Seedling Survival at Timberline Is Critical to Conifer Mountain Forest Elevation and Extent

Craig R Brodersen,Gerhard Wieser,Lara M Kueppers,Lynn M Resler,Maaike Y Bader,David M Cairns,William K Smith,Keith Reinhardt,Daniel M Johnson,Gabriele Broll,Anna Sala,Friedrich-Karl Holtmeier,Matthew J Germino

doi:10.3389/ffgc.2019.00009

Abstract

Conifer mountain forests influence a large portion of the World’s human population by providing a host of critical economic, sociological, and ecosystem services. Although the causes of the elevational, transitional boundaries of these forests (i.e. upper and lower timberlines) have been questioned for over a century, these investigations have focused predominately on the growth limitations of saplings or mature trees at the upper alpine boundary. Yet, the elevational movement of timberlines is dependent initially on new seedling establishment in favorable microsites that appear to be generated by ecological facilitation. Recent evidence suggests that this facilitation is critical during the initial 1-2 years of growth when survival may be less than a few percent, only cotyledons are present, and survival occurs only in favorable microsites created by inanimate objects (e.g. boulders, dead stems), microtopography, or already established vegetation. Dramatic changes in tree form (e.g. krummholz mats) across the timberline ecotone also plays an important role in generating microsite facilitation. These favorable, facilitated microsites have been characterized broadly as experiencing low sky exposure during summer (day and night) and leeward wind exposure during winter that generates protective snow cover, all of which are needed for new seedling survival. Thus, determining the specific microclimate and edaphic characteristics of favorable microsites, and their frequency at timberline, will provide a more mechanistic understanding and greater predictability of the future elevation and extent of conifer mountain forests. In addition, although the ecophysiological advantages of a needle-like leaf morphology is well established for adult conifer trees, the advantage of this phylogenetically unique trait in emergent seedlings has not been thoroughly evaluated. Understanding seedling ecophysiology and the functional morphology that contributes to survival, plus the nature and frequency of favorable microsites at timberline, will enable more reliable estimates of future elevational shifts in conifer mountain forests. This approach could also lead to the development of a valuable and sensitive tool for forest managers interested in evaluating future changes in these forests under increased large-scale infestation and drought mortality, as well as for current scenarios of predicted climate change.

Highlights

Ecosystem services provided by conifer mountain forests include water capture and supply, anthropogenic carbon sequestration, forest products, biodiversity, recreation and esthetics (e.g., Funnell and Parish, 2005; Grêt-Regamey et al, 2012)
Correlating predicted changes in future macroclimate with the abundant data at the mesoclimate level, and extrapolating to the seedling microsite level could be fundamental for estimating future effects of global change on the elevations and extent of our conifer mountain forests (Smith et al, 2009)
Knowing how each of the dominant timberline species within these regions will respond to climate change at each life history stage influencing forest regeneration is critical

Summary

INTRODUCTION

Ecosystem services provided by conifer mountain forests include water capture and supply, anthropogenic carbon sequestration, forest products, biodiversity, recreation and esthetics (e.g., Funnell and Parish, 2005; Grêt-Regamey et al, 2012). Emergent seedling survival at the alpine timberline appears constrained to favorable microsites facilitated by microtopography (e.g., depressions), existing vegetation (Wheeler et al, 2011), or inanimate structures such as boulders, rocks, and fallen logs (Smith et al, 2003; Körner, 2012; Wagner et al, 2018) These structures within the environment serve multiple roles such as accumulating leeward snow and snowmelt water that is available later in summer, buffering seedlings from extreme maximum and minimum temperature fluctuations, and protecting seedlings from high winds that often carry abrasive ice crystals that can damage the waxy needle cuticle and lead to desiccation death (Hadley and Smith, 1986; Renard et al, 2016). Co-occurring herbaceous and woody species, site characteristics (e.g., slope, aspect, soil type, snowpack, etc.) and both stochastic weather events and longterm trends must all be considered

IMPORTANCE OF SEEDLING FORM

CHANGES IN TREE FORM AND SPACING ACROSS THE TIMBERLINE LANDSCAPE

SEEDLING ECOPHYSIOLOGY

SUMMARY AND CONCLUSIONS

AUTHOR CONTRIBUTIONS