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Aim To determine the functional relationships between, and the relative importance of, different driver variables (mean annual precipitation, soil properties, fire and herbivory) in regulating woody plant cover across broad environmental gradients in African savannas. Location Savanna grasslands of East, West and Southern Africa. Methods The dependence of woody cover on mean annual precipitation (MAP), soil properties (texture, nitrogen mineralization potential and total phosphorus), fire regimes, and herbivory (grazer, browser + mixed feeder, and elephant biomass) was determined for 161 savanna sites across Africa using stochastic gradient boosting, a refinement of the regression tree analysis technique. Results All variables were significant predictors of woody cover, collectively explaining 71% of the variance in our data set. However, their relative importance as regulators of woody cover varied. MAP was the most important predictor, followed by fire return periods, soil characteristics and herbivory regimes. Woody cover showed a strong positive dependence on MAP between 200 and 700 mm, but no dependence on MAP above this threshold when the effects of other predictors were accounted for. Fires served to reduce woody cover below rainfall-determined levels. Woody cover showed a complex, non-linear relationship with total soil phosphorus, and was negatively correlated with clay content. There was a strong negative dependence of woody cover on soil nitrogen (N) availability, suggesting that increased N-deposition may cause shifts in savannas towards more grassy states. Elephants, mixed feeders and browsers had negative effects on woody cover. Grazers, on the other hand, depressed woody cover at low biomass, but favoured woody vegetation when their biomass exceeded a certain threshold. Main conclusions Our results indicate complex and contrasting relationships between woody cover, rainfall, soil properties and disturbance regimes in savannas, and suggest that future environmental changes such as altered precipitation regimes, N-enrichment and elevated levels of CO2 are likely to have opposing, and potentially interacting, influences on the tree–grass balance in savannas.

This paper investigates the performance of medium access control (MAC) protocols in a wavelength-division multiplexing (WDM) based optical access network consisting of a backbone ring interconnecting several passive-star-based clusters of optical networking units (ONUs) at customer premises. Each cluster is connected to the backbone through an access node (AN). A scheduler located in each AN, executes two MAC protocols, one for the intracluster traffic and the other for the intercluster traffic. In order to maintain the quality of service, the scheduler in the AN employs, priority-based queuing for the intercluster traffic on pre-assigned wavelengths. For controlling the intracluster traffic, the scheduler employs pre-transmission coordination with ranging and look-ahead functionalities in the MAC protocol. The performance of MAC protocol for intracluster traffic is evaluated through event-driven simulation, while for intercluster traffic the MAC performance is evaluated through analytical modeling of the queuing system employing two dynamic bandwidth management schemes. Performance of the intracluster MAC protocol is shown to be improved by novel use of subcarrier multiplexing on the wavelength used for the control packet transmission. A comparative study of the two intercluster schemes in terms of end-to-end delay is carried out, to understand the effect of priority queuing on the real-time and non-real-time service packets.

Several explanations for the persistence of tree–grass mixtures in savannas have been advanced thus far. In general, these either concentrate on competition-based mechanisms, where niche separation with respect to limiting resources such as water lead to tree–grass coexistence, or demographic mechanisms, where factors such as fire, herbivory and rainfall variability promote tree–grass persistence through their dissimilar effects on different life-history stages of trees. Tests of these models have been largely site-specific, and although different models find support in empirical data from some savanna sites, enough dissenting evidence exists from others to question their validity as general mechanisms of tree–grass coexistence. This lack of consensus on determinants of savanna structure and function arises because different models: (i) focus on different demographic stages of trees, (ii) focus on different limiting factors of tree establishment, and (iii) emphasize different subsets of the potential interactions between trees and grasses. Furthermore, models differ in terms of the most basic assumptions as to whether trees or grasses are the better competitors. We believe an integration of competition-based and demographic approaches is required if a comprehensive model that explains both coexistence and the relative productivity of the tree and grass components across the diverse savannas of the world is to emerge. As a first step towards this end, we outline a conceptual framework that integrates existing approaches and applies them explicitly to different life-history stage of trees.

We review the distribution, ecology, and environmental controls of savanna structure and function, with a focus on African and Asian savannas. Savannas or mixed tree–grass systems occupy most of Africa but cover less of Asia. On both continents, they support unique faunal and floral assemblages. Rainfall, soil nutrients, fire, and herbivory are important determinants of savanna structure, their relative effects driving the observed variation in the cover of trees and grasses. Given that trees and grasses use different photosynthetic pathways, and are differently limited by water and nutrients, this biome is likely to be very sensitive to future changes in precipitation, nutrient deposition, and atmospheric CO 2 .

In this paper we examine the resource provisioning aspects in a passive optical network (PON) employing wavelength-division multiplexing (WDM) and optical code-division multiple access (OCDMA) techniques. A judicious resource allocation scheme is proposed for a given set of optical codes and wavelengths with due consideration to the traffic asymmetry between upstream and downstream transmissions. The proposed scheme employs some initial heuristic estimates, followed up by a subsequent open search mode towards reaching an optimal solution. The search mechanism in the provisioning scheme is carried out by employing some overall network performance metrics, evaluated using appropriate analytical models.

