A new study, published recently in Nature, has found that the native biodiversity of natural forests largely buffers the severity of non-native tree invasions.
The bad news, however, is that humans remain mostly responsible for introducing non-native tree species to an area in the first place – either intentionally or accidentally.
These are two of the key findings from a global study to determine the relative importance of human activity, environmental conditions, and biological diversity as drivers of tree invasions worldwide. The study, titled “Native diversity buffers against severity of non-native tree invasions” was published in the journal Nature on Wednesday, 23 August 2023.
This is because of their size, long life span and important role in forestry, foraging, city landscaping and reforestation, as well as carbon sequestration and climate regulation. Yet invasion biologists have long been struggling to identify the ecological mechanisms driving the invasion success of a small portion of non-native tree species.
Their findings support the biotic resistance hypothesis, which holds that greater diversity in the native community will fill the ecological niches and reduce available resources, thereby limiting non-native species to take up niche spaces.
The prominent role of human activities, however, came as a surprise: “Our findings suggest that human activity may overwhelm ecological drivers of invasions and even reduce the influence of ecological processes,” he warns.
Repeated human introductions of plant species, especially close to ports and airports, play an important role in the initial introduction process. The severity of the invasion, however, is predominantly a result of the intrinsic diversity of the native community.
It is therefore important to conserve natural forests to maintain high native tree diversity, they write in the paper.
Furthermore, because many tree species are introduced purposefully for forestry or to support local livelihoods, they recommend that local stakeholders are included when making decisions about how best to benefit from these managed forests.
Some of the other findings include:
The most frequent non-native trees in the GFBI database are Black Locust (Robinia pseudoacacia), Scots pine (Pinus sylvestris), Osage orange (Maclura pomifera), Norway spruce (Picea Abies) and Tree of Heaven (Ailanthus altissima).
The regions with the greatest likelihood of being invaded are North America, Europe and East Asia.
In environments experiencing cold or hot temperature extremes, the non-native trees were more similar to the native community.
Moderate environments allowed a wider range of species to survive.
Alien invasive species a massive threat to biodiversity
The severe global threat posed by invasive alien species is underappreciated, underestimated, and often unacknowledged. According to a major new report by the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES), more than 37,000 alien species have been introduced by many human activities to regions and biomes around the world. This conservative estimate is now rising at unprecedented rates. More than 3,500 of these are harmful invasive alien species – seriously threatening nature, nature's contributions to people and good quality of life. Too often ignored until it is too late, invasive alien species are a significant challenge to people in all regions and in every country.
Approved on Saturday in Bonn, Germany, by representatives of the 143 member States of IPBES, the Assessment Report on Invasive Alien Species and their Control finds that alongside dramatic changes to biodiversity and ecosystems, the global economic cost of invasive alien species exceeded $423 billion annually in 2019, with costs having at least quadrupled every decade since 1970.
In 2019, the IPBES Global Assessment Report found that invasive alien species are one of the five most important direct drivers of biodiversity loss – alongside changes in land and sea use, direct exploitation of species, climate change and pollution. On the basis of this finding, Governments tasked IPBES to provide the best available evidence and policy options to deal with the challenges of biological invasions. The resulting report was produced by 86 experts from 49 countries, working for more than four and a half years. It draws on more than 13,000 references, including very significant contributions from indigenous peoples and local communities, making it the most comprehensive assessment ever carried out of invasive alien species around the world.
"Invasive alien species are a major threat to biodiversity and can cause irreversible damage to nature, including local and global species extinctions, and also threaten human wellbeing," said Professor Helen Roy (United Kingdom), co-chair of the Assessment with Prof. Anibal Pauchard (Chile) and Prof. Peter Stoett (Canada).
The authors of the report emphasize that not all alien species become invasive – invasive alien species are the subset of alien species that are known to have become established and spread, which cause negative impacts on nature and often also on people. About 6% of alien plants; 22% of alien invertebrates; 14% of alien vertebrates; and 11% of alien microbes are known to be invasive, posing major risks to nature and to people. People with the greatest direct dependence on nature, such as indigenous peoples and local communities, are found to be at even greater risk. More than 2,300 invasive alien species are found on lands under the stewardship of indigenous peoples – threatening their quality of life and even cultural identities.
