems from conversion and extensive, wildlife friendly agriculture with lower yields. The comparison between these two extreme ends of the land-use spectrum could potentially be applied to a whole host of problems relating to how we use land, such as urban planning, electricity production and timber production. While making changes to a manuscript I have been pondering the last of these problems a bit, in the context of tropical selective logging.

Our recent preprint, as well as in 2 other papers in the last year (here and here), showed how the impact of logging biodiversity and carbon storage vary over a gradient of logging intensity. Where large volumes of wood are extracted species richness of trees and animals are negatively impacted, animal populations are reduced as is carbon storage in tree biomass. This gradient of logging extraction represents potential different intensities at which tropical forests could be logged, extensive and low intensity, or high intensity and spatially concentrated. Though it is a topical subject (indeed there has been an NCEAS working group set up to deal with it and who have a flashy website here) there has been little empirical study of land-sparing/sharing in the context of tropical forests, with the only study published so far suggesting that land-sparing presents a better option for birds, dung beetles and ants in Borneo. Typically the gradient of timber extraction is calculated as the volume of trees felled per hectare. However, there are a number of problems that make this metric far from ideal.

Firstly, it tends to be calculated at very large scales, often covering an entire forest concession of hundreds of hectares. To get a better idea of the impact of logging across a gradient the scale of the measurement needs to be reduced so that variation between plots can be examined. Also, though the volume of trees felled obviously tells us quite a lot about the gradient of disturbance, it doesn’t actually tell us what we want to know – the yield. Just as crop yields are what is most important for a farmer the yield of timber from a logging concession is the primary concern of logging companies. Importantly the volume of trees harvested is not always very good at measuring this, since some logging operations are more efficient than others. For example, some trees that are felled but never actually make it to the sawmill. Such wastage is more likely in unplanned logging when lack of co-ordination can result in logs being left behind after being cut, and as a result the yields per hectare can be lower than measures of logging intensity might otherwise suggest.

Though there is currently some discussion of whether sharing or sparing are likely to result in better outcomes in tropical logged forests, the truth is that we currently don’t know much since we lack the sufficient evidence. In order to get this much needed evidence we need to make sure that when studies are designed to answer the land sparing/sharing question in tropical forests they use timber yield, not logging intensity as their gradient and species density as their response variable. Doing this will require closer collaboration with logging companies in order to get detailed information. Some people have cited the fact that logging appears to have relatively little effect on species richness at low intensities, however as I have discussed these relatively modest reductions in species richness may mask large changes in what species are present. As such species richness has no place in the debate about the configuration of landscapes in the context of tropical logging.

In addition to the populations of priority conservation species any future assessment of land-sharing/sparing must recognise that recovery times for carbon and timber tree populations are likely to be longer when logging intensities are high. Given this it seems likely that in order to reach as many goals as possible logging intensity should be high enough to reduce the area impacted but low enough to allow recovery within cutting periods – often around 30 years. Finding this balance will be difficult in the current data vacuum.