Cloud cover and delayed herbivory relative to timing of spring onset interact to dampen climate change impacts on net ecosystem exchange in a coastal Alaskan wetland

Abstract

Rapid warming in northern ecosystems over the past four decades has resulted in earlier spring, increased precipitation, and altered timing of plant-animal interactions, such as herbivory. Advanced spring phenology can lead to longer growing seasons and increased carbon (C) uptake. Greater precipitation coincides with greater cloud cover possibly suppressing photosynthesis. Timing of herbivory relative to spring phenology influences plant biomass. None of these changes are mutually exclusive and their interactions could lead to unexpected consequences for Arctic ecosystem function. We examined the influence of advanced spring phenology, cloud cover, and timing of grazing on C exchange in the Yukon-Kuskokwim Delta of western Alaska for three years. We combined advancement of the growing season using passive-warming open-top chambers (OTC) with controlled timing of goose grazing (early, typical, and late season) and removal of grazing. We also monitored natural variation in incident sunlight to examine the C exchange consequences of these interacting forcings. We monitored net ecosystem exchange of C (NEE) hourly using an autochamber system. Data were used to construct daily light curves for each experimental plot and sunlight data coupled with a clear-sky model was used to quantify daily and seasonal NEE over a range of incident sunlight conditions. Cloudy days resulted in the largest suppression of NEE, reducing C uptake by approximately 2 g C m −2 d −1 regardless of the timing of the season or timing of grazing. Delaying grazing enhanced C uptake by approximately 3 g C m −2 d −1 . Advancing spring phenology reduced C uptake by approximately 1.5 g C m −2 d −1 , but only when plots were directly warmed by the OTCs; spring advancement did not have a long-term influence on NEE. Consequently, the two strongest drivers of NEE, cloud cover and grazing, can have opposing effects and thus future growing season NEE will depend on the magnitude of change in timing of grazing and incident sunlight. Shifting plant phenology in response to global change Trends Ecology Evol. 22 357-65 Comiso J C 2003 Warming trends in the Arctic from clear sky satellite observations J. Clim. 16 3498-510 Comiso J C and Hall D K 2014 Climate trends in the Arctic as observed from space WIREs Clim. Change 5 389-409 Doiron M, Gauthier G and Levesque E 2015 Trophic mismatch and its effects on the growth of young in an Arctic herbivore Glob. Change Biol. 21 4364-76 Elmendorf S C et al 2012 Global assessment of experimental climate warming on tundra vegetation: heterogeneity over space and time Ecology Lett. 15 164-75 Fischer J B, Stehn R A

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