Tropical timber harvesting, biodiversity loss and environmental change

Forest profile diagram before and after selective logging in primary hill dipterocarp forests. For only three exploitable emergent trees, damage to the residual trees and the canopy was considerable,top.

This paper examines the possible biodiversity loss and environmental changes following tropical forest harvesting and looks at practical approaches to address them.
In tropical forest, most of the biodiversity studies have been done on primary forest. Very little is known on the biodiversity of production areas that have been subjected to logging. This is particularly true for plants against the multitude of studies on fauna particularly birds and mammals. Currently scientists realised that both conservation and current style of timber exploitation are not compatible in managing tropical forests in terms of maintenance of structure, species composition and diversity. As a result, some scientists emphasised the need for preliminary baseline assessment followed by monitoring to achieve an optimum practice that is compatible in both timber production and biodiversity conservation. Others looked at measuring forest biodiversity for deciding priorities by which certain areas of forest can be protected and the remaining converted to other uses with minimum impact on biodiversity.
However, in both cases, emphasis has been put to regulate human interventions to minimise the impact on biodiversity and environment. But to what extent is it achieved? How much of the forest is damaged by tropical logging? How much of species diversity is lost? What effects of logging on forest environment so far detected? What actions can be taken to address biodiversity loss and environmental consequences? Before describing the above issues, first, it is important to see the ingredients of the word, biodiversity and its relation to the conservation measures.
The word 'biodiversity' has recently been widely used to relate it with the conservation and global environmental change. It refers to the entire range of variation among plants and animals across all levels of the biological hierarchy from genes to ecosystems and it is therefore a property of living systems of being distinct, that is, different, unlike. Discussion on biodiversity thus usually focuses on the number of existing species in a site and their ecological aspects. Currently, presence of rare species is the most often cited criteria for conservationists, and as such conservation measures to the whole ecosystem are getting priority as holistic approach. In the past, saving prominent species and threatened areas were the major focus for conservation decision.
Though timber harvesting is considered as secondary cause of deforestation, but uncontrolled logging had caused many tropical forest severely degraded, losing its original composition, and thus gradually deforested. Even before collecting and documenting biodiversity data, many tropical forest species meanwhile possibly eliminated due mainly to over exploitation and habitat alteration. According to an estimate, about 20 percent of the global carbon dioxide emissions come from deforestation and forest fires, with tropical America being the chief contributor.
At global level, tropical forests are extremely rich in plants and animal species. About two-thirds of approximately 250,000 species of flowering plants in the world occur in the tropics. Trees are the major component in tropical forest ecosystem that represents varieties of economic, social and environmental values. Unfortunately, for high export earning, the tropical forest is regarded predominately as a source of timber and this tendency has caused the species-rich forest of the tropics to be converted into species-poor secondary forest.
Based on the Wilson and Peter's conservative estimate of a tropical deforestation rate of 0.7 per cent per annum, about 50 species are being lost per day. According to an estimate of FAO, at least 5-10 per cent of tropical forest species would face extinction in the next 30 years. Setting priorities for conservation therefore requires a better understanding of the process of deforestation, the amount and spatial heterogeneity of forest altered and their implications on species extinction.
Environmental aspects of tropical forest logging are also less known. Few scanty data are available. However, intensity of disturbance followed by changes in soil property and microclimate could have a profound effect not only on regeneration in early stage of succession but also on structure and species composition and richness of secondary forests. But information on how physical components of the ecosystem are affected by different intensity of selective logging remains largely untapped.
Based on the available published literature, it is difficult to ascertain the contribution of tropical forest harvesting towards rate of forest destruction, species loss and environmental change. It is because of the fact that study on the same population or samples before and after timber harvesting was rarely done. In other words, most studies were restricted to compare the logged forest with the adjacent un-logged forests. Comparisons in such case may be confounded and valid changes cannot be detected if there are differences in topography, soils, and species composition and disturbance history between the un-logged and logged forest. As a result, sweeping statement cannot be made on how much of species diversity is lost as a result of selective logging, though overall species richness (i.e. number of species) may decline in the logged-over forest than the adjacent un-logged stands.
However, one extensive study on census of the same samples before and after timber harvesting over 1000 hectares of selectively logged primary dipterocarp forest using heavy bulldozers and following systematic sampling design showed that about 47 percent of trees were totally injured and 40 percent trees were smashed or dead. The relative disturbance index (RDI) also showed significant negative relationship with the tree density per hectare. There were also drastic changes in the forest structure and 43 percent canopy was open followed by invasion of pioneer species. The resulting changes in the forest structure possibly displaced many forms of animal life, particularly birds.
24.1 percent of the total tree species were locally lost from the study site in the first cut that encompasses only rare tree species including highly valued timber trees. Again, about 50 percent of the residual species were under very rare category in the logged-over forest. The increase in rarity was due to the fact that some species of common and frequent status were newly added in the rare category by reduction of individuals following logging. These rare species would be lost even forever from the area if they cannot survive as a result of destructive harvesting. Per plot species richness and Shannon diversity index fell by 42.2 percent and 20.9 percent, respectively after logging.
Environmental aspect of logging suggests much delaying of the recovery process for the original composition. Severe soil compaction with bulk density exceeding 1.4 g/cm³ was observed in the logging tracks and log landing sites and is unfavourable for normal tree growth. But the other area remained below the critical level as in the case of undisturbed forest condition. The logging also reduced the moisture content to 21.2 percent from the original level. The logging compartments with higher canopy damage had significantly low percentage of moisture content.
Significant changes in microclimates were also detected due to canopy opening. For example, selective logging significantly altered all microclimatic variables such as air temperature, soil temperature and relative humidity by increasing air and soil temperature but decreasing relative humidity. On a regional scale, however such heavily logged forest in the tropics might disrupt rainfall patterns, create hotter and drier environment. The regional cumulative environmental effects of tropical forest harvesting may also accelerate global climate change.
To address the effects of tropical timber harvesting, there is a real necessity for intervention not only in the management system but also in the extraction operation. These are as follows:
Integration of biodiversity in the management system: There is no evidence for long term success of the existing management system in the tropics as the system is solely dependent on timber production. With the indications of rapid decline in biodiversity in the tropics, there is a great necessity for integration of biodiversity survey with the existing management inventory so that the results of the inventory could be applied directly to resource management and conservation.
Execution of improved logging practices: Execution of harvesting guidelines is also more important than simply incorporating these in the management plan. For example, trees retained as seed bearers were also reported to be damaged due to lack of directional felling. Reduced impact logging reported elsewhere is of considerable importance towards efforts to reduce logging damage by about half in the tropical forest.
Environmental Impact Assessment (EIA) of forestry operations: Forestry operations can have negative environmental consequences for the forest area itself as well as for the surrounding environment. These consequences should be assessed in advance of operations to take necessary measures against the possible impact of logging. Studies showed that one of the major shortcomings in EIA reports in the tropics is the lack of detailed baseline data supported by sound ecological content and hence poor prediction of impacts and insufficient mitigation measures.
Policy support: At the policy level many tropical countries had already a biodiversity policy and plan of action proposed. The adoption of the national policy on biological diversity is a follow-up action of the nation's commitment to make operational the Convention on Biological Diversity. The current state of forest and biodiversity affairs in the tropics provide the proper setting and need for a better approach to biodiversity conservation especially in the timber production areas. The recent Bali Action Plan emphasised the enhanced national action on mitigation of climate change and acknowledged that forest degradation due to human interference also leads to emissions and if needs to be addressed through policy approaches and thus enhancing forest carbon stocks in developing countries.
Dr. Saiful Islam is Director, Forest Academy, Chittagong.