Monsoonal influences on evapotranspiration of savanna vegetation of northern Australia

Data from savannas of northern Australia are presented for net radiation, latent and sensible heat, ecosystem surface conductance (G_s) and stand water use for sites covering a latitudinal range of 5° or 700 km. Measurements were made at three locations of increasing distance from the northern coastline and represent high- (1,750 mm), medium- (890 mm) and low- (520 mm) rainfall sites. This rainfall gradient arises from the weakened monsoonal influence with distance inland. Data were coupled to seasonal estimates of leaf area index (LAI) for the tree and understorey strata. All parameters were measured at the seasonal extremes of late wet and dry seasons. During the wet season, daily rates of evapotranspiration were 3.1-3.6 mm day^-1 and were similar for all sites along the rainfall gradient and did not reflect site differences in annual rainfall. During the dry season, site differences were very apparent with evapotranspiration 2-18 times lower than wet season rates, the seasonal differences increasing with distance from coast and reduced annual rainfall. Due to low overstorey LAI, more than 80% of water vapour flux was attributed to the understorey. Seasonal differences in evapotranspiration were mostly due to reductions in understorey leaf area during the dry season. Water use of individual trees did not differ between the wet and dry seasons at any of the sites and stand water use was a simple function of tree density. G_s declined markedly during the dry season at all sites, and we conclude that the savanna water (and carbon) balance is largely determined by G_s and its response to atmospheric and soil water content and by seasonal adjustments to canopy leaf area.     

Influence of climate-driven shifts in biomass allocation on water transport and storage in ponderosa pine

Conifers decrease the amount of biomass apportioned to leaves relative to sapwood in response to increasing atmospheric evaporative demand. We determined how these climate-driven shifts in allocation affect the aboveground water relations of ponderosa pine growing in contrasting arid (desert) and humid (montane) climates. To support higher transpiration rates, a low leaf:sapwood area ratio (A_L/A_S) in desert versus montane trees could increase leaf-specific hydraulic conductance (K_L). Alternatively, a high sapwood volume:leaf area ratio in the desert environment may increase the contribution of stored water to transpiration. Transpiration and hydraulic conductance were determined by measuring sap flow (J_S) and shoot water potential during the summer (June-July) and fall (August-September). The daily contribution of stored water to transpiration was determined using the lag between the beginning of transpiration from the crown at sunrise and J_S. In the summer, mean maximum J_S was 31.80LJ.74 and 24.34Dž.05 g m^-2 s^-1 for desert and montane trees (a 30.6% difference), respectively. In the fall, J_S was 25.33NJ.52 and 16.36dž.64 g m^-2 s^-1 in desert and montane trees (a 54.8% difference), respectively. J_S was significantly higher in desert relative to montane trees during summer and fall (P<0.05). Predawn and midday shoot water potential and sapwood relative water content did not differ between environments. Desert trees had a 129% higher K_L than montane trees in the summer (2.41᎒^-5 versus 1.05᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001) and a 162% higher K_L in the fall (1.97᎒^-5 versus 0.75᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001). Canopy conductance decreased with D in all trees at all measurement periods (P<0.05). Maximum g_C was 3.91 times higher in desert relative to montane trees averaged over the summer and fall. Water storage capacity accounted for 11 kg (11%) and 10.6 kg (17%) of daily transpiration in the summer and fall, respectively, and did not differ between desert and montane trees. By preventing xylem tensions from reaching levels that cause xylem cavitation, high K_L in desert ponderosa pine may facilitate its avoidance. Thus, the primary benefit of low leaf:sapwood allocation in progressively arid environments is to increase K_L and not to increase the contribution of stored water to transpiration.     

