Tree species mixing can amplify microclimate offsets in young forest plantations.
Macroclimate warming is affecting ecosystems world-wide. Tree canopies, however, can significantly buffer temperature fluctuations, giving rise to lower maximum temperatures, higher minimum temperatures and enhanced water availability at the forest floor. Forests, therefore, can act as refugia for heat-sensitive species with important implications for climate-change responses in the understorey of forests. These forest microclimate offsets have been well-studied, but far less is known about how microclimate offsets are modulated by tree species mixing. We installed temperature and air humidity loggers in a multisite tree diversity experiment to quantify the role of tree species richness and composition on below-canopy offsets and investigate mechanisms underlying these effects. Microclimate offsets highly depended on tree species identity and mixing. Not only monocultures of larch Larix × eurolepis and Douglas fir Pseudotsuga menziesii had a high performance for thermal buffering, but also mixtures with species such as birch Betula pendula and pine Pinus sylvestris. Indirect effects brought about by tree species mixing were found to amplify microclimate offsets, mostly via increased canopy cover. Synthesis and applications. Our findings show that microclimate offsetting highly depends on tree species identity and diversity, and that buffered forest microclimates can be achieved rapidly in young plantations, depending on the species being planted. Based on our findings, we recommend planting mixtures instead of monocultures. Not only because species mixing was found to amplify microclimate offsets in some of the investigated stands, but also because tree species mixing is likely to increase the resilience of forests to climate change. Hence, we expect that also on the long-term species mixing might be beneficial for microclimate buffering. Forest managers and policy makers can use our finding to optimise the design of tree plantations to minimise climate-change impacts on below-canopy biodiversity and functioning.