Leaf Carbon Isotope Ratios Research Articles

Stable carbon and nitrogen isotope ratios as indicators of water status and nitrogen effects on peach trees

Carbon and nitrogen isotope ratios are tools that are widely used in ecophysiological studies of forest and herbaceous plants. However, the use of isotope discrimination techniques with fruit trees has been very limited. Three irrigation strategies were applied to peach trees (Prunus persica (L.) Batsch. cv. Andross): full irrigation throughout the growing season; restricted irrigation during stage-II of fruit growth (70% restriction); and restricted irrigation during stage-III (30% restriction). These irrigation treatments were combined with three different nitrogen fertilization treatments: 0, 60 and 120kgNha−1. The stable carbon isotope ratio (δ13C) in fruit proved a reliable indicator of peach crop water status and physiological water use efficiency. The leaf carbon isotope ratio was not affected by either the irrigation or N treatments. δ13C in endocarp tissues exhibited a powerful discriminatory capacity for water stress during phase II of fruit growth, whereas δ13C in the mesocarp was an interesting tracer for water stress in phase III of fruit growth. There was an important relationship between leaf N content and photosynthetic efficiency during period GSII, which enhanced leaf WUE. The stable nitrogen isotope ratio (δ15N) in the fruit mesocarp was significantly related to the amount of N applied and the origin of this N. The nitrogen isotope ratio did not, however, exhibit a good discriminative capacity when we studied the influence of water and nitrogen on plant response.

The effect of strobilurins on leaf gas exchange, water use efficiency and ABA content in grapevine under field conditions

Strobilurins are one of the most important classes of agricultural fungicides. In addition to their anti-fungal effect, strobilurins have been reported to produce simultaneous effects in plant physiology. This study investigated whether the use of strobilurin fungicide improved water use efficiency in leaves of grapevines grown under field conditions in a Mediterranean climate in southern Spain. Fungicide was applied three times in the vineyard and measurements of leaf gas exchange, plant water status, abscisic acid concentration in sap ([ABA]), and carbon isotope composition in leaves were performed before and after applications. No clear effect on stomatal conductance, leaf water potential and intrinsic water use efficiency was found after three fungicide applications. ABA concentration was observed to increase after fungicide application on the first day, vanishing three days later. Despite this transient effect, evolution of [ABA] matched well with the evolution of leaf carbon isotope ratio, which can be used as a surrogate for plant water use efficiency. Morning stomatal conductance was negatively correlated to [ABA]. Yield was enhanced in strobilurin treated plants, whereas fruit quality remained unaltered.

Evidence of a universal scaling relationship for leaf CO2 drawdown along an aridity gradient

The leaf carbon isotope ratio (δ(13) C) of C3 plants is inversely related to the drawdown of CO2 concentration during photosynthesis, which increases towards drier environments. We aimed to discriminate between the hypothesis of universal scaling, which predicts between-species responses of δ(13) C to aridity similar to within-species responses, and biotic homoeostasis, which predicts offsets in the δ(13) C of species occupying adjacent ranges. The Northeast China Transect spans 130-900 mm annual precipitation within a narrow latitude and temperature range. Leaves of 171 species were sampled at 33 sites along the transect (18 at ≥ 5 sites) for dry matter, carbon (C) and nitrogen (N) content, specific leaf area (SLA) and δ(13) C. The δ(13) C of species generally followed a common relationship with the climatic moisture index (MI). Offsets between adjacent species were not observed. Trees and forbs diverged slightly at high MI. In C3 plants, δ(13) C predicted N per unit leaf area (Narea) better than MI. The δ(13) C of C4 plants was invariant with MI. SLA declined and Narea increased towards low MI in both C3 and C4 plants. The data are consistent with optimal stomatal regulation with respect to atmospheric dryness. They provide evidence for universal scaling of CO2 drawdown with aridity in C3 plants.