Primary production, light and vertical mixing in Potter Cove, a shallow bay in the maritime Antarctic

Phytoplankton photosynthesis was measured during spring-summer 1991-1992 in the inner and outer part of the shallow Potter Cove, King George Island. Strong winds characterise this area. Wind-induced turbulent mixing was quantified by means of the root-mean square expected vertical displacement depth of cells in the water column, Z_t. The light attenuation coefficient was used as a measure of the influence of the large amount of terrigenous particles usually present in the water column; 1% light penetration ranged between 30 and 9 m, and between 30 and 15 m for the inner and outer cove, respectively. Obvious differences between photosynthetic capacity [P*_max; averages 2.6 and 0.6 µg C (µg chlorophyll-a)^-1 h^-1] and photosynthetic efficiency {!*; 0.073 and 0.0018 µg C (µg chlorophyll-a)^-1 h^-1 [(µmol m^-2 s^-1)^-1]} values were obtained for both sites during low mixing conditions (Z_t from 10 to 20 m), while no differences were found for high mixing situations (Z_t>20 m). This suggests different photoacclimation of phytoplankton responses, induced by modifications of the light field, which in turn are controlled by physical forcing. Our results suggest that although in experimental work P*_max can be high, wind-induced mixing and low irradiance will prevent profuse phytoplankton development in the area.     

In situ oxygen microelectrode measurements of bottom-ice algal production in McMurdo Sound, Antarctica

In-situ estimates of fast-ice algal productivity at Cape Evans, McMurdo Sound, in 1999 were lower than at the same site in previous years. Under-ice irradiance was between 0 and 8 µmol photons m^-2 s^-1; the ice was between 1.9 and 2.0 m thick and the algal biomass averaged 150 mg chl a m^-2, although values as high as 378 mg chl a m^-2 were recorded. Production on 11 and 12 November was between 0.053 and 1.474 mg C m^-2 h^-1. When the data from 11 November were fitted to a hyperbolic tangent function, a multilinear regression gave estimates for P_max of 0.571 nmol O₂ cm^-2 s^-1, an ! of 0.167 nmol O₂ cm^-2 s^-1 µmol^-1 photons m^-2 s^-1 and an E_k of 3.419 µmol photons m^-2 s^-1. A P_max of 2.674 nmol O₂ cm^-2 s^-1, an ! of 0.275 nmol O₂ cm^-2 s^-1 µmol^-1 photons m^-2 s^-1, r of 0.305 nmol O₂ cm^-2 s^-1 and an E_k of 9.724 µmol^-1 photons m^-2 s^-1 were estimated from the 12 November data. The sea-ice algal community was principally comprised of Nitzschia stellata, Entomoneis kjellmanii and Berkeleya adeliensis. Other taxa present included N. lecointei, Fragilariopsis spp., Navicula glaciei, Pleurosigma spp. and Amphora spp. Variations in the method for estimating the thickness of the diffusive boundary layer were not found to significantly affect the measurements of oxygen flux. However, the inability to accurately measure fine-scale variations in biomass is thought to contribute to the scatter of the P versus E data.     

Growth, photosynthesis and ozone uptake of young beech (Fagus sylvatica L.) in response to different ozone exposures

Beech seedlings (Fagus sylvatica L.) were exposed to episodes of O₃ in environmentally controlled growth chambers during one growing season. Three treatments were applied: charcoal-filtered air, charcoal-filtered air with the addition of 40 ppb O₃ for seven episodes of 5 days' duration (9000-1700 hours), and charcoal-filtered air with the addition of 100 ppb O₃ for seven episodes of 5 days' duration (9000-1700 hours). The accumulated exposure over a threshold of 40 ppb in the last treatment reached 13,911 ppb h. Throughout the growing season we measured growth as well as photosynthetic properties and related effects to external and calculated internal doses of O₃, using stomatal conductance (g_s) data. Growth, measured as diameter increment and biomass, was not significantly affected by the O₃ treatments. In the 100-ppb treatment, light-saturated CO₂ assimilation rates and chlorophyll content were significantly reduced, and the chlorophyll fluorescence parameter F_v/F_m was significantly reduced at times of high uptake rates and coincided with strong reductions of assimilation rates. O₃ uptake was lowered in the 100-ppb treatment due to reduced g_s. There was serious visible damage by the end of the exposure period in the 100-ppb treatment, while the treatment with 40 ppb O₃ did not seem to cause any significant changes.     

