Surface albedo is a critical parameter that controls surface energy balance. In dryland ecosystems, fires play a significant role in decreasing surface albedo, resulting in positive radiative forcing. Here we investigate the long‐term effect of fire on surface albedo. We devised a method to calculate short‐, medium‐, and long‐term effect of fire‐induced radiative forcing and their relative effects on energy balance. We used Moderate Resolution Imaging Spectroradiometer (MODIS) data in our analysis, covering different vegetation classes in sub‐Saharan Africa (SSA). Our analysis indicated that mean short‐term fire‐induced albedo change in SSA was −0.022, −0.035, and −0.041 for savannas, shrubland, and grasslands, respectively. At regional scale, mean fire‐induced albedo change in savannas was −0.018 and −0.024 for northern sub‐Saharan of Africa and the southern hemisphere Africa, respectively. The short‐term mean fire‐induced radiative forcing in burned areas in sub‐Saharan Africa (SSA) was 5.41 W m−2, which contributed continental and global radiative forcings of 0.25 and 0.058 W m−2, respectively. The impact of fire in surface albedo has long‐lasting effects that varies with vegetation type. The long‐term energetic effects of fire‐induced albedo change and associated radiative forcing were, on average, more than 19 times greater across SSA than the short‐term effects, suggesting that fires exerted far more radiative forcing than previously thought. Taking into account the actual duration of fire's effect on surface albedo, we conclude that the contribution of SSA fires, globally and throughout the year, is ~0.12 W m−2. These findings provide crucial information on possible impact of fire on regional climate variability.

[1] The Southern Hemisphere shows relatively low levels of atmospheric dust concentrations. Dust concentrations could, however, increase as a result of losses of vegetation cover in the southern Kalahari. There is some evidence of an ongoing remobilization of stabilized dunefields in the southern Kalahari where dune crests with sparse vegetation cover are reactivated during dry and windy periods, a phenomenon that is predicted to intensify with increased land degradation, overgrazing, and droughts. Despite the potentially important climatic and biogeochemical implications of dust emissions from the Kalahari, it is still unclear whether the predicted remobilization of the Kalahari dunes could be associated with increased dust emissions from this region. The dependence of sediment fluxes and dust emissions on vegetation cover in the Kalahari dunelands remains poorly understood, which prevents a quantitative assessment of possible changes in aeolian activity in this region under different land use and land cover scenarios. In this study, we report the results of an aeolian sediment sampling campaign over a variety of land covers in the southern Kalahari. We use these results to quantify the potential rate of dust emissions and its dependence on vegetation cover and to make an estimate of dust fluxes from a portion of the southern Kalahari. The results show that the loss of vegetation could lead to substantial increases in dust emission and nutrient loss.

Many dune fields around the world have undergone alternating periods of mobilization and stabilization in response to changes in wind power and rainfall. However, in modern times disturbances associated with land use are believed to be a dominant factor contributing to the activation of stabilized vegetated dunes in drylands, while the reduction in human activities such as grazing and farming may lead to stabilization of once active dune fields. The Kalahari region of southern Africa has recently begun to exhibit visible signs of dune mobilization, a process that could lead to an activation of aeolian transport in the region with important implications for the biogeochemistry of downwind terrestrial and marine ecosystems. It is still unclear whether the region is poised at a tipping point between its current state (i.e., vegetated fixed linear dunes), and a “degraded” state (i.e., barren and active dunes). Here we investigate the ability of the landscape to recover from the degraded state by assessing the resilience of duneland vegetation and evaluating the vegetation and soil characteristics. Using field observations and soil seed bank experiments, we show that palatable perennial grass cover is reduced while the seedbank is depleted on grazed dunefields. Conversely, the interdunes generally exhibit relatively rich seed banks. Soils from grazed and ungrazed sites show that plant available nutrient contents are not significantly different; therefore, soil nutrients are likely not a major factor limiting the recovery of perennial vegetation in this region. It is observed that the perennial grasses reestablish on the recovering dunes after grazers have been excluded, indicating that the landscape is still able to recover after years of denudation and that any irreversible shift to a stable degraded state is likely during extended periods of disturbance and/or climatic shifts that promote the degraded state. We also find that changes in grass cover, grass community composition, and seed bank can serve as indicators of whether the system has irreversibly shifted from a vegetated to a bare dune state.