While many alien species were historically introduced on purpose for their perceived benefits to people, the IPBES report finds that the negative impacts of those that do become invasive are enormous for nature and people. "Invasive alien species have been a major factor in 60% and the only driver in 16% of global animal and plant extinctions that we have recorded, and at least 218 invasive alien species have been responsible for more than 1,200 local extinctions. In fact, 85% of the impacts of biological invasions on native species are negative," said Prof. Pauchard. Examples of such impacts include the ways that North American beavers (Castor canadensis) and Pacific Oysters (Magallana gigas) change ecosystems by transforming habitats – often with severe consequences for native species.
Nearly 80% of the documented impacts of invasive alien species on nature's contributions to people are also negative – especially through damage to food supplies – such as the impact of the European shore crab (Carcinus maenas) on commercial shellfish beds in New England and the damage caused by the Caribbean false mussel (Mytilopsis sallei) to locally important fishery resources in India.
Similarly, 85% of documented impacts negatively affect people's quality of life – for instance through health impacts, including diseases such as malaria, Zika and West Nile Fever, spread by invasive alien mosquito species like Aedes albopictus and Aedes aegyptii. Invasive alien species also damage livelihoods, for example in Lake Victoria where fisheries have declined due to the depletion of tilapia, as a result of the spread of water hyacinth (Pontederia crassipes), which is the world's most widespread terrestrial invasive alien species. Lantana (Lantana camara), a flowering shrub, and the black rat (Rattus rattus) are the second and third most widespread globally, with far-reaching impacts on people and nature.
"It would be an extremely costly mistake to regard biological invasions only as someone else's problem," said Pauchard. "Although the specific species that inflict damages vary from place to place, these are risks and challenges with global roots but very local impacts, facing people in every country, from all backgrounds and in every community – even Antarctica is being affected."
The report shows that 34% of the impacts of biological invasions were reported from the Americas, 31% from Europe and Central Asia, 25% from Asia and the Pacific and about 7% from Africa. Most negative impacts are reported on land (about 75%) – especially in forests, woodlands and cultivated areas – with considerably fewer reported in freshwater (14%) and marine (10%) habitats. Invasive alien species are most damaging on islands, with numbers of alien plants now exceeding the number of native plants on more than 25% of all islands.
"The future threat from invasive alien species is a major concern," said Prof. Roy. "37% of the 37,000 alien species known today have been reported since 1970 – largely caused by rising levels of global trade and human travel. Under 'business-as-usual' conditions, we project that total numbers of alien species will continue to increase in this way."
"But business-as-usual is actually unlikely," continues Roy. "With so many major drivers of change predicted to worsen, it is expected that the increase of invasive alien species and their negative impacts, are likely to be significantly greater. The accelerating global economy, intensified and expanded land- and sea-use change, as well as demographic changes are likely to lead to increases in invasive alien species worldwide. Even without the introduction of new alien species, already established alien species will continue to expand their ranges and spread to new countries and regions. Climate change will make the situation even worse." The report underscores that interactions between invasive alien species and other drivers of change will be likely to amplify their impacts – for example invasive alien plants can interact with climate change, often resulting in more intense and frequent fires, such as some of the devastating wildfires experienced recently around the world, releasing even more carbon dioxide into the atmosphere.
The IPBES experts point to the generally insufficient measures in place to tackle these challenges. While 80% of countries have targets related to managing invasive alien species in their national biodiversity plans, only 17% have national laws or regulations specifically addressing these issues. This also increases the risk of invasive alien species for neighbouring States. The report finds that 45% of all countries do not invest in the management of biological invasions.
On a more positive note, the report highlights that future biological invasions, invasive alien species, and their impacts, can be prevented through effective management and more integrated approaches. "The good news is that, for almost every context and situation, there are management tools, governance options and targeted actions that really work," said Prof. Pauchard. "Prevention is absolutely the best, most cost-effective option – but eradication, containment and control are also effective in specific contexts. Ecosystem restoration can also improve the results of management actions and increase the resistance of ecosystems to future invasive alien species. Indeed, managing invasive alien species can help to mitigate the negative effects of other drivers of change."
Prevention measures – such as border biosecurity and strictly enforced import controls – are identified by the report as having worked in many instances, such as the successes achieved in Australasia in reducing the spread of the brown marmorated stink bug (Halyomorpha halys). Preparedness, early detection and rapid response are shown to be effective at reducing rates of alien species establishment, and to be especially critical for marine and connected water systems. The PlantwisePlus programme, assisting smallholder farmers in Africa, Asia and Latin America is spotlighted by the report as a good example of the importance of general surveillance strategies to detect new alien species.
Eradication has been successful and cost-effective for some invasive alien species, especially when their populations are small and slow-spreading, in isolated ecosystems such as islands. Some examples of this are in French Polynesia where the black rat (Rattus rattus) and rabbit (Oryctolagus cuniculus) have been successfully eradicated. The report indicates that eradication of alien plants is more challenging due to the length of time that seeds may lie dormant in soil. The authors add that successful eradication programmes depend on, amongst other elements, the support and engagement of stakeholders and Indigenous Peoples and local communities.
When eradication is not possible for different reasons, invasive alien species can often be contained and controlled – especially in land-based and closed water systems, as well as in aquaculture – an example being the containment of the invasive alien Asian tunicate (Styela clava) in aqua-cultured blue mussels in Canada. Successful containment can be physical, chemical or biological – although the appropriateness and effectiveness of each option is dependent on local context. The use of biological control for invasive alien plants and invertebrates, such as introducing a rust fungus (Puccinia spegazzinii) to control bitter vine (Mikania micrantha) in the Asia-Pacific region, has been effective – with success in more than 60% of known cases.
"One of the most important messages from the report is that ambitious progress in tackling invasive alien species is achievable," said Prof. Stoett. "What is needed is a context-specific integrated approach, across and within countries and the various sectors involved in providing biosecurity, including trade and transportation; human and plant health; economic development and more. This will have far-reaching benefits for nature and people." Options explored in the report include considering coherent policies and codes of conduct across sectors and scales; commitment and resourcing; public awareness and engagement, such as citizen science campaigns like those promoting 'check, clean and dry'; open and interoperable information systems; filling knowledge gaps (the authors identify more than 40 areas where research is needed); as well as inclusive and fair governance.
"The immediate urgency of invasive alien species, with extensive and growing harm to nature and people, makes this report so valuable and timely," said Dr. Anne Larigauderie, the Executive Secretary of IPBES. "The Governments of the world agreed, in December last year, as part of the new Kunming-Montreal Global Biodiversity Framework, to reduce the introduction and establishment of priority invasive alien species by at least 50% by 2030. This is a vital, but also very ambitious commitment. The IPBES Invasive Alien Species Report provides the evidence, tools and options to help make this commitment more achievable."
Top 10 Most Widespread Invasive Alien Species Worldwide
Number of regions
Pontederia crassipes (water hyacinth)
Lantana camara (lantana)
Rattus rattus (black rat)
Leucaena leucocephala (leucaena)
Mus musculus (house mouse)
Rattus norvegicus (brown rat)
Ricinus communis (castor bean)
Ailanthus altissima (tree-of-heaven)
Robinia pseudoacacia (black locust)
Chromolaena odorata (Siam weed)
The number of regions where a species has been recorded and classified as invasive based on GRIIS (Pagad et al., 2022). Note this table only refers to the distribution of invasive alien species and not their impacts.
Celebrating International Day of Biological Diversity
Forestry South Africa (FSA) celebrates International Day of Biological Diversity (Monday 22 May 2023) with a video that emphasises the need to take action to conserve biological diversity for future generations and the role multifunctional landscapes can play...
The South African forestry landscape is a multifunctional environment, where grasslands, indigenous forests, wetlands, rivers and streams weave their way between the productive forestry compartments. These natural spaces provide important habitats and migratory routes for all kinds of species, some of which are endangered and others that are only found in South Africa (endemic). These natural corridors are important, enabling the free movement of species both within the forestry landscape and between conservation areas that neighbour it. When these unplanted natural areas are managed in a way that promotes biodiversity, they can play a really important role in the conservation of both individual species and whole ecosystems.
"Across the forestry landscape, there are over 170 000 hectares of grasslands, with their associated wetlands and pans, and over 60 000 hectares of indigenous forests, all of which are sustainably managed by forestry companies. Very few Agri-sector businesses would leave 30% of their landholdings unplanted, however, in forestry this is common practice and as a result, the forestry landscape is made up of a mosaic of planted compartments and natural spaces. FSA is intensely proud of the onus forestry companies place on being custodians of the natural spaces found within their landholdings and the role these are having in maintaining and preserving biodiversity." – FSA Head of Communication and Director of Research and Protection, Dr Ronald Heath.
The Forestry Sector provides the perfect example of how it's possible to incorporate environmental stewardship into a production landscape without compromising on productivity. This not only benefits biological diversity, but it also produces a patchwork landscape that provides all manner of recreational activities that benefit our mental and physical health. FSA commissioned this video in celebration of the International Day of Biological Diversity to encourage other land users to take a multifunctional landscape approach when planning their management activities. We hope this will inspire them to do their bit in conserving South Africa's precious biodiversity for future generations to come.
"We are only now realising the true extent of the impact human activity is having on our planet. The dramatic loss in global biodiversity combined with the changing weather patterns attributed to climate change, are a warning to us all that now is the time to act. Small changes make big differences and if we all take responsibility now, as individuals and companies, we still have time to reverse these worrying trends. We have gone past the point where biodiversity can be conserved in dedicated nature reserves alone, we need to adopt a multifunctional landscape approach where production landscapes are managed in a sustainable way that promotes environmental and social benefits alongside economic gains. The video illustrates this, showcasing the important role multifunctional landscapes play in the conservation of biological diversity and how adopting this approach can make a big difference", Dr Heath concludes.
*Video shot and edited by SA Forestry / Green Forest Films
Massive mangrove restoration project launched in Mozambique
Around 100 million mangrove trees to be planted; total project area 185,000 ha; 200,000 tons of CO2 to be offset annually...
The largest mangrove reforestation project in Africa has been launched by Mozambique’s Ministry of Sea, Inland Waters and Fisheries (MIMAIP) in partnership with Blue Forest, a UAE-based mangrove reforestation specialist.
The project will be implemented in the biodiversity-sensitive provinces of Sofala and Zambezia, spread across 185,000 hectares of mangrove forests. It is expected that between 50-100 million trees will be planted as part of this long-term partnership. This project will offset approximately 200,000 tons of CO emissions annually, equivalent to taking 50,000 cars off the road.
Mozambique has over 300,000 hectares of mangroves along its coast, which is one of the largest tracts of mangrove forest in Africa
The partners will utilize high resolution satellite imagery, LiDAR technology and remote sensing data to identify key ‘hot spots’ where the need for restoration is highest. Artificial Intelligence (AI) algorithms will then be used to decipher the satellite data and field measurement to customize the reforestation activities in an accurate, efficient and transparent manner.
The reforestation work will be carried out in collaboration with several stakeholders tackling the issue of mangrove forest restoration in Mozambique. Public institutions such as the National Directorate for Forrest (DINAF) and the National Fund for Sustainable Development (FNDS), as well as universities and NGOs will be engaged in this flagship campaign.
The project will be financed through carbon credits that will be generated through the reforestation and conservation activities over the 30-year period of this partnership. The proceeds will be shared between the local and national stakeholders as per the guidelines set by FNDS.
Xavier Munjovo, Permanent Secretary of MIMAIP, commented: “Mozambique has over 300,000 hectares of mangroves along its coast, which is one of the largest tracts of mangrove forest in Africa. We are delighted to partner with Blue Forest and to introduce innovative technology in the way we map and restore our vital mangrove forests for generations to come.”
Vahid Fotuhi, Founder and CEO of Blue Forest, added: “Mozambique is a hugely strategic country when it comes to mangrove forests. We are thrilled to partner with MIMAIP and to work in coordination with all the public and private national and provincial institutions, as well as the local communities in Sofala and Zambezia on this historic project. Tens of thousands of people and endless marine life will benefit from this project.”
Mangroves are a rare ecosystem that support biodiversity and provide vital ecological services including coastal protection from floods and storm surges, water filtration, carbon sequestration and nursery grounds for thousands of species of fish and crustaceans.
Mangroves all around the world are under threat due to urban development and poor farming practices upstream which release excess sediment into rivers.
KZN forest needs your help
A local artist and environmental NPO are rallying efforts to conserve a rare and shrinking patch of indigenous mistbelt forest on the northern edge of Pietermaritzburg, KwaZulu-Natal, which is under threat from alien plant invasion.
Ferncliffe is the precious remnant of a biome that originally stretched over 2 000 hectares. This magical tangle of vegetation, situated right at the edge of the city’s urban sprawl, is blessed with high rainfall and is often swathed in mist.
