Climate drivers of dieback in Mediterranean urban pine forests

Abstract

Droughts trigger forest dieback worldwide, but these events are still understudied in cities. However, conditions of urban forests can aggravate drought stress due to heat island effects, poor growing conditions or lack of management. We investigated a recent dieback event affecting four Pinus halepensis Mediterranean forests located in Málaga, south eastern Spain. We sampled declining (D) and non-declining (ND) trees showing high and low defoliation levels, respectively, and measured their diameter, stem water content (SWC) and radial growth. We used dendrochronology to obtain basal area increment (BAI) series in 40 trees per forest corresponding to D and ND trees growing in N- and S-oriented slopes. These data were correlated with climate variables, teleconnection indices such as the North Atlantic Oscillation (NAO) index and a drought index. Remote sensing indices were used to evaluate changes in cover and greenness. We found that D trees significantly grew more than ND trees in three out of four forests, regardless slope orientation. However, D trees showed a stronger growth decline in the past 10 years and lower SWC than ND trees. Growth was enhanced by high precipitation in the prior December and high minimum temperatures in April. Long winter droughts constrained growth. The dieback was triggered by very warm and dry conditions starting in the winter 2021–2022 and leading to sharp declines in cover and growth from 2022 to 2024. Such dry winter conditions were linked to positive NAO phases. Fast-growing trees with lower SWC were prone to show dieback in response to severe and long winter droughts. These two characteristics could be further explored as early-warning signals of dieback and tree death. Adaptive urban silviculture should consider thinning but also planting tree and shrub species showing higher drought tolerance.