Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    On the importance of root traits in seedlings of tropical tree species
    Boonman, Coline C.F. ; Langevelde, Frank van; Oliveras, Imma ; Couédon, Jeremy ; Luijken, Natascha ; Martini, David ; Veenendaal, Elmar M. - \ 2019
    New Phytologist 227 (2019)1. - ISSN 0028-646X - p. 156 - 167.
    biomass allocation - root morphology - rooting depth - savanna - specific root length - tropical forest - vertical root distribution

    Plant biomass allocation may be optimized to acquire and conserve resources. How trade-offs in the allocation of tropical tree seedlings depend on different stressors remains poorly understood. Here we test whether above- and below-ground traits of tropical tree seedlings could explain observed occurrence along gradients of resources (light, water) and defoliation (fire, herbivory). We grew 24 tree species occurring in five African vegetation types, varying from dry savanna to moist forest, in a glasshouse for 6 months, and measured traits associated with biomass allocation. Classification based on above-ground traits resulted in clusters representing savanna and forest species, with low and high shoot investment, respectively. Classification based on root traits resulted in four clusters representing dry savanna, humid savanna, dry forest and moist forest, characterized by a deep mean rooting depth, root starch investment, high specific root length in deeper soil layers, and high specific root length in the top soil layer, respectively. In conclusion, tree seedlings in this study show root trait syndromes, which vary along gradients of resources and defoliation: seedlings from dry areas invest in deep roots, seedlings from shaded environments optimize shoot investment, and seedlings experiencing frequent defoliation store resources in the roots.

    Improving the precision and accuracy of animal population estimates with aerial image object detection
    Eikelboom, Jasper A.J. ; Wind, Johan ; Ven, Eline van de; Kenana, Lekishon M. ; Schroder, Bradley ; Knegt, Henrik J. de; Langevelde, Frank van; Prins, Herbert H.T. - \ 2019
    Methods in Ecology and Evolution 10 (2019)11. - ISSN 2041-210X - p. 1875 - 1887.
    computer vision - convolutional neural network - deep machine learning - drones - game census - image recognition - savanna - wildlife survey

    Animal population sizes are often estimated using aerial sample counts by human observers, both for wildlife and livestock. The associated methods of counting remained more or less the same since the 1970s, but suffer from low precision and low accuracy of population estimates. Aerial counts using cost-efficient Unmanned Aerial Vehicles or microlight aircrafts with cameras and an automated animal detection algorithm can potentially improve this precision and accuracy. Therefore, we evaluated the performance of the multi-class convolutional neural network RetinaNet in detecting elephants, giraffes and zebras in aerial images from two Kenyan animal counts. The algorithm detected 95% of the number of elephants, 91% of giraffes and 90% of zebras that were found by four layers of human annotation, of which it correctly detected an extra 2.8% of elephants, 3.8% giraffes and 4.0% zebras that were missed by all humans, while detecting only 1.6 to 5.0 false positives per true positive. Furthermore, the animal detections by the algorithm were less sensitive to the sighting distance than humans were. With such a high recall and precision, we posit it is feasible to replace manual aerial animal count methods (from images and/or directly) by only the manual identification of image bounding boxes selected by the algorithm and then use a correction factor equal to the inverse of the undercounting bias in the calculation of the population estimates. This correction factor causes the standard error of the population estimate to increase slightly compared to a manual method, but this increase can be compensated for when the sampling effort would increase by 23%. However, an increase in sampling effort of 160% to 1,050% can be attained with the same expenses for equipment and personnel using our proposed semi-automatic method compared to a manual method. Therefore, we conclude that our proposed aerial count method will improve the accuracy of population estimates and will decrease the standard error of population estimates by 31% to 67%. Most importantly, this animal detection algorithm has the potential to outperform humans in detecting animals from the air when supplied with images taken at a fixed rate.

