Temperature memory and non-structural carbohydrates mediate legacies of a hot drought in trees across the southwestern USA

Abstract

Trees are long-lived organisms that integrate climate conditions across years or decades to produce secondary growth. This integration process is sometimes referred to as ‘climatic memory.’ While widely perceived, the physiological processes underlying this temporal integration, such as the storage and remobilization of non-structural carbohydrates (NSC), are rarely explicitly studied. This is perhaps most apparent when considering drought legacies (perturbed post-drought growth responses to climate), and the physiological mechanisms underlying these lagged responses to climatic extremes. Yet, drought legacies are likely to become more common if warming climate brings more frequent drought. To quantify the linkages between drought legacies, climate memory, and NSC, we measured tree growth (via tree ring widths) and NSC concentrations in three dominant species across the southwestern US. We analyzed these data with a hierarchical mixed effects model to evaluate the time-scales of influence of past climate (memory) on tree growth. We then evaluated the role of climate memory and the degree to which variation in NSC concentrations were related to forward-predicted growth during the hot 2011–2012 drought and subsequent 4-year recovery period. Populus tremuloides exhibited longer climatic memory compared to either Pinus edulis or Juniperus osteosperma, but following the 2011–2012 drought, P. tremuloides trees with relatively longer memory of temperature conditions showed larger (more negative) drought legacies. Conversely, P. edulis trees with longer temperature memory had smaller (less negative) drought legacies. For both species, higher NSC concentrations followed more negative (larger) drought legacies, though the relevant NSC fraction differed between P. tremuloides and P. edulis. Our results suggest that differences in tree NSC are also imprinted upon tree growth responses to climate across long time scales, which also underlie tree resilience to increasingly frequent drought events under climate change.

Root:shoot balance controls flush phenology and carbohydrate translocation dynamics in citrus (Citrusxsinensis) trunk

“…A utilization pool between current flush shoot and mature shoots grown in last growth cycle is similar to the buffer pool within current new growth and the nearby mature needles in gymnosperms (Kozlowski, 1992). More recent studies indicate that stem nonstructural carbohydrates are and active and important intermediate pool, of sufficient importance to impact evolution of temperate tree species (Furze et al, 2021; Peltier et al, 2021), indicating that low transport and high stem storage may be common among a number of tree species, depending on ecological niche adaptation. Thus, slow photoassimilate translocation speed in citrus could be associated with this utilization strategy of distribution among local organs within the plant.…”

Contemporary tree growth shows altered climate memory

“…More details on chronology length are provided in Figure S1. Because we only estimate shifts in memory at the species-level, to address H2, we assess how spatial variation in memory quantified in a different study using the same data (Peltier et al, 2021) is related to mortality in the most recent survey at nearby Forest inventory and analysis (FIA) plots. of all RWI values (i.e., across cores) in a given year, producing 109 age-detrended chronologies containing a total of 8714 annual ring-width indices.…”

“…To address differences in sample depth across time and concerns about over-representation of "sensitive" trees in the ITRDB (Klesse et al, 2018;Nehrbass-Ahles et al, 2014), we repeated our analysis (see Model description) with an "ecologically sampled" network of 11 piñon sites across the southwestern US (Peltier et al, 2021). Briefly, we sampled dominant and co-dominant trees of varying sizes and ages at 11 sites in Arizona, New Mexico, Colorado and Utah.…”

Predicting resilience through the lens of competing adjustments to vegetation function

“…Multiple evidence streams—satellites (Wigneron et al, 2020), eddy covariance (Schwalm et al, 2017), tree‐rings (Peltier et al, 2021; Vanoni et al, 2016), and manipulation experiments (Zweifel et al, 2020)—have shown that drought impacts on forest carbon stocks and tree growth can last several years after droughts have concluded. One of the mechanisms most often invoked to explain these legacies is hydraulic damage.…”