Ecophysiology of riparian cottonwood and willow before, during, and after two years of soil water removal

Riparian cottonwood/willow forest assemblages are highly valued in the southwestern United States for their wildlife habitat, biodiversity, and watershed protection. Yet these forests are under considerable threat from climate change impacts on water resources and land-use activities to support human enterprise. Stream diversions, groundwater pumping, and extended drought have resulted in the decline of cottonwood/willow forests along many riparian corridors in the Southwest and, in many cases, the replacement of these forests with less desirable invasive shrubs and trees. Nevertheless, ecophysiological responses of cottonwood and willow, along with associated ecohydrological feedbacks of soil water depletion, are not well understood. Ecophysiological processes of mature Fremont cottonwood and coyote willow stands were examined over four consecutive growing seasons (2004-2007) near Salt Lake City, Utah, USA. The tree stands occurred near the inlet of a reservoir that was drained in the spring of 2005 and remained empty until mid-summer of 2006, effectively removing the primary water source for most of two growing seasons. Stem sap flux density (Js) in cottonwood was highly correlated with volumetric soil moisture (theta) in the upper 60 cm and decreased sevenfold as soil moisture dropped from 12% to 7% after the reservoir was drained. Conversely, Js in willow was marginally correlated with 0 and decreased by only 25% during the same period. Opposite patterns emerged during the following growing season: willow had a lower whole-plant conductance (kt) in June and higher leaf carbon isotope ratios (delta13C) than cottonwood in August, whereas k(t) and delta13C were otherwise similar between species. Water relations in both species recovered quickly from soil water depletion, with the exception that sapwood area to stem area (As:Ast) was significantly lower in both species after the 2007 growing season compared to 2004. Results suggest that cottonwood has a greater sensitivity to interannual reductions in water availability, while willow is more sensitive to longer periods of soil water depletion. These data shed light on the linkage between soil water deficits and ecophysiological processes of threatened riparian forests given potential land-use and long-term drought impacts on freshwater resources.

Effects of tree height on branch hydraulics, leaf structure and gas exchange in California redwoods

We examined changes in branch hydraulic, leaf structure and gas exchange properties in coast redwood (Sequoia sempervirens) and giant sequoia (Sequoiadendron giganteum) trees of different sizes. Leaf-specific hydraulic conductivity (k(L)) increased with height in S. sempervirens but not in S. giganteum, while xylem cavitation resistance increased with height in both species. Despite hydraulic adjustments, leaf mass per unit area (LMA) and leaf carbon isotope ratios (delta(13)C) increased, and maximum mass-based stomatal conductance (g(mass)) and photosynthesis (A(mass)) decreased with height in both species. As a result, both A(mass) and g(mass) were negatively correlated with branch hydraulic properties in S. sempervirens and uncorrelated in S. giganteum. In addition, A(mass) and g(mass) were negatively correlated with LMA in both species, which we attributed to the effects of decreasing leaf internal CO(2) conductance (g(i)). Species-level differences in wood density, LMA and area-based gas exchange capacity constrained other structural and physiological properties, with S. sempervirens exhibiting increased branch water transport efficiency and S. giganteum exhibiting increased leaf-level water-use efficiency with increasing height. Our results reveal different adaptive strategies for the two redwoods that help them compensate for constraints associated with growing taller, and reflect contrasting environmental conditions each species faces in its native habitat.

Gender Differences in Reproductive and Physiological Traits in a Gynodioecious Species, Geranium maculatum (Geraniaceae)

Plant species with separate genders often exhibit gender differences in traits related to reproductive allocation. In gynodioecious species, females often produce more seeds than do hermaphrodites, leading to a higher reproductive cost. The mechanisms that allow females to meet the high costs of reproduction are currently under debate. In this study, we test the hypothesis that there are genetically based gender differences in physiological traits that enable females to finance these costs through higher photosynthetic carbon gain in the gynodioecious perennial Geranium maculatum. Females and hermaphrodites were compared in a greenhouse study that minimized environmental and selfing rate differences between the genders. We found that females produced smaller flowers but more of them and more fruits than did hermaphrodites. However, genders did not differ in their seed number, seed mass, fruit set, and reproductive allocation. In addition, genders did not differ in photosynthetic rate (A), leaf N, and water use efficiency inferred from leaf carbon isotope ratio (δ13C). Overall, G. maculatum shows no genetically based gender differences for most of the reproductive traits or any of the physiological traits measured. Our results suggest that for G. maculatum, the gender fitness differences previously identified in natural populations may be caused by gender differences in microhabitat and/or selfing rate.

