Overstory #188 - Helping forests to help themselves – Accelerated natural regeneration
What is accelerated natural regeneration (ANR)?
Accelerated natural regeneration (ANR) covers any set of activities that enhance the natural processes of forest regeneration. These include promoting the natural establishment and subsequent growth of indigenous forest trees, whilst preventing any factors that might harm them e.g. competition from weeds, browsing by cattle, fire, etc.
Because ANR relies on existing natural processes, it requires less labour input than tree planting and is therefore a very cheap way to restore forest ecosystems. However, ANR and tree planting should not be regarded as two exclusive alternatives to forest restoration. More often than not, forest restoration depends on the clever combination of tree planting with ANR techniques. Under certain circumstances, ANR may be sufficient alone to restore forest ecosystems, but tree planting should always be implemented in combination with whatever ANR techniques may be appropriate.
Where is ANR appropriate?
ANR is appropriate wherever the natural processes of forest regeneration are, to some extent, already happening. For example, at least a few seed trees should exist nearby and seeddispersing animals should remain common in the vicinity. Sites, which already support a high density of tree saplings or sprouting tree stumps, are particularly suited to ANR.
A detailed site assessment is necessary to decide if ANR might be sufficient, on its own, to restore forest and (if it is) to select the most appropriate techniques.
The site assessment should-
i) determine the existing potential for natural forest regeneration and
ii) identify which factors might be limiting natural forest regeneration.
Direct observations of site conditions should be combined with interviews with local people to address the following questions-
- What is the density of tree seedlings, saplings and tree stumps in the site? Are they evenly distributed or confined to a few parts of the site?
- How recently was the site deforested? Ask local people about the history of land use practices on the site.
- Does the site show signs of fire e.g. blackened tree stumps etc.? Ask local people how frequently fires occur in the area.
- Are there any signs that cattle use the site? Ask local people about cattle rearing practices in the area.
- How far away are the nearest sources of forest tree seeds?
- What is the status of seed-dispersing birds and mammals in surrounding areas?
The density of naturally occurring tree saplings and stumps (number of stems per hectare) provides a good prediction of whether ANR, on its own, may be sufficient to restore forest on any particular site. However, it is also important to consider the sizes of the saplings and stumps. Tall saplings are more likely to survive than small ones. The chances of a sapling growing into a mature tree increases greatly once it overtops surrounding weeds. So, it is also useful to record whether saplings are taller or shorter than the weed canopy.
Randomly place circular sample plots across the site. Use a pole to mark the centre of each plot and a 5-m-long piece of string as the radius. Count, identify and measure all stumps and saplings taller than 1 m, closer than 5 m to the pole. Calculate density by dividing the total number of saplings/stumps counted by the total area surveyed.
As a rough guide; if the density of saplings + live stumps exceeds 250 stems per rai (1,562 per hectare), ANR alone may be sufficient to restore basic forest structure within 5 years, provided fire, cattle and other limiting factors are controlled. Where density of saplings + live stumps is lower, ANR alone is unlikely to be successful, unless the site is very close to intact forest and seed-dispersing animals are common. If these conditions are not met, ANR should be combined with tree planting.
The intensity of tree planting required may vary within sites, since the distribution of natural saplings and tree stumps is often patchy or clumped. For those parts of the site undergoing vigorous regeneration, such as forest edges or around remnant, fruiting trees, tree planting would be a waste of resources. In the centers of large deforested areas, where tree seedling recruitment may be limited by distances from seed sources, the need to augment ANR with tree planting will be greater.
What are the limitations of ANR?
ANR acts mostly on trees that are already established in deforested areas. Unfortunately, most of the tree species that are capable of colonizing such areas are light-demanding pioneers, with seeds dispersed by the wind or small birds. They represent only a small fraction of the tree species richness of climax forest. Therefore, whilst ANR, might be sufficient to restore tree cover and to some extent forest structure, full recovery of biodiversity may require additional measures. Where large seed-dispersing animal species have become extirpated, planting large-seeded climax forest tree species may be the only way to convert secondary forest, created by ANR, back into primary forest.
Taking care of what's already there
The most thoroughly tested and widely practiced ANR techniques are those, which increase survival and growth of the woody plants that are already established on a site. Various methods are used to manipulate the environmental conditions around the seedlings and saplings of woody plants, as well as sprouting tree stumps, to accelerate their growth and to protect them from harm.