Oxygen transfer characteristics of a centrifugal, packed-bed reactor during viscous xanthan fermentation

The oxygen transfer properties of a novel, centrifugal, packed-bed reactor (CPBR) during viscous xanthan fermentation were determined with respect to the effects of the arrangement of the centrifugal, packed bed (CPB) and the recirculation loop (RL). Characterized by the maximum volumetric transfer coefficient (k_La) in xanthan broth, the aeration efficiency of CPBR was compared to those in stirred-tank reactors (STR) equipped with disc turbines (DT) or marine propellers (MP), and to that in a water-in-oil emulsion (WIO). As expected, STR-WIO showed the highest k_La (0.038 s^-1 at 2%) among all systems studied due to reduced broth viscosity; however, practical difficulties exist in product recovery. It was found that, at 3.5% xanthan the k_La in CPBR (0.018 s^-1) was higher than that of STR (0.005 s^-1) and close to that of STR-WIO (0.020 s^-1), indicating improved oxygen transfer at such a xanthan concentration. The exterior baffles along the rotating fibrous matrix offer additional agitation in the viscous broth. A gas-continuous arrangement, in which the CPB was kept above the broth, was able to elevate k_La to 0.023 s^-1, higher than that of STR-WIO. The external RL operated by a peristaltic pump was found to play an important role in CPBR aeration by providing better gas-liquid contact. With the improved oxygen transfer efficiency in CPBR at high xanthan concentrations, the CPBR system is practically the preferred system for xanthan fermentation. The characteristic roles of CPB arrangement and the RL should be considered primarily during scale-up operation.     

Photosynthesis, photoinhibition, and nitrogen use efficiency in native and invasive tree ferns in Hawaii

Photosynthetic gas exchange, chlorophyll fluorescence, nitrogen use efficiency, and related leaf traits of native Hawaiian tree ferns in the genus Cibotium were compared with those of the invasive Australian tree fern Sphaeropteris cooperi in an attempt to explain the higher growth rates of S. cooperi in Hawaii. Comparisons were made between mature sporophytes growing in the sun (gap or forest edge) and in shady understories at four sites at three different elevations. The invasive tree fern had 12-13 cm greater height increase per year and approximately 5 times larger total leaf surface area per plant compared to the native tree ferns. The maximum rates of photosynthesis of S. cooperi in the sun and shade were significantly higher than those of the native Cibotium spp (for example, 11.2 and 7.1 µmol m^-2 s^-1, and 5.8 and 3.6 µmol m^-2 s^-1 respectively for the invasive and natives at low elevation). The instantaneous photosynthetic nitrogen use efficiency of the invasive tree fern was significantly higher than that of the native tree ferns, but when integrated over the life span of the frond the differences were not significant. The fronds of the invasive tree fern species had a significantly shorter life span than the native tree ferns (approximately 6 months and 12 months, respectively), and significantly higher nitrogen content per unit leaf mass. The native tree ferns growing in both sun and shade exhibited greater photoinhibition than the invasive tree fern after being experimentally subjected to high light levels. The native tree ferns recovered only 78% of their dark-acclimated quantum yield (F_v/F_m), while the invasive tree fern recovered 90% and 86% of its dark-acclimated F_v/F_m when growing in sun and shade, respectively. Overall, the invasive tree fern appears to be more efficient at capturing and utilizing light than the native Cibotium species, particularly in high-light environments such as those associated with high levels of disturbance.     

极端干旱条件下锡林河流域羊草草原净生态系统碳交换特征

采用涡度相关法对2005年生长季内蒙古锡林河流域羊草(Leymus chinensis)草原净生态系统交换(Net ecosystem exchange, NEE)进行了观测。观测结果表明:作为生长季降雨量仅有126 mm的干旱年,锡林河流域羊草草原生态系统受到强烈的干旱胁迫,其净生态系统碳交换的日动态表现为具有两个吸收高峰,净吸收峰值出现在8∶00和18∶00左右。最大的CO₂吸收率为-0.38 mg CO₂·m^-2·s^-1,出现在6月底,与丰水年相比生态系统最大CO₂吸收率下降了1倍。就整个生长季而言,不管是白天还是晚上2005年都表现为净CO₂排放,整个生长季CO₂净排放量为372.56 g CO₂·m^-2,是一个明显的CO₂源。土壤含水量和土壤温度控制着生态系统CO₂通量的大小,尤其是在白天,CO₂通量和土壤含水量的变化呈现出显著的负相关关系,和土壤温度表现为正相关关系。     

