Determining the mechanisms that cause woody plants to moderate microclimate in grasslands.

Forests and grasslands experience shifts in woody plant cover creating a continuum of woody plants across space. Global change accelerates this, causing many ecosystems to experience the redistribution of woody plants. There is growing interest in understanding how these ecological changes influence ecosystem function including climate regulation. Research shows that woody plant expansion generally moderates microclimate but can impact regional macroclimate differently, while the loss of woody plants may lead to hotter regional macroclimates. However, the mechanisms in grasslands are largely speculative. Changes in shade, evapotranspiration, and wind associated with woody plants may drive changes in microclimate. Because changes in temperature can impact ecosystem function, it is critical that we understand the mechanisms that drive this to determine how the redistribution of woody plants impacts grassland ecosystems. Our objective was to determine the mechanisms that cause woody plants to moderate microclimate in grasslands by testing specific hypotheses that may drive how individual woody plants influence microclimate. We performed a 2 x 2 x 2 factorial experiment in a fallow field across 3 independent variables (shade, pan evaporation, and no wind) during the summer of 2023 and measured the microclimate. We analyzed the data using a linear-mixed modeling and model selection approach. We determined that the presence of shade alone best described microclimate temperature and vapor pressure deficit. During the daytime, shade moderated temperature, especially during high temperature extremes, and reduced vapor pressure deficit, while during the nighttime shade slightly increased temperature, but largely had little effect on vapor pressure deficit except during conditions with high vapor pressure deficit. Our findings show that ecosystems experiencing woody plant expansion could experience lower temperature and vapor pressure deficit, while ecosystems experiencing a loss in woody plant cover may experience higher temperature and vapor pressure deficit, which could impact ecosystem function.