Determining the effect and tradeoffs of woody plant expansion on grassland ecosystems

Global change is causing grasslands to experience the expansion of woody plants. There is growing interest in understanding how this influences ecosystem services. Afforestation is being promoted as a strategy to mitigate climate change further increasing woody plants in grasslands. There is considerable debate on how to manage woody plant expansion given the negative impacts. However, growing evidence suggests that woody plants could combat climate change by moderating climate extremes, increasing carbon sequestration, providing thermal refuge for biodiversity, and increasing groundwater recharge. Here, 1. we assess the impacts of woody plant expansion on microclimate moderation, belowground carbon sequestration, water regulation, and biodiversity, 2. determine how woody plants in grasslands moderate microclimate, 3. determine how the landscape microclimate compares to the weather station microclimate, and 4. determine the effect of species on microclimate. We collected microclimate, soil carbon, water infiltration, and herbaceous vegetation data in Oklahoma during 2022-23 using a combination of field and mechanistic experiments. We determined that Juniperus virginiana (hereafter; Juniperus) create cooler, wetter microclimates particularly during hot, dry conditions, but did not increase soil carbon compared to grass. Water infiltration within Juniperus was highly variability suggesting low predictability. Within Juniperus there was two times less grass and forb cover than in grass-dominated areas. Weather stations did not accurately model landscape temperature during temperature extremes highlighting the importance of woody plants. We determined that the presence of shade alone determines how woody plants moderate microclimate and that some species provide tremendous microclimate moderation while some provide none. Our findings show that the impact of woody plants on grasslands is complex. Woody plant expansion has the potential to mitigate climate extremes by moderating microclimate; however, we found no evidence that this increases soil carbon sequestration. Despite this, the ability to moderate temperature indicates that woody plants could protect ecological processes critical to stability of grassland ecosystems in the future; however, this would cause declines in grassland-obligate biodiversity and livestock production. As climate change intensifies, a balanced perspective is needed—one that recognizes both the potential costs and the ecological benefits of woody vegetation when developing grassland management and policy frameworks.