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.

Franca_logging
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.


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.

Logging intensity drives species richness loss

An area bigger than the entire Indian landmass is now used for timber production in the tropics. This logging is largely selective and leads to degradation with loss of specialist species and ecosystem services like carbon storage. However, many have also argued that these forests should be considered a priority for protection since they are at danger of conversion to other land-uses such as agriculture. In addition the impact of logging on tropical forest biodiversity appears to be less negative than other human impacts.

However, simply saying that logging is a less damaging option when compared to other way in which humans exploit forests misses a lot of what is going on. Logging operations differ massively from place to place in terms of the volume of trees cut for timber, the area affected by logging, the distance between logged and unlogged areas, I could go on… All of these differences have the potential to influence the the effect of logging on biodiversity.

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Forest regeneration provides cheap carbon and biodiversity benefits

First of all, hello again and apologies for my sporadic posting on here recently. I have now successfully defended my viva and have a few corrections to make but hopefully should be able to post on here a bit more regularly from now on.

One paper I read that really impressed me while on my hiatus from the blog was by my old commuting buddy James Gilroy and colleagues. This paper attempted to identify the potential biodiversity and carbon benefits of forest recovering in the Tropical Andes in Colombia, an area full of species found nowhere else many of which are under threat from agricultural conversion. The paper also attempted to look at the cost effectiveness of carbon payments for landowners who converted farmland to forest when compared to different land-use options like cattle farming.

Gilroy et al - Fig 1
Recovery of secondary forest carbon stock compared to that of pasture and primary forest (Taken from Gilroy et al. 2014)

I was actually quite surprised by what Gilroy and his team found. Their results suggested that carbon storage in recovering forests was fairly similar to that in mature forests in the area after around 30 years, much less than the 100 years or so that I estimated these stocks would take to recover in a previous study.

Gilroy et al - Fig 4

Gilroy et al - Fig 3
Relationships between carbon stocks and similarity of dung beetle and bird communities to primary forest communities (Taken from Gilroy et al. 2014)

 

More surprising still was that bird and dung beetle communities in the regenerating forests were fairly similar to those of mature forests, suggesting that they have high conservation value. Again previous studies have generally estimated that animal species that are forest specialists may take a long time to colonise secondary forests, and plants probably take even longer. The fast recovery times may be attributable to the relative closeness of recovering forest to intact forests in the study area, allowing immigration of  forest animals and increased likelihood of transportation of seeds from long lived tree species.

Gilroy et al - Fig 2
Relationship between the additional cost of undertaking forest regeneration and the price paid for carbon per tonne. The solid horizontal line shows where costs are equal to zero. This graph indicates that there are potentially net economic benefits for people undertaking forest regeneration projects when the carbon price is greater than $4 per tonne. (Taken from Gilroy et al. 2014)

More important than these findings though was the discovery that if forest regeneration schemes were implemented in the area, they could be more profitable to land-owners than current land-uses like cattle farming. This was true for all pastures in the area when carbon trading prices were greater than $4 per tonne of CO2 and given that the median price of carbon in 2013 was around $7.80 per tonne, paying for the carbon benefits of regeneration in these locations works out cheaply. This is the part that I thought was really neat, because all too often restoration schemes fail to account for the costs and benefits associated with such projects.

Given that the study area has fairly representative socioeconomic conditions to those found in the wider Colombian Andes, the results suggest that regeneration of cloud forest may provide a great opportunity for REDD+ carbon based conservation, which can deliver multiple environmental benefits at minimal cost. Though REDD+ has its critics it has the potential to transform forest conservation so we need to work hard to make sure it is done in the right way.

Looking to the past for insights into tropical forest resilience

A few weeks back Lydia Cole and colleagues published a really cool paper exploring recovery rates of tropical forests. Seeing as it’s something I’ve covered a here before in relation to my work on secondary forests recovering after agricultural clearance and recovery from selective logging, I invited Lydia to write a guest post giving a different perspective to a topic I have discussed here before. Thanks to Lydia for stepping up to the plate and I hope you find her post as interesting as I did.


