A study by Woody Weeds MSc student Samson Aman Samson Kiswaga, published in Ecology and Evolution, reveals no relationship between spread of a range of alien plants and the seed germination, seedling survival or seedling communities of native trees in the Amani Botanical Garden (ABG) in Tanzania.

Samson, a student at Sokoine University of Agriculture, assessed the effects of up to 29 alien plant species that differed in the extent of spread on germination, abundance, and diversity of native tree species. He specifically sought to determine if the spread of planted alien species – such as the Mexican Rubber Tree (Castilla elastica Cerv), the African Oil Palm (Elaeis guineensis Jacq) and the coffee plant (Coffea canephora Pierre ex A.Froehner) – related to their per capita impact on germination, establishment, and survival of three native tree species and the abundance and species richness of seedlings growing in the understory of these alien species. He also studied possible mechanisms underlying the impact on native tree species, in particular the effect of allelochemistry.

Samson Aman Samson Kiswaga, lead author of the paper in Ecology and Evolution, said, “We found no relationship between the extent of spread of an alien tree species and their impact on seed germination, seedling survival, and seedling communities of native trees in their understory, and no indication that allelochemicals consistently explain their effects on recruitment of the studied species.

“These results suggest that extent of spread cannot be used as a proxy for impact. Hence, managers should continue assessing both the spread and the impact of alien species when prioritizing alien species for management.”

The ABG was established in 1902 and is one of the oldest botanic gardens in Africa, with more than 1,000 species of plants from all over the world, many of which were planted within two decades following the establishment of ABG. The native tree species which formed part of the study were Funtumia africana (Benth.) Stapf, Macaranga capensis (Baill.) Sim and Isoberlinia scheffleri (Harms) Greenway.

In these experimental studies, we worked with tree species belonging to different stages of succession, but the germination rate of M. capensis and I. scheffleri was very low, which, in addition to the small number of species sown, makes it impossible to generalize the results.

It is unclear why the germination rate of the untreated seeds was low, but M. capensis seeds are eaten by birds and passage of the gut may stimulate germination, while I. scheffleri seeds swell and produce mucus when soaked in water, which results in very rapid germination, and the experimental conditions in the Petri dish study may have been suboptimal for these species. However, the seedlings of 50 native species recorded under alien trees in the ABG allowed us to make this comparison and the results indicate that there was no difference in the effect of the extent of spread of alien plant species on seedling establishment among native tree species of different successional stages.

One of the most striking findings of the study, the scientists say, was the difference in the relationships between seedling abundance and species richness of alien and native species. The positive relationship between native seedling species richness and abundance appears typical for “normal” ecological systems: With increasing sample size, the number of detected species increases too.

Samson Aman Samson Kiswaga concluded that more multispecies studies that assess patterns in impacts are needed, because management should target alien species, such as Psidium guajava, that both spread rapidly and cause major impact on native species, biodiversity, and ecosystem functioning.

Full paper reference

Kiswaga, S.A.S., Mbwambo, J.R., Shirima, D., Mndolwa, A.S., Schaffner, U., Eschen, R., ‘More widespread alien tree species do not have larger impacts on regeneration of native tree species in a tropical forest reserve’, 12 April 2020, Ecology and Evolution, DOI: 10.1002/ece3.6256

Reblogged from https://www.cde.unibe.ch/ Picture by Hanspeter Liniger/CDE

Invasive alien species are threatening large tracts of rangeland and causing biodiversity loss in Eastern Africa. An international team of scientists, including of CDE, are now warning that woody weeds like Prosopis juliflora risk invading vast new areas. They recommend a coordinated management plan to curb their spread.

Lead authors of a new study in Ecosphere, Sandra Eckert, Centre for Development and Environment, University of Bern, and CABI’s Urs Schaffner, say that unless steps are taken to control Prosopis juliflora and Lantana camara it could spread to unreached areas of Kenya, Ethiopia and Tanzania.

