Abstract
Hot drought is a climate phenomenon that has lately received much attention for its potential to disrupt forest function worldwide. A sharp increase in tree mortality associated with this climate pattern are often cast as a disturbance, in which high temperature is responsible for causing exceptional rates of mortality. The alternative interpretation is simply that drought kills trees in a density-regulating manner and within the bounds of normal forest function. To evaluate the evidence for disturbance versus regulating dynamics, we conducted censuses across 30 plots in the Edwards Plateau region of central Texas, USA, four years after the hot drought of 2011. The purpose was to explain variation in population responses to drought, including crown mortality, resprouting rate and sapling survivorship in terms of physical site factors, community characteristics and local climate data. Through model selection analysis, we identified the most parsimonious binomial regression models for the three most common species. In Ashe juniper populations, overall crown mortality was 20% and all predictive factors indicated the influence of population-regulating dynamics. In live oak (Quercus virginiana & Q. fusiformis), which had a crown mortality rate of 23%, the influence of regulating factors was less prominent, but there was also no evidence that crown death was linked to heat exposure. However, resprouting in both species appeared to be inhibited by heat exposure, as was crown mortality in the understory species Texas persimmon (Diospyros texana). Along with overall high levels of sapling survivorship, these patterns suggest that Ashe juniper woodlands, despite having been relatively hard hit by the 2011 drought, are not particularly threatened by hot drought events, although some subordinate species may be. Density-regulating mechanisms of forest drought response are often underplayed, but they are far easier to represent in vegetation models than disturbance dynamics.
Highlights
There is growing concern that episodes of extreme temperature related to global warming in interaction with episodic drought embody a new threat to the world’s forests, which are already decimated by intentional deforestation (Pravalie, 2018)
The dynamic occurs in self-regenerating populations after disturbance, if recruitment occurs within a narrow timeframe (Bi and Turvey, 1996; Coomes and Allen, 2007)
There has been no clear road map for developing mortality algorithms. They could be developed from trait-based models of individual stress tolerance, emulate the stochastic disturbance framework of fire models, or build upon frameworks of density-dependent resource supply and demand
Summary
There is growing concern that episodes of extreme temperature related to global warming in interaction with episodic drought embody a new threat to the world’s forests, which are already decimated by intentional deforestation (Pravalie, 2018). Numerous studies to date have documented forest die-off events, caused either directly or indirectly by hot and dry conditions (Allen et al, 2010; Anderegg et al, 2016). These events have often been interpreted as disturbances, in which harsh conditions kill many trees through direct effects on physiological function (Zeppel et al, 2013). Debate has predominantly focused on proximate causes of death, for example, whether trees died because of irreversible desiccation, carbon starvation or the interaction of these factors (Zeppel et al, 2013; Adams et al, 2017). We suggest that the classic ecological division between density-dependent and -independent mortality can help frame the question in a different way and thereby guide the development of new mortality algorithms for dynamic vegetation models or determine whether new algorithms are necessary at all (Fisher et al, 2018)
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