Contrasted effects of climate change on low-altitude relict Pinus uncinata stands in the Northern French Alps

This paper reports on climate-induced growth changes in relict, low-altitude mountain pines (Pinus uncinata Mill. ex. Mirb.) from two refugia with cold microclimates located in the Northern French Alps. The P. uncinata stands analyzed grow at the lower bound of their ecological limit and are thus thought to be sensitive indicators of ongoing climate change. Using dendroecological approaches, we compare tree-ring growth at two closely spaced low-altitude stands in the Chartreuse massif (French Alps): La Plagne and Cirque de Bresson. La Plagne is a N-NW-exposed, ventilated slope with cold air circulating in the scree during summer, and the presence of sporadic permafrost as well as ground overcooling, whereas Cirque de Bresson is located on a small, S-exposed fan with sporadic avalanche activity. At both sites, growth responses of P. uncinata to changes in twentieth and twenty-first centuries temperature and precipitation conditions were investigated by means of moving correlation analyses. At Cirque de Bresson, a significant and rapid decline in tree-ring widths has been observed since the early 1990s. We attribute this decline to (i) increasing air temperatures at the beginning of the growing season (May–June) as well as to (ii) a decrease in soil water potential. At La Plagne, we do not detect any significant trend between the higher summer temperatures and tree growth, presumably as a result of the circulation of cold air in the scree slope, which is thought to maintain fresh and humid soil conditions and therefore favor tree growth. These forest stands provide prime examples on how dendroecology can contribute to the study of the dynamics and local variability of tree growth and climate change in relict forest populations with high ecological and conservation values.

Needle water potential of selected pine species during the off-season: A case study

In this study we investigated the needle water potential of different pine species using a pressure chamber during winter months. Totally, seven pine species Pinus jeffreyi Greville & Balfour, Pinus ponderosa P. & C. Lawson, Pinus uncinata Ramond ex de Candolle, Pinus nigra Arnold, Pinus sylvestris Linnaeus, Pinus heldreichii Christ, Pinus cembra Linnaeus representing ecological variability of this genus were selected for measurements. Although the water potential is well documented in many tree species during the vegetation season, data from the off-season are scarce. During the investigated seasons (November 2014, January and February 2016), days with air temperature (T) above and below 0°C were selected for the measurement. During the days with air T above 0°C and global radiation reaching 3,000 kJ·m^–2·day^–1, the water potential averaged –0.6 ± 0.4 MPa. On the contrary, under T below 0°C and global radiation above 6,000 kJ·m^–2·day^–1, the needle water potential was close to zero (–0.3 ± 0.2 MPa) regardless of the pine species. The reason why the needle water potential reflects air T and is negatively correlated with the global radiation in winter will be an objective of our future investigations.