Abstract: Forest canopies create distinct microclimates, buffering extreme temperatures and altering soil moisture conditions relative to ambient. These microclimates could serve as climate refugia that enable persistence of climate-sensitive species in future climate regimes. Recent work has shown generality of forest effects on temperatures below the canopy, but whether forest canopies have globally consistent effects on soil moisture has not been explored as extensively. Conventional wisdom suggests that soil moisture will be higher below the canopy because cooler, darker conditions will reduce evapotranspiration. On the other hand, evapotranspiration might be elevated due to high vegetation biomass and associated demand for water, resulting in lower soil moisture under forest canopies. We conducted a systematic literature review and meta-analysis to test the hypothesis that soil moisture is higher under forest canopies. We identified 63 studies that contain measurements of soil moisture under forest canopies and at adjacent sites with open vegetation. Studies spanned multiple forest biomes (boreal, temperate, and tropical) and 6 continents. We found that soil moisture was lower under forest canopies than in the open in ~2/3 of studies, with forests on average 6% drier than adjacent open sites. Soil moisture was less reduced relative to open sites under coniferous canopies than under broadleaf canopies across latitudes. Forest canopy microclimates may create refugia from temperature extremes under a changing climate, but our results suggest that their ability to buffer against dry conditions are limited. Future research should consider temperature and moisture microclimates, and their role in creating climate refugia, jointly.