Can tree stumps be encouraged to sprout?
Sprouting tree stumps provide the most rapid means of re-establishing forest cover in seasonal tropical forests. Consequently, where they exist, they should be the focus of initial ANR efforts. Coppicing shoots can grow much faster than tree seedlings, since they can draw on large food reserves through the stumps' existing root systems. They are less susceptible to drought than seedlings, and they are thus less affected by weed competition. Held above the weed canopy, coppicing shoots are less likely to burn during a fire, but even if they do, they can recover rapidly.
However, almost no practical techniques have been tested to enhance the role of tree stumps in ANR, besides general recommendations that they should be protected from chopping, burning or browsing. Would application of plant hormones encourage sprouting? Could chemicals be used to prevent fungi or termites attacking tree stumps? Would laying mulch or applying fertilizer around tree stumps have the same beneficial effect as they do on tree seedlings? If many coppice shoots are growing from a single tree stump, would trimming back the weaker, smaller shoots enhance growth of the taller, stronger ones? These questions would make interesting topics for future research. Practitioners of ANR are encouraged to experiment.
How can competition with weeds be reduced?
Weeding, to reduce competition with herbs and grasses, is just as beneficial for naturally established trees as it is for planted ones. The smaller the tree seedlings or saplings, the more they benefit from weeding, especially during the rainy season. In the dry season, a weed canopy may help to protect small tree seedlings from desiccation, but this potentially beneficial effect must be weighed against the fire risk posed by the dried vegetation. Weeding around tree stumps is unlikely to be very beneficial, since tree stumps already have deep root systems that extend well below those of herbaceous weeds.
Before weeding, tree seedlings or saplings should be clearly marked with brightly coloured tape or poles, to make them more visible. This prevents accidentally trampling or cutting them during weeding. Weeding should first be concentrated around the marked trees, before clearing weeds from the rest of the site. Around small seedlings, it is better to hand-pull weeds than to use tools, since digging can damage the seedlings' delicate root systems.
One weeding method that seems particularly suited to ANR is "lodging", i.e. flattening weeds with a board, rather than cutting them or digging them out. This does not kill the weeds immediately but each time the weeds grow back, they use up food reserves stored in their root systems. If the weeds are flattened often enough, food reserves are eventually exhausted and the plants die. Lodging weeds does not disturb the soil surface and, by shading the soil, the flattened weeds suppress germination of light-dependent weed seeds. This technique is particularly effective against grasses and bracken fern.
Use a wide plank of hard but lightweight wood (about 5 x 25 x 130 cm). Carve out semicircles at both ends of the plank so that it can be used to flatten weeds growing close to tree saplings. Attach a piece of sturdy rope and a shoulder pad to both ends of the plank, making a loop, long enough to pass over your shoulders. Lift the plank onto the weed canopy and step on it with full body weight. Repeat this action, moving forward in short steps (for more information please log on to http// www.fs.fed.us/psw/publications/documents/ other/3.pdf). The method has been used to great effect in the Philippines to clear Imperata grass and accelerate forest regeneration on abandoned slash and burn sites there.
Can mycorrhizae increase tree growth?
The dependence of tropical trees on symbiotic relationships with mycorrhizal fungi is significant. The prevalence of such relationships raises the question could inoculating naturally establishing trees with mycorrhizal fungi improve their performance?
Recently, commercial preparations of mycorrhizal spores have become available. Usually such products contain a mixture of spores of several ubiquitous fungus species, adsorbed onto an inert substrate. However, as far as we are aware, the use of such products to improve performance of naturally establishing tree seedlings in ANR sites has never been tested. This is clearly another topic worthy of further research.
Should cattle be removed?
Ultimately, the decision to reduce the number of cattle or to remove them altogether depends on careful consideration of their economic value to the community, balanced against their effects on the regenerating trees.
In Nepal, villagers do not allow cattle to roam freely in their community forests. To protect the trees, villagers keep their cattle at home. They cut forage from the forests and bring it to their villages to feed their cattle. Not only does this feed the cattle without damaging the trees, but it also encourages effective weeding of forest plots. On the other hand, in Central America, cattle are used as an important tool in the early stages of forest restoration. They are regarded as "living lawn mowers", releasing young trees from competition with grasses and providing a vital seed dispersal service for some of the dominant forest tree species. Using cattle to control herbaceous vegetation also reduces fire risk.