몬순기후와 복잡지형의 특성을 갖는 광릉 산림유역의 물과 탄소순환에 대한 교차규모 연구로부터의 교훈

광릉 슈퍼사이트에서 대기와 식생간의 에너지와 물질교환 규명은 지형과 식생의 복잡성으로 인해 많은 기술적인 어려움에 직면하고 있다. 따라서 KoFlux 연구에서는 상호보완적인 다중 분야의 연구를 통해 이러한 어려움을 극복하기 위한 시도를 해 오고 있다. 본 논문에서는 현재 진행중인 KoFlux 연구의 예비결과 중 특히 광릉사이트에서의 물/탄소수지, 물과 탄소순환의 상호관련성 그리고 몬순기후하의 생태계에서 탄소순환에 미치는 수문학적 요인의 중요성에 대해 중점적으로 논의하고자 한다. 광릉낙엽활엽수림의 순생태생산량 (NEP)은 광범위한 생태학적 연구결과에 따르면 약 <TEX>${\sim}2.6\;t\;C/ha/y$</TEX>로 예상된다. 이를 미기상 플럭스 관측 결과와 함께 고려할 때 광릉 산림은 탄소의 중요한 육상흡원으로서 기능함을 예상할 수 있다. 다양한 생태수문 관측에 의해 규명된 유역단위 물수지에 의하면 연단위 전체강수량의 약 30-40%가 증발산(ET)에 해당함을 나타내고 있다. 광릉산림의 대표적인 수종인 졸참과 서어나무의 잎에서 얻어진 탄소 안정동위원소 조성에 근거하여 계산된 식물 성장기간의 평균 물이용효율(WUE)은 약 <TEX>${\sim}12\;{\mu}mol\;CO_2/mmol\;H_2O$</TEX>에 해당된다. 얻어진 증발산량과 물이용효율은 유역단위의 생태계 생산량을 산출하는데 이용될 수 있다. 나이테 성장량과 토양호흡량의 연단위 변화는 강수량과 강수의 패턴에 의해 크게 영향을 받음을 보이고, 이는 몬순기후에 영향을 받는 동아시아 지역 생태계에서 탄소 순환과정이 수문학적 조건과 밀접한 상관관계를 가짐을 지시한다. 연구지역의 공간적 특성을 정량화하기 위해 관측지의 구조 및 기능적 단위를 규명하기 위한 연구가 진행되고 있고 이를 통해 서로 다른 시공간 규모에서 진행되고 있는 연구의 결과물을 체계적으로 연결 통합하고 나아가 보다 광역적인 규모에서 대표적인 물/탄소 수지를 산출하기 위한 연구가 시도되고 있다. KoFlux Gwangneung Supersite comprises complex topography and diverse vegetation types (and structures), which necessitate complementary multi-disciplinary measurements to understand energy and matter exchange. Here, we report the results of this ongoing research with special focuses on carbon/water budgets in Gwangneung forest, implications of inter-dependency between water and carbon cycles, and the importance of hydrology in carbon cycling under monsoon climate. Comprehensive biometric and chamber measurements indicated the mean annual net ecosystem productivity (NEP) of this forest to be <TEX>${\sim}2.6\;t\;C\;ha^{-1}y^{-1}$</TEX>. In conjunction with the tower flux measurement, the preliminary carbon budget suggests the Gwangneung forest to be an important sink for atmospheric <TEX>$CO_2$</TEX>. The catchment scale water budget indicated that <TEX>$30\sim40%$</TEX> of annual precipitation was apportioned to evapotranspiration (ET). The growing season average of the water use efficiency (WUE), determined from leaf carbon isotope ratios of representative tree species, was about <TEX>$12{\mu}mol\;CO_2/mmol\;H_2O$</TEX> with noticeable seasonal variations. Such information on ET and WUE can be used to constrain the catchment scale carbon uptake. Inter-annual variations in tree ring growth and soil respiration rates correlated with the magnitude and the pattern of precipitation during the growing season, which requires further investigation of the effect of a monsoon climate on the catchment carbon cycle. Additionally, we examine whether structural and functional units exist in this catchment by characterizing the spatial heterogeneity of the study site, which will provide the linkage between different spatial and temporal scale measurements.

