Ecological impacts of Prosopis invasion in Riverine forests of Kenya

Drylands occupy over 41% of the global land surface, with Africa and Asia accounting for 32% of the global total each. Because of poor resource management, resource overexploitation and periodic droughts, drylands have experienced severe land degradation. Land degradation is manifested in vegetation loss or deterioration, soil erosion and salinization of soil and water. In Kenya, drylands occupy over 87% of the land surface, and support about 30% of the national population, over 70% of national livestock and the bulk of wildlife that support the tourist sector. Following the prolonged sahelian droughts of the 1970’s Kenya’s drylands were seriously degraded through extensive loss of ground vegetation cover; thus threatening the survival of local populations, livestock production and sustenance of tourism sector. Subsequently, exotic trees and shrubs were introduced for land rehabilitation and fodder supply. Trees from from Prosopis genus emerged as the most adapted and were widely planted. Since introduction, Prosopis species have spread from target rehabilitation sites and invaded riverine and wetlands ecosystems but invasion mechanisms and impacts are not yet well understood. In this study we combined geographical information systems techniques; field, greenhouse and laboratory studies, to evaluate riverine habitat invasibility, invasion impacts, invasiveness of Prosopis species and the composition of invasive Prosopis species in Kenya. The following questions were addressed: 1) What abiotic factors make riverine forests vulnerable to Prosopis invasion?, 2) What are the ecological implications of Prosopis invasion in riverine forests?, 3) What mechanisms underlie inhibition of A. tortilis regeneration by Prosopis species invasion?, 4) What are the species composition in Prosopis invaded areas of Kenya, and 5) What are the implications of our results? The present study revealed indiscriminate Prosopis invasion in all land cover and land use types identified through satellite image analysis, field surveys and historical site information provided by local informants. As a result of this trend, we found a contrasting occurrence increase of Prosopis species and decrease of Acacia tortilis between 1998 and 2007. Accordingly, the study has demonstrated that Prosopis species invasion in the Turkwel Riverine forest is invoked more by species invasiveness rather than habitat susceptibility. Consequently, we investigated the invasiveness of Prosopis species by studying invasion impacts and the underpinning mechanisms Our study has shown reduction of herbaceous species ground vegetation cover and herbaceous species diversity, and termination of A. tortilis regeneration by Prosopis invasion. The negative regression coefficients found between herbaceous species ground cover or between herbaceous species diversity and Prosopis canopy dummy, clarifies the partial direct negative effect of Prosopis on herbaceous species. We corroborate this finding by greenhouse studies that show stronger inhibition A. tortilis and Prosopis seed germination by increasing the concentration of fresh Prosopis litter than by increasing the concentration A. tortilis litter in the soil. Indeed, our study demonstrates potential of seed germination termination at 50% fresh Prosopis litter concentration in the soil. After one month of watering of soil-litter mixture, we found no litter effect on seed germination. Since water leaching decreased the concentration of soluble phenols and leached litter had no effect on seed germination, our study has clarified that the inhibition of A. tortilis regeneration by Prosopis canopy was partially the result of allelopathic effect of Prosopis litter on A. tortilis seed germination. There has been great confusion on Prosopis species identity in Prosopis invaded areas of Kenya, because of similar morphology and introduction of several species within sites. Species misidentification may hamper invasion management. In this study we used Random Amplified DNA markers to differentiate species according to sites. Our study shows that only one species or a hybrid is adapted to any one site, despite the number of species that were introduced to any site. We have further clarified that P. juliflora and its hybrid are the most invasive germplasm in Kenya. However, P. juliflora and the hybrid trees tended to have similar tree characteristics in riverine forests and wetlands as we could predict tree volumes in wetlands from equations developed from a distant riverine site. Our study demonstrates potential for perpetual replacement of A. tortilis by Prosopis species in riverine ecosystems. A notable consequence is reduction of both herbaceous species productivity and diversity. Since both A. tortilis and herbaceous species are used for fodder; invasion may have severe consequences on the pastoral economy but this can be reversed by intensified utilization of Prosopis biomass for fuelwood and pods for fodder.   </p

Genetic diversity of Kenyan Prosopis populations based on random amplified polymorphic DNA markers

Several Prosopis species and provenances were introduced in Kenya, either as a single event or repeatedly. To date, naturally established Prosopis populations are described as pure species depending on site, despite the aforementioned introduction of several species within some sites. To determine whether naturally established stands consist of a single or mixture of species, six populations from Bamburi, Bura, Isiolo, Marigat, Taveta and Turkwel were compared for relatedness with reference to Prosopis chilensis, Prosopis juliflora and Prosopis pallida using random amplified polymorphic DNA markers. Cluster analysis based on Nei’s genetic distance clustered Kenyan populations as follows: Marigat, Bura and Isiolo with P. juliflora, Bamburi with P. pallida and Taveta with P. chilensis, whereas the Turkwel population is likely to be a hybrid between P. chileneis and P. juliflora. Four populations had private markers, revealing germplasm uniqueness. Expected heterozygosity tended to be larger for Kenyan populations (ranging from 0.091 to 0.191) than in the three reference (ranging from 0.065 to 0.144). For the six Kenyan populations and two P. juliflora provenances from the Middle East, molecular variation was larger within populations than between population. Higher molecular variance among populations is attributed to their geographical separation and the low variation within populations is due to gene flow between individuals within a population. Overall, this study shows that (1) the Kenyan Prosopis populations are genetically isolated, (2) multiple introductions enhanced genetic diversity within sites and (3) P. juliflora and its hybrid are the most aggressive invaders

