Below-ground carbon input to soil is controlled by nutrient availability and fine root dynamics in loblolly pine.

• Availability of growth limiting resources may alter root dynamics in forest ecosystems, possibly affecting the land-atmosphere exchange of carbon. This was evaluated for a commercially important southern timber species by installing a factorial experiment of fertilization and irrigation treatments in an 8-yr-old loblolly pine (Pinus taeda) plantation. • After 3 yr of growth, production and turnover of fine, coarse and mycorrhizal root length was observed using minirhizotrons, and compared with stem growth and foliage development. • Fertilization increased net production of fine roots and mycorrhizal roots, but did not affect coarse roots. Fine roots had average lifespans of 166 d, coarse roots 294 d and mycorrhizal roots 507 d. Foliage growth rate peaked in late spring and declined over the remainder of the growing season, whereas fine roots experienced multiple growth flushes in the spring, summer and fall. • We conclude that increased nutrient availability might increase carbon input to soils through enhanced fine root turnover. However, this will depend on the extent to which mycorrhizal root formation is affected, as these mycorrhizal roots have much longer average lifespans than fine and coarse roots.

Elevated CO(2) studies: past, present and future.

Increasing concentrations of atmospheric CO(2) are predicted to impact both current and future ecosystems. Elevated CO(2) is also predicted to affect biological processes at many levels of organization. In this overview, we summarize the responses of plants to elevated CO(2) including primary physiological and molecular responses, growth and reproductive responses, effects on plant-plant competition and interactions with other organisms, evolutionary responses, and effects at the ecosystem level. The objectives of this paper are to: (a) overview studies in this issue that were presented at a 1997 meeting entitled "Critical Assessment of the Response of Forest Ecosystems to Elevated Atmospheric Carbon Dioxide," which was sponsored by the Global Change and Terrestrial Ecosystems (GCTE) group of the International Geosphere Biosphere Program (IGBP), (b) review areas of recent progress in CO(2) research, (c) generalize patterns arising from past research, and (d) list critical areas of research for the future.

The effect of elevated CO2 and N availability on tissue concentrations and whole plant pools of carbon-based secondary compounds in loblolly pine (Pinus taeda).

We examined the extent to which carbon investment into secondary compounds in loblolly pine (Pinus taeda L.) is changed by the interactive effect of elevated CO2 and N availability and whether differences among treatments are the result of size-dependent changes. Seedlings were grown for 138 days at two CO2 partial pressures (35 and 70 Pa CO2) and four N solution concentrations (0.5, 1.5, 3.5, and 6.5 mmol l-1 NO3NH4) and concentrations of total phenolics and condensed tannins were determined four times during plant development in primary and fascicular needles, stems and lateral and tap roots. Concentrations of total phenolics in lateral roots and condensed tannins in tap roots were relatively high regardless of treatment. In the smallest seedlings secondary compound concentrations were relatively high and decreased in the initial growth phase. Thereafter condensed tannins accumulated strongly during plant maturation in all plant parts except in lateral roots, where concentrations did not change. Concentrations of total phenolics continued to decrease in lateral roots while they remained constant in all other plant parts. At the final harvest plants grown at elevated CO2 or low N availability showed increased concentrations of condensed tannins in aboveground parts. The CO2 effect, however, disappeared when size differences were adjusted for, indicating that CO2 only indirectly affected concentrations of condensed tannins through accelerating growth. Concentrations of total phenolics increased directly in response to low N availability and elevated CO2 in primary and fascicular needles and in lateral roots, which is consistent with predictions of the carbon-nutrient balance (CNB) hypothesis. The CNB hypothesis is also supported by the strong positive correlations between soluble sugar and total phenolics and between starch and condensed tannins. The results suggest that predictions of the CNB hypothesis could be improved if developmentally induced changes of secondary compounds were included.

Effects of CO(2) enrichment on growth and root (15)NH(4) (+) uptake rate of loblolly pine and ponderosa pine seedlings.

