18.104.22.168 Invasive species: plants, pathogens, insect pests and grazing animals
Invasive species, including plants, insect pests and microbial pathogens, are increasingly being identified and noted as major threats to ecosystem integrity and individual species, including trees. In the USA, for example, 46% of all federally-listed threatened and endangered species are considered at risk primarily due to competition with or predation by invasive species, and interactions with other threat factors.
In the case of invasive plant species the main threat comes from ‘transformer’ plant species which have the capacity to invade natural or slightly disturbed forest associations, becoming the dominant canopy species and totally modifying and displacing entire ecosystems, with the loss of many of the existing tree and other species. In East Africa the introduced tropical American tree Prosopis juliflora is taking over large swathes of natural forest and woodlands, considerably negatively impacting on native tree populations (both species and genetic diversity) and also damaging local livelihoods in the process (e.g. Mwangi and Swallow 2005).
Since its introduction in the early 1900’s, including later plantings to drain swamps, the !ustralasian tree
Melaleuca quinquenervia has invaded up to 200,000 ha in South Florida (USA). In the process M. quinquenervia has transformed various ecosystems in the Florida everglades and causing major environmental and economic damage (Carter-Finn et al. 2006). Even minor climatic changes can seemingly result in native tree species becoming more invasive, spreading into neighbouring regions and dramatically changing the forest dynamics, structure and species composition, e.g. sweet pittosporum (Pittosporum undulatum) and coast tea-tree (Leptospermum laevigatum) in south-eastern Australia. This is likely to be a portent of future developments and challenges for in situ FGR management with predicted more extreme climatic changes favouring disturbance-adapted pioneer and early secondary tree species.
Island ecosystems are especially vulnerable to invasives: in a just a few decades African tulip tree (Spathodea campanulata), introduced as an ornamental, has taken over large areas of secondary and primary rainforest, and abandoned agricultural fields, in Fiji and threatens to become a major invasive tree in many Pacific Islands, including Australia and Papua New Guinea. The tropical American velvet tree (Miconia calvescens) has become one of the world’s most invasive species and has completely taken over more than a quarter of rainforest in Tahiti, French Polynesia. The spread and impacts of invasives are frequently exacerbated by climate change or other major environmental disturbances. In the Southwestern Pacific, excessive opening of forest canopy due to intensive logging, coupled with major cyclones has greatly favoured the spread of the light-loving Merremia peltata. This native vine has now taken over large swathes of Pacific Islands’ forest ecosystems, thickly draping all trees and shrubs, and maintaining these communities in a state of arrested natural succession in Samoa and Vanuatu.
There are many well-documented cases in the northern Hemisphere where virulent introduced pathogenic fungi have wreaked havoc on economically and environmentally important tree species. One often cited example is the accidental introduction of Asian chestnut blight fungus (Cryphonectria parasitica) into USA early last century which wiped out almost the entire population of American chestnut (Castanea dentata) including more than three billion trees over 70 million hectares; this was accompanied by the extinction of other species dependent on chestnuts including ten species of moths. Ironically, early salvation logging may have removed some of the few American chestnut trees which showed resistance to the disease. Programs have been implemented to backcross surviving American chestnuts with blight resistant chestnuts from Asia for reintroduction into the former natural range of the American chestnut. Since its introduction into North America around 1930, Dutch elm disease (Ophiostoma) has killed more than 95% of American elms (Ulmus americana), millions of trees, and it is estimated that only 1 in 100,000 trees is naturally resistant. A few resistant individuals in Canada have recently been cloned (Shukla et al. 2012), and along with newly identified resistant diploids and triploids in USA (Whittemore and Olsen 2011) and interspecific hybrids derived from crossing with resistant Asian Ulmus species are paving the way for American elms to be reintroduced in North America. In 1967 a virulent strain of Dutch elm disease (Ophiostoma novo-ulmi) introduced into the UK wiped out most of the elm (Ulmus procera) trees, although they often survive as suckers and in hedgerows, in UK and continental Europe (http://en.wikipedia.org/wiki/Dutch_elm_disease; accessed November 2012). Various selection and breeding programs with Ulmus, including development of interspecific hybrids, have produced clones which are resistant to the fungus in Europe. These two examples for elms in USA and Europe are illustrations of the benefits of retaining genetic diversity in tree species in order to deal with introduced exotic diseases. Various pathogenic diseases, many only identified or found over the past ten years, are now threatening important tree species in the United Kingdom ( http://www.forestry.gov.uk/ accessed November 2012) including Aesculus hippocastanum (horse chestnut) – a new bacterial bleeding canker (Pseudomonas syringae pathovar aesculi) which was first detected around 2002, and now afflicts 70% of trees and likely to eventually kill them; Fraxinus species (ash) – chalara dieback caused by the introduced fungus (Hymenoscyphus pseudoalbidus), a serious disease first identified in 2012 and often resulting in tree death and spreading throughout Europe; and Quercus species (oaks) -recently a new disease, Acute Oak Decline, of bacterial origin which threatens to wipe out oaks in the UK.
In the past there have been fewer reported outbreaks of exotic pathogens causing major damage in natural and planted forests in the tropics and Southern Hemisphere, but the situation seems to have been changing over the past decade, perhaps as a result of increased movement of goods and people with more opportunities for disease to be spread, and accelerated by climate change and other environmental disturbances. Poplar rust was one of the first major exotic tree diseases to be reported from the Southern Hemisphere. Two species of poplar rust (Melampsora medusae and M. larici-populina) appeared in Australia in 1972-73 and rapidly spread in eastern Australia and across the Tasman Sea to New Zealand and devastating poplar plantations. However, considerable genetic variation in resistance to poplar rusts has been found between poplar species and clones (and alternate conifer hosts) and disease impacts can be managed by planting mixtures of more resistant clones. Selection for poplar rust resistance has been complicated by the appearance of different races. Another rust fungus (Atelocauda digitata), and other fungal pathogens, is a major concern for the productivity of the more than one million hectares of Acacia plantations in Asia, mainly A. mangium and A. auriculiformis and their hybrids and A. crassicarpa (See Old et al. 2000) Nevertheless there appears to be considerable variation between different Acacia provenances in susceptibility to disease, indicating a potential for selection of resistant genotypes and underscoring the importance of genetic diversity in dealing with forestry diseases of economic importance. A new disease to California, pine pitch canker, caused by the fungus Fusarium circinatum, has become established along the coast, having a devastating effect on all three mainland populations.
The native mainland California (USA) stands of radiata pine (Pinus radiata) are being devastated by the introduced pine pitch canker (Fusarium circinatum) with more than 90% of the trees likely to succumb to the disease (Devey et al., 1999). In the Republic of South Africa, pitch canker has recently been isolated from P. radiata (Coutinho et al. 2007), and seriously threatens the future of the pine plantation industry in
South Africa, comprising 670,000 ha and half the country’s wood and fibre assets/ Pitch canker has also
been recently identified on Pinus species in Colombia (Steenkamp et al.). In southern Africa and Colombia, and in other parts of the world where pitch canker has spread to (perhaps originally from Mexico) there will be a need to alter management practices, but also to change to more pitch canker-resistant Pinus species and provenances, such as to P. tecunumanii from low-elevation sources and P. maximinoi in Colombia. In 2010 a new pathogen myrtle rust or guava rust (Puccinia psidii), originating in South America, was detected in New South Wales that could fundamentally alter !ustralia’s forest ecology/ There are more than 2000 plant species in the family Myrtaceae, Australia's dominant plant family, including eucalypts, and most have the potential to become infected to some degree by Puccinia (Morin et al. 2012). Myrtle rust will likely alter the composition, function and diversity of many of !ustralia’s eucalypt-dominated forest and woodland ecosystems and impact severely on forest industries. Doran et al. (2012) have recently identified resistance to myrtle rust in one family of lemon myrtle (Backhousia citriodora) an economically-important essential oil producing plant through evaluation of a comprehensive provenance/ family/clone trial. These authors have recommended further germplasm collections and evaluations of this seed source, once again illustrating the importance of genetic diversity, its conservation in native stands, and provenance/family trials in combating threats from pathogens, especially newly introduced strains and species. Because pathogens are continuously evolving, a combination of management measures is needed to deal with forest pathogens including deployment of diverse resistant genetic materials and continuing breeding programs with access to genetic diversity. Other successful examples of breeding for pathogen resistance include radiata pine (Pinus radiata) for resistance to red band needle blight (Mycosphaerella pini) in New Zealand (Carson 1990) and western white pine (Pinus monticola) for resistance to white pine blister rust (Cronartium ribicola) in North America (Sniezko 2006).
As part of the Global Forest Resources Assessment 2005 (FRA 2005), countries reported on area affected by insect pests, diseases and other disturbances, and this information was used to undertake a global review of forest pests and diseases (FAO 2009). This review revealed major and increasing threats to forests from insect pests, both native and exotic. Some examples of how exotic pests threaten FGR, and the economic and environmental values of forests are discussed below, and are mainly derived from the FAO review.
The invasive European wood wasp (Sirex noctilio) has affected thousands of hectares of plantation pine forests around the globe including South Africa, South America and Australia, and is continuing to spread and is now threatening native pine and Douglas fir in North America. The leucaena psyllid (Heteropsylla cubana) is a significant pest of Leucaena leucocephala, a fast-growing multipurpose tree legume native to Mexico and Central America that has been widely planted throughout the tropics. In the mid-1980s, this insect spread rapidly across the Asia and the Pacific region (FAO, 2001); the spread of the psyllid was especially rapid as most leucaena plantings consisted of a very narrow, near identical genetic base. The Asian longhorned beetle (Anoplophora glabripennis) has increased in range in Chinese plantation forests as a result of widespread planting of susceptible poplar hybrids (EPPO, 1999). In China more than 200 million infested trees have been removed to control outbreaks of the Asian longhorned beetle, and authorities in USA and Canada have implemented emergency control measures anytime the pest has been detected. Strains of black poplar (Populus nigra) resistant to attack by the Asian longhorned beetle have been developed, through inserting a Cry1Ac gene from Bacillus thuringiensis, in China (Hu et al., 2001). Around 1986 the cypress aphid (Cinara cupressi) reached Malawi, and soon spread to Kenya where it rapidly caused major damage to Cupressus lusitanica (cypress) plantations which constituted half of Kenya’s plantation estate. The cypress aphid killed a total of USD27.5 million worth of trees in 1991 and was causing a loss in annual growth of around USD9 million per year (Murphy et al. 1996). This is one example, of many, of the perils and risks of plantation and farm forestry becoming too reliant on a single exotic species, especially when grown in monocultures, cf. planting more diverse polycultures. In Malawi the cypress aphid also attacks and kills the highly endangered conifer and national tree Widdringtonia nodiflora (Bayliss et al. 2007), but genetic resistance has yet to be found. Mountain pine beetle (Dendroctonus ponderosae) is a bark beetle indigenous to western North America that primarily feeds on lodgepole pine (Pinus contorta var. latifolia), that can erupt into large-scale outbreaks and cause significant losses of mature healthy stands. A devastating outbreak, initiated in the 1990s, has affected over 14 million hectares of forest land in western Canada (Nealis and Peter 2008) killing 50% of the standing volume in British Columbia. Increased warming associated with climate change is enabling the beetle to expand its range, including into Alberta in 2006, and may eventually cause large scale destruction to jackpine (Pinus banksiana) in boreal forest (Cullingham et al. 2011). The extent to which jackpine might show genetic resistance to mountain pine beetle is unknown, but natural hybrids with lodgepine are expected to display some resistance. The blue gum chalcid (Leptocybe invasa) is a relatively new threat to planted eucalypt forests in Africa, reported first from Kenya in 2002 and from South Africa in 2007, and this pest has also been reported in Asia and the Pacific, Europe and the Near East. The FAO review (2009) identified major insect pests of trees introduced into African continent in the past decade, including Cinara pinivora in Malawi, Coniothyrium zuluense in Ethiopia, Thaumastocoris peregrinus and Coryphodema tristis in South Africa and Gonometa podocarpi in the United Republic of Tanzania, and noted that these insect pests all pose threats to adjacent countries. The severity and frequency of insect pest outbreaks is projected to increase in concert with extreme climatic factors. China has already reported increased forest pest outbreaks in 2009 following a major snowstorm in South China, and the severe widespread drought of 2008 (China p 18-19).
Grazing animals, especially introduced goats and rats have wrought havoc on tree vegetation in many parts of the globe, especially on island communities. Radiata pine (Pinus radiata) is amongst the most important plantation forestry trees species in the world, but the unique island population on Guadalupe Island (Mexico) is now highly threatened with surviving trees very old, and predation by goats removing any regeneration (Spencer et al. 1999). Whilst the Guadalupe provenance is secured through ex situ conservation efforts, a loss of the tree in its natural habitats would exclude continued evolution and adaptation in the environment that has resulted in highly drought tolerant germplasm.
