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  • Publication
    Accès libre
    Enhancing tree performance through species mixing: Review of a quarter-century of TreeDivNet experiments reveals research gaps and practical insights
    (2024)
    Leen Depauw
    ;
    Emiel De Lombaerde
    ;
    Els Dhiedt
    ;
    Haben Blondeel
    ;
    Luis Abdala-Roberts
    ;
    Harald Auge
    ;
    Nadia Barsoum
    ;
    Jürgen Bauhus
    ;
    Chengjin Chu
    ;
    Abebe Damtew
    ;
    Nico Eisenhauer
    ;
    Marina V. Fagundes
    ;
    Gislene Ganade
    ;
    Benoit Gendreau-Berthiaume
    ;
    Douglas Godbold
    ;
    Dominique Gravel
    ;
    Joannès Guillemot
    ;
    Peter Hajek
    ;
    Andrew Hector
    ;
    Bruno Hérault
    ;
    Hervé Jactel
    ;
    Julia Koricheva
    ;
    Holger Kreft
    ;
    Xiaojuan Liu
    ;
    Simone Mereu
    ;
    Christian Messier
    ;
    Bart Muys
    ;
    Charles A. Nock
    ;
    Alain Paquette
    ;
    John D. Parker
    ;
    William C. Parker
    ;
    Gustavo B. Paterno
    ;
    Michael P. Perring
    ;
    Quentin Ponette
    ;
    Catherine Potvin
    ;
    Peter B. Reich
    ;
    Boris Rewald
    ;
    Michael Scherer-Lorenzen
    ;
    Florian Schnabel
    ;
    Rita Sousa-Silva
    ;
    Martin Weih
    ;
    Zemp, Clara 
    ;
    Kris Verheyen
    ;
    Lander Baeten
    Purpose of Review International ambitions for massive afforestation and restoration are high. To make these investments sustainable and resilient under future climate change, science is calling for a shift from planting monocultures to mixed forests. But what is the scientific basis for promoting diverse plantations, and what is the feasibility of their establishment and management? As the largest global network of tree diversity experiments, TreeDivNet is uniquely positioned to answer these pressing questions. Building on 428 peer-reviewed TreeDivNet studies, combined with the results of a questionnaire completed by managers of 32 TreeDivNet sites, we aimed to answer the following questions: (i) How and where have TreeDivNet experiments enabled the relationship between tree diversity and tree performance (including productivity, survival, and pathogen damage) to be studied, and what has been learned? (ii) What are the remaining key knowledge gaps in our understanding of the relationship between tree diversity and tree performance? and (iii) What practical insights can be gained from the TreeDivNet experiments for operational, real-world forest plantations? Recent Findings We developed a conceptual framework that identifies the variety of pathways through which target tree performance is related to local neighbourhood diversity and mapped the research efforts for each of those pathways. Experimental research on forest mixtures has focused primarily on direct tree diversity effects on productivity, with generally positive effects of species and functional diversity on productivity. Fewer studies focused on indirect effects mediated via biotic growing conditions (e.g. soil microbes and herbivores) and resource availability and uptake. Most studies examining light uptake found positive effects of species diversity. For pests and diseases, the evidence points mostly towards lower levels of infection for target trees when growing in mixed plantations. Tree diversity effects on the abiotic growing conditions (e.g. microclimate, soil properties) and resource-use efficiency have been less well studied to date. The majority of tree diversity experiments are situated in temperate forests, while (sub)tropical forests, and boreal forests in particular, remain underrepresented. Summary TreeDivNet provides evidence in favour of mixing tree species to increase tree productivity while identifying a variety of different processes that drive these diversity effects. The design, scale, age, and management of TreeDivNet experiments reflect their focus on fundamental research questions pertaining to tree diversity-ecosystem function relationships and this scientific focus complicates translation of findings into direct practical management guidelines. Future research could focus on (i) filling the knowledge gaps related to underlying processes of tree diversity effects to better design plantation schemes, (ii) identifying optimal species mixtures, and (iii) developing practical approaches to make experimental mixed plantings more management oriented.
