To Chop, or Not to Chop? The Issue of Exotic Invasive Trees in the Western Ghats

Before: 1970s
Dr. Manfred Laun
1973: Berijam lake (Palni Hills) and surrounding landscape with grasslands and non-native tree plantations.
Now: 2010s
Ian Lockwood
2012: Berijam lake (Palni Hills) and surrounding landscape with practically all grasslands invaded by non-native trees.

The Tamil Nadu Forest department is required to respond to a petition filed in the Madurai High court that asked the Forest Department to act to solve a problem. This problem has been in the making for over 50 years and has no simple solution. I define the problem, its complexities, and some interventions that could help guide a response to the court order in the short-term, and possibly address the problem in the long-term.

The problem

Over fifty years ago the Tamil Nadu Forest Department began planting exotic (non-native) trees in the natural grasslands that comprised the dominant component of the ancient shola-grassland ecosystem, an ecosystem that is found in the higher elevations of the southern Western Ghats (like the Nilgiris and the Palni Hills) and nowhere else in the world.

Initially these non-native trees were planted in small numbers to meet the increasing demand for firewood. Around the time of India’s independence the department began increasing the extent of these plantations to satisfy the growing demand for tanning bark by the leather tanning industry, and a demand for wood pulp by the paper industry (India’s trade sanctions on South Africa after independence stopped the supply of tanning bark from South Africa, which was the major reason for the increased demand for the tanning bark). What followed was a tree planting rush, and, by 1988, over 11,000 hectares of grasslands had been converted to plantations. These plantations were actively harvested for over 40 years, but in the mid 1990s the Forest Department stopped this activity. The main reason for this policy change was a growing awareness within the Forest Department that converting natural ecosystems to non-native monoculture plantations was not a good idea. When the plantation harvesting stopped, the non-native trees had an unfair advantage over grasses for resources (soil nutrients and water) and continued to spread and invade the grasslands. This invasion has been so successful that only a few grassland patches remain in the Palni Hills. The former shola-grassland landscape in the Palni Hills has more-or-less been completely replaced by a shola-exotic plantation landscape. It is only natural that this large-scale landscape modification has impacted the ecology of the landscape and affected animals that were dependent on the grasslands for resources, particularly food.

The three non-native invasive trees

There are three major exotic invasive species to deal with: wattle (Acacia mearnsii), eucalytpus (Eucalyptus globulus), and pine (Pinus patula). Wattle was initially introduced for firewood and then to satisfy the demand for tanning bark; eucalyptus and pine were introduced mainly to satisfy the demand for wood pulp. Wattle has turned out to be the dominant invasive tree species in the region, which is not surprising as it is considered to be one of the most invasive tree species in the world. Wattle in some areas are so dense that people claim to have found carcasses of deer that appear to have starved to death after being trapped trying to navigate through dense thickets of young wattle. It is well known that eucalyptus trees absorb large quantities of water to support their fast growth rate, which is detrimental to water tables. There is evidence of drought stress in wattle and eucalyptus trees, which suggests that some patches of these species could be going through a bust after an initial boom phase, i.e., reaching high densities that have led to detrimental competition and depletion of resources. There is evidence that a fungus is killing, or at least managing to stem the growth of many wattle trees. Shola trees have successfully colonized plantation patches of all three species; some shola trees appear to have done so nearly 50 years ago. It should be noted that while the exotic non-native trees have invaded grasslands, they have been unsuccessful in invading shola forest patches. Wattle trees are best killed by cutting their stem below the ground where the stem has less wood density, eucalyptus trees are difficult to remove as they can regenerate from cut stumps and their roots are usually very extensive, sometimes reaching over a hundred feet, and pine are the easiest to kill. Wattle will require the most attention because of its wide scale expansion, but it is also the preferred tree for firewood in the region.

Is there a solution?

The petition filed in the courts asks that the Forest Department get rid of the exotic invasive trees to restore the grasslands. The predominant idea to accomplish this goal is rather simple – cut the invasive trees and the grasslands will return. Although there is a poor understanding of the exact mechanism that was responsible for establishing the shola-grassland ecosystem, there is little debate that the process took hundreds, if not thousands of years. Intensive plantation activity for over 40 years, followed by wide-spread invasion by non-native trees for 20 years have surely modified the soils and water tables in the region significantly. Therefore, is it reasonable to expect a system that took thousands of years to evolve, but has been extensively modified for over 60 years, to easily restore itself to a former state? The short answer to this question is “Probably not”, which is why we need to acknowledge that we are dealing with a complex issue that probably requires more than the simple solution of chopping down the invasive trees.

