Light availability predicts mortality probability of conifer saplings in Swiss mountain forests better than radial growth and tree size

Abstract

Radial and height growth rates are suitable indicators of impending tree mortality risk of adult trees, but their applicability to saplings remains unknown. We compared radial growth of living and dead saplings of different heights and quantified the effects of light availability, growth and tree size on mortality. We sampled an equal number of living and dead saplings of four coniferous tree species (Pinus cembra, Larix decidua, Picea abies, Abies alba) in nine forests along an elevational gradient of the Swiss Alps. Based on tree-ring widths reconstructed from stem disks at multiple tree heights, we calculated radial growth rates. We observed a divergent pattern in radial growth of living and dead saplings, with reduced growth of dead saplings starting several years prior to death. By matching living and dead saplings of similar ages, we tested whether mortality probabilities of saplings were influenced by light availability, recent growth rates and diameter. Mortality of coniferous saplings in mountain forests was mainly influenced by light availability, with changing effects along the elevational gradient. Recent radial growth rate and tree size were only weakly associated with sapling mortality. Our study establishes the importance of long-term predisposing factors for the mortality probability of conifer saplings in mountain forests, thus extending well-established findings from the adult stage to saplings, which represent a critical stage of forest dynamics.