Water availability and carbon isotope discrimination in conifers

The stable C isotope composition ('¹³C) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and '¹³C. A significant component of the variation in '¹³C was related to altitude (discrimination decreased with altitude in stemwood, 2.53‰ km^-1 altitude, r²=0.49, and in foliage, 1.91‰ km^-1, r²=0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO₂ partial pressure into the leaf (P_a-P_i) and altitude were generally unrelated. The ratio of precipitation to evaporation (P/E) explained significant variation in P_a-P_i of stemwood (r²=0.45) and foliage (r²=0.27), but only at low (<0.8) P/E. At greater P/E there was little or no relationship, and other influences on '¹³C probably dominated the effect of water availability. We also examined the relationship between plant drought stress (O) and '¹³C within annual rings of stemwood from Pinus radiata and Pinus pinaster in south-western Australia. Differential thinning and fertiliser application produced large differences in the availability of water, nutrients and light to individual trees. At a density of 750 stems ha^-1, O and '¹³C were less (more negative) than at 250 stems ha^-1 indicating greater drought stress and less efficient water use, contrary to what was expected in light of the general relationship between discrimination and P/E. The greater '¹³C of trees from heavily thinned plots may well be related to an increased interception of radiation by individual trees and greater concentrations of nutrients in foliage - attributes that increase rates of photosynthesis, reduce P_i and increase '¹³C. '¹³C was thus modified to a greater extent by interception of radiation and by nutrient concentrations than by water availability and the '¹³C-O relationship varied between thinning treatments. Within treatments, the relationship between '¹³C and O was strong (0.38<r²<0.58). We conclude that '¹³C may well be a useful indicator of water availability or drought stress, but only in seasonally dry climates (P/E<1) and where variation in other environmental factors can be accounted for.     

Soil microbial community and bacterial functional diversity at Machu Picchu, King George Island, Antarctica

The objective of this study was to evaluate the potential contribution of the soil microbial community in the vicinity of two plant covers, Sanionia uncinata and Deschampsia antarctica, at Machu Picchu Station, King George Island, Antarctica. Soil samples were collected at the study site during the southern (pole) summer period from 0-5 cm and 5-10 cm depths, for chemical and biological analyses. Soil microbial biomass reached a maximal value of 144 µg g^-1 in soil samples taken from under the S. uncinata upper layer plant. qCO₂ ranged from 167 to 239 µg CO₂^.mgC_mic^.h^-1 at the 0-5 and 5-10 cm depths, respectively. CO₂ evolution showed values of 54.3 mg^.m^-2 h^-1 beneath plant cover and 55.9 mg^.m^-2 h^-1 in the open space. CO₂ evolved by substrate induced respiration in the soil samples taken under the plant cover in the summer period, oscillated between 0.25 and 4.78 µg CO₂ g^-1 h^-1. The data obtained from this short study may provide evidence that both activity and the composition and substrate utilization of the microbial community appear to change substantially across the moisture level and sample location.     

13C content of ecosystem respiration is linked to precipitation and vapor pressure deficit

Variation in the carbon isotopic composition of ecosystem respiration ('¹³C_R) was studied for 3 years along a precipitation gradient in western Oregon, USA, using the Keeling plot approach. Study sites included six coniferous forests, dominated by Picea sitchensis, Tsuga heterophylla, Pseudotsuga menziesii, Pinus ponderosa, and Juniperus occidentalis, and ranged in location from the Pacific coast to the eastern side of the Cascade Mountains (a 250-km transect). Mean annual precipitation across these sites ranged from 227 to 2,760 mm. Overall '¹³C_R varied from -23.1 to -33.1‰, and within a single forest, it varied in magnitude by 3.5-8.5‰. Mean annual '¹³C_R differed significantly in the forests and was strongly correlated with mean annual precipitation. The carbon isotope ratio of carbon stocks (leaves, fine roots, litter, and soil organic matter) varied similarly with mean precipitation (more positive at the drier sites). There was a strong link between '¹³C_R and the vapor saturation deficit of air (vpd) 5-10 days earlier, both across and within sites. This relationship is consistent with stomatal regulation of gas exchange and associated changes in photosynthetic carbon isotope discrimination. Recent freeze events caused significant deviation from the '¹³C_R versus vpd relationship, resulting in higher than expected '¹³C_R values.     