Although Ferncliffe is small, it still contains an astonishing diversity of life, ranging from large mammals like bushpig and caracal, to unusual millipedes and amphibians, a species of carnivorous snail, and the enormous monkey-catching crowned eagle. It harbours unexpected, often secretive creatures that dwell in, and depend upon, the profusion of indigenous plants that grow there.
Unfortunately, a tide of alien plants is threatening the ecosystem's survival. This environmental degradation is an enormous problem. Now members of the public are invited to assist efforts to restore the biodiversity of this mist-drenched wonder.
To help fund the registered NPO’s vital work, local fine artist Connor Cullinan is producing a series of original art prints that are sold online as open editions via https://ferncliffe.org. Since 1991, Cullinan has participated in several solo and group exhibitions at a number of respected galleries - these include Obert Contemporary, Erdman Contemporary, Barnard, whatiftheworld and Daor Contemporary. His screenprints have been shown at the FNB Joburg Art Fair, Cape Town Art Fair and Turbine Art Fair. Outside of South Africa, he has showcased his work in Queretaro and Oaxaca in Mexico; his paintings and prints form part of the Nando's permanent collection and are on show in various countries; and his work is held in private collections in Europe and the United States.
His beautifully illustrated images in aid of Ferncliffe are based on the fauna and flora that can be found in the forest and on its fringes. The first two prints in this ongoing series have already been released and were produced at Black River Studio in Cape Town. They depict a tenderly hand-drawn porcupine and the vibrantly yellow Forest Weaver. These art prints make a meaningful acquisition, whether for a formal art collection or to grace the walls of your home.
There are other ways of participating in the restoration of Ferncliff too. You can adopt an existing tree, plant a tree (which comes with an exquisite tree certificate appointing you as an honorary forester), make a straightforward donation, or contribute to unemployment alleviation by sponsoring a day’s wage to clear invasive aliens. Whether it’s for conscious corporate gifting, or a thoughtful gesture for a friend or loved one, you’ll be reaffirming how much the world needs forests, and how much these forests need us…
Grassland dynamics & bush encroachment in forestry plantations
By Lize Joubert-van der Merwe, Veldtology (Pty) Ltd I really like grasslands. I especially like how they ripple in waves up and down hill and mountain slopes when there is an approaching thunderstorm; how they change to that rich golden color in the final sunspots just before the dark-grey, rolling thunderclouds and lightning chases you indoors; and how their inflorescences hold rain drops from the previous night like a chandelier of diamonds. My fascination with grasslands extends beyond their aesthetic appearance to also include their ecology, management, and why this matters to forestry.
Moisture and temperature shape grasslands Grasslands are so vast that we often accept their presence as ubiquitous, yet, they are constantly changing in response to natural and anthropogenic drivers of diversity. I interpret grassland diversity from an understanding that moisture and temperature influenced broad vegetation patterns over the past few thousand years, as outlined by Frank Neumann. Did you know that the current wet-and-warm climatic period has only been around for 800-1000 years? A cooler period with less fires (more than 13 000 years ago) caused grassland to have many more fynbos elements, and we still see relics of Protea and Erica communities growing on cooler, south-facing slopes (Figure 1 above). Much later (~4600 to 3500 years ago), there was a drier period when grasslands saw an increase in karroid elements, specifically Pentzia incana (Ankerkaroo) that nowadays dominates sheep farms in the central Karoo.
In the current wet-and-warm climatic period, grasses dominate in grasslands (hence, the name), but they still have to compete with flowering forbs, trees and alien plants to remain numero uno. For this, they use various competitive strategies. Grasses keep flowering forbs at bay by rapidly growing into a dense layer that intercepts heat and sunlight from (s)lower-growing plants. This strategy to monopolize access to sunlight is quite a dicey move, because grasses are themselves not tolerant of shading. In fact, it happens in the absence of fire and grazing that build-up of leaf litter and moribund grass blocks sunlight from reaching live buds and leaves, which causes die-back of grass tussocks – a phenomenon known as ‘self-shading’. This is why fire is such an important part of grassland management. Fire is truly the exfoliating treatment that removes dead and dry cells from grasslands, so that new life can flourish.
