- Some studies report a negative effect of exotic tree plantations on native species, while others suggest that exotics allow for native and endangered species to thrive.
- A recent study in the Upper Palani Hills indicates the possibility of native shola tree regeneration in existing stands of exotic tree species.
- It finds that the average density of shola tree seedlings was highest in the acacia plantations, along with species richness. Although acacia has been previously found to deter forest restoration, that was not the case in this study.
Native shola forest species could regenerate under exotic tree plantations, found a team of scientists that investigated the possibility of restoring shola forests under abandoned exotic tree plantations in the Western Ghats.
Exotic tree plantations in India have been prevalent since the late eighteenth century and were established for different purposes ranging from their ornamental presence to providing timber and firewood to planting specific trees such as acacia for tannins.
While some studies report a negative effect of exotic tree plantations on the presence and range of native species, others suggest that these plantations allow for native and endangered species to thrive.
The team comprising scientists from the University of Freiburg, Germany; TERI School of Advanced Studies, India; Karlsruhe Institute of Technology, Germany; and the Faculty of Forestry, Hans-Carl-von-Carlowitz-Platz, Germany, assessed the abundance, diversity and composition of the regeneration of shola forest tree species in plantations with different dominant trees.
The study area was situated in the Upper Palani Hills (1,500m – 2,450m), which comprise diverse landscapes including “montane forests, grasslands, agricultural lands including diverse agro-forestry systems, water bodies, and forest plantations.” Currently, the Tamil Nadu Forest Department is permitted to remove black wattle (Acacia mearnsii) plantations in the Palani Hills and work towards restoring the tropical montane forests in this region by planting shola tree species samplings.
Exotic tree plantations are at the heart of two key debates: the loss of historic grasslands that these plantations replaced and the non-conducive environmental conditions that they create, which deter native tree species from regenerating. How exotic plantation species affect native tree regeneration can vary based on location, environmental factors and forest types. For instance, previous research in the Western Ghats has shown that shola tree species can regenerate under eucalyptus and teak plantations. The forest canopy species is a key factor affecting this regeneration. Other research has shown that sapling growth rates are higher in pine canopies than in eucalyptus. Moreover, the species that make up the forest canopy also influence tree regeneration.
Native species undeterred by Acacia
The study began by investigating how similar the regeneration of native tree species was between plantations of acacia, pine and eucalyptus and how the density and diversity of shola species varied based on “different aspects, slopes, the openness of canopy, plantation type, distance from the nearest shola forests, structural diversity, and presence of herbivores.”
The term “seedlings” or “tree regeneration” was used to refer to all trees with a diameter at breast height less than 7 cm. The study comprised 33 plantation sets of five field plots per plantation while ensuring that different conditions, such as elevation and exposition, were represented.
The average density of shola tree seedlings was highest in the acacia plantations, along with species richness. Although acacia as a genus has been previously found to deter forest restoration, that was not the case in this study.
Joachim Schmerbeck, the lead author of the study, said, “We are talking here about the high elevation Western Ghats above 1800 m. Here, Acacia mearnsii spreads on the grassland and triggers the regeneration of the shola forest. From what I observed and what is evident from the trades these plants have, there will be no or very few shola trees if A. mearnsii or other exotics are removed. This is simply because A. mearnsii is a disturbance indicator. When it regenerates, it says: “Hey, there is too much sun on the ground! I grow and make shade!” Shola trees will regenerate only to a minimal extent in an open area: for most of them, it is too much sunlight and too much pressure from herbivores. Shola tree species need to be planted under a tree canopy and protected from herbivores. In the open, some shola tree species that are less attractive to herbivores like Pittosporum neilgherrense, will establish, but this young forest will not be very dense.”
The density of native tree regeneration in eucalyptus plantations was comparable to acacia plantations. The species richness was second highest in eucalyptus plantations, but this was still lower than previously found in a similar study in 2009 on the effects of eucalyptus plantations on native species regeneration in the southern Western Ghats.
Although the regeneration density was lowest in pine plantations, the authors state that these numbers are at the lower end of what has been reported by other studies in Kenya and Sri Lanka. The tallest seedlings were found in the pine plantations and although canopy openness needed to be removed from the analysis, it is likely that light availability played a role in the regeneration of shola tree seedlings.
When asked about the feasibility of restoration of forests alongside invasive species, Schmerbeck explains that plantations of A. mearnsii can be considered ideal sites for the restoration of local forests. “You see this in our paper and in the research that we cited. So, it is not ‘alongside’; it is an integrated system. The science behind it is called silviculture. Silvicultural techniques will include the gradual removal of the tree canopy to first trigger and then enable the establishment of young shola trees. Here it is very important to fence off herbivores,” he adds.
