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• We investigated long-term drivers of species population dynamics in the natural forest reserve network of Switzerland. • We modeled ingrowth and tree mortality for seven abundant species based on forest structure and climate. • Tree density declined while species richness increased over the 60-year measurement period. • Competition and climate variables were the main drivers of population dynamics. Changes in tree species composition can have strong effects on ecosystem functioning and the services provided by forests. Empirical observations can provide an improved understanding of the drivers of species population dynamics, yet long-term datasets are scarce in natural forests. This study used inventory data of the Swiss forest reserve network to assess tree population dynamics over the past 60 years. Tree status, species and dbh were repeatedly measured between 1956 and 2018 in 211 permanent plots of 34 forest reserves that covered a wide environmental gradient. Differences in species richness and tree density were compared between the first and last inventory. Furthermore, we used generalized linear mixed effect models to estimate the processes of ingrowth and tree mortality of seven abundant species (Abies alba, Acer pseudoplatanus, Fagus sylvatica, Fraxinus excelsior, Picea abies, Pinus sylvestris and Quercus spp.) separately as a function of stand structural attributes and climate. We found a general decline in the density of the dominant species, with a low recruitment of light-demanding species and an increase in species richness, particularly in the colline vegetation zone. Both species-specific ingrowth and tree mortality were influenced by stand density and climate variables. Tree mortality increased with live tree density and increasing temperature. Ingrowth increased with tree density and decreased with higher temperature. Therefore, forest structural properties and climate have a strong influence on species population dynamics, and both are essential for predicting ingrowth and tree mortality in forest ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: The impact of ungulate herbivores on tree regeneration and its possible consequences for long-term forest dynamics has raised concerns worldwide. In many countries, ungulate management aims at constant animal densities, whereas unmanaged ungulate populations tend to fluctuate over time. The ecosystem consequences of constant vs. varying ungulate densities are largely unknown, and the exact density that is acceptable from a forestry point of view is highly uncertain as well. We used the gap model ForClim v2.9.5 to examine the effects of three browsing-related phenomena: (a) temporal changes in animal densities and thus oscillations in browsing intensity; (b) changes in the importance of browsing as a limiting factor relative to other limitations for ingrowth; and (c) growth suppression by browsing and hence different ingrowth rates for slow- vs. fast-growing trees. Results showed that ungulate herbivory can induce profound compositional and structural changes in forest stands: (a) oscillations in the browsing intensity led to compositional shifts that were less severe than under the corresponding constant browsing intensity; (b) an increase in the importance of browsing relative to other environmental factors caused a decrease in the incidence of palatable species; and (c) growth suppression strongly affected the numbers and composition of small trees of all species. We conclude that browsing can cause a shift not only in the structure and composition of tree regeneration, but also of the upper canopy in the long term. Management can manipulate forest ecosystems through the control of animal densities, and our results suggest that alternative management strategies for ungulate populations may be worth considering so as to provide “windows of opportunity” for forest regeneration in time and/or space. [Copyright &y& Elsevier]

Recent applications of forest gap models have shown weak performancealong drought gradients. Using the gap model FORCLIM, we examine twopossible explanations for this: (1) the parameters denoting the drought tolerance of the tree species need to be estimated more accurately, and (2) the soil water balance estimation schemes used in most gapmodels are not capable of tracking soil moisture content under dry conditions sufficiently well to reflect its influence on tree growth. The behaviour of the model along two drought gradients in Europe and eastern North America is used as a test case. Parameter adjustments alone turned out to be ineffective for improving the performance of FORCLIM. A theoretical analysis of the soil water balance model by Thornthwaite and Mather [Thornthwaite, C.W.. Mather. J.R., 1957, Instructions and tables for computing potential evapotranspiration and the water balance. Publ. Climatol. 10, pp. 183-311], which is used in FORCLIM and in many other gap models, showed that it yields inconsistent results when applied to simulate the year-to-year variability of drought occurrence. We therefore have developed a new water balance model with the goal of adequate performance in connection with a forest gapmodel. similar simplicity and low computational demand as in previous models. The new scheme uses a bucket approach and monthly time steps of temperature and precipitation alone. The performance of FORCLIM did not improve when the new water balance model was used together with the original estimates of the species' drought tolerance. However,when both a small number of drought tolerance parameters were adjusted and the new water balance model was incorporated, the performance of FORCLIM increased strongly along the drought gradients on both continents. In Europe, the transition from beech to oak forests could besimulated correctly, whereas in eastern North America the simulated pattern of aboveground biomass became much more realistic. We conclude that [ABSTRACT FROM AUTHOR]