    MODIS VCF should not be used to detect discontinuities in tree cover due to binning bias. A comment on Hanan et al. (2014) and Staver and Hansen (2015)
    Gerard, France ; Hooftman, Danny ; Langevelde, Frank van; Veenendaal, Elmar ; White, Steven M. ; Lloyd, Jon - \ 2017
    Global Ecology and Biogeography 26 (2017)7. - ISSN 1466-822X - p. 854 - 859.
    alternative stable states - forest - frequency distribution - MODIS VCF - remote sensing - savanna - tree cover

    In their recent paper, Staver and Hansen (Global Ecology and Biogeography, 2015, 24, 985–987) refute the case made by Hanan et al. (Global Ecology and Biogeography, 2014, 23, 259–263) that the use of classification and regression trees (CARTs) to predict tree cover from remotely sensed imagery (MODIS VCF) inherently introduces biases, thus making the resulting tree cover unsuitable for showing alternative stable states through tree cover frequency distribution analyses. Here we provide a new and equally fundamental argument for why the published frequency distributions should not be used for such purposes. We show that the practice of pre-average binning of tree cover values used to derive cover values to train the CART model will also introduce errors in the frequency distributions of the final product. We demonstrate that the frequency minima found at tree covers of 8–18%, 33–45% and 55–75% can be attributed to numerical biases introduced when training samples are derived from landscapes containing asymmetric tree cover distributions and/or a tree cover gradient. So it is highly likely that the CART, used to produce MODIS VCF, delivers tree cover frequency distributions that do not reflect the real world situation.

    To Tree or Not to Tree : Cultural Views from Ancient Romans to Modern Ecologists
    Holmgren, Milena ; Scheffer, Marten - \ 2017
    Ecosystems 20 (2017)1. - ISSN 1432-9840 - p. 62 - 68.
    ecosystem services - forest - grassland - land degradation - savanna - tree encroachment - visions of nature

    Few things are more defining in a landscape compared to the absence or presence of trees, both in aesthetic and in functional terms. At the same time, tree cover has been profoundly affected by humans since ancient times. It is therefore not surprising that opinions about deforestation and colonization of landscapes by trees have always been strong. Although loss of forests is often lamented, there is also profound cultural affection for open landscapes including some that have been deforested in the past. Here we take a historical view on perceptions of changing tree cover, and subsequently argue that the current ecological literature on forest-savanna-grassland transitions is not immune to value-laden perspectives. So far, ecosystem science has not done enough to analyze the effects of tree cover changes on ecosystem services and indicators of human well-being. Until these analyses are done, debates about forested versus open landscapes will be clashes of values rather than scientific evaluations. We discuss how ecosystem science may contribute to developing this field.

    Change in Mesoherbivore Browsing Is Mediated by Elephant and Hillslope Position
    Lagendijk, D.D.G. ; Thaker, M. ; Boer, W.F. de; Page, B.R. ; Prins, H.H.T. ; Slotow, R. - \ 2015
    PLoS ONE 10 (2015)6. - ISSN 1932-6203 - 15 p.
    kruger-national-park - loxodonta-africana - woodland regeneration - aepyceros-melampus - foraging behavior - east-africa - dry-season - savanna - herbivores - population
    Elephant are considered major drivers of ecosystems, but their effects within small-scale landscape features and on other herbivores still remain unclear. Elephant impact on vegetation has been widely studied in areas where elephant have been present for many years. We therefore examined the combined effect of short-term elephant presence (<4 years) and hillslope position on tree species assemblages, resource availability, browsing intensity and soil properties. Short-term elephant presence did not affect woody species assemblages, but did affect height distribution, with greater sapling densities in elephant access areas. Overall tree and stem densities were also not affected by elephant. By contrast, slope position affected woody species assemblages, but not height distributions and densities. Variation in species assemblages was statistically best explained by levels of total cations, Zinc, sand and clay. Although elephant and mesoherbivore browsing intensities were unaffected by slope position, we found lower mesoherbivore browsing intensity on crests with high elephant browsing intensity. Thus, elephant appear to indirectly facilitate the survival of saplings, via the displacement of mesoherbivores, providing a window of opportunity for saplings to grow into taller trees. In the short-term, effects of elephant can be minor and in the opposite direction of expectation. In addition, such behavioural displacement promotes recruitment of saplings into larger height classes. The interaction between slope position and elephant effect found here is in contrast with other studies, and illustrates the importance of examining ecosystem complexity as a function of variation in species presence and topography. The absence of a direct effect of elephant on vegetation, but the presence of an effect on mesoherbivore browsing, is relevant for conservation areas especially where both herbivore groups are actively managed.