Species differences in carbon isotope ratios, specific leaf area and nitrogen concentrations in leaves of Eucalyptus growing in a common garden compared with along an aridity gradient

Leaves produced in 2004 of 422 species of Eucalyptus whose natural habitat is southern Australia were sampled at the Currency Creek Arboretum in South Australia where the annual (mainly winter) rainfall is about 400 mm. Tree height, leaf area, leaf dry weight, leaf nitrogen (N) concentration and leaf carbon isotope ratio (δ13C) were measured and the specific leaf area (SLA) calculated. Among the 422 species, the SLA varied from 1.5 to 8.8 m2 kg−1 and N concentration varied from 0.6 to 2.1%, much greater than in 64 species collected along an aridity transect from southwestern Western Australia to central Australia in 2003. Also, the range of leaf δ13C values was similar in the common garden to that across the aridity transect. For the 45 species present in both studies, the SLA and leaf N concentration in the common garden were similar to those measured in leaves along the aridity transect, indicating that these characteristics are inherent in the species and vary little with environment. The variation in leaf δ13C in the common garden was just as great as along the transect, but the values measured in the one location were poorly correlated with those along the transect. This was not expected, as the variation in δ13C at one common site in South Australia was anticipated to be less than along the aridity gradient where annual rainfall varied from 250 to 1200 mm. Path analysis on the 45 species common to both studies indicated that rainfall did not have a direct effect on δ13C, but the differences in δ13C resulted from indirect effects of rainfall on SLA and N concentration. δ13C was negatively correlated with SLA but positively correlated with N. Thus, both effects may compensate for each other so that no significant relationship between δ13C and rainfall was observable. However, there is a large degree of variation of δ13C at any level of rainfall. The origin and ecological implications of this observation are discussed.

Coordination of crown structure, leaf plasticity and carbon gain within the crowns of three winter-deciduous mature trees

We examined the vertical profiles of leaf characteristics within the crowns of two late-successional (Fagus crenata Blume and Fagus japonica Maxim.) and one early-successional tree species (Betula grossa Sieb. et Zucc.) in a Japanese forest. We also assessed the contributions of the leaves in each crown layer to whole-crown instantaneous carbon gain at midday. Carbon gain was estimated from the relationship between electron transport and photosynthetic rates. We hypothesized that more irradiance can penetrate into the middle of the crown if the upper crown layers have steep leaf inclination angles. We found that such a crown has a high whole-crown carbon gain, even if leaf traits do not change greatly with decreasing crown height. Leaf area indices (LAIs) of the two Fagus trees (5.26-5.52) were higher than the LAI of the B. grossa tree (4.50) and the leaves of the F. crenata tree were more concentrated in the top crown layers than were leaves of the other trees. Whole-crown carbon gain per unit ground area (micromol m(-2) ground s(-1)) at midday on fine days in summer was 16.3 for F. crenata, 11.0 for F. japonica, and 20.4 for B. grossa. In all study trees, leaf dry mass (LMA) and leaf nitrogen content (N) per unit area decreased with decreasing height in the crown, but leaf N per unit mass increased. Variations (plasticity) between the uppermost and lowermost crown layers in LMA, leaf N, the ratio of chlorophyll to N and the ratio of chlorophyll a to b were smaller for F. japonica and B. grossa than for F. crenata. The light extinction coefficients in the crowns were lower for the F. japonica and B. grossa trees than for the F. crenata tree. The leaf carbon isotope ratio (delta(13)C) was higher for F. japonica and B. grossa than for F. crenata, especially in the mid-crown. These results suggest that, in crowns with low leaf plasticity but steep leaf inclination angles, such as those of F. japonica and B. grossa trees, irradiance can penetrate into the middle of the crowns, thereby enhancing whole-crown carbon gain.

