According to the Inter-government Panel on Climate Change (IPCC) Working Group II report land use change and cover (LUCC) is both a cause and consequence of climate change and constitutes a major driver of changes in a natural ecosystem and biodiversity. The major ongoing changes in subtropical and tropical regions are clearance of forests and woodlands for conversion to agriculture, pasture and commercial cash crops (such as soy, palm oil and rubber). These changes have been causing green house gas (GHG) emissions and their increased concentrations in the atmosphere. The green vegetation is both a source and sink of GHGs. Deforestation for many people in tropical countries creates economic opportunities that is encouraged by growing demand for food products and also creates export potential.
Forests help mitigate climate change impact through removal of large quantities of carbon from the atmosphere, through absorption or reflection of solar radiation (albedo) the production of aerosols that form clouds and also some cooling effect through evapotranspiration.
The IPCC fifth assessment report (AR5) claims with high confidence that the global forests currently are net sink. The intake of carbon by well-stocked and regenerating forests was counterbalanced by release of GHG due to land use change, deforestation and forest degradation between 2000 and 2007 resulting in a net balance of 1.1± 0.8GtCyr-1. However, there are also recent scientific findings that point out that the forest carbon sink is gradually becoming weaker as a number of complex drivers are causing deforestation and forest degradation.
Interestingly, the impact of climate change on temperate forests is that there is an increase in growth rates of trees and corresponding carbon stocks. This is attributed to increased length of growing season, atmospheric CO2 concentration, nitrogen deposition, and forest management involving regeneration of area heavily harvested in the past. On the contrary the impact of climate change on tropical forests is still not very clear with degree of certainty and there are conflicting views. There is no clarity regarding the rates of photosynthesis due to increased CO2 concentrations, erratic patterns of precipitation, frequency of drought, forest fires and pests and pathogen epidemics. Climate models have not been able to provide reasonable conclusions. However, there is agreement that many tree species in moist tropical forests are sensitive to drought and may face migration and mortality. There is also agreement that forest fire frequency and severity is also increasing.
Dry tropical forests have characteristics that develop under seasonal rainfall regime. Monsoon rains occur only in a certain part of the year for 2-3 months and these forests face long periods of heat and drought. There is likelihood that significant parts of these forests are going to be under severe climate change stress due to over exposure to high temperatures and lower rainfall. The productivity of these forests is likely to decrease due to climate change and associated increased risk of fires, pest and pathogen attacks and other drivers.
There are evidences that plantation forests created by afforestation and reforestation are showing increased growth rates and productivity due to increased CO2 the atmosphere. However, the effect of increased fire frequency and intensity, drought, pathogen, and storms have not been taken into account that will affect or offset the increased productivity. The plantation forests are also vulnerable due to their even aged and single species composition that makes them less resilient. It is believed by scientists that uneven aged and mixed species tree plantations are better equipped to face climate change stress, are more resilient and have better chance of surviving.
It is well recognized that earth’s forest ecosystems are vulnerable to climate change impacts; the only question remains is the severity in different regions. Certain changes are being observed in forest ecosystems and are being attributed to rising global temperature and changed precipitation patterns. On the other hand, there are indications that the productivity of boreal forests is increasing due to increased temperature. There is likelihood that the climate change will lead to species migration, tree mortality, early tree flowering and seed production, early leave fall among deciduous species, pest and pathogen epidemic, droughts, absence of natural regeneration as well as loss of biodiversity and even species extinction. Changes are likely to increase both forest fire frequency and intensity.
Some forests in temperate regions may expand northward and in some tropical regions forest cover and area may reduce. Unsustainable land use and non-availability of water in many tropical regions may result in forest species die back. There are observations in south Asia that Tectona grandis (teak) and Shorea robusta, two valuable timber species, are showing top dying phenomenon and it is expanding to large areas. Although scientific research and data are lacking, the changes are closely associated with climate change.
The adverse impact on forest health and productivity will reduce availability of ecosystem services and goods from forests that will adversely affect local communities that depend on forests for sustenance and livelihood in the tropical countries of Africa, Asia and Latin America. These countries are likely to suffer the worst impact that may be difficult to visualize today.
It is a matter of concern that there appears to be no preparedness to face the climate challenges. The approach is one of watching passively and even a feeling of skepticism about the scientific forecast as emerges from climate change models. The adaptation strategies for forest ecosystems are lacking.
There is also a sense of lack of clarity as to what exactly would be the impacts, and what would be the intensity and which species and ecosystems will be more vulnerable than the others. It remains difficult to make exact or tenable forecasts on these issues. In view of the complexities of the natural ecosystems, modeling for scenario forecasting has its own limitations. Many people tend to argue that the best course is watch and observe the changes taking place and at what speed as well as resilience of these systems. The predictions point out that the impacts will be highly conspicuous and even irreversible in the second half of this century.