Boreal tree growth exhibits decadal‐scale ecological memory to drought and insect defoliation, but no negative response to their interaction

Abstract

Interactions between drought and insect defoliation may dramatically alter forestfunction under novel climate and disturbance regimes, but remain poorly understood. We empirically tested two important hypotheses regarding tree responses to drought and insect defoliation: (a) trees exhibit delayed, persistent, and cumulative growth responses to these stressors; (b) physiological feedbacks in tree responses to these stressors exacerbate their impacts on tree growth. These hypotheses remain largely untested at a landscape scale, yet are critical for predicting forest function under novel future conditions, given the connection between tree growth and demographic processes such as mortality and regeneration.2. We developed a Bayesian hierarchical model to quantify the ecological memory of tree growth to past water deficits and insect defoliation events, derive antecedent variables reflecting the persistent and cumulative effects of these stressors on current growth, and test for their interactive effects. The model was applied to extensive tree growth, weather, and defoliation survey data from western and eastern regions of the Canadian boreal forest impacted by recent drought and defoliation events and characterized by contrasting tree compositions, climates, and insect defoliators.3. Results revealed persistent negative tree growth responses to past water (all trees) and defoliation (host trees) stress lasting 3-6 and 10-12 years, respectively, depending on study region. Accounting for the ecological memory of tree growth to water and defoliation stress allowed for detection of interactions not previously demonstrated. Contrary to expectations, we found evidence for positive interactions among non-host trees likely due to reduced water stress following defoliation events. Regional differences in ecological memory to water stress highlight the role of climate in shaping forest responses to drought.

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

“…Drought legacy effects appear to be extremely common in temperate and boreal forests globally (Anderegg et al 2015a), even in mesic ecosystems and in response to relatively mild droughts (Kannenberg et al 2019a). These effects can last for 4 + years in some systems (Anderegg et al 2015a, Peltier et al 2016; Itter et al 2019) and the length of this lag in growth is contingent on broad climatic factors and ecosystem types (Gazol et al 2017). Despite their ubiquity, legacy effects are highly variable across taxa (Camarero et al 2018) and can even vary within the same species due to differences in age (Gao et al 2018), site conditions such as stand density and diversity (Gazol & Camarero 2016; Bottero et al 2017), and drought characteristics such as severity and seasonal timing (Merlin et al 2015; Huang et al 2018; Kannenberg et al 2019a).…”

“…Thus, the post‐drought trajectory that a tree follows is likely dependent on numerous factors, but especially contingent on the severity, duration and timing of the drought period itself, and the post‐drought climatic conditions that could facilitate or inhibit recovery. Additional disturbances such as frost or pest‐pathogen dynamics could also compound the impacts of drought (Gazol et al 2019; Itter et al 2019; Vitasse et al 2019), though this question remains understudied in the context of legacy effects. These findings highlight the need for future studies to consider exactly what types of drought give rise to legacy effects and why.…”

“…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.…”

EcoMem: An R package for quantifying ecological memory

“…The utility of the EcoMem package for ecological inference is demonstrated through a case study assessing the memory of boreal tree growth to insect defoliation events. Recent work has shown that boreal tree growth exhibits negative responses to insect defoliation for several years following a moderate-to-severe defoliation event (Itter et al, 2018). We apply the EcoMem package to annual tree growth and insect defoliation survey data for 34 sites across Alberta, Canada to assess the ecological memory of tree growth to defoliation (see Itter et al, 2018, for detailed description of dataset).…”

Physiological and PIP Transcriptional Responses to Progressive Soil Water Deficit in Three Mulberry Cultivars

“…Although enormous studies have investigated the above-mentioned responses to drought in a long list of economic tree species at different levels from species ( Zaher-Ara et al., 2016 ), population ( Kreyling et al., 2008 ) to ecosystems ( Golinski et al., 2008 ; Itter et al., 2019 ), our understanding of the physiological and transcriptional responses in mulberry, a relatively less mainstream tree species, is still limited compared to its counterparts such as the model tree poplar. Mulberry generally shows strong endurance to various environmental stresses, and large variations in drought tolerance exist among different mulberry species/cultivars ( Sharma and Zote, 2010 ; Vijayan et al., 2011 ).…”