Ecosystem Gross Primary Productivity After Autumn Snowfall and Melt Events in a Mountain Meadow.

Vegetation productivity is increasing in the U.S. Northern Great Plains but decreasing in some nearby Northern Rocky Mountain grasslands due to increases in aridity. It is unclear if decreases to montane grassland productivity from drying autumns can be partly offset by late‐season green‐ups after precipitation events. These include the multiple snowfall and snowmelt periods that often characterize the summer‐to‐winter transition, but are difficult to observe due to logistical constraints. Here, we quantify changes to vegetation indices and ecosystem carbon uptake that occurs after snowfall and melt in climatological autumn in a montane grassland in Montana, USA using eddy covariance, phenological camera, and remote sensing analyses. Carbon dioxide flux follows a diurnal pattern after autumn snowmelt events despite overall ecosystem C loss, suggesting that post‐snowmelt photosynthesis helps dampen C loss during autumn and provides fresh photosynthate to support ecosystem functioning. Light‐saturated photosynthesis after three snow events was not statistically different than before snowfall (∼6 μmol CO2 m−2 s−1 in 2016 and ∼2.5 μmol CO2 m−2 s−1 in 2017). Observations are consistent with the notion that canopy photosynthesis is resistant, rather than resilient, to autumn snow. MODIS indicates that post‐snow greenups can occur, but not every year. Such events likely play a small role in the annual ecosystem carbon balance but may be disproportionately important for organisms faced with dwindling late‐season forage. Future efforts should seek to understand the community and ecosystem consequences of vegetation functioning during autumn as part of an expanded effort to understand phenological changes during this under‐studied and changing time of year. Plain Language Summary: The productivity of some montane meadows in the U.S. Northern Rockies has decreased over the past 50 years due to increases in aridity. This is of concern for the ecosystem services that these grasslands provide like forage to help animals overwinter. Our understanding of the full annual carbon cycle of many montane ecosystems is incomplete because research sites can be difficult to access when snow is intermittent, as is often the case during autumn in the Northern Rockies. This is one of the many reasons why autumn is underrepresented in climate change studies. We used automated measurement systems to measure ecosystem carbon dioxide flux and vegetation greenness during the autumn‐to‐winter transition in a montane meadow in Montana, USA. Ecosystem carbon uptake after two or more snowfall and melt periods was the same as before snowfall during two consecutive years. This suggests that montane grassland photosynthesis is resistant to short‐lived snow events. Our results demonstrate that autumn snow and melt events neither help nor substantially hinder mountain meadow grass photosynthesis, which helps us understand the ecosystem services they provide. Key Points: Mountain meadow carbon uptake persists after autumn snowfall and melt eventsAutumn snowfall appears to be a minor ecosystem disturbance against which grass photosynthesis is resistant [ABSTRACT FROM AUTHOR]