TY - JOUR A1 - Atmani, Farid A1 - Bookhagen, Bodo A1 - Smith, Taylor T1 - Measuring vegetation heights and their seasonal changes in the Western Namibian Savanna using spaceborne lidars JF - Remote sensing / Molecular Diversity Preservation International (MDPI) N2 - The Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) with its land and vegetation height data product (ATL08), and Global Ecosystem Dynamics Investigation (GEDI) with its terrain elevation and height metrics data product (GEDI Level 2A) missions have great potential to globally map ground and canopy heights. Canopy height is a key factor in estimating above-ground biomass and its seasonal changes; these satellite missions can also improve estimated above-ground carbon stocks. This study presents a novel Sparse Vegetation Detection Algorithm (SVDA) which uses ICESat-2 (ATL03, geolocated photons) data to map tree and vegetation heights in a sparsely vegetated savanna ecosystem. The SVDA consists of three main steps: First, noise photons are filtered using the signal confidence flag from ATL03 data and local point statistics. Second, we classify ground photons based on photon height percentiles. Third, tree and grass photons are classified based on the number of neighbors. We validated tree heights with field measurements (n = 55), finding a root-mean-square error (RMSE) of 1.82 m using SVDA, GEDI Level 2A (Geolocated Elevation and Height Metrics product): 1.33 m, and ATL08: 5.59 m. Our results indicate that the SVDA is effective in identifying canopy photons in savanna ecosystems, where ATL08 performs poorly. We further identify seasonal vegetation height changes with an emphasis on vegetation below 3 m; widespread height changes in this class from two wet-dry cycles show maximum seasonal changes of 1 m, possibly related to seasonal grass-height differences. Our study shows the difficulties of vegetation measurements in savanna ecosystems but provides the first estimates of seasonal biomass changes. KW - ICESat-2 KW - GEDI KW - canopy height KW - lidar KW - savanna Y1 - 2022 U6 - https://doi.org/10.3390/rs14122928 SN - 2072-4292 VL - 14 IS - 12 SP - 1 EP - 20 PB - MDPI CY - Basel, Schweiz ET - 12 ER - TY - JOUR A1 - Irob, Katja A1 - Blaum, Niels A1 - Baldauf, Selina A1 - Kerger, Leon A1 - Strohbach, Ben A1 - Kanduvarisa, Angelina A1 - Lohmann, Dirk A1 - Tietjen, Britta T1 - Browsing herbivores improve the state and functioning of savannas BT - A model assessment of alternative land-use strategies JF - Ecology and evolution N2 - Changing climatic conditions and unsustainable land use are major threats to savannas worldwide. Historically, many African savannas were used intensively for livestock grazing, which contributed to widespread patterns of bush encroachment across savanna systems. To reverse bush encroachment, it has been proposed to change the cattle-dominated land use to one dominated by comparatively specialized browsers and usually native herbivores. However, the consequences for ecosystem properties and processes remain largely unclear. We used the ecohydrological, spatially explicit model EcoHyD to assess the impacts of two contrasting, herbivore land-use strategies on a Namibian savanna: grazer- versus browser-dominated herbivore communities. We varied the densities of grazers and browsers and determined the resulting composition and diversity of the plant community, total vegetation cover, soil moisture, and water use by plants. Our results showed that plant types that are less palatable to herbivores were best adapted to grazing or browsing animals in all simulated densities. Also, plant types that had a competitive advantage under limited water availability were among the dominant ones irrespective of land-use scenario. Overall, the results were in line with our expectations: under high grazer densities, we found heavy bush encroachment and the loss of the perennial grass matrix. Importantly, regardless of the density of browsers, grass cover and plant functional diversity were significantly higher in browsing scenarios. Browsing herbivores increased grass cover, and the higher total cover in turn improved water uptake by plants overall. We concluded that, in contrast to grazing-dominated land-use strategies, land-use strategies dominated by browsing herbivores, even at high herbivore densities, sustain diverse vegetation communities with high cover of perennial grasses, resulting in lower erosion risk and bolstering ecosystem services. KW - browsing KW - ecohydrology KW - land use KW - plant community KW - savanna KW - wildlife KW - management Y1 - 2022 U6 - https://doi.org/10.1002/ece3.8715 SN - 2045-7758 VL - 12 IS - 3 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Moustakas, Aristides A1 - Günther, Matthias A1 - Wiegand, Kerstin A1 - Müller, Karl-Heinz A1 - Ward, David A1 - Meyer, Katrin M. A1 - Jeltsch, Florian T1 - Long-term mortality patterns of the deep-rooted Acacia erioloba BT - The middle class shall die! JF - Journal of vegetation science N2 - Question: Is there a relationship between size and death in the Iona-lived, deep-rooted tree, Acacia erioloba, in a semi-arid savanna? What is the size-class distribution of A. erioloba mortality? Does the mortality distribution differ from total tree size distribution? Does A. erioloba mortality distribution match the mortality distributions recorded thus far in other environments? Location: Dronfield Ranch, near Kimberley, Kalahari, South Africa. Methods: A combination of aerial photographs and a satellite image covering 61 year was used to provide long-term spatial data on mortality. We used aerial photographs of the study area from 1940, 1964, 1984, 1993 and a satellite image from 2001 to follow three plots covering 510 ha. We were able to identify and individually follow ca. 3000 individual trees from 1940 till 2001. Results: The total number of trees increased over time. No relationship between total number of trees and mean tree size was detected. There were no trends over time in total number of deaths per plot or in size distributions of dead trees. Kolmogorov-Smirnov tests showed no differences in size class distributions for living trees through time. The size distribution of dead trees was significantly different from the size distribution of all trees present on the plots. Overall, the number of dead trees was low in small size classes, reached a peak value when canopy area was 20 - 30 m(2), and declined in lamer size-classes. Mortality as a ratio of dead vs. total trees peaked at intermediate canopy sizes too. Conclusion: A. erioloba mortality was size-dependent, peaking at intermediate sizes. The mortality distribution differs from all other tree mortality distributions recorded thus far. We suggest that a possible mechanism for this unusual mortality distribution is intraspecific competition for water in this semi-arid environment. KW - competition KW - long-term data KW - remote sensing KW - savanna KW - size dependent mortality KW - size distribution KW - tree death Y1 - 2006 U6 - https://doi.org/10.1111/j.1654-1103.2006.tb02468.x SN - 1100-9233 VL - 17 SP - 473 EP - 480 PB - Blackwell CY - Malden ER -