The combined effects of climate and canopy cover changes on understorey plants of the Hyrcanian forest biodiversity hotspot in northern Iran.

Understanding forest understorey community response to environmental change, including management actions, is vital given the understorey's importance for biodiversity conservation and ecosystem functioning. The Natural World Heritage Hyrcanian temperate forests (Iran) provide an ideal template for furnishing an appreciation of how management actions can mitigate undesired climate change effects, due to the forests' broad environmental gradients, isolation from colonization sources and varied light environments. We used records of 95 understorey plant species from 512 plots to model their probability of occurrence as a function of contemporary climate and soil variables, and canopy cover. For 65 species with good predictive accuracy, we then projected two climate scenarios in the context of either increasing or decreasing canopy cover, to assess whether overstorey management could mitigate or aggravate climate change effects. Climate variables were the most important predictors for the distribution of all species. Soil and canopy cover varied in importance depending on understorey growth form. Climate change was projected to negatively affect future probabilities of occurrence. However, management, here represented by canopy cover change, is predicted to modify this trajectory for some species groups. Models predict increases in light-adapted and generalist forbs with reduced canopy cover, while graminoids and ferns still decline. Increased canopy cover is projected to buffer an otherwise significant decreasing response of cold-adapted species to climate change. However, increasing canopy cover is not projected to buffer the predicted negative impact of climate change on shade-adapted forest specialists. Inconsistent responses of different species and/or growth forms to climate change and canopy cover reflect their complicated life histories and habitat preferences. Canopy cover management may help prevent the climate change induced loss of some important groups for biodiversity conservation. However, for shade-adapted forest specialists, our results imply a need to adopt other conservation measures in the face of anticipated climate change.