Graziosi et al. (full citation at the end of the blog) point out that trees are crucial for Africa’s future. Eight hundred of the 4,500–6,000 indigenous tree species provide significant food. As elsewhere, trees provide wood and other extractive resources essential for economic growth. They also support biodiversity and mitigate current and impending climatic variations. Africa– especially the Sub-Saharan countries – is already considered highly vulnerable to climate change.
According to Graziosi et al., the cumulative economic impact of all invasive species in Africa is expected to exceed $1.2 billion per year. The total invasion cost as a proportion of GDP for many African countries is among the highest in the world. This raises the stakes for developing locally appropriate management strategies across the continent.
Responding effectively to this threat is hampered by gaps in data as well as some countries’ limited capacity for biosecurity. Graziosi et al. say that improved knowledge of taxonomy, distribution, and damage caused by these organisms is essential. Such knoledge will be crucial to develop continent-wide strategies to manage this emergency and to enhance capacity for country-level interventions.
Native and alien pests. Indigenous and plantation trees
Africa’s trees and their services are threatened by both native pests and accelerating introductions of pests and diseases from elsewhere. Long-established and new invaders increasingly affect planted forests of exotic eucalypts, pines, and Australian acacias, as well as important indigenous trees. Graziosi et al. note that the U.N. Food and Agriculture Organization (FAO) in an annex to a report issued in 2009 recorded about100 species of forest pests affecting trees in planted and natural forests across Africa. Half are native insects and pathogens, a third are alien; about 15% are of unknown origin. Considering all pests, broadleaf trees (predominantly native) are most affected.
The result is damage from the local – e.g., to rural livelihoods – to the continental – e.g., to economic development and biological diversity across Africa. Moreover, pests exacerbate widespread loss of forest cover. Overall, African forests are shrinking at the rate of almost 0.5% annually. This deforestation is affecting particularly natural forests; planted forests are actually growing 1.3% annually.
Exotic plantation trees face severe threats. More than 47 native and 19 non-indigenous defoliators, sap-feeders, wood- and shoot-borers attack plantations of Acacia spp., Eucalyptus spp., Pinus spp., and teak (Tectona grandis). About 90% of pathogens of plantation forestry are either non-indigenous or of uncertain origin. Eucalyptus alone are severely damaged by 15 species of pathogens. These organisms are listed in Tables 1 and 2.
Numerous native insect species, known as pests of indigenous trees, have reportedly widened their host range and now damage exotic trees too. Some introduced insects appear to pose significant threats to native tree species. One example is the Cypress aphid Cinara cupressi, which is attacking both exotic cypress plantations and the native African cedar Juniperus procera. Some fungi in the family Botryosphaeriaceae are latent pathogens infecting a wide range of hosts including indigenous Acacia. Dieback of large baobab trees was recently reported from southern Africa. While various microorganisms are associated with these symptoms, the specific cause is still uncertain.
The risk currently appears to be particularly high in South Africa. The country’s flora is highly diverse and has a high level of endemism. In fact, South Africa is home to the Earth’s smallest floral kingdom, the Cape Floral Kingdom. It is also the apparent hot spot for pest introductions from overseas (see below). Phytophthora cinnamomi is attacking native Proteaceae in South Africa. According to Graziosi et al., an “incredible diversity” of Phytophthora taxa is present, portending threats to additional plant species. Other pathogens are attacking native conifers in the Podocarpus genus, Ekebergia capensis (Meliaceae), and Syzygium trees.
There is a clear pattern to further spread: pests first introduced to South Africa often spread. Examples include several insects and pathogens on Eucalyptus and the wood-boring pest of pine Sirex noctilio. This pattern is explained by two main factors. South Africa has a high capacity to detect introduced species. Also, there is an active plantation forestry sector that imports propagules. This offers opportunities for contaminating organisms to be introduced simultaneously.
Furthermore, as Graziosi et al. note, determining the geographic origin of significant proportion of pathogens is extremely difficult – an issue I will discuss in a separate blog based on a publication by primarily South African scientists. Some non-indigenous pathogens have been on the African continent for a long time. The Armillaria root rot pathogen apparently was introduced to South Africa with potted plants from Europe in the 1600s! They note also that many non-indigenous pathogens are probably already established on the continent but not yet detected due to the organisms’ cryptic nature and lagging detection abilities.
The future of African forests
African countries expect economic growth with associated increased trade with countries off-continent. The probable result will be to accelerate the rate of species introductions and spread. However, as climate change worsens, managers will find it increasingly difficult both to predict introduced species’ impact and to implement management programs.
This led Graziosi et al. to call for urgent improvements in plant biosecurity across the continent. They advocate improved coordination at regional and international levels. The list of needed actions is a familiar one: development and application of improved diagnostic tools, updated plant exchange regulations, and revised trade policies.
Graziosi et al. also call for development of effective control and management options. They suggest biocontrol, innovative silviculture practices, and selection of resistant trees. The good news is that African countries have already initiated programs to conserve tree germplasm and domesticate indigenous species, including establishment of field gene banks of high-priority indigenous trees. I have previously praised South African efforts, specifically reports here and here.
Mudada, Mapope, and Ngezimana (2022) describe the risk from introduced species to agriculture and human well-being in southern Africa beyond forestry. The region is already ravaged by food insecurities and hidden hunger. It would be devastated if the global average of crop loss due to plant diseases (10-16%) occurs there. They say these losses can be avoided with improved biosecurity mechanisms focused primarily on pest exclusion and plant quarantine regulations.
SOURCES
Graziosi, I. M. Tembo, J. Kuate, A. Muchugi. 2020 Pests and diseases of trees in Africa: A growing continental emergency. Plants People Planet DOI: 10.1002/ppp3.31
Mudada, N. Mapope, N., and Ngezimana, W. 2022 – The threat of transboundary plant pathogens to agricultural trade in Southern Africa: a perspective on Zimbabwe’s plant biosecurity – A review. Plant Pathology & Quarantine 12(1), 1–33, Doi 10.5943/ppq/12/1/1
Posted by Faith Campbell
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For a detailed discussion of the policies and practices that have allowed these pests to enter the United States and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm