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.

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.

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
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 yearspost 1976.
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.
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.

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!

Friday Linkfest: Vagueness in ecology, and was I wrong about tropical forest recovery?

Those of you who read my latest post on defining resilience will know I just spent two days discussing what resilience is. It is somewhat ironic, then, that over on Dynamic Ecology on Jeremy Fox just put up a blog post discussing situations when vagueness in ecology can be useful. Considering that he has previously been a stickler for precision he is very open to vagueness, but only when it is productive. For example the concepts of ‘species,’ ‘ecosystem’ and, even ‘ecology’ are fairly vaguely defined – but very few people would argue that they aren’t useful…

Thompson-Reuters have just released their Impact Factors for the year. Exciting, right?  Methods in Ecology and Evolution have moved up to be the 9th highest ranked journal in ecology, and have a blog post about this. Worth reading if only for Bob O’Hara’s recognition that the whole system is a bit ridiculous.

A really nice looking new paper out this week shows that animals with bigger prey, that forage in three dimension tend to have bigger home ranges. Not usually my topic, but I was blown away by how good the figures looked in this paper.

In case you missed it, our paper on the response of different functional groups to forest recovery just came out a week or two ago, you can read a blog post on it here and see the open-access paper here.

For those of you into r and stats, here is a nice ggplot2 cheat sheet that is easily searchable (HT Dynamic Ecology).

A paper that challenges the conclusions of some of my work on forest regeneration has just come out in PNAS. The paper suggests that the characteristics of tropical forests recovering from clearance are highly idiosyncratic. One of our previous studies suggested that these recovery trajectories are relatively predictable, but I am open to the possibility we were wrong. I think this is one paper I need to have a proper read of and write a full post about…

Just what is resilience, anyway?

Last week I organised a workshop bringing together researchers interested in resilience from across the Biodiversity and Ecosystem Service Sustainability (BESS) programme run by NERC. I’ll write about things that came out of it over the next few weeks. Here is my first missive

“Is it real? Is it an obscure object of desire?” my boss Adrian asked during our workshop . Given that  nearly 20 years ago there were 163 different definitions of ecosystem resilience, it is perhaps no wonder that we we having a few problems. Part of this problem is that resilience is a boundary object – a term that is interpreted differently by different communities. During our meeting it became clear, for example, that ecological researchers and policy-makers did not necessarily mean the same thing when they were talking about resilience.

Generally, researchers see resilience as a property of a system. Being researchers, we want to quantify this resilience. However, it turns out that resilience can’t really be viewed as a single thing, since it is made up of a number of different qualities. Along with the expert guidance of Volker Grimm our workshop came up with 3 different elements that are important when assessing resilience for research:

  1. Recovery – The return of a variable to the reference state after a disturbance.
  2. Resistance – A variable staying essentially unchanged despite disturbances.
  3. Persistence – Persistence of the system over time.

Using these three different properties allows researchers to look at different aspects of resilience and compare across systems. Making such comparisons is actually very difficult due to constrains on time and funding, as well as logistical problems. For example, to compare resistance of different communities you would ideally apply different intensities of disturbance in different locations. This may be possible in some relatively ‘fast’ systems such as grasslands but it is unlikely that you would get permission to do this to a woodland where you might have to cut trees down. In order to resolve this, mechanistic models can be exceptionally useful for investigating different scenarios of change. Combining this with empirical data collection in the same system can help us gain a more detailed understanding of resilience. This is something we are aiming to do in our current project as part of my post-doc work.

Policy makers on the other hand generally view resilience as a goal. Recently policy documents have begun to mention the importance of resilience. For example, one of the Convention on Biological Diversity’s 2020 aims is to:

By 2020, ecosystem resilience and the contribution of biodiversity to carbon stocks has been enhanced, through conservation and restoration, including restoration of at least 15 per cent of degraded ecosystems, thereby contributing to climate change mitigation and adaptation and to combating desertification.

Similarly the environment white paper in published by the UK government in 2010 mentions resilience 36 times and the Welsh government is aiming to create:

A biodiverse natural environment with healthy functioning ecosystems that support social, economic and ecological resilience and the capacity to adapt to change.

It is also included in US and Australian policy. So in the case of policy-makers it becomes clear that resilience is seen as a target. While for researchers resilience can mean something very specific policy-makers probably consider it to be closest to the previous definition of persistence.  At our workshop there were plenty of anecdotes about policy-makers saying things like “resilience is the new sustainability” and telling civil servants to “stick some resilience in your report, it’s the new thing.” There were also reports that some policy-makers wanted the production of maps of resilience. I think this is potentially dangerous. Given that the ratio of empirical work to conceptual stuff/reviews and perspectives pieces is about 1:1000 we simply don’t have enough evidence to produce these maps at the moment. If push came to shove then we could probably come up with a best guess based on ecological theory, but even then there would be all sorts of caveats.

I think it’s clear we will never reach a point where there is one definition of resilience that fits everyone’s need. However, when we talk about resilience we need to be clearer about what we mean by it. So next time you use it in a paper, for the love of god, define it.

*Edit #1 – I just came across this nice post by Jeremy Fox on defining stability concepts in ecology, which, if might be a useful companion piece to what I said here.

*Edit #2 – Ambroise Baker who helped organise the workshop with me has a short summary of the meeting over on the Lake BESS blog, you can see that here.