Canopy structure in savannas along a moisture gradient on Kalahari sands

Abstract

Measurements of tree canopy architecture were made at six savanna sites on deep, sandy soils, along a gradient of increasing aridity. There was substantial variation in the leaf area estimated within each site, using the same sample frame, but different measurement techniques. The trends in canopy properties in relation to the aridity gradient were consistent, regardless of the technique used for estimating the properties. The effective plant area index for the tree canopy (the sum of the stem area index and the leaf area index (LAI)) declined from around 2 to around 0.8 m 2 m À 2 over a gradient of mean annual rainfall from 1000 to 350 mm. Stems contributed 2-5% of the tree canopy plant area index. Since the tree canopy cover decreased from 50% to 20% over this aridity range, the leaf area index within the area covered by tree canopies remained fairly constant at 3-4 m 2 m À 2. Tree leaves tended from a horizontal orientation to a more random orientation as the aridity increased. On the same gradient, the leaf minor axis dimension decreased from around 30 mm to around 3 mm, and the mean specific leaf area decreased from 14 to 5 m 2 kg ha À 1 . There was good agreement between LAI observed in the field using a line ceptometer and the LAI inferred by the MODIS sensor on the Terra satellite platform, 2 months later in the same season.

Comparing the Dry Season In-Situ Leaf Area Index (LAI) Derived from High-Resolution RapidEye Imagery with MODIS LAI in a Namibian Savanna

“…Compared to other biomes, savannas are underrepresented in studies focusing on the indirect sampling of LAI [64], where even fewer deal with dry season LAI. For example, Boulain et al [10] investigated a shrub land fallow in Western Niger (MAP = 479 mm) with the annual minima of LAI ranging between 0.1 and 0.2.…”

“…From Southern Africa, Privette et al [21] report a mean dry season LAI of 0.8 in Zambia (MAP = 950 mm), whereas a mean dry season LAI of 0.79 (±0.13) was found in several Miombo woodland sites in Northern Mozambique (MAP = 900 mm) [65]. Within the Kalahari Transect [64], two shrub land sites located in Tsane (MAP = 350 mm) and Okwa (MAP = 424 mm) in Botswana were sampled during a drought period within the growing season. Here, LAI ranged between 0.51 and 0.83 in Tsane and between 0.19 and 1.75 in Okwa.…”

Precipitation as driver of carbon fluxes in 11 African ecosystems

“…The study covers ecosystems from the very dry Sahel (Sudan, Mali and Niger), to the semiarid and sub-humid regions in Southern Africa (Botswana, Zambia and South Africa; Scholes et al, 2004) and to the humid environments towards the equator (Republic of Congo, see Fig. 1 and Table 1 for details).…”

“…The first focused on the biocomplexity of savannas (Sankaran et al, 2005(Sankaran et al, , 2008Bucini and Hanan, 2007) and followed a long tradition of ground-based ecophysiological research trying to understand the role of water, nutrients, herbivory and fire in savanna dynamics, and in particular on tree-grass interactions (Walter, 1939(Walter, , 1971Scholes and Walker, 1993;Scholes and Archer, 1997;Jeltsch et al, 2000;Scholes et al, 2004;Bond and Keeley, 2005;. This approach supports to interpret functional properties of savannas by understanding their structure.…”

“…A limited number of eddy covariance-based analyses of seasonal variation of ecosystem-atmosphere carbon exchange have been published over the last few years, describing single ecosystem types such as Mopane woodlands (Colophospermum mopane), broad-leaved deciduous Combretum-dominated savannas, fine-leaved Acacia-dominated savannas, as well as plantations of clonal Eucalypt in tropical Africa Hanan et al, 1998;Epron et al, 2004Epron et al, , 2006Veenendaal et al, 2004;Arneth et al, 2006;Bruemmer et al, 2008;Kutsch et al, 2009). Some studies that seek to provide a broader integration over latitude or along rainfall gradients have been based on short-term campaign measurements Dolman et al, 2003;Scanlon and Albertson, 2004;Scholes et al, 2004;Shugart et al, 2004;Schuttemeyer et al, 2006).…”