Long-term growth and functioning of high-elevation Pinus uncinata forests and trees inferred through dendroecology = Creixement i funcionament a llarg termini de boscos i individus de Pinus uncinata inferits mitjançant dendroecologia

Resumen

Iberian Pinus uncinata tree-growth variability and its responses to climate are analyzed by means of dendrochonological methods from an individual- to a population-level scale across its distribution area in the Iberian Peninsula. This multiple approach provides new knowledge about the site-tree-climate interactions at an individual scale, and about the species performance at a population scale. Age-related changes in sapwood area were the main drivers of BAI in mountain P. uncinata forests. Thus, any potential climate-induced effect on BAI will be mainly driven by sapwood production, which is mediated by tree age and altitude. Because slow-growing high-elevation trees get older than fast-growing low-elevation trees, we expect differential age-mediated BAI responses along the altitudinal gradient. Once trees reach a maximum age- or size-related functional threshold linked to a stagnant sapwood production, they can become relatively insensitive to climate variability. Climate plays a secondary role in controlling TRWi variability among coexisting trees even in these high-elevation environments. Actually, altitude plays a major role affecting P. uncinata TRWi responses to climate at the site and tree scales. This suggests that the altitude-mediated decrease in air temperatures is the major driver of TRWi at both the site and tree levels determining the maximum elevation of the tree growth form. These results also stress that a more realistic projection of future growth and productivity responses of mountain forests to climate warming can be strongly affected by individual tree features and local factors modulating or buffering the regional effects of climate stress on growth. Both (a) a tree-scale approach to quantify growth-index responses to climate and (b) a detailed characterization of the potential drivers of those individual tree responses are requisites for applying an individual-based framework in dendroecology. Such increased focus on individual tree responses would improve the ecological knowledge of the individuals¿ vulnerability against climatic stressors. Following a population-based approach, a weakness of the theoretically temperature-sensitive TRW proxy to capture recent warming trends is observed. Instead, summer drought is increasingly influencing TRW along the 20th century, which agrees with observations from Iberian mountain forests. Rising temperatures might have led to an increase in drought stress of Pyrenean and Iberian high-elevation forests as has been observed in other Mediterranean similar ecosystems. We may be attending how a physiological threshold in terms of optimal temperature for growth is surpassed, reinforcing the role of drought as a plausible growth-limiting factor of Iberian high-elevation forests during the last decades. The intricate topography and diverse climate of the Mediterranean Basin produce varied and often opposite trend signs in dendrochronological proxies like TRW or BAI even between neighbouring sites, during the last decades. In spite of this local complexity, our findings reveal a pattern acting at synoptic scales where tree growth across the Mediterranean Basin is limited by drought or low water availability during the growing season. Likely biased by selective sampling efforts towards higher elevations and older trees within a few countries in the north-west of the Mediterranean Basin, we emphasize the need in dendroecological research of a more evenly distribution of study sites and age classes that better reflect ecological rather than political and methodological criteria.¿¿¿¿¿