Empirical and process-based approaches to climate-induced forest mortality models

Henry D. Adams,Sara A. Rauscher,Xiaoyan Jiang,Nate G. McDowell,Chonggang Xu,A. Park Williams

doi:10.3389/fpls.2013.00438

Abstract

Globally, forests store ∼45% of car-bon sequestered terrestrially, contributemore to the terrestrial sink per areathan any other land cover type, andassimilate an important portion ofanthropogenic emissions (Bonan, 2008).Forests exert strong biophysical controlon climate via surface energy balance(Bonan, 2008; Rotenberg and Yakir, 2010;Houspanossian et al., 2013), and thehydrological cycle (Zhang et al., 2001;Brown et al., 2005). Widespread for-est mortality in response to drought,increased temperatures, and infestationof tree pests has been observed globally,potentially threatening forests’ regula-tion of climate (Kurz et al., 2008; Adamset al., 2010; Allen et al., 2010; Anderegget al., 2013a). This threat has promptedgreat interest in understanding and pre-dicting tree mortality due to climatevariability and change, especially drought.Initial tests of hydraulic failure (mortal-ity caused by irreversible loss of xylemconductivity from air embolism), carbonstarvation (mortality due to carbohy-drate limitation), insect attacks, wildﬁre,and their interdependence (Allen, 2007;McDowell et al., 2008, 2011, 2013a), sug-gest proximal causes of mortalityare likelycomplex, co-occurring, interrelated, andvariable with tree species (supported byAdams et al., 2009, 2013; Sala et al.,2010; Piper, 2011; Zeppel et al., 2011;Anderegg et al., 2012a, 2013b; Adamsetal.,2013;AndereggandAnderegg,2013;Galvez et al., 2013; Gaylord et al., 2013;Hartmann et al., 2013a,b; Mitchell et al.,2013; Quirk et al., 2013; Williams et al.,in review). While the interdependent rolesof carbon and water in plant mortalityare consistently observed, this work iscontinuously prompting new questions(Sala et al., 2010; McDowell et al., 2013b;O’Grady et al., 2013).The justiﬁcation for physiologicalresearch on drought-induced tree mortal-ity is often stated as a need to improve thepredictive capability of vegetation mod-els through incorporation of mortalitymechanisms(Fisheretal.,2010;McDowellet al., 2011, 2013a; Powell et al., 2013). Yetif mortality is particularly complicatedand associated with failure of multiplephysiological processes (Manion, 1981;McDowell et al., 2011; Anderegg et al.,2012b), then a key question emerges: is amechanistic approach necessary for accu-rate prediction of future mortality? Theanswertothisquestionultimatelydependson the application and goal of the model.At issue is whether increasing modelcomplexitywillimproveprediction,whichis inﬂuenced in part by the modelingapproach employed. Two endpoints ona theoretical continuum of approachto mechanism are process-based andempirical model types. The process-based approach focuses on simulatingdetailed physical or biological processesthat explicitly describe system behav-ior, while the empirical approach relieson correlative relationships in line withmechanistic understanding, but with-out fully describing system behaviorsand interactions (Korzukhin et al., 1996;

Highlights

The justification for physiological research on drought-induced tree mortality is often stated as a need to improve the predictive capability of vegetation models through incorporation of mortality mechanisms (Fisher et al, 2010; McDowell et al, 2011, 2013a; Powell et al, 2013)
The processbased approach focuses on simulating detailed physical or biological processes that explicitly describe system behavior, while the empirical approach relies on correlative relationships in line with mechanistic understanding, but without fully describing system behaviors and interactions (Korzukhin et al, 1996; Table 1)
The advantages and disadvantages of both approaches have been well acknowledged in ecology (Korzukhin et al, 1996; Levin et al, 1997; Makela et al, 2000; Green et al, 2005; Van Nes and Scheffer, 2005)

Summary

Introduction

The justification for physiological research on drought-induced tree mortality is often stated as a need to improve the predictive capability of vegetation models through incorporation of mortality mechanisms (Fisher et al, 2010; McDowell et al, 2011, 2013a; Powell et al, 2013). An ensemble approach for forest mortality should include models from across the spectrum of empirical to process-based types (e.g., McDowell et al, 2013a). To quantify the feedback of vegetation impacts upon future climate, reliable predictions of forest mortality at the global scale should be simulated.

Results

Conclusion