  • Publication
    Accès libre
    Combining planting trees and natural regeneration promotes long-term structural complexity in oil palm landscapes
    (2024)
    Tatsuro Kikuchi
    ;
    Dominik Seidel
    ;
    Martin Ehbrecht
    ;
    Zemp, Clara 
    ;
    Fabian Brambach
    ;
    Bambang Irawan
    ;
    Leti Sundawati
    ;
    Dirk Hölscher
    ;
    Holger Kreft
    ;
    Gustavo Brant Paterno
    Vegetation structural complexity has been identified as a vital factor for forest ecosystem function, stability, and resilience. However, agricultural land with much reduced structural complexity has largely replaced natural forests in the tropics. Therefore, restoring structural complexity in large-scale plantation monocultures by introducing agroforestry systems may counteract the loss of biodiversity and ecosystem functions. However, we still have limited knowledge of how the structural complexity of agroforests develops under different restoration treatments. We established a large-scale biodiversity enrichment experiment in a conventional monoculture oil palm plantation in Sumatra, Indonesia. In this experiment, agroforests were implemented by planting clusters of native trees (“tree islands”) within the oil palm plantation, systematically varying initially planted tree richness ranging from 0–6 (0 corresponding to natural regeneration only) and plot area (25–1600 m2). We tested the effect of the experimental treatments on nine years of the development of local structural complexity using a stand structural complexity index (SSCI) derived from terrestrial laser scanning. We found that tree planting and natural regeneration treatments promoted structural complexity by creating denser and more complex vegetation structures. Plots with a tree planting treatment tended to show greater structural complexity than plots with natural regeneration only. However, during the study period, oil palms still dominated heights, and the temporal change in structural complexity among plots with or without a tree planting treatment did not differ. As for plot area, our results indicate that structural complexity looking from the center of plots did not necessarily depend on the area during the study period, and even small tree islands can increase local structural complexity in a monoculture oil palm plantation. Initially planted tree richness did not significantly affect the development of structural complexity. Nine years after establishment, not planted trees but regenerated woody plants strongly positively affected vegetation density and structural complexity. Our findings highlight that sustaining vegetation density below oil palm canopies is a key strategy to increase the structural complexity of oil palm landscapes.
  • Publication
    Accès libre
    Comparing airborne and terrestrial LiDAR with ground-based inventory metrics of vegetation structural complexity in oil palm agroforests
    (2024)
    Vannesa Montoya-Sánchez
    ;
    Nicolò Camarretta
    ;
    Martin Ehbrecht
    ;
    Michael Schlund
    ;
    Gustavo Brant Paterno
    ;
    Dominik Seidel
    ;
    Nathaly Guerrero-Ramírez
    ;
    Fabian Brambach
    ;
    Dirk Hölscher
    ;
    Holger Kreft
    ;
    Bambang Irawan
    ;
    Leti Sundawati
    ;
    Zemp, Clara 
    Vegetation structural complexity is an important component of forest ecosystems, influencing biodiversity and functioning. Due to the heterogeneous distribution of vegetation elements, structural complexity underpins ecological dynamics, species composition, microclimate, and habitat diversity. Field measurements and Light Detection and Ranging (LiDAR) data, such as airborne (ALS) and terrestrial (TLS), can assess structural characteristics of forest and agroforestry systems at various spatial scales. This assessment is urgently needed for monitoring ecosystem restoration in degraded lands (e.g., in oil palm landscapes), where it is not well-known how structural measures derived from these different approaches relate to each other. Here, we compared the degree of correlation between individual and multivariate datasets of vegetation structural complexity metrics derived from ALS, TLS, and ground-based inventory approaches. The study was conducted in a 140 ha oil palm monoculture, enriched with 52 plots in the form of tree islands representing agroforestry systems of varying sizes and planted diversity levels in Sumatra, Indonesia. Our datasets comprised 25 ALS, five TLS, and nine ground-based inventory metrics. We studied correlations among metrics related to traditional stand summary, heterogeneity, and vertical and horizontal stand structure. We used principal component analysis for data dimensionality reduction, correlation analysis to quantify the strength of relationships between metrics, and Procrustes analysis to investigate the agreement between datasets. Significant correlations were found between ALS and TLS metrics for canopy density (r = 0.79) and maximum tree height (r = 0.58) and between ALS and ground-based inventory measures of stand heterogeneity and height diversity (r between 0.60 and −0.63). Further, we observed significant agreements between the ordinations of multivariate datasets (r = 0.56 for ALS − TLS; and r = 0.46 for ALS – ground-based inventory). Our findings underline the ability of ALS to capture structural complexity patterns, especially for canopy gap dynamics and vegetation height metrics, as captured by TLS, and for measures of heterogeneity and vertical structure as captured by ground-based inventories. Our study highlights the strength of each approach and underscores the potential of integrating ALS and TLS with ground-based inventories for a comprehensive characterization of vegetation structure in complex agroforestry systems, which can provide guidance for their management and support ecosystem restoration monitoring efforts.