Can the restoration of natural vegetation be accelerated on the Chinese Loess Plateau? A study of the response of the leaf carbon isotope ratio of dominant species to changing soil carbon and nitrogen levels

AbstractFor the heavily degraded ecosystem on the Chinese Loess Plateau, it would be of great significance if vegetation restoration could be accelerated anthropogenically. However, one major concern is that if the late successional species were planted or sown in degraded habitats, would they still be competitive in terms of some critical plant traits associated with specific habitats? Water use efficiency (WUE) is a major plant trait shaping the pattern of species turnover in vegetation secondary succession on the Loess Plateau. We hypothesized that if late successional stage plants could still hold a competitive advantage in terms of WUE, the prospects for an acceleration of succession by sowing these species in newly abandoned fields would be good. We tested this hypothesis by comparing the leaf C isotope ratio (δ13C) value (a surrogate of WUE) of dominant species from different successional stages at given soil C and N levels. Results indicated that leaf δ13C of the two dominant species that co‐dominated in the second and third stages were significantly more positive than that of the dominant species from the first stage regardless of changing soil C and N. Yet the dominant species from the climax stage is a C4 grass assumed to have the highest WUE. In addition, increasing soil nutrition had no effects on leaf δ13C of two dominant species in the late successional stage, indicating that dominant species from the late successional stages could still have a competitive advantage in terms of WUE in soil C‐ and N‐poor habitats. Therefore, from the perspective of plant WUE, there are great opportunities for ecosystem restoration by sowing both dominant species and other species that co‐occur in late successional stages in newly abandoned fields, for the purpose of enhancing species diversity and optimising species composition.

The influence of N addition on nutrient content, leaf carbon isotope ratio, and productivity in a Nothofagus forest during stand development

To test whether increased nitrogen (N) availability might increase productivity in maturing mountain beech (Nothofagus solandri var. cliffortioides (Hook. f.) Poole) forest in central South Island, New Zealand, we applied N to 25-year-old sapling and 125-year-old pole stands. Nitrogen fertilizer increased foliar and fine-root N concentrations, fine-root growth, and leaf litter production in both sapling and pole stands but had no effect on stem basal area increment or individual leaf area, and it decreased individual leaf mass marginally. Heavy flowering and seeding occurred in the second year after fertilizer was applied, and N increased production of both. Leaf litter production and flowering responded similarly to N in sapling and pole stands, but N increased fine-root and seed productivity more in pole stands than in sapling stands, confirming our hypothesis that productivity of pole stands was more limited by low N availability. Resource allocation to fine roots and seed production may have restricted stem basal area increment response to N in the short term. Pole stands had higher leaf δ13C values than sapling stands. It is concluded that both low N availability and moisture stress may contribute to the decline in productivity and wood biomass previously found in mature mountain beech stands.

Effects of an Experimental Increase of Temperature and Drought on the Photosynthetic Performance of Two Ericaceous Shrub Species Along a North?South European Gradient

Plant ecophysiological changes in response to climatic change may be different in northern and southern European countries because different abiotic factors constrain plant physiological activity. We studied the effects of experimental warming and drought on the photosynthetic performance of two ericaceous shrubs (Erica multiflora and Calluna vulgaris) along a European gradient of temperature and precipitation (UK, Denmark, The Netherlands, and Spain). At each site, a passive warming treatment was applied during the night throughout the whole year, whereas the drought treatment excluded rain events over 6–10 weeks during the growing season. We measured leaf gas exchange, chlorophyll a fluorescence, and leaf carbon isotope ratio (δ13C) during the growing seasons of 1999 and 2000. Leaf net photosynthetic rates clearly followed a gradient from northern to southern countries in agreement with the geographical gradient in water availability. Accordingly, there was a strong correlation between net photosynthetic rates and the accumulated rainfall over the growing season. Droughted plants showed lower leaf gas exchange rates than control plants in the four sites. Interestingly, although leaf photosynthetic rates decreased along the precipitation gradient and in response to drought treatment, droughted plants were able to maintain higher leaf photosynthetic rates than control plants in relation to the accumulated rainfall over the months previous to the measurements. Droughted plants also showed higher values of potential photochemical efficiency (Fv/Fm) in relation to controls, mainly at midday. The warming treatment did not affect significantly any of the studied instantaneous ecophysiological variables.

