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This chapter introduces the different agroforestry systems (AFSs) as part of the diversification of agricultural landscapes and gives examples of their use in different related crop production systems in southern Africa. The introduction of trees into agriculture has several benefits and can mitigate the effects of climate change. For example nitrogen-fixing trees and shrubs contribute significantly to nutrient recycling and benefit soil conservation, which is particularly important for smallholder farms. In addition, shelterbelts play an important role in reducing wind speeds, and thus, evapotranspiration, and modifying the microclimatic conditions, which is an important factor for the adaptation of cropping systems to climate change. These integrated AFS landscapes provide important ecosystem services for soil protection, food security and for biodiversity. However, deficiencies in the institutional and policy frameworks that underlie the adoption and stimulus of AFS in the southern African region were identified. Furthermore, the following factors must be considered to optimise AFS: (1) selection of tree species that ensure maximum residual soil fertility beyond 3 years, (2) size of land owned by the farmer, (3) integrated nutrition management, where organic resources are combined with synthetic inorganic fertilisers and (4) tree-crop competition in the root zone for water.
This chapter aims at providing an overview of the diversity of agroecological conditions, features of main farming systems, agricultural land use, its dynamics and drivers during the last two decades as well as major threats in ten countries of southern Africa (SA10). Based on this, we attempt to identify the resultant challenges for sustainable land management and outline potential interventions with a focus on smallholder farmers. By analyzing cropland dynamics during 2000–2019, we show how land use has been shaped by climate, demographic development, economic imperatives and policy realities. Concrete examples of these complex interactions illustrate both considerable shrinkage in South Africa and Zimbabwe or expansion of cropland in Mozambique and Zambia. During the past 20 years, cropland increased by 37% on average across SA10 mainly at the expense of forestland—showing huge spatiotemporal heterogeneity among countries. Most smallholders face shrinking farm size and other resource limitations that have resulted in soil nutrient mining and low agricultural productivity—a highly unsustainable situation. We conclude with an outlook on potential transformation pathways (“TechnoGarden” and “AdaptiveMosaic”) for the near future and thereby provide a frame for further studies on sustainable land management options under given local settings.
Ecosystems in southern Africa are threatened by numerous global change forces, with climate change being a major threat to the region. Many climate change impacts and environmental-based mitigation and adaptation options remain poorly researched in this globally important biodiversity hotspot. This book is a collection of chapters covering research undertaken in southern Africa by the German Federal Ministry of Education and Research’s (BMBF) SPACES and SPACES II programs. SPACES II covered a wide range of global change-linked environmental issues ranging in scope from the impacts of ocean currents on global climate systems through to understanding how small-scale farmers may best adapt to the impacts of climate change. All the research has identified policy implications, and the book strives for a balance between presenting the detailed science underpinning the conclusions as well as providing clear and simple policy messages. To achieve this, many chapters in the book contextualize the issues through the provision of a mini-review and combine this with the latest science emulating out of the SPACES II program of research. The book therefore consolidated both past and the most current research findings in a way that will be of benefit to both academia and policy makers.
The sustainability of southern Africa’s natural and managed marine and terrestrial ecosystems is threatened by overuse, mismanagement, population pressures, degradation, and climate change. Counteracting unsustainable development requires a deep understanding of earth system processes and how these are affected by ongoing and anticipated global changes. This information must be translated into practical policy and management interventions. Climate models project that the rate of terrestrial warming in southern Africa is above the global terrestrial average. Moreover, most of the region will become drier. Already there is evidence that climate change is disrupting ecosystem functioning and the provision of ecosystem services. This is likely to continue in the foreseeable future, but impacts can be partly mitigated through urgent implementation of appropriate policy and management interventions to enhance resilience and sustainability of the ecosystems. The recommendations presented in the previous chapters are informed by a deepened scientific understanding of the relevant earth system processes, but also identify research and knowledge gaps. Ongoing disciplinary research remains critical, but needs to be complemented with cross-disciplinary and transdisciplinary research that can integrate across temporal and spatial scales to give a fuller understanding of not only individual components of the complex earth-system, but how they interact.