We examined changes in root growth and (15)NH(4) (+) uptake capacity of loblolly pine (Pinus taeda L.) and ponderosa pine (Pinus ponderosa Douglas. Ex Laws.) seedlings that were grown in pots in a phytotron at CO(2) partial pressures of 35 or 70 Pa with NH(4) (+) as the sole N source. Kinetics of (15)N-labeled NH(4) (+) uptake were determined in excised roots, whereas total NH(4) (+) uptake and uptake rates were determined in intact root systems following a 48-h labeling of intact seedlings with (15)N. In both species, the elevated CO(2) treatment caused a significant downregulation of (15)NH(4) (+) uptake capacity in excised roots as a result of a severe inhibition of the maximum rate of root (15)NH(4) (+) uptake (V(max)). Rates of (15)NH(4) (+) uptake in intact roots were, however, unaffected by CO(2) treatment and were on average 4- to 10-fold less than the V(max) in excised roots, suggesting that (15)NH(4) (+) absorption from the soil was not limited by the kinetics of root (15)NH(4) (+) uptake. Despite the lack of a CO(2) effect on intact root absorption rates, (15)NH(4) (+) uptake on a per plant basis was enhanced at high CO(2) concentrations in both species, with the relative increase being markedly higher in ponderosa pine than in loblolly pine. High CO(2) concentration increased total (15)NH(4) (+) uptake and the fraction of total biomass allocated to fine roots (< 2 mm in diameter) to a similar relative extent. We suggest that the increased uptake on a per plant basis in response to CO(2) enrichment is largely the result of a compensatory increase in root absorbing surfaces.

Direct and indirect effects of elevated CO(2) on whole-shoot respiration in ponderosa pine seedlings.

We determined the short-term direct and long-term indirect effects of CO(2) on apparent dark respiration (CO(2) efflux in the dark) in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings grown in 35 or 70 Pa CO(2) partial pressure for 163 days in naturally lit, controlled-environment chambers. Two soil N treatments (7 and 107 ppm total N, low-N and high-N treatments, respectively) were imposed by watering half the plants every 2 weeks with 15/15/18 fertilizer (N,P,K) and the other half with demineralized water. Direct effects of ambient CO(2) partial pressure on apparent dark respiration were measured during short-term manipulations (from minutes to hours) of the CO(2) environment surrounding the aboveground portion of individual seedlings. Short-term increases in the ambient CO(2) partial pressure consistently resulted in significant decreases in CO(2) efflux of seedling in all treatments. Efflux of CO(2) decreased by 3 to 13% when measurement CO(2) partial pressure was increased from 35 to 70 Pa, and by 8 to 46% over the entire measurement range from 0 to 100 Pa. No significant interactions between the indirect effects of growth CO(2) partial pressure and the direct effects of the measurement CO(2) partial pressure were found. Seedlings grown in the high-N treatment were significantly less sensitive to short-term changes in CO(2) partial pressures than seedlings grown in the low-N treatment. Apparent respiration tended to decrease in seedlings grown in elevated CO(2), but the decrease was not significant. Nitrogen had a large effect on CO(2) efflux, increasing apparent respiration more than twofold on both a leaf area and a leaf or shoot mass basis. Both the direct and indirect effects of elevated CO(2) were correlated with changes in the C/N ratio. A model of cumulative CO(2) efflux for a 160-day period demonstrated that, despite a 49% increase in total plant biomass, seedlings grown in the high-N + high-CO(2) treatment lost only 2% more carbon than seedlings grown in the high-N + low-CO(2) treatment, suggesting increased carbon use efficiency in plants grown in elevated CO(2). We conclude that small changes in instantaneous CO(2) efflux, such as those observed in ponderosa pine seedlings, could scale to large changes in carbon sequestration.

Contrasting patterns of photosynthetic acclimation and photoinhibition in two evergreen herbs from a winter deciduous forest.