How can fire be prevented?
Fire is the most serious hindrance to forest regeneration in the seasonally dry tropics. Where the fire risk is considered to be significant, fire prevention is a vital activity for ANR. Firebreaks around ANR sites must be cut at the beginning of the hot, dry season and a fire warning and suppression system must be maintained until the rainy season begins.
What other techniques can be used to encourage tree growth?
Mulching and fertilizer application can be used to enhance growth and survival of naturally established trees. Small seedlings or saplings are more likely to respond positively to such treatments than large trees. It is probably a waste of effort and expense to apply such treatments to older saplings and tree stumps, since they would already have developed deep root systems.
Increasing the seed rain
After severe and prolonged disturbance, remnant tree stumps, seedlings and the soil seed bank will be sparse or absent, so the potential for natural regeneration will depend critically on the seed rain.
Can seed-dispersers be attracted to ANR sites?
Yes. The seed rain can be dramatically increased by adding very simple structures to ANR sites that attract the most common seed-dispersing animals i.e. birds (particularly bulbuls) and fruit bats.
Research in FORRU's experimental plots has shown that simple artificial bird perches, made from bamboo, placed randomly across sites, can significantly increase the seed rain. Moving such perches around, from time to time, could help to distribute seeds over wider areas. Adding bait to the perches may increase their attractiveness (but is labour intensive) and clearing weeds beneath them increases survival of germinating tree seedlings. Bird nesting boxes may have a similar effect.
Provision of roosting boxes might attract seed-dispersing fruit bats into ANR sites. The boxes can be hung from remnant trees or mounted on tall poles. Bats have very particular requirements so it is important that boxes are made from rough timber and that the entrance to the box matches the body size of the target bat species. Unlike bird perches, the effects of bat boxes on forest regeneration in the tropics have never been tested.
Structurally diverse vegetation, especially fruiting shrubs or remnant trees, act as natural attractants to seed-dispersing animals. So, protecting such vegetation will greatly help to increase the seed rain.
In seasonally dry climates, water is a strong attraction to wildlife, so digging artificial ponds might also attract potential seed dispersers.
Can large, seed-dispersing animals be brought back?
Large seed-dispersing animals (e.g. elephants, rhinos and wild cattle) have been extirpated over much of their former ranges or reduced to populations too small to play a significant role in seed dispersal. So might it be possible to bring them back? After all, human beings must be paid to plant trees, whereas elephants not only plant tree seeds, but provide them with a generous dose of fertilizer, for free!
Unfortunately, re-introduction of large seed-dispersing mammals is a difficult and expensive process. It is only worthwhile where the problem, which caused the species to become extirpated in the first place, has been solved. This usually means persuading local people not to hunt re-introduced animals.
Re-introducing captive animals back into their natural habitats is particularly difficult because captive animals often lose the skills needed to survive in the wild. A lengthy rehabilitation process is usually necessary. However, even translocating wild animals, from conservation areas where they are common, to those where they have become extirpated, is not easy. The risk of death or injury during capture is high and the source population may be seriously disturbed or depleted. Veterinary care, maintaining genetic diversity within small populations, monitoring animals after release and, most crucial, preventing hunting, are all vital components of any animal reintroduction programme.
In addition to the technical issues, local people may object to the return of large animals, which might damage crops, compete with domestic animals or threaten human life.
However, such obstacles are not insurmountable. For example in northern Thailand, domestic elephants have been successfully returned to the wild. So, despite the difficulties, re-introduction programmes are worth careful consideration. For further information, please refer to the guidelines issued by the Species Survival Commission of the International Union for the Conservation of Nature.
Can people be seed dispersers?
Yes. One method of forest restoration is to collect seeds from nearby forest trees and sow them in deforested sites. This is called "direct seeding". The technique can rapidly increase tree density as well as tree species richness, but it has several drawbacks. The hot, dry conditions, of most deforested sites can rapidly desiccate seeds on the soil surface. In addition, seed predators, such as rodents and ants, are particularly common in deforested sites and can cause complete loss of some tree seed species, within a few days after sowing.