It is commonly accepted that CO2 efflux increases with soil water content in aerated soils and that rewetting after periods of soil drying can result in respiration pulses. It has further been shown that soil pores may become water logged which can impede soil gas exchange. The present study aimed to quantify the carbon response of moss (Polytrichum piliferum) and lichen (Cladonia coniocraea) biocoenoses to different quantities of rain in an artificial catchment, which granted that the starting point of the development of both biocoenoses was the same. To address this aim, we conducted in situ soil moisture and soil respiration measurements, where soil respiration was hypothesised to emerge from a cryptogamic vegetation layer and from mineral soil beneath. We found that higher water-holding capacity of the moss layer and higher accumulation of organic matter in the upper mineral soil under mosses result in higher amounts of water stored near the surface. As a consequence, evaporation of water as well as pulses of CO2 efflux after moderate rain following a period of drought were higher in the moss biocoenosis, where the upper mineral soil was of key importance. In contrast, the lichen biocoenosis facilitated penetration of rainwater into the deep soil. Superimposing rewetting pulses, near-saturation of soil pores with water after severe rain resulted in gas exchange inhibition and diminished soil respiration until subsequent aeration in both biocoenoses
Biocrusts are a worldwide phenomenon, contributing substantially to ecosystem functioning. Their growth and survival depend on multiple environmental factors, including climatic ones, and the relations of these factors to physiological processes. Responses of biocrusts to individual environmental factors have been examined in a large number of field and laboratory experiments. These observational data, however, have rarely been assembled into a comprehensive, consistent framework that allows quantitative exploration of the roles of multiple environmental factors and physiological properties for the performance of biocrusts, in particular across climatic regions. Here we used a data-driven mechanistic modelling framework to simulate the carbon balance of biocrusts, a key measure of their growth and survival. We thereby assessed the relative importance of physiological and environmental factors for the carbon balance at six study sites that differ in climatic conditions. Moreover, we examined the role of seasonal acclimation of physiological properties using our framework, since the effects of this process on the carbon balance of biocrusts are poorly constrained so far. We found substantial effects of air temperature, CO2 concentration, and physiological parameters that are related to respiration on biocrust carbon balance, which differ, however, in their patterns across regions. The ambient CO2 concentration is the most important factor for biocrusts from drylands, while air temperature has the strongest impact at alpine and temperate sites. Metabolic respiration cost plays a more important role than optimum temperature for gross photosynthesis at the alpine site; this is not the case, however, in drylands and temperate regions. Moreover, we estimated a small annual carbon gain of 1.5 by lichen-dominated biocrust and 1.9 by moss-dominated biocrust at a dryland site, while the biocrusts lost a large amount of carbon at some of the temperate sites (e.g. −92.1 for lichen-dominated and −74.7 for moss-dominated biocrust). These strongly negative values contradict the observed survival of the organisms at the sites and may be caused by the uncertainty in environmental conditions and physiological parameters, which we assessed in a sensitivity analysis. Another potential explanation for this result may be the lack of acclimation in the modelling approach, since the carbon balance can increase substantially when testing for seasonally varying parameters in the sensitivity analysis. We conclude that the uncertainties in air temperature, CO2 concentration, respiration-related physiological parameters, and the absence of seasonal acclimation in the model for humid temperate and alpine regions may be a relevant source of error and should be taken into account in future approaches that aim at estimating the long-term biocrust carbon balance based on ecophysiological data.
In nutrient-poor ecosystems, certain plant families have evolved specific adaptations for phosphate acquisition, namely cluster roots or proteoid roots. Cluster rooted species have a unique suite of adaptations that enable them to survive in nutrient poor ecosystems, such as the Fynbos biome of South Africa. The Fynbos biome has heterogeneous soils which caused radiation of varying soil pH within small localised areas. Previous studies on cluster rooted species ignored the possibility that cluster root functioning may differ at varying soil pH levels and soil types. Therefore, two native Fynbos protea species, Protea cynaroides (L.) L. and Protea obtusifolia H.Buek ex Meisn. were compared to assess the impact of different soil pH levels on organic acid metabolism and exudation in cluster rooted species. Our results show that cluster roots of the acidic-soil grown P. cynaroides is more effective and more efficient than that of the alkaline-soil grown P. obtusifolia. Our findings indicate that the mechanisms for P acquisition vary greatly, depending on substrate and plant organ type. This suggests that the metabolism of each species is adapted to specific soil pH and that these species have evolutionary adapted their P acquisition mechanisms for these extreme soils. The limited nutrient availability and variation in soil pH South African soils has led to various localised adaptations for cluster rooted species.