The relationship between the microclimate within an Oak-Hickory forest and photosynthetic characters of two resident evergreen herbs with contrasting leaf phenologies was investigated on a monthly basis for 1 full year. Heuchera americana has leaf flushes in the spring and fall, with average leaf life spans of 6-7 months. Hexastylis arifolia produces a single cohort of leaves each spring with a leaf life span of 12-13 months. We predicted that among evergreen plants inhabiting a seasonal habitat, a species for which the frequency of leaf turnover is greater than the frequency of seasonal extremes would have a greater annual range in photosynthetic capacity than a species that only produced a single flush of leaves during the year. Photosynthetic parameters, including apparent quantum yield, maximum photosynthetic capacity (Pmax), temperature of maximum photosynthesis, photochemical efficiency of PSII and leaf nitrogen (N) and chlorophyll concentrations, were periodically measured under laboratory conditions in leaves sampled from natural populations of both species. Mature leaves of both species acclimated to changing understory conditions with the mean seasonal differences being significantly greater for Heuchera than for Hexastylis. Area based maximum photosynthetic rates at 25°C were approximately 250% and 100% greater in winter leaves than summer leaves for Heuchera and Hexastylis respectively. Nitrogen concentrations were highest in winter leaves. Chlorophyll concentrations were highest in summer leaves. Low Pmax/N values for these species suggest preferential allocation of leaf nitrogen into non-photosynthetic pools and/or light-harvesting function at the expense of photosynthetic enzymes and electron transport components. Despite the increase in photosynthetic capacity, there was evidence of chronic winter photoinhibition in Hexastylis, but not in Heuchera. Among these ecologically similar species, there appears to be a trade-off between the frequency of leaf production and the balance of photosynthetic acclimation and photoinhibition.

Effects of CO2 enrichment on whole-plant carbon budget of seedlings of Fagus grandifolia and Acer saccharum in low irradiance.

Carbon exchange rates (CER) and whole-plant carbon balances of beech (Fagus grandifolia) and sugar maple (Acer saccharum) were compared for seedlings grown under low irradiance to determine the effects of atmospheric CO2 enrichment on shade-tolerant seedlings of co-dominant species. Under contemporary atmospheric CO2, photosynthetic rate per unit mass of beech was lower than for sugar maple, and atmospheric CO2 enrich ment enhanced photosynthesis for beech only. Aboveground respiration per unit mass decreased with CO2 enrichment for both species while root respiration per unitmass decreased for sugar maple only. Under contemporary atmoapheric CO2, beech had lower C uptake per plant than sugar maple, while C losses per plant to nocturnal aboveground and root respiration were similar for both species. Under elevated CO2, C uptake per plant was similar for both species, indicating a significant relative increase in whole-seedling CER with CO2 enrich ment for beech but not for sugar maple. Total C loss per plant to aboveground respiration was decreased for beech only because increase in sugar maple leaf mass counterbalanced a reduction in respiration rates. Carbon loss to root respiration per plant was not changed by CO2 enrichment for either species. However, changes in maintenance respiration cost and nitrogen level suggest changes in tissue composition with elevated CO2. Beech had a greater net daily C gain with CO2 enrichment than did sugar maple in contrast to a lower one under contemporary CO2. Elevated CO2 preferentially enhances the net C balance of beech by increasing photosynthesis and reducing respiration cost. In all cases, the greatest C lost was by roots, indicating the importance of belowground biomass in net C gain. Relative growth rate estimated from biomass accumulation was not affected by CO2 enrichment for either species possibly because of slow growth under low light. This study indicates the importance of direct effects of CO2 enrichment when predicting potential change in species distribution with global climate change.

Effects of nitrogen supply and elevated carbon dioxide on construction cost in leaves of Pinus taeda (L.) seedlings.

Seedlings of loblolly pine (Pinus taeda L.) were grown under varying conditions of soil nitrogen and atmospheric carbon dioxide availability to investigate the interactive effects of these resources on the energetic requirements for leaf growth. Increasing the ambient CO2 partial pressure from 35 to 65 Pa increased seedling growth only when soil nitrogen was high. Biomass increased by 55% and photosynthesis increased by 13% after 100 days of CO2 enrichment. Leaves from seedlings grown in high soil nitrogen were 7.0% more expensive on a g glucose g-1 dry mass basis to produce than those grown in low nitrogen, while elevated CO2 decreased leaf cost by 3.5%. Nitrogen and CO2 availability had an interactive effect on leaf construction cost expressed on an area basis, reflecting source-sink interactions. When both resources were abundant, leaf construction cost on an area basis was relatively high (81.8±3.0 g glucose m-2) compared to leaves from high nitrogen, low CO2 seedlings (56.3±3.0 g glucose m-2) and low nitrogen, low CO2 seedlings (67.1±2.7 g glucose m-2). Leaf construction cost appears to respond to alterations in the utilization of photoassimilates mediated by resource availability.

