So one of the main ideas behind the development of iLand is, that to understand and simulate forest ecosystem dynamics with particular consideration of disturbances, an integrated view of climate, disturbance agents, and the vegetation is needed. The idea is that what we observe as forest dynamics is the result of the complex interactions between those three elements.
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In a recent effort we’ve been putting this framework to the test with regard to disturbance changes in Europe’s forests. The background: Disturbances have been increasing throughout the continent over the last decades, as my colleague Mart-Jan Schelhaas first reported a few years ago. While the trend is pretty clear its drivers have been subject to some discussion: For western North America, for instance, colleagues recently showed that similar increases were mostly driven by changes in the climate system. But European forests, unlike their western US counterparts, have a long and intensive management history, so also management-induced vegetation changes could be hypothesized as drivers behind observed disturbance increases.
Following the scheme outlined above (which is a modification of an earlier conceptual figure by Virginia Dale and co-workers), our hypothesis was that it wouldn’t be either climate or vegetation changes driving the disturbance increase, but a complex interplay of both. Thus we set out to test this hypothesis on disturbance data for continental scale Europe from 1958 to 2001, using machine learning and structural equation modeling as our main methodological tools.
The results, just published in Global Change Biology, support our general framework, with about two thirds of continental scale disturbance damage from wind and bark beetles occurring under coinciding elevated climate conditions AND increased susceptibility of the forest vegetation. In other words, it’s the interplay of both climate and forest change that caused the recent increase in forest disturbances in Europe. And although the factors differ somewhat between agents (increases in area burnt were most strongly driven by climate, while increases in bark beetle damages were most strongly related to increasing forest susceptibility), this pattern seems to hold for most areas of Europe (read the full results here).
Besides the fact that this is one of the first quantitative studies on the causes of the widespread increase in forest disturbances, there are two broader implications that I deduce from our findings: First, disturbance damages are likely going to increase further, since both the atmosphere as well as forest ecosystems have a long system memory, i.e. the past and current anthropogenic changes to both systems are going to have a continuing effect for many years to come. In our analysis of temporal trends we didn’t find a reversal of a trend towards increasing forest susceptibility and climatic triggers of disturbances yet, quite the opposite actually. From the data it looks like we’re more and more moving towards a highly disturbed forest future. Thus it is highly important to mainstream disturbance thinking in our ecological and management considerations.
The second implication for me is that we’ll need models that are able to incorporate and address those interactions between the climatic environment, the forest vegetation, and a variety of disturbance agents. Models that help us address these effects, and ultimately inform management planning and thus support the sustainable provision of ecosystem services. As our recent review showed, this is still quite a challenge for modeling, but within the iLand project we’re diligently working towards advancing our capacities in this regard.
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