  • Publication
    Accès libre
    Tree identity and canopy openness mediate oil palm biodiversity enrichment effects on insect herbivory and pollination
    (2023)
    Kevin Li
    ;
    Ingo Grass
    ;
    Zemp, Clara 
    ;
    Hendrik Lorenz
    ;
    Lena Sachsenmaier
    ;
    Fuad Nurdiansyah
    ;
    Dirk Hölscher
    ;
    Holger Kreft
    ;
    Teja Tscharntke
    As the extent of oil palm (Elaeis guineensis) cultivation has expanded at the expense of tropical rainforests, enriching conventional large‐scale oil palm plantations with native trees has been proposed as a strategy for restoring biodiversity and ecosystem function. However, how tree enrichment affects insect‐mediated ecosystem functions is unknown. We investigated impacts on insect herbivory and pollination in the fourth year of a plantation‐scale, long‐term oil palm biodiversity enrichment experiment in Jambi, Sumatra, Indonesia. Within 48 plots systematically varying in size (25–1600 m2) and planted tree species richness (one to six species), we collected response data on vegetation structure, understory insect abundances, and pollinator and herbivore activity on chili plants (Capsicum annuum), which served as indicators of insect‐mediated ecosystem functions. We examined the independent effects of plot size, tree species richness, and tree identity on these response variables, using the linear model for random partitions design. The experimental treatments were most associated with vegetation structure: tree identity mattered, as the species Peronema canescens strongly decreased (by approximately one standard deviation) both canopy openness and understory vegetation cover; whereas tree richness only decreased understory flower density. Further, the smallest plots had the lowest understory flower density and richness, presumably because of lower light availability and colonization rates, respectively. Enrichment influenced herbivorous insects and natural enemies in the understory to a lesser extent: both groups had higher abundances in plots with two enrichment species planted, possibly because higher associated tree mortality created more habitat, while herbivores decreased with increasing tree species richness, in line with the resource concentration hypothesis. Linking relationships in structural equation models showed that the negative association between P. canescens and understory vegetation cover was mediated through canopy openness. Likewise, canopy openness mediated increases in herbivore and pollinator insect abundances. Higher pollinator visitation increased phytometer yield, while impacts of insect herbivores on yield were not apparent. Our results demonstrate that even at an early stage, different levels of ecological restoration influence insect‐mediated ecosystem functions, mainly through canopy openness. These findings suggest that maintaining some canopy gaps while enrichment plots develop may be beneficial for increasing habitat heterogeneity and insect‐mediated ecosystem functions.
  • Publication
    Accès libre
    Landscape heterogeneity and soil biota are central to multi-taxa diversity for oil palm landscape restoration
    (2023)
    Vannesa Montoya-Sánchez
    ;
    Holger Kreft
    ;
    Isabelle Arimond
    ;
    Johannes Ballauff
    ;
    Dirk Berkelmann
    ;
    Fabian Brambach
    ;
    Rolf Daniel
    ;
    Ingo Grass
    ;
    Jes Hines
    ;
    Dirk Hölscher
    ;
    Bambang Irawan
    ;
    Alena Krause
    ;
    Andrea Polle
    ;
    Anton Potapov
    ;
    Lena Sachsenmaier
    ;
    Stefan Scheu
    ;
    Leti Sundawati
    ;
    Teja Tscharntke
    ;
    Zemp, Clara 
    ;
    Nathaly Guerrero-Ramírez
    Enhancing biodiversity in monoculture-dominated landscapes is a key sustainability challenge that requires considering the spatial organization of ecological communities (beta diversity). Here, we tested whether increasing landscape heterogeneity, through establishing 52 tree islands in an oil-palm landscape, is a suitable restoration strategy to enhance the diversity of six taxa (multi-taxa diversity). Further, we elucidated whether patterns in the spatial distribution of above- and below-ground taxa are related, and their role in shaping multi-taxa beta diversity. After five years, islands enhanced diversity at the landscape scale by fostering unique species (turnover). Partial correlation networks revealed that dissimilarity, in vegetation structural complexity and soil conditions, impacts multi-taxa beta diversity and turnover. In addition, soil fauna, bacteria, and fungi were more strongly associated with the overall community than aboveground taxa. Thus, strategies aiming to enhance multi-taxa diversity should consider the central role of landscape heterogeneity and soil biota.