By 1945 goat predation on Three Kings Island of the north coast of New Zealand, had reduced the entire population of Three Kings kaikomako (Pennantia baylisiana) to one individual female tree incapable of sexually reproducing itself. Treatment of latent pollen with hormones by researchers in 1985 induced some seed including a self-fertile individual and the future of this species has now been secured. In French Polynesia, rats have prevented the natural regeneration of Eastern Polynesian sandalwood (Santalum insulare) by eating more than 99% of fruits before ripening (Meyer and Butaud 2009).
Given the major and increasing threats posed to FGR from invasive species (animals, plants and microorganisms) a key strategic priority is to promote national assessments of invasive alien species, networking and collaboration among concerned countries and IPCC and research to avoid their further spread.
22.214.171.124 Unsustainable harvesting and use
Many country reports have detailed over-exploitation and unsustainable harvesting threats to FGR. Overharvesting by itself rarely leads to extinction, but can seriously erode genetic diversity and recovery can be very slow for species which occur naturally at low frequency. However, for narrowly distributed and naturally rare species, overharvesting can directly lead to or threaten extinction. In China, Sichuan thuja (Thuja sutchuenensis), a critically endangered narrowly distributed endemic tree in Chongqing Municipality was driven to the brink of extinction from overharvesting for its precious scented wood and only rediscovered in 1999 and accorded protection. Overharvesting usually involves highly valuable species such as ebonies, sandalwoods, agarwoods and frankincense, but in areas with high population pressure and poverty, overharvesting may be associated with lower value products such as fuelwood and charcoal. Even an activity as seemingly innocuous as harvesting trees for Christmas trees may threaten FGR, e.g. in Guatemala, uncontrolled cutting of pinabete (Abies guatemalensis) branches for use as Christmas trees is reducing the regenerative capacity of the species which has now disappeared from some areas (Lopez 1999), while in Tonga, harvesting of ‘ahi sandalwood (Santalum yasi) saplings for Christmas trees is limiting recruitment and one of the major threats to the species (Tuisese et al. 2000).
One remedy to overharvesting can be the greater involvement of indigenous and local communities in management of the forest and their FGR, especially if this is backed by appropriate technical support. Technical support may include application of improved silvicultural practices to ensure sustainable production of desired products and regeneration of preferred species. There is also need for better legislative protection, including implementation and monitoring of the legislation, and development of alternative sources of wood, NWFPs and AFTPs, such as through highly productive plantation systems and improved agroforestry systems.
126.96.36.199 Mixing of gene pools and hybridization
A major risk to FGR conservation and utilization is the uncontrolled and undocumented mixing of genepools of forest tree species. This can occur at the within-species levels whereby genetically diversified local populations, which may possess valuable attributes, interbreed with non-local germplasm introduced for forest plantation establishment. Hybridization of local and introduced gene pools may reduce local adaptation in subsequent tree generations (Millar and Libby 1989; Palmberg-Lerche 1999). Mixing of genepools can also inadvertently lead to incorporation of undesirable genes, resulting in a diminished economic value for production forests, and vastly complicating and increasing the costs of tree breeding in cases where breeders may need to ‘unscramble the omelette’/
Interbreeding can also occur when formerly allopatric related species are brought together. If the taxa are not fully reproductively isolated and share the same flowering times and pollinators, then hybridization is likely, and if the resulting progeny are fertile, then the eventual outcome can be loss of a species through assimilation. Factors that threaten extinction by hybridisation, viz. habitat destruction, fragmentation, and species introductions are all increasing and often act synergistically (Rhymer 1996). The threats from ‘genetic pollution’, as specifically related to tree species, are discussed in Potts et al. (2001). Outbreeding depression from detrimental gene flow may reduce the fitness of a locally rare species making it vulnerable to extinction. Alternatively, pollen swamping may result in its loss of genetic integrity and it may become assimilated into the gene pool of the common species/ F!O’s International Poplar Commission’s Working
Party on Poplar and Willow Genetics, Conservation and Improvement has drawn attention to the fact that populations of some native poplar species were rapidly disappearing, because they spontaneously hybridize with cultivars and/or are being displaced by agriculture or other land uses. Natural stands of black poplar (Populus nigra) have almost disappeared in Europe and the situation for eastern cottonwood (P. deltoides) in North America has become very serious as a result of interbreeding.