Anyone reading this blog probably doesn’t need reminding of how important tropical forests are!  Birds, bees, berries and a whole load of other plants, animals and services that we probably underestimate our reliance on.  Despite the many arguments in favour of keeping tropical forests standing, vast areas continue to be deforested at rapid rates resulting in changes like that shown below (Fig 1), under pressures of expanding human population, rising consumption and the agricultural footprint to match (Geist & Lambin, 2002).

Borneo-forest
Fig 1 – Forest disturbance like logging can lead to forests such as this one in Borneo being converted from intact (left) to heavily degraded (right).

Disturbance and recovery in tropical forests Despite this widespread clearance as a result of  recent international forest conservation initiatives and rising rural-to-urban migration (Mather, 1992), some degraded tropical forests are being given a chance to recover.  But how long does it take them to recover?  Much recent research has attempted to answer this question (e.g. the great work of Chazdon et al., 2007) but little has monitored change over time scales of >50 years. Since many tropical trees have lifespans much longer than this previous studies have only captured a snap-shot of the ecological process of recovery.  In our study, we attempted to answer the question again; this time by looking into the past to gather data over longer time scales that could offer a more complete picture of forest recovery post disturbance.

The palaeoecological approach

Palaeoecology, otherwise known as long-term ecology, uses fossils to decipher how plants and animals interacted with their environment in the past.  Fossil pollen grains come in all shapes and sizes, and their morphological characteristics can be used to identify the plant family, genus or even the species to which they belong.  When a collection of these grains are identified and counted from a layer of sediment, we can reconstruct what the vegetation was like at that point in time when those grains were deposited. In our project, we were interested in studies that documented disturbance-induced changes in fossil pollen from forested communities across the Tropics, over the last 20,000 years.  Types of disturbances ranged from climatic drying events and landslides, to shifting cultivation and human-induced biomass burning.  We found 71 studies published on tropical forest palaeoecology that satisfied our selection criteria (e.g. within 23oN/S of the equator, possessing a sufficient chronology), documenting 283 disturbance and associated recovery events.  The rate at which recovery was occurring across the different forests and disturbance events was the key variable of interest and was calculated as the percentage increase in forest pollen abundance per year relative to the pre-disturbance level.

How far and how fast have tropical forests recovered in the past?

Our results demonstrate that in the past the majority of forests regrew to less than 100% of pre-disturbance levels, prior to declining again or reaching a new baseline; the median recovery was to 95.5%.  They also recovered at a variety of speeds, ranging from rates that would lead to 95.5% regrowth in less than 10 years to those taking nearly 7,000 years; the average was 503 years.  This is significantly longer than the periods adopted by logging companies between extraction cycles!

What affects the rate of recovery?

Three of the different factors we investigated for their potential effect on the forest recovery rate seemed to be of particular importance: geographical location, disturbance type and frequency of disturbance events. Of the four key tropical regions, Central American forests recovered the fastest and those in Asia the slowest (Figs. 2 & 3).  This is concerning, given that forests in Southeast Asia are currently experiencing some of the greatest rates of deforestation of all tropical regions, primarily due to the economic profitability of oil palm agriculture (check out mongabay for details).

Tropical forest recovery
Fig. 2  Map of tropical forest distribution, the location of studies and relative recovery rates across regions.

The most common form of disturbance, and one from which forest regrowth happened relatively slowly, was anthropogenic impact, i.e. via logging, burning and/or for agriculture (Fig. 3).  The slowest rates of recovery occurred after climatic disturbances and the fastest after large infrequent events, e.g. landslides, hurricanes and natural fire.  This latter result is somewhat intuitive given that these perturbations are a natural part of all ecosystems, leading to the evolution of a dynamic response in the native plant communities.  

Figure 3
Fig. 3  Composite figure showing how the recovery rate varies with different variables.

Insights into resilience

When we looked at the standardised rate of disturbance events (SRD), i.e. the number of disturbance events per 1,000 years, we found that the greater the frequency events occurred in the past, the more quickly the forest responded to each subsequent disturbance.  This runs counter to contemporary theories on resilience that describe slowing rates and diminishing ability to recover with each subsequent perturbation (e.g. Veraart et al., 2012).  Our results suggest that over ecologically meaningful timescales, i.e. over the life-span of entire forest communities rather than single trees, increased exposure results in adaptation to that disturbance over time, leading to a greater ability to recover quickly from the perturbation.

What does this all mean for tropical forests?

From looking back into the past, it seems that tropical forests can take a long time to recover from disturbances, and that different regions may require different management regimes to encourage more complete reforestation after natural or anthropogenic events, such as fire.  Central American and African forests may bounce back from impacts more quickly than the other regions, with disturbances such as tropical hurricanes and climatic fluctuations being a more common component of these ecosystems than in the other tropical regions.  However, all of the forests we looked at demonstrated a greater vulnerability to anthropogenic impacts and climatic changes than large infrequent disturbances: the two major forms of disturbance occurring today and at levels that far exceed those experienced over the past 20,000 years – reasons for caution.

Sustainable management

Identifying and understanding the different ecological requirements of forests across the different geographical regions, and of the forest-types within those regions, is vital for developing more sustainable landscape management plans.  With increasing international concern over deforestation rates, the associated loss of biodiversity and elevated carbon dioxide emissions, the conservation and restoration of tropical forests is becoming more politically and economically feasible.  Indonesia, for example, has introduced ‘ecosystem restoration concessions’ in the last decade, providing a legal means for forest protection from the further expansion of industrial agriculture.  And the potential of Reducing Emissions from Deforestation and Forest Degradation (now REDD+) to save the World’s forests continues to generate international debate. Of importance to all of these programmes and initiatives, is the suggestion from our study that forests take time to recover, and if we give them that time, they will persist, and continue to provide their faunal inhabitants, including us, the greatest collection of biological riches on Earth.

Tropical forest carbon storage is related to tree richness and traits. Or is it?

There’s been a lot said about relationships between species diversity and ecosystem function over the last two decades. The general view of these relationships is that diverse ecosystems are more productive, use resources more efficiently and are more stable.

But, and it’s a big but, almost none of this work has been done in forests and even less in mega-diverse tropical forests. Because of this diversity productivity relationships can’t really be described as general. How do we know what is general across the globe if we have only concentrated on temperate grasslands?

This is something a new paper in Global Ecology and Biogeography by Kyle Cavanaugh and colleagues hopes to set straight. Their study drew on a dataset of carbon storage and tree biodiversity from 59 plots across the tropics produced by members of the Terrestrial Ecosystem Assessment Monitoring  (TEAM) Network.

The great thing about this work is that all plots were surveyed using the same methods, meaning they should be reasonably comparable. All sites collected measures of aboveground carbon storage, genus diversity, functional diversity – by measuring wood density of trees and potential maximum diameter, and the mean value for wood density and maximum diameter for each plot. All of this was then analysed while trying to account for climatic differences between sites.

The general findings of the study were that both genus diversity and the mean potential maximum diversity of species appear to be  positively related to aboveground carbon storage.

Relationships between
Relationships between site (a) elevation and genus richness, (b) precipitation and functional diversity, (c) carbon storage and genus richness and (d) carbon richness and mean maximum diameter of trees. Stolen from the paper.

This enforces the view that diverse ecosystems are more productive and that large species may contribute a disproportionate amount of biomass – as I have written before. Very few studies have shown a relationship between diversity and biomass in tropical systems before, so this is exciting stuff.

And yet, I still have a few queries about some findings. The study failed to find any relationship between climate and carbon storage – a connection that is fairly well established. Also it uses stepwise model selection, which is beginning to become one of my (and others’) pet peeves . I am of the feeling that testing all possible models and then averaging amongst them based on the ones that have greatest support is the best way to do things, and this often comes up with very different findings to stepwise selection.

Previous similar work has suggested that carbon – biodiversity relationships are scale-dependant, with positive relationships in small plots and mixed results at larger plot sizes. Given the increasing number of tropical forest research networks I am sure this study will not be the last of its type. Once these get published we will have a better idea of how general these findings are.  At the moment I am not completely convinced.

Are large, old trees in decline?

I’ve banged on enough about the crisis in forests for people here to know what the deal is.

Anyway, there has recently been a bit of back-and-forth regarding the state of large, old trees at a global scale.

These trees are key in both forest and non-forest ecosystems. The definition of what is ‘old’ and ‘large’ is specific to each region but it is widely accepted that these trees tend to store lots of carbon and are valuable for many species because of their structural complexity.

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David Lindenmayer and colleagues published a note last year on the importance of large, old trees and evidence for their declines, and they expanded on that with an article discussing policy options to deal with these declines.

This is all important stuff and they had me convinced. It makes sense. Large, long lived species are disproportionately vulnerable to threats because they take a long time to reach maturity and they are targeted simply because they are large – for animals see hunting of ungulates, for trees see selective logging.

However, a recent letter by Edward Faison has made me doubt the claims of Lindenmayer. Faison points out that there have been increases in the abundance of large trees in forests in Sweden, Spain, Hungary, Italy and Switzerland. In addition there have apparently been relatively few declines of large trees in North America.

Lindenmayer and colleagues have since rebutted this letter, saying that there is a difference between large old trees and simply large trees.  They point out that there have been increases in Europe and North America but that these increases have been from a very low point since both regions have historically cleared large swathes of forest for agriculture. They also point out the loss of large trees as a result of logging in the tropics as well as in Australia, North America and Siberia.

And yet I am still not entirely convinced.

Don’t get me wrong, I believe that large old trees are probably in decline in the ecosystems they talk about, but is this a general trend all over the place?

I am also a little scared that all of the discourse on this so far has been in the form of reviews/essays that could easily cherry-pick some cases and then craft a nice narrative around them. It is easy to believe the stories we tell ourselves and this is where we as scientists should be most self critical. After all how many beautiful sounding theories have been seen to have nothing to do with how things work in the real world? The only way to confront such problems is with cold, unemotional statistical analysis.

How long does tropical forest take to recover from agricultural clearance?

Secondaryforest4
Intermediate secondary forest in Paragominas, Para, Brazil – Photo credit to the fantastic Ricardo Solar, you can see more of his pics here

Today our work on the recovery of secondary tropical forests got published in Royal Society Proceedings B. I’m really chuffed with this piece of work and in this blog I’m going to summarise what we found out and why I think it’s important. If you want to read the paper you can get it here.

Large areas of tropical forest have been cleared for agriculture over the last 100 years.

Why does this matter? Well it matters because these forests are vital for the unique biodiversity in the tropics but also because humans can benefit from them remaining intact.

Their loss causes extinction, release of carbon into the atmosphere – worsening climate change, and changes the ecosystem services we get from these forests.

Because of the importance of these forests their restoration is seen as a priority by some. There are valiant attempts to restore tropical forests in Brazil and various Central American countries. In addition there are also international initiatives that aim to encourage the restoration of carbon and biodiversity (E.g. CBD & REDD+). These are great and ambitious aims but, until now, we didn’t really know how long these recoveries took, or whether recovery was different for different disturbance and forest types.

To solve all this we collected the biggest dataset yet compiled on recovery of aboveground, belowground and soil carbon as well as plant species richness and community composition following agricultural clearance. All this data came from previous studies.

Carbon_pools_colour

We found that after about 80 years aboveground carbon storage was around 85% of that found in undisturbed forests, while belowground carbon storage seemed to recover more slowly. Soil carbon showed no relationship with time since clearance.

Richness_colour

In terms of biodiversity both tree and epiphyte species richness seemed to increase over time, with tree richness recovering after around 50 years since disturbance but epiphytes took around 100 years.

