Ulrike Hiltner

Last Name

Hiltner

First Name

Ulrike

ORCID

0000-0001-5663-7068

Organisational unit

08701 - Gruppe Waldbau / Group Silviculture

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Publications 1 - 6 of 6

Importance of the forest state in estimating biomass losses from tropical forests: combining dynamic forest models and remote sensing
Hiltner, Ulrike; Huth, Andreas; Fischer, Rico (2022)
Biogeosciences
Disturbances, such as extreme weather events, fires, floods, and biotic agents, can have strong impacts on the dynamics and structures of tropical forests. In the future, the intensity of disturbances will likely further increase, which may have more serious consequences for tropical forests than those we have already observed. Thus, quantifying aboveground biomass loss of forest stands due to stem mortality (hereafter biomass loss rate) is important for the estimation of the role of tropical forests in the global carbon cycle. So far, the long-term impacts of altered stem mortality on rates of biomass loss have not been adequately described. This study aims to analyse the consequences of long-term elevated stem mortality rates on forest dynamics and biomass loss rate. We applied an individual-based forest model and investigated the impacts of permanently increased stem mortality rates on the growth dynamics of humid, terra firme forests in French Guiana. Here, we focused on biomass, leaf area index (LAI), forest height, productivity, forest age, quadratic mean stem diameter, and biomass loss rate. Based on the simulation data, we developed a multiple linear regression model to estimate biomass loss rates of forests in different successional states from the various forest attributes. The findings of our simulation study indicated that increased stem mortality altered the succession patterns of forests in favour of fast-growing species, which increased the old-growth forests' gross primary production, though net primary production remained stable. The stem mortality rate had a strong influence on the functional species composition and tree size distribution, which led to lower values in LAI, biomass, and forest height at the ecosystem level. We observed a strong influence of a change in stem mortality on biomass loss rate. Assuming a doubling of stem mortality rate, the biomass loss rate increased from 3.2 % yr−1 to 4.5 % yr−1 at equilibrium. We also obtained a multidimensional relationship that allowed for the estimation of biomass loss rates from forest height and LAI. Via an example, we applied this relationship to remote sensing data on LAI and forest height to map biomass loss rates for French Guiana. We estimated a countrywide mean biomass loss rate of 3.0 % yr−1. The approach described here provides a novel methodology for quantifying biomass loss rates, taking the successional state of tropical forests into account. Quantifying biomass loss rates may help to reduce uncertainties in the analysis of the global carbon cycle.
Perspectives for forest modeling to improve the representation of drought-related tree mortality
Fischer, Rico; Anders, Tim; Bugmann, Harald; et al. (2025)
Journal für Kulturpflanzen
We are currently observing increased tree mortality following multi-year drought events, particularly in low mountain ranges like the Harz Mountains in Germany, where over 70% of spruce stands have died. Forest models are useful tools for understanding the long-term effects of climate change on forest ecosystems, yet struggle to reproduce this massive dieback. In this study, we simulated spruce mortality in the Harz Mountains using five forest models (ForClim, FORMIND, 3-PG-Hydro, LPJ-GUESS, GOTILWA+) of very different complexity. Estimated from the crown condition survey, spruce mortality in the Harz region increased to values above 30% during recent drought years (2018-2020). We found that most models failed to capture these observed high mortality rates, although they showed a clear signal in reduced forest productivity during drought. This discrepancy between the observed high spruce mortality and simulated forest dynamics highlights the need for improved modelling approaches to accurately represent tree mortality processes during and after extreme drought events. We discuss several perspectives for enhancing dynamic forest models by integrating missing processes prospectively. This includes novel (i) process-based drought mortality approaches, (ii) enhanced description of eco-physiological processes like plant hydraulics, (iii) data-driven and AI approaches, and (iv) improved representation of biotic damaging agents (i.e., insects and pathogens). Incorporating these perspectives into forest models has the potential to improve their ability to simulate forest dynamics under extreme drought, ultimately con tributing to the assessment of forest resilience and informing adaptive management strategies in Germany and beyond.
Climate change alters the ability of neotropical forests to provide timber and sequester carbon
Hiltner, Ulrike; Huth, Andreas; Hérault, Bruno; et al. (2021)
Forest Ecology and Management
Logging is widespread in tropical regions, with approximately 50% of all humid tropical forests (1.73 × 109 ha) regarded as production forests. To maintain the ecosystem functions of carbon sequestration and timber supply in tropical production forests over a long term, forest management must be sustainable under changing climate conditions. Individual-based forest models are useful tools to enhance our understanding about the long-term effects of harvest and climate change on forest dynamics because they link empirical field data with simulations of ecological processes. The objective of this study is to analyze the combined effects of selective logging and climate change on biomass stocks and timber harvest in a tropical forest in French Guiana. By applying a forest model, we simulated natural forest dynamics under the baseline scenario of current climate conditions and compared the results with scenarios of selective logging under climate change. The analyses revealed how substantially forest dynamics are altered under different scenarios of climate change. (1) Repeated logging within recovery times decreased biomass and timber harvest, irrespective of the intensity of climate change. (2) With moderate climate change as envisaged by the 5th IPCC Assessment Report (representative concentration pathway 2.6), the average biomass remained the same as in the baseline scenario (−1%), but with intensive climate change (RCP 8.5), the average biomass decreased by 12%. (3) The combination of selective logging and climate change increased the likelihood of changes in forest dynamics, driven mainly by rising temperatures. Under RCP 8.5, the average timber harvest was almost halved, regardless of the logging cycle applied. An application-oriented use of forest models will help to identify opportunities to reduce the effects of unwanted ecosystem changes in a changing environment. To ensure that ecosystem functions in production forests are maintained under climate change conditions, appropriate management strategies will help to maintain biomass and harvest in production forests.
Contrasting impacts of climate change on protection forests of the Italian Alps
