Maturation of encroaching juniper woodland elevates gross primary productivity and water use but reduces net ecosystem exchange relative to native tallgrass prairie

Abstract

Woody plant encroachment (WPE) is transforming grassland ecosystems, with important consequences for carbon sequestration and water balance. This study assessed the long-term impacts of eastern redcedar (Juniperus virginiana, juniper) encroachment by comparing ecosystem carbon and water fluxes between a mature juniper‐dominant woodland and an adjacent tallgrass prairie in the Southern Great Plains, USA. Paired eddy covariance systems (2022–2024) revealed that the juniper woodland was a weaker carbon sink, with a mean annual net ecosystem CO₂ exchange (NEE) of -162 g C/m², compared to -182 g C/m² in the tallgrass prairie. This occurred despite higher annual gross primary productivity (GPP: 2164 vs. 1475 g C/m²), aboveground net primary productivity (ANPP: 281 vs. 142 g C/m²), and evapotranspiration (ET: 762 vs. 589 mm) of the woodland because WPE increased ecosystem respiration (R_e: 2001 vs. 1294 g C/m²). These results suggest a decoupling of water loss from carbon gain in juniper woodlands and underscore the importance of evaluating full ecosystem carbon budgets – beyond aboveground biomass – to guide integrated carbon and water management in a transitional landscape in the prairies.