Global vegetation models: incorporating transient changes to structure and composition

Abstract. We describe an approach for developing a Dynamic Global Vegetation Model (DGVM) that accounts for transient changes in vegetation distribution over a decadal time scale. The DGVM structure is based on a linkage between an equilibrium global vegetation model and smaller scale ecosystem dynamics modules that simulate the rate of vegetation change. Vegetation change is classified into four basic types, based largely on the projected change in above-ground biomass of the vegetation. These four types of change are: (1) dieback of forest, shrubland or grassland; (2) successional replacement within forest, shrubland or grassland; (3) invasion of forest, shrubland or grassland; (4) change in tree/grass ratio. We then propose an approach in which the appropriate ecosystem dynamics module for each type of change is applied and the grid cells of the global model updated accordingly. An approach for accounting for fire, as an example of a disturbance which may strongly influence the rate and spatial pattern of forest dieback, is incorporated. We also discuss data needs for the development, calibration and validation of the model.     

A predictive framework to assess spatio‐temporal variability of infestations by the European spruce bark beetle

Natural disturbances are key factors for the development of forest ecosystems. In forests of central Europe and Scandinavia, the European spruce bark beetle Ips typographus is the most devastating biotic disturbance agent in Norway spruce Picea abies, but our understanding of the factors determining its spatio‐temporal dynamics is still quite limited. To quantify the drivers of bark beetle dynamics, we analyzed a survey dataset with annual resolution that covers 17 yr and 469 forest districts (10 860 km² of forest area) all over Switzerland. We used Poisson log‐normal models in a Bayesian framework to analyze the spatio‐temporal dynamics of bark beetle infestation spots at the forest district level. Bark beetle infestations increased with increasing heat sum (> 8.3°C), volume of standing Norway spruce stock, and the number of infestation spots of the previous year. Precipitation tended to slightly affect the risk of bark beetle infestations. Two major storm events further increased the spatio‐temporal variability of bark beetle infestations. Spruce abundance, storm damage and temperature are known to be important factors influencing the population dynamics of the European spruce bark beetle. Our study is the first to quantify the combined effects of spruce abundance and heat sum, whereby the heat sum turned out to be the most important and consistent predictor. Because our study area encompasses large ecological and climatological gradients, our model is likely to be applicable to Norway spruce forests in other regions of central Europe and Scandinavia.     

Early emergence increases survival of tree seedlings in Central European temperate forests despite severe late frost

Global warming is expected to result in earlier emergence of tree seedlings that may experience higher damages and mortality due to late frost in spring. We monitored emergence, characteristics, and survival of seedlings across ten tree species in temperate mixed deciduous forests of Central Europe over one and a half year. We tested whether the timing of emergence represents a trade‐off for seedling survival between minimizing frost risk and maximizing the length of the growing period. Almost two‐thirds of the seedlings died during the first growing period. The timing of emergence was decisive for seedling survival. Although seedlings that emerged early faced a severe late frost event, they benefited from a longer growing period resulting in increased overall survival. Larger seedling height and higher number of leaves positively influenced survival. Seedlings growing on moss had higher survival compared to mineral soil, litter, or herbaceous vegetation. Synthesis. Our findings demonstrate the importance of emergence time for survival of tree seedlings, with early‐emerging seedlings more likely surviving the first growing period.     

Temporal stability in forest productivity increases with tree diversity due to asynchrony in species dynamics

Theory predicts a positive relationship between biodiversity and stability in ecosystem properties, while diversity is expected to have a negative impact on stability at the species level. We used virtual experiments based on a dynamic simulation model to test for the diversity–stability relationship and its underlying mechanisms in Central European forests. First our results show that variability in productivity between stands differing in species composition decreases as species richness and functional diversity increase. Second we show temporal stability increases with increasing diversity due to compensatory dynamics across species, supporting the biodiversity insurance hypothesis. We demonstrate that this pattern is mainly driven by the asynchrony of species responses to small disturbances rather than to environmental fluctuations, and is only weakly affected by the net biodiversity effect on productivity. Furthermore, our results suggest that compensatory dynamics between species may enhance ecosystem stability through an optimisation of canopy occupancy by coexisting species.     

Assessing the response of forest productivity to climate extremes in Switzerland using model–data fusion

The response of forest productivity to climate extremes strongly depends on ambient environmental and site conditions. To better understand these relationships at a regional scale, we used nearly 800 observation years from 271 permanent long‐term forest monitoring plots across Switzerland, obtained between 1980 and 2017. We assimilated these data into the 3‐PG forest ecosystem model using Bayesian inference, reducing the bias of model predictions from 14% to 5% for forest stem carbon stocks and from 45% to 9% for stem carbon stock changes. We then estimated the productivity of forests dominated by Picea abies and Fagus sylvatica for the period of 1960–2018, and tested for productivity shifts in response to climate along elevational gradient and in extreme years. Simulated net primary productivity (NPP) decreased with elevation (2.86 ± 0.006 Mg C ha^?1 year^?1 km^?1 for P. abies and 0.93 ± 0.010 Mg C ha^?1 year^?1 km^?1 for F. sylvatica). During warm–dry extremes, simulated NPP for both species increased at higher and decreased at lower elevations, with reductions in NPP of more than 25% for up to 21% of the potential species distribution range in Switzerland. Reduced plant water availability had a stronger effect on NPP than temperature during warm‐dry extremes. Importantly, cold–dry extremes had negative impacts on regional forest NPP comparable to warm–dry extremes. Overall, our calibrated model suggests that the response of forest productivity to climate extremes is more complex than simple shift toward higher elevation. Such robust estimates of NPP are key for increasing our understanding of forests ecosystems carbon dynamics under climate extremes.     