Windbreaks, trees, structures, and shade nets are used in South Africa to protect citrus orchards from wind scar damage to avoid major losses in export quality. Severe southeasterly winds in the Western Cape during spring and summer are the main causes of significant external peel damage. In this study, wind scar damage of ‘Tango’ mandarin (Citrus reticulata) fruit was quantified at different distances from established Casuarina cunninghamiana (Beefwood) and Populus simonii (Chinese Poplar) windbreaks around Stellenbosch, Western Cape, South Africa. The yield efficiency was not significantly affected by distance from the windbreak in either of the trials. Fruit at 2 H and 3 H had significantly lower ratios of total soluble solids to titratable acidity, but export standards for mandarins were still met. The wind speed was consistently higher at 16 H, compared with 3 H during summer. The highest packout (class 1 fruit) was observed near the windbreak (3 H) and decreased as the distance increased towards 10 H and 16 H. There were no significant differences for severe wind scar damage incidence between the distance treatments. Our results confirmed the impact of wind on external fruit quality and the limitation of tree-based windbreaks with an increase in distance, which partly supports the current movement towards full enclosure of orchards with shade nets.
Nach extremen Störungen durch Brände, Vulkanausbrüchen und Hangrutschungen, aber auch menschlichen Eingriffen werden Böden mehr oder weniger zerstört und vegetationsarm. Es entstehen sogenannte initiale Rohböden, die durch Veränderung der Bodenstruktur, Minderung der Wasserhaltekapazität als auch durch Nährstoffarmut extrem beeinflusst werden. Dabei haben anfängliche und nährstoffarme Ökosysteme, wie Grasländer und Offenlandschaften, aus Sicht des Naturschutzes einen hohen Wert für die Landschaft und sind in der intensiv genutzten Kulturlandschaft Mitteleuropas selten geworden. Insbesondere bieten Binnendünen, wie sie in Norddeutschland häufig sind, Potenzial für die Entwicklung von initialen Lebensräumen mit ihren spezifisch angepassten Pflanzen. Neben den Pionierarten werden die Rohböden, wie sie nach Störungen entstehen, zuerst durch verschiedene blütenlose Pflanzen, den sogenannten Kryptogamen, besiedelt.
Biocrusts are distributed over all climate zones of the world and they substantially contribute to ecosystem functioning. Their growth, determined by their carbon balance, can be affected by various climatic drivers. The effects of individual drivers are clear from laboratory experiments, but the relative importance of different drivers along climatic gradients and their underlying mechanisms are largely unknown. Moreover, the effects of seasonal acclimation on the annual carbon balance are not fully understood either. Therefore, we aim at determining the level and variation of annual biocrust carbon balances and their connection to climatic drivers along environmental gradients. In addition, we explore the role that acclimation plays in the carbon balance of biocrusts
Non-vascular photoautotrophs (NVP), including bryophytes, lichens, terrestrial algae, and cyanobacteria, are increasingly recognized as being essential to ecosystem functioning in many regions of the world. Current research suggests that climate change may pose a substantial threat to NVP, but it is highly uncertain to what extent this will affect the associated ecosystem functions and services. Here, we propose a research agenda to address this urgent question, focusing on physiological and ecological processes that link NVP to ecosystem functions while also taking into account the substantial taxonomic diversity across multiple ecosystem types. Accordingly, we developed a new categorization scheme, based on microclimatic gradients, which simplifies the high physiological and morphological diversity of NVP and worldwide distribution with respect to several broad habitat types. We found that habitat-specific ecosystem functions of NVP will likely be substantially affected by climate change, and more quantitative process understanding is required on (1) potential for acclimation (2) response to elevated CO2 (3) role of the microbiome and (4) feedback to (micro)climate. We suggest an integrative approach of innovative, multi-method laboratory and field experiments and eco-physiological modelling, for which sustained scientific collaboration on NVP research will be essential.