Ecological impacts of Prosopis invasion in Riverine forests of Kenya

Drylands occupy over 41% of the global land surface, with Africa and Asia accounting for 32% of the global total each. Because of poor resource management, resource overexploitation and periodic droughts, drylands have experienced severe land degradation. Land degradation is manifested in vegetation loss or deterioration, soil erosion and salinization of soil and water. In Kenya, drylands occupy over 87% of the land surface, and support about 30% of the national population, over 70% of national livestock and the bulk of wildlife that support the tourist sector. Following the prolonged sahelian droughts of the 1970’s Kenya’s drylands were seriously degraded through extensive loss of ground vegetation cover; thus threatening the survival of local populations, livestock production and sustenance of tourism sector. Subsequently, exotic trees and shrubs were introduced for land rehabilitation and fodder supply. Trees from from Prosopis genus emerged as the most adapted and were widely planted. Since introduction, Prosopis species have spread from target rehabilitation sites and invaded riverine and wetlands ecosystems but invasion mechanisms and impacts are not yet well understood. In this study we combined geographical information systems techniques; field, greenhouse and laboratory studies, to evaluate riverine habitat invasibility, invasion impacts, invasiveness of Prosopis species and the composition of invasive Prosopis species in Kenya. The following questions were addressed: 1) What abiotic factors make riverine forests vulnerable to Prosopis invasion?, 2) What are the ecological implications of Prosopis invasion in riverine forests?, 3) What mechanisms underlie inhibition of A. tortilis regeneration by Prosopis species invasion?, 4) What are the species composition in Prosopis invaded areas of Kenya, and 5) What are the implications of our results? The present study revealed indiscriminate Prosopis invasion in all land cover and land use types identified through satellite image analysis, field surveys and historical site information provided by local informants. As a result of this trend, we found a contrasting occurrence increase of Prosopis species and decrease of Acacia tortilis between 1998 and 2007. Accordingly, the study has demonstrated that Prosopis species invasion in the Turkwel Riverine forest is invoked more by species invasiveness rather than habitat susceptibility. Consequently, we investigated the invasiveness of Prosopis species by studying invasion impacts and the underpinning mechanisms Our study has shown reduction of herbaceous species ground vegetation cover and herbaceous species diversity, and termination of A. tortilis regeneration by Prosopis invasion. The negative regression coefficients found between herbaceous species ground cover or between herbaceous species diversity and Prosopis canopy dummy, clarifies the partial direct negative effect of Prosopis on herbaceous species. We corroborate this finding by greenhouse studies that show stronger inhibition A. tortilis and Prosopis seed germination by increasing the concentration of fresh Prosopis litter than by increasing the concentration A. tortilis litter in the soil. Indeed, our study demonstrates potential of seed germination termination at 50% fresh Prosopis litter concentration in the soil. After one month of watering of soil-litter mixture, we found no litter effect on seed germination. Since water leaching decreased the concentration of soluble phenols and leached litter had no effect on seed germination, our study has clarified that the inhibition of A. tortilis regeneration by Prosopis canopy was partially the result of allelopathic effect of Prosopis litter on A. tortilis seed germination. There has been great confusion on Prosopis species identity in Prosopis invaded areas of Kenya, because of similar morphology and introduction of several species within sites. Species misidentification may hamper invasion management. In this study we used Random Amplified DNA markers to differentiate species according to sites. Our study shows that only one species or a hybrid is adapted to any one site, despite the number of species that were introduced to any site. We have further clarified that P. juliflora and its hybrid are the most invasive germplasm in Kenya. However, P. juliflora and the hybrid trees tended to have similar tree characteristics in riverine forests and wetlands as we could predict tree volumes in wetlands from equations developed from a distant riverine site. Our study demonstrates potential for perpetual replacement of A. tortilis by Prosopis species in riverine ecosystems. A notable consequence is reduction of both herbaceous species productivity and diversity. Since both A. tortilis and herbaceous species are used for fodder; invasion may have severe consequences on the pastoral economy but this can be reversed by intensified utilization of Prosopis biomass for fuelwood and pods for fodder

Prediction of Prosopis species invasion in Kenya using geographical information system techniques