The contribution of savannas to global carbon storage is poorly understood, in part due to lack of knowledge of the amount of belowground biomass. In these ecosystems, the coexistence of woody and herbaceous life forms is often explained on the basis of belowground interactions among roots. However, the distribution of root biomass in savannas has seldom been investigated, and the dependence of root biomass on rainfall regime remains unclear, particularly for woody plants. Here we investigate patterns of belowground woody biomass along a rainfall gradient in the Kalahari of southern Africa, a region with consistent sandy soils. We test the hypotheses that (1) the root depth increases with mean annual precipitation (root optimality and plant hydrotropism hypothesis), and (2) the root-to-shoot ratio increases with decreasing mean annual rainfall (functional equilibrium hypothesis). Both hypotheses have been previously assessed for herbaceous vegetation using global root data sets. Our data do not support these hypotheses for the case of woody plants in savannas. We find that in the Kalahari, the root profiles of woody plants do not become deeper with increasing mean annual precipitation, whereas the root-to-shoot ratios decrease along a gradient of increasing aridity.

Savannah ecosystems are important carbon stocks on the Earth, and their quantification is crucial for understanding the global impact of climate and land‐use changes in savannahs. The estimation of aboveground/belowground plant biomass requires tested allometric relationships that can be used to determine total plant biomass as a function of easy‐to‐measure morphological indicators. Despite recent advances in savannah ecology, research on allometric relations in savannahs remains confined to a few site‐specific studies where basal area is typically used as the main morphometric parameter with plant biomass. We investigate allometric relations at four sites along a 950‐km transect in the Kalahari across mean rainfall gradient 170 mm yr−1–550 mm yr−1. Using data from 342 harvested trees/shrubs, we relate basal area, height and crown diameter to aboveground biomass. These relationships are strongest in trees and weakest in small shrubs. Strong allometric relationships are also determined for morphologically similar groups of woody vegetation. We show that crown diameter can be used as an alternative to basal area in allometric relationships with plant biomass. This finding may enhance the ability to determine aboveground biomass over large areas using high‐resolution aerial or satellite imagery without requiring ground‐based measurements of basal area.

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2015. A quantitative description of the interspecies diversity of belowground structure in savanna woody plants. Ecosphere 6(9):154. http:// dx.Abstract. The relative importance of resource competition and disturbance in limiting woody cover is one of the most basic questions in savanna ecology. Modeling approaches that seek to address this question are limited in their ability to accurately represent resource competition, which occurs belowground, by the limited detail of existing data on root system structure. Using compressed air, we excavated individual trees and shrubs and mapped their coarse roots on a three-dimensional grid system up to 1.5 m depth. We excavated four woody savanna species at three sites spanning a climate gradient on the Kalahari Transect in southern Africa and determined functions to describe the distribution of root biomass within the root systems. Overall, most of the variation in both large-scale and small-scale characteristics of root system structure was related to species. The species excavated fell into two groups that coexisted across the climate gradient. Acacia mellifera and Terminalia sericea had straight roots in a laterally extensive system that was shallow relative to the aggregate root profile for woody plants at the sites. Boscia albitrunca and Ochna pulchra had sinuous roots that were mostly concentrated beneath the canopy and were more prevalent in deep than near-surface soil layers, departing from the conventional model of decreasing root abundance with depth. Based on our results, we suggest a reframing of the theoretical characterization of the root zone used in modeling approaches to consider diversity within the woody plant community.

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