Seed production and dispersal limit treeline advance in the Pyrenees

Prepared by Alba Anadon-Rosell, Marc Talavera, Josep M. Ninot, Empar Carrillo & Enric Batllori

Alpine treelines represent the upper altitudinal limit of the forest in mountain regions. This boundary is globally constrained by temperature, and therefore potentially sensitive to climate change. Many studies have reported a treeline upward shift in the last decades associated to increased temperatures. In many cases, however, such upward shifts were also explained by the decrease and cessation of anthropogenic activities (i.e., land use changes) throughout the last century. Contrastingly, other studies have not reported upslope shifts but rather densification (i.e. increased tree density) of the treeline.

The mountain pine, Pinus uncinata, is the major treeline-forming species in the Pyrenees. Decades of research have led to a broad comprehension of treeline dynamics in this area, with studies mainly focusing on tree growth, seedling recruitment dynamics, and forest expansion. Yet, the role of reproductive processes such as seed production, dispersal and germination success as drivers of treeline dynamics remains unknown. Here, we examine whether such reproductive processes may constrain the foreseen treeline advance in the Pyrenees in the context of ongoing global change.

We carried out our study in the Central Pyrenees, Catalonia, NE Spain. We selected five study sites, and at each site, we established four plots along the elevation gradient, one each at subalpine forest at middle elevation, subalpine forest at high elevation, scattered trees over the closed forest, and the krummholz zone (stunted trees). We collected cones from five trees at each plot, measured their length, and counted and weighed the seeds. We used the collected seeds in a germination experiment under controlled conditions to determine potential differences in seed germination success depending on elevation and site of origin. Additionally, in three of the study sites, we installed seed traps at four elevation positions along the forest-alpine grassland transition (subalpine dense forest, subalpine fragmented forest, krummholz zone and alpine grassland) to estimate seed dispersal over three consecutive years.

We found that elevation does not limit the viability of mountain pine seeds since neither seeds’ weight nor their germination success differed significantly between elevations of origin (i.e., the elevation position of the mother tree). However, we found that the number of seeds per cone significantly decreased with elevation and that seed dispersal was very limited. No seeds arrived into any of the alpine grassland traps consistently across study years. Therefore, our study reveals that seed production and dispersal are constraining the ongoing rates of treeline advance in the Pyrenees. This may explain why, over this mountain range, treeline densification has been previously reported as a more common response to global change (temperature and land use changes) than upslope shift.

This is a plain language summary for the paper of Anadon-Rosell et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.12849).