Importantly, the ability of grasses to outgrow forbs and intercept limiting resources is directly tied to the current climate. During periodic droughts, when grasses cannot maintain their productivity levels, forbs are quite capable of recruiting successfully from seeds (Figure 2). Similarly, grasslands subjected to severe overgrazing are not able to keep forbs in check, leading to an overabundance of flowering forbs that is sometimes even visible on satellite images (Figure 3).
The role of fire If we shift our focus to the woodies in our midst, grasses keep shrubs and trees in check by sustaining a ‘fire trap’ from which tree seedlings hardly ever emerge unscathed (Figure 4). A fire trap is essentially the fire flame zone of the grassy layer. Unlike grasses, most indigenous shrubs and trees are sensitive to fire, especially as seedlings. So, fire gives grasses the competitive edge over shrubs and trees, just like climate gives grasses the competitive edge over forbs.
Grassland with a well-developed grass layer that burns at the correct intervals (when biomass ~ 4 tons / ha) should have no problem with invading trees. However, where the grass layer is jeopardized by too frequent burning, overgrazing or shading by large trees, fire intensity will be lower with consequently less killing power to aim at invading tree seedlings. For example, in communal rangeland (with heavier grazing → less grass → cooler fires), it often happens that tree seedlings escape the fire trap and grow into bigger trees that are more fire tolerant. Shading by timber trees also play an important role in advancing bush encroachment into grassland, especially in narrow corridors of forestry plantations. In fact, shading might explain much of the ‘edge effect’ of timber on adjacent vegetation, previously reported by Prof. James Pryke.
The role of atmospheric CO2 Interrogations of the local and global drivers of bush encroachment have led to a growing consensus among researchers that elevated atmospheric CO2 levels is an important global driver of bush encroachment. The exact mechanism is still unknown, but possibilities include the fertilizer effect of atmospheric CO2 on woody shrubs and trees, or an indirect effect on soil water content and its depletion in the surface soil layers where grass roots sit. Encouragingly though, a team of researchers led by Prof. Sally Archibald and Prof. William Bond found that bush do not encroach as rapidly in protected areas with elephants – the big giants of Africa that create their favored grassland habitat by pushing over trees. Although I am not advocating for the introduction of elephants to eradicate bush, this shows that local actions can trump global drivers in shaping vegetation dynamics. This is indeed encouraging.
Practical solutions customized to local context The trick is to find practical management solutions that can be applied in forestry plantations to help control bush encroachment. Such solutions will probably involve a combination of management actions sustained for longer periods of time, rather than single once-off interventions. For example, it would be pointless to do a once-off clearing of dense stands of Ouhout trees, with no follow-up burning and thinning operations to keep shrubs and trees in check. Moreover, instead of looking for a silver-bullet strategy that works well everywhere, management actions would probably need to be customized to fit local context and challenges.
Key local issues will include the shape and size of conservation areas. By virtue of their close proximity, any management intervention inside a narrow, small or irregularly-shaped conservation area has a greater probability of affecting adjacent timber compartments, than if you had a wider or larger conservation area. Thus, when a decision has to be made to control bush encroachment in one conservation area (but not another), shape and size is a useful starting point. In fact, it is non-negotiable that the conservation area must be the correct shape and of reasonable size to allow for safe burning.
Additional considerations include proximity to important conservation areas (e.g., with Red-Listed species or threatened ecosystems) and level of wetness. If bush encroachment threatens the functionality of a threatened grassy ecosystem, this is a good reason to prioritize bush thinning operations. Even more so when that threatened ecosystem contains threatened species, such as Long-toed Tree Frogs (Leptopelis xenodactylus), Swamp Nightjars (Caprimulgus natalensis) or African Grass Owls (Tyto capensis) that all depend on grassy habitats.
Lastly, level of wetness seems to influence vegetation succession (grassland -> bushy thickets or forest) and / or how the wetland ecosystem responds to bush thinning and burning. This is beautifully shown in the delineated areas of Zululand, where wetter wetlands have a greater tendency to remain grassy, whereas drier wetlands have a greater tendency to become bush encroached. We do not quite understand the mechanism of this phenomenon - it might be that grasses (with shallower roots) respond quicker than shrubs and trees when there is a shallow water table present. If this is the case, it will mean a better ecosystem response to burning, because a healthy grass layer is better able to sustain a fire trap that kills tree seedlings. Personally, I would consider shortlisting wetter wetlands for bush thinning and burning.