Forest dieback can be both a consequence and a cause of climate change. The changing climate does not only lead to temperature increases but also changes in the precipitation regime. Extreme events have increased sharply in recent years, making drought and heat waves ubiquitous. Meanwhile, for temperate forests, drought stress is considered one of the most serious impacts of climate change. In this context, forest soils are of great importance in their hydrological functions, as well as their feedbacks with ground vegetation. In this context, biological soil crusts are key drivers of functional processes and ecosystem development, also under forest, where they have been less studied so far. Bryophyte and lichen dominated communities can importantly affect e.g. water storage and discharge as well as soil development and stabilization. Moreover, they contribute to carbon and nitrogen cycling and play an important role in biogeochemical processes. Their species composition depends on soil properties such as texture and pH, on microclimate and as poikilohydric plants, their ecophysiology is strongly dependent on water availability, differing in time and space.
The generally accepted assumption that CAM is an important adaptation to the drought conditions in arid deserts is mainly based on studies in controlled environments comparing the respective photosynthetic pathways in plants of different morphology. Comparative long-term field studies on the ecophysiology of C3- and CAM succulents co-occurring in the same desert habitat are too rare to satisfactorily answer the old question whether the metabolic pathway or the succulence is more important to guarantee survival in such a harsh habitat. Thus, relevant ecophysiological parameters were repeatedly monitored on plants of the leaf succulents Augea capensis (C3) and Malephora purpureo-crocea (CAM), which commonly dominate a habitat in the Knersvlakte, Namaqualand, South Africa. Under identical conditions in the same habitat, carbon gain of the C3 species was higher than that of the CAM plant, but their WUE lower and maximum photosynthetic activity nearly the same. In M. purpureo-crocea, irrigation induced rapid whole day stomatal opening at drastically increased daytime but decreased night-time CO2-uptake, while respective responses of the C3 species were delayed. These different response dynamics were due to different rooting depth, root morphology and water acquisition strategy of the flat rooting CAM and the deep-rooting C3 species. This, however, may not explain the highly similar growth success. Anyway, the results verify that CAM provides no advantage over C3 for long-term survival in the same desert environment in morphologically similar succulents.
In the context of ongoing climate change and increasing population, there is an urgent need to optimize the water consumption of surface and groundwater in agricultural production. In recent years, intensive irrigated viticulture and horticulture have faced increasing demand pressure in many water-limited areas including the Western Cape Province in South Africa. Shelterbelts of trees are often used to reduce wind speed and water demands as an eco-engineering measure directly influencing soil evaporation and crop transpiration. Objectives are (i) to evaluate the extent of impacts of wind speed from shelterbelts at canopy level in citrus orchards and vineyards (ii) to assess the wind effects at leaf level including leaf temperature and related ecophysiological performance in irrigated vineyards.
Biological soil crusts (BSCs) are thin microbiological vegetation layers that naturally develop in unfavorable higher plant conditions (i.e., low precipitation rates and high temperatures) in global drylands. They consist of poikilohydric organisms capable of adjusting their metabolic activities depending on the water availability. However, they, and with them, their ecosystem functions, are endangered by climate change and land-use intensification. Remote sensing (RS)-based studies estimated the BSC cover in global drylands through various multispectral indices, and few of them correlated the BSCs’ activity response to rainfall. However, the allocation of BSCs is not limited to drylands only as there are areas beyond where smaller patches have developed under intense human impact and frequent disturbance. Yet, those areas were not addressed in RS-based studies, raising the question of whether the methods developed in extensive drylands can be transferred easily. Our temperate climate study area, the ‘Lieberoser Heide’ in northeastern Germany, is home to the country’s largest BSC-covered area. We applied a Random Forest (RF) classification model incorporating multispectral Sentinel-2 (S2) data, indices derived from them, and topographic information to spatiotemporally map the BSC cover for the first time in Central Europe. We further monitored the BSC response to rainfall events over a period of around five years (June 2015 to end of December 2020). Therefore, we combined datasets of gridded NDVI as a measure of photosynthetic activity with daily precipitation data and conducted a change detection analysis. With an overall accuracy of 98.9%, our classification proved satisfactory. Detected changes in BSC activity between dry and wet conditions were found to be significant. Our study emphasizes a high transferability of established methods from extensive drylands to BSC-covered areas in the temperate climate. Therefore, we consider our study to provide essential impulses so that RS-based biocrust mapping in the future will be applied beyond the global drylands
In recent years, the importance of soil health for ecosystem functions has come further into the scientific focus (Lehmann et al., 2020). Especially after severe ecosystem disturbances, soil formation has to start anew. Such disturbances, which reset ecosystem development to the starting point, can be of natural (volcanoes, mobile sand dunes, floods, glaciers) (La Farge et al., 2013; Lan et al., 2014) or human origin (post-mining landscapes, military training areas, agricultural lands) (Belnap et al., 2007; Schaaf et al., 2011). In these young ecosystems, the interactions between the initial colonizers, the inorganic matter, and the subsequent biogeochemical processes are an important prerequisite for the development of elemental fluxes, soil genesis, and thus for further ecosystem development.