Photosynthetic responses of Miconia species to canopy openings in a lowland tropical rainforest.

We examined the photosynthetic acclimation of three tropical species of Miconia to canopy openings in a Costa Rican rainforest. The response of photosynthesis to canopy opening was very similar in Miconia affinis, M. gracilis, and M. nervosa, despite differences in growth form (trees and shrubs) and local distributions of plants (understory and gap). Four months after the canopy was opened by a treefall, photosynthetic capacity in all three species had approximately doubled from closed canopy levels. There were no obvious signs of high light damage after treefall but acclimation to the gap environment was not immediate. Two weeks after treefall, Amax, stomatal conductance, apprarent quantum efficiency, and dark respiration rates had not changed significantly from understory values. The production of new leaves appears to be an important component of light acclimation in these species. The only variables to differ significantly among species were stomatal conductance at Amax and the light level at which assimilation was saturated. M. affinis had a higher stomatal conductance which may reduce its water use efficiency in gap environments. Photosynthesis in the more shade-tolerant M. gracilis saturated at lower light levels than in the other two species. Individual plant light environments were assessed after treefall with canopy photography but they explained only a small fraction of plant variation in most measures of photosynthesis and growth. In conclusion, we speculate that species differences in local distribution and in light requirements for reproduction may be more strongly related to species differences in carbon allocation than in carbon assimilation.

Effects of drought and CO2 enrichment on competition between two old-field perennials.

We studied the effects of drought stress and CO2 enrichment on the competition between Aster pilosus Willd. (aster, C3) and Andropogon virginicus L. (broomsedge, C4) under two CO2 concentrations (350 and 650 µl l-1 CO2 ) and two water treatments (well-watered and water-limited). Although broomsedge is the more drought-tolerant species, this did not increase its competitive ability against aster under drought conditions. With CO2 enrichment, aster was a stronger competitor than broomsedge and comprised 75% of above-ground pot biomass in both water treatments. CO2 enrichment also increased aster survival when competing with broomsedge under extreme drought conditions. Although drought stress and CO2 enrichment interacted to affect the two species in different ways, there was no interaction of drought stress and competition; aster was a stronger competitor than broomsedge under CO2 enrichment in both well-watered and water-limited conditions. With future increases in the atmospheric CO2 concentration, aster may delay broomsedge dominance in old-field communities.

Effects of temperature and CO2 enrichment on kinetic properties of NADP+-malate dehydrogenase in two ecotypes of Barnyard grass (Echinochloa crus-galli (L.) Beauv.) from contrasting climates.

The apparent energy of activation (E a), Michaelis-Menten constant (K mfor oxaloacetate), V max/K mratios and specific activities of NADP+-malate dehydrogenase (NADP+-MDH; EC 1.1.1.82) were analyzed in plants of Barnyard grass from Québec (QUE) and Mississippi (MISS) acclimated to two thermoperiods 28/22°C, 21/15°C, and grown under two CO2 concentrations, 350 µl l-1 and 675 µl l-1. E avalues of NADP+-MDH extracted from QUE plants were significantly lower than those of MISS plants. K mvalues and V max/K mratios of the enzyme from both ecotypes were similar over the range of 10-30°C but reduced V max/K mratios were found for the enzyme of QUE plants at 30 and 40°C assays. MISS plants had higher enzyme activities when measured on a chlorophyll basis but this trend was reversed when activities were expressed per fresh weight leaf or per leaf surface area. Activities were significantly higher in plants of both populations acclimated to 22/28°C. CO2 enrichment did not modify appreciably the catalytic properties of NADP+-MDH and did not have a compensatory effect upon catalysis or enzyme activity under cool acclimatory conditions. NADP+-MDH activities were always in excess of the amount required to support observed rates of CO2 assimilation and these two parameters were significantly correlated. The enhanced photosynthetic performance of QUE plants under cold temperature conditions, as compared to that of MISS plants, cannot be attributed to kinetic differences of NADP+-malate dehydrogenase among these ecotypes.