The role of roads in shaping fire It makes logical sense that all management operations should be aligned with clearly-visible, on-the-ground features so that operators know where to work. Such features can be roads, trace belts, streams or fence lines, depending on what is available. Where fire management is concerned, roads (mostly vegetated or dirt tracks) work exceptionally well, because they also provide access to vehicles and fire-fighting equipment, and should have low fuel loads (due to routine road maintenance). This makes it possible for foresters to set alight vegetation along the road, so that the fire burns from the periphery towards the interior of a conservation area, with minimal risk to adjacent timber. This is probably why we find grassland vegetation in larger conservation areas with roads along their edges, but bushy thicket in those without roads (or wrongly placed roads) (Figure 5). Exceptions include narrow or irregularly-shaped conservation areas that will probably not burn, regardless of presence or absence of roads, because of risk to adjacent timber. Another exception is conservation areas on steep slopes, where management (also roads placement and burning) would be adjusted to fit the soil erosion risk profile.
The value of well-placed roads is not new and already embedded in wetland delineation procedures for some forestry companies. Especially in Zululand (where terrain is not a problem), valley-bottom cut-off roads are routinely implemented at the edge of conservation areas (where they join commercial timber) to mark new compartment boundaries, to provide access, and to enable the use of fire in alien plant control (Figure 6). Getting control of alien plants within the first few years after felling timber is a major delineation goal, because it feeds into water security and sets the direction of ecosystem recovery in terms of biodiversity. Most of the roads around conservation areas have vegetated surfaces and are not expensive to maintain (Figure 6), but they make the world’s difference in restoring delineated land to a semi-natural state.
I think many environmentally-minded people (including myself) have been blinded by the negative impacts of roads, notably in connection with soil erosion and sedimentation. Perhaps, it is time to recognize that well-designed road networks (with roads that are well-placed, well-drained and well-maintained) can be conservation assets too.
Using fire in alien plant control Fire, along with foliar herbicide sprays and cut-stump applications are your cost-effective tools in the fight against alien plant invasions, notably American Bramble. Of these, fire followed by foliar sprays is the most cost-effective treatment option available in grasslands, but it is seldom (if ever) used in dense thickets.
Fire in grassland simplifies access, reduces the size of alien plants and causes a flush of new growth, which is more susceptible to foliar sprays than stems and mature leaves (Figure 7). However, of critical importance is the timing of post-fire follow-up sprays to hit the flush of new vegetative growth just at the right time, i.e., when plants are between knee and hip height. Get the timing wrong, and it is back to square one. No alien plant control operation should start without a viable follow-up plan that can be implemented with available resources - money, manpower and the necessary expertise to guide effective alien plant control.
Different stages of bush encroachment Bush encroachment is a gradual process of indigenous shrubs and trees replacing grasses often over a period of >10 years. Drought with uncontrolled grazing and shading of the grass layer in narrow corridors can increase the rate of bush encroachment, while expeditious burning can delay or stop the process. At the end of the day, there will be different stages of bush encroachment in a forestry landscape, with at least some alien plants that need to be controlled.
Early stages of bush encroachment (when shrub and tree cover is still sparse) should be prioritized for intervention, because the cost-effective management of alien plants with fire and foliar sprays is still possible. Basically, foliar sprays can be used until alien plants are about shoulder height, but do remember that bigger plants → more herbicide → greater cost. For alien plants above shoulder height, cut-stump applications are your next-best option, but at a far greater expense. Even larger specimens can be frilled or ring-barked, which are labor-intensive and time-consuming operations. Here, you must ensure it is done correctly to ensure maximum effectiveness.
For late stages of bush encroachment (dense thicket or early-successional forest), a different mechanism is used to effectively control alien plants. Here, the tree canopy effectively intercepts sunlight from reaching the soil surface, which prevents alien seeds from germinating. Dense thickets with an intact tree canopy generally do not have a problem with alien plants. It is only where there are gaps in the canopy (due to windfall or along thicket edges) that alien plants can establish, and where they need to be controlled.