Nitrogen fertilization of trembling aspen seedlings grown on soils of different pH

Seedlings from three open-pollinated aspen (Populus tremuloides Michx.) families were grown in a greenhouse with four nitrogen (N) sources, each at two N levels and three pH levels. Nitrogen sources were ammonium sulfate ((NH4)2SO4), calcium nitrate (Ca(NO3)2), ammonium nitrate (NH4NO3), and urea ((NH2)2CO); N levels were 50 and 200 mg·L–1 (100-mL aliquots three times per week); and pH levels were 5, 6, and 7. Shoot dry mass and leaf area increased with pH in (NH4)2SO4 and (NH2)2CO treatments, but were greatest at pH 6 in the NH4NO3 and Ca(NO3)2 treatments as a result of a pH × N source interaction. N level was directly related to growth, net assimilation, water-use efficiency, and leaf carbon isotope ratio, while N source had no effect on these parameters. Seedlings from families 2 and 3 grew larger than seedlings from family 1, which allocated relatively more dry matter to roots. Growth of the three families interacted with soil pH so that family 1 was largest at pH 7, but families 2 and 3 were largest at pH 6. Seedling boron (B) uptake was reduced by increasing pH and by the Ca(NO3)2 fertilizer. These results were interpreted to show that although the three families seemed well adapted to all the N and pH conditions to which they were exposed, growth could be increased by selection of a fertilizer most suitable to the pH and availability of other nutrients in the soil.

The effect of internal Co2 conductance on leaf carbon isotope ratio

We estimated internal CO2 conductance (gi ) for C3 plant species of different life forms (annual herbs, deciduous trees and evergreen trees) grown in a variety of environments to examine the effect of g i on their leaf carbon isotope ratio (i13C). The purpose of this study was to test the validity of using i13C as an index of photosynthetic water use efficiency (WUE). When comparing deciduous tree species grown in contrasting light environments, there was a strong positive relationship between i13C and WUE. Similarly, i13C was positively correlated to WUE when comparing the different species of evergreen trees. However, the difference in WUE between evergreen and deciduous tree species did not relate to that in leaf i13C. In addition, WUE was similar between highland and lowland herbaceous plants, although the former had a much higher i13C. The positive relationship between i13C and WUE did not hold across different life forms and different altitudes when differences in gi did not relate to those in stomatal conductance, resulting in independence of chloroplast CO2 partial pressure from intercellular CO2 partial pressure.

Predicting daytime carbon isotope ratios of atmospheric CO2 within forest canopies

Summary While measurements of leaf carbon isotope ratios (13C/12C) in terrestrial ecosystems have become more frequent, interpreting these data can remain a challenge in well developed canopies: the variation in leaf δ13C (δ13Cp) values is influenced by both the variation in δ13C of source air (δ13Ca) and by photosynthetic carbon isotope discrimination (Δ). However, source air information is often unavailable, limiting the interpretation of δ13Cp, particularly in dense stands. In this synthesis we found that about 70% of the observed variation in δ13Cp values within the canopy was influenced by changes in Δ, and that about 30% was determined by source air effects. Significant shifts in δ13Ca occur in canopies with high leaf area, predominantly within 1 m above the forest floor. In complex canopies, particularly in the understorey, source air effects cannot be neglected if δ13Cp measurements are used to calculate Δ and ci/ca ratios [ratio of internal CO2 concentration in the mesophyll airspaces (ci) to the ambient atmospheric concentration of CO2 (ca)]. We modelled δ13Ca of daytime source air for deciduous and coniferous forests in boreal, temperate and tropical biomes. An inverse regression model with easily available input variables accounted for about 90% of the variation in daytime δ13Ca values throughout the canopy. In open canopies with leaf area index (L) of &lt;2·5 or at canopy heights ≥1 m, the within‐canopy daytime δ13Ca differences are negligible, and variations in δ13Cp are associated primarily with changes in Δ. Then, one can use the easily available carbon isotope ratio of the troposphere (δ13Ctrop) as a substitute for δ13Ca to calculate Δ to within ±0·4‰. In canopies with L values &gt;2·5, and at canopy heights &lt;1 m, our model is recommended for calculating canopy δ13Ca values when direct measurements are not feasible. Although δ13Ca is highly variable near the forest floor in those dense forests, our model is more accurate and precise for estimating δ13Ca within 1 m above the forest floor than using δ13Ctrop throughout the canopy (−0·2‰ ± 1·5 versus −1·4‰ ± 1·1).

