Short-term impacts of prescribed fire and invasive grass on net ecosystem CO2 exchange (NEE) in sagebrush ecosystems: quantifying NEE drivers through automated plot-scale chamber measurements

It is widely known that the increasing presence of invasive annual grasses (IAG), such as Bromus tectorum (cheatgrass), is reshaping landscapes through positive fire-IAG feedback loops across the ~520,000 km² sagebrush ecosystems of the western US. However, the short-term effects of invasive annuals and fire cycles on net ecosystem CO₂ exchange (NEE) and their controlling mechanisms remain poorly quantified, partly due to limited observations able to resolve heterogeneity in plant community composition, fire effects, and soil-vegetation interactions. The objectives of this study were (1) to quantify how fire, plant community species composition, IAG presence, plant canopy greenness (NDVI), and environmental drivers influence late season NEE; and (2) to compare 4x3 meter plot-level NEE values measured with an automated mobile transparent ecosystem gas exchange chamber with simultaneously collected NEE measured across the entire study site using eddy covariance.  We measured diel NEE using the automated chamber system on 20 4x3 m experimental plots, which were categorized into pairs according to their plant compositions, namely the varying amounts of B. tectorum, perennial native herbaceous species, and native perennial grasses. In late autumn 2025, one plot within each pair was experimentally burned, allowing for comparison of burned and unburned plots with similar pre-fire vegetation.  Repeated pre- and post-burn CO₂ flux measurements were collected and analyzed in relation to key NEE drivers, including photosynthetically active radiation (PAR), air temperature, and vegetation composition. Nighttime NEE ranged from 0.62 to 1.48 µmol CO₂ m⁻² s⁻¹, consistent with net CO₂ release via ecosystem respiration, while daytime NEE ranged from 0.32 to −4.36 µmol CO₂ m⁻² s⁻¹, indicating net CO₂ uptake. Negative daytime NEE was observed on all measurement dates for most plots, including post-fire burned plots, coincident with late-season germination of B. tectorum. Daytime NEE measured in burned plots actually became more negative relative to values measured in these plots before they were burned, whereas nighttime values remained unchanged. These patterns highlight the importance of IAGs in mediating short-term carbon exchange in recently disturbed sagebrush ecosystems. This work aims to provide new insight into the short-term carbon dynamics of fire-prone sagebrush systems and improve mechanistic understanding of disturbance-driven changes in ecosystem CO₂ exchange.