Establishing why some tree populations survive drought while others decline
Climate change-type drought is disrupting tree populations and the distribution of tree species across the globe. Predictions suggest that rising global temperatures should drive tree species poleward and upward in elevation, as they track the climates to which they are adapted. It therefore also follows that population loss and regional extinctions should occur at species rear range-edges, that is, the most drought-prone areas of species distributions. Although this prediction is well supported in the literature, evidence of rear-edge population persistence is also accumulating. The PERS-RELICT-CLIM project looked especially at relict populations. This is the most impressive example of persistence where populations remain geographically isolated in marginal climates, significantly dryer than that tolerated by the species. Field, molecular and modelling approaches The EU-funded PERS-RELICT-CLIM project was set up to propose a framework for research design and analysis of species' rear range-edges, and to demonstrate how such an analysis can improve the understanding of rear-edge population responses to increased drought conditions. The researchers developed the methodological framework, drawing upon habitat, functional, genetic and demographic information. They established the study at the rear range-edge of the European beech tree (Fagus sylvatica L.), a species with significant ecological importance, yet which is highly drought-sensitive. The field-based research assessed the impact of increased drought occurrence on the demographic responses of populations and the physiological performance of trees, and evaluated the contribution of microclimatic conditions to population persistence. The team also assessed whether habitat fragmentation resulted in genetic isolation and the loss of genetic diversity, evaluating the implications for future population decline. The modelling approach studied tree growth dynamics under different scenarios of climate change, using an extensive network of dendroecological data from across Europe. The first results point to a higher resilience to drought than expected, in geographically isolated populations found in the most drought-prone habitats. At the same time, researchers discovered the core of the range was more strongly impacted by recurrent droughts. Summarising the project results, the project coordinator Prof. Alistair Jump says, “Our project strongly suggests that we need to assess population decline from a different perspective, with an improved methodology. Our proposed framework has great potential to improve our predictive understanding of species rear range-edge dynamics.” The empirical evidence that the project gathered suggested that rear-edge population dynamics are determined by complex ecological and evolutionary factors, and that they do not always respond neatly to predictions about the consequences of climate warming. Extending the reach The results of PERS-RELICT-CLIM are likely to benefit a range of disciplines, from population ecology to population genetics and biogeography. The project’s information will improve our understanding of climate change impacts at the rear-edge of species distributions, helping to predict the consequences from regional extinctions and trophic cascades (food chain interactions), to carbon and water dynamics. The project Research Fellow, Dr Albert Vilà-Cabrera, concludes, “The proposed framework and the information gained in this project have the potential to increase the predictability of climate change impacts across a wide range of taxa and ecosystems. It can be used to optimise conservation strategies to maintain the functions and services that rear-edge populations provide to human societies, such as the regulation of water and nutrient cycling, habitats for biodiversity.” It is anticipated that factors, such as land-use changes or biological invasions, can be incorporated within the PERS-RELICT-CLIM framework. Finally, the combination of field-based approaches with remote sensing methodologies also has the potential to improve the predictive understanding of rear-edge population dynamics.
Keywords
PERS-RELICT-CLIM, trees, species, climate change, adaptation, evolution, range edges, ecology, conservation, population, drought, resilience