Overstorey–Understorey Interactions Intensify After Drought-Induced Forest Die-Off: Long-Term Effects for Forest Structure and Composition

Severe drought events increasingly affect forests worldwide, but little is known about their long-term effects at the ecosystem level. Competition between trees and herbs (‘overstorey–understorey competition’) for soil water can reduce tree growth and regeneration success and may thereby alter forest structure and composition. However, these effects are typically ignored in modelling studies. To test the long-term impact of water competition by the herbaceous understorey on forest dynamics, we incorporated this process in the dynamic forest landscape model LandClim. Simulations were performed both with and without understorey under current and future climate scenarios (RCP4.5 and RCP8.5) in a drought-prone inner-Alpine valley in Switzerland. Under current climate, herbaceous understorey reduced tree regeneration biomass by up to 51%, particularly in drought-prone landscape positions (i.e., south-facing, low-elevation slopes), where it also caused a shift in forest composition towards drought-tolerant tree species (for example, Quercus pubescens). For adult trees, the understorey had a minor effect on growth. Under future climate change scenarios, increasing drought frequency and intensity resulted in large-scale mortality of canopy trees, which intensified the competitive interaction between the understorey and tree regeneration. At the driest landscape positions, a complete exclusion of tree regeneration and a shift towards an open, savannah-like vegetation occurred. Overall, our results demonstrate that water competition by the herbaceous understorey can cause long-lasting legacy effects on forest structure and composition across drought-prone landscapes, by affecting the vulnerable recruitment phase. Ignoring herbaceous vegetation may thus lead to a strong underestimation of future drought impacts on forests.     

Disappearing refuges in time and space: how environmental change threatens species coexistence

Understanding the impacts of environmental changes on species survival is a major challenge in ecological research, especially when shifting from single- to multispecies foci. Here, we apply a spatially explicit two-species simulation model to analyze the effects of geographic range shifting and habitat isolation on different coexistence mechanisms. The model explicitly considers dispersal, local competition, and growth on a single resource. Results highlight that both range shifting and habitat isolation severely impact coexistence. However, the strength of these impacts depends on the underlying coexistence mechanisms. Neutrally coexisting species are particularly sensitive to habitat isolation, while stabilized coexistence through overcompensatory density regulation is much more sensitive to range shifting. We conclude that, at the community level, the response to environmental change sensitively depends on the underlying coexistence mechanisms. This suggests that predictions and management recommendations should consider differences between neutral versus stabilized community structures whenever possible.     

Predicting decay and ground vegetation development in <Emphasis Type="Italic">Picea abies</Emphasis> snag stands

In a Picea abies (L.) Karst. (Norway spruce) mountain forest on the Gandberg site in the northern Swiss Alps, trees were killed by bark beetles in 1992–1997. A combination of field studies and dynamic modelling was used to project snag decay and future ground vegetation succession in these steep, unharvested stands. In permanent plots, ground vegetation cover and natural tree regeneration have been monitored annually since 1994. To obtain additional information on the abundance of snags, logs, boulders and other microsite types in these stands, the relative frequency of the microsite types was quantified along four strip transects on the montane and subalpine elevational levels. A dynamic model of snag decay and ground vegetation development was constructed (modified matrix model approach). Based on field data and literature values, the model was parameterised and initialised separately for the montane and the subalpine level. For model validation, microsite types were quantified in 2001 with the line-intercept method on both elevational levels. Starting from the conditions in the stands before the bark beetle attacks, it was possible to project short-term succession and to accurately simulate the decay and ground vegetation patterns eight years after tree die-back. Long-term simulations suggest that on the montane level, raspberries (Rubus idaeus L.) will be replaced by Picea abies, while on the subalpine level ferns will dominate for a long time.     

Tree species richness promotes productivity in temperate forests through strong complementarity between species

Understanding the link between biodiversity and ecosystem functioning (BEF) is pivotal in the context of global biodiversity loss. Yet, long-term effects have been explored only weakly, especially for forests, and no clear evidence has been found regarding the underlying mechanisms. We explore the long-term relationship between diversity and productivity using a forest succession model. Extensive simulations show that tree species richness promotes productivity in European temperate forests across a large climatic gradient, mostly through strong complementarity between species. We show that this biodiversity effect emerges because increasing species richness promotes higher diversity in shade tolerance and growth ability, which results in forests responding faster to small-scale mortality events. Our study generalises results from short-term experiments in grasslands to forest ecosystems and demonstrates that competition for light alone induces a positive effect of biodiversity on productivity, thus providing a new angle for explaining BEF relationships.