    Elephant (Loxodonta africana) impact on trees used bynesting vultures and raptors in South Africa
    Vogel, S.M. ; Henley, M.D. ; Rode, S.C. ; Vyver, D. van de; Simmons, G. ; Boer, W.F. de - \ 2014
    African Journal of Ecology 52 (2014)4. - ISSN 0141-6707 - p. 458 - 465.
    national-park - savanna - population - plants
    Negative influences on the establishment and persistence of large trees used by tree-nesting birds as nesting sites represent a potential threat to vultures and raptors. We monitored large trees and their surrounding vegetation and analysed whether trees with nesting sites are at risk due to elephant impact. Trees with nests did not differ in elephant impact from control trees without nests, and the survival rates of trees with nests and the actual nests within the trees showed that nests decreased at a faster rate than the trees themselves. Elephant damage did not affect the persistence of nests over the 5-year monitoring period. However, the presence of insects and fungus on large trees was negatively related to tree survival, thereby indicating that elephant impact could indirectly facilitate insect and fungus attack and shorten the lifespan of a tree.
    Why trees and shrubs but rarely trubs?
    Scheffer, M. ; Vergnon, R.O.H. ; Cornelissen, J.H.C. ; Hantson, S. ; Holmgren, M. ; Nes, E.H. van; Xu, C. - \ 2014
    Trends in Ecology and Evolution 29 (2014)8. - ISSN 0169-5347 - p. 433 - 434.
    savanna - forest - transitions - height
    An analysis of the maximum height of woody plant species across the globe reveals that an intermediate size is remarkably rare. We speculate that this may be due to intrinsic suboptimality or to ecosystem bistability with open landscapes favouring shrubs, and closed canopies propelling trees to excessive tallness.
    Which farmers benefit most from sustainable intensification? An ex-ante impact assessment of expanding grain legume production in Malawi
    Franke, A.C. ; Brand, G.J. van den; Giller, K.E. - \ 2014
    European Journal of Agronomy 58 (2014). - ISSN 1161-0301 - p. 28 - 38.
    soil fertility gradients - smallholder farms - crop production - phosphorus - maize - management - savanna - yield - environments - responses
    Legume technologies are widely promoted among smallholders in southern Africa, providing an opportunity for sustainable intensification. Farms and farming strategies of smallholders differ greatly within any given locality and determine the opportunities for uptake of technologies. We provide an ex-ante assessment of the impact of grain legumes on different types of farms and identify niches for grain legumes in Malawi. After creation of a farm typology, detailed farm characterisations were used to describe the farming system. The characterisations provided the basis for the construction of simplified, virtual farms on which possible scenarios for expanding and intensifying grain legume production were explored using the farm-scale simulation model NUANCES-FARMSIM. Observed yields and labour inputs suggested that maize provides more edible yield per unit area with a higher calorific value and greater labour use efficiency than groundnut and soybean. Crop yields simulated by the model partly confirmed these yield trends, but at farm level maize-dominated systems often produced less food than systems with more grain legumes. Improved management practices such as addition of P-based fertiliser to grain legumes and inoculation of soybean were crucial to increase biological nitrogen fixation and grain yields of legumes and maize, and created systems with increased area of legumes that were more productive than the current farms. Improved legume management was especially a necessity for low resource endowed farmers who, due to little past use of P-based fertiliser and organic inputs, have soils with a poorer P status than wealthier farmers. Economic analyses suggested that legume cultivation was considerably more profitable than continuous maize cropping. Highest potential net benefits were achieved with tobacco, but the required financial investment made tobacco cultivation riskier. Grain legumes have excellent potential as food and cash crops particularly for medium and high resource endowed farmers, a role that could grow in importance as legume markets further develop. For low resource endowed farmers, legumes can improve food self-sufficiency of households, but only if legumes can be managed with P fertiliser and inoculation in the case of soybean. Given that low resource endowed farmers tend to be risk averse and have few resources to invest, the ability of poorer farmers to adopt legume technologies could be limited
    Impact of African elephants on baobab (Adansonia digitata L.) population structure in northern Gonarezhou National Park, Zimbabwe
    Kupika, O.L. ; Kativu, S. ; Gandiwa, E. ; Gumbie, A. - \ 2014
    Tropical Ecology 55 (2014)2. - ISSN 0564-3295 - p. 159 - 166.
