The timing of spring green up, trajectories of summer leaf area and photosynthesis, and the timing of fall senescence have profound impacts to the water, carbon, and energy balance of ecosystems, and are expected to be influenced by global warming. In particular, a longer growing season has been observed to increase carbon uptake by temperate forests. However, if soils dry out and/or nutrients become more limiting, the impact of growing season length is less pronounced due to reductions in summer-time photosynthesis. A gradual summer green down has been observed in most time series of summer-time forest phenology (e.g., from remote sensing, flux towers, webcams, and leaf-level observations), which has been interpreted as either an artifact of declining solar radiation and/or a more complicated set of processes implicating water and nutrient availability and leaf aging through the summer months. Thus, summer green down has potential as a metric for predicting limits on changes in carbon assimilation associated with a longer growing season. We explored spatial patterns in summer green down and growing season length for mid-Atlantic forests at spatial resolutions appropriate for understanding a suite of ecological processes. Two decades of 30m-resolution remote sensing observations of greenness were stacked by day of year and a continuous function was fit to the data (one average annual time series), which included a sloped line (representing summer green down) to connect spring-increasing and fall-decreasing logistic functions. We analyzed the resulting spatial patterns of summer green down, growing season length, and the integral of summer greenness (sumGV) in the context of topography and forest structure, provided by a detailed LiDAR survey of a portion of our study area.
Results/Conclusions

Our results show that summer green down is greatest on more exposed ridgelines and least on valley bottoms. Tree height and sumGV were also found to be greater in local depressions where water availability is greater throughout the late summer months. In the extreme case, sites in the vicinity of springs, seeps, and headwater streams exhibited the most reduced summer green down, implicating the availability of groundwater in controlling summer green down. Ridgelines where exposure to wind and summer time soil drying is more extreme, exhibited increased green down. The sumGV was found to be most responsive to growing season length wherever summer green down was low, suggesting that the spatial pattern of summer green down is integral to predicting the carbon-response of forests to global warming.