Implications of mechanistic modeling of drought effects on growth and competition in forest landscape models
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Ecosphere. 7(4): 10.1002/ecs2.1253. 23 p.
The incidence of drought is expected to increase worldwide as a factor structuring forested landscapes. Ecophysiological mechanisms are being added to Forest Landscape Models (FLMs) to increase their robustness to the novel environmental conditions of the future (including drought), but their behavior has not been evaluated for mixed temperate forests. We evaluated such an approach by assessing the ability of physiological mechanisms to predict susceptibility to tree mortality as a function of drought in the upper Midwest (USA) through controlled site-level drought simulation experiments using the PnET-Succession extension of the LANDIS-II FLM. We also conducted a landscape-level experiment to study landscape response to drought treatments in the presence of the spatial processes of seed dispersal and stand-replacing disturbance. At the site level we found that net photosynthesis and carbon reserves showed a clear negative response to both the length of drought and the alternating pulses of normal precipitation and drought events, with soils holding more water moderating this response. The effect of the drought treatments varied somewhat depending on the assemblage of competitors and their specific life-history traits such as ability to compete for light, maximum photosynthetic capacity and water use efficiency. A large diversity of assemblages were simulated at the landscape level, and species abundance generally sorted by photosynthetic capacity (foliar nitrogen) and life form (deciduous vs. evergreen), with faster growing species and deciduous species suffering less decline because of drought. Soil type also had an impact on total productivity (biomass), with soils having higher available water being more productive through time. We conclude that (1) the mechanistic, first principles approach is advantageous for global change research because the combination of life-history traits of competitors interact to cause a site-specific dynamic response to fundamental drivers (e.g., precipitation, temperature), and these site-level responses interact spatially to create landscape responses that are complex and difficult to project with less mechanistic approaches, and (2) published findings that increasing drought length (rather than severity) increases tree mortality in the upper Midwest are clearly consistent with a mechanism of acute photosynthetic depression resulting in increased likelihood of carbon starvation as droughts lengthen.
Keywordsclimate change, drought-induced mortality, first principles, forest succession, LANDIS-II, mechanistic landscape model, PnET-Succession, water stress
Gustafson, Eric J.; De Bruijn, Arjan M. G.; Miranda, Brian R.; Sturtevant, Brian R.; Thompson, J. 2016. Implications of mechanistic modeling of drought effects on growth and competition in forest landscape models. Ecosphere. 7(4): 10.1002/ecs2.1253. 23 p. https://doi.org/10.1002/ecs2.1253.