Ecological Impacts of Prosopis Invasion in Riverine Forests of Kenya
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ECOLOGICAL IMPACTS OF PROSOPIS INVASION IN RIVERINE FORESTS OF KENYA Gabriel Mukuria Muturi III Thesis committee Thesis supervisor Prof. dr. ir. G.M.J. Mohren Professor of Forest Ecology and Forest Management, Wageningen University Thesis co-supervisors Dr.ir. L. Poorter Associate professor, Forest Ecology and Forest Management Group Wageningen University Dr. B.N. Kigomo Deputy Director, Research and Development Kenya forestry Research Institute Other members Prof.dr. M. Herold, Wageningen University Prof.dr. M. van Kleunen, University of Konstanz Prof.dr. T.M. Kuyper, Wageningen University Dr. W.F. de Boer, Wageningen University This research was conducted under the auspices of the C.T. de Wit Graduate School of Production Ecology and Resource Conservation (PE&RC) IV ECOLOGICAL IMPACTS OF PROSOPIS INVASION IN RIVERINE FORESTS OF KENYA Gabriel Mukuria Muturi Thesis submitted in fulfillment of the requirements for degree of doctor at Wageningen University by the authority of Rector Magnificus Prof.dr. M.J. Kropff in the presence of the Thesis Committee appointed by the Academic Board to be defended in public on Wednesday 5 September 2012 at 1.30 p.m. in the Aula. V Gabriel Mukuria Muturi Ecological impacts of Prosopis invasion in Riverine forests of Kenya Thesis, Wageningen University, Wageningen, NL (2012) With references, with summaries in English and Dutch ISBN: 978-94-6173-402-0 VI To my parents: Mr. John Muturi Njehu & Mrs Anastacia Wanja Muturi for taking me to school and encouragements in the academic life. To my wife Beatrice, and Children Winnie, Yvonne, John, Leah and Abigail for your understanding during my absence while pursuing this study. To all those who be inspired by this Thesis to advance academically. v vi Abstract Prosopis chilensis, Prosopis juliflora and Prosopis pallida were widely introduced in tropical drylands, where they have become naturalized and invasive. Invasion management is partially hampered by species misidentification because of morphological similarities between species and between species and their hybrids. In Kenya, species identification is even more challenging as several species were introduced within sites. Since biological invasions are invoked by either the susceptibility of a habitat to invasion (invasibility) or the invasive traits of a species (invasiveness), proper knowledge of these two factors is required for proper invasion management. In this dissertation, we used field, greenhouse and laboratory studies to: 1) evaluate invasibility of Turkwel riverine forest, and 2) invasiveness of Prosopis species by determining: 2) ecological impacts of Prosopis invasion, 3) underlying mechanisms for observed ecological impacts and 4) species composition and genetic diversity of Prosopis populations in Kenya. Invasibility was evaluated by comparing Prosopis occurrence between undisturbed invasion resistant forests with invasion susceptible forest gaps; ecological impacts of Prosopis invasion determined by comparing herbaceous species and tree regeneration among Acacia tortilis, Prosopis and a mixture of A. tortilis and Prosopis species canopies; underlying mechanisms of observed ecological impacts determined by assessing the impact of A. tortilis and Prosopis litter on A. tortilis and Prosopis species seed germination and seedling growth; and species composition and genetic diversity of Kenyan Prosopis populations determined by comparing Kenyan populations at Bamburi, Bura, Isiolo, Marigat, Taveta and Turkwel with P. chilensis, P. juliflora and P. pallida references for relatedness and genetic diversity. We found that both the forest and the forest gaps were equally susceptible to Prosopis invasion, suggesting that invasion was spontaneous and independent of the assumed habitat invasibility variation. The ecological impact study revealed reduction of ground vegetation cover, herbaceous species diversity and termination of A. tortilis regeneration by Prosopis. Termination of A. tortilis regeneration may be attributed to strong reduction of A. tortilis and Prosopis species seed germination by increasing Prosopis litter concentration in the vii soil in greenhouse studies. Relatedness studies between the Kenya Prosopis populations and the reference species revealed the clustering of P. chilensis with Taveta population, P. juliflora with Bura, Marigat and Isiolo and P. pallida with Bamburi. Results suggested that Turkwel population was a likely hybrid between P. chilensis and P. juliflora. Genetic diversity of populations at Bamburi, Isiolo and Marigat was higher than that of the reference species that they clustered with. Bamburi, Isiolo, Taveta and Turkwel populations revealed genetic uniqueness, as demonstrated by generation of private markers by a specific primer in each of the population. Prosopis juliflora and its hybrid occurred in areas currently classified as most invaded. Based on the results, we concluded that P. juliflora and its hybrid are the most aggressive invaders and that riverine forest invasion was invoked more by species invasiveness and not habitat invasibility. The invasiveness traits found in our study were: 1) Allelopathy of Prosopis litter which reduces herbaceous species ground vegetation cover and herbaceous species diversity; and inhibited regeneration of A. tortilis. 