Prosopis juliflora and L. camara are both considered to be amongst the world’s 100 worst invasive species with the former being introduced in the 1970s and 1980s as part of dryland reforestation programmes and the latter in the 19th century as an ornamental plant. However, the scientists, including those from the University of Nairobi, Sokoine University of Agriculture in Morogoro, and CDE’s partner institution, the Water and Land Resource Centre in Addis Ababa, suggest that these noxious weeds – exacerbated by climate change – will seriously impact upon grazing land and drive out native flora and fauna if left unmanaged.

“Lowland regions are seriously threatened”

Sandra Eckert said, “The semi-arid to arid regions of Ethiopia and Kenya, essentially all areas below about 1600m, are either currently or at risk of being invaded by P. julifora. It has also started to invade the north of Tanzania and could even affect neighbouring regions such as Sudan and South Sudan.”

A previous study co-authored by Sandra Eckert and Urs Schaffner showed that P. juliflora can spread very rapidly; in the Afar region in Ethiopia, this weed has invaded 1.2 million ha of rangeland, shrubland and crop fields within 35 years.

Sandra Eckert added, “Our data clearly show that the whole Afar and large parts of Somali region are suitable for invasion by P. juliflora, so these lowland regions, once known for their grasslands and pastoralist communities, are seriously threatened.

“In respect of L. camara, the Ethiopian and Kenyan highlands as well as northern and central Tanzania have the highest risk of invasion. Our model suggests that Zambia could also be a target in the future.”

The researchers believe that a “spatially explicit management strategy that ensures concerted communication and management across national and subnational borders” is needed to nip the woody weeds scourge in the bud.
They have been investigating the spread of invasive woody species in Eastern Africa since 2015, in a project funded by the Swiss Programme for Research on Global Issues for Development (r4d).

“Early detection and rapid response will be key”

Urs Schaffner said, “Our results reveal that ‘early detection and rapid response’, i.e. the identification of the first trees in new areas and a rapid removal of these trees, will be key for slowing down the fast spread of P. juliflora. Surveillance of areas still free of P. juliflora is warranted not only within the boundaries of the current climate, but also in areas which will become suitable in the context of climate change. In the case of P. juliflora, the planting of saplings and dispersal of seeds should be prohibited in areas bordering the current range of suitable habitats – for example south-western Tanzania and northern Zambia.”

“In respect of L. camara our findings indicate that this species has already colonized a significant part of its ecological niche in Eastern Africa and that prevention measures may be less warranted than with P. julifora. Rather, management should be targeted at preventing L. Camara from building up high local densities, as currently observed in areas bordering Lake Victoria. We recommend that management is in line with more detailed maps of the invasive species’ current distribution – ideally with information on local abundance or cover.”

Planting of alien tree species has been promoted by national and international organisations, including NGOs, with the aim of addressing social and environmental problems in Eastern Africa. These species are promoted for agroforestry purposes, to re-green degraded landscapes or to alleviate fuelwood shortages. Some of the promoted trees have spread widely and now cause serious environmental problems that impact human wellbeing, including losses of grazing land, reduced water availability and increased land management costs.

Despite the well-known unintended impacts of such actions in the past, planting of alien tree species continues to be promoted in developing countries, while leaving these countries to deal with the unintended consequences. Project proposals and funding calls should therefore be critically reviewed by experts from diverse backgrounds to avoid promoting any alien tree species that could have serious impacts. A new policy brief provides recommendations to support decision making about funding for projects that aim to introduce new, or promote established alien woody plant species.

The policy brief can be downloaded here.

A meeting of national institutions involved in the development of the Kenyan National Strategy for Prosopis juliflora was held on 24th- 29th November 2019. The participating institutions were the Kenya Forestry Research Institute (KEFRI), Kenya Forest Service (KFS), Ministry of Agriculture (MOE), National Environment Management Authority (NEMA), the Council of Governors, Kenya Wildlife Service (KWS) and CABI Kenya.

The objectives of the strategy will be to 1) enhance awareness of Prosopis juliflora invasion and impacts, 2) detect and prevent new Prosopis juliflora invasions, 3) manage naturally established Prosopis juliflora invasions and restore degraded landscapes, and 4) enhance research and development on Prosopis juliflora in Kenya.