Hybridisation, with or without introgression, can easily threaten a rare species’ existence (Rhymer and
Simberloff 1996). However, there are few documented examples in the literature for tree species, although presumably this has happened often during angiosperm evolution. The main cited example of this risk is Catalina Island mountain mahogany (Cercocarpus traskiae), a rare island endemic in California, USA, which has been reduced to about seven mature pure individuals and hybridises with the more abundant California mountain mahogany (C. betuloides; Rieseberg et al. 1993). In Fiji and Tonga, ahi sandalwood (Santalum yasi) hybridises freely with the introduced East Indian sandalwood (S. album) producing more vigorous F1 hybrid off-spring (Bulai and Nataniela 2005) which may eventually lead to the disappearance of pure yasi due to natural selective pressures and the commercial choices of smallholder sandalwood growers (Huish 2009).
There is an increased awareness amongst the forestry profession of the risks posed by hybridization on local gene pools. For example in order to protect the genetic integrity of the national tree of Lebanon (Lebanon cedar, Cedrus libani), a Ministerial decision has been taken which prohibits the import of Cedrus germplasm into Lebanon (Lebanon country report, p 19). In Australia, Barbour et al. (2008) have formulated a framework for managing the risk of gene flow from exotic Tasmanian blue gum (Eucalyptus globulus) plantations into native eucalypt populations in southern states which could serve as a useful model for other tree genera and species. The same authors ascertained there was a low risk of genetic pollution of large-scale planting of E. globulus for pulpwood plantations in southern Australia on other native eucalypts species in the same sub-genus Symphyomyrtus: however, there are clearly risks of loss of genetic integrity of native blue gum populations near plantations of different and limited variability, including to different sub-species such as eurabbie (E. globulus ssp. bicostata) in central Victoria.
1.4.2 Loss of ecosystems, species and intraspecific diversity
188.8.131.52 Loss of ecosystems
There are increasing threats to loss of FGR due to disappearance or significant modification of the ecosystems of which they are a constituent. For much of last century and until recently the major threat came from habitat conversion of forest to a different landuse, mainly an agricultural landuse. Examples of major habitat loss include Brazil’s !tlantic forest (2-5% of original), Ethiopia’s forests (most forest types reduced down to fractions of their former extent, including conversion to more open woodland formations), eastern !ustralia’s sub-tropical lowland rainforest (7.2% of original;
http://www.environment.gov.au/biodiversity/threatened/communities/pubs/101-listing-advice.pdf accessed December 2012), and many smaller countries, such as Haiti, and Samoa which have lost almost their entire lowland tropical forests. Forest ecosystems are breaking down and with dramatically changed function and structure, increasingly attributable to climate change, and associated extreme events such as uncontrolled wild fires, and alien invasive species. Isolated montane forest ecosystems, including cloud forests, in tropical and subtropical zones will be impacted differentially by climate change, especially as often these forests have a high proportion of unique endemic associated species, which may have no possibility of migrating to other climatically suitable habitats, e.g. tropical Central and South America and the Caribbean, East and Central Africa, the Philippines, Malaysia, Indonesia and Papua-New Guinea. Foster (2001) has described a scenario of complete replacement of many of the narrow altitude range cloud forests by lower altitude ecosystems, as well as the expulsion of peak residing cloud forests into extinction.
Other forest ecosystem changes are associated with changes to keystone animal species. In their recent literature review, Ripple and Beschta (2012) concluded that predation by large mammalian carnivores, notably sympatric grey wolves (Canis lupus) and bears (Ursus spp.), limit densities of large mammalian herbivores in boreal and temperate forests of North America and Eurasia with impacts on tree and shrub recruitment. The same authors have previously reported that cougars (Puma concolor) limit mule deer (Odocoileus hemionus) densities releasing woody plants from browsing and maintaining biodiversity in western North America (Ripple and Beschta 2008). Increasingly large carnivores, such as the tiger (Panthera tigris) and lions (P. leo), are being threatened in many parts reducing their natural ranges, and reductions in top-chain predator populations and changes to other keystone species, such as elephants (Loxodonta spp.) in Africa, will result in changes, both major and subtle, to forest and woodland ecosyems and alter the FGR contained in them.