Interactive effects of ozone and climate on tree growth and water use in a southern Appalachian forest in the USA.

* A lack of data on responses of mature tree growth and water use to ambient ozone (O(3)) concentrations has been a major limitation in efforts to understand and model responses of forests to current and future changes in climate. * Here, hourly to seasonal patterns of stem growth and sap flow velocity were examined in mature trees from a mixed deciduous forest in eastern Tennessee (USA) to evaluate the effects of variations in ambient O(3) exposure and climate on patterns of stem growth and water use. * Ambient O(3) caused a periodic slowdown in seasonal growth patterns that was attributable in part to amplification of diurnal patterns of water loss in tree stems. This response was mediated by statistically significant increases in O(3)-induced daily sap flow and led to seasonal losses in stem growth of 30-50% for most species in a high-O(3) year. * Decreased growth and increased water use of mature forest trees under episodically high ambient O(3) concentrations suggest that O(3) will amplify the adverse effects of increasing temperatures on forest growth and forest hydrology.

Interactive effects of ozone and climate on water use, soil moisture content and streamflow in a southern Appalachian forest in the USA.

* Documentation of the degree and direction of effects of ozone on transpiration of canopies of mature forest trees is critically needed to model ozone effects on forest water use and growth in a warmer future climate. * Patterns of sap flow in stems and soil moisture in the rooting zones of mature trees, coupled with late-season streamflow in three forested watersheds in east Tennessee, USA, were analyzed to determine relative influences of ozone and other climatic variables on canopy physiology and streamflow patterns. * Statistically significant increases in whole-tree canopy conductance, depletion of soil moisture in the rooting zone, and reduced late-season streamflow in forested watersheds were detected in response to increasing ambient ozone levels. * Short-term changes in canopy water use and empirically modeled streamflow patterns over a 23-yr observation period suggest that current ambient ozone exposures may exacerbate the frequency and level of negative effects of drought on forest growth and stream health.

High-value renewable energy from prairie grasses.

Projected economic benefits of renewable energy derived from a native prairie grass, switchgrass, include nonmarket values that can reduce net fuel costs to near zero. At a farm gate price of $44.00/dry Mg, an agricultural sector model predicts higher profits for switchgrass than conventional crops on 16.9 million hectares (ha). Benefits would include an annual increase of $6 billion in net farm returns, a $1.86 billion reduction in government subsidies, and displacement of 44-159 Tg/year (1 Tg = 1012 g) of greenhouse gas emissions. Incorporating these values into the pricing structure for switchgrass bioenergy could accelerate commercialization and provide net benefits to the U.S. economy.

Seasonal changes in shoot water relations of Picea rubens at two high elevation sites in the Smoky Mountains.

Seasonal changes in water relations of current-year shoots of red spruce (Picea rubens Sarg.) were examined in relation to climatic conditions in trees growing at elevations of 1720 and 1935 m on Clingman's Dome, Tennessee, USA, where increment core data have shown that red spruce decline increases with elevation. Relative height growth of trees at 1720 m was 68% greater than in trees at 1935 m. Following two weeks in July with only traces of precipitation, trees at both sites showed decreased saturated osmotic potentials. The magnitude of the reduction was greater in trees at the high elevation site than in trees at the low elevation site. However, during August and September, shoot water relations of trees at both sites were similar. Precipitation patterns and water relations measurements suggested that, at both sites, trees experienced water stress only briefly during the growing season and to a degree that could not account for the lower growth rates of trees at the high elevation site. During the period of cold hardening in October and November, trees at the low elevation site exhibited saturated osmotic potentials that were lower by 0.2 MPa and solute accumulation (osmol kg(dw) (-1)) that was 48% greater than in trees at the high elevation site.

An analysis of climate and competition as contributors to decline of red spruce in high elevation Appalachian forests of the Eastern United states.

Long-term growth patterns of red spruce (Picea rubens Sarg.) were analyzed from increment cores collected from over 1000 trees at 48 sites in the eastern United States. Principal objectives were the evaluation of the distribution, timing, and uniqueness of observed patterns of decreasing radial growth during the past 25 years and the examination of stand competition and climate as factors contributing to observed changes.Our analyses focused on historical records of spruce mortality and approximately 200 years of radial growth data to search for historical precedents for current trends. In this work we have used time series analysis to detect the temporal frequency of significant negative or positive shifts in radial growth rates, an analysis of relationships between a stand competition index and observed changes in growth and mortality, and modeling of past growth-climate relationships to determine whether recent growth changes could be predicted based on climate.Collectively, these analyses indicate that the observed growth decreases of surviving red spruce trees at northeastern sites with high mortality have been anomalous during the past 20 to 25 years with respect to both historical annual growth patterns and past relationships to climate or stand development at these sites. In general, reductions in radial increment that have also been noted at southern high elevation sites but not at low elevations occurred 5 to 10 years later than at northern sites and represent less substantive departures from growth trends predicted by linear climate models.These results suggest that regional and not local stresses have triggered the observed decline in radial growth of red spruce at these sites. While climatic change may have contributed to observed changes, the degree of radial growth suppression observed is greater than would be expected based on past growth-climate relationships. This unique relationship of growth to climate suggests the influences of either recent, unique combinations of climatic stresses or the possibly interactive intervention of other regional-scale stresses, such as atmospheric pollution.

