Soil nitrogen and chronic ozone stress influence physiology, growth and nutrient status of Pinus taeda L. and Liriodendron tulipifera L. seedlings.

The effects of soil nitrogen availability and chronic ozone stress on carbon and nutrient economy were investigated in loblolly pine (Pinus. taeda L.) and yellow-poplar (Liriodendron tulipifera L.). One-year-old seedlings were planted individually in pots in forest soil of low (58 µg g-1 ), medium (96 µg g-1 ) or high (172 µg g-1 ) initial concentrations of soluble nitrogen. The seedlings were exposed to ozone in open-top field chambers at sub-ambient (charcoal-filtered air), ambient, and elevated (ambient + 60 nl 1-1 O3 ) (32, 56, 108 nl 1-1 O3 , 1 h seasonal mean, respectively) levels for 18 weeks. At final harvest loblolly pine dry matter increased by 50% at the highest soil K level relative to the low with the largest gains in new needle biomass. Elevated ozone reduced the biomass of current-year needles by 20% in plants grown at the highest N level. Higher soil N supply increased the concentration of nitrogen in needles, stimulated current-year needle photosynthesis and increased needle and whole-plant water-use efficiencies. Ozone treatment had no significant effect on photosynthesis or water-use efficiency in either species, although ozone exposure tended to reduce- stomatal conductance in loblolly pine. The low N treatment increased the proportion of dry matter allocated to fine roots in yellow-poplar, but whole-plant dry weight had not responded to N fertilization at the final harvest, suggesting other limitations on growth. Ozone exposure increased leaf abscission and doubled leaf turnover m yellow-poplar. Although yellow-poplar was highly sensitive to ozone-induced leaf abscission, final whole-plant dry weights were not affected. The indeterminate growth habit of yellow-poplar permitted compensatory leaf growth which may have ameliorated effects of chronic ozone stress on biomass gain. Ozone exposure also decreased shoot weight more than root weight, resulting in higher root:leaf ratios in loblolly pine and a similar trend m higher fine roor:leaf ratios in yellow-poplar. Greater proportional allocation of carbon to roots in response to nutrient deficiency may preclude an increased allocation to shoots often observed in response to air pollution stress. Interspecific differences in growth response to chronic ozone and nutrient stress may be influenced by differences in leaf growth habit.

A source-sink framework for coupling water, carbon, and nutrient dynamics of vegetation.

Feedbacks between photosynthesis and growth and the influence on these relationships of environmental factors are reviewed. The evidence indicates a strong dependence of photosynthesis on photosynthate utilization. It also indicates that growth is more sensitive than photosynthesis to water and mineral nutrient stress. As a consequence, many relationships between photosynthesis and external driving variables are confounded by internal feedback effects that reflect the influence of external factors on growth. A source-sink framework for modeling carbon dynamics with coupled water and nutrient interactions in soil-plant systems is outlined. Simulations based on these concepts suggest that sink feedback regulation of photosynthesis at various times in diurnal, wetting-drying, and annual cycles is probably a common occurrence in soil-plant systems.

Approaches to scaling up physiologically based soil-plant models in space and time.

Many broad-scale, environmental phenomena can be investigated by extrapolating from detailed study of events at a small scale. This paper evaluates approaches to the use of physiologically based soil-plant models for addressing broad-scale, environmental issues. When the space and time domains of a soil-plant simulator are extended, there is an increase in the variability of soil, plant, and weather variables, which can be dealt with by what is called extended-range modeling, ERM. There may also be a gain of phenomena not represented at the small scale, which can be dealt with by what is called phenomena-added modeling, PAM. As an example of ERM, a Monte Carlo procedure, called Latin hypercube sampling, is used to estimate annual photosynthate production of an oak-hickory forest under three atmospheric CO(2) concentrations. Phenomena-added modeling is illustrated by scaling up spatially from a vegetated plot to a watershed, and scaling up temporally from a physiological model with hourly time steps to a forest-succession model operating on annual time steps. Where large-scale processes take place on a time scale similar to, or faster than, that of small-scale processes (plot-watershed case), less computation is required if the small-scale processes are built into the large-scale model and ERM is conducted with the expanded model. Phenomena-added modeling may be conducted by information transfer from a small-scale simulator to a large-scale simulator. This is also possible with Latin hypercube sampling by using the output frequency distributions from the small-scale model as input distributions for the large-scale model. The final outputs at the large scale are also frequency distributions, and these can be used to determine confidence intervals for statistical comparisons among modeling scenarios. The ERM and PAM methods are data and computer intensive; nevertheless, they can fill an important need for addressing large-scale issues that cannot be adequately addressed through other scaling up methods.

