Impacts of more frequent droughts on a relict low-altitude Pinus uncinata stand in the French Alps

Abstract

Cold microclimatic conditions provide exceptional microhabitats to Pinus uncinata stands occurring at abnormally low altitudes in seven relict stands of the northern French Alps. Here, P. uncinata is located at the lower bounds of its ecological limits and therefore expected to provide a sensitive indicator of climate change processes. We used dendrochronological analysis to study the growth patterns of closely spaced chronologies across an elevational transect and compare a relict low-altitude to a P. uncinata stand located at the alpine treeline. Two detrending procedures are used to reveal high-and low-frequencies embedded in annually resolved ring-width series. Growth response of P. uncinata to instrumental temperature and precipitation data is investigated by means of moving response function analyses. Results show an increase in the sensitivity of tree-ring widths to drought during previous summer in both stands. At the treeline stand, an increasing response to fall temperature is observed whereby fall temperature and radial tree growth increased in two synchronous steps around ∼1930 and from ∼1980-present. At the low-altitude stand, P. uncinata appears more drought sensitive and exhibits a sharp growth decline since the mid-1980s, coinciding with increasing summer temperatures. Growth divergence between the two stands can be observed since the mid-1980s. We argue that the positive growth trend at the high-altitude stand is due to increasing fall temperatures which would favor the formation of metabolic reserves in conjunction with atmospheric CO 2 enrichment that in turn would facilitate improved water use efficiency. At the relict low-altitude stand, in contrast, it seems that improved water use efficiency cannot compensate for the increase in summer temperatures.

Aged but withstanding: Maintenance of growth rates in old pines is not related to enhanced water-use efficiency

“…Even if some periods exhibited declining growth, the overall positive BAI trends from to 2008 do not support those studies considering old forests as carbon neutral (Yoder et al 1994); rather the opposite, we conclude that old trees remain active carbon sinks (Carey et al 2001;Luyssaert et al 2008;Stephenson et al 2014). Increased growth of high-elevation conifer forests has been observed also in other mountain ranges and atributted to increased temperatures and an extended growing season enhancing net photosynthesis in these cold-limited ecosystems (Rolland et al 1998;Salzer et al 2009;Corona et al 2015).…”

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

“…At these sites, the ground thermal regime is controlled by the interaction of climatic conditions with topography (such as north exposure, limited amount of incoming solar radiation, shadow effect of mountains, frequent temperature inversions in winter, or distribution/duration of snow cover) as well as surface and subsurface factors (organic layers, blocky material, water supply in scree slope). In the French Alps, the P. uncinata stands of Cirque de Saint-Même (Chartreuse massif) and La Grande Aiguille (Oisans massif; Corona et al 2015) show this setting. The second category of cold scree slopes is attributed to a process of reversible internal air circulation in scree slopes, also referred to as "chimney effect" or "wind tube" (Sawada et al 2003;Morard et al 2010;Stiegler et al 2014;Kneisel et al 2015;Wicky and Hauck 2017).…”