Photosynthesis in an invasive grass and native forb at elevated CO2 during an El Niño year in the Mojave Desert

Annual and short-lived perennial plant performance during wet years is important for long-term persistence in the Mojave Desert. Additionally, the effects of elevated CO₂ on desert plants may be relatively greater during years of high resource availability compared to dry years. Therefore, during an El Niño year at the Nevada Desert FACE Facility (a whole-ecosystem CO₂ manipulation), we characterized photosynthetic investment (by assimilation rate-internal CO₂ concentration relationships) and evaluated the seasonal pattern of net photosynthesis (A_net) and stomatal conductance (g_s) for an invasive annual grass, Bromus madritensis ssp. rubens and a native herbaceous perennial, Eriogonum inflatum. Prior to and following flowering, Bromus showed consistent increases in both the maximum rate of carboxylation by Rubisco (V_Cmax) and the light-saturated rate of electron flow (J_max) at elevated CO₂. This resulted in greater A_net at elevated CO₂ throughout most of the life cycle and a decrease in the seasonal decline of maximum midday A_net upon flowering as compared to ambient CO₂. Eriogonum showed significant photosynthetic down-regulation to elevated CO₂ late in the season, but the overall pattern of maximum midday A_net was not altered with respect to phenology. For Eriogonum, this resulted in similar levels of A_net on a leaf area basis as the season progressed between CO₂ treatments, but greater photosynthetic activity over a typical diurnal period. While g_s did not consistently vary with CO₂ in Bromus, it did decrease in Eriogonum at elevated CO₂ throughout much of the season. Since the biomass of both plants increased significantly at elevated CO₂, these patterns of gas exchange highlight the differential mechanisms for increased plant growth. The species-specific interaction between CO₂ and phenology in different growth forms suggests that important plant strategies may be altered by elevated CO₂ in natural settings. These results indicate the importance of evaluating the effects of elevated CO₂ at all life cycle stages to better understand the effects of elevated CO₂ on whole-plant performance in natural ecosystems.     

Physiological energetics of the protobranch bivalve <Emphasis Type="Italic">Yoldia hyperborea</Emphasis> in a cold ocean environment

Yoldia hyperborea is a deposit-feeding circumpolar protobranch that also inhabits muddy sediments of the cold water boreal system of Conception Bay, Newfoundland, Canada. Little is known about this species, despite its wide distribution and frequent high density in the benthos. The present work deals with oxygen consumption and ammonia excretion under cold ambient conditions. Y. hyperborea showed low basal metabolism [0.051 ml O₂ h^у·(g dry weight)^у, T=у°C] and low ammonia excretion rates [4.212 µg·NH₄-N·h^у·(g dry weight)^у, T=у°C]. Low metabolic activity could prove a useful strategy during periods of low food availability. In addition, Y. hyperborea was able to regulate its O₂ consumption rate at very low pO₂ levels, which may be advantageous for a species that may experience periods of hypoxia.     

Microsite characteristics of Muhlenbergia richardsonis (Trin.) Rydb., an alpine C4 grass from the White Mountains, California

