Reversible Impacts of a Cold Spell on Forest Cover, Tree Growth and Carbohydrates in Mediterranean Pine and Oak Forests

Climate extremes such as cold spells are becoming more frequent as climate variability increases. However, few studies have evaluated the impacts of winter cold spells on forest cover, tree growth and leaf and sapwood non-structural carbohydrate (NSC) concentrations. We analyzed changes in tree cover using remote sensing data and compared the radial growth of coexisting and defoliated Pinus halepensis trees and non-defoliated P. halepensis and Pinus pinaster trees. We also compared NSC concentrations in leaves and sapwood of defoliated and non-defoliated P. halepensis and Quercus ilex trees. In January 2021, a rapid drop in temperatures led to minimum values (-21.3 degrees C) in eastern Spain and triggered canopy defoliation in several planted (P. halepensis) and native (Q. ilex) tree species. The cold spell led to a decrease in forest cover in the most defoliated stands and reduced radial growth of defoliated P. halepensis and sapwood NSC concentrations in P. halepensis and Q. ilex, particularly starch. Prior to the cold spell, defoliated P. halepensis trees significantly (p < 0.05) grew more (2.73 +/- 1.70 mm) in response to wetter winter conditions than non-defoliated P. halepensis (2.29 +/- 1.08 mm) and P. pinaster (1.39 mm) trees. Those P. halepensis individuals which grew faster at a young age were less resilient to the winter cold spell in later years. The study stands showed a high recovery capacity after the cold spell, but the Mediterranean drought-avoiding P. halepensis was the most affected species.