Brookshire, E. N. J. | Search bioRxiv

bioRxiv2022

Ecosystem gross primary productivity after autumn snowfall and melt events in a mountain meadow

Vegetation productivity is increasing across much of the U.S. Northern Great Plains but is decreasing in some nearby Northern Rocky Mountain grasslands due to increases in aridity. Mountain grasslands support critical ecosystem services that are under threat from ongoing land use and climate changes, and it is important to understand their function across all changing seasons. Observing the full range of montane ecosystem productivity is challenging because site access is often difficult during the "shoulder seasons" in spring and autumn if the snowpack is not fully developed or degrading. It is unclear if decreases to montane grassland productivity from drying autumns can be offset in part by late-season green-ups after precipitation events. These include the snowfall/snowmelt periods that often characterize the summer-to-winter transition in the Northern Rockies. Here, we quantify the ecosystem carbon uptake that occurs after snowfall and melt in climatological autumn (September, October, and November) in a montane grassland in Montana, USA using a combination of eddy covariance, phenological camera, and remote sensing analyses. Carbon dioxide flux follows a diurnal pattern after autumn snowmelt events despite overall ecosystem losses of C, suggesting that post-snowmelt photosynthesis helps dampen C loss during autumn and provides fresh photosynthate to support ecosystem functioning. Light-saturated photosynthesis after two snow events was not different than before snowfall ([~]6 {micro}mol CO2 m-2 s-1 in 2016 and [~]2.5 {micro}mol CO2 m-2 s-1 in 2017); observations are consistent with the notion that canopy photosynthesis is resistant, rather than resilient, to the first snow disturbances. MODIS observations also suggest that post-snowfall increases in NDVI can occur but do not happen every year, such that late-season photosynthesis is not a reliable source of fresh photosynthate. These late-season carbon uptake 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 and regrowth in spring. 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.

ecology↗

bioRxiv2019

Top-down and bottom-up controls on soil carbon and nitrogen cycling with repeated burning across four ecosystems

Fires shape the biogeochemistry and functioning of many ecosystems, but fire frequencies are changing across large areas of the globe. Frequent fires can change soil carbon (C) and nitrogen (N) storage through both "top-down" pathways, by altering inputs through shifting plant composition and biomass, and "bottom-up" ones, by altering losses through decomposition and turnover of soil organic matter. However, the relative importance of these different pathways and the degree to which they regulate ecosystem responses to decades of changing fire frequencies is uncertain. Here, we sampled soils and plant communities in four North American and African sites spanning tropical savanna, temperate coniferous savanna, temperate broadleaf savanna, and temperate coniferous forest that each contained multiple plots repeatedly burned for 33-61 years and nearby plots that were protected from fire over the same period. The sites varied markedly in temperature, precipitation, species composition, fire history and soil chemistry; thus they represent a broad test for the generality of fire impacts on biogeochemical cycling. For all four sites, bulk soil C and N by were 25-180% higher in unburned vs. frequently burned plots, with greater soil losses occurring in areas with greater declines in tree cover and biomass inputs into soils. Fire reduced the activity of soil extracellular enzymes that hydrolyze labile C and N from soil organic matter by two- to ten-fold, whereas tree cover was the predominant control on the oxidation of recalcitrant C compounds. C-acquisition enzyme activity tended to decline with decreasing soil N, suggesting that N losses may contribute to limited decomposition, buffering systems against increased losses of soil C with fire. In conclusion, variability in how fire alters soil C and N across ecosystems can be explained partly by fire-driven changes in tree cover and biomass, but the slower turnover of organic matter we observed may offset some of the reduction of C inputs from plants after fire.

ecology↗