C₄ plants are uncommon in cold environments and do not generally occur in the alpine tundra. In the White Mountains of California, however, the C₄ grass Muhlenbergia richardsonis is common in the alpine zone at 3,300-3,800 m, with the highest population observed at 3,960 m (13,000 feet) above sea level. This is the highest reported C₄ species in North America and is near the world altitude limit for C₄ plants (4,000-4,500 m). Above 3,800 m, M. richardsonis is largely restricted to southern slope aspects, with greatest frequency on southeast-facing slopes. In open tundra, M. richardsonis formed prostrate mats with a mean height of 2.5 cm. Neighboring C₃ grasses were two to three times taller. Because of its short stature, leaf temperature of M. richardsonis was greatly influenced by the boundary layer of the ground, rising over 20°C above air temperature in full sun and still air and over 10°C above air temperature in full sun and wind velocity of 1-4 m s^-1. Thus, although air temperatures did not exceed 15°C, midday leaf temperatures of M. richardsonis were routinely between 25°C and 35°C, a range favorable to C₄ photosynthesis. At night, leaf temperature of M. richardsonis was often 5-12°C below air temperature, resulting in regular exposure to subzero temperatures and frosting of the leaves. No visible injury was associated with exposure to freezing night temperatures. The presence of M. richardsonis in the alpine zone demonstrates that C₄ plants can tolerate extreme cold during the growing season. The localization to microsites where leaf temperatures can exceed 25°C during the day, however, indicates that even when cold tolerant, C₄ plants still require periods of high leaf temperature to remain competitive with C₃ species. In this regard, the prostrate growth form of M. richardsonis compensates for the alpine climate by allowing sufficient heating of the leaf canopy during the day.     

Water relations of coastal and estuarine Rhizophora mangle: xylem pressure potential and dynamics of embolism formation and repair

Physiological traits related to water transport were studied in Rhizophora mangle (red mangrove) growing in coastal and estuarine sites in Hawaii. The magnitude of xylem pressure potential (P_x), the vulnerability of xylem to cavitation, the frequency of embolized vessels in situ, and the capacity of R. mangle to repair embolized vessels were evaluated with conventional and recently developed techniques. The osmotic potential of the interstitial soil water (?_sw) surrounding the roots of R. mangle was c. -2.6LJ.52᎒^-3 and -0.4Lj.13᎒^-3 MPa in the coastal and estuarine sites, respectively. Midday covered (non-transpiring) leaf water potentials (O_L) determined with a pressure chamber were 0.6-0.8 MPa more positive than those of exposed, freely-transpiring leaves, and osmotic potential of the xylem sap (?_x) ranged from -0.1 to -0.3 MPa. Consequently, estimated midday values of P_x (calculated by subtracting ?_x from covered O_L) were about 1 MPa more positive than O_L determined on freely transpiring leaves. The differences in O_L between covered and transpiring leaves were linearly related to the transpiration rates. The slope of this relationship was steeper for the coastal site, suggesting that the hydraulic resistance was larger in leaves of coastal R. mangle plants. This was confirmed by both hydraulic conductivity measurements on stem segments and high-pressure flowmeter studies made on excised leafy twigs. Based on two independent criteria, loss of hydraulic conductivity and proportions of gas- and liquid-filled vessels in cryo-scanning electron microscope (cryo-SEM) images, the xylem of R. mangle plants growing at the estuarine site was found to be more vulnerable to cavitation than that of plants growing at the coastal site. However, the cryo-SEM analyses suggested that cavitation occurred more readily in intact plants than in excised branches that were air-dried in the laboratory. Cryo-SEM analyses also revealed that, in both sites, the proportion of gas-filled vessels was 20-30% greater at midday than at dawn or during the late afternoon. Refilling of cavitated vessels thus occurred during the late afternoon when considerable tension was present in neighboring vessels. These results and results from pressure-volume relationships suggest that R. mangle adjusts hydraulic properties of the water-transport system, as well as the leaf osmotic potential, in concert with the environmental growing conditions.     

Symbiotic N2 fixation of Alnus incana ssp. rugosa in shrub wetlands of the Adirondack Mountains, New York, USA

Surface waters in forested watersheds in the Adirondack Mountains and northern New York State are susceptible to nitrogen (N) saturation. Atmospheric deposition of N to watersheds in this region has been measured but the extent of internal N inputs from symbiotic N₂ fixation in alder-dominated wetlands is not known. We estimated N₂ fixation by speckled alder in these wetlands by the ¹⁵N natural abundance method and by acetylene reduction using a flow-through system. Foliar N derived from fixation (%N_dfa) was estimated for five wetlands. The '¹⁵N of speckled alder foliage from four of the five sites did not differ significantly (PА.05) from that of nodulated speckled alders grown in N-free water culture (-1.2ǂ.1‰). Estimates from the ¹⁵N natural abundance method indicated that alders at these sites derive 85-100% of their foliar N from N₂ fixation. At one of the sites, we also measured biomass and N content and estimated that the alder foliage contained 43 kg N ha^-1 of fixed N in 1997. This estimate was based on a foliar N content of 55.4lj kg N ha^-1 (mean-SE), 86dž%N_dfa, and an assumption that 10% of foliar N was derived from reserves in woody tissues. At this site, we further estimated via acetylene reduction that 37ᆞ kg N ha^-1 was fixed by speckled alders in 1998. This estimate used the theoretical 4:1 C₂H₂ reduction to N₂ fixation ratio and assumed no night-time fixation late in the season. Nitrogen inputs in wet and dry deposition at this site are approximately 8 kg N ha^-1 year^-1. We conclude that speckled alder in wetlands of northern New York State relies heavily on N₂ fixation to meet N demands, and symbiotic N₂ fixation in speckled alders adds substantial amounts of N to alder-dominated wetlands in the Adirondack Mountains. These additions may be important for watershed N budgets, where alder-dominated wetlands occupy a large proportion of watershed area.     

Leaf δ<Superscript>13</Superscript>C variability with elevation,slope aspect,and precipitation in the southwest United States

Leaves from several desert and woodland species, including gymnosperms and angiosperms with both C₃ and C₄ physiology, were analyzed to detect trends in '¹³C_leaf with elevation and slope aspect along two transects in southeastern Utah and south-central New Mexico, USA. The main difference between the two transects is the steeper elevational gradient for mean annual and summer precipitation in the southern transect. For any given species, we found that isotopic differences between individual plants growing at the same site commonly equal differences measured for plants along the entire altitudinal gradient. In C₃ plants, '¹³C_leaf values become slightly enriched at the lowest elevations, the opposite of trends identified in more humid regions. Apparently, increasing water-use efficiency with drought stress offsets the influence of other biotic and abiotic factors that operate to decrease isotopic discrimination with elevation. For some species shared by the two transects (e.g., Pinus edulis and Cercocarpus montanus), '¹³C_leaf values are dramatically depleted at sites that receive more than 550 mm mean annual precipitation, roughly the boundary (pedalfer-pedocal) at which soils commonly fill to field capacity in summer and carbonates are leached. We hypothesize that, in summer-wet areas, this may represent the boundary at which drought stress overtakes other factors in determining the sign of '¹³C_leaf with elevation. The opposition of isotopic trends with elevation in arid versus humid regions cautions against standard correction for elevation in comparative studies of '¹³C_leaf.     

Photosynthetic characteristics in canopies of Quercus rubra, Quercus prinus and Acer rubrum differ in response to soil water availability

Photosynthesis and related leaf characteristics were measured in canopies of co-occurring Quercus rubra L. (red oak), Quercus prinus L. (chestnut oak) and Acer rubrum L. (red maple) trees. Mature (20+ m tall) trees were investigated at sites of differing soil water availability within a catchment (a drier upper site and a wetter lower site). Leaf photosynthetic characteristics differed significantly between species and in response to site and position in the canopy. Photosynthetic capacity (A_max) was significantly greater at the wetter site in all canopy strata in A. rubrum but not in Q. rubra or Q. prinus. Our findings for A. rubrum are generally consistent with those predicting that species with higher specific leaf area (SLA) will have higher A_max per unit leaf nitrogen (N) and that species with leaves with lower SLA (e.g. Q. rubra and Q. prinus) will have shallower slopes of the A_max-N relationship. Importantly, the relationships between A_max and N_area (and by implication photosynthetic nitrogen-use efficiency, PNUE) differed in A. rubrum between the sites, with PNUE significantly lower at the drier site. The lower photosynthetic capacity and PNUE must substantially reduce carbon acquisition capacity in A. rubrum under these field conditions. Maximum stomatal conductance (g_smax) differed significantly between species, with g_smax greatest in Q. rubra and Q. prinus. In Q. rubra and Q. prinus, g_smax was significantly lower at the upper site than the lower site. There was no significant response of g_smax to site in A. rubrum. These stomatal responses were consistent with the C_i/C_a ratio, which was significantly lower in leaves of Q. rubra and Q. prinus at the upper site, but did not differ between sites in A. rubrum. Leaf '¹³C was significantly lower in A. rubrum than in either Q. rubra or Q. prinus at both sites. These findings indicate differences in stomatal behaviour in A. rubrum which are likely to contribute to lower water use efficiency at both sites. Our results support the hypothesis that the two Quercus species, in contrast to A. rubrum, maintain photosynthetic capacity at the drier site whilst minimising transpirational water loss. They also suggest, based primarily on physiological evidence, that the ability of A. rubrum to compete with other species of these deciduous forests may be limited, particularly in sites of low moisture availability and during low rainfall years.     

