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  • Publication
    Accès libre
    Global patterns and climatic controls of forest structural complexity
    (2021)
    Martin Ehbrecht
    ;
    Dominik Seidel
    ;
    Peter Annighöfer
    ;
    Holger Kreft
    ;
    Michael Köhler
    ;
    Zemp, Clara 
    ;
    Klaus Puettmann
    ;
    Reuben Nilus
    ;
    Fred Babweteera
    ;
    Katharina Willim
    ;
    Melissa Stiers
    ;
    Daniel Soto
    ;
    Hans Juergen Boehmer
    ;
    Nicholas Fisichelli
    ;
    Michael Burnett
    ;
    Glenn Juday
    ;
    Scott L. Stephens
    ;
    Christian Ammer
    The complexity of forest structures plays a crucial role in regulating forest ecosystem functions and strongly influences biodiversity. Yet, knowledge of the global patterns and determinants of forest structural complexity remains scarce. Using a stand structural complexity index based on terrestrial laser scanning, we quantify the structural complexity of boreal, temperate, subtropical and tropical primary forests. We find that the global variation of forest structural complexity is largely explained by annual precipitation and precipitation seasonality (R² = 0.89). Using the structural complexity of primary forests as benchmark, we model the potential structural complexity across biomes and present a global map of the potential structural complexity of the earth´s forest ecoregions. Our analyses reveal distinct latitudinal patterns of forest structure and show that hotspots of high structural complexity coincide with hotspots of plant diversity. Considering the mechanistic underpinnings of forest structural complexity, our results suggest spatially contrasting changes of forest structure with climate change within and across biomes.
  • Publication
    Accès libre
    Mixed-species tree plantings enhance structural complexity in oil palm plantations
    (2019)
    Zemp, Clara 
    ;
    Martin Ehbrecht
    ;
    Dominik Seidel
    ;
    Christian Ammer
    ;
    Dylan Craven
    ;
    Joshua Erkelenz
    ;
    Bambang Irawan
    ;
    Leti Sundawati
    ;
    Dirk Hölscher
    ;
    Holger Kreft
    Conversion of structurally complex rainforests into simplified oil palm monocultures leads to dramatic losses of biodiversity and ecosystem functioning. To alleviate negative ecological impacts, enrichment with native tree species may rapidly restore structural complexity in existing oil palm plantations. However, the mechanisms underlying the recovery of structural complexity in mixed-species tree plantings remain poorly understood. We measured structural complexity from terrestrial laser scanning in a biodiversity enrichment experiment with multiple tree species planted in an oil palm monoculture, forming agroforestry plots of varying tree species diversity and plot size. We find that three years after tree planting, structural complexity in oil palm increased by one third, representing 25% of the increase needed to restore the structural complexity of tropical forests. Changes in structural complexity were associated with denser and more complex filling of three-dimensional space, whereas vertical stratification was mainly influenced by oil palm. Furthermore, structural complexity increased with tree species diversity in the agroforestry plots. This relationship was mainly due the presence of well-performing species that contributed to higher levels of structural complexity. However, interactions among multiple species independently from the species identity were also detected. Finally, increasing plot size had a positive effect on a scale-independent measure of structural complexity. Our results provide evidence that planting multiple tree species in large agroforestry plots is a suitable strategy to rapidly enhance structural complexity in oil palm plantations.
  • Publication
    Accès libre
    How a measure of tree structural complexity relates to architectural benefit-to-cost ratio, light availability, and growth of trees
    (2019)
    Dominik Seidel
    ;
    Peter Annighöfer
    ;
    Melissa Stiers
    ;
    Zemp, Clara 
    ;
    Katharina Burkardt
    ;
    Martin Ehbrecht
    ;
    Katharina Willim
    ;
    Holger Kreft
    ;
    Dirk Hölscher
    ;
    Christian Ammer
    Aboveground tree architecture is neither fully deterministic nor random. It is likely the result of mechanisms that balance static requirements and light-capturing efficiency. Here, we used terrestrial laser scanning data to investigate the relationship between tree architecture, here addressed using the box-dimension (Db), and the architectural benefit-to-cost ratio, the light availability, and the growth of trees. We detected a clear relationship between Db and the benefit-to-cost ratio for the tested three temperate forest tree species (Fagus sylvatica L., Fraxinus excelsior L., and Acer pseudoplatanus L.). In addition, we could also show that Db is positively related to the growth performance of several tropical tree species. Finally, we observed a negative relationship between the strength of competition enforced on red oak (Quercus rubra L.) trees and their Db. We therefore argue that Db is a meaningful and integrative measure that describes the structural complexity of the aboveground compartments of a plant as well as its relation to structural efficiency (benefit-to-cost ratio), productivity, and growing conditions (competition or availability of light).
  • Publication
    Accès libre
    Tree performance in a biodiversity enrichment experiment in an oil palm landscape
    (2019)
    Zemp, Clara 
    ;
    Anne Gérard
    ;
    Dirk Hölscher
    ;
    Christian Ammer
    ;
    Bambang Irawan
    ;
    Leti Sundawati
    ;
    Miriam Teuscher
    ;
    Holger Kreft
    ;
    Marney Isaac
    1. Large‐scale conversion of tropical forests into oil palm monocultures has led to dramatic losses of biodiversity and ecosystem functions. While ecological restoration is urgently needed in many oil palm landscapes, there is a lack of scientific knowledge of sustainable management strategies. 2. We established experimental tree islands of varying sizes (25 m2 to 1,600 m2) and diversity levels (1, 2, 3 and 6 species) in an oil palm plantation in Sumatra, Indonesia. Six native multi‐purpose tree species including Archidendron pauciflorum (Jengkol), Durio zibethinus (Durian), Parkia speciosa (Petai), Shorea leprosula (Meranti), Peronema canescens (Sungkai) and Dyera polyphylla (Jelutung) were planted between living and felled oil palms. Here, we analyse the controlling factors of tree growth and survival during the first 4 years at the level of local neighbourhood and tree island. 3. We found a significant effect of diversity levels on tree productivity, that is, basal area was higher in multi‐species than in single‐species tree islands. This overyielding was attributed to enhanced tree growth, while mortality had no effect. In the local neighbourhood, tree species richness had a positive effect on tree growth during the first year only, indicating that selection and dominance of well‐performing species at high level of diversity are most likely driving overyielding. 4. Trees grew better away from living oil palms, suggesting tree‐palm competition. Proximity to felled oil palms increased growth especially during the first years, during which the planted trees might have benefited from the additional available space and resources. Despite positive edge effects from the conventional oil palm management in the surrounding, tree island size had an overall positive effect on tree growth. 5. Synthesis and applications. We planted native trees in an oil palm landscape following a tree island pattern. The establishment success differed widely among the six planted species. The selection of particular species is a decisive factor to foster a positive relationship between diversity and tree growth. Planting larger tree islands (e.g. 1,600 square metres) is a better option to enhance tree growth, but tree‐palm competition implies a trade‐off between tree growth and palm oil production in the tree islands.