Experiment, monitoring, and gradient methods used to infer climate change effects on plant communities yield consistent patterns

Abstract

Inference about future climate change impacts typically relies on one of three approaches: manipulative experiments, historical comparisons (broadly defined to include monitoring the response to ambient climate fluctuations using repeat sampling of plots, dendroecology, and paleoecology techniques), and space-for-time substitutions derived from sampling along environmental gradients. Potential limitations of all three approaches are recognized. Here we address the congruence among these three main approaches by comparing the degree to which tundra plant community composition changes (i) in response to in situ experimental warming, (ii) with interannual variability in summer temperature within sites, and (iii) over spatial gradients in summer temperature. We analyzed changes in plant community composition from repeat sampling (85 plant communities in 28 regions) and experimental warming studies (28 experiments in 14 regions) throughout arctic and alpine North America and Europe. Increases in the relative abundance of species with a warmer thermal niche were observed in response to warmer summer temperatures using all three methods; however, effect sizes were greater over broad-scale spatial gradients relative to either temporal variability in summer temperature within a site or summer temperature increases induced by experimental warming. The effect sizes for change over time within a site and with experimental warming were nearly identical. These results support the view that inferences based on space-for-time substitution overestimate the magnitude of responses to contemporary climate warming, because spatial gradients reflect long-term processes. In contrast, in situ experimental warming and monitoring approaches yield consistent estimates of the magnitude of response of plant communities to climate warming.

Keywords: climate change; space-for-time substitution; thermophilization; tundra; warming experiment.

Fig. 1.

Map of study sites, grouped by region. Red triangles indicate regions used in the experimental warming analyses only (n = 5), blue circles indicate monitoring studies only (n = 19), and purple squares indicate regions with data in both analyses (n = 9). The regions (each indicated by a single dot) often contained multiple experiments or monitoring sites.

Fig. 2.

Thermophilization of plant communities in response to variability in temperature over space (orange) or time (blue) or induced by experimental warming (purple). The y-axis shows the change in CTI per 1 °C. Points are the predicted magnitude of effect; error bars encompass 95% CI based on the parametric bootstrap. Bars with different letters (a, b) are significantly different (95% CI of difference in effect size >0, based on the parametric bootstrap of differences in effect size of the plots not experimentally warmed). Parametric bootstrapping across datasets (monitoring vs. experimental warming) was not feasible, so a formal statistical comparison of effect size between experiments and other sources is not presented. Shown are 85 studies in 28 regions for long-term monitoring (orange and blue bars) and 12 studies in 8 regions for experimental warming (purple bars).

Fig. 3.

Thermophilization of plant communities in response to multiple years of experimental warming scaled to a 1 °C increase in temperature. The purple reference line shows effect size after four summers of warming, for direct comparison with Fig. 2 (8 regions, 12 studies, 320 plots).