Adding to this, Anita Varghese, director at Keystone Foundation in Kotagiri, said, “There are several successful efforts of restoration of forests which have been covered with invasive species. Depending on the number of years that the invasive species have been on the site, the efforts needed to push them back vary because of seed banks in the soil, which persist and can lead to new seedlings sprouting when conditions are favourable. In our experience, we have only removed invasives by pulling them out manually, and that too in phases. In the case of shola planting, we have also seen that the pioneer species, after they have been established, help suppress the regeneration of the invasives.”
Shades of growth
Canopy openness plays a key role in these dynamics.
The present study found that eucalyptus plantations had the most open canopies, whereas pine plantations had the most closed. “The canopy openness plays a key role in the establishment of plant species. Those species that need a lot of light benefit from open canopies, while species that can grow with a reduced availability of light can grow under a more closed canopy. Fast-spreading, non-native plant species are, in general, light-demanding, and so a closed tree canopy is needed to reduce widespread species like Lantana camara. Forests that are degraded by grazing and regular fires cannot replace the canopy. L. camara can withstand grazing, but it cannot withstand regular fires. Once the frequency of fires is reduced, L. camara can establish, grow, and produce seeds. The trees regenerating under its canopy are protected from grazing, but only a few can grow through the canopy of L. camara. To bring back the tree canopy, people would have to dig their way through the Lantana thicket, plant large saplings of fast-growing tree species, and wait for the shade from canopies to reduce the Lantana cover,” says Schmerbeck.
However, this might not always work for all native species. Manaswi Raghurama, a researcher at the National Centre for Biological Sciences, Bengaluru, who has worked on the effects of invasive species on shola grasslands, says, “Recent observations and studies suggest that forest regeneration is indeed possible in the understorey of invasive and exotic plantations. Many forest tree species are late-successional and require shade, especially in the early growth stages. This is why researchers are suggesting that plantations of exotic woody plants and woody invasive species could act as a nursery for the initial regeneration of forest species. Once the native forest species are established, they suggest that the woody exotic and invasive species can be removed,” he says.
“I would like to highlight here that shola forests exist alongside natural grasslands in the Western Ghats. As such, our work shows that the grass community in invasive and plantation understorey is extremely different from the native grasslands, and as such, I would not expect the shade to serve as a nursery for the native grass community (which are mostly C4 grasses, and hence, shade-intolerant),” he adds.
Moreover, depending on the type of landscape that is being targeted, efforts to restore it would vary. Varghese says, “Certainly, the approach should be based on whether one is restoring a grassland or a closed canopy forest. Open natural ecosystems are the ones that got invaded heavily in the Nilgiris. We need to learn from the experiences of practitioners across different habitats in managing invasive plants on restoration sites. Removal of invasive plants by cutting them without uprooting them has only led to more thickly invaded habitats when they coppice and regenerate.”
Future directions
The study found that the regeneration capacity of the shola forest in the study area is likely to be sufficient, provided there is proper management.
Varghese says, “When exotic trees are “removed’ en masse, it is done by cutting them. In some species, this only causes them to regrow with multiple stems. This is seen in the case of Senna spectabilis and Acacia spp. When the effort has been to plant the native shola tree saplings between the trees within a certain period of time, the shola trees are able to shade out and push back the exotic trees.”
Local community involvement can make or break these processes. “Communities are key. Local people do shape the ecosystems and are interwoven with their landscape. So, understanding the interaction between forest ecosystems and the local communities is very crucial for any forest management. In the case of A. mearnsii, we found in another study that these plantations satisfy the local people’s demand for fuel to a great extent. By this practice, they protect the shola forest. So here, local people need to be involved in planning the use of the landscape,” Schmerbeck says.
Consistency and local effort are necessary, Varghese adds. “It would be very meaningful to involve local communities because restoration is not a one-time effort; there is a need for monitoring, replanting, and protection, and in all of these activities, there is a role for local communities to be actively involved in the restoration. Through the MNREGA scheme, we could even think of mechanisms for communities to be paid for the work. Importantly, restoration is also about restoring our relationship with nature, ecological restoration is a great way to reconnect with the forests and grasslands,” she says.
Banner image: Shola trees regenerating under a Eucalyptus plantation (Road to Kodaikanal near Perumalmalai). Photo by Joachim Schmerbeck.