Tree species from Prosopis genus were widely planted for rehabilitation of degraded drylands of Kenya. However, they have invaded riverine ecosystems where they cause negative socio-economic and ecological impacts. GIS was used to estimate the reverine area threatened by Prosopis invasion in Kenya. Landsat satellite images, field surveys and past studies were also used to assess the resulting potential ecological impacts in the Turkwel ecosystem in Kenya. The study revealed that 3.0 to 27.7¿million hectares are threatened by invasion, based on documented riverine forests width of 0.5-3¿km. Image analysis showed that 34% of the sites under positive change were invaded, with most invasions occurring in natural forests and abandoned farms. Prosopis had overall occurrence of 39% in all the sampled sites in 2007, in contrast to 0% in 1990 that was reported in an earlier study. In these areas, Acacia tortilis occurrence dropped from 81% in 1990 to 43% in 2007, suggesting that Prosopis could be displacing it. Utilization of Prosopis for fodder, fuel wood and pods for animal feeds is recommended as a management tool to reverse the trend. The methods used in this study are also recommended for invasion prediction and management in other similar ecosystem

Ecological impact of Prosopis species invasion in Turkwel riverine forest, Kenya

The impact of Prosopis species invasion in the Turkwel riverine forest in Kenya was investigated under three contrasting: Acacia, Prosopis and Mixed species (Acacia and Prosopis) canopies. Variation amongst canopies was assessed through soil nutrients and physical properties, tree characteristics and canopy closure. Invasion impact was evaluated by comparing herbaceous species cover and diversity, and occurrence of indigenous tree seedlings. Soil characteristics under Prosopis and Mixed species canopies were similar except in pH and calcium content, and had lower silt and carbon contents than soil under Acacia canopy. Tree density was higher under Prosopis intermediate under Mixed and lower under Acacia canopies. Prosopis trees had lower diameters than Acacia tortilis trees. Diameter classes' distribution in Mixed species canopy revealed invasion of Prosopis into mature A. tortilis stands. Herbaceous species cover and diversity were negatively correlated to Prosopis tree density; thus explaining the lower herbaceous species cover and diversity under Prosopis than under Acacia and Mixed species canopies. The study suggests a gradual conversion of herbaceous rich A. tortilis woodland to herbaceous poor Prosopis species woodland or thickets, through indiscriminate Prosopis invasion. Highlights - ¿ We assessed herbs and tree regeneration under Prosopis and Acacia tortilis canopies. ¿ Herbs diversity and productivity were lower under Prosopis than under A. tortilis. ¿ We also found lack of A. tortilis seedlings under Prosopis canopy. ¿ Apparently, Prosopis invasion has severe repercussions to riverine forest ecology. ¿ We recommend Prosopis management to mitigate ecological and livelihoods threats

Allometric equations for estimating biomass in naturally established Prosopis stands in Kenya

Forty five Prosopis stems of 2.5-18.0 cm diameter at breast height (DBH) were sampled at Nadapal along the Turkwel riverine forest for development of biomass and volume prediction equations for naturally established stands. Basal diameter (D30), DBH and heights were measured, felled trees and their volumes, fresh and dry weights determined. Linear and power models were evaluated for volume and biomass prediction through regression analysis of measured tree parameters. Power models yielded better results than linear models in volume and biomass prediction, with D30 and DBH being more reliable than height. Validation of models at two sites in Marigat and Bura, revealed strong significant correlations between predicted and measured tree biomass and volumes, suggesting effectiveness of the models in biomass prediction across sites. Subsequently, model development and model validation data were pooled to develop national models. Basal diameter was found to be the best variable in the development of power models for biomass and volume prediction across the country. When logarithmically transformed, biomass and volume per tree had strong significant linear relationship with basal diameter, and are accordingly recommended for quick biomass and volume estimation in the fiel

The DesertMargins Programme Approaches in Upscaling Best-Bet Technologies in Arid and Semi-arid Lands in Kenya

Kenya’s land surface is primarily arid and AQ1 semi-aridlands(ASALs)whichaccountfor84%ofthe totallandarea.TheDesertMarginsProgramme(DMP) inKenyahasmadesomecontributiontounderstanding which technology options have potential in reducing land degradation in marginal areas and conserving biodiversity through demonstrations, testing of the most promising natural resource management options, developing sustainable alternative livelihoods and policy guidelines, and replicating successful models. In extension of sustainable natural resource management, two types of strategies were used: (i) strategies for the promotion of readily available technologies and (ii) approaches for participatory learning and action research. Thus DMP-Kenya initiated upscaling of four ‘best-bet’ technologies. Under the rangeland/livestock management options, scaling-up activities include improvement of rangeland productivity, rangeland resource management through communitybasedrangeresourcesmonitoring/assessment,andfodder conservation for home-based herds. Restoration of degraded lands included rehabilitation of rangelands using the red paint approach in conservation of Acacia tortilis, control of Prosopis, planting of Acacia senegal trees in micro-catchments, and rehabilitation of degraded areas through community enclosures. Improved land, nutrient, and water management involved upscaling water harvesting and integrated nutrient management (INM) technologies. Activities A.O. Esilaba ( ) Desert Margins Programme, Kenya Agricultural Research Institute, Nairobi, Kenya e-mail: [email protected] under tree-crop/livestock interactions included upscaling of Melia volkensii and fruit trees (mangoes) and enhancing biodiversity conservation through support of beekeeping as a viable alternative livelihood. Participatory learning and action research (PLAR) was used for technology development and dissemination. Capacity building and training was a major component of upscaling of these best-bet technologies