Intellectual Merit

Coweeta research has shown that the effects of climate change on forest composition, structure, hydrology, and biogeochemistry will be complex, multi-dimensional, and probably slow, involving interactions among many processes and drivers. Long-term (LT) ecological monitoring and research is conducted to discover unexpected dynamics, to observe slow ecosystem responses, and to generate hypotheses that can be tested with ecosystem manipulation studies. Recent Coweeta research has found examples of each motivation. For example, our LT climate data indicate that precipitation is becoming more variable, with longer and more frequent summer drought periods, more hot summer days, and larger storms. We would thus expect more drought-resistant oaks and hickories to become more dominant, but instead we have seen a greater proportion of tulip poplars and maples. This has raised multiple competing hypotheses: 1) prior forest disturbance (clearcutting of early 1900s or the chestnut blight of the 1930s) has altered resource availability in ways favoring poplars and maples; 2) fire suppression has altered competitive balances; 3) competing effects of climate change on forest composition are too slow to be observed yet; or 4) as yet unexplored interactions are affecting competitive balances. Our research has shown that climate change can accelerate tree growth, causing younger trees to be more fecund and older trees to be less so. Furthermore, small scale variation in soil moisture retention and soil fertility strongly affect vegetation responses to climate changes. In turn, changes in forest composition have substantial, but slow-acting, effects on nitrogen fixation and watershed water budgets. Retrospective synthetic analysis of the iconic 1976 Watershed 7 clearcut experiment indicates that forest succession and enhanced nitrogen fixation following natural regeneration of a large component of black locust continue to alter water budgets and elevate dissolved inorganic nitrogen concentrations 40 years later. Subsequent measurement of chronosequences of nitrogen nodule densities indicates that disturbance of southern Appalachian forests can affect N availability and cycling for many decades. Data from other LT watersheds at Coweeta show that streamflows are responsive to changes in species composition, stand age, and climate change. The loss of chestnuts and eastern hemlock has apparently spurred the spread of the native understory rhododendron, with effects on tree recruitment, soil microbiology, stream basal resources, and even stream temperature. The effects of climate change on southern Appalachian forest ecosystems are layered upon the effects of long-term successional responses to past disturbance, species loss due to disease (e.g. chestnut and eastern hemlock), fire suppression, invasive species introductions, and conversion of the valley floors to agricultural and residential uses. Locally, climate change effects are mediated by topographic position and soils.

Coweeta research has continued to connect forest dynamics to stream ecosystems. Experimental additions of nitrogen and phosphorus to detrital-based forested streams accelerated breakdown of detrital material, accelerated resource flows to primary consumers, and increased carbon exports. Consequently, higher nutrient levels in detritus-based streams may result in basal resource scarcity in late summer and early fall. Our regional studies have shown that increases in stream nutrient concentrations due to forest change and atmospheric are very small relative to those observed downstream in the rural valleys where stream nutrients are subsidized by fertilizers. The more direct effects of fertilization on stream nutrient concentrations are orders of magnitude larger than those observed due to forest disturbance.

Through cross-site synthesis, Coweeta contributed to a broader understanding of how global climate change is affecting disturbance patterns and ecosystem conditions and processes and how social/governance feedbacks might alter these trends. Cross-site synthesis has revealed both commonalities and important distinctions regarding how climate change is manifested in different ecosystems. It has also shown the prevalence and importance of time lags in ecosystem responses.

Broader Impacts

Findings from this research have helped guide ecosystem restoration and management by land management agencies and NGOs and also the development of water quality policies at multiple levels of governance. For instance, the Mainsprings Land Trust developed their "Shade Your Stream" public campaign based on results from CWT research. The research team has communicated our work to the local community through several symposia. Our research has created education, training and engagement opportunities for undergraduate and graduate students, students from 5^th through 8^th grade, and various segments of society within the greater southern Appalachian biome, including the Eastern Band of the Cherokee. Our field-based middle-school education programs have served several thousand students from 14 public schools in the study region. We have mentored approximately 30 undergraduates through the REU program.

Last Modified: 05/26/2021
Modified by: C R Jackson