Feeding rates of the woodlouse Armadillidium vulgare on herb litters produced at two levels of atmospheric CO2

The consumption and assimilation rates of the woodlouse Armadillidium vulgare were measured on leaf litters from five herb species grown and naturally senesced at 350 and 700 µl l^-1 CO₂. Each type of litter was tested separately after 12, 30 and 45 days of decomposition at 18°C. The effects of elevated CO₂ differed depending on the plant species. In Medicago minima (Fabaceae), the CO₂ treatment had no significant effect on consumption and assimilation. In Tyrimnus leucographus (Asteraceae), the CO₂ treatment had no significant effect on consumption, but the elevated CO₂ litter was assimilated at a lower rate than the ambient CO₂ litter after 30 days of decomposition. In the three other species, Galactites tomentosa (Asteraceae), Trifolium angustifolium (Fabaceae) and Lolium rigidum (Poaceae), the elevated CO₂ litter was consumed and/or assimilated at a higher rate than the ambient CO₂ litter. Examination of the nitrogen contents in these three species of litter did not support the hypothesis of compensatory feeding, i.e. an increase in woodlouse consumption to compensate for low nitrogen content of the food. Rather, the results suggest that in herbs that were unpalatable at the start of the experiment (Galactites, Trifolium and Lolium), more of the the litter produced at 700 µl l^-1 CO₂ was consumed than of that produced at 350 µl l^-1 because inhibitory factors were eliminated faster during decomposition.     

Plant frequency,stem and root characteristics,and CO<Subscript>2</Subscript> uptake for <Emphasis Type="Italic">Opuntia acanthocarpa</Emphasis>: elevational correlates in the northwestern Sonoran Desert

A common cylindropuntia in the northwestern Sonoran Desert, Opuntia acanthocarpa, was investigated for the following hypotheses: its lower elevational limit is set by high temperatures, so its seedlings require nurse plants; its upper elevational limit is set by freezing; spine shading is the least at intermediate elevations; and changes in plant size and frequency with elevation reflect net CO₂ uptake ability. For four elevations ranging from 230 m to 1,050 m, the mean height of O. acanthocarpa approximately doubled and its frequency increased 14-fold. Nurse plants were associated with only 4% of O. acanthocarpa less than 20 cm tall at the two lower elevations compared with 57% at 1,050 m, where putative freezing damage was especially noticeable, suggesting that nurse plants protect from low temperature damage. Spine shading of the stem doubled from the lowest to the highest elevation. Net CO₂ uptake, which followed a Crassulacean acid metabolism pattern, was maximal at day/night air temperatures of 25/15°C and was halved by 4 weeks of drought and by reducing the photosynthetic photon flux from 30 to 12 mol m^-2 day^-1. The root system of O. acanthocarpa was shallow, with a mean depth of only 9 cm for the largest plants. Root growth was substantial and similar for plants at 25/15°C and 35/25°C, decreasing over 70-fold at 15/5°C and 45/35°C. Based on cellular uptake of the vital stain neutral red, neither roots nor stems tolerated tissue temperatures below -5°C for 1 h while both showed substantial high temperature acclimation, roots tolerating 1 h at 61°C and stems 1 h at 70°C for plants grown at 35/25°C. The increase in height and frequency of O. acanthocarpa with elevation apparently reflected both a greater ability for net CO₂ uptake and greater root growth and hence water uptake. This species achieves its greatest ecological success at elevations where it becomes vulnerable to low temperature damage.