Alexander Röll

Alexander Röll
Georg-August-Universität Göttingen | GAUG · Tropical Silviculture and Forest Ecology

PhD

About

43
Publications
22,373
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734
Citations
Citations since 2017
35 Research Items
709 Citations
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Introduction
Alex Röll studied Forest Sciences and Forest Ecology with a focus on tropical forests. For his PhD, he focused on the ecohydrological consequences of rainforest transformation to rubber and oil palm plantations in Indonesia. As a PostDoc, he focuses on ground-validated remote sensing applications from drones and satellites to study ecosystem structures, dynamics and key exchange processes such as evapotranspiration across spatial scales.
Education
April 2012 - October 2015
Georg-August-Universität Göttingen
Field of study
  • Forest Ecology, Ecohydrology
October 2007 - October 2010
Georg-August-Universität Göttingen
Field of study
  • Tropical and International Forestry
October 2004 - October 2007
Georg-August-Universität Göttingen
Field of study
  • Forest Science and Forest Ecology

Publications

Publications (43)
Article
Farmland tree cultivation is considered an important option for enhancing wood production. In South India, the native leaf-deciduous tree species Melia dubia is popular for short-rotation plantations. Across a rainfall gradient from 420 to 2170 mm year-1 , we studied 186 farmland wood-lots between one and nine years in age. The objectives were to i...
Article
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 an...
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Plant transpiration is a key element in the hydrological cycle. Widely used methods for its assessment comprise sap flux techniques for whole-plant transpiration and porometry for leaf stomatal conductance. Recently emerging approaches based on surface temperatures and a wide range of machine learning techniques offer new possibilities to quantify...
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In India, short-rotation plantations have increasingly been promoted over the last decades in response to altered forest management policy and increased wood demand. Understanding how short-rotation forest species coordinate fast growth with water demand and carbon investment will help improving management decisions. We investigated the intra-speci...
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Rubber plantations are an economically viable land-use type that occupies large swathes of land in Southeast Asia that have undergone conversion from native forest to intensive plantation forestry. Such land-use change has a strong impact on carbon, energy, and water fluxes in ecosystems, and uncertainties exist in the modeling of future land-use c...
Preprint
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1In India, short-rotation plantations have increasingly been promoted over the last decades in 2 response to altered forest management policy and increased wood demand. Understanding how 3 short-rotation forest species coordinate fast growth with water demand and carbon investment 4 will help improving management decisions. 5
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Microclimate and Land Surface Temperature (LST) are important analytical variables used to understand complex oil palm agroforestry systems and their effects on biodiversity and ecosystem functions. In order to examine 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 co...
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Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological, energy, and carbon budgets at the land–atmosphere interface. However, despite being the main land evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observatio...
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For the assessment of evapotranspiration, near-surface airborne thermography offers new opportunities for studies with high numbers of spatial replicates and in a fine spatial resolution. We tested drone-based thermography and the subsequent application of the DATTUTDUT energy balance model using the widely accepted eddy covariance technique as a r...
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The efficiency of the water transport system in trees sets physical limits to their productivity and water use. Although the coordination of carbon assimilation and hydraulic functions has long been documented, the mutual inter-relationships between wood anatomy, water use and productivity have not yet been jointly addressed in comprehensive field...
Article
Full-text available
Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological,energy and carbon budgets at the land-atmosphere interface. However, despite being the main land evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observations...
Article
Full-text available
The rapid expansion of rubber and oil palm plantations in Jambi Province, Sumatra, Indonesia, is associated with large-scale deforestation and the impairment of many ecosystem services. According to villagers' observations, this land use change has, together with climate change, led to an increase in the magnitude and frequency of river flood event...
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Moso bamboo is widespread in natural forests and is cultivated over large areas in China. This study investigated how climate controls its distribution, about which little is known. We collected moso bamboo presence-absence data from 674 sites with long-term climate data in Mainland China. Generalized additive models that included location and clim...
Preprint
Full-text available
Abstract. For the assessment of evapotranspiration, near-surface airborne thermography offers new opportunities for studies with high numbers of spatial replicates and in a fine spatial resolution. We tested drone-based thermography and the subsequent application of three energy balance models (DATTUTDUT, TSEB-PT, DTD) using the widely accepted edd...
Article
Evapotranspiration (ET) is a central flux in the hydrological cycle. Various approaches to compute ET via energy balance models exist, but their handling is often complex and challenging. We developed QWaterModel as an easy-to-use tool to make ET predictions available to broader audiences. QWaterModel is based on the DATTUTDUT energy balance model...
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Land-use transitions can enhance the livelihoods of smallholder farmers but potential economic-ecological trade-offs remain poorly understood. Here, we present an interdisciplinary study of the environmental, social and economic consequences of land-use transitions in a tropical smallholder landscape on Sumatra, Indonesia. We find widespread biodiv...
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The potential of palm-oil biofuels to reduce greenhouse gas (GHG) emissions compared with fossil fuels is increasingly questioned. So far, no measurement-based GHG budgets were available, and plantation age was ignored in Life Cycle Analyses (LCA). Here, we conduct LCA based on measured CO2, CH4 and N2O fluxes in young and mature Indonesian oil pal...
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Tropical rainforests comprise complex 3D structures and encompass heterogeneous site conditions; their transpiration contributes to climate regulation. The objectives of our study were to test the relationship between tree water use and crown metrics and to predict spatial variability of canopy transpiration across sites. In a lowland rainforest of...
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Complementary use of resources is considered a strong driver of enhanced performance in mixed-species assemblages. In income-producing agroforestry systems, economically valuable species will ideally benefit from resource partitioning. In agroforests in southern India, we assessed soil water uptake depths of coffee and different shade tree species...
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Bamboo water transport comprises the pathway rhizomes-culms-leaves as well as transfer among culms via connected rhizomes. We assessed bamboo water transport in three big clumpy bamboo species by deuterium tracing. The tracer was injected into the base of established culms, and water samples were collected from leaves of the labeled culms and from...
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Bamboo culms are connected to neighboring culms via rhizomes, which enable resource exchange between culms. We assessed water transfer between established and neighboring, freshly sprouted culms by thermal dissipation probes (TDP) inserted into culms and the connecting rhizome. During the early phase of sprouting, highest sap flux densities in fres...
Article
Transpiration at the stand level is often estimated from water use measurements on a limited number of plants and then scaled up by predicting the remaining plants of a stand by plant size related variables. Today, drone‐based methods offer new opportunities for plant size assessments. We tested crown variables derived from drone‐based photogrammet...
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By mediating evapotranspiration processes, plant canopies play an important role in the terrestrial water cycle and regional climate. Substantial uncertainties exist in modeling canopy water interception and related hydrological processes due to rainfall forcing frequency selection and varying canopy traits. Here we design a new time interpolation...
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Land-use change has a strong impact on carbon, energy and water fluxes and its effect is particularly pronounced in tropical regions. Uncertainties exist in the prediction of future land-use change impacts on these fluxes by land surface models due to scarcity of suitable measured data for parametrization and poor representation of key biogeochemic...
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Key message Water content fluctuations in bamboo culms significantly influence sap flux measurements with thermal dissipation probes, as indicated and quantified by experimental, monitoring and model analyses. Abstract Bamboos and other plants may substantially rely on stem water storage for transpiration. Fluctuations in wood water content (θwood...
Article
Rubber tree mono-cultural plantations are expanding. Also, there is an increasing search for 'green' rubber production. Rubber tree cultivation in stands with admixed, spontaneously established native trees, referred to as jungle rubber, has a long tradition on Sumatra. For rubber tree monocultures on mainland Asia, concerns have been raised becaus...
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The expansion of rubber cultivation in Southeast Asia raises concerns about the integrity of the hydrological cycle. From mainland Asia, high evapotranspiration from rubber plantations was reported. Our study was conducted in the Sumatran lowlands (Indonesia), where rubber is grown by small-holders under maritime climate. We assessed patterns of wa...
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Oil palm is rapidly expanding, particularly in Indonesia, but there is still very limited information on water and energy fluxes in oil palm plantations, and on how those are affected by varying environmental conditions or plantation age. In our study, we measured turbulent fluxes of sensible (H) and latent (LE) heat and gross primary productivity...
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Oil palm and rubber plantations extend over large areas and encompass heterogeneous site conditions. In periods of high rainfall, plants in valleys and at riparian sites are more prone to flooding than plants at elevated topographic positions. We asked to what extent topographic position and flooding affect oil palm and rubber tree water use patter...
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Oil palm cultivation is expanding rapidly across the globe. Current discussions on ecological impacts focus on the destruction of forests, greenhouse gas emissions and biodiversity loss. Little attention has been paid to concerns that oil palm expansion has severe impacts on local water resources.
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Ölpalmplantagen nehmen weltweit zu. In aktuellen Diskussionen über deren Umweltauswirkungen stehen die Abholzung von Regenwäldern, Treibhausgasemissionen sowie Biodiversitätsverlust im Vordergrund. Wenig Aufmerksamkeit wurde bisher den Auswirkungen auf lokale Wasserkreisläufe gewidmet.
Article
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Conversions of natural ecosystems, e.g., from rain forests to managed plantations, result in significant changes in the hydrological cycle including periodic water scarcity. In Indonesia, large areas of forest were lost and extensive oil palm plantations were established over the last decades. We conducted a combined social and environmental study...
Article
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Bamboos are grasses (Poaceae) that are widespread in tropical and subtropical regions. We aimed at exploring water use patterns of four tropical bamboo species (Bambusa vulgaris, Dendrocalamus asper, Gigantochloa atroviolacea, and G. apus) with sap flux measurement techniques. Our approach included three experimental steps: (1) a pot experiment wit...
Article
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Oil palm (Elaeis guineensis Jacq.) plantations cover large and continuously increasing areas of humid tropical lowlands. Landscapes dominated by oil palms usually consist of a mosaic of mono-cultural, homogeneous stands of varying age, which may be heterogeneous in their water use characteristics. However, studies on the water use characteristics o...
Article
Full-text available
Oil palm (Elaeis guineensis Jacq.) plantations cover large and continuously increasing areas of humid tropical lowlands. Landscapes dominated by oil palms usually consist of a mosaic of mono-cultural, homogeneous stands of varying age, which may be heterogeneous in their water use characteristics. However, studies on the water use characteristics o...
Article
Full-text available
Oil palm (Elaeis guineensis Jacq.) water use was assessed by sap flux density measurements with the aim to establish the method and derive water-use characteristics. Thermal dissipation probes were inserted into leaf petioles of mature oil palms. In the laboratory, we tested our set-up against gravimetric measurements and derived new parameters for...
Chapter
Full-text available
Auf Basis einer flächendeckenden permanenten Stichprobeninventur wurde 1996 eine Erstaufnahme von Gehölzbestand und Vegetation in der Naturwaldzelle Hellerberg durchgeführt. Nach 14 Jahren erfolgte eine Wiederholungsinventur. Wesentliche Veränderungen werden vorgestellt.

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Projects

Projects (4)
Project
Across steep gradients in precipitation and different land-use types such as forests and plantations, key ecosystem exchange processes related to carbon and water are being investigated. This includes the PhD projects 'Water use and water stress of Melia dubia plantations across a rainfall gradient' (finished in Sept 2022) and 'Forest structure and evapotranspiration in the Western Ghats: effects of climate and land use' (ongoing).
Project
current project stage: awaiting final project approval after budget cuts expected start of project: March 2023 (3 year duration) Abstract: Because of the progressing climate change, central European forests are increasingly prone to water and heat stress. Tree and ecosystem vitality and ecophysiological relationships are assessed on monitoring plots and in current projects. Modern remote sensing methods for measuring surface temperatures via drones and satellites offer opportunities for integrating such studies and for further advancing forest monitoring. They can assess the spatial patterns and temporal dynamics of water and heat stress. In the proposed project, temp-2-stress, we will assess surface temperatures of forest at the tree, stand and landscape scale. The assessment will encompass mixed broadleaf forest stands on level-II monitoring plots along a north-south gradient in Lower Saxony and Hesse, as well as complementary projects funded by the ‘Waldklimafonds’ (SiZuRi, F3). These sites will be analyzed with a drone and a satellite method for a large-scale analysis of water and heat stress at high spatial resolution, including sites with different tree species, soil characteristics and management. The workflow from raw data to stress index maps will be implemented in user-friendly and automized workflows basing on open-source solutions. The methods of both platforms largely base on thermal imaging. Regarding the drone method, our working group has broad experience in terms of data acquisition and water flux modeling, which was implemented in a much-downloaded QGIS plugin (QWaterModel). One of our current foci is a further QGIS plugin allowing the calculation of tree-level stress indices. For satellite analysis we postulate ECOSTRESS, which is implemented on the ISS and is freely available. Exploratory analyses for the Göttinger Wald show promising results. The applied methods are similar and complement each other regarding their spatial resolution (drone 10 cm, satellite 70 m). The overall goal of temp-2-stress is to test and further develop these integrating and complementary methods for forests in Lower Saxony and Hesse, and to make them available to potential users. We expect a substantial contribution for rapid monitoring of water and heat stress of forests on different spatial scales, which in association with our project partners will allow for an enhanced ecosystemic understanding and future silvicultural management.
Project
Eco-hydrological sub-project of the large, interdisciplinary EFForTS project (CRC 990) as a cooperation of Göttingen University and Indonesian partner universities in Jambi, Bogor and Tadukalo. PI: Prof. Dr. Dirk Hölscher Goals: 1. Develop an appropriate sap flux measurement scheme for rainforests, rubber plantations and particularly oil palm plantations. 2. Derive and compare water use characteristics in different land-use types (rainforest, 'jungle rubber' agroforest, rubber and oil palm monocultures). 3. Assess the consequences of rainforest transformation on landscape hydrological functioning. 4. Scale the results to landscape and regional scales via remote sensing