Hillebrand, Laurin; Marzini, Sebastian; Crespi, Alice; et al. (2023)
Frontiers in Forests and Global Change
Protection forests play a key role in protecting settlements, people, and infrastructures from gravitational hazards such as rockfalls and avalanches in mountain areas. Rapid climate change is challenging the role of protection forests by altering their dynamics, structure, and composition. Information on local- and regional-scale impacts of climate change on protection forests is critical for planning adaptations in forest management. We used a model of forest dynamics (ForClim) to assess the succession of mountain forests in the Eastern Alps and their protective effects under future climate change scenarios. We investigated eleven representative forest sites along an elevational gradient across multiple locations within an administrative region, covering wide differences in tree species structure, composition, altitude, and exposition. We evaluated protective performance against rockfall and avalanches using numerical indices (i.e., linker functions) quantifying the degree of protection from metrics of simulated forest structure and composition. Our findings reveal that climate warming has a contrasting impact on protective effects in mountain forests of the Eastern Alps. Climate change is likely to not affect negatively all protection forest stands but its impact depends on site and stand conditions. Impacts were highly contingent to the magnitude of climate warming, with increasing criticality under the most severe climate projections. Forests in lower-montane elevations and those located in dry continental valleys showed drastic changes in forest structure and composition due to drought-induced mortality while subalpine forests mostly profited from rising temperatures and a longer vegetation period. Overall, avalanche protection will likely be negatively affected by climate change, while the ability of forests to maintain rockfall protection depends on the severity of expected climate change and their vulnerability due to elevation and topography, with most subalpine forests less prone to loosing protective effects. Proactive measures in management should be taken in the near future to avoid losses of protective effects in the case of severe climate change in the Alps. Given the heterogeneous impact of climate warming, such adaptations can be aided by model-based projections and high local resolution studies to identify forest stand types that might require management priority for maintaining protective effects in the future.
A harmonized database of European forest simulations under climate change
Grünig, Marc; Rammer, Werner; Albrich, Katharina; et al. (2024)
Data in Brief
Process -based forest models combine biological, physical, and chemical process understanding to simulate forest dynamics as an emergent property of the system. As such, they are valuable tools to investigate the effects of climate change on forest ecosystems. Specifically, they allow testing of hypotheses regarding long-term ecosystem dynamics and provide means to assess the impacts of climate scenarios on future forest development. As a consequence, numerous localscale simulation studies have been conducted over the past decades to assess the impacts of climate change on forests. These studies apply the best available models tailored to local conditions, parameterized and evaluated by local experts. However, this treasure trove of knowledge on climate change responses remains underexplored to date, as a consistent and harmonized dataset of local model simulations is missing. Here, our objectives were (i) to compile existing local simulations on forest development under climate change in Europe in a common database, (ii) to harmonize them to a common suite of output variables, and (iii) to provide a standardized vector of auxiliary environmental variables for each simulated location to aid subsequent investigations. Our dataset of European stand- and landscape -level forest simulations contains over 1.1 million simulation runs representing 135 million simulation years for more than 13,0 0 0 unique locations spread across Europe. The data were harmonized to consistently describe forest development in terms of stand structure (dominant height), composition (dominant species, admixed species), and functioning (leaf area index). Auxiliary variables provided include consistent daily climate information (temperature, precipitation, radiation, vapor pressure deficit) as well as information on local site conditions (soil depth, soil physical properties, soil water holding capacity, plant -available nitrogen). The present dataset facilitates analyses across models and locations, with the aim to better harness the valuable information contained in local simulations for large-scale policy support, and for fostering a deeper understanding of the effects of climate change on forest ecosystems in Europe. (c) 2024 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )
Maintaining rockfall protection in mountain forests under climate change: optimizing management for sustainable stem size distributions
Hiltner, Ulrike; Glatthorn, Jonas; Thrippleton, Timothy; et al. (2025)
Ecological Indicators
Climate change threatens the long-term effectiveness of mountain forests, which provide crucial protection against rockfall. Maintaining this protection function requires a sustainable stem size distribution, yet how to adapt forest management for this purpose remains unclear. This study uses a simulation-based optimization approach, integrating the dynamic forest model ForClim with the Simulated Annealing optimization algorithm, to identify adaptive management strategies for Swiss forests. We first established sustainable stem size distributions for managed protection forests in four elevation zones − lower montane to subalpine − under historical climate, leading to a so-called target profile. These represent a novel indicator enabling foresters to tailor silvicultural interventions towards improving rockfall protection. Subsequently, we assessed climate change impacts on these distributions. Our simulations show that climate change will alter stem size distributions, particularly at higher elevations where a reduction of soil water availability will hinder regeneration and growth. This leads to fewer trees, especially smaller ones. We developed optimized management regimes to counteract this effect, recommending specific adjustments depending on elevation zone and management type, such as less frequent and less intensive harvesting with larger minimum removal DBH in higher-elevation mountain forest plentering, and adjustments to target DBH and harvest intensity in lower-elevation plentering. This study demonstrates that adapting silvicultural interventions can preserve the desired forest structure under climate change, without fundamental regime shifts. These findings provide practical guidance for forest managers, enabling them to proactively respond to climate change impacts and ensure the long-term functionality of rockfall protection across elevation zones.

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Ulrike Hiltner

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