  • Publication
    Accès libre
    Tree islands enhance biodiversity and functioning in oil palm landscapes
    (2023)
    Zemp, Clara 
    ;
    Nathaly Guerrero-Ramirez
    ;
    Fabian Brambach
    ;
    Kevin Darras
    ;
    Ingo Grass
    ;
    Anton Potapov
    ;
    Alexander Röll
    ;
    Isabelle Arimond
    ;
    Johannes Ballauff
    ;
    Hermann Behling
    ;
    Dirk Berkelmann
    ;
    Siria Biagioni
    ;
    Damayanti Buchori
    ;
    Dylan Craven
    ;
    Rolf Daniel
    ;
    Oliver Gailing
    ;
    Florian Ellsäßer
    ;
    Riko Fardiansah
    ;
    Nina Hennings
    ;
    Bambang Irawan
    ;
    Watit Khokthong
    ;
    Valentyna Krashevska
    ;
    Alena Krause
    ;
    Johanna Kückes
    ;
    Kevin Li
    ;
    Hendrik Lorenz
    ;
    Mark Maraun
    ;
    Miryam Sarah Merk
    ;
    Carina C. M. Moura
    ;
    Yeni A. Mulyani
    ;
    Gustavo B. Paterno
    ;
    Herni Dwinta Pebrianti
    ;
    Andrea Polle
    ;
    Di Ajeng Prameswari
    ;
    Lena Sachsenmaier
    ;
    Stefan Scheu
    ;
    Dominik Schneider
    ;
    Fitta Setiajiati
    ;
    Christina Ani Setyaningsih
    ;
    Leti Sundawati
    ;
    Teja Tscharntke
    ;
    Meike Wollni
    ;
    Dirk Hölscher
    ;
    Holger Kreft
    In the United Nations Decade on Ecosystem Restoration1, large knowledge gaps persist on how to increase biodiversity and ecosystem functioning in cash crop-dominated tropical landscapes2. Here, we present findings from a large-scale, 5-year ecosystem restoration experiment in an oil palm landscape enriched with 52 tree islands, encompassing assessments of ten indicators of biodiversity and 19 indicators of ecosystem functioning. Overall, indicators of biodiversity and ecosystem functioning, as well as multidiversity and ecosystem multifunctionality, were higher in tree islands compared to conventionally managed oil palm. Larger tree islands led to larger gains in multidiversity through changes in vegetation structure. Furthermore, tree enrichment did not decrease landscape-scale oil palm yield. Our results demonstrate that enriching oil palm-dominated landscapes with tree islands is a promising ecological restoration strategy, yet should not replace the protection of remaining forests.
  • Publication
    Accès libre
    Nine actions to successfully restore tropical agroecosystems
    (2022)
    Michael David Pashkevich
    ;
    Francisco d’Albertas
    ;
    Anak Agung Ketut Aryawan
    ;
    Damayanti Buchori
    ;
    Jean-Pierre Caliman
    ;
    Adrian David González Chaves
    ;
    Purnama Hidayat
    ;
    Holger Kreft
    ;
    Mohammad Naim
    ;
    Appolinaire Razafimahatratra
    ;
    Edgar Clive Turner
    ;
    Zemp, Clara 
    ;
    Sarah Helen Luke
    Well-designed approaches to ecological restoration can benefit nature and society. This is particularly the case in tropical agroecosystems, where restoration can provide substantial socioecological benefits at relatively low costs. To successfully restore tropical agroecosystems and maximise benefits, initiatives must begin by considering ‘who’ should be involved in and benefit from restoration, and ‘what’, ‘where’, and‘how’ restoration should occur. Based on collective experience of restoring tropical agroecosystems worldwide,we present nine actions to guide future restoration of these systems, supported by case studies that demonstrate our actions being used successfully in practice and highlighting cases where poorly designed restoration has been damaging. We call for increased restoration activity in tropical agroecosystems during the current UN Decade on Ecosystem Restoration.
