Growth history of pollarded black poplars in a continental Mediterranean region: A paradigm of vanishing landscapes

Pollarded woodlands are iconic components of European rural landscapes. Pollarding is a traditional management technique used to obtain timber and firewood. However, these woodlands are subjected to different stressors in rapidly depopulating rural regions under continental Mediterranean areas where riparian black poplar (Populus nigra) pollards are widespread but still understudied. First, the rapid rural depopulation has lead to the abandonment of pollarding creating trees with abundant, thick and heavy branches. Second, climate warming and alteration of river flows reduce soil moisture which could contribute to growth decline of riparian pollards. We investigated how these stress factors have impacted radial growth in seven sites located in four river basins of southern Aragón, north eastern Spain. We used dendrochronology to reconstruct the changes in radial growth during the period 1890–2020 and to assess how growth has responded to changes in temperature, precipitation and river flow variability. Since 1900 growth suppressions, probably corresponding to pollarding events, occurred in several decades (1910s, 1960s, 1980s and 1990s) and showed a high variability among sites and river basins. Suppressions peaked during the 1940s, probably associated to intense pollarding related to high timber and firewood demand after the Spanish Civil War. Poplar radial growth increased in response to high precipitation and river flow from March to April, particularly in sites located away of river banks. Pollarding abandonment and declines in soil moisture availability threaten the persistence of veteran black poplars. Active management through pollarding and protection should be implemented to preserve these iconic, anthropogenic woodlands due to their multiple ecological and cultural services.