A synthesis of tree functional traits related to drought‐induced mortality in forests across climatic zones

Abstract

1. Forest dieback caused by drought-induced tree mortality has been observed worldwide. Forecasting which trees in which locations are vulnerable to drought-induced mortality is important to predict the consequences of drought on forest structure, biodiversity and ecosystem function. 2.In this paper, our central aim was to compile a synthesis of tree traits and associated abiotic variables that can be used to predict drought-induced mortality. 3.We reviewed the literature that specifically links drought mortality to functional traits and site conditions (i.e. edaphic variables and biotic conditions), targeting studies that show clear use of tree traits in drought analysis. We separated the review into five climatic-zones to determine global versus regionally restricted relationships between traits and mortality. 4.Our synthesis identifies a number of traits that have clear relationships with drought-induced mortality (e.g. wood density at the species level and tree size and growth at the individual level). However, the lack of direct relationships between most traits and drought-induced mortality highlights areas where future research should focus to broaden our understanding. 5.Synthesis and applications. Our synthesis highlights established relationships between traits and drought-induced mortality, presents knowledge-gaps for future research focus and suggests monitoring and research avenues for improving our understanding of drought-induced mortality. It is intended to assist ecologists and natural resource managers choose appropriate and measurable parameters for predicting local and regional scale tree mortality risk in different climatic-zones within constraints of time and funding availability

The effect of environmental filtering on variation in functional diversity along a tropical elevational gradient

“…At the lowest elevation the habitat is characterized by high temperatures, relatively high soil nutrient availability and low levels of water in the soil. Water availability is often the most important factor in determining forest distribution in tropical areas with seasonal precipitation (O'Brien et al., ). In the case of drought, limited water availability greatly affects the forest composition and structure at lower elevations, more so than high temperatures, especially for the seasonal deciduous monsoon forest at the lowest elevation, where most canopy trees lose their leaves during the dry season (Zang, Ding, Zhang, Deng, & Mao, ).…”

“…On the contrary, water availability is an extremely important resource for trees in tropical regions. At lower elevations, the seasonal distribution of precipitation and maximum temperature influences the morphology and phenology of tree species, which could significantly affect a tree species’ performance and distribution (O'Brien et al., ). Different stressors may affect species strategies differently.…”

Droughts and the ecological future of tropical savanna vegetation

“…Thus, multiple consecutive years with below average rainfall are not uncommon in most savannas (Fensham et al, ). Although savanna vegetation has evolved a diverse range of strategies to cope with such variability (O'Brien et al, ), extreme and/or protracted periods of rainfall deficit, especially when coupled with warmer temperatures, can nevertheless result in the widespread mortality of both savanna grasses (Staver, Wigley‐Coetsee, & Botha, ; Swemmer et al, ; Walker, Emslie, Owen‐Smith, & Scholes, ) and trees (Fensham et al, ; Fensham, Laffineur, & Allen, ). Here, I primarily focus on the effects of such extreme droughts on savanna vegetation.…”

“…For the purposes of this review, I use the term drought to broadly refer to periods where lowered rainfall during the growing season reduces plant water availability to levels significantly below average annual conditions (Greenwood et al, ; O'Brien et al, ). I do not specifically consider how vegetation responses change with drought intensity per se, but acknowledge that the magnitude of observed responses will undoubtedly be more pronounced as droughts become more extreme and protracted (Greenwood et al, ; Ruppert et al, ; Young et al, ).…”

“…I draw on both observational studies that have recorded responses of savanna trees and grasses to droughts, as well asexperimental studies that have manipulated plant wateravailability under controlled conditions. Although experimentsallow for clearer attribution of causes (Greenwood et al, ; Smith, ), manipulating water availability for adult savanna trees is challenging given their size and longevity (O'Brien et al, ). Consequently, experiments are typically limited to grasses and tree seedlings, and do not capture adult tree responses, which observational studies do.…”