Last-century forest productivity in a managed dry-edge Scots pine population: the two sides of climate warming
Abstract
Climate change in the Mediterranean, associated with warmer temperatures and more frequent droughts, is expected to impact forest productivity and the functioning of forests ecosystems as carbon reservoirs in the region. Climate warming can positively affect forest growth by extending the growing season, whereas increasing summer drought generally reduces forest productivity and may cause growth decline, trigger dieback, hamper regeneration, and increase mortality. Forest management could potentially counteract such negative effects by reducing stand density and thereby competition for water. The effectiveness of such interventions, however, has so far mostly been evaluated for short time periods at the tree and stand levels, which limits our confidence regarding the efficacy of thinning interventions over longer time scales under the complex interplay between climate, stand structure, and forest management. In this study, we use a century‐long historical data set to assess the effects of climate and management on forest productivity. We consider rear‐edge Scots pine (Pinus sylvestris) populations covering continental and Mediterranean conditions along an altitudinal gradient in Central Spain. We use linear mixed‐effects models to disentangle the effects of altitude, climate, and stand volume on forest growth and ingrowth (recruitment and young trees’ growth). We find that warming tends to benefit these tree populations, warmer winter temperature has a significant positive effect on both forest growth and ingrowth, and the effect is more pronounced at low elevations. However, drought conditions severely reduce growth and ingrowth, in particular when competition (stand volume) is high. We conclude that summer droughts are the main threat to Scots pine populations in the region, and that a reduction of stand volume can partially mitigate the negative impacts of more arid conditions. Mitigation and adaptation measures could therefore manage stand structure to adopt for the anticipated impacts of climate change in Mediterranean forest ecosystems.
What the paper shows and why it matters
(AI-generated)
Climate warming can help forests and harm them at once. Warmer winters lengthen the growing season, whereas hotter, drier summers reduce growth and recruitment, especially at a species’ dry southern range edge. Thinning, which lowers the number of trees competing for water, is therefore proposed as a way to buffer drought. That evidence, however, comes mostly from short-term experiments and from tree-level studies, while tree-ring records, which span long periods, often lack information on past management. The open question is whether management changes how climate affects forest productivity over a century and across a whole forest, not only in single trees or short trials.
The paper shows that warming has two sides for a Scots pine forest at the dry edge of the species’ range: warmer winters raised growth and recruitment, whereas the harm done by dry summers increased with stand volume, the wood standing per hectare, which management controls. The study digitises ten-yearly forest inventories from 1896 to 2008 for 45 stands of one managed forest in the Guadarrama mountains of central Spain, at 1,300 to 2,000 m. With linear mixed models, it relates growth (the increase in wood volume of adult trees) and ingrowth (new trees above 20 cm trunk diameter, a measure of recruitment and of the growth of young trees) to winter temperature, a summer drought index, altitude and the stand volume of the previous decade. Warmer winters raised both, unexpectedly more at low than at high elevation, a pattern the authors consider possibly spurious. Stand volume mattered most in dry summers: in wet summers it was nearly inconsequential, whereas in dry ones growth and ingrowth fell markedly as volume increased.
The study concludes that summer drought is the main threat to these populations and that keeping stand volume low can partly offset it, so stand structure is a lever for adapting forests to a drier climate. It also suggests that the moderate, long-rotation shelterwood cutting practised in this forest is effective in reducing competition, so strong and frequent thinning, which other work presents as necessary, is not the only effective option. The evidence has limits: it covers one forest, has ten-yearly resolution, includes no mortality data, and the fixed effects (climate, altitude, stand volume) explain only about 10% of the variance in growth and 30% in ingrowth.