A Tree-Centered Approach to Assess Impacts of Extreme Climatic Events on Forests

摘要

Introduction A major task of our society is to manage forests in a way that their resources are preserved to meet future generation needs (Forest Europe et al., 2015 ). Current scenarios of climate change effects are making this task extremely challenging (Kirilenko and Sedjo, 2007 ). Climate shifts will impact forest vitality and affect goods and services forests provide, including carbon sequestration and climate change mitigation (IPCC, 2014 ). To guide sustainable forest management, forest researchers are asked to provide concrete answers about forest resilience in response to expected climatic trends, and extreme climatic events (Lindner et al., 2014 ). This is not an easy task, because responses of trees and forest ecosystems to environmental conditions are often non-linear and moreover vary on spatial and temporal scales (Smith, 2011 ; Anderegg et al., 2012 ; Reichstein et al., 2013 ). For instance, although drought is one of the most frequent and widespread climatic extremes affecting forests worldwide (e.g., Allen et al., 2010 ), the assessment of its impact on future forests is currently under intense debate. Mechanisms behind tree growth and mortality are complex (McDowell et al., 2008 , 2011 ; Fatichi et al., 2014 ; Anderegg et al., 2015 ; Meir et al., 2015 ). Besides strength or frequency of external factors, such as extreme events, also the tree's ability to resist and recover is relevant, which, in turn, is largely determined by intrinsic factors such as the tree's life stage, life history, and genetic characteristics.In this paper, we advocate for a tree-centered approach. By providing an improved mechanistic understanding of physiological and growth responses of trees growing under various conditions we can define the tree's capacity to respond to external stress factors. This concept can valuably contribute to the debate on how to shape future forests toward resilient forest ecosystems. A tree-centered approach Current spatiotemporal simulations on future forest growth responses to changing climate conditions are performed with dynamic global vegetation models (DGVMs; Wullschleger et al., 2014 ). These models—usually generalizing tree species as plant functional types (PFTs)—provide valuable descriptions of the evolution of natural vegetation at a grid cell level under several climate scenarios. Such approaches are powerful in assessing growth responses related to the interaction between vegetation and atmosphere (including anthropogenic impact). However, although first steps toward representation of tree species, size classes, and forest structure in a DGVM were recently made (e.g., Naudts et al., 2016 ) they often lack to explain the variability between and within species, and often do not adequately explain growth responses (Fatichi et al., 2014 ) under varying site conditions, and to climatic extremes (Anderegg et al., 2015 ). These aspects are however extremely relevant to evaluate plasticity of tree individuals and tree species and the resilience of forests under changing climatic conditions, especially considering changing frequencies and intensities of climatic extremes (Reyer et al., 2013 ).The tree-centered approach proposed here considers the individual tree as main source of information for understanding variability in growth responses. Comprehensive investigations using well-selected trees growing under different environmental conditions foster a better understanding of projected large-scale forest responses to changing climate. In comparison to generalizing approaches using PFTs, the tree-centered approach yields information with less spatial coverage but with the potential to convey more details on specific tree responses to a given climatic factor. This knowledge complements other approaches and can for instance support forest managers in tree species and/or provenance selection to better prepare specific forest stands to cope with expected challenges. Four important elements The incentive for the tree-centered approach is gaining a process-based understanding on tree responses to changing environmental conditions on temporal scales varying from short-term responses to climatic extreme events to long time periods matching the life cycles of tree populations. This can be achieved through an ensemble of observational studies on a selection of trees from different species and life histories, growing in diverse settings (forest types, species composition, successional stages, management regimes), and exposed to different climates and extreme climatic events. Establishing such a model framework requires the following elements: In-depth understanding of causal processes occurring within the tree in response to environmental changes. The cascade of physiological and growth responses is assessed by an integration of real-time observations of physiological and structural growth responses. Assessment of the link between tree structure and function. This allows for evaluation of the