Dr. Heike Lischke

High-Arctic environments are facing an elevated pace of warming and increasing human activities, making them more susceptible to the introduction and spread of alien species. We investigated the role of human disturbance in facilitating the spread of a native plant (Papaver dahlianum) in a high-Arctic natural environment close to Isfjord Radio station and along adjacent hiking trails at Kapp Linné, Svalbard. We reconstructed the spatial pattern of the arrival and spread of P. dahlianum at Kapp Linné by combining historical records of the species occurrence (1928–2018) with a contemporary survey of the plant abundance along the main hiking trail (2023 survey) and tested the relative effects of altitude and proximity to hiking trails on the species density via a generalised linear model (GLM). We then compared historical records with the simulated annual spread of the species by assuming either only local spread or local spread plus spread from hiking trails. Finally, we used a fine-scale UAV-derived brightness index to test for terrain preference by applying a randomisation test. Distance from the station (56% explained variation) and minimum distance from the trail (28%) significantly explained the species density across the research area (best GLM R² = 0.755). The modelled species spread including the trail effect (fitted spread ~30 m yr.⁻¹) managed to capture the maximum extent of the occupied area, whereas simulations assuming only local spread (~2 m yr.⁻¹) underestimated the historical extent. A randomisation test showed that P. dahlianum has a significant preference for gravel soils with low vegetation cover due to either trail trampling and/or natural processes. Along with climate warming, human activities can increase the rate of species range shift by providing hot spots of introduction (human settlements) and spreading corridors (hiking trails). Our results show that spatially explicit models can be applied to accurately predict the potential spread of species, leading to a more efficient monitoring plan. Systematic monitoring of alien species and sanitisation measures should be prioritised in polar habitats with a high incidence of human disturbances.

Aim: Our primary aim was to assess how dispersal limitation affected forest changes in the Northern Hemisphere during the last glacial cycle, and especially after the last deglaciation. Location: Northern Hemisphere (between 20°N and 80°N). Time Period: 50,000–0 years before present.
Major Taxa Studied: Angiosperms and gymnosperms.
Methods: We used the LPJ-GM 2.0 dynamic global vegetation model to simulate the vegetation dynamics of the Northern Hemisphere over the past 50,000 years. We compared two dispersal modes by allowing establishment as soon as suitable environmental conditions were realised (free dispersal), or by additionally constraining establishment based on potential migration speed and maturation time (dispersal limitation). For both dispersal modes, we inferred (mega-)biomes and calculated the fraction of forest cover, earliest post-glacial forest establishment, and the difference between dispersal modes (dispersal lag). To evaluate our results, we compared simulated tree cover with pollen-based reconstructions of European Holocene vegetation and of North American vegetation since the Last Glacial Maximum.
Results: We simulated multi-millennial dispersal lags in post-glacial establishment of temperate forests in Europe, boreal forests in Central Siberia, and scattered forest areas close to mountain chains (Rocky Mountains and north of Himalaya). Overall, the simulation with limited dispersal had lower forest extents compared to the free-dispersal simulation, especially after the Last Glacial Maximum, with greater mismatches following intervals of rapid warming, notably the onsets of GI-1 and the Holocene. Comparisons with pollen reconstructions showed that the simulation with limited dispersal better captured both the expansion of European temperate trees and the distribution of broadleaved and needleleaved trees in North America during the early Holocene.
Main Conclusions: The lag in response of forests to rapid climate change has implications for past ecosystems and projections of future ecosystem services.

Frost drought refers to the chronic or acute desiccation of trees exposed to high evaporative pressures while being rooted in cold or frozen soils. This phenomenon has been known for more than a century but is still poorly characterized. Summer desiccation manifests itself as long-term stem contractions. Similar contractions have been reported in winter. In this study, we investigated the causes of total winter stem contraction (WSC) using 14 years of dendrometer data from evergreen (P.abies) and deciduous (L.decidua) mature trees growing along an elevational transect (from 800 to 2200 m asl) in the Swiss Alps. Results indicated that WSC varied between and and were strongly dependent on species, elevation, and tree height. Moreover, the magnitude of contractions was strongly associated with stem contractions subsequent to freeze–thaw events (F). We suggest that both F and WSC are the consequences of water losses due to ice blockage associated frost drought, occurring when the distal parts of the tree are thawed and transpiring, while the larger basal parts remain frozen, thus inhibiting water uptake and creating a hydraulic imbalance.