Response to long- and short-term salinity in populations of the C4 nonhalophyte Andropogon glomeratus Walter B.S.P.

This research was undertaken to investigate differences in salt tolerance under conditions in which salinity is increased gradually and maintained for long periods or increased rapidly and maintained for shorter periods. The responses of populations of a C4 nonhalophytic grass, Andropogon glomeratus, to long- and short-term salinity were measured under controlled environment conditions. Additionally, plants from a salt marsh population and an inland population were transplanted into a salt marsh and their survival compared. The relative growth reductions in the salt marsh and the inland populations under long-term salinity were similar. Survival of seedlings of 4 populations inundated with full-strength seawater over a relatively short period indicated differential capacities to tolerate soil salinities imposed in a manner similar to tidal inundation in a salt marsh. The greater survival of plants from the marsh population transplanted into the salt marsh further indicated genetic differentiation between the populations. These results indicate that genetic differentiation to salt tolerance in A. glomeratus is better reflected by survival after shortterm salinity events, rather than growth inhibition due to long-term salinity imposed gradually.

Physiological responses in two populations of Andropogon glomeratus Walter B.S.P. to short-term salinity.

Andropogon glomeratus is a C4 nonhalophytic grass which exhibits population differentiation for tolerance to short-term salinity exposure. To investigate possible physiological mechanisms whch enable salt-tolerant individuals to survive short-term inundation, gas exchange and water relations parameters were measured before and during a 5-day watering treatment of half-strength synthetic seawater in plants from a tolerant and a non-tolerant population. Photosynthetic recovery was followed for 10 days after the salinity treatment. Photosynthetic CO2 uptake was substantially inhibited in both populations. Stomatal conductances decreased and intercellular CO2 concentrations increased, indicating non-stomatal factors were primarily responsible for the decrease in CO2 uptake. After termination of the salinity treatment photosynthetic capacity increased more rapidly in the tolerant population and reached the pretreatment level after 6 days, whereas the nontolerant population did not recover fully after 10 days. A-Ci curves measured before and after the salinity treatment indicated a decrease in the carboxylation efficiency, and suggested a proportionately greater metabolic inhibition relative to the increase in the stomatal limitation. Osmotic adjustment occurred in a 2-day period in the tolerant population, but there was no change in the osmotic potentials or the water potential at the point of turgor loss in the nontolerant population. Thus short-term salt tolerance in the marsh population is associated with rapid osmotic adjustment and recovcry of photosynthetic capacity shortly after the end of the salinity exposure, rather than maintenance of greater photosynthesis during the salinity treatment.

Effect of low temperature on the photosynthetic metabolism of the C4 grass Echinochloa crus-galli.

CO2 curves of photosynthesis and activities of the four C4 enzymes and Ribulose bisphosphate carboxylase (RUBPc) were compared in two populations of the C4 grass Echinochloa crus-galli from contrasting thermal environments (Québec and Mississippi). Analyses were conducted both before and after 14 h of chilling at 7°C under high light conditions. This comparison provides the opportunity to assess which steps of the C4 pathway are more susceptible to become limiting at low temperatures. Both populations maintained, after chilling, a pattern of CO2 fixation typical of C4 plants with photosynthesis saturating at low external CO2 concentrations. However, the chilling treatment led to reductions in carbon uptake and in the activities of the C4 enzymes. RUBPc activity was not significantly affected by chilling. Reductions in photosynthesis and in C4 enzyme activities following the chilling treatment were significantly larger for plants of the Mississippi population. The enzyme data suggest that two steps of the C4 pathway, NADP+-malate dehydrogenase and pyruvate Pi dikinase, are likely to be associated with the reduction of CO2 uptake in C4 plants under cool conditions. When the experiment was replicated under enriched atmospheric CO2 (675 µl l-1 CO2), similar differences were observed between the two populations. CO2 enrichment resulted in an increase of activity for phospho-enol-pyruvate carboxylase and NADP+-malate dehydrogenase while activities of phospho-enol-pyruvate carboxylase and NADP+-malic enzyme were less reduced following chilling. Such an interaction was not observed for gas exchange parameters but net photosynthesis was lower when plants were grown under enriched CO2.

