Growth and quality of Fagus sylvatica saplings depend on seed source, site, and browsing intensity

Abstract

Local climate and ungulate browsing are two major factors that affect tree regeneration and genetic adaptation in Central European forests. Owing to climate change and increasing ungulate numbers, the abiotic and biotic environments of trees are changing remarkably, making it necessary to investigate the separate and joint effects of seed source (i.e., location of tree population origin) and ungulate herbivory. We used a common garden experiment to study the growth and morphology of Fagus sylvatica saplings from 77 Swiss seed sources. The experiment was set up at two sites and included a clipping treatment (i.e., terminal shoot clipped at two intensities) applied before budburst to simulate winter ungulate browsing. We studied F. sylvatica sapling growth and morphology before and two years after clipping. Measured growth traits included sapling height, stem diameter, and biomass. Morphological traits included multi-stemming, stem and crown form, stem quality, and reaction to clipping. Seed source, test site, and simulated leader browsing were all important in determining the growth and quality of F. sylvatica saplings. The effects of seed source on growth and quality indicate that F. sylvatica possesses a large pool of diverse genotypes across Switzerland and thus has the potential to adapt to local conditions through gene flow. Growth and morphology differed significantly between the two test sites, indicating that local environments should be considered carefully when a new plantation is established. The effect of the single simulated browsing event disappeared over time for the growth traits, owing to growth compensation. However, sapling quality decreased after clipping, suggesting that browsing may lead to persistent quality losses in production forests. Neither the growth nor the morphological reaction after clipping depended on the effect of population, meaning that resilience to browsing was independent of seed source. Consequently, interactions with ungulate browsing do not have to be taken into account when selecting F. sylvatica populations for particular climatic and site conditions.