Tree regeneration is a key process in forest dynamics, particularly in the context of forest resilience and climate change. Models are pivotal for assessing long-term forest dynamics, and they have been in use for more than 50 years. However, there is a need to evaluate their capacity to accurately represent tree regeneration. We assess how well current models capture the overall abundance, species composition, and mortality of tree regeneration. Using 15 models built to capture long-term forest dynamics at the stand, landscape, and global levels, we simulate tree regeneration at 200 sites representing large environmental gradients across Central Europe. The results are evaluated against extensive data from unmanaged forests. Most of the models overestimate recruitment levels, which is compensated only in some models by high simulated mortality rates in the early stages of individual-tree dynamics. Simulated species diversity of recruitment generally matches observed ranges. Models simulating higher stand-level species diversity do not feature higher species diversity in the recruitment layer. The effect of light availability on recruitment levels is captured better than the effects of temperature and soil moisture, but patterns are not consistent across models. Increasing complexity in the tree regeneration modules is not related to higher accuracy of simulated tree recruitment. Furthermore, individual model design is more important than scale (stand, landscape, and global) and approach (empirical and process-based) for accurately capturing tree regeneration. Despite the mismatches between simulation results and data, it is remarkable that most models capture the essential features of the highly complex process of tree regeneration, while not having been parameterized with such data. We conclude that much can be gained by evaluating and refining the modeling of tree regeneration processes. This has the potential to render long-term projections of forest dynamics under changing environmental conditions much more robust.

Aim: How the ability of plants to move towards newly favourable habitats (dispersal limitation) impacts the change of biome distribution and transition under fast climate warming is still debated. Analysing vegetation change in the past may help to clarify the relative importance of underlying ecological processes such as climate, biotic interactions, and dispersal. In this study, we investigated how dispersal limitation affected the distribution of European forests in the last 18,000 years.
Location: Southern and Central Europe.
Taxon: Spermatophyta.
Methods: Using the LPJ-GM 2.0 model (an extension of LPJ-GUESS), we simulated European vegetation from the end of the Last Glacial Maximum (18.5 ka) to the current time (0 ka). Using biome reconstructions from pollen data as reference, we compared the performance of two dispersal modes: with no migration constraints or seed limitation (free dispersal mode), and with plant establishment depending on seed dynamics and dispersal (dispersal limitation mode).
Results: The model run, including migration processes, was better at capturing the post-glacial expansion of European temperate forests (and the longer persistence of boreal forests) than the setting assuming free dispersal, especially during periods of rapid warming. This suggests that a number of (temperate) tree taxa experienced delayed occupancy of climatically suitable habitats due to a limited dispersal capacity, i.e., post-glacial migration lags.
Main conclusions: Our results show that including migration processes in model simulations allows for more realistic reconstructions of forest patterns under rapid climate change, with consequences for future projections of carbon sequestration and climate reconstructions with vegetation feedback, assisted migration and forest conservation.

Dendrometers recording stem diameter variations (SDV) at high-resolution are useful to assess trees' water relation since water reserves are stored in the elastic tissue of the bark. These tissues typically shrink during the day as they release water when evaporative demand is high and swell during the night as they are replenished when evaporative demand is low, generating the typical SDV profile known as the diel SDV cycle. However, similar SDV cycles have been observed on dead trees due to the hygroscopic shrinking and swelling of the dead bark tissues. In order to remove this hygroscopic effect of the bark, dendrometers are applied as close as possible to the living bark tissues by removing the outer dead layer, however with questionable success. In this study, we used SDV time series from 40 point dendrometers applied on dead-bark-removed mature trees to assess and quantify the remaining hygroscopic effect on individual trees. To do so, we checked SDV behavior in the cold season and explored the relation between the diel SDV cycle and changes in relative humidity (RH). Our results showed that (a) the hygroscopic effect in SDV can be well-detected based on the amplitude of the diel SDV cycle (diel SDV_ampl) and the correlation between SDV and RH during both the cold and the warm season; (b) the level of the hygroscopic effect varies strongly among individuals; (c) diel SDV_ampl is proportional to both changes in RH and transpiration so that the hygroscopic effect on the diel SDV cycle can be quantified using a linear model where (diel SDV_ampl) is a function of RH changes and transpiration. These results allow the use of the model to correct the amplitude of the diel SDV cycles and suggest that this method can be applied to other ecological relevant water-related SDV variables such as tree water deficit.

