Bioclimatic Envelopes for Individual Demographic Events Driven by Extremes: Plant Mortality from Drought and Warming

Abstract

The occurrence of plant species around the globe is largely constrained by climate. Ecologists use plant-climate relationships such as bioclimatic envelopes to determine environmental conditions promoting probable species occurrence. Traditional bioclimatic envelopes exclude disturbance or include disturbance as infrequent and small scale effects, assuming that the net effect of climate on demographic processes predicts longer-term equilibrial responses of biota. Due to increasing frequency and extent of extreme events associated with climate change, ecologists may need to increase focus on individual demographic events driven by extreme events such as large-scale tree die-off. Existing approaches that predict traditional equilibrial biogeographic responses associated with long-term trends in mean climate could be complemented with an expanded focus on how extreme events catalyze individual demographic events. Extreme conditions of drought are often a prerequisite for abrupt demographic events such as large-scale tree die-off, with effects of extremes often exacerbated by climatic trends such as warming. In this PERSPECTIVE, we illustrate the use of bioclimatic envelopes for predicting individual demographic events. Currently, data on conditions that drive individual demographic events are usually aggregated across time and/or are correlative. We highlight this approach with a case study of experimentally drought-induced mortality in Pinus edulis trees resulting from a combination of ecologically extreme conditions in one parameter and a shifting distribution in another: drought under higher temperatures. Based on this example, we predict a more than five-fold increase in the frequency of die-off events under a global change scenario of high emissions. This general approach complements traditional bioclimatic envelopes, and more detailed physiological approaches currently being refined to address climate change challenges. Notably, this approach could be developed for other climate conditions and plant species, and may improve predictions of abrupt demographic events that are altering ecosystems globally.

Small-scale water deficits after wildfires create long-lasting ecological impacts

“…Therefore, these types of changes do not qualify as ecological drought. In contrast, loss of dominant species in a plant community does constitute a threshold change if the loss is permanent or long-term (Bestelmeyer et al 2018, Law et al 2018. Drought indices do not relate well at the individual plot or local community scale, and thus are poor predictors of within-season plant responses and demographic processes such as germination, establishment, or mortality, which can lead to restructuring of plant communities.…”

Drought supersedes warming in determining volatile and tissue defenses of piñon pine (Pinus edulis)

“…Field study sites Field campaigns took place monthly during the 2010 May through September growing season at two experimental sites in northern Arizona, USA: A piñon pinejuniper woodland at 35.49°N 111.85°W (Ambient), and a hotter-drier site at 35.45°N 111.50°W (HFotter-Drier) ca. 35 kilometers east of the Ambient site (Law et al 2019). Air temperature at the Hotter-Drier site was 1.5°C warmer on average than the Ambient site when measurements were made.…”

“…In October 2008, piñon pine trees of similar size and age were randomly selected from the Ambient site and, using a 2.3 m tree spade, were immediately transplanted into open areas of the site as well as at the lower elevation Hotter-Drier site. The transplanted trees were regularly watered until August 2009 when the drought treatments began, resulting in 100% survival following the transplant at both sites (Law et al 2019). All selected trees had nearest neighbor canopy-tocanopy distances of at least one meter and were transplanted in a grid of squares with a spacing of at least 10 m. Eighteen trees at each site were randomly assigned to three moisture treatment groups: watered (n=3), ambient (n=6), and barrier (n=9).…”

“…Pre-dawn water potential measurements were made within one week of gas exchange and monoterpene flux measurements, which are described below. See appendix A in Law et al (2019) for additional details on the field transplant experiment.…”

“…We focused our study on the piñon pine Pinus edulis, a widespread, well-studied species that has undergone extensive drought-herbivore-related mortality (Breshears et al 2005(Breshears et al , 2018. Trees were transplanted downhill to a hot and dry site outside the species' range (hereafter, 'Hotter-Drier'), and to a site near their origin which served as an ambient temperature control (hereafter, 'Ambient') (Law et al 2019). Within each site, piñon pine water status was altered by watering trees and by adding impervious barriers to prevent water from entering the soil as precipitation or leaving through soil evaporation.…”