Under the conditions of climate change in South Africa, ecological and technical measures are needed to reduce the water consumption of irrigated crops. Windbreak hedges are long-rated systems in agriculture that significantly reduce wind speed. Their possibilities to reduce evapotranspiration and water demand are being investigated at a vineyard in the Western Cape Province, South Africa. Detailed measurements of meteorological parameters relevant for the computation of reference and crop-specific evapotranspiration following the FAO 56 approaches within a vineyard in the Western Cape Province of South Africa have shown the beneficial effect of an existing hedgerow consisting of 6 m high poplars (Populus simonii (Carrière) Wesm.). With reference to a control station in the open field, the mean wind speed in a position about 18 m from the hedgerow at canopy level (2 m) was reduced by 27.6% over the entire year and by 39.2% over the summer growing season. This effect leads to a parallel reduction of reference evapotranspiration of 15.5% during the whole year and of 18.4% over the growing season. When applying empirical crop-specific Kc values for well-irrigated grapes, the reduction of evapotranspiration is 18.8% over the summer growth period. The introduced tree shelterbelts are a suitable eco-engineering approach to reduce water consumption and to enhance water saving in vineyards.
Agroforestry: An Appropriate and Sustainable Response to a Changing Climate in Southern Africa?
(2020)
Agroforestry is often discussed as a strategy that can be used both for the adaptation to and the mitigation of climate change effects. The climate of southern Africa is predicted to be severely affected by such changes. With agriculture noted as the continent’s largest economic sector, issues such as food security and land degradation are in the forefront. In the light of such concerns we review the current literature to investigate if agroforestry systems (AFS) are a suitable response to the challenges besetting traditional agricultural caused by a changing climate. The benefits bestowed by AFS are multiple, offering ecosystem services, influence over crop production and positive impacts on rural livelihoods through provisioning and income generation. Nevertheless, knowledge gaps remain. We identify outstanding questions requiring further investigation such as the interplay between trees and crops and their combination, with a discussion of potential benefits. Furthermore, we identify deficiencies in the institutional and policy frameworks that underlie the adoption and stimulus of AFS in the southern African region. We uphold the concept that AFS remains an appropriate and sustainable response for an increased resilience against a changing climate in southern Africa for the benefit of livelihoods and multiple environmental values
Due to disturbances of soils after open-cast lignite mining activities in the Lusatian mining district, Northeast Germany, large areas covered with excavated and dumped material. These initial soils are characterized by a lack of nutrients and organic matter which is why Robinia pseudoacacia was established during reclamation of those marginal sites. As a pioneer species, Robinia is well suitable for the establishment on reclaimed forest ecosystems due to the N fixation capability and its ability to grow on a nutrient-poor sandy substrate with low water holding capacity. Since the N requirement of the Robinia can be covered by symbiotic nodule bacteria, P is important as a growth-limiting nutrient. Even though the total and available P in soils increased with the age of the plantings (planted between 1995 and 2007), the ratio of available to total P decreased relative to younger plantings. Despite the accumulation of organic matter and the decomposition of organic bound P, the bioavailability is affected by ad- and desorption processes, especially on Fe- and Al-hydroxides. It was hypothesized that the P demand of young trees, initially provided by a one-time fertilizer application, exceeds the P supply by mineralization over time. As a first step, kinetics of P desorption from synthetic Fe- and Al-hydroxides were characterized by using humic acid as a model organic compound, to detect fast and slow releasable P over time. At the Robinia sites, soil samples were examined at different depth with regard to their C, N and P concentrations. From the first results, we can conclude that the P availability and its limitation influences the growth performance of Robinia as well as the N-fixing symbiotic bacteria and, thus, the C:N:P ratio as a long-term effect on an ecosystem scale.