Arctic ecosystems are experiencing rapid advancement of spring due to considerable warming over the past several decades, amplification of storm cycles, and changes in the arrival timing of herbivorous migratory geese. These changes can potentially influences net ecosystem exchange (NEE): season-advancement promotes early growth and more rapid leaf expansion, cloud cover reduces energy available for C uptake, and herbivores remove photosynthetic tissue. These processes interact in a biome critical for carbon cycle feedbacks to global climate due to large soil C pools. We ask the question: which of these factors plays the largest role in altering NEE in a coastal wetland? For three summers we conducted an experiment altering timing of spring green-up using open-top warming chambers, and timing of grazing using exclosures and captive Pacific Black Brant geese. In a subset of the experimental units, we monitored NEE using an automated chamber system taking hourly flux measurements. From these data, we estimated changes in NEE due to earlier growing seasons, the reduction in carbon uptake associated with grazing events at different times of the year, and the effect of cloudiness using physiological response curves. Data were collected between late May and mid-August 2014-2016.
Results/Conclusions
We found that advancing the growing season reduced season-long NEE by 18%, from 268 to 218 g C m-2, by enhancing ecosystem respiration more than C uptake. However, the reduction in NEE with an advanced growing season was only 16%, from 318 to 266 g C m-2, when plots were grazed in contrast to a reduction of 22%, from 218 to 170 g C m-2, when grazing was excluded. In plots that were not grazed, median cloud cover reduced NEE by ca. 45%, from 485 to 268 g C m-2, from potential NEE, but reduction in NEE due to clouds was not uniform throughout the summer. Increased clouds in late summer had a greater influence on NEE than increased clouds in early summer because plants are operating under reduced light conditions due to already cloudy and shorter days. Reduction in NEE due to clouds also depended on timing of grazing: early-season grazing caused cloud cover to reduce NEE by ca. 55% while late-season grazing caused cloud cover to reduce NEE by 35%. Through interactive responses, changes in timing of phenological events and weather can lead to complex alterations of the carbon cycle in Arctic coastal ecosystems.