What do we do?

There are no clear answers to the restoration process. Any management interventions should be implemented with caution, patience, and initially on a small scale. The landscape is variable, which means different sections on the landscape should get different levels of priority and interventions. The long term needs to be kept in mind – modifications to the landscape lasted 60 years and we have waited twenty years since the end of plantation activity to intervene. We, therefore, need to be patient with the restoration process and not expect large-scale changes in the short-term. Any removal of trees has to be done keeping in mind the needs of the local people for firewood, both for cooking and heating. Unless some effort is made to reduce the dependency that the local people have had on firewood for hundreds of years in the region, we cannot expect this dependency to disappear any time soon. Fortunately the Mukurthi Wildlife Sanctuary in the Niligiri Hills and the Kodaikanal Wildlife Sanctuary in the Palni Hills provide the department with ample opportunity to experiment with management interventions while provisioning for the needs of local people using buffer regions for firewood. For the grasslands to make a comeback they will require assistance and a strong long-term commitment from us. The following suggestions could help address the court order in the short-term and the restoration process in the long-term (the suggestions are targeted at the Palni Hills, but are applicable to the Nilgiris too):

  1. Prioritize the remaining grassland patches:  There are a few remaining grassland patches. These, however, are not completely free of invading non-native trees. Many of these patches are found at the western region of the newly declared Kodaikanal sanctuary, and are far away from human habitation. However, by the same token they are generally difficult to access, often only by foot. Maintaining these remote grasslands patches free of invasive trees and shrubs might turn out to be an expensive endeavour, which requires a strong commitment from the Government to bear these costs.
  2. Thinning of plantations:  Shola trees are regenerating within many plantation patches — an invasion of native trees into patches of non-native trees. Ideally we would prefer grasslands to make a comeback, but grasses cannot compete as well as native shola trees can with the invasive trees for sunlight. It makes little sense to kill colonizing shola trees especially since there is no guarantee that grasslands will return to their entire former range. To help shola trees succeed in their colonization, we could help by thinning, i.e., cutting select invasive trees around them. This management intervention is relatively inexpensive and we could experiment with different strategies, i.e., cutting select trees with no additional intervention in some areas, and in others areas cutting select trees, but following up with removal of saplings. Trying different methods will allow the Department to compare the effectiveness and cost to benefit ratios of different intervention strategies.
  3. Begin mass tree removal with a pilot phase:
    • Chopping down all the invasive trees would be a staggering endeavour and could lead to further ecological issues. It is common knowledge that large-scale tree removal always affects the soil layer for the worse, either by modifying soil composition or by soil loss. The shola-plantation/grassland landscape plays an important role as a watershed that supplies water to millions of people. It is likely that the plantations have altered the water table for the worse, but it is unlikely that large-scale cutting of plantations would improve the situation. Therefore, it would be best to begin mass tree removal with a pilot phase.
    • It would be a good idea to remove trees en masse in 1-2 sizeable (~10 hectares) experimental plots deep inside the Kodaikanal Sanctuary that preferably do not have invading shola tree species (in general, further the distance from a shola patch, less the chance of finding colonizing shola trees). Keeping in mind that this restoration process is meant to benefit wildlife, and that we need buffers of wattle to satisfy the prevailing high demand of firewood, it is important that these plots are not within easy reach of people. It would be best to choose plots that are easily accessible, for example besides a road (an ideal location for both plots would be around Berijam lake). These plots will require constant support to provide the best conditions for grasses to make a comeback, mainly the regular (every 3-4 weeks) removal of seedlings of non-native trees and native woody shrubs. It is highly likely that supplementary planting of native grasses will be required.
    • Based on the lessons we learn from restoring grasses in these initial experimental plots for a period of 2-3 years, we can then expand the scope of removal to other adjacent non-native tree plantations. There are also lessons waiting to be learnt from a few mass tree cuttings that the Forest Department has conducted over the last decade.

About the author

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Dr. Farshid S. Ahrestani is a wildlife and quantitative ecologist interested in spatiotemporal questions related to community and population ecology. He is also deeply passionate about biodiversity conservation.



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