Collaborative Research: Thresholds and mechanisms of net ecosystem production (NEP) resilience following moderate disturbance: Why does one ecosystem recover and another one crash?

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Forests of the United States are primary sources of food, fiber and energy. They play a fundamental role in the earth's climate system by sequestering in plant biomass carbon that might otherwise form the molecular backbone of atmospheric greenhouse gases such as carbon dioxide. Forests' capacity to capture atmospheric carbon dioxide and build biomass may change substantially with age and disturbance. Scientists have long theorized a decline in growth and carbon uptake as forests age. New observations, however, suggest that low levels of disturbance, such as those originating from insect pests, fungal pathogens, and extreme weather, in aging forests may, counter-intuitively, sustain or even increase forest carbon sequestration and growth. The mechanisms underlying these higher-than-expected rates of forest carbon sequestration are unknown. This study seeks to identify the mechanisms underpinning forest growth resilience to disturbance, and their thresholds. The researchers will also evaluate if, how, and why different computer simulations, critical to predicting future forest carbon storage and growth and yield, fail to replicate this resilience. Furthermore, they will determine whether evergreen forests in the western United States and deciduous forests in the East, with different prevailing disturbance regimes and climates, follow unique age-forest growth trajectories. The benefits of this project to society, forest and land managers, grade school educators, university students, and forest scientists are far-reaching. By combining biologically-informed field and simulation experiments with a synthesis of North American forests, this study will significantly advance our ecological thinking about forest disturbance, while producing results immediately relevant and accessible to ecosystem and earth system simulations, and to forest managers working to maximize carbon storage, growth, and timber production in increasingly disturbed forest landscapes. The project will produce openly available instructional materials for grade school teachers, train several graduate and undergraduate students, provide open and transparent sources of data and computer code to scientists and land managers, and form a student training partnership between a United States Department of Energy laboratory and an academic institution.  The future terrestrial carbon sink is uncertain as forests of the United States upper Midwest and east broadly advance from early to middle forest succession. With this transition, early successional canopy dominants are senescing and giving way to more biologically and structurally complex forests that are increasingly subject to moderate severity disturbance. Recent studies suggest that net primary production may be sustained in such forests at higher-than-expected rates, but the limits of and mechanisms behind such functional resilience cannot be predicted from present knowledge, which is derived almost entirely from studies of severe, stand-replacing disturbance dynamics in recently disturbed forests. Ecosystem and global models, developed from the same intellectual foundations, also have trouble reproducing the effects of moderate disturbances. The three core research objectives of this work are to: 1) identify mechanisms supporting net primary production resilience to disturbance, and their thresholds; understand if, how, and why different forest models fail to replicate this NPP resilience; and 3) elucidate whether temperate deciduous and coniferous forests, with different disturbance regimes and climates, follow unique age-production trajectories. The project uses a 3-pronged approach of field experiments, model testing, and large-scale data synthesis to transform understanding of how resilient the carbon cycle will be to a range of moderate disturbance intensities in aging forests, elucidating the underlying mechanisms that determine the threshold between net primary production resilience and decline. The field component uses a fully replicated gradient of disturbance severity, from 0 to 85 % defoliation, to systematically determine how and why the carbon cycle shifts in response to rising disturbance levels. The PIs will employ a suite of carbon and nitrogen cycling measurements, focusing on canopy structure, leaf physiology, and canopy nitrogen reallocation, to identify the mechanisms that cause rapid net primary production resilience or decline following disturbance. The modeling component of the project uses data assimilation experiments, running two very different ecophysiological models within an open source, NSF-supported ecoinformatics toolbox, to identify the processes most responsible for the models' hypothesized failure to simulate net primary production resilience to disturbance, and iteratively inform the next field season's sampling priorities. Finally, a data synthesis component uses newly available observations to characterize disturbance effects on age-net ecosystem production trajectories for North American's temperate forests.