Decametric-scale buffering of climate extremes in forest understory within a riparian microrefugia: the key role of microtopography

Riparian corridors often act as low-land climate refugia for temperate tree species in their southern distribution range. A plausible mechanism is the buffering of regional climate extremes by local physiographic and biotic factors. To test this idea, we deployed over 3 years a network of 39 microclimate sensors along the Ciron river, a refugia for European beech (Fagus sylvatica) in southwestern France. Sensor locations spread along the main geomorphological landscapes, vegetation types and microtopographic situations. Across the whole network, canopy gap fraction was the main predictor for spatial microclimatic variations. Two landscape features (elevation above the river and woodland fraction within a 300m radius) were also strong predictors, while geographical variables such as altitude or distance to the river mouth were marginally important, and mainly contributed to explain offsets in winter and spring minimum temperature. However, within the riparian forest only (canopy gap fraction < 25%, distance to the river < 150m), variations of up to -4°C and +15% in summertime daily maximum air temperature and minimum relative humidity, respectively, were still found from the plateau to the cooler, moister river banks, only ~5-10m below. Elevation above the river was then identified as the main predictor, and explained the marked variations from the plateau to the banks much better than canopy gap fraction. The microclimate measured near the river is as cool but moister than the macroclimate encountered at 700-1000m asl further east in F. sylvatica's main distribution range. Indeed, at all locations, we found that air relative humidity was much higher than expected from a temperature-only effect, suggesting that extra moisture is brought by the river. Such strong microclimatic influence of fine-scale topography and river moisture may well explain the distribution of beech trees in this riparian refugium, restricted to the river gorges where microtopographic variations are the strongest.