ECOLOGICAL DIFFERENTIATION OF COMBINED AND SEPARATE SEXES OFWURMBEA DIOICA(COLCHICACEAE) IN SYMPATRY

The evolution and maintenance of combined vs. separate sexes in flowering plants is influenced by both ecological and genetic factors; variation in resources, partic- ularly moisture availability, is thought to play a role in selection for gender dimorphism in some groups. We investigated the density, distribution, biomass allocation, and physi- ology of sympatric monomorphic (cosexual) and dimorphic (female and male) populations of Wurmbea dioica in relation to soil moisture on the Darling Escarpment in southwestern Australia. Populations with monomorphic vs. dimorphic sexual systems segregated into wet vs. dry microsites, respectively, and biomass allocation patterns and physiological traits reflected differences in water availability, despite similarities in total ramet biomass between the sexual systems. Unisexuals flowered earlier at lower density, and they allocated sig- nificantly more biomass below ground to roots and corms than did cosexuals, which al- located more biomass above ground to leaves, stems, and flowers. Females, males, and cosexuals produced similar numbers of flowers per ramet, but unisexuals produced more ramets than cosexuals, increasing the total number of flowers per genet. Contrary to ex- pectation, cosexuals had significantly higher (more positive) leaf carbon isotope ratios and lower leaf nitrogen content than unisexuals, suggesting that cosexuals are more water-use efficient and have lower rates of photosynthesis per unit leaf mass despite their occurrence in wetter microsites. Cosexuals appear to adjust their stomatal behavior to minimize water loss through transpiration while maintaining high investment in leaves and reproductive structures. Unisexuals apparently maximize the acquisition and storage of both water and nitrogen through increased allocation to roots and corms and enhance the uptake of CO2 by keeping stomata more open. These findings indicate that the two sexual systems have different morphological and physiological features associated with local-scale variation in water availability.

Dwarf mistletoe affects whole-tree water relations of Douglas fir and western larch primarily through changes in leaf to sapwood ratios.

Dwarf mistletoes induce abnormal growth patterns and extreme changes in the biomass allocation of their hosts as well as directly parasitizing them for resources. Because biomass allocation can affect the resource use and efficiency of conifers, we studied the influences of dwarf mistletoe infection on above-ground biomass allocation of Douglas fir and western larch, and the consequences of such changes on whole-tree water use and water relations. Sap flow, tree water potentials, leaf:sapwood area ratios (A L:A S), leaf carbon isotope ratios, and nitrogen content were measured on Douglas fir and western larch trees with various degrees of mistletoe infection during the summer of 1996 in western Montana. Heavy dwarf mistletoe infection on Douglas fir and western larch was related to significant increases in A L:A S. Correspondingly, water transport dynamics were altered in infected trees, but responses were different for the two species. Higher A L:A S ratios in heavily infected Douglas firs were offset by increases in sapwood area-based sap flux densities (Q SW) such that leaf area-based sap flux densities (Q L) and predawn leaf water potentials at the end of the summer did not change significantly with mistletoe infection. Small (but statistically insignificant) decreases of Q L for heavily infected Douglas firs were enough to offset increases in leaf area such that whole-tree water use was similar for uninfected and heavily infected trees. Increased A L:A S ratios of heavily infected western larch were not offset by increases of Q SW. Consequently, Q L was reduced, which corresponded with significant decreases of water potential at the end of the summer. Furthermore, mistletoe-infection-related changes in A L:A S as a function of tree size resulted in greater whole-tree water use for large infected larches than for large uninfected trees. Such changes may result in further depletion of limited soil water resources in mature infected stands late in the growing season. Foliage from infected trees of both species had lower water use efficiencies than non-infected trees. Our results demonstrate substantial changes of whole-tree processes related to mistletoe infection, and stress the importance of integrating whole-tree physiological and structural processes to fully understand the mechanisms by which pathogens suppress forest productivity.