Report on discussion of paper by P. G. Jarvis.

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Report on discussion of paper by H. G. Miller.

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Carbon dioxide assimilation and growth of red spruce (Picea rubens Sarg.) seedlings in response to ozone, precipitation chemistry, and soil type.

The influence of ozone, mist chemistry, rain chemistry, and soil type on CO2 assimilation and growth of red spruce (Picea rubens Sarg.) seedlings was investigated over a 4-month period under controlled laboratory and glasshouse conditions. Growth was evaluated through interval estimates of aboveground relative growth rates (RGR) and the partitioning of biomass components at harvest to root, stem, and needle fractions. Precipitation chemistry treatments and O3 exposure dynamics were based on reported characteristics of air chemistry and/or deposition in high-elevation forests of eastern North America. The two soils were collected from Camels Hump in the Green Mountains of Vermont and Acadia National Park on the Maine coast. Soil from Acadia had higher organic content, higher levels of extractable base cations, and lower levels of extractable aluminum and heavy metals. The only treatment variables that consistently influenced the growth of P. rubens were soil type and rain chemistry. In comparison with seedlings grown in soil from Acadia National Park, those grown in Camels Hump soil had significantly less needle (27%), stem (33%), and root (26%) biomass at harvest and statistically lower aboveground RGR within 2 months after initiation of the treatments. Seedlings grown in Camels Hump soil had significantly higher levels of aluminum (6.5X), copper (1.4X), and nickel (2.7X) in new needle tissue. The only influence of precipitation chemistry on the growth of P. rubens was a pattern of greater root and shoot biomass in seedlings experiencing the more acidic rain treatments. Interactive effects among the main treatment variables (e.g., acidic mist and O3, acidic rain and soil type) on seedling growth were not notable. Rates of CO2 assimilation and transpiration on a per gram needle dry weight basis [mol·g-1·s-1] were not influenced by any of the main treatment variables or their interaction. Because neither soil type nor precipitation chemistry influenced the efficiency of CO2 assimilation per gram dry weight of needle tissue, the physiological mechanism underlying the growth response of P. rubens is attributed to a change in either whole-plant allocation of carbon resources or a direct toxic effect in the rhizosphere on root growth.

Trace elements in tree rings: evidence of recent and historical air pollution.

Annual growth rings from short-leaf pine trees in the Great Smoky Mountains National Park show suppressed growth and increased iron content between 1863 and 1912, a period of smelting activity and large sulfur dioxide releases at Copperhill, Tennessee, 88 kilometers upwind. Similar growth suppression and increases of iron and other metals were found in rings formed in the past 20 to 25 years, a period when regional fossil fuel combustion emissions increased about 200 percent. Metals concentrations in phloem and cambium are high, but whether they exceed toxic thresholds for these tissues is not known.

Effects of SO(2) and O(3) on Allocation of C-Labeled Photosynthate in Phaseolus vulgaris.

A series of laboratory exposures of two varieties of bush bean (Phaseolus vulgaris L., var 274 and var 290) was conducted to determine the sensitivity of [(14)C]photosynthate allocation patterns to alteration by SO(2) and O(3). Experiments with the pollution-resistant 274 variety demonstrated short-term changes in both (14)C and biomass allocation to roots of (14)CO(2)-labeled plants but no significant effect on yield by up to 40 hours of exposure to SO(2) at 0.50 microliters per liter or 4 hours of O(3) at 0.40 microliters per liter. Subsequent experiments with the more sensitive 290 variety demonstrated significant alteration of photosynthesis, translocation, and partitioning of photosynthate between plant parts including developing pods. Significant increases in foliar retention of photosynthate (+40%) occurred after 8 hours of exposure to SO(2) at 0.75 microliters per liter (6.0 microliters per liter-hour) and 11 hours of exposure to O(3) at 0.30 microliters per liter-hour (3.3 microliters-hours). Time series sampling of labeled tissues after (14)CO(2) uptake showed that the disruption of translocation patterns was persistent for at least 1 week after exposures ceased. Subsequent longer-term exposures at lower concentrations of both O(3) (0.0, 0.10, 0.15, and 0.20 microliters per liter) and SO(2) (0.0, 0.20, and 0.40 microliters per liter) demonstrated that O(3) more effectively altered allocation than SO(2), that primary leaves were generally more sensitive than trifoliates, and that responses of trifoliate leaves varied with plant growth stage. Altered rates of allocation of photosynthate by leaves were generally associated with alterations of similar magnitude and opposite direction in developing pods. Collectively, these experiments suggest that allocation patterns can provide sensitive indices of incipient growth responses of pollution-stressed vegetation.

Relative humidity: important modifier of pollutant uptake by plants.

Laboratory measurements of foliar uptake of sulfur dioxide and ozone by red kidney beans demonstrated a strong effect of relative humidity on internal pollutant dose. Foliar uptake was enhanced two- to threefold for sulfur dioxide and three- to fourfold for ozone by an increase in relative humidity from 35 to 75 percent. For the same exposure concentration, vegetation growing in humid areas (such as the eastern United States) may experience a significantly greater internal flux of pollutants than that in more arid regions.