Prop_sp

However, when we looked at species that are found in the undisturbed forests, relatively few of them are found in the recovering forests. They didn’t seem to accumulate over time either. Given that these species are likely to be more prone to extinction it is worrying that they don’t seem to be doing very well in secondary forests.

We think that carbon recovers relatively well following abandonment of land since there tends to be a rapid influx of woody species. However, we also think that complete recovery of carbon is likely to take more than a century since this is likely to be dependent upon large, slow-growing trees.

Differences between tree species richness recovery and that of epiphytes is likely to be because tree seeds are more easily transported between forests than those of epiphytes. Also epiphytes seem to be found more on big trees, and there don’t tend to be many of these in secondary forests.

The lack of recovery of species found in undisturbed forests is perhaps the most disturbing thing that we found. We think that to improve this situation there may be a need for management of these forests by planting trees and helping to increase dispersal of seeds throughout the non-forest areas.

Disturbed forests like this are not worthless.
Regrowing forests like this are vital if we wish to conserve biodiversity in our human dominated world. Photo credit again to Ricardo Solar

There’s been lots of great work recently on the value of disturbed forests. We hope our work goes into a bit more detail where the soon-to-be-classic work of Luke Gibson etl al  left off which showed that primary forest has greater conservation value than any types of disturbed forest in the tropics. We agree with this, particularly for specialist species. However, most tropical forests are not primary forests and have been logged, cut down or burnt at some point in recent history. Because of this we think that older secondary forests need to be recognised as important for conservation and carbon storage and their clearance should be avoided. These forests are not worthless.

How well do logged forests recover?

Photo credit to tlupic of flickr
Photo credit to tlupic on flickr

Logging  is one of the most widespread threats to tropical forests. It doesn’t seem to be disastrous for forest biodiversity, although that is somewhat unclear as I have discussed previously. However, it does reduce carbon storage because of the removal of trees – causing carbon emissions, which is bad – no matter what some people would prefer you to believe.

Even if you think climate change is green hype there are reasons to worry about logging.  The time given to logged foreststo recover is often not enough to allow timber species to recover properly. If this is widespread, it would put the long term sustainability of logging practices in the tropics in doubt.

Recovery of biodiversity, carbon, timber stocks and a whole host of other things are vital to work out how long forests should be left to recover between each logging period. Despite this there is actually relatively little data on recovery following logging, and this is particularly lacking from Africa.

To fill this gap researchers from France, Belgium, Central African Republic and Gabon looked at the recovery of a logged forest in Central Africa over more than 20 years. This involved setting up a logging experiment  in the forests near M’Biaki in the Central African Republic, which looks something like this. The area has been monitored for changes in biomass and timber stocks since logging took place in 1984 in forest that was unlogged, logged and logged and then trees thinned out during recovery to encourage growth.

Change in (a) aboveground biomass and (b) timber stock over time after logging. Figure taken from Gourlet-Fleury et al 2013.
Change in (a) aboveground biomass and (b) timber stock over time after logging. Figure taken from Gourlet-Fleury et al 2013.

Predictably logging forests sharply reduced their biomass and timber stock, with biomass reduced by about 30% and timber stock by 50%. More interestingly biomass then increased back to levels similar to undisturbed forests in  by 2011, while timber stock did not.

This is alarming because this forest was logged  much less intensively than those on other continents, but still did not recover its timber stocks. We should be worried by this. It means that even at the relatively low intensities of logging that happen in Central Africa, it might not be sustainable.

The authors’ argue that in order to make logging more sustainable the diameter at which trees are cut should be increased, whilst encouraging thinning to promote regrowth. I agree. However, we also need innovative solutions that go beyond those proposed already.

Reduced Impact Logging, which aims to reduce the amount of damage done to non-timber trees may help carbon stocks, but is less likely to aid timber stock recovery. A possible solution for tropical logging may be a combination of reduced logging techniques with planting of timber species which have been grown off site, in a similar way to that done by restoration projects. This could be lead by more researchers engaging with logging companies and encouraging projects to benefit sustainability of biodiversity and timber.

