Are savannas patch-dynamic systems? A landscape model

Moustakas, Aristides; Sakkos, K.; Wiegand, Kerstin; Ward, David; Meyer, Katrin; Eisinger, Dirk

doi:10.1016/j.ecolmodel.2009.06.036

Cited by 28 publications

(17 citation statements)

References 87 publications

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“…A related issue is the scale at which vegetation patchiness is observed in savannas (Wiegand et al, 2006;Moustakas et al, 2009;Levick and Rogers, 2011). Spatial patterns of savanna trees and shrubs begin at the scale of the plants themselves, with canopies typically 3-6 m both in height and crown diameter and mean canopy cover of 10-40 %, although trees over 10 m and much higher canopy cover are not uncommon (Scholes and Walker, 1993).…”

Section: Introductionmentioning

confidence: 99%

Topo-edaphic controls over woody plant biomass in South African savannas

et al. 2012

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Abstract. The distribution of woody biomass in savannas reflects spatial patterns fundamental to ecosystem processes, such as water flow, competition, and herbivory, and is a key contributor to savanna ecosystem services, such as fuelwood supply. While total precipitation sets an upper bound on savanna woody biomass, the extent to which substrate and terrain constrain trees and shrubs below this maximum remains poorly understood, often occluded by local-scale disturbances such as fire and trampling. Here we investigate the role of hillslope topography and soil properties in controlling woody plant aboveground biomass (AGB) in Kruger National Park, South Africa. Large-area sampling with airborne Light Detection and Ranging (LiDAR) provided a means to average across local-scale disturbances, revealing an unexpectedly linear relationship between AGB and hillslopeposition on basalts, where biomass levels were lowest on crests, and linearly increased toward streams (R 2 = 0.91). The observed pattern was different on granite substrates, where AGB exhibited a strongly non-linear relationship with hillslope position: AGB was high on crests, decreased midslope, and then increased near stream channels (R 2 = 0.87). Overall, we observed 5-to-8-fold lower AGB on clayey, basalt-derived soil than on granites, and we suggest this is due to herbivore-fire interactions rather than lower hydraulic conductivity or clay shrinkage/swelling, as previously hypothesized. By mapping AGB within and outside fire and herbivore exclosures, we found that basalt-derived soils support tenfold higher AGB in the absence of fire and herbivory, suggesting high clay content alone is not a proximal limitation on AGB. Understanding how fire and herbivory contribute to AGB heterogeneity is critical to predicting future savanna carbon storage under a changing climate.

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Section: Introductionmentioning

confidence: 99%

Topo-edaphic controls over woody plant biomass in South African savannas

et al. 2012

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“…However, OBIA has not yet been extensively tested in natural landscapes where continuous variation in vegetation features makes it difficult to determine boundaries between 'objects' (Yu et al 2008;Blaschke 2010). Further, in heterogeneous natural systems, landscape patches can vary in size, shape and structure that may require characterization at multiple spatial scales (Moustakas et al 2009). Within the OBIA approach, 'segmentation scale' parameter used in segmentation controls the maximum allowable object-level spectral heterogeneity and object size (Trimble 2011a).…”

Section: Introductionmentioning

confidence: 98%

Estimating fractional land cover in semi-arid central Kalahari: the impact of mapping method (spectral unmixing vs. object-based image analysis) and vegetation morphology

Mishra

Crews

2014

Geocarto International

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Focusing on the central Kalahari, this study utilized fractional cover of photosynthetic vegetation ( f PV ), non-photosynthetic vegetation ( f NPV ) and bare soil ( f BS ), derived in situ and estimated from GeoEye-1 imagery using Multiple Endmember Spectral Mixture Analysis (MESMA) and object-based image analysis (OBIA) to determine superior method for fractional cover estimation and the impact of vegetation morphology on the estimation accuracy. MESMA mapped fractional cover by testing endmember models of varying complexity. Based on OBIA, image was segmented at five segmentation scales followed by classification. MESMA provided more accurate fractional cover estimates than OBIA. The increasing segmentation scale in OBIA resulted in a consistent increase in error. Different vegetation morphology types showed varied responses to the changing segmentation scale, reflecting their unique ecology and physiognomy. While areas under woody cover produced lower error even at coarse segmentation scales, those with herbaceous cover provided low error only at the fine segmentation scale.

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“…Landscapes that are mosaics of grasslands, savannas and woodlands typically are dynamic, with the physiognomy of individual patches changing on time-scales from decades to centuries (Callaway & Davis, 1993;Moustakas et al, 2009). Such shifts in physiognomy can be associated with substantial changes in ecosystem functioning, such as changes in hydrological and biogeochemical processes (Archer et al, 2001;Huxman et al, 2005), and in ecosystem structure, such as levels of native biodiversity (Ratajczak et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Historical patterns of oak population expansion in the Chautauqua Hills, Kansas

Russell

2014

Journal of Biogeography

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Aim Rates of tree population expansion have increased in many North American landscapes that were mosaics of grasslands, savannas and woodlands historically. Consequences of woodland expansion include reduced economic return from grazing and changes in native biodiversity, but causes of woodland expansion are poorly understood. We address historical timing of blackjack oak (Quercus marilandica) and post oak (Quercus stellata) population expansion in tree–grass ecosystems, roles of climate and land use in driving this expansion, and future stability of these woodlands. Location The Cross Timbers ecosystem in Kansas, USA. Methods Using increment cores, we quantified blackjack oak and post oak age structures in four woodlands on sites that were not wooded in the 1860s. We compared timing of oak regeneration with climate fluctuations (using the Palmer drought severity index) and land‐use history. We quantified tree species composition within 5‐m radii of sampled oaks. Results Recruitment of both oak species increased between 1925 and 1945. Modal age classes recruited from 1935 to 1960. Recruitment was associated with dry intervals at the two sites with north‐ or east‐facing aspects. This association was driven by blackjack oak recruitment in dry intervals. Woodlands on the sites with south‐ and west‐facing aspects contained only oaks, whereas those on the sites with north‐ and east‐facing aspects contained saplings of fire‐intolerant, shade‐tolerant tree species. Main conclusions Our results contribute to growing evidence for woodland expansion in the region during dry climate intervals. The association between drought and recruitment was influenced by slope aspect and was more pronounced in the less fire‐tolerant oak species. Although woodland expansion coincided with regional increases in fire frequency, drought and greater use of prescribed burning are likely to have reduced fire intensities by reducing fuel loads. These oak woodlands, which have developed during the 20th century, appear to form stable communities on xeric slopes but to be undergoing succession towards a mesophytic tree community on mesic slopes.

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Are savannas patch-dynamic systems? A landscape model

Cited by 28 publications

References 87 publications

Topo-edaphic controls over woody plant biomass in South African savannas

Topo-edaphic controls over woody plant biomass in South African savannas

Estimating fractional land cover in semi-arid central Kalahari: the impact of mapping method (spectral unmixing vs. object-based image analysis) and vegetation morphology

Historical patterns of oak population expansion in the Chautauqua Hills, Kansas

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