Environmental influences on climate-driven growth of Mediterranean pollarded willows

Anthropogenic woodlands dominated by pollarded trees are iconic landscapes common in Europe, where they represent unique cultural, aesthetic, ecological and economic values. A special type of these woodlands are riparian pollarded willows ( Salix spp.), which show a strong coupling with shallow soil water sources and streamflow. Willows are fast-growing, pioneer species formerly used to obtain key resources for rural communities. However, the long-term responses of these species to pollarding abandonment and warmer and drier conditions are unknown, particularly in Mediterranean, seasonally dry areas. We also lack a good understanding on the diameter, age and growth rates of pollarded willows growing under different ecohydrological situations. Here, we sampled five willow ( Salix alba ) stands, located in north eastern Spain and growing under Mediterranean conditions, characterized by warm and dry summers, and analyzed them using tree-ring data. Lower-basin dry sites near intermittent streams and upper-basin cold-wet sites near permanent streams were compared. We compared these stands with two coexisting poplar ( Populus nigra ) populations. Growth in lower-basin sites located near intermittent streams was constrained by low precipitation and soil moisture from spring to summer, whereas growth in upper-basin sites situated near permanent streams and with deep soils was constrained by winter waterlogging, dry spring conditions and low summer temperatures. Willow growth declined in response to short-term spring-summer droughts. The different responses of Mediterranean pollarded willows to climate variables and river flow depend on their contrasting ecohydrological situations which condition their use and management.