Carbon allocation, root exudation and mycorrhizal colonization of Pinus echinata seedlings grown under CO(2) enrichment.

Increased exudation of carbon compounds from roots may provide a mechanism for enhancement of nutrient availability to plants growing in a CO(2)-enriched atmosphere. Therefore, the effect of atmospheric CO(2) concentration on carbon allocation and root exudation was investigated in Pinus echinata Mill. (shortleaf pine) seedlings. After 34 and 41 weeks, seedlings growing in 695 microl l(-1) CO(2) allocated proportionately more (14)C-labeled photosynthate to fine roots than did seedlings growing in ambient air. This was associated with greater fine root mass and mycorrhizal density in CO(2)-enriched plants after 34 weeks. Exudation of soluble, (14)C-labeled compounds from roots also was greater in these plants at 34 weeks, but the effect of CO(2) concentration on exudation did not persist at 41 weeks.

Effects of Atmospheric CO(2) Enrichment on the Growth and Mineral Nutrition of Quercus alba Seedlings in Nutrient-Poor Soil.

One-year-old dormant white oak (Quercus alba L.) seedlings were planted in a nutrient-deficient forest soil and grown for 40 weeks in growth chambers at ambient (362 microliters per liter) or elevated (690 microliters per liter) levels of CO(2). Although all of the seedlings became severely N deficient, CO(2) enrichment enhanced growth by 85%, with the greatest enhancement in root systems. The growth enhancement did not increase the total water use per plant, so water-use efficiency was significantly greater in elevated CO(2). Total uptake of N, S, and B was not affected by CO(2), therefore, tissue concentrations of these nutrients were significantly lower in elevated CO(2). An increase in nutrient-use efficiency with respect to N was apparent in that a greater proportion of the limited N pool in the CO(2)-enriched plants was in fine roots and leaves. The uptake of other nutrients increased with CO(2) concentration, and P and K uptake increased in proportion to growth. Increased uptake of P by plants in elevated CO(2) may have been a result of greater proliferation of fine roots and associated mycorrhizae and rhizosphere bacteria stimulating P mineralization. The results demonstrate that a growth response to CO(2) enrichment is possible in nutrient-limited systems, and that the mechanisms of response may include either increased nutrient supply or decreased physiological demand.

Passive nighttime warming facility for forest ecosystem research.

A nighttime warming experiment is proposed. Over the last four decades a significant rise in nighttime minimum temperature has been determined from analysis of meteorological records from a global distribution of locations. The experiment involves nighttime deployment of infrared (IR) reflecting curtains around four sides of a forest canopy and across the top of the forest to mimic the top-down warming effect of cloud cover. The curtains are deployed with cable and pulley systems mounted on a tower and scaffolding structure built around the selected forest site. The trunk space is not enclosed except as an optional manipulation. The curtains reflect long-wave radiation emitted from the forest and ground back into the forest warming the trees, litter, and soil. Excellent infrared reflection can be obtained with commercially available fabrics that have aluminum foil bonded to one side. A canopy warming of 3 to 5 degrees C is expected on cloudless nights, and on cloudy nights, a warming of 1 to 3 degrees C is anticipated relative to a control plot. The curtains are withdrawn by computer control during the day and also at night during periods with precipitation or excessive wind. Examples of hypothesized ecosystem responses to nighttime warming include: (1) increase in tree maintenance respiration (decreasing carbon reserves and ultimately tree growth), (2) increase in the length of the growing season (increasing growth), (3) increase in soil respiration, (4) increase in litter decomposition, (5) increase in mineralization of N and other nutrients from soil organic matter, (6) increase in nutrient uptake (increasing growth), and (7) increase in N immobilization in litter. Hypothesis 1 has the opposite consequence for tree growth to Hypotheses 2 and 6, and thus opposite consequences for the feedback regulation that vegetation has on net greenhouse gas releases to the atmosphere. If Hypothesis 1 is dominant, warming could lead to more warming from the additional CO(2) emissions. Site-specific meteorological, ecophysiological, and phenological measurements are obtained in the warming treatment and in a carefully selected control plot to investigate site-specific hypotheses. Measurements made on both plots for a baseline period and during the period of curtain deployment provide data to test the hypotheses statistically by the "before-after-control-impact" method applicable to unreplicated experiments. The enclosure has a modular design that can be adapted and combined with other forest-scale manipulation experiments such as free air CO(2) enrichment and throughfall displacement.