What effect does an individual tree in a grassland have on microclimate, soil organic carbon, and water infiltration?

Global change is causing woody plants to expand into grasslands. Grassland afforestation is being implemented and promoted as a strategy to mitigate climate change further increasing woody plants. There is considerable debate on the merits of this policy given its impact on ecosystem services. Our objective was to assess the impacts of grassland afforestation on three ecosystem services (microclimate moderation, belowground carbon sequestration, water regulation) at the scale of an individual tree. We collected data in the Southern Great Plains of North America during 2022-23 and used mixed models to analyze the data. We determined that individual Juniperus virginiana (hereafter, Juniperus) trees in grasslands create cooler, wetter microclimates particularly during hot, dry conditions. We found no statistical evidence that individual Juniperus increases soil organic carbon at 7.5 cm depth. Despite water infiltration being highest within individual Juniperus, high variability suggested low predictability. Despite these findings, it remains unclear whether grassland afforestation leads to regional cooling. Further, our findings suggest grassland afforestation may not increase belowground carbon sequestration, and water regulation at the ecosystem level but this remains speculative. Despite the ecological benefits of trees moderating microclimate in grasslands, it is critical to recognize that large-scale woody plant expansion threatens many ecosystem services. The benefits of establishing trees in grasslands may not outweigh the negative impacts to these ecosystems. It is critical that in regions where afforestation projects are proposed or where woody plant expansion is occurring to consider the ecological and societal impacts of such policy to ensure sustainable management.