Forest Resilience Threshold Experiment (FoRTE)

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Thresholds and Mechanisms of NPP resilience following moderate disturbance: Why does one ecosystem recover and another one crash?

Scientists have long theorized that growth and carbon uptake decline as forests age. This long-held hypothesis, however, is largely supported by observations from old coniferous forests of the Pacific Northwest that experience high severity disturbances such as fire. New observations suggest that lower levels of disturbance – those that kill some but not all trees and are prompted by insects, pathogens, and extreme weather – may counter-intuitively sustain or even increase forest carbon storage and growth in aging eastern deciduous forests, a biome that on average is subjected to these much more moderate disturbances. But, at what level of severity does disturbance cause forest carbon storage and growth to decline?

The Forest Resilience Threshold Experiment (FoRTE) program seeks to identify the mechanisms underpinning forest net primary productivity (NPP) – the rate of carbon accumulated in plant biomass – resilience and, when pushed too far by higher levels of disturbance, its breaking point, beyond which NPP declines. It will also examine why different computer simulations, which currently play a critical role in predicting future forest carbon storage and growth and yield, fail to replicate observed resilience or decline in response to disturbance. Finally, it will determine whether evergreen forests in the western United States and deciduous forests in the east -- two different ecosystems shaped by different disturbance regimes -- follow unique age-forest carbon storage trajectories.

FoRTE will rely on field experiments, model testing, and large-scale data synthesis to enhance understanding of how resilient the carbon cycle will be to a range of moderate disturbance intensities. The field component will stem girdle targeted tree species to create a gradient of disturbance severity, from 0 to 85% defoliation and then employ a suite of carbon and nitrogen cycling measurements – as well as an assessment of canopy structure, leaf physiology, and canopy nitrogen reallocation – to systematically determine when and why NPP exhibits resilience or decline. Primary Investigators Chris Gough (Virginia Commonwealth University) and Ben Bond-Lamberty (Pacific Northwest National Laboratory) hypothesize that more structurally complex canopies resulting from moderate disturbances use light more efficiently than simpler canopies, sustaining NPP until a disturbance threshold is exceeded and canopy defoliation cannot be offset by improved resource-use efficiency. They expect to see rapid leaf physiological and morphological changes in sub-canopy vegetation as newly available light reaches deeper into the canopy following disturbance. Finally, they expect to see the greatest levels of resilience to disturbance where nitrogen from senescent trees is reallocated in support of new leaf construction.

The computer modeling component of the project will identify the processes most responsible for the Ecosystem Demography model’s hypothesized failure to simulate NPP resilience to disturbance. Gough and Bond-Lamberty hypothesize that incorporation of metrics of canopy structure and more sophisticated representation of nitrogen cycling will improve model performance. Results from this modeling will be used to iteratively inform the next field season’s sampling priorities as well, supporting a tight coupling between field and modeling activities.

Finally, the data synthesis component of the FoRTE project will use newly available observations from an open dataset, including data from UMBS, to characterize disturbance effects on age-net ecosystem production trajectories for North American temperate forests.