Ecotone response to climatic variability depends on stress gradient interactions

Abstract

Background: Variability added to directional climate change could have consequences for ecotone community responses, or positive and negative variations could balance. The response will depend on interactions among individuals along environmental gradients, further affected by stress gradient effects. Methods: Two instantiations of the stress gradient hypothesis, simple stress and a size-mediated model, are represented in a spatially explicit agent based simulation of an ecotone derived from observations of Abies lasiocarpa, Picea engelmannii, and Pinus albicaulis in the northern Rocky Mountains. The simple model has two hierarchically competitive species on a single environmental gradient. The environment undergoes progressive climate change and increases in variability. Because the size model includes system memory, it is expected to buffer the effects of extreme events. Results: The interactions included in both models of the stress gradient hypothesis similarly reduce the effects of increasing climatic variability. With climate amelioration, the spatial pattern at the ecotone shows an advance of both species into what had been a higher stress area, but with less density when variation increases. In the size-mediated model the competitive species advances farther along the stress gradient at the expense of the second species. The memory embedded in the size-mediated model does not appear to buffer extreme events because the interactions between the two species within their shifting ecotone determine the outcomes. Conclusions: Ecotone responses are determined by the differences in slopes of the species response to the environment near their point of intersection and further changed by whether neighbor interactions are competitive. Interactions are more diverse and more interwoven than previously conceived, and their quantification will be necessary to move beyond simplistic species distribution models.

Intraspecific variability may not compensate for increasing climatic volatility

“…The processes of recruitment and mortality were simulated by a Monte Carlo process with the probabilities determined by the environmental gradient that changes through time. The probabilities were modiied by neighbors in a feedback that actualized a version (Malanson et al 2017) of the stress gradient hypothesis (Bertness and Callaway 1994) in which the density of neighbors was competitive at high relative itness and linearly changed to facilitative at low relative itness (in Fig. 1a the stress is the opposite of relative itness).…”

“…The plant's-eye-view of the environment was deined by the itness of two hypothetical species (hereafter S1 and S2, subscript s) as hierarchical Gaussian curves on the long axis of the grid (Fig. 1) (following Keddy 1989; Malanson 1997Malanson , 2015Malanson et al 2017): where the mode of the curves, φ µ , was 1.0 and 0.65 and their breadth, twice the variance or 2σ 2 , was 55,000 and 110,000 for S1 and S2, respectively; the coeicients were tuned so that the two species were at similar equilibrium numbers in iterations 200-300 in the case with constant volatility and heterogeneity. The initial environmental gradient was then individualized for every virtual organism.…”

“…where d = number of neighbors in a 3 ⋅ 3 cell window. Given that no mechanistic quantiication exists for most instances of the stress gradient hypothesis and that a number of reiications have been tried Resler 2015, 2016;Malanson et al 2017), these coeicients arbitrarily produced a logarithmic increase in efect with the density of neighbors so that the irst neighbors have greater efect than the later ones. This efect was then apportioned so that as a multiplier of Eys (see below) it ranged from 0.5 at high itness to 2.0 at lowest itness.…”

“…This approach collapses the various dimensions of niche to a single gradient appropriate for a simple exploratory model that has generality. The model emphasizes the ecotone between two species and extends two-species model analyses of general ecological principles seen at alpine treeline ecotones (Malanson and Resler 2015;Malanson 2017b;Malanson et al 2017). The environmental variability in this model is based on novel derivations from tree ring records of Pinus albicaulis.…”

Assessing the relative vulnerabilities of Mid‐Atlantic freshwater wetlands to projected hydrologic changes

“…Increasing temperatures, altered patterns of precipitation, and related increases in evapotranspiration can result in changes in surface and groundwater levels, where a change in only a few centimeters can result in changes in wetland size, in loss of wetlands to drylands, or in conversions to other wetland types (Schouten et al 1992, Burkett andKusler 2000). Climate changes also interact with the aforementioned anthropogenic stressors such as land use change (Ghedini et al 2015, Malanson et al 2017, potentially exacerbating existing wetland problems.…”