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The persistence of relict populations under climate change

Periodic Reporting for period 1 - PERS-RELICT-CLIM (The persistence of relict populations under climate change)

Reporting period: 2016-02-01 to 2018-01-31

Climate change is disrupting population performance and the distribution of tree species across the globe. Biogeographical theory suggests that rising global temperatures should drive species to move poleward and upward in elevation as they track the climates to which they are adapted. Consequently, one fundamental climate change prediction is that population loss and regional extinctions of tree species should occur in the most drought prone areas of their ranges (i.e. the rear range-edge). This prediction assumes that rear-edge populations are marginal with higher risk of extinction than range-core populations because they occur at reduced population sizes along less favorable habitats. This assumption is well supported in the literature documenting altered tree growth and mortality in response to increased drought stress. However, evidence of rear-edge populations persisting in the most drought-prone habitats is increasingly accumulating. For example, relict populations are one of the most impressive cases of persistence as they persist geographically isolated in climates significantly hotter and dryer than that tolerated by the species. The disparity of documented responses is potentially a result of complex ecological and evolutionary factors that determine population dynamics, and points out the need for a proper understanding and assessment of marginality and the identification of the key mechanisms implied in rear-edge population persistence.

Forests provide essential ecosystem functions and services, including regulation of nutrient and water cycling, atmospheric composition and climate, as well as the maintenance of biodiversity and human well-being. However, climate change is expected to alter forest functioning, with profound consequences that range from genes to ecosystems. Rear-edge populations occur along the transition between biomes and contribute to the impressive diversity of ecosystems and species found in these regions. Furthermore, rear-edge populations hold a singularity that makes them very relevant for the maintenance of ecosystem services and biodiversity, as well as very attractive for the uses and profits by human societies. In this context, the progress that this project provides is essential to refine and advance our ability to predict, monitor and manage the impacts of climate change on the function and fate of rear range-edge populations.

Here we aim to propose a framework for research design and analysis at species' rear range-edges, and we then aim to demonstrate how the framework improves our understanding of marginality and rear-edge population dynamics. In order to put into practise the framework we have established our study system at the rear range-edge of the European beech tree (Fagus sylvatica L.), a species with a high ecological importance yet highly drought sensitive. The species occurs along the transition between the Mediterranean and temperate biomes, where beech populations are distributed across a fragmented landscape and along highly heterogeneous ecological conditions. In this region, relict beech populations persist out of the species' physiological tolerances. The main objectives of this project are:

1) To propose a conceptual and methodological framework for a better understanding and assessment of marginality in order to refine predictions of rear-edge population decline.
2) To test if populations inhabiting the most drought-prone habitats show higher levels of mortality and canopy decline, and altered tree growth and limited physiological performance to increased drought stress.
3) To monitor and characterize the microclimate of rear-edge populations, and to test if microclimates contribute to the local persistence of populations under unfavourable regional climates.
4) To study the amount genetic variation of rear-edge populations across ecological and habitat fragmentation gradients, and to test if geographical isolation is associated to the loss of genetic diversity.
5) To model tree growth dynamics and predict population loss under different scenarios of climate change.
A novel framework is proposed for research design and analysis and it encompasses three fundamental dimensions of marginality: ecological, geographic, and genetic. Population distribution along these dimensions and their extremes (e.g. climatic conditions, habitat fragmentation, species composition, soil type, etc.) are inferred from existing data and incorporated into experimental study design and analyses.

The framework is applied to field-based studies across 40 populations distributed across the species rear range-edge. Preliminary results suggest that adult mortality and canopy decline (i.e. defoliation and early leaf yellowing and browning) is concentrated in drought prone areas within the core-range, while relict populations show signs of high persistence (low mortality and canopy decline). Dendroecological data and physiological parameters are measured, while population microclimatic conditions monitored. All this information complements in-deep the preliminary results found and provides key data to understand population performance in terms of tree growth and water-use efficiency in response to drought stress.

At the same time, leaf tissue is sampled and its DNA sequenced across multiple rear-edge populations distributed along gradients of fragmentation and climatic drought stress. This data allows to (1) evaluate the role of geographical isolation determining the amount of genetic diversity across populations and its temporal trends by comparing the genetic characteristics among tree age classes; (2) to contribute on the production of a reference genome sequence for the European beech tree; and (3) to contribute to identify patterns of local adaptation of rear-edge populations inhabiting the more drought-prone areas of the species range.

Finally, an extensive network of dendroecological data collected across the entire species distribution including rear-edge, range-core and leading-edge populations is used to project future growth trends under different climate change scenarios. The outputs of the models will help to predict which populations are the most vulnerable to increased aridity, as well as where population loss will occur under future climatic conditions.
This project represents a significant advance in Ecology, especially in relation to the predictive understanding of rear-edge populations in a context of climate change. The presented framework will inspire scientists of different disciplines to redefine theory at the rear-edge of species distributions with the concept of marginality at its heart. The results obtained will signify a relevant progress for population ecology, population genetics and biogeography, while the information gained will be relevant to improve our ability to understand the impacts of climate change at the rear-edge of species distributions and to predict their consequences, from regional extinctions and trophic cascades to carbon and water dynamics. Furthermore, given that the effects of global warming are affecting rear-edge populations across the globe, the framework developed here can be useful to increase the predictability of the effects of climate change across a wide range of taxa and ecosystems, and optimize management and conservation strategies to maintain the functions and services that rear-edge populations provide to human societies.