Biotic and abiotic drivers of tree seedling recruitment across an alpine treeline ecotone

Esther R Frei,Giulietta Bernareggi,Melissa A Dawes,Peter Bebi,Eva Bianchi,Andrew J Trant,Christian Rixen,Steven D Mamet,Carissa D Brown

doi:10.1038/s41598-018-28808-w

Abstract

Treeline responses to climate change ultimately depend on successful seedling recruitment, which requires dispersal of viable seeds and establishment of individual propagules in novel environments. In this study, we evaluated the effects of several abiotic and biotic drivers of early tree seedling recruitment across an alpine treeline ecotone. In two consecutive years, we sowed seeds of low- and high-elevation provenances of Larix decidua (European larch) and Picea abies (Norway spruce) below, at, and above the current treeline into intact vegetation and into open microsites with artificially removed surface vegetation, as well as into plots protected from seed predators and herbivores. Seedling emergence and early establishment in treatment and in control plots were monitored over two years. Tree seedling emergence occurred at and several hundred metres above the current treeline when viable seeds and suitable microsites for germination were available. However, dense vegetation cover at lower elevations and winter mortality at higher elevations particularly limited early recruitment. Post-dispersal predation, species, and provenance also affected emergence and early establishment. This study demonstrates the importance of understanding multiple abiotic and biotic drivers of early seedling recruitment that should be incorporated into predictions of treeline dynamics under climate change.

Highlights

Plant species are responding to recent global temperature increases[1] by shifting their ranges as populations track their fundamental niche[2,3]
Establishment is limited by temperature and water availability[31,32,33], but other abiotic factors, such as snow cover duration and desiccating winds, may affect seedling recruitment[34,35,36]
Germination of experimentally sown seeds was highest at the uppermost site, where 981 seedlings emerged (8.0 ± 0.8%, values represent mean ± 1 standard error of the percentage of viable seeds per seeded subplot, for absolute numbers see Supplementary Table S1a), followed by the mid-elevation site with 734 seedlings (5.8 ± 0.8%), and lowest at the forest site, where only 12 seedlings emerged (0.1 ± 0.04%; Psite < 0.001; Table 1; Fig. 1)

Summary

Introduction

Plant species are responding to recent global temperature increases[1] by shifting their ranges as populations track their fundamental niche[2,3]. Climate change-induced range expansion of treeline populations depends on successful recruitment, which requires dispersal of viable seeds followed by successful establishment of individual propagules[22]. Viable seed availability commonly declines with elevation[13,23] due to lower abundance of seed bearing trees and less frequent mast years, i.e. synchronous production of large seed crops[24,25,26]. Biotic interactions, such as pre-dispersal predation, may further constrain seed productivity at treeline[27], impacting future treeline range expansion. Provenance may be important in early life stages of seedlings but has rarely been evaluated (but see[57])

Methods

Results

Discussion

Conclusion