It is perplexing when species-rich ecosystems change abruptly and, for example, dominant or economically interesting species populations collapse. Although various aspects of such ecosystem regime shift at tipping points have been studied, little attention has been paid to the possible dependence of community stability on the intrinsic growth rates of their species. Intrinsic growth rates of species can vary, e.g., due to evolution, environmental changes or fluctuations, disturbances, or human influences such as exploitation of certain species. We show theoretically and computationally that under certain conditions changing relative intrinsic growth rates of competing species have a strong effect on community stability. In addition, we are investigating which species characteristics favour such changes of communities from stable to unstable and vice versa.

Introduction: The ability of species to colonize newly suitable habitats under rapid climate change can be constrained by migration processes, resulting in a shift of the leading edge lagging behind the ameliorating climate, i.e. migration lag. The importance and extent of such migration lags during the forest expansion after the Last Glacial Maximum (LGM) are still debated. Similarly, the relative importance of the main drivers of post-glacial vegetation dynamics (temperature, dispersal limitation, and competition) is still discussed in the literature.

Methods: We used the dynamic global vegetation model LPJ-GM 2.0 to reconstruct the range shifts of 16 competing major European tree species after the LGM (18.5 ka BP) until recent times (0 ka BP). We simulated two dispersal modes by allowing free establishment whenever the climatic conditions suited the species (free dispersal), or by accounting for migration processes in the simulated vegetation dynamics (dispersal limitation). We then calculated thermal and range shift velocities, competition at establishment, thermal and dispersal lags for each species and dispersal mode. Finally, we compared our simulated range shift velocities with pollen-derived migration rates.

Results: The simulation assuming limited dispersal resulted in more accurate migration rates as compared to pollen-derived migration rates and spreading patterns. We found no marked migration lags in the post-glacial establishment of pioneer species (Pinus sylvestris and Betula pubescens). Under the free dispersal mode, the remaining temperate species expanded rapidly and almost synchronously across central Europe upon climate warming (Bølling-Allerød interstadial). Differently, the northward spread of temperate species simulated under dispersal limitation happened mainly during the Holocene and in successive waves, with late spreaders (e.g. Fraxinus excelsior) experiencing multi-millennial dispersal lags and higher competition.

Discussion: Our simulation under dispersal constraints suggests that the post-glacial tree expansion in Europe was mainly driven by species-specific thermal requirements and dispersal capacity, which in turn affected the order of taxa establishment and thus the degree of competition. Namely, taxa with less cold-tolerance and relatively low dispersal ability experienced the highest migration lags, whereas the establishment of pioneer species was mostly in equilibrium with the climate.

In response to rapid climate change and increasing human pressure, a wide variety of taxa have shifted their distribution in the past decades (range shift), with important consequences for ecosystem services and human health and economy. However, it is not yet clear whether most species will be able to track their favourable habitats or lag behind the climate signal (migration lag). Studying the paleo-vegetation response to past climatic fluctuations may help to understand the ecological processes underlying range shift dynamics. This thesis aims to implement an efficient model to hind- and fore-cast the range shift of the vegetation at large temporal and spatial scales. To this end, we used the dynamic global vegetation model (DGVM) LPJ-GM, which couples a migration module to the widely-used DGVM LPJ-GUESS, thus allowing plant species to migrate while interacting with each other. [...]

