Exotic Pines and Eucalypts: From demonised trees to partners in forest restoration?
Hirikatu Oya Pinus plantation in Ratnapura Forest Division clear-felled leading to heavy soil erosion and failed restoration with native species.
Exotic single species plantations have been the bane of environmentalists world over for several decades so much so that, in a landmark decision, the Madurai bench of the Madras high court in India has decided that eucalypts, along with another Australian tree species – black wattle (Acacia mearnsii - which is also planted in Sri Lankan highlands), need to be removed from their colonized heartland forests in the Western Ghats, due to ill-effects they have on tropical rainforests and water supply in the hills.
Similar concerns have been raised in countries where extensive commercial eucalypt plantations exist viz. Brazil, China, India, and South Africa.
Consequently, a number of management options are being implemented in these countries to reduce their environmental impacts. In several tropical countries, eucalypts are being raised as a constituent of mixed species agroforestry stands (as opposed to large-scale monoculture plantations) thus providing benefits to smallholder farmers.
However, the Forest Department of Sri Lanka, having raised monocultures of Caribbean pine and Australian eucalypt plantations in denuded and infertile landscape so successfully for over half a century or more, has been somewhat reluctant still to yield to the conservationists’ and environmentalists’ pressures. This is mainly due to the repeated failures and setbacks in their past approaches with native species restoration and possibly inadequacy of knowledge and training in ecological restoration methodology.
The primary reason for past failures in raising mixed native species plantations was that the native species selected for restoration were mostly from among mature phase canopy species of the respective forest type. Seedlings or saplings of these canopy species are ecologically ill-adapted to establish themselves in larger forest openings resulting from deforestation. Furthermore, these forest gaps often get quickly overgrown with shade intolerant and faster growing pioneers and climbers smothering the advance regeneration of the saplings. Even though seed sources are available within the surrounding degraded forest landscape, the natural regeneration of mature phase native species has been painstakingly slow for these reasons.
Three Pine rows removal treatment creating canopy gaps (width 10 m) along N-S direction and planting of native species of utility vallue in Sinharaja IBR buffer zone. This photo-sequence shows the growth performance of site generalist- and restricted rain forest species over a period of 21 years.
At the same time, most of the fernlands and grasslands in these regions are maintained by frequent human-induced fires during short-dry periods each year. Consequently, the natural successional trajectories towards reaching a climatic climax of a natural forest are almost completely stalled in them.
Pines and eucalypts with their superlative ability to tolerate strong sunlight and poor soil conditions as seedlings/saplings show impressive growth as compared with native canopy species of the mature forests. This is clearly evident from the research trials being conducted by the Forest Department in Welanhinna (near Welimada) and Conical Hill Forest Reserve in Nuwara Eliya comparing the growth of exotic eucalypts vs. native forest canopy species of those climatic regions.
A recent research article published in the Journal of Applied Ecology (https://doi.org/10.1111/1365-2664.13513 [2020]) has reported some very relevant findings using exotic eucalypts globally branded as ‘demons’ or ‘devils’ in the Atlantic Forests of Brazil where selective logging of strips or rows of plantations for timber, bioenergy, pulp and paper, has been able to offset 45% – 75% of restoration implementation and maintenance costs partially overcoming the financial hurdle in undertaking active restoration projects by prospective investors.
In our own studies in Sri Lanka, equally demonised exotic pines have been used as partners in ecological restoration in the NW buffer zone of Sinharaja World Heritage Site and also in Lower Hantana in Kandy District for about 30 years. This healthy partnership between exotic and native species opens up a conciliatory middle path as the proverbial ‘rawulath theagene kendath bema’- i.e., raising exotic and naturalized/native fast-growing plantations (some of which are demonised as invasive), as the first step in forest restoration initiatives. These exotics or naturalised ‘nurse’ trees can then be replaced sequentially by suitable native species along an ecological successional gradient leading to a successful forest restoration programme.
In both Sinharaja and Lower Hantana Peradeniya University campus land we have successfully converted pine plantations into native and naturalized plant species of utility value using these successional methods. They are being regularly used for training restoration ecologists and others for over two decades.
During this ‘Decade of Ecosystem Restoration 2021 – 2030’, ecologically driven Forest Landscape Restoration is being promoted as the main driving force for global scale forest restoration. Sri Lanka has pledged to restore 200,000 ha of degraded forests as its “Nationally Determined Contribution’ towards the Bonn Challenge which has set a global target of restoring 350 million ha during this decade. While the monoculture plantations of pines and eucalypts have provided superior growth and impressive timber volumes, the downside is that they are not the best for the provision of enhanced ecosystem services such as biodiversity enhancement, water and soil conservation, reduction of fire and landslide hazards and amelioration of the environment. As such, we sincerely hope the Pinus and Eucalypt plantations already established particularly in critical watersheds in Sri Lanka would be given a high priority in systematically converting them to native species using sound ecological principles with a view to increase the ecosystem services they provide to downstream communities.
Forest landscape restoration can best be done using ecological succession-based principles. One of these is known in silviculture (the science and art of growing and cultivating forest trees, based on a knowledge of their ecological characteristics) as relay floristic method of forest restoration. Both eucalypts and pines are ‘early successional’ species that have the ability to grow in relatively poor soil and under strong light conditions common in degraded landscapes like cultivated and abandoned pathanagrasslands and kekillafernlands. The other suitable candidates for this step/lap of the relay are Havari nuga, Kenda, Gedumba, Lunu midella, Ipil Ipil, Acacia spp., Caliandra sp. Such common plants can colonize the open and degraded lands rapidly and provide some partial shade facilitating the introduction of native species of the second lap. At the same time, they keep away the aggressive grasses and ferns by providing shade to the ground rather quickly due to their rapid growth.
The pine and eucalypt stands which have reached maturity in mountain landscapes particularly in the Central, Sabaragamuwa and Uva Provinces can be manipulated by creating appropriately sized openings simulating gaps in natural forests. In native forests, such gaps originate from natural phenomena such as standing tree deaths due to old age, lightning strikes, wind throws and landslides. The native species selected based on their ecological characteristics either as habitat generalists or specialists for each climatic/floristic region can be introduced in successive laps as in a relay in these artificially created gaps in pine stands. Both timber and non-timber species of utility value such as hora, hal, bedi del, yakahalu, dun, nawada, kitul, weniwel, wevel, enasal, thippili, etc. have been introduced in our demonstration restoration trials in Sinharaja (30 years old, now) and Hantana (15 years old, now).
Unfortunately, despite these pilot-scale successful trials which have the potential for scaling up, there are silvicultural operations going on that are evidently harmful to the environment as in Hirikatu Oya region in Ratnapura forest division. Large scale clear felling of Pinusplantations on steep slopes has resulted in extensive environmental damage not only to the site itself but also to the downstream areas. Subsequent reforestation efforts have not progressed as expected and resulted in heavy gulley erosion and soil degradation.
There are moves to repeat similar large-scale clear-felling operations in mature pine stands in Diyatalawa-Bandarawela regions. While this is likely to be environmentally harmful as in Hirikatu Oya, it also prevents an excellent opportunity to make these so called ‘demonis ed’ exotic plantations partnering in systematic conversion of them to native species forests.
The ecological principles and methods showcased in our demonstration models in Sinharaja and Lower Hantana could be extended to this region, initially as a trial conversion. These mature pine plantations would serve as the first lap in a relay floristic model. The key is to select the right mix of native species for second and third laps of the relay floristic silvicultural succession suitable for this climatic and floristic region. They could include not only woody species but others of traditional utility value such as medicinal, ornamental, artisanal and food value. We are in the process of preparing a list of such potential species for each subsequent lap.
It is our fervent hope that we learn from the environmentally costly experiments of clear felling of forest plantations in steep slopes as evident from Hirikatu Oya example in Ratnapura/Balangoda forest region and not repeat the same mistake in Diyatalawa/Bandarawela or any other similar landscapes. Proven silvicultural methods such as Relay Floristics methods are available to be adapted to suit the site conditions of this climatic region.
(The writer is Emeritus Professor, University of Peradeniya)