Physiological Effects of Air Pollutants on Western Trees. In: Olson RK, Binkley D, Böhm M, editors. The Response of Western Forests to Air Pollution. New York: Springer; 1992. pp. 183-233. [DOI: 10.1007/978-1-4612-2960-5_6]

A review of the relationships between drought and forest fire in the United States

The historical and presettlement relationships between drought and wildfire are well documented in North America, with forest fire occurrence and area clearly increasing in response to drought. There is also evidence that drought interacts with other controls (forest productivity, topography, fire weather, management activities) to affect fire intensity, severity, extent, and frequency. Fire regime characteristics arise across many individual fires at a variety of spatial and temporal scales, so both weather and climate - including short- and long-term droughts - are important and influence several, but not all, aspects of fire regimes. We review relationships between drought and fire regimes in United States forests, fire-related drought metrics and expected changes in fire risk, and implications for fire management under climate change. Collectively, this points to a conceptual model of fire on real landscapes: fire regimes, and how they change through time, are products of fuels and how other factors affect their availability (abundance, arrangement, continuity) and flammability (moisture, chemical composition). Climate, management, and land use all affect availability, flammability, and probability of ignition differently in different parts of North America. From a fire ecology perspective, the concept of drought varies with scale, application, scientific or management objective, and ecosystem.

Stricter ozone ambient air quality standard has beneficial effect on ponderosa pine in California

Ambient air quality standards and control strategies are implemented to protect humans and vegetation from adverse effects. We used a process-based tree-growth model (TREGRO) to show that over the past 37 years, changes in O(3) exposure, with accompanying variation in climate, are reflected in changes in the growth of Pinus ponderosa Dougl. ex Laws. in the San Bernardino Mountains near Los Angeles, California, USA. Despite variation in temperature and precipitation over the study period (1963-1999), O(3) exposure consistently reduced simulated tree growth. Simulated growth reductions increased concurrent with increasing O(3) exposure. The maximum growth reduction occurred in 1979. As O(3) exposures decreased during the 1980s and 1990s, effects on growth also decreased. This implies that emission control strategies taken to reduce exposures to attain O(3) standards benefited P. ponderosa growth in the San Bernardino Mountains. This modeling approach provides a powerful tool for solving the difficult problem of evaluating regulatory effectiveness by simulating plant response using long-term climate and air pollution exposure records for a given region.

Causal mechanisms by which sulphate, nitrate and acidity influence frost hardiness in red spruce: review and hypothesis

This paper summarizes results from four experiments in which red spruce seedlings (Picea rubens Sarg.) were exposed to simulated acid mist containing SO₄ ^2- , NH_4- , NO₃ ^- and H⁺ ions. Seedlings were grown in compost, with or without fertilizer, in charcoal filtered air in open-top chambers near Edinburgh, Scotland. Plants were sprayed from bud burst between May and November with mist containing different concentrations and combinations of the four major ions to provide a range of doses, which were applied at different frequencies. Reductions in frost hardiness expressed in terms of the temperature which killed 50% of shoots (LT₅₀ ) were significantly correlated with the dose of S received by the seedlings. Differences in foliar S concentrations between the controls and treated plants were correlated with S dose. Absolute S concentrations were, however, of limited use for predictive purposes. Seedlings appear to be more sensitive than older trees to S toxicity because the former have the greatest proportion of newly expanding needles which optimize conditions for foliar uptake. Seedlings are also least well equipped to export SO₄ ^2- ions since they have a smaller resource of older foliage to supplement their assimilate pool. In conditions which promote uncontrolled SO₄ ^2- ion uptake by foliage, i.e. high external SO₄ ^2- concentrations and incompletely formed cuticles, the potential exists for the internal build up of SO₄ ^2- ions. It is proposed that in the absence of sufficient assimilate and K the presence of these high concentrations of SO₄ ^2- ions in the apoplast or cytosol can lead to protein denaturation and loss of membrane integrity. Reductions in frost hardiness appear to result through direct attack by SO₄ ² ions on membrane proteins which impairs their function. Indirect effects on hardiness occur through both an increased consumption of sugars reducing the'pool'available for cryoprotection and a reduction in photosynthetic function, the ability to produce sugars. The presence of NO_3- N mitigates the toxic effects of SO₄ ² because SO₄ ² ions are consumed in assimilation processes which both utilize and are facilitated by the presence of large amounts of fertilizer N. High concentrations of SO₄ ² and H+ are found to be particularly toxic because of the synergistic effects these ions exert on their mutual uptake with devastating consequences for the control of cellular pH. Trees growing at high altitude sites are likely to be particularly sensitive to SO₄ ^2- toxicity because (1) their carbon balance is low, (2) cuticle development is poor and (3) levels of soil available Ca² tend to be low relative to Al³⁺ so that membranes may already be weakened as a result of insufficient Ca²⁺ ions for protein bridging.