Mountain forests below rocky cliffs are regularly impacted by falling blocks. Rockfall thus increases tree mortality and can locally alter the forest structure. At the same time, trees can decelerate or stop falling blocks and play an important role in protecting settlements and infrastructure from rockfall impacts. Furthermore, trees in the upper part of a slope protect trees further downslope. Considering the interaction between forest dynamics and rockfall disturbance in dynamic forest models is necessary to accurately predict the development of rockfall protection forests in the long term. In this study, we integrated the disturbing effect of rockfall on trees in the dynamic forest landscape model TreeMig through a coupling with three-dimensional rockfall simulations and analyzed the rockfall-forest feedback over time. We introduced an additional mortality per cell, based on the probability and severity of rockfall disturbance derived from rockfall simulations. We implemented the potential feedback effect between rockfall disturbance and forest development using a meta-model of the rockfall simulations and analyzed the sensitivity of forest development to varying disturbances for a case study in the Swiss Alps. With increasing disturbance, the total biomass of the forest decreased, whereby differences were relatively small at the scale of the forest complex, but more pronounced at local (cell) scale. Generally, the comparison to light detection and ranging (LiDAR)-derived forest data showed a better agreement between the modeled forest and reality when considering the rockfall disturbing effect. The coupled simulations further revealed a positive feedback effect of rockfall disturbance and forest development. Disturbance probability and severity clearly decreased with advancing forest growth, which, however, lead to an overestimated recolonization of the disturbed areas. Still, the rockfall disturbance module clearly improved the simulations of a rockfall protection forest with TreeMig. Future improvement of the model should include a better representation of soil formation and water availability in the disturbed areas and the consideration of long-term effects of the rockfall disturbances, such as pests and diseases. The consideration of the rockfall disturbance in forest modeling is particularly relevant for small-scale studies requiring a detailed representation of spatial differences in forest cover, as this is the case in protection forest management.

The prediction of species geographic redistribution under climate change (i.e. range shifts) has been addressed by both experimental and modelling approaches and can be used to inform efficient policy measures on the functioning and services of future ecosystems. Dynamic global vegetation models (DGVMs) are considered state-of-the art tools to understand and quantify the spatio-temporal dynamics of ecosystems at large scales and their response to changing environments. They can explicitly include local vegetation dynamics relevant to migration (establishment, growth, seed (propagule) production), species-specific dispersal abilities and the competitive interactions with other species in the new environment. However, the inclusion of more detailed mechanistic formulations of range shift processes may also widen the overall uncertainty of the model. Thus, a quantification of these uncertainties is needed to evaluate and improve our confidence in the model predictions. In this study, we present an efficient assessment of parameter and model uncertainties combining low-cost analyses in successive steps: local sensitivity analysis, exploration of the performance landscape at extreme parameter values, and inclusion of relevant ecological processes in the model structure. This approach was tested on the newly implemented migration module of the state-of-the-art DGVM LPJ-GM, focusing on European forests. Estimates of post-glacial migration rates obtained from pollen and macrofossil records of dominant European tree taxa were used to test the model performance. The results indicate higher sensitivity of migration rates to parameters associated with the dispersal kernel (dispersal distances and kernel shape) compared to plant traits (germination rate and maximum fecundity) and highlight the importance of representing rare long-distance dispersal events via fat-tailed kernels. Overall, the successful parametrization and model selection of LPJ-GM will allow plant migration to be simulated with a more mechanistic approach at larger spatial and temporal scales, thus improving our efforts to understand past vegetation dynamics and predict future range shifts in a context of global change.

It has long been anticipated that relating functional traits to species demography would be a cornerstone for achieving large-scale predictability of ecological systems. If such a relationship existed, species demography could be modeled only by measuring functional traits, transforming our ability to predict states and dynamics of species-rich communities with process-based community models. Here, we introduce a new method that links empirical functional traits with the demographic parameters of a process-based model by calibrating a transfer function through inverse modeling. As a case study, we parameterize a modified Lotka–Volterra model of a high-diversity mountain grassland with static plant community and functional trait data only. The calibrated trait–demography relationships are amenable to ecological interpretation, and lead to species abundances that fit well to the observed community structure. We conclude that our new method offers a general solution to bridge the divide between trait data and process-based models in species-rich ecosystems.

1. Understanding the processes that shape forest functioning, structure, and diversity remains challenging, although data on forest systems are being collected at a rapid pace and across scales. Forest models have a long history in bridging data with ecological knowledge and can simulate forest dynamics over spatio‐temporal scales unreachable by most empirical investigations.
2. We describe the development that different forest modelling communities have followed to underpin the leverage that simulation models offer for advancing our understanding of forest ecosystems.
3. Using three widely applied but contrasting approaches - species distribution models, individual‐based forest models, and dynamic global vegetation models - as examples, we show how scientific and technical advances have led models to transgress their initial objectives and limitations. We provide an overview of recent model applications on current important ecological topics and pinpoint ten key questions that could, and should, be tackled with forest models in the next decade.
4. Synthesis. This overview shows that forest models, due to their complementarity and mutual enrichment, represent an invaluable toolkit to address a wide range of fundamental and applied ecological questions, hence fostering a deeper understanding of forest dynamics in the context of global change.