A word of caution, though. Chopping down a large, solitary pine or eucalypt tree inside a dense thicket patch will create a gap in the tree canopy that presents an opportunity for alien plant recruitment. It is better to ring bark or frill such trees, so that surrounding indigenous trees are not damaged and so that there is not a sudden flush of sunlight available at the forest floor. The longer ‘time-to-kill’ for ring barked or frilled alien plant specimens also leaves a window of opportunity for indigenous tree species to fill the tree canopy gap, which effectively removes the alien plant recruitment opportunity. The effective control of alien plants in dense thickets considers treatments along with this careful manipulation of shade and sunlight on the forest floor.
In contrast to early and late stages of bush encroachment, there is an intermediary bushy state that presents a problem to management, and also has less biodiversity value than both more-grassy and more-forested states, according to Dr. René Gaigher. Here, fire cannot be used anymore (due to lack of grass cover) and the tree canopy has not yet locked out sunlight. This stage is susceptible to alien invasion, but it is difficult to gauge extent of invasion due to poor visibility and accessibility. For the same reasons, alien plants are difficult to find and treat. Viable treatment options in this context are expensive and time-consuming: cut-stump applications and frilling. Foliar herbicide spraying is an option along edges, but cannot be recommended for the interior of these bushy patches.
Where it makes sense to do so, the long-term strategy to control alien plants would be to reverse the intermediary bushy state back to grassland, so that fire and foliar spraying can again be used as treatment options. This will also benefit biodiversity. However, it will be an expensive and difficult journey of sustained effort for many years, which necessitates careful consideration of the points raised earlier (size and shape, important conservation areas, slope direction, and level of wetness). For all other conservation areas that is still in a predominantly grassy state, it is of utmost importance to maintain that grassy state with appropriate management.
Grassland for water production Probably the greatest benefit of functional grasslands in conservation corridors involves their ecological function in the sense of water production. Grasslands use less water than bushy thickets, and much less than alien vegetation. According to the National Water Act (Act 36 of 1998), commercial forestry is a stream-flow reduction activity that requires a water use license to safeguard national water security. It is this legal framework that enforces wetland delineation and control of alien plants along waterways, but it does not stipulate desired natural vegetation type (grassland vs. bushy thickets vs. forest) once timber along streams and around wetlands is felled. If the objective of legislation is to safeguard water security for downstream users, it would seem advantageous to have more grassland and less bushy vegetation in riparian and wetland buffer zones.
However, conservation corridors are not just water production areas. They also conserve biodiversity and ecosystem function, specifically ecological values representative of the historic state before timber dominated these landscapes. If the historic state in Zululand is coastal forest along streams, with grassland a bit further way, there is no reason for bush thinning operations in the riparian zone. Burning of grassland adjacent the forested riparian zone will maintain a functional ecotone and ensure that the coastal forest do not expand to dominate the entire drainage line. Maintaining this natural range of habitat types (grassy and woody types in wetter and drier areas) will tick the ‘biodiversity conservation’ block along with the one for water production.
A bit of practical wisdom also goes a long way for the management of rugged, south-facing, bush encroached hillslopes in the KZN Midlands. If the terrain is too rugged to have roads (to help safe burning) and if the microclimate on that hillslope is too cold and wet to sustain a fire, it might be best not to intervene with bush thinning operations. However, conservation areas on warmer hillslopes that jut down to rivers and with bush encroachment that can be traced back to a clear starting point (such as a change in ownership or retirement of an experienced forester) are good candidates for bush thinning operations that will probably also benefit water production.
Lize Joubert-van der Merwe is an independent consultant specialising in sustainable agriculture and forestry through improved management of natural resources.
Drone-mounted tool for sampling tree canopies
A University of Sherbrooke research team specialising in aerial robotics has developed a self-powered, drone-mounted tool that is able to collect foliage samples from high up in tree canopies, according to a report from Friday Offcuts.
The DeLeaves canopy sampling tool is suspended beneath a drone, is equipped with an HD camera, and has two robotic arms to collect foliage samples from trees.
The tool was first used by a group of horticulturists to sample foliage from tropical forests in Vietnam.It has been deployed by the Canadian Airborne Biodiversity Observatory to study the spectral and functional trait differences within tree crowns; and has been used to sample Douglas Fir, Silver Firs, and Western Hemlocks by the National Ecological Observatory Network team.
Since then it has been used to collect samples of tree canopies in North America and Europe. It has further potential for crop sampling in agriculture. The DeLeaves canopy sampling tool will be showcased at the ForestTECH 2021 event being run in Rotorua, New Zealand in November.