Global change and terrestrial plant community dynamics

Abstract

Anthropogenic drivers of global change include rising atmospheric concentrations of carbon dioxide and other greenhouse gasses and resulting changes in the climate, as well as nitrogen deposition, biotic invasions, altered disturbance regimes, and land-use change. Predicting the effects of global change on terrestrial plant communities is crucial because of the ecosystem services vegetation provides, from climate regulation to forest products. In this paper, we present a framework for detecting vegetation changes and attributing them to global change drivers that incorporates multiple lines of evidence from spatially extensive monitoring networks, distributed experiments, remotely sensed data, and historical records. Based on a literature review, we summarize observed changes and then describe modeling tools that can forecast the impacts of multiple drivers on plant communities in an era of rapid change. Observed responses to changes in temperature, water, nutrients, land use, and disturbance show strong sensitivity of ecosystem productivity and plant population dynamics to water balance and long-lasting effects of disturbance on plant community dynamics. Persistent effects of land-use change and human-altered fire regimes on vegetation can overshadow or interact with climate change impacts. Models forecasting plant community responses to global change incorporate shifting ecological niches, population dynamics, species interactions, spatially explicit disturbance, ecosystem processes, and plant functional responses. Monitoring, experiments, and models evaluating multiple change drivers are needed to detect and predict vegetation changes in response to 21st century global change.

Keywords: climate change; drought; forests; global change; land-use change.

Fig. 1.

Framework for understanding effects of global change drivers—climate change, altered disturbance regimes, invasive species leading to novel species assemblages, and land-use change—on terrestrial ecosystems using multiple lines of evidence from observations (detection) for attribution, experiments for elucidating mechanisms, and models deployed at multiple ecological scales for verification.

Fig. 2.

In the Tehachapi Mountains in southern California, the brown-needled conical crowns on the midslope are Pinus ponderosa that are dying as a result of the most severe drought ever recorded in California (Tejon Ranch Conservancy, 9 June 2014).

Fig. 3.

Relative changes in final population abundance for three plant functional types based on species in southern California, under altered fire regime, climate change, and land-use change (urban growth). The plant functional types span shrubs, trees, and herbs and are long (obligate seeders and resprouter) and short (annual herb) lived. The obligate seeders accrue seed banks. The resprouting tree is the only species with seed dispersal beyond a few meters. All species are adversely affected by frequent fire. The reliance of obligate seeders on optimally timed fires for germination renders frequent fire their most serious threat. Urban growth tends to occur near the coast in this southern Californian scenario and only affects the species with a coastal distribution. The climate change effects differ substantially depending on the functional type and location, strongly affecting the interior resprouting and obligate seeding species; long distance dispersal by the resprouter is insufficient to match the pace of habitat shifts. The coastal obligate seeding species is less affected by climate change than the interior species due to different climatic conditions and projected changes therein. Conversely, population abundance of the interior annual herb increases with climate change as occupied patches of suitable habitat become larger. The only consistent global change effect occurs with very frequent fires which reduce populations in all cases, whereas the effects of urban growth and climate change are context dependent (results based on refs. , , , , and 142).

Fig. 4.

Multispecies modeling showing vegetation dynamics departing from species climate equilibrium as a result of disturbance, species traits, and competition for light (detailed in ref. 143). Eight species represent a combination of physiological and dispersal traits. A–D are cold adapted species with identical initial distributions in the upper (cold) part of the idealized landscape, and E–H are warm adapted species with identical initial distributions in the lower (warm) part of the landscape. Simulated climate change displaces the climate niche of all species upward. Species interactions, disturbance dynamics, and spatial heterogeneity of climate (refugia) result in patchy distributions after 100 y of climate warming (simulation output). (A and E) Short distance dispersing, shade tolerant with establishment following disturbance. (B and F) Long dispersing, shade tolerant. (C and G) Short dispersing, shade intolerant. (D and H) Long dispersing, shade intolerant. Cold adapted species may be able to persist in warmer areas if enough environmental heterogeneity is present (refugia), especially if they can disperse long distances and/or are shade tolerant (A, B, and D).