Ozone and Its Known and Potential Effects on Forests in Western United States

Berlin

1997

S. 39-68

zahlr. Lit. Ang.

Bandaufführung

77948

Chronic ozone damage to forests of the western part of the United States is confined to the western slope of the central and southern Sierra Nevada, and to the sections of the transverse mountain ranges in southern California nearest the source of ozone pollution. California's summer climate, topography, and densely urbanized air basins combine to create ideal conditions for the accumulation of pollution and its long-range transport to adjacent forest areas. The species comprising the Californian mixed conifer forest type, particularly ponderosa pine and Jeffrey pine, are the most affected by chronic ozone exposure. Air pollution control measures have resulted in a gradual decrease of ozone concentrations in both urban and rural locations. An evaluation of whether the current situation should be considered in the context of "forest decline" must be based on an agglomeration of information ranging from mechanisms of ozone injury at the leaf and single tree level to the response of competing species in the mixed conifer forest. The role of interacting stressors, particularly periodic droughts, must also be evaluated. There has been no compelling incentive to examine changes in leaf antioxidant systems or to identify biochemical markers of early ozone injury to foliage since the appearance of visible symptoms (chlorotic mottle) serves for detection. Physiological studies with ponderosa and Jeffrey pine show that stomatal conductance must be high enough to permit the entrance of ozone into the leaf during the times when ozone polluted air is transported to forests. The concept that there is an "effective dose" required to cause leaf injury is very difficult to measure and interpret. Therefore, we cannot clearly describe the ozone exposure-response relationship for ponderosa and Jeffrey pine. The results of experiments with seedlings exposed to ozone in open-top chambers have provided valuable information on exposure response, but these results can not be extrapolated to larger trees in forest stands where other stressors cannot be easily evaluated. Drought stress and its relationship to chronic ozone injury has been investigated the most by inducing drought in seedling experiments and by interpreting tree ring patterns over an extended time. These approaches suggest that simultaneous drought and ozone stress leads to a significant reduction of ozone injury in controlled experiments; in forest stands, the various sequential combinations of drought and ozone stress result in "episodes" of tree decline due first to extended drought followed by a more "effective" dose of ozone during recovery periods. Since there is a range of sensitivity to ozone in a species population of ponderosa and Jeffrey pine, the most sensitive individuals will suffer preater competition from companion trees of the same or other species. White fir and incense cedar in all age classes and especially newly established seedlings are the most successful competitors in stands with the most ozone exposure. Tree-ring patterns, including consecutive years of missing rings, and records of higher rates of mortality suggest that "forest decline" is episodic, depending on the frequency of consecutive drought years and the levels of ozone exposure. Mature forest experimental manipulations need to be conducted to test this hypothesis and to more precisely determine the connections between single stressor seedling and sapling studies and mature forest observational studies. Underlying the ozone effect is the simultaneous accumulation of nitrogen in leaf litter and soils. The greater abundance of nitrogen in forest ecosystems that evolved under lower levels of nitrogen is an additional variable the interactions of which with long-term ozone injury are yet to be determined (Fenn et al. 1996). Early indications of lower soil pH and lower base saturation have been reported at heavily polluted sites in the San Bernardino mountains. Seedling experiments have shown a .