Constraints to and conservation implications for c

Authors: Matthew J Christmas Martin F Breed Andrew J Lowe

Publish Date: 2015/09/24

Volume: 17, Issue: 2, Pages: 305-320

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Abstract

Contemporary climate change is having widespread impacts on plant populations Understanding how plants respond to this change is essential to our efforts to conserve them The key climate responses of plant populations can be categorised into one of three types migration in situ adaptation or extirpation If populations are to avoid extirpation then migration and/or in situ adaptation is essential In this review we first articulate the current and future constraints of plant populations but trees in particular to the different adaptation strategies eg space availability rate of change habitat fragmentation niche availability Secondly we assess the use of the most appropriate methods eg natural environmental gradients genome and transcriptome scans for assessing and understanding adaptive responses and the capacity to adapt to future challenges Thirdly we discuss the best conservation approaches eg assisted migration biodiversity corridors ex situ strategies to help overcome adaptive constraints in plants Our synthesis of plant and particularly tree responses and constraints to climate change adaptation combined with the identification of conservation strategies designed to overcome constraints will help deliver effective management actions to assist adaptation in the face of current and future climate changePlant responses to climate change constraints on these responses and conservation strategies best suited to overcoming these constraints Coloured circles represent populations Shade of green indicates how well adapted the average phenotype is to the environment bold green well adapted light green maladapted Arrows in c represent gene flow The use of seed banks could be employed under all scenarios to safeguard against loss of genetic diversity due to population extirpationThe ‘first line’ response to a changing climate may therefore be migration—a geographic shift in distribution via seed dispersal into climatically suitable areas Fig 1b However such shifts may not be straightforward In an analysis of USDA Forest Inventory and Analysis data from across the eastern United States 587  of 92 tree species showed indications of range contraction at both northern and southern boundaries of their distribution Zhu et al 2012 Only 207  of species exhibited patterns consistent with a northward shift and crucially evidence was particularly lacking for population spread in areas where climate had changed the most This suggests that migration may not be as common as expected and potentially puts populations at higher risk of becoming increasingly maladapted over time Under the projected high rates of future climate change successful migration may also rely on the evolution of ever longer dispersal distances in order to enable species to reach suitable new habitatIf species fail to sufficiently migrate then adaptation in situ will be essential for persistence Fig 1c–d The ability to adapt in situ will rely upon a wide range of factors These include 1 heritable trait variation ie trait variation attributable to genes within populations for selection to act upon 2 levels of gene flow from populations at lower latitudes/altitudes introducing ‘preadapted’ alleles to or more generally increasing genetic variation within populations at higher latitudes/altitudes Sexton et al 2011 Kremer et al 2012 and 3 the chance occurrence of new advantageous mutations arising within populations However the likelihood of new adaptive alleles arising particularly those of large effect as may be required under rapid climate change Reed et al 2011 is very low The presence of standing heritable genetic variation that can provide a fitness advantage under new climatic conditions would bestow populations with the best chances of adapting in situAn alternative to geneticbased adaptation is epigenetic or plastic responses Fig 1e Trait plasticity can result in a wide range of phenotypes occurring in the same genetic background acting as a buffer to environmental changes Ghalambor et al 2007 Nicotra et al 2010 There is now increasing evidence that some heritable variation in ecologically relevant traits is not caused by genes but is instead stimulated by epigenetic mechanisms that alter gene activity through targeted molecular processes Fieldes and Amyot 1999 Molinier et al 2006 Yakovlev et al 2011 In particular DNA methylation and histone posttranslational modifications are key mechanisms for geneexpression regulation and development in plants These mechanisms can bring about a rapid 1 generation adaptive response to environmental change Bossdorf et al 2008 Rival et al 2012If populations fail to adapt or migrate under climate change then as conditions become more unsuitable extirpation and extinction may be unavoidable Fig 1a Mass extinction events have had a strong association with major climatic change over the past 520 million years with low levels of global biodiversity and high extinction rates prevalent immediately following periods of major temperature change Mayhew et al 2008 Extinctions linked to contemporary climate change have already been observed for a number of animal groups eg checkerspot butterflies McLaughlin et al 2002 and frogs Pounds et al 2006 but as yet there are no reports of climate driven extinctions in plants There is of course the possibility that some species will persist without migration or genetic adaptation by for example exploiting microclimatic variation within their current range eg populations move from sun to shade from dry slopes to seeps through facilitative interactions with other local plant species Brooker et al 2008 or endophytic relationships with soil microbes Lau and Lennon 2012 Increasing our understanding of the ways that plant species are likely to respond under climate change and particularly of the constraints that restrict the adaptive responses of species and populations is integral to our attempts to conserve speciesThere have been a number of recent reviews on plant responses to climate change each with a different emphasis eg Davis and Shaw 2001 Walther 2003 Jump and Peñuelas 2005 Aitken et al 2008 Anderson et al 2012 Corlett and Westcott 2013 For example Aitken et al 2008 looked specifically at trees and Corlett and Westcott 2013 predominantly considered migrational responses across all taxa Here we provide an updated appraisal of the most recent literature on plant responses to contemporary climate change We focus mainly on the threats caused by direct climate effects ie changes in temperature and rainfall with climate envelopes shifting towards the poles while acknowledging that climate change is complex and its effects will not be felt equally across all environments and locations More complex/synergistic impacts of global change have been discussed elsewhere eg Brook et al 2008 While we consider the responses of both short and longlived species our review pays particular attention to the responses of and potential impacts on tree species Our review is focussed on three largely neglected aspects Firstly we focus on key constraints to responses to climate change Secondly we discuss the best approaches to assess adaptive potential in plants highlighting the use of environmental gradients and genome and transcriptome scans to understand the type magnitude and rate of adaptive responses Thirdly we consider the best conservation approaches to help overcome adaptive constraints in plants We discuss how this information can be integrated into conservation actions to help prevent species extinctions as well as facilitate ongoing adaptation to climate

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