Effects of temperature and CO2 enrichment on kinetic properties of phospho-enol-pyruvate carboxylase in two ecotypes of Echinochloa crus-galli (L.) Beauv., a C4 weed grass species.

Two populations of Echinochloa crus-galli (Québec, Mississippi) were grown at the Duke University Phytotron under 2 thermoperiods (28°/22°C, 21°/15°C day/night) and 2 CO2 regimes (350 and 675 µl l-1). Thermostability, energy of activation (E a ),K m (PEP), K m (Mg++), and specific activity of phospho-enol-pyruvate carboxylase (PEPc) were analyzed in partially purified enzyme preparations of plants grown for 5 weeks. Thermostability of PEPc from extracts (in vitro) and leaves (in situ) was significantly higher in Mississippi plants. In vitro denaturation was not appreciably modified by thermal acclimation but CO2 enrichment elicited higher thermostability of PEPc. In situ thermostability was significantly higher than that of in vitro assays and was higher in Mississippi plants acclimated at 28°/22°C and in plants of the two ecotypes grown at 675 µl l-1 CO2. E a (Q 10 30°/20°C) for PEPc was significantly lower in Québec plants as compared to Mississippi and no acclimatory shifts were observed. Significantly higher K m's (PEP) in 20°C assays were obtained for Mississippi as compared to Québec plants but values were similar at 30°C and 40°C assays. K m (Mg++) decreased at higher assay temperatures and were significantly lower for PEPc of the Québec ecotype. No significant changes in K m (Mg++) values were associated with modifications in temperature on CO2 regimes. PEPc activity measured at 30°C was significantly higher for Québec plants when measured on a leaf fresh weight, leaf area or protein basis but not on a chlorophyll basis. Significantly higher PEPc activity for both genotypes was observed for plants acclimated at 21°/15°C or grown at 675 µl l-1 CO2. Net photosynthesis (Ps) and net assimilation rates (NAR) were higher in Québec plants and were enhanced by CO2 enrichment. NAR was higher in plants acclimated at low temperature, while an opposite trend was observed for Ps. PEPc activities were always in excess of the amounts required to support observed rates of CO2 assimilation.

Dynamic behavior of CO2 uptake as affected by light: system identification based on spectral analysis.

The dynamic behavior of CO2 uptake in sunflower plants was measured in a computer controlled apparatus and identified systematically using spectral analysis.To obtain impulse responses of net CO2 uptake, power spectra were estimated by the Fast Fourier Transform (FFT) method. FFT was done by computing 512 separate data points sampled at one minute intervals. From examination of coherency, it was determined that dynamic behavior of CO2 uptake could be estimated by linear filtering based on impulse response as affected by the light in changes slower than 0.08 cycle min-1. This suggests that other dynamic phenomena in physiological ecology could be estimated effectively by using advanced system theory involving many algorithms.

Effects of light regime on the growth, leaf morphology, and water relations of seedlings of two species of tropical trees.

An experiment was conducted with Heliocarpus appendiculatus, a pioneer or large gap species of tropical moist forest in Costa Rica, and Dipteryx panamensis, a small gap species. Seedlings were grown in full sun, partial (80%) shade, and full (98%) shade. After one month of growth they were switched between environments and grown for two more months.Growth in height of Heliocarpus was greatly affected by irradiance, being increased in response to full shade and decreased in full sun. Height of Dipteryx was unaffected by irradiance level. Survival of Heliocarpus seedlings was only 49% in full shade, whereas Dipteryx had 100% survival. Biomass of Heliocarpus was not significantly greater in full sun than in partial shade whereas it was for Dipteryx. The response of root: shoot ratio was similar for both species. They were lowest in full shade and highest in full sun. Heliocarpus exhibited greater changes in leaf thickness, specific leaf weight, and stomatal density than did Dipteryx. Stomatal conductance of both species was lower in full shade and full sun than in partial shade.The results of the experiment indicate that growth of Heliocarpus is more plastic than that of Dipteryx in response to changes in irradiance. Previous environment did not affect the response to the present environment in either species. Both species responded positively to increases in irradiance.

