• Energy Research

Evaluations of the local effects of global change are often confounded by the interactions of natural and anthropogenic factors that overshadow the effects of climate changes on ecosystems. Long-term watershed and natural elevation gradient studies at the Hubbard Brook Experimental Forest and in the surrounding region show surprising results demonstrating the effects of climate change on hydrologic variables (e.g., evapo- transpiration, streamflow, soil moisture); the importance of changes in phenology on water, carbon, and nitrogen fluxes during critical seasonal transition periods; winter climate change effects on plant and animal community composition and ecosystem services; and the effects of anthro- pogenic disturbances and land-use history on plant community composition. These studies highlight the value of long-term integrated research for assessments of the subtle effects of changing climate on complex ecosystems. unraveling this daunting complexity is long-term studies, including those in which natural elevation gradients are exploited, as a foundation for detailed studies of critical and often unexpected climate-induced changes in forest struc- ture and function. In this article, results from the Hubbard Brook Experimental Forest (HBEF) and the surrounding region are used to illustrate how long-term studies can serve as a foundation for addressing the complex interactions that ultimately determine the effects of climate change on ecosystems. We combine data from long-term (50-year) measurements of multiple aspects of climate and ecosystem structure and function to highlight important but poorly studied inter- actions that could be critical determinants of the responses of plant and animal communities, fluxes of water, element dynamics, and services in northern hardwood forest eco- systems. Our objective is to demonstrate how a combina- tion of long-term and in-depth measurements facilitates A dominant approach in climate change research has been to focus on the effects of changes in temperature and precipitation on broadscale ecosystem properties over large areas and long periods. This body of research suggests that climate change will substantially alter the distribution of species and the function of ecosystems (e.g., Iverson and Prasad 2001), with important effects on ecosystem services. These analyses are based on well-described effects of tem- perature and precipitation on the distribution and activity of organisms. However, climate change is playing out over the complex and dynamic hydrobiogeological structure of the landscape—that is, the intertwined patterns of soils, vegetation, and hydrologic flowpaths, with a spatially variable history of land use and a wide range of current human activities and concurrent environmental changes. The climate effects on ecosystem structure and function may be modified by interactions with these patterns and histories over a range of time scales. We assert that a key approach to

Species distribution models typically use correlative approaches that characterize the species-environment relationship using occurrence or abundance data for a single species. However, species distributions are determined by both abiotic conditions and biotic interactions with other species in the community. Therefore, climate change is expected to impact species through direct effects on their physiology and indirect effects propagated through their resources, predators, competitors, or mutualists. Furthermore, the sign and strength of species interactions can change according to abiotic conditions, resulting in context-dependent species interactions that may change across space or with climate change. Here, we incorporated the context dependency of species interactions into a dynamic species distribution model. We developed a multi-species model that uses a time-series of observational survey data to evaluate how abiotic conditions and species interactions affect the dynamics of three rocky intertidal species. The model further distinguishes between the direct effects of abiotic conditions on abundance and the indirect effects propagated through interactions with other species. We apply the model to keystone predation by the sea star Pisaster ochraceus on the mussel Mytilus californianus and the barnacle Balanus glandula in the rocky intertidal zone of the Pacific coast, USA. Our method indicated that biotic interactions between P. ochraceus and B. glandula affected B. glandula dynamics across >1000 km of coastline. Consistent with patterns from keystone predation, the growth rate of B. glandula varied according to the abundance of P. ochraceus in the previous year. The data and the model did not indicate that the strength of keystone predation by P. ochraceus varied with a mean annual upwelling index. Balanus glandula cover increased following years with high phytoplankton abundance measured as mean annual chlorophyll-a. M. californianus exhibited the same pattern to a lesser degree, although this pattern was not significant. This work bridges the disciplines of biogeography and community ecology to develop tools to better understand the direct and indirect effects of abiotic conditions on ecological communities.

Climate change is expected to favor exotic plant species over native species, because exotics tend to have wider climatic tolerances and greater phenological plasticity, and also because climate change may intensify enemy release. Here, we examine direct effects of warming (+ 1.8 °C above ambient) on plant abundance and phenology, as well as indirect effects of warming propagated through herbivores, in two heavily invaded plant communities in Michigan, USA, separated by approximately three degrees latitude. At the northern site, warming increased exotic plant abundance by 19% but decreased native plant abundance by 31%, indicating that exotic species may be favored in a warmer world. Warming also resulted in earlier spring green-up (1.65 ± 0.77 days), earlier flowering (2.18 ± 0.92 days), and greater damage by herbivores (twofold increase), affecting exotic and native species equally. Contrary to expectations, native and exotic plants experienced similar amounts of herbivory. Warming did not have strong ecological effects at the southern site, only resulting in a delay of flowering time by 2.42 ± 0.83 days for both native and exotic species. Consistent with the enemy release hypothesis, exotic plants experienced less herbivory than native plants at the southern site. Herbivory was lower under warming for both exotic and native species at the southern site. Thus, climate warming may favor exotic over native plant species, but the response is likely to depend on additional environmental and individual species' traits.