    loxodonta-africana - damage - vegetation - woodland - tanzania - savanna - tree - ethiopia - western - cover
    The impact of African elephant (Loxodonta africana) on population structure of baobab trees (Adansonia digitata L.) was assessed in northern Gonarezhou National Park (GNP), southeast Zimbabwe. Baobabs were sampled in March 2008 and September 2012 using 11 randomly laid belt transects of variable length within 1 km of the eastern and western sections of the Runde River and also away (> 1 km) from the water sources. A total of 223 baobabs, 130 near permanent water sources and 93 away from permanent water sources, were sampled. Baobab density did not significantly differ across the two study sites. Furthermore, there were no significant differences in girth at breast height between the two study sites. Results of the present study suggest that elephants target large baobabs (girth = 5 m). In contrast, significant difference in baobab damage was recorded between the two sites. A single dead baobab tree was encountered at a site away from water sources. A larger proportion of elephant damaged baobabs was located closer to permanent water sources. However, baobab recruitment and regeneration was higher in areas close to permanent water sources than in distant areas. Management should come up with strategies to monitor vegetation changes in order to avoid loss of baobabs and other tree species.
    Tipping points in tropical tree cover: linking theory to data
    Nes, E.H. van; Hirota, M. ; Holmgren, M. ; Scheffer, M. - \ 2014
    Global Change Biology 20 (2014)3. - ISSN 1354-1013 - p. 1016 - 1021.
    critical transitions - global resilience - climate-change - stable states - savanna - fire - forest - ecosystems - amazon - deforestation
    It has recently been found that the frequency distribution of remotely sensed tree cover in the tropics has three distinct modes, which seem to correspond to forest, savanna and treeless states. This pattern has been suggested to imply that these states represent alternative attractors, and that the response of these systems to climate change would be characterized by critical transitions and hysteresis. Here, we show how this inference is contingent upon mechanisms at play. We present a simple dynamical model that can generate three alternative tree cover states (forest, savanna and a treeless state), based on known mechanisms, and use this model to simulate patterns of tree cover under different scenarios. We use these synthetic data to show that the hysteresis inferred from remotely sensed tree cover patterns will be inflated by spatial heterogeneity of environmental conditions. On the other hand, we show that the hysteresis inferred from satellite data may actually underestimate real hysteresis in response to climate change if there exists a positive feedback between regional tree cover and precipitation. Our results also indicate that such positive feedback between vegetation and climate should cause direct shifts between forest and a treeless state (rather than through an intermediate savanna-state) to become more likely. Lastly, we show how directionality of historical change in conditions may bias the observed relationship between tree cover and environmental conditions.
    Changes in lion (Panthera leo) home range size in Waza National Park, Cameroon
    Tumenta, P.N. ; Van't Zelfde, M. ; Croes, B.M. ; Buij, R. ; Funston, P.J. ; Haes, H.A.U. de; longh, H.H. De - \ 2013
    Mammalian Biology - Zeitschrift für Säugetierkunde 78 (2013)6. - ISSN 1616-5047 - p. 461 - 469.