  • Publication
    Accès libre
    Land-use trajectories for sustainable land system transformations: Identifying leverage points in a global biodiversity hotspot
    (2022)
    Dominic Andreas Martin
    ;
    Fanilo Andrianisaina
    ;
    Thio Rosin Fulgence
    ;
    Kristina Osen
    ;
    Anjaharinony Andry Ny Aina Rakotomalala
    ;
    Estelle Raveloaritiana
    ;
    Marie Rolande Soazafy
    ;
    Annemarie Wurz
    ;
    Rouvah Andriafanomezantsoa
    ;
    Harilala Andriamaniraka
    ;
    Aristide Andrianarimisa
    ;
    Jan Barkmann
    ;
    Saskia Dröge
    ;
    Ingo Grass
    ;
    Nathaly Guerrero-Ramirez
    ;
    Hendrik Hänke
    ;
    Dirk Hölscher
    ;
    Bakolimalala Rakouth
    ;
    Hery Lisy Tiana Ranarijaona
    ;
    Romual Randriamanantena
    ;
    Fanomezana Mihaja Ratsoavina
    ;
    Lala Harivelo Raveloson Ravaomanarivo
    ;
    Dominik Schwab
    ;
    Teja Tscharntke
    ;
    Zemp, Clara 
    ;
    Holger Kreft
    Finding entry points where policy has strong leverage to transform land systems for people and nature is pivotal. We develop an innovative framework to identify and evaluate such leverage points along land-use trajectories that account for path dependency. Applied to the biodiversity hotspot Madagascar, the framework reveals three leverage points: Two leverage points are associated with trade-offs between biodiversity, ecosystem services, and agricultural productivity, while the third entails cobenefits. Swift policy action is required, as path dependency caused by forest loss may soon put two leverage points out of reach. We argue that such closing windows of opportunity may be common, but often overlooked, calling for a wider consideration of path dependency in land-system science.
  • Publication
    Accès libre
    Microclimate and land surface temperature in a biodiversity enriched oil palm plantation
    (2021)
    Laura Somenguem Donfack
    ;
    Alexander Röll
    ;
    Florian Ellsäßer
    ;
    Martin Ehbrecht
    ;
    Bambang Irawan
    ;
    Dirk Hölscher
    ;
    Alexander Knohl
    ;
    Holger Kreft
    ;
    Eduard J. Siahaan
    ;
    Leti Sundawati
    ;
    Christian Stiegler
    ;
    Zemp, Clara 
    Agroforestry options such as mixed-species tree planting and natural regeneration in oil palm plantations may alleviate negative effects of forest loss on biodiversity and ecosystem functioning. The effects of agroforestry on microclimate and land surface temperatures (LST) remain largely unknown despite their central role in controlling abiotic and biotic factors and in buffering climate at a larger scale. We assessed spatial and temporal microclimate and LST variability in a biodiversity enrichment experiment, in which tree islands have been planted in an oil palm plantation in Sumatra (Indonesia). Four years after establishment of the experiment, we measured microclimate and LST using mini microclimate sensors and drone-recorded thermal images. We examined experimental effects of tree species richness (0, 1, 2, 3 or 6), plot size (25 m2, 100 m2, 400 m2, 1600 m2) and stand structural complexity on microclimate and LST. Diurnal patterns showed ambient air temperature peaks and relative humidity (RH) minima at 3 pm, whereas diurnal soil temperatures peaked around 6 pm. The lowest LST were observed from oil palm canopy leaves and the highest from bare soils and understorey vegetation (including trees). Spatial and temporal ranges of ambient air temperature were smaller than LST ranges, and average ambient air temperature and LST were positively correlated. Tree species diversity had no overall significant effect neither on microclimate nor LST, but humidity was higher in planted tree islands compared to natural regeneration only. Smaller plots were characterized by higher mean air, soil and LST, compared to larger plots. Structurally complex plots were associated with low mean and maximum values of ambient air temperature, soil temperature and LST and high mean and minimum RH. Still, conditions were hotter and drier in several experimental plots compared to conventional oil palm plantations, considering a higher transpiration in the latest. We conclude that stand structural complexity and tree island size control microclimate and LST in the experimental oil palm agroforests, but alleviating the harsh microclimate conditions in oil palm plantations might take longer to occur.
  • 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.