Green and senesced leaf nitrogen (N) and phosphorus (P) concentrations of different plant functional groups in savanna communities of Kruger National Park, South Africa were analyzed to determine if nutrient resorption was regulated by plant nutritional status and foliar N:P ratios. The N and P concentrations in green leaves and the N concentrations in senesced leaves differed significantly between the dominant plant functional groups in these savannas: fine-leaved trees, broad-leaved trees and grasses. However, all three functional groups reduced P to comparable and very low levels in senesced leaves, suggesting that P was tightly conserved in this tropical semi-arid savanna ecosystem. Across all functional groups, there was evidence for nutritional control of resorption in this system, with both N and P resorption efficiencies decreasing as green leaf nutrient concentrations increased. However, specific patterns of resorption and the functional relationships between nutrient concentrations in green and senesced leaves varied by nutrient and plant functional group. Functional relationships between N concentrations in green and senesced leaves were indistinguishable between the dominant groups, suggesting that variation in N resorption efficiency was largely the result of inter-life form differences in green leaf N concentrations. In contrast, observed differences in P resorption efficiencies between life forms appear to be the result of both differences in green leaf P concentrations as well as inherent differences between life forms in the fraction of green leaf P resorbed from senescing leaves. Our results indicate that foliar N:P ratios are poor predictors of resorption efficiency in this ecosystem, in contrast to N and P resorption proficiencies, which are more responsive to foliar N:P ratios.

In this paper an analytical model has been developed for a packet switched multi-wavelength optical CDMA network to study its system performance, while employing various two-dimensional optical codes. The model adopts Poisson distributed traffic implying infinite population assumption to reduce the computational complexity. It takes into account the packet errors due to chip interference between the code words. Further, guard time between chip pulses is introduced in order to relax the tunability requirements of the lasers in the wavelength hopped 2-D OCDMA scheme. Guard time reduces network speed and lowers the effective data transport. However the probability of receiving correct data packets is improved due to reduced chip interference. The combined impact of both on system behavior (throughput and delay) for different multi-wavelength codes is investigated.

Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than approximately 650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered 'stable' systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of approximately 650 mm, savannas are 'unstable' systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics.

This paper explores the scope and applicability of optical code division multiple access schemes, in catering to the ambitious network requirements of the future subscriber population. A comprehensive report is made of the latest scenario that optical CDMA offers to the local area network environment in particular, and to broadband service sector in general. Different aspects of OCDMA communication systems are presented, tracing the chronological evolution spanning over the past two decades. The review concludes that notwithstanding the yet-to-mature optical component technology, OCDMA stands as a viable alternative to the prevalent multiple access methods in some broadband applications.

This paper deals with the performance evaluation of a passive optical network (PON) using wavelength division multiplexing (WDM) by employing an arrayed-waveguide grating (AWG) in the remote node (RN). In such WDM-based PONs (WDMPONs) the RN demultiplexes the downstream WDM signal via distinct output ports through a static wavelength-routing mechanism. An analytical model is developed for evaluating the bit error rate (BER) performance of the demultiplexed channels as a function of the output port location in the AWG by incorporating a novel spectral-to-spatial domain transformation technique. The proposed model captures the impact of finite laser linewidth as well as the AWG characteristics in determining the received signal power at the individual optical network unit (ONU) attached to the output port (due to spectral-to-spatial mapping). Furthermore, the BER model accounts for several transmission impairments, including beat noise due to interchannel crosstalk in the AWG. Results indicate that the signal power captured by the output ports of AWGs and hence their BER values are not only determined by the Gaussian focal-field pattern of the AWG but also by the power spillover from adjacent ports for broader linewidths and higher data rates. A significant interplay is observed between data rate, source linewidth, and relevant device parameters while examining the loss and BER characteristics of a given AWG-based WDMPON.

The News of the Week article that reports on Senator Kay Bailey Hutchison (R-TX) questioning the need to fund social science research at the National Science Foundation is alarming and shortsighted (“Senate panel chair asks why NSF funds social sciences,” 12 May, p. [829][1]). Social science research is at the fundamental core of basic research and has much to contribute to the economic viability of the United States. Twenty years of direct and jointly funded social and ecosystem science research at Colorado State University's Natural Resource Ecology Laboratory has produced deep insights into environmental and societal impacts of political upheaval, land use, and climate change in parts of Africa, Asia, and the Americas. Beyond greatly advancing our understanding of the coupled human-environmental system, the partnership of social and ecosystem science has brought scientists and decision-makers together to begin to develop solutions to difficult problems. Insights from the social sciences about the vulnerability, sustainability, and resilience of social and environmental systems have greatly increased our understanding of the complex interactions of the world on which we depend. The contributions of social and ecological science for data analyses associated with spatial relationships, transfer of goods and services, valuation, and decision-making processes have improved our understanding of how human activities are altering environmental systems worldwide. These findings have become the core of policy and development decisions used throughout the world. It is critical that the NSF support social science funding and that, as budget decisions are made, the social science allocation should move forward equally with increases as in other basic and applied research. Failure to do so will further hinder U.S. competitiveness in the future and will slow transfer of knowledge and usefulness to the public. [1]: /lookup/doi/10.1126/science.312.5775.829a