Seasonal patterns of growth, tissue acid fluctuations, and 14CO2 uptake in the crassulacean acid metabolism epiphyte Tjllandsia usneoides L. (Spanish moss).

Seasonal patterns of growth, 14CO2 uptake, and fluctuations in tissue titratable acidity were studied over the course of a year at a study site in the coastal plain of North Carolina.Elongation rates of Spanish moss strands were maximal in the summer and minimal in the winter. Summer maximal biomass addition rates were calculated to be 3.4 mg·month-1. Mortality of the strands was greatest in the winter months. Rates of 14CO2 uptake and fluctuations in tissue acidity were greatest in the summer over a fairly broad spectrum of environmental conditions (day and night temperatures, irradiance, length of drought). Maximal 14CO2 uptake rates (1.2 mg CO2·mg Chl-1 ·h01) were measured in May 1978. Rates of 14CO2 uptake and fluctuations in titratable acidity were inhibited below 5°C and eliminated at 0°C air temperatures.Isothermal diurnal conditions resulted in low rates of 14CO2 uptake. Tissue water content did not appear to be a major factor controlling 14CO2 uptake rates. However, tissue wetting by rain severely reduced nighttime uptake yet stimulated low rates of daytime 14CO2 uptake. This was the only condition in which daytime 14CO2 uptake occurred, excluding the early morning and late afternoon 14CO2 uptake typical of many Crassulacean Acid Metabolism (CAM) plants.The results suggest that tissue water content is not the major factor controlling CO2 uptake as has been found in many other CAM species; and that low temperatures limit the growth of Spanish moss in North Carolina.

Seasonal patterns of leaf water relations in four co-occurring forest tree species: Parameters from pressure-volume curves.

Leaf water relationships were studied in four widespread forest tree species (Ilex opaca Ait., Cornus florida L., Acer rubrum L., and Liriodendron tulipifera L.). The individuals studied all occurred on the same site and were selected to represent a range of growth forms and water relationships in some of the principal tree species of the region. The water relations of the species were analyzed using the concept of the water potential-water content relationship. The pressure-volume method was used to measure this relationship using leaf material sampled from naturally occurring plants in the field. Water potential components (turgor, osmotic, and matric) were obtained by analysis of the pressure-volume curves.Initial osmotic potentials (the value of the osmotic component at full turgidity) were highest (least negative) at the start of the growing season. They decreased (becoming progressively more negative) as the season progressed through a drought period. Following a period of precipitation at the end of the drought period, initial osmotic potentials increased toward the values measured earlier in the season.Seasonal osmotic adjustments were sufficient in all species to allow maintenance of leaf turgor through the season, with one exception: Acer appeared to undergo some midday turgor loss during the height of the July drought period.In addition to environmental influences, tissue stage of development played a role; young Ilex leaves had higher early season initial osmotic potentials than overwintering leaves from the same tree.The seasonal pattern of initial osmotic potential in Liriodendron and the observed pattern of leaf mortality suggested a possible role of osmotic potentials in the resistance of those leaves to drought conditions. The fraction of total leaf water which is available to affect osmotic potentials, called the osmotic water fraction in this study, was greatest in young tissue early in the season and declined as the season progressed.The results of this study showed that the water potential-water content relationship represents a dynamic mechanism by which plant internal water relations may vary in response to a changing external water-availability regime. The measured water relationships confirmed the relative positions of the species along a water-availability gradient, with Cornus at the wettest end and Ilex at the driest end of the gradient. Acer and Liriodendron were intermediate in their water relations. The spread of these species along a water-availability gradient on the same site suggested that coexistence is partially based on differential water use patterns.