Complex climate constraints of upper treeline formation in the Pyrenees

Abstract

Alpine treelines are often induced by a threshold of minimum temperatures above which tree growth is generally not possible anymore. However, this worldwide pattern may not account for more drought-prone mountains, where changes in precipitation can also affect ecosystem functioning and productivity at higher elevations. Here, we aim to determine if tree ring formation in ecotones along the Pyrenees indeed primarily depends on temperature means or also reflects hydroclimatic changes. The mean duration of the growing season at Pyrenean treelines was 177 days lasting from mid May to early November, which corresponds to a mean root-zone temperature of 8.2 ºC.Temperatures during the early and late growing season were most critical for mountain pine growth at highest elevations, and above average spring temperatures also contributed to radial enlargement of tracheids. Sites showing more similar soil temperatures also presented more alike long-term growth trends indicating that local conditions trace regional influences on growth. Nevertheless, more wet conditions in summer enhanced growth at treelines subjected to more Mediterranean influence and therefore having a higher summer water deficit. A broader perspective reveals that soil moisture availability during spring and summer, however, also influenced ring width formation at other treelines located in mid-latitude mountains separating temperate and semi-arid biomes. Our results suggest re-evaluating the idea of purely temperaturelimited treelines, especially in mid-latitude mountains that experience moisture seasonality or where precipitation decreases with increasing elevation.

Climate-induced reversal of tree growth patterns at a tropical treeline

“…At the same time, these shifts point to a potential link between physiological responses and the future distribution of pine forests at low latitudes. Along with other recent studies from Mediterranean and temperate regions ( 11 , 20 , 27 , 49 ), the low-latitude sites studied here indicate that as CO 2 and temperatures continue to rise, NFS may become climatic holdouts or microrefugia. Incorporating these and other relevant findings into niche theory and species distribution models should be a priority for conservation strategies for protected areas and management plans.…”

Forests dynamics in the montane–alpine boundary: a comparative study using satellite imagery and climate data

“…The temperature trend is consistent with the general constant increasing trend in average temperatures across European mountain ranges (Kullman 2007, Leonelli et al 2011, González de Andrés et al 2015. The impacts of temperature on tree lines are well known (Körner 1998), but the significant responses are mostly related to early or late growing seasons (Rammig et al 2010, González de Andrés et al 2015 or even winter temperatures (Kullman 2007). Our findings suggest that increasing annual average temperatures favour the expansion of tree cover and line in the Austrian Alps, Low Tatra and the Roma nian Car pathians.…”

Spatial dynamics of alpine tree lines under global warming: What explains the mismatch between tree densification and elevational upward shifts at the tree line ecotone?

“…Since the mountain pine has a short growing season, this explains its strong presence in the eastern Pyrenees (Riou‐Nivert, ). Based on temperature records at six tree line sites of the eastern half of the range from 2005 to 2008, de Andrés, Camarero, and Büntgen () estimated that the mean duration of the growing season was 177 days (but according to Ninot et al () only 90 days at 2,300 m), lasting from mid‐May to early November, corresponding to a mean root‐zone temperature of 8.2°C.…”