Ecological contingency in the effects of climatic warming on forest herb communities

Abstract

Downscaling from the predictions of general climate models is critical to current strategies for mitigating species loss caused by climate change. A key impediment to this downscaling is that we lack a fully developed understanding of how variation in physical, biological, or land-use characteristics mediates the effects of climate change on ecological communities within regions. We analyzed change in understory herb communities over a 60-y period (1949/1951-2007/ 2009) in a complex montane landscape (the Siskiyou Mountains, Oregon) where mean temperatures have increased 2°C since 1948, similar to projections for other terrestrial communities. Our 185 sites included primary and secondary-growth lower montane forests (500-1.200 m above sea level) and primary upper montane to subalpine forests (1,500-2,100 m above sea level). In lower montane forests, regardless of land-use history, we found multiple herbcommunity changes consistent with an effectively drier climate, including lower mean specific leaf area, lower relative cover by species of northern biogeographic affinity, and greater compositional resemblance to communities in southerly topographic positions. At higher elevations we found qualitatively different and more modest changes, including increases in herbs of northern biogeographic affinity and in forest canopy cover. Our results provide communitylevel validation of predicted nonlinearities in climate change effects.climate change | elevation | land use | plant community | topography U pward and poleward shifts of species and vegetation zones are expected under climatic warming, and considerable evidence has been found in support of these broad predictions (1-5). However, large differences among communities in the magnitude, rate, and direction of responses to climatic warming are also expected, based on factors such as topography and substrate, landuse history, and community-level variation in species functional traits (e.g., 6, 7-10). Anticipating ecological contingency in responses to climate change is especially critical for managers of natural resources, who are well aware of the potential for major nonlinearities ("surprises") in community change and of the particular difficulty of making predictions for physically and biotically complex landscapes (e.g., 11-13). Our ability to test and refine predictions about contingent changes over time is limited by the scarcity of both appropriate data sets and metrics for comparisons across ecological communities.One of the earliest and best-known expectations about contingency is that climate change effects should be most pronounced at high elevations where plant growth is most strongly limited by temperature (14), specifically by the length of the snow-free growing season. This expectation is based on studies in the alpine and nival zones of the European Alps and elsewhere, where warming temperatures have been observed to lead to increases in plant productivity and species richness, although with losses of high-elevation specialist species, presumably as ...

Intensifying postfire weather and biological invasion drive species loss in a Mediterranean-type biodiversity hotspot

“…The decline in graminoid and herb species richness is consistent with findings from longterm studies of climate change impacts on Californian grasslands (1) and the Siskiyou mountain herb flora of southern Oregon (30). The observed differences in the response of major growth forms and fire-response types to climate change could well drive major shifts in ecosystem structure and function (13,17,18,31).…”

“…Shrubs are perhaps the least likely to be affected by climate extremes because they are deeper-rooted than other Fynbos growth forms, with roots reaching depths of 17 cm within 16 wk of germination (29). Shallow-rooted perennial species, including most graminoids and herbs in Fynbos, are likely to be most sensitive, as reported in other ecosystems (30). A recent global review proposes that species that resprout are more resilient to drought stress, but reports that empirical studies are few and conflicting (31).…”

Climate-driven diversity loss in a grassland community

“…We measured SLA (mm 2· g −1 of dry mass) because of its widely documented links to water balance (33-37) and to the community-level consequences of aridification (38,39). For clarity in interpretation, we focused this analysis on native annual forbs, the functional group largely responsible for the time trend in species richness.…”

“…We also repeated this analysis by using abundance-weighted community mean SLA (as in ref. 39 (47), located near the center of our 2,776-ha (roughly 4 × 7 km) study landscape.…”

“…Species with high SLA and RGR, and low LL and WUE, are more prevalent in wetter climates (34) and in wetter years (35) and tend to increase disproportionately in response to experimental watering (36) and natural precipitation increase (37). Several previous studies have found that high-SLA species are especially vulnerable to decline or loss under aridification (38,39).…”