At the moment the problem of logging is similar to that of fisheries:  forests provide a resource that is difficult limit access to and is difficult to track once exploited. As well as improving logging practices it is vital that we improve governance, but if I knew how to do that I wouldn’t be sat here writing a blog for free.

We do need to make progress on this issue though, we can’t stick everything in protected areas – that smacks of green colonialism and people need to get resources in one way or another. Idealists may accuse those who work with logging companies of dining with the devil. Fair enough. But we’re not going to solve these problems by ignoring them.

Guns, birds and squirrels

Tropical forests are getting ever more fragmented, human population in the tropics is increasing and guns are now widely available.

All of this has led to an explosion in the number of people hunting for food in the tropics.

This hunting can cause local extinction of bird and mammal species, with large bodied species being particularly at risk. This can lead to loss of species that eat fruit and therefore act as dispersers of plant seeds.

This dispersal is important since it means that species are dispersed widely around forest, rather that just being concentrated in small areas around their parent plant. However, those species that eat seeds, causing them to be damaged and therefore unable to germinate, may also be lost as a result of hunting. The balance between the losses of these two types of species will determine their effects on plant reproduction.

In general, it appears that losses of animal species, particularly larger species, as a result hunting tends to lead to an increase in the abundance of plant species which don’t require animals for dispersion. However, the results of these studies can sometimes be unclear due to lack of replication and because they have tended to be over a relatively short period of time.

A new study in Ecology Letters aims to tidy up our view of how hunting affects plant species. This work studied the dynamics of Lambir forest in Malaysian Borneo, which looks like this:

Lambir forest
photo credit: berniedup on flickr

Though it looks nice, this forest has been hunted for over 15 years and this has caused the local extinction of seed dispersing species like the white crested hornbill, which looks like this:

White crested hornbill
photo credit: berniedup on flickr

as well as the red giant flying squirrel, which looks like this:

Red giant flying squirrel
photo credit: vil.sandi on flickr

However, seed predators such as the sambar deer have also become locally extinct

5180069064_f5ba447792_b
photo credit: Smithosian Wild on flickr

This situations mirrors that of other study sites and makes it hard to determine how hunting will affect plant biodiversity.

For their study Rhett Harrison and colleagues investigated the changes in diversity and distribution of plant species in Lambir by monitoring nearly 500,000 (!) individual trees. They found that the density of seedlings tended to increase – suggesting a reduction in the amount of seed predation going on as well as a reduction in dispersal by animals. They also found that tree richness was reduced, though this reduction was relatively modest.

Figure 1 - richness and seedlings
Number of seedlings (a) and tree species richness (b) change during the study period. Error bars are 95% confidence intervals.

Most interestingly the study also suggests that plant species that need animals to disperse their seed tended to become relatively more clustered than species which didn’t rely on animals.

Figure 2- seed mode
Degree of clustering by dispersal mode during study. Lines around dots represent 95% confidence intervals.

All of these results suggest that hunting can have marked effects on tropical forest plant biodiversity – in the long run leading to a potential decline in some animal dispersed species.

Reading this study reminded me of attempts to link traits of species which determine the probability of extinction and those which affect ecosystem functions and services. In this case large body size is associated with a dietary preference for fruit or seeds – with obvious consequences for seed dispersal. What really sets this study apart is the length and size of it which means it is the most precise study of its kind. Linking these traits will allow us to generalise about the ecology of hunting in tropical forests but this is only part of the solution.

Large areas of South East Asia, West Africa and the Atlantic forest in Brazil are facing similar pressures from hunting, so this phenomenon may be quite widespread. Though it is obviously less of a threat to biodiversity when compared to deforestation and other more dramatic degradation the subtle effects of hunting may occur both inside and outside protected areas going relatively unnoticed. To tackle this problem effectively we need to know the motivation for this hunting. Only then can we start to deal with what to do to stop it.