By Kerryn O’Conor and Carla Catterall
Restoring forests on cleared lands is a global conservation priority, particularly in tropical regions where extensive deforestation persists. Restoration can be achieved in a variety of ways, from planting trees to allowing forest to regrow naturally, and each method has inherent pros and cons.
Professor Carla Catterall from Griffith University and Dr Luke Shoo from the University of Queensland have been investigating different rainforest restoration approaches as part of a National Environmental Research Program (NERP) project. Their study is exploring the ecological and financial benefits and costs of reforestation methods in the Wet Tropics region. The research is highlighting the need for strategic approaches to restoration to maximise ecological and financial returns.
A global imperative
The world is losing an average of thirteen million hectares of forest annually, an area the size of England or nearly fifteen times the size of the Wet Tropics World Heritage Area. This is despite deforestation rates having slowed in recent years.
Tropical forests have fared particularly badly. Half of the world’s tropical forests have disappeared since World War II and tropical deforestation accounts for approximately twenty percent of global carbon emissions. With the Tropics being home to eighty percent of the world’s terrestrial biodiversity, it is imperative that we find cost effective ways to successfully restore tropical forests on a large scale.
The cost and benefits of regrowth
Forest restoration can occur either actively or passively. Passive regeneration occurs when forests regrow naturally, without human intervention. Active restoration involves people intervening to speed up the process, most often by planting trees. Both approaches have merit. For example, both types of restored forest can provide habitat for fauna, re-establish connectivity by linking remnant forest patches, help prevent erosion, improve soil and water quality and sequester carbon.
However, there are also significant differences between the two approaches and one clear distinction is in cost. Active rainforest restoration is relatively expensive because it involves planting a high diversity of advanced seedlings at high densities. These types of plantings are therefore limited to small areas. On the other hand, they enable very rapid forest re-establishment, accelerating ecological recovery. Initial costs could be reduced by planting less advanced seedlings further apart, but this allows greater light penetration which promotes grass and weed growth, resulting in much higher and longer term maintenance costs.
It is a much cheaper option to simply enable rainforest to regenerate naturally (for example, by excluding livestock) but this regrowth takes much longer to re-establish. Also, natural regrowth can only occur by overcoming a range of ecological barriers. First, it relies on fruit-eating rainforest birds and bats to disperse new seeds into cleared areas. However, the most effective seed dispersers are bird species that rarely leave the refuge of intact forest to fly over clearings or to perch in open pasture. Second, even when rainforest seeds are successfully transported, they can fail to germinate or survive because of a range of physical limitations such as insufficient light, water and nutrients, as well as the risk of being consumed by seed-eating or leaf-feeding animals.
Woody weeds: benefits and risks
In some instances, natural regrowth can be assisted by the presence of woody weeds. This occurs when the seedlings of some weed species are able to successfully establish and compete with dense pasture grasses. As they grow, these woody weeds provide perches and food resources, attracting birds that bring in rainforest seeds which then germinate and grow in their shade.
In the Wet Tropics, woody weeds such as lantana, wild tobacco and camphor laurel may act as catalysts for overcoming several of the barriers to rainforest regeneration. More investigations are needed to reveal in what situations they either aid the process of native tree regrowth, as described above, or suppress it, for example by forming a dense canopy that may shade-out native seedlings.
Assisting natural regeneration
Given the costs and benefits of the different approaches, researchers and land managers are now looking at a more varied menu of restoration methods, such as assisting natural regrowth in some areas while planting trees in others.
Combined approaches which involve interventions to remove barriers to regrowth are also being explored. In areas without much existing regeneration this may be achieved by increasing seed supply through methods such as direct seeding, installing artificial bird perches or by using herbicide to suppress pasture grasses. In areas where regeneration processes have already progressed to a stage where there are some established trees or a native seed or seedling bank, methods of ‘assisted natural regeneration’ (also known as ‘bush regeneration’) can accelerate the regrowth process. For example, in some areas where woody weeds have been established long enough to attract seed-dispersing birds and develop an understorey of native seedlings, herbicide treatment to kill these weeds may further accelerate the development of emerging native forest.
Professor Catterall and Dr Shoo’s research demonstrates the importance of weighing up the costs and benefits of different restoration approaches before making a strategic decision about which approach to employ in any given situation. It also highlights the need for further research to increase our knowledge about the effectiveness of the different methods. Lastly, it confirms the significant role that remnant forest plays in assisting natural regrowth, emphasising the importance of protecting the remaining areas of old-growth forest.
For more information
Please refer to the fact sheet:
Catterall, C.P., Shoo, L.P. and Freebody, K. 2014. Natural Regeneration and Rainforest Restoration – Outcomes, Pathways and Management of Regrowth. National Environmental Research Program. Reef and Rainforest Research Centre Limited, Cairns.
This blog is posted with permission from the Wet Tropics Management Authority. The original blog is posted here: http://www.wettropics.gov.au/restoring-rainforests-naturally-when-le-is-more.html