Forests below rocky cliffs often play a very important role in protecting settlements against rockfall. The structure and development of these forests are expected to be substantially affected by the disturbance of the falling rocks. Knowing about this effect is important to predict the development of protection forests and consider potential effects of the falling blocks in management strategies. The goal of this study is to quantify differences in forest structure depending on rockfall activity in four different sites in the Swiss Alps. For this, we collected data on forest structure in zones of different rockfall activity and derived rockfall impact probabilities based on rockfall simulations. We assessed whether differences in forest structure and signs of rockfall disturbance could be observed between the rockfall zones. We additionally built mixed-effects models to identify the key variables explaining the forest characteristics described by diameter (DBH) and basal area (bA). The forest structure differs between the rockfall zones, however, with varying effects amongst the sites. DBH tends to decrease with increasing rockfall activity, whereas tree density appears to be little impacted by rockfall. For most sites, the number of deposited blocks and the simulated tree impact probability have a significant effect in the models along with the species, whereas for one site, hardly any effect of rockfall was found. Our results, obtained either from direct measurements or modelling, show that rockfall can locally influence the structure of forests, whereas the influence depends on the frequency and intensity of the rockfall disturbance. Impact probabilities obtained by simulations can serve as a good proxy for rockfall disturbances.

In steep terrain, forests play an important role as natural means of protection against natural hazards, such as rockfall. Due to climate warming, significant changes in the protection service of these forests have to be expected in future. Shifts of current to more drought adapted species may result in temporary or even irreversible losses in the reduction of rockfall risk provided by these forests. In this study, we assessed how the protective capacity against rockfall of a protection forest in the western part of the Valais in the Swiss Alps may change in future, by combining dynamic forest modelling with a quantitative risk analysis. Current and future forest development was modelled with the spatially explicit forest model TreeMig under a moderate (RCP4.5) and an extreme (RCP8.5) climate change scenario. The simulated forest scenarios were compared to ground-truth data from the current forest complex. We quantified the protective effect of the different forest scenarios based on the reduction of rockfall risk for people and infrastructure at the bottom of the slope. Rockfall risk was calculated on the basis of three-dimensional rockfall simulations. The forest simulations predicted a clear decrease in basal area of most of the currently occuring species (Fagus sylvatica, Picea abies, Larix decidua, and Abies alba) in future. The forest turned into a Quercus pubescens dominated forest, for both climate scenarios, mixed with Pinus sylvestris under RCP4.5. With climate warming, a clear increase in risk is expected for both climate change scenarios. In the long-term (>100 years), a stabilization of risk, or even a slight decline may be expected due to an increase in biomass of the trees. The results of this study further indicate that regular forest interventions may promote regeneration and thus accelerate the shift in species distribution. Future research should explore into more details the long-term effect of different adaptive forest management strategies on the protection service of forests under climate change.

1. Modelling is often confronted with scaling problems, because modelling—directly or indirectly—always implies scaling. This is because models simplify. Simplification usually means aggregation, and aggregation is a scaling process. As scaling cannot be avoided in modelling, it should carefully be addressed and resolved, at least to the degree possible.
2. In this paper, we give an overview of scaling approaches in ecological modelling. We propose to classify scaling approaches into pre‐model scaling, in‐model scaling and post‐model scaling depending on the timing of the scaling relative to the main modelling process.
3. We show general approaches, examples and potential application problems for each category. We suggest that scaling problems might be more widespread than previously thought. These scaling problems are matched with a range of solutions, but often these solutions will have to be adapted and tailored to the specific scaling case.
4. Thus, we recommend that ecologists be aware of scaling challenges especially where models do not explicitly aim at scaling. Developing general test systems for scaling methods may help to broaden and enhance the application of scaling methods in ecology.