Relationship between leaf characteristics, tree sizes and species distribution along a slope in a warm temperate forest

The relationships between some leaf characteristics, tree size and species distribution were investigated for evergreen tree species along a slope in a warm temperate forest in Japan. Tree species were classified into three groups based on their dominance on the slope: ‘ridge species’ that were aggregated in an uppersite, ‘valley species’ that were aggregated in a lowersite, and ‘uniform species’ that were distributed almost uniformly. The ridge species had a more positive leaf carbon isotope ratio than the valley species, which suggests that the ridge species have larger water use efficiency than the valley species. This may give some advantage to the ridge species over the valley species in the uppersite where water availability would be limited. However, the ridge species had smaller leaf nitrogen content on a mass basis and larger leaf mass per area than the valley species, which suggests that the ridge species had a smaller mass‐based leaf photosynthetic capacity than the valley species. This may be disadvantageous to ridge species in the lowersite, because smaller leaf photosynthetic capacities cause lower leaf carbon gain and thus lower growth than the valley species. These differences in leaf characteristics between the ridge and the valley species were affected by microenvironments, and were also partly affected by the difference in species specific responses to microenvironments on the slope.

Germination and growth responses of hybridizing Carpobrotus species (Aizoaceae) from coastal California to soil salinity

Germination, growth, and physiological responses of hybridizing Carpobrotus from coastal California to soil salinity were studied. Hybrids are presumably the result of hybridization and introgression between the exotic Carpobrotus edulis, a succulent perennial invading coastal habitats, and the native or long-naturalized C. chilensis. Germination responses were investigated at 0, 10, 20, and 50% seawater. Seedling growth and physiology were compared by irrigating seedlings with solutions of the same seawater concentrations and in low and high nutrients. Germination was inhibited in the presence of salt, but recovered after transferring the seeds to fresh water. Seeds exposed to salt had higher final germination rates than control. Growth of Carpobrotus was slightly enhanced by low seawater concentrations but reduced at high salinity at both nutrient regimes. Leaf cell sap osmolarity increased with increasing soil salinity, and taxa did not differ significantly in this physiological adjustment. Leaf carbon isotope ratios (∂(13)C) ranged from -28 to -22‰ and became less negative at higher salinities, indicating an improved water use efficiency in the seedlings at high salt concentrations. In addition, ∂(13)C values were generally less negative at high than at low nutrients. Differences among taxa were generally small. The results show that salinity affects both establishment and growth of hybridizing Carpobrotus. The overall weak species differences in salt tolerance indicate that the exotic C. edulis can occupy the same sites as C. chilensis in terms of salinity. The similarity of hybrids in their response to salinity suggests that they may contribute to the invasion by Carpobrotus.

Carbon Isotope Ratio in Street Tree Leaves: An Indicator for the Environmental Assessment of the Atmosphere

When plants fix atmospheric CO2 according to the process of photosynthesis, carbon isotope fractionation associated with carboxylation and diffusion occurs. Therefore, plants are enriched in 12C relative to the atmosphere. The isotopic discrimination between plants and atmospheric CO2 is shown in the following equation (after Farquhar et al., 1989), written as: ~13Cp = ~13C a - (4.4 + 22.6  Ci/Ca) where 5~3Cp, is the isotope ratio of plants, 8~3Ca the isotope ratio of atmospheric CO2, Ci the internal gasphase concentration Of CO2, and Ca the external CO2 concentration. When isotope discrimination between plants and atmospheric CO2 is established, the leaf carbon isotope ratio depends on that of the CO2