Burying seeds can substantially reduce both desiccation and predation, but it also increases the labour input required. Selecting tree species with seed characteristics that make them resistant to predation (e.g. small size, tough seed coat, etc.) can increase the success of direct seeding. Treating seeds with chemical repellents is also worth exploring, but further research is needed to identify compounds that deter seed predators without harming the seeds. Since prolonged dormancy increases the chances that seed predators will find seeds, treating seeds to break dormancy (e.g. soaking, scarification etc.), before direct seeding, might shorten that vulnerable period, during which predation could occur. However, sometimes such treatments can increase the risk of desiccation or make seeds more attractive to ants by exposing the cotyledons.
As with nearly all ANR techniques, experiments must be carried out to determine the most successful techniques to use in any particular site. Naturally, any animals, which prey on rodents (e.g. birds of prey, wild cats etc.), should be regarded as valuable assets on ANR sites. Preventing the hunting of such animals can help control rodent populations and reduce seed predation.
If it is decided to include direct seeding in an ANR programme, try the procedures in the diagrams below. At the beginning of the rainy season, collect seeds from fruiting forest trees, near to the ANR site. Dig out weeds in "seeding spots", approximately 30 cm across, spaced about 1.5-2 m apart (the spacing can be wider where saplings or tree stumps are common).
Dig a small hole in the soil and loosely fill it with forest soil (dug up from where the seeds were collected). This ensures that beneficial symbiotic micro-organisms (e.g. mycorrhizal fungi etc.) are present when the seed germinates. Finally, press several seeds into each hole, to a depth of about twice the diameter of the seed and cover with more forest soil.
What if ANR doesn't work?
ANR is a very young science. Provided ANR techniques are applied to a suitable site, they are unlikely to be a complete failure, but they might not yield desired results quickly enough, especially biodiversity recovery.
Another approach is to use a "nurse crop" of trees to re-establish canopy cover, whilst also implementing ANR techniques. This approach is called "foster ecosystem" or "plantations as catalysts". Almost any tree crop will encourage the processes that accelerate forest regeneration, by ameliorating the micro-climate and attracting seed-dispersing birds. Even an exotic tree species can be used, especially where economic benefits are required. The nurse crop is gradually thinned, to yield an economic return, as the plantation becomes colonized by forest trees.
This article was excerpted with the kind permission of the publisher from
Forest Restoration Research Unit. 2006. How to Plant a Forest The Principles and Practice of Restoring Tropical Forests. Biology Department, Science Faculty, Chiang Mai University, Thailand.
Copies (in English or Thai) are available from The Forest Restoration Research Unit, c/o Dr. Stephen Elliott or Dr. Sutthathorn Suwannaratana, at the address below. The book can also be downloaded in pdf format from http//www.forru.org.
About the author
Chiang Mai University's Biology Department established the Forest Restoration Research Unit (FORRU) in collaboration with Doi Suthep- Pui National Park Headquarters in 1994. Supported by BRT and Eden Project FORRU carries out research, to develop appropriate methods to accelerate regeneration of natural forest ecosystems on degraded land, within protected areas of Northern Thailand. This research generates original knowledge to enable local people to improve forest restoration projects for biodiversity conservation, environmental protection and enhancement of human well being.
FORRU's first task was to gather basic ecological data about the very large number of tree species, which occur in Northern Thailand, to determine which ones might be most useful for restoring damaged forest ecosystems. FORRU teamed up with Britain's Horticulture Research International to design a nursery and field research program to test the suitability of various indigenous tree species for planting in deforested areas. More recently FORRU is adapting the framework species method of forest restoration, originally conceived in Australia to the environmental and social conditions of Northern Thailand. This has involved selecting mixtures of 20-30 native forest trees species for their ability to grow fast, shade out weeds, recover rapidly after burning and attract seed-dispersing wildlife.
Since 2002, with support from Britain's Darwin Initiative, FORRU has been disseminating it's research results to all those involved in restoring Thailand's forest. These include Royal Forestry Department officials, village communities living in or near conservation areas, non-government organizations and most importantly, school children, which have most to gain from environmental conservation.
The Forest Restoration Research Unit c/o Dr. Stephen Elliott or Dr. Sutthathorn Suwannaratana Biology Department Faculty of Science Chiang Mai University Chiang Mai Thailand 50200
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