Parallel but different deaths: sudden drought-trigged mortality in Scots pine versus slow climate- and mistletoe-driven mortality in silver fir

Abstract

In recent decades, forest die-off events have increased worldwide due to warmer droughts, with Southwestern Europe emerging as a mortality hotspot. This region is notably affected by widespread decline and die-off episodes, particularly in mountain forests dominated by Abies alba (silver fir) and Pinus sylvestris (Scots pine). We study four stands (two Scots pine, two silver fir) showing ongoing die-off in the Spanish Pyrenees, near the southwestern edge of both species’ distributions. Our objectives were to analyze growth patterns of declining trees in mortality hotspots, and to study the climatic (drought) and biotic (mistletoe) factors contributing to this decline. Using dendroecological methods and a recently developed change-point analysis approach, we assessed differences in growth patterns at stand and individual tree levels. Results showed that mortality events were recent, especially for Scots pine. Although mortality patterns were tree-specific, Scots pine typically exhibited a sudden growth decline before death or a sudden mortality without prior growth decline, whereas silver fir generally showed gradual and prolonged growth decline preceding mortality. Drought and elevated temperatures were the primary triggers for Scots pine and the warmer-drier silver fir site, with secondary contributors such as historical management and microsite conditions (soil, aspect, slope, moisture, density, size, age, competition). Conversely, in the colder-wetter silver fir site, mistletoe was the sole driver of decline and mortality. These findings highlight the complex interplay of primary and secondary stressors underlying forest die-off.