2) Unique site adaptations of introduced germplasm evident from the genetic differentiation of introduced germplasm depending on site, and 3) hybridization that was evident from a P. chilensis –P. juliflora hybrid in Turkwel. viii Contents Abstract vii Chapter 1 1 Introduction Chapter 2 17 Prediction of Prosopis Species invasion in Kenya using Geographical Information System (GIS) Techniques. Published in African Journal of Ecology 48:628-636 (2010) Gabriel M. Muturi, Godefridus M.J. Mohren and Jacob N. Kimani Chapter 3 33 Ecological impact of Prosopis species invasion in Turkwel Riverine Forest, Kenya (Submitted for publication) Gabriel M. Muturi, Lourens Poorter, Godefridus M.J. Mohren and Bernard N. Kigomo Chapter 4 59 Prosopis litter inhibits germination and stimulates seedling growth of dry woodland species (Submitted for publication) Gabriel M. Muturi, Lourens Poorter, Godefridus M.J. Mohren and Pauline Bala Chapter 5 79 Genetic diversity of Kenyan Prosopis populations based on Random Amplified Polymorphic DNA markers (Submitted for publication) Gabriel M. Muturi, Joseph M. Machua, Godefridus M.J. Mohren, Lourens Poorter, John M. Gicheru and Lorna W. Maina ix Chapter 6 99 Allometric equations for estimating biomass in naturally establishedProsopis stands in Kenya Published in Journal of Horticulture and Forestry 4:69-77 (2012) Gabriel M. Muturi, Jason G. Kariuki, Lourens Poorter and Godefridus M.J. Mohren Chapter 7 115 Synthesis References 129 Summary 147 Samenvatting 149 Acknowledgements 153 Short biography 157 List of publications 159 Education Certificate 161 x Chapter 1 Introduction Turkwel riverine forest near Lodwar 1 Chapter 1- Introduction Drylands degradation, desertification and mitigation Drylands are globally important, as they occupy 41% of the world land surface. The majority of the drylands are found in Africa and Asia (each accounting for 32% of the global total), whereas the remainder is distributed over, Australia, Europe, North America and South America (Table 1.1). According to Sombroek et al., (1982), drylands occupy 87% of the total land in Kenya (Table 1.1). Of the Kenyan drylands, 46% are classified as very arid, 22% as arid, 15% as semi-arid and 5% as semi humid to semi-arid (Sombroek et al., 1982). In both Africa and Asia, over 40% of the population lives in drylands, compared to 25 – 30% in other continents (White and Nackoney 2003). In Kenya, drylands support 30% of human population and 70% of livestock and the bulk of wildlife that supports the tourism sector (GoK 2007). Pastoralism is the main economic activity in drylands, whereas agropastoralism is also practiced but has high incidents of crop failures (Darkoh 1998, Speranza et al., 2008 Eriksen and Lind 2009). As livestock depends on the natural range resource and crop failure is frequent, then local communities rely more on natural resources for livelihoods than on crop production (Eriksen and Lind 2009, Sietz et al., 2011). Because of overstocking, the range resource is often degraded. Resource degradation is further exacerbated by cyclic droughts (Darkoh 1998, UNEP 2000, Dregne 2002). Land degradation in drylands, also called desertification, leads to loss of biological productivity (Dregne 2002). Land degradation includes vegetation deterioration, soil erosion by wind and water, and salinization of water and soil. Because these environmental challenges are widespread (Darkoh 1998, Dregne 2002), the United Nations Convention to combat desertification was specifically dedicated to mitigate desertification. The introduction of drought tolerant multipurpose trees and shrubs has been an important tool to mitigate large-scale loss of ground vegetation cover in drylands. Trees from the genus Prosopis were extensively introduced in many tropical drylands (Pasiecznik et al., 2001), because they met the criteria of environmental stresses tolerance, fast growth, soil amelioration, fodder provision and fuelwood supply (Rosenschein et al., 1999, Pasiecznik et al., 2001, Gallaher and Merlin 2010). Prosopis species are now naturalized in many introduced regions where they have become invasive (Zimmerman 1991, Pasiecznik et al., 2001, Shiferaw et al., 2004, van Klinken et al, 2006, Mwangi and Swallow 2008). 2 Chapter