Last year, the Woody Weeds project has presented a summary of its key results to the director of KEFRI, who has received the insights about the impacts of Prosopis on livelihoods and environment with great interest. More recently, several partners of the Woody Weeds project have been awarded a Transformation Accelerating Grant by the r4d Programme, which allows the project to participate in the development of the Kenyan National Prosopis Strategy.

A next meeting to continue working on the draft strategy is planned for the end of January 2020.

In October 2019, a group of rangers and community members have started removing several hundred trees of the highly invasive Prosopis juliflora from the land of the Ruko Conservancy in Baringo County, Kenya. The Woody Weeds project supports Ruko Conservancy in implementing an ‘Early detection and rapid response’ management strategy against Prosopis. In a meeting earlier this year, the board members of the Ruko Conservancy decided that Prosopis should be considered as a threat to the peace-building process in the region and will negatively affect the Conservancy’s efforts to integrate sustainable livestock management with wildlife conservation.

Last week, Gasto Mushi successfully defended his MSc thesis at Sokoine University of Agriculture. Gasto’s research was about local communities’ perceptions on Lantana camara focusing on its origin, pathways, socio-economic impacts and management responses in East Usambara, Tanzania. The work was supervised by Prof. George Kajmenbe and Dr Charles Kilawe. Gasto presented his work the same week at the annual meeting of the Tanzanian Wildlife Research Institute (TAWIRI). Congratulations, Gasto, on these achievements!

The Woody Weeds project tests different sustainable land management (SLM) practices for management of the invasive species Prosopis juliflora and Lantana camara. Among the practices is removal of Prosopis and establishment of grassland. In Kahe, near the town of Moshi in Tanzania, members of the Local Implementation Group (LIG) selected to test removal of Prosopis followed by intensive agriculture as SLM practice.

The test implementation is being done on a single area that was heavily infested with Prosopis, on which six plots were laid out that will each be cultivated by one of six farmers. Each farmer cleared the Prosopis on her or his piece of land and uprooted stumps from the field, followed by ploughing and land preparation for growing crops. A single well was dug from which all farms will be irrigated. All farming activities are closely monitored by two master students from Sokoine University of Agriculture, Kosei Masaka and Omega Kaaya, to document practice’s environmental and economic sustainability and its effectiveness in Prosopis control.

We congratulate Dr Maria Loreto Castillo with the successful defense of her PhD, which was carried out in the Woody Weeds project at the Centre for Invasion Biology in South Africa. The thesis, entitled “Processes and drivers of Prosopis invasions in Eastern Africa”, was supervised by Jaco Le Roux, Brian van Wilgen and Urs Schaffner.

Maria’s thesis investigated the ecological and evolutionary processes underlying Prosopis invasions in Eastern Africa. She first tackled age-old issues around the problematic taxonomy of the genus using molecular genetic approaches. This part of her work illustrated that many Prosopis species can hybridise freely and that existing taxonomic issues are unlikely to be resolved using genetic data. Maria also found invasive Prosopis juliflora in Kenya to, not only have flexible ecological strategies (i.e. high phenotypic plasticity), but also to have undergone rapid post-introduction evolution. Her comparative research between invasions in Ethiopia and Kenya also found demographic processes, like dispersal, to differ markedly between these two countries.

In July 2019, Woody Weeds project partners participated in the Board Meeting of the RUKO conservancy on the Eastern shore of Lake Baringo, Kenya. This conservancy was formed some ten years ago to bring an end to the conflict between the Pokot and Il Chamus communities and to reintroduce the rare Rothschild giraffe in the region. Based on the information provided by the Woody weeds project partners, the Board decided to initiate management of the recently established alien tree Prosopis juliflora, which is considered as a threat to the peace-building process in the region.

Prosopis has recently started invading the conservancy from the south and RUKO board members were shown the densely invaded areas south of Lake Baringo. The leaders recognised the likelihood that RUKO is under severe threat from the invasion and that, if no urgent action is undertaken soon, Prosopis will possibly invade a large part of the conservancy within the next few years. The decision was therefore taken to map Prosopis juliflora on the territory of the conservancy, which has been done by the conservancy rangers and Woody Weeds partners, with the aim to remove the trees. In the near future, tools will be bought and the trees removed.