Legacies of more frequent drought in ponderosa pine across the western United States

Abstract

Despite widespread interest in drought legacies—multiyear impacts of drought on tree growth—the key implication of reported drought legacies remains unaddressed: as impaired growth and slow recovery associated with drought legacies are pervasive across forest ecosystems, what is the impact of more frequent drought conditions? We investigated the assumption that either multiple drought years occurring during a short period (multiyear droughts), or droughts occurring during the recovery period from previous drought (compounded droughts), are detrimental to subsequent growth. There is evidence that drought responses may vary among populations of widespread species, leading us to examine regional differences in responses of the conifer Pinus ponderosa to historic drought frequency in the western United States. More frequent drought conditions incurred additional growth declines and shifts in growth–climate sensitivities in the years following drought relative to single‐drought events, with ‘triple‐droughts' being worse than ‘double‐droughts'. Notably, prediction skill was not strongly reduced when ignoring compounded droughts, a consequence of the temporally comprehensive formulation of our stochastic antecedent model that accounts for the climatic memory of tree growth. We argue that incorporating drought‐induced temporal variability in tree growth sensitivities can aid inference gained from statistical models, where more simplistic models could overestimate the severity of drought legacies. We also found regional differences in response to repeated drought, and suggest plastic post‐drought sensitivities and climatic memory may represent beneficial physiological adjustments in interior regions. Within‐species variability may thus mediate forest responses to increasing drought frequency under future climate change, but experimental approaches using more species are necessary to improve our understanding of the mechanisms that underlie drought legacy effects on tree growth.

Patterns in nonstructural carbohydrate contents at the tree organ level in response to drought duration

“…To reduce the proportion of tissues with negative carbon balance, they re‐allocate carbohydrates from older to younger leaves or to tissues associated with the capture and transport of water (roots and sapwood; Chaves et al, 2009). In addition, insufficient carbohydrate reserves will limit full leaf area production and growth in the years following the drought (Peltier & Ogle, 2019; van der Molen et al, 2011). Hence, all of these factors contribute to the significant variation of NSCs of evergreen trees.…”

“…The increasing severity and frequency of drought events not only significantly reduce forest productivity but also cause rapid forest collapse and lead to further changes in forest composition, structure, and geographical distribution, converting the world's forests from a net carbon sink into a large carbon source (Anderegg et al, 2013; Brendan et al, 2012; Herrera‐Estrada, Satoh, & Sheffield, 2017; Peltier & Ogle, 2019; van der Molen et al, 2011; Wang, Zhuang, & Lähteenoja, 2018). Nonstructural carbohydrates (NSCs), as important substances involved in trees' life processes, greatly affect the growth and even the survival of trees.…”

Tree growth sensitivity to climate is temporally variable

“…Other studies have followed, including quantification of legacies occurring at the ecosystem scale from eddy-flux covariance data sets (Schwalm et al 2017), and numerous other tree-ring based studies focused on the southwestern US (Peltier et al 2016;Gao et al 2018;Peltier & Ogle 2019a), the western US (Peltier & Ogle 2019b), the eastern US (Kannenberg et al 2018), the conterminous US (Dannenberg et al 2019), the Northern Hemisphere (Wu et al 2018), and global syntheses (Yin & Bauerle 2017;Huang et al 2018). Drought legacies have also been shown in tree seedlings in the tropics (O'Brien et al 2017).…”

“…masting or nutrient pulses) leading to variable growth‐climate sensitivity may be apparent (Reimchen & Arbellay 2019), others (e.g. drought legacies, decline or lagged mortality, climate change forcings) are still poorly understood (Peltier & Ogle 2019b). The impacts of these drivers, however, are all potentially quantifiable through their effects on the sensitivity of tree growth to climate, with potentially strong links to key quantities of interest, such as aboveground biomass, carbon fluxes and transpiration.…”

Ghosts of the past: how drought legacy effects shape forest functioning and carbon cycling

“…However, it is important to note that these studies rarely report the uncertainty associated with the ring width predictions. Finally, several studies have used more complex statistical methods such as lagged autocorrelation or cross correlation models to understand the influence of antecedent climatic conditions on tree growth (Ogle et al 2015; Peltier et al 2018; Szejner et al 2018; Itter et al 2019; Peltier & Ogle 2019). These models have the advantage of not relying on a semi‐arbitrary cutoff for defining a ‘severe drought’ and can also be sensitive enough to identify the influence of water availability many years in the past.…”