Der Klimawandel verändert die Lebensbedingungen von Bäumen. Verschlechtern sich für bestimmte Baumarten die Umweltbedingungen stark, bedeutet dies nicht unbedingt deren sofortiges lokales Aussterben. Oft können die Baumarten noch eine Weile überdauern, bevor sie wegen fehlender Verjüngung verschwinden. Neue, an die klimatischen Bedingungen angepasste Baumarten sind jedoch nicht immer schon an Ort und Stelle, um sich selbstständig und natürlich zu verjüngen. Das Waldlandschaftsmodell TreeMig beschreibt die Walddynamik im Raum: In Zellen eines regelmässigen Gitters wird die Änderung der Populationsdichte der verschiedenen Baumarten in den einzelnen Höhenklassen mit klimaabhängigen populationsdynamischen Prozessen formuliert. Ergänzt durch Samenproduktion, -transport, Samenbankdynamik und Entwicklung der Jungbäume, simuliert Tree-Mig so die Migration der Baumarten. Simulationen mit Klimaänderungsszenarien deuten darauf hin, dass die räumlich-zeitliche Walddynamik wegen der begrenzten Wandergeschwindigkeit der Bäume der klimawandelbedingten Veränderung der Standortbedingungen hinterherhinkt. Der Rückstand ist in Gebieten mit flachen Temperaturgradienten, zum Beispiel in der borealen Zone, ausgeprägt. In der Schweiz mit ihren steilen Temperaturgradienten ist er geringer. Es zeigt sich zudem, dass die Alpenpässe als Flaschenhals für die Ausbreitung von trockenheitsadaptierten submediterranen Arten wirken können.

Projections of landscape dynamics are uncertain, partly due to uncertainties in model formulations. However, quantitative comparative analyses of forest landscape models are lacking. We conducted a systematic comparison of all forest landscape models currently applied in temperate European forests (LandClim, TreeMig, LANDIS-II, iLand). We examined the uncertainty of model projections under several future climate, disturbance, and dispersal scenarios, and quantified uncertainties by variance partitioning. While projections under past climate conditions were in good agreement with observations, uncertainty under future climate conditions was high, with between-model biomass differences of up to 200 t ha^-1. Disturbances strongly influenced landscape dynamics and contributed substantially to uncertainty in model projections (~25-40% of observed variance). Overall, model differences were the main source of uncertainty, explaining at least 50% of observed variance. We advocate a more rigorous and systematic model evaluation and calibration, and a broader use of ensemble projections to quantify uncertainties in future landscape dynamics.

1. Species range limits are expected to be dramatically altered under future climate change and many species are predicted to shift their distribution upslope to track their suitable conditions (i.e. based on their niche). However, there might be large discrepancies between the speed of the upward shift of the climatic niche and the actual migration velocity of the species, especially in long‐lived organisms such as trees. In fact, most studies did not find any significant upward shift of the distributional limits of temperate forest trees over the last decades. It therefore beckons the questions why trees are moving upslope much slower than their bioclimatic envelope and what are the implications for ecosystem functioning.
2. Here, we compared the simulations of the upslope displacement of the bioclimatic envelope of 16 tree species inhabiting temperate mountain forests under ongoing and future climate change obtained by correlative species distribution models (SDMs) to those from a dynamic forest model accounting for dispersal, competition and demography. We then partitioned the discrepancy in upslope migration velocity between the SDMs and the dynamic forest model into different components by manipulating dispersal limitation, interspecific competition and demography.
3. Tree species in the dynamic forest model migrated only slowly upslope in contrast to the SDMs. Most of the difference in migration velocity can directly be attributed to tree's demography (long life cycle), followed by effects of competition and only a marginal contribution of dispersal limitation. Additionally, lower elevation species ('non‐treeline') shifted slower upslope than high elevation species ('treeline') indicating a strong effect of interspecific competition at their leading edge.
4. Synthesis. Forests have a high inertia to climate change because of their longevity and ability to acclimatize to high climatic fluctuations. Lower elevation tree species (deciduous) only slowly establish in stands at higher elevation where coniferous species dominate and likely profit from facilitation by disturbance events. Therefore, forest ecosystems seem to persist, even if climate becomes unfavourable, until they approach a tipping point at which an extreme event (e.g. drought, storm or insect attack) leads to a large dieback and resource change enabling new suitable species to spread and establish.