Drought-induced tree mortality: ecological consequences, causes, and modeling

Abstract

Drought-induced tree mortality, which rapidly alters forest ecosystem composition, structure, and function, as well as the feedbacks between the biosphere and climate, has occurred worldwide over the past few decades, and is expected to increase pervasively as climate change progresses. The objectives of this review are to (1) highlight the likely ecological consequences of drought-induced tree mortality, (2) synthesize the hypotheses related to drought-induced tree mortality, (3) discuss the implications of current knowledge for modeling tree mortality processes under climate change, and (4) highlight future research needs. First, we emphasize the likely ecological consequences of tree mortality from ecosystem to biome to continental scales. We then document and criticize multiple non-exclusive tree mortality hypotheses (e.g., carbon starvation — carbon supply is less than carbon demand; and hydraulic failure — desiccation from failed water transport) from a more comprehensive ecological perspective. Next, we extend a forest decline concept model, Manion’s framework, by considering new emerging environmental conditions, for a more thorough understanding of the effects of climate change on forest decline. We find that an increase in drought frequency and (or) climate-change-type droughts may trigger increased background tree mortality rates and severe forest dieback events, accelerating species turnover and ecological regime shifts. The contribution of CO2 fertilization, rising temperature within the optimal growth range, and increased nitrogen deposition may defer or reduce this trend in tree mortality, but such contributions will vary between locations, species, and tree sizes. Multiple hypotheses proposed for drought-induced tree mortality are discussed, but coupling carbon and water cycles could help resolve the debate. The absence of a physiological understanding of tree mortality mechanisms limits the predictive ability of current models from stand-level process-based models to dynamic global vegetation models. We thus suggest that long-term observations, experiments, and models should be tightly interwoven during the research process to better forecast future climate changes and evaluate their impacts on forests.

Review of broad-scale drought monitoring of forests: Toward an integrated data mining approach

“…In forests, the reduced moisture levels and high temperatures associated with drought contribute directly to tree stress and mortality (Wang et al, 2012;Anderegg et al, 2013;Millar and Stephenson, 2015). Drought also impacts forests indirectly, by altering the frequency and severity of disturbances such as insect outbreaks or wildfire (Schowalter et al, 1986;Mattson and Haack, 1987;Meyn et al, 2007;Raffa et al, 2008;Trouet et al, 2010;Martínez-Vilalta et al, 2012).…”

“…Additionally, because climate conditions vary across time scales, the appropriate timespan for meaningful climatic baselines is debatable (Lamb and Changnon, 1981;Livezey et al, 2007;Wilks, 2013). Drought-associated tree mortality can sometimes cause rapid changes in forests (Mueller et al, 2005;Wang et al, 2012), but a long-term successional perspective may be necessary to discern the relative importance of observed changes over the lifespans of trees. Having a relevant temporal context is particularly critical because applicable remotely sensed data have only been available for a few decades, or roughly a third of the time that ground-station-based meteorological data have been available on a wide scale (Choi et al, 2013).…”

Can't see the trees for the forest: complex factors influence tree survival in a temperate second growth forest

“…This concern is motivated by the important suite of ecosystem services provided by forest trees as foundation species (Ellison et al 2005). A number of studies and reviews have highlighted regional (van Mantgem et al 2009, Peng et al 2011, Luo and Chen 2013 and global (Allen et al 2010, Wang et al 2012) decreases in tree survival, particularly in large trees (Dolanc et al 2013, McIntyre et al 2015, and some of these studies implicate climate change. In the arid West of the United States, studies of declining tree survival focus on old-growth forests, where endogenous changes in forest structure and species composition are minimal and assumed not to influence demographic trends Stephenson 2007, van Mantgem et al 2009, but see Lutz et al 2014).…”

Mapping Mountain Pine Beetle Mortality through Growth Trend Analysis of Time-Series Landsat Data

“…Understanding patterns of forest disturbances is important for assessing past and future ecosystem structure, function, productivity and diversity [1,2]. In recent years, mountain pine beetle (MPB) outbreaks have occurred over extensive areas, from British Columba in Canada to New Mexico in the United States.…”