Forests under threat

30/03/2001

Forests under threat smoggy air damages plants, causes leaf injury, adversely affects plant growth and reduces fruit yield. Air pollutants such as sulphur, fluorides, ozone affect forest ecosystems adversely. This book brings out 30-35 years of research experience of around 30 scientists who have studied the effect of pollutants on plant life. The knowledge attained over the years has enabled the first ecosystem-level interpretation of chronic ozone exposure effects on forest ecosystem in the United States Environment Protection Agency (usepa) criteria documents, used in support of the evaluation of National Air Quality Standards (naqs) for ozone in the us.

Combination of various factors such as topography, climate, geology and vegetation influence the soils, underground water table, and hence the nature of the forest ecosystem. These have been discussed in case of forests of San Bernardino Mountains (sbm) in chapter one of this case study, while chapter two discusses the local climatic factors and their effects on ambient air pollution levels. Latitude, altitude, winds, ocean currents and mountain barriers along with storms of various kinds are some of the major factors that control climate. In this chapter, the effects of each of such factors on the climate of the concerned region has been discussed.

Forest fires play a vital role affecting species composition, vegetation structure and biogeography. Chapter 3 of the book evaluates the fire ecology of major eco-systems while describing the vegetation of sbm . The Mediterranean climate of winter rain and dry summers result in inefficient decomposition, rapid fuel build-up and high fire hazard. The long-term effect of air pollution varies depending on the intrinsic sensitivity of species to pollutants, spatial variation of pollutants and spatial variations in fire regimes.

Chapter 4 of the book documents ozone, particulate, nitric acid and sulphuric acid ambient concentrations in the sbm and in California's South Coast Air Basin (scab) and other California forests. While nitric oxide and ammonia gases rapidly deposit to the earth's surface, particles of 2.5 micron size and below (pm 25 ) remain in the atmosphere five to 10 times longer and can be transported for hundreds of kilometres. Air quality measurements taken at sbm and scab help understand the concentration of these interacting components, how they vary with the time of the day and year, how they are transported from scab region to the forests under study.

Visibility parameters of the region have been studied using both aerosol and optical monitors. The haziest days are found to occur during spring due to high nitrates and sulphates concentrations. Visibility is also found to have a strong diurnal component, with most mornings being relatively clear and most evenings being hazy. During clear winter days (winter experiencing the clearest conditions) the visual ranges are about 211 km, while during haziest days occurring during spring, they drop down to 23 km. These have been analysed and rationalised in chapter five of the book.

Plant processes that gain carbon, nutrients, and water are affected by air pollutants. Oxidant pollution adversely affect photosynthetic capacity, photo-electron transport system, rubisco carboxylation efficiency and apparent quantum efficiency of the forest trees. Most conifers have a higher threshold to the air pollutants relative to broad-leaved deciduous species. The effects of environmental stressors on physiological responses of plants have been analysed in chapter six of the book and it has been concluded that there is no unique response of plants to oxidant stress and that it is the collection of various individual plant's success or failure in the face of multiple stressors.

Ozone injury to tree species and air pollution effects on growth has been discussed in chapter 7 and 8. Severity of ozone exposure in sbm forests seems to be sufficient to maintain continuing stress on ponderosa and jeffrey pines. Rising levels of greenhouse gases have the potential to increase global temperatures and reduce precipitation.

The relatively thin leaves and fast growth rates of most under story vegetations (annuals, herbaceous, perennials and deciduous shrubs) suggest that they may be more susceptible to ozone than most coniferous or deciduous forest trees (over story trees). The species most susceptible to ozone injury in sbm forests have been studied and details of the experimental exposure studies are given in chapter 9; while chapter 10 reviews investigations conducted on Lichen communities in the sbm region. This field data provides the first evidence that lichens may also respond to oxidant air pollutants, apart form their well established use as indicators of sulphur dioxide pollution.

Studies on wet deposition (rain, snow, fog etc) and dry pollutant (large air pollutants emitting from mobile and stationary sources during long dry periods) deposition characteristics in the sbm region are discussed in chapter 11. It seems that ozone concentrations in the mountainous terrain need not necessarily increase continually with increasing altitude (as found in earlier European studies) but that ozone concentration distribution patterns vary with climate and terrain features of individual mountain ranges. It was also determined that bulk deposition of nitrogen and sulphur compounds ( no 3 , so 4 , nh 4 etc) in late summer was proportional to the rain precipitation.

Chapter 12 deals with the direct effects of nitric acid on forest trees. Excessive nitrogen deposition causes major perturbations in bio-chemical cycling of nitrogen. Nitrogen deposition and cycling in the Mediterranean forests has been discussed in chapter 13. Limited field studies indicate that chaparral and mixed conifer forests with nitrogen deposition of around 20-25 kg/ha/year for around 40 years or so show symptoms of nitrogen saturation. Further research is needed to verify this threshold. Trees and their insects and pathogens affect each other's survival and reproduction. Pollution may have direct or indirect effects on insect, their hosts and pathogens. The pest-host air pollution interactions in the sbm is discussed in chapter 14, while chapter 15 reviews the results of a study initiated in the 1970s to evaluate the long term impact of chronic air pollution on plant succession in the mixed conifer forest of sbm .

In chapter 16 two models used to simulate the effect of nitrogen deposition, ozone, and climate change on biogeochemical processes of forest growth at a site in the eastern sbm , have been discussed. The simulations are intended to show the logical implications of the present state of knowledge of carbon and nutrient flows and accumulation in the mixed conifer forest system of sbm . Neither model has adequate scope to simulate all complexities of this ecosystem.

Chapter 17 focuses on the human aspects of air quality in sbm . Poor air quality affects both the ecosystem and humans within it. Apart from the residents in sbm ranges, more than 6 million visitors come to this second most visited wilderness in California annually. Both year round and seasonal residents may have definite interests in the health and sustainability of the flora and fauna of the region and as such should have a major commitment to a plan of action to achieve clean air environment.

The last two chapters of the book summarise briefly the ecological risks and implications of chronic air pollution for forest management policy formulation and research. Areas of major concern identified for research are potential problems arising from global climate changes, long term response of the mixed conifer forest ecosystem to continued air pollution, the interaction of plant succession, fire risk and air pollution, potential synergistic interactions of ozone and nitrogen pollutants and forest management activities to reduce the impacts of air pollution on the mixed conifer forest eco system.

The book presents the collective knowledge of eminent researchers who have worked together for around 30 years studying the structure and function of a mixed conifer forest ecosystem and its responses to climatic changes and long term exposure to photochemical oxidant air pollution. Well illustrated, well presented with exhaustive references, it provides a wealth of technical and scientific information through a case study for all advanced students.

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