    carnivore conservation - social-behavior - africa - savanna - ecology - west
    The spatial ecology of Africa lions (Panthera leo) was studied from 2007 to 2009 in Waza National Park, Cameroon, by equipping individual lions with GPS/VHF radio-collars. Mean home range estimates using 100% minimum convex polygons (MCP) and 95% kernel-density estimation (KDE) were respectively 1015 km2 and 641 km2. The lions spent a considerable amount of time out of the park during the study period (21%), resulting in significantly larger wet season home ranges than in the hot dry season when they were largely within the park. Time spent outside of the park coincided with increased livestock predation, especially by males. The seasonal variation observed in home range appeared to be mainly due to prey dispersal, flooding and migrating livestock. Mean home range size was observed to have increased by 58.6% within the last decade. This observed increase in home range could possibly be attributed to recent declines in wild prey abundance and also, may be indicative of a trend of general degradation of the park due to intense human pressure. The change observed in lions’ ranging behaviour was remarkable, with lions crossing the highway parallel to the park to the Cameroon-Nigerian borders. Measures to restore the integrity of the park are urgently needed, which could include the construction of a partial fence along the western boundary of the park to prevent lions moving across the parallel highway.
    Breeding Biology and Diet of the African Swallow-Tailed Kite (Chelictinia riocourii) in Senegal and Cameroon
    Buij, R. ; Cavaillés, S. ; Mullié, W.C. - \ 2013
    Journal of Raptor Research 47 (2013)1. - ISSN 0892-1016 - p. 41 - 53.
    savanna - raptors - birds - hawk - populations - predation - survival - site
    We studied the breeding biology of the African Swallow-tailed Kite (Chelictinia riocourii) in two study areas located 3400 km apart in the central (Cameroon) and western (Senegal) portions of the species' breeding range. With 110 nests in 2.8 km2 of suitable breeding habitat, Kousmar islet (23 km2) in Senegal supports the largest documented colony of African Swallow-tailed Kites known to date. Breeding kites in Senegal nested in a single large colony near a massive winter roost. In Cameroon, breeding colonies averaged seven pairs/colony, with nest densities of 0.3 nests/km2 in protected woodland and 0.9 nests/km2 in cultivated habitat. Egg-laying coincided with the end of the dry season in Cameroon, but eggs were recorded from the middle of the dry season in Senegal. Eggs hatched between April and June in both study sites in 2010, but from March 2012 in Senegal. The incubation period was estimated at 27–31 d based on two nests, and the fledging period was 32–35 d (n ¿=¿ 3 fledglings). Mean clutch size was 2.5 eggs (n ¿=¿ 32) in Cameroon and 2.1 in Senegal (n ¿=¿ 29); one clutch of four eggs was recorded in Cameroon. Nest success estimated with the Mayfield method was low at 17% in Cameroon and exceptionally low at 4% in Senegal, possibly related to a combination of suboptimal food conditions, high predation pressure, intraspecific aggression, and lack of experience among breeding pairs. Prey items at nests were made up primarily of lizards (30–54% of items) and insects (27–49%), notably grasshoppers, whereas the diet at the winter roost in Senegal was predominantly Orthoptera (55%) and Solifugids (43%). Our study suggested that African Swallow-tailed Kites were able to adapt to moderate land transformation near floodplains.
    Effects of land-use change and rainfall in Sudano-Sahelian West Africa on the diet and nestling growth rates of an avian predator
    Buij, R. ; Folkertsma, I. ; Kortekaas, K. ; longh, H.H. De; Komdeur, J. - \ 2013
    Ibis 155 (2013)1. - ISSN 0019-1019 - p. 89 - 101.
    field energetics - breeding success - protected areas - hatching order - climate-change - foraging mode - national-park - raptors - habitat - savanna
    Raptor populations in Sudano-Sahelian West Africa are being severely affected by widespread habitat alteration which depletes prey populations, potentially aggravated by changing rainfall patterns. We studied Grasshopper Buzzards Butastur rufipennis at nests in natural and transformed habitats in the Sudano-Sahelian region of northern Cameroon to assess the effects of habbitat transformation and rainfall on nestling diet and growth.
    Assessing the impacts of livestock production on biodiversity in rangeland ecosystems
    Alkemade, R. ; Reid, R.S. ; Berg, M. van den; Leeuw, J. de; Jeuken, M. - \ 2013
    Proceedings of the National Academy of Sciences of the United States of America 110 (2013)52. - ISSN 0027-8424 - p. 20900 - 20905.
    land-use changes - south-africa - diversity - conservation - assemblages - grassland - management - scenarios - responses - savanna
    Biodiversity in rangelands is decreasing, due to intense utilization for livestock production and conversion of rangeland into cropland; yet the outlook of rangeland biodiversity has not been considered in view of future global demand for food. Here we assess the impact of future livestock production on the global rangelands area and their biodiversity. First we formalized existing knowledge about livestock grazing impacts on biodiversity, expressed in mean species abundance (MSA) of the original rangeland native species assemblages, through metaanalysis of peer-reviewed literature. MSA values, ranging from 1 in natural rangelands to 0.3 in man-made grasslands, were entered in the IMAGE-GLOBIO model. This model was used to assess the impact of change in food demand and livestock production on future rangeland biodiversity. The model revealed remarkable regional variation in impact on rangeland area and MSA between two agricultural production scenarios. The area of used rangelands slightly increases globally between 2000 and 2050 in the baseline scenario and reduces under a scenario of enhanced uptake of resource-efficient production technologies increasing production [high levels of agricultural knowledge, science, and technology (high-AKST)], particularly in Africa. Both scenarios suggest a global decrease in MSA for rangelands until 2050. The contribution of livestock grazing to MSA loss is, however, expected to diminish after 2030, in particular in Africa under the high-AKST scenario. Policies fostering agricultural intensification can reduce the overall pressure on rangeland biodiversity, but additional measures, addressing factors such as climate change and infrastructural development, are necessary to totally halt biodiversity loss.
    Assessing the effects of subtropical forest fragmentation on leaf nitrogen distribution using remote sensing data
    Cho, M.A. ; Ramoelo, A. ; Debba, P. ; Mutanga, O. ; Mathieu, R. ; Deventer, H. van; Ndlovu, N. - \ 2013
    Landscape Ecology 28 (2013)8. - ISSN 0921-2973 - p. 1479 - 1491.
    red-edge - tropical deforestation - spatial heterogeneity - chlorophyll content - multispectral data - eucalyptus leaves - land-use - landscape - reflectance - savanna
    Subtropical forest loss resulting from conversion of forest to other land-cover types such as grassland, secondary forest, subsistence crop farms and small forest patches affects leaf nitrogen (N) stocks in the landscape. This study explores the utility of new remote sensing tools to model the spatial distribution of leaf N concentration in a forested landscape undergoing deforestation in KwaZulu-Natal, South Africa. Leaf N was mapped using models developed from RapidEye imagery; a relatively new space-borne multispectral sensor. RapidEye consists of five spectral bands in the visible to near infra-red (NIR) and has a spatial resolution of 5 m. MERIS terrestrial chlorophyll index derived from the RapidEye explained 50 % of the variance in leaf N across different land-cover types with a model standard error of prediction of 29 % (i.e. of the observed mean leaf N) when assessed on an independent test data. The results showed that indigenous forest fragmentation leads to significant losses in leaf N as most of the land-cover types (e.g. grasslands and subsistence farmlands) resulting from forest degradation showed lower leaf N when compared to the original indigenous forest. Further analysis of the spatial variation of leaf N revealed an autocorrelation distance of about 50 m for leaf N in the fragmented landscape, a scale corresponding to the average dimension of subsistence fields (2,781 m2) in the region. The availability of new multispectral sensors such as RapidEye thus, moves remote sensing closer to widespread monitoring of the effect of tropical forest degradation on leaf N distribution.
    Deciduous and evergreen trees differ in juvenile biomass allometries because of differences in allocation to root storage
    Tomlinson, K.W. ; Langevelde, F. van; Ward, D. ; Bongers, F.J.J.M. ; Alves da Silva, D. ; Prins, H.H.T. ; Bie, S. de; Sterck, F.J. - \ 2013
    Annals of Botany 112 (2013)3. - ISSN 0305-7364 - p. 575 - 587.
    optimal partitioning theory - shade-tolerance - general-model - dry forest - plants - water - savanna - light - carbohydrate - communities
    Background and Aims - Biomass partitioning for resource conservation might affect plant allometry, accounting for a substantial amount of unexplained variation in existing plant allometry models. One means of resource conservation is through direct allocation to storage in particular organs. In this study, storage allocation and biomass allometry of deciduous and evergreen tree species from seasonal environments were considered. It was expected that deciduous species would have greater allocation to storage in roots to support leaf regrowth in subsequent growing seasons, and consequently have lower scaling exponents for leaf to root and stem to root partitioning, than evergreen species. Itwas further expected that changes to root carbohydrate storage and biomass allometry under different soil nutrient supply conditions would be greater for deciduous species than for evergreen species. Methods - Root carbohydrate storage and organ biomass allometrieswere compared for juveniles of 20 savanna tree species of different leaf habit (nine evergreen, 11 deciduous) grown in two nutrient treatments for periods of 5 and 20 weeks (total dry mass of individual plants ranged from 0.003 to 258.724 g). Key Results - Deciduous species had greater root non-structural carbohydrate than evergreen species, and lower scaling exponents for leaf to root and stem to root partitioning than evergreen species. Across species, leaf to stem scaling was positively related, and stem to root scaling was negatively related to root carbohydrate concentration. Under lower nutrient supply, trees displayed increased partitioning to non-structural carbohydrate, and to roots and leaves over stems with increasing plant size, but this change did not differ between leaf habits. Conclusions - Substantial unexplained variation in biomass allometry of woody species may be related to selection for resource conservation against environmental stresses, such as resource seasonality. Further differences in plant allometry could arise due to selection for different types of biomass allocation in response to different environmental stressors (e.g. fire vs herbivory)
    Effects of interannual climate variability on tropical tree cover
    Holmgren, M. ; Hirota, M. ; Nes, E.H. van; Scheffer, M. - \ 2013
    Nature Climate Change 3 (2013). - ISSN 1758-678X - p. 755 - 758.
    el-nino - critical transitions - shrub encroachment - south-america - woody cover - savanna - forest - determinants - ecosystems - africa
    Climatic warming is substantially intensifying the global water cycle1 and is projected to increase rainfall variability2. Using satellite data, we show that higher climatic variability is associated with reduced tree cover in the wet tropics globally. In contrast, interannual variability in rainfall can have neutral or even positive effects on tree cover in the dry tropics. In South America, tree cover in dry lands is higher in areas with high year-to-year variability in rainfall. This is consistent with evidence from case studies suggesting that in these areas rare wet episodes are essential for opening windows of opportunity where massive tree recruitment can overwhelm disturbance effects, allowing the establishment of extensive woodlands. In Australia, wet extremes have similar effects, but the net effect of rainfall variability is overwhelmed by negative effects of extreme dry years. In Africa, effects of rainfall variability are neutral for dry lands. It is most likely that differences in herbivore communities and fire regimes contribute to regulating tree expansion during wet extremes. Our results illustrate that increasing climatic variability may affect ecosystem services in contrasting, and sometimes surprising, ways. Expansion of dry tropical tree cover during extreme wet events may decrease grassland productivity but enhance carbon sequestration, soil nutrient retention and biodiversity.
    Ecological impact of Prosopis species invasion in Turkwel riverine forest, Kenya
    Muturi, G.M. ; Poorter, L. ; Mohren, G.M.J. ; Kigomo, B.N. - \ 2013
    Journal of Arid Environments 92 (2013)may. - ISSN 0140-1963 - p. 89 - 97.
    leguminosae-subfam-mimosoideae - acacia-tortilis - western-australia - juliflora - patterns - mesquite - trees - floodplain - woodland - savanna
    The impact of Prosopis species invasion in the Turkwel riverine forest in Kenya was investigated under three contrasting: Acacia, Prosopis and Mixed species (Acacia and Prosopis) canopies. Variation amongst canopies was assessed through soil nutrients and physical properties, tree characteristics and canopy closure. Invasion impact was evaluated by comparing herbaceous species cover and diversity, and occurrence of indigenous tree seedlings. Soil characteristics under Prosopis and Mixed species canopies were similar except in pH and calcium content, and had lower silt and carbon contents than soil under Acacia canopy. Tree density was higher under Prosopis intermediate under Mixed and lower under Acacia canopies. Prosopis trees had lower diameters than Acacia tortilis trees. Diameter classes' distribution in Mixed species canopy revealed invasion of Prosopis into mature A. tortilis stands. Herbaceous species cover and diversity were negatively correlated to Prosopis tree density; thus explaining the lower herbaceous species cover and diversity under Prosopis than under Acacia and Mixed species canopies. The study suggests a gradual conversion of herbaceous rich A. tortilis woodland to herbaceous poor Prosopis species woodland or thickets, through indiscriminate Prosopis invasion. Highlights - ¿ We assessed herbs and tree regeneration under Prosopis and Acacia tortilis canopies. ¿ Herbs diversity and productivity were lower under Prosopis than under A. tortilis. ¿ We also found lack of A. tortilis seedlings under Prosopis canopy. ¿ Apparently, Prosopis invasion has severe repercussions to riverine forest ecology. ¿ We recommend Prosopis management to mitigate ecological and livelihoods threats.
    Hydrological implications of desertification: Degradation of South African semi-arid subtropical thicket
    Luijk, G. van; Cowling, R.M. ; Riksen, M.J.P.M. ; Glenday, J. - \ 2013
    Journal of Arid Environments 91 (2013). - ISSN 0140-1963 - p. 14 - 21.
    succulent thicket - eastern cape - interception loss - land-use - runoff - savanna - carbon - water - transformation - sequestration
    Almost half of the 16,942 km2 of South Africa's subtropical thicket with a substantial Portulacaria afra (spekboom) component has been heavily degraded by domestic herbivores. The subtropical thicket biome is a drought-prone and water-stressed area, and many of the region's watersheds comprise of eroded landscapes clothed in degraded spekboom thicket. The objective of this study was to determine the impact of degradation of spekboom thicket on hydrological processes. We hypothesised that degradation of spekboom thicket would reduce infiltration and, hence, reduce soil moisture retention and increase run-off and erosion. We tested this hypothesis by collecting data on rainfall, infiltration, soil moisture retention and run-off in degraded thicket, and – as a reference site – in an adjacent stand of relatively intact thicket. The results showed clear trends in the impacts of spekboom thicket degradation on hydrological processes. The more than hundred-fold lower infiltration in soils associated with degraded thicket relative to the soils beneath the intact, spekboom canopy, resulted in lower levels and less retention of soil moisture, almost double the amount of runoff, and an almost six-fold increase in sediment load. Thus, restoring degraded thicket will reduce erosion and likely improve baseflows, in addition to sequestering carbon.
    Thresholds for Boreal Biome Transitions
    Scheffer, M. ; Hirota, M. ; Holmgren, M. ; Nes, E.H. van; Chapin, F.S. - \ 2012
    Proceedings of the National Academy of Sciences of the United States of America 109 (2012)52. - ISSN 0027-8424 - p. 21384 - 21389.
    arctic forest-tundra - climate-change - fire regime - global resilience - interior alaska - tropical forest - white spruce - tree - savanna - canada
    Although the boreal region is warming twice as fast as the global average, the way in which the vast boreal forests and tundras may respond is poorly understood. Using satellite data, we reveal marked alternative modes in the frequency distributions of boreal tree cover. At the northern end and at the dry continental southern extremes, treeless tundra and steppe, respectively, are the only possible states. However, over a broad intermediate temperature range, these treeless states coexist with boreal forest (~75% tree cover) and with two more open woodland states (~20% and ~45% tree cover). Intermediate tree covers (e.g., ~10%, ~30%, and ~60% tree cover) between these distinct states are relatively rare, suggesting that they may represent unstable states where the system dwells only transiently. Mechanisms for such instabilities remain to be unraveled, but our results have important implications for the anticipated response of these ecosystems to climatic change. The data reveal that boreal forest shows no gradual decline in tree cover toward its limits. Instead, our analysis suggests that it becomes less resilient in the sense that it may more easily shift into a sparse woodland or treeless state. Similarly, the relative scarcity of the intermediate ~10% tree cover suggests that tundra may shift relatively abruptly to a more abundant tree cover. If our inferences are correct, climate change may invoke massive nonlinear shifts in boreal biomes.
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