RESUMEN
Western Iberia has recently shown increasing frequency of drought conditions coupled with heatwave events, leading to exacerbated limiting climatic conditions for plant growth. It is not clear to what extent wood growth and density of agroforestry species have suffered from such changes or recent extreme climate events. To address this question, tree-ring width and density chronologies were built for a Pinus pinaster stand in southern Portugal and correlated with climate variables, including the minimum, mean and maximum temperatures and the number of cold days. Monthly and maximum daily precipitations were also analyzed as well as dry spells. The drought effect was assessed using the standardized precipitation-evapotranspiration (SPEI) multi-scalar drought index, between 1 to 24-months. The climate-growth/density relationships were evaluated for the period 1958-2011. We show that both wood radial growth and density highly benefit from the strong decay of cold days and the increase of minimum temperature. Yet the benefits are hindered by long-term water deficit, which results in different levels of impact on wood radial growth and density. Despite of the intensification of long-term water deficit, tree-ring width appears to benefit from the minimum temperature increase, whereas the effects of long-term droughts significantly prevail on tree-ring density. Our results further highlight the dependency of the species on deep water sources after the juvenile stage. The impact of climate changes on long-term droughts and their repercussion on the shallow groundwater table and P. pinaster's vulnerability are also discussed. This work provides relevant information for forest management in the semi-arid area of the Alentejo region of Portugal. It should ease the elaboration of mitigation strategies to assure P. pinaster's production capacity and quality in response to more arid conditions in the near future in the region.
RESUMEN
Mediterranean plants cope with cold wet winters and dry hot summers, with a drought gradient from northwest to southeast. Limiting climatic conditions have become more pronounced in the last decades due to the warming trend and rainfall decrease. Juniperus thurifera L., a long-lived conifer tree endemic to the western Mediterranean region, has a disjunct distribution in Europe and Africa, making it a suitable species to study sensitivity to climate in both sides of the Mediterranean Basin. Tree-ring width chronologies were built for three J. thurifera stands at Spain (Europe) and three in Morocco (Africa) and correlated with monthly temperature and precipitation. The temporal stability of climate-growth relationships was assessed using moving correlations; the drought effect on growth was calculated using the monthly standardized precipitation-evapotranspiration index (SPEI) at different temporal scales. In the wettest stands, increasing spring temperature and summer precipitation enhanced growth, while in the driest stands, growth was enhanced by higher spring precipitation and lower summer temperature. The climate-growth correlations shifted during the twentieth century, especially since the 1970s. Particularly noticeable is the recent negative correlation with previous autumn and winter precipitation in the wettest stands of J. thurifera, probably related with an effect of cloud cover or flooding on carbon storage depletion for next year growth. The driest stands were affected by drought at long time scales, while the wettest stands respond to drought at short time scales. This reveals a different strategy to cope with drought conditions, with populations from drier sites able to cope with short periods of water deficit.