Microclimate and ecological threshold responses in a warming and wetting experiment following whole tree harvest

Ecosystem climate manipulation experiments (ECMEs) are a key tool for predicting the effects of climate change on ecosystems. However, the strength of inferences drawn from these experiments depends on whether the manipulated conditions mimic future climate changes. While ECMEs have examined mean temperature and moisture conditions, ecosystem processes may respond more to microclimatic thresholds (e.g., freeze-thaw events). We reported the mean and microclimatic thresholds from a post-clearcut ECME in a temperate, mixed deciduous forest. Target treatments were ambient, warmed (+~2 °C), wetted (+~20 % precipitation), and warmed + wetted. Wetted treatments increased mean monthly precipitation by 23 %, but did not change the amount of time the soil water potential was below the permanent wilting point. Relative to ambient, warmed treatments increased the mean temperatures of the surface and soil by 1.8 and 2.5 °C, respectively. Warming decreased the number of soil freeze-thaw events and increased the number of growing degree days, frost-free days, and amount of time leaf surface temperatures were in the optimal photosynthetic range. Our results showed that, even when ECMEs mimic mean predicted climate conditions, their effect on microclimatic thresholds can be variable. We suggest that measuring these and other microclimatic thresholds will be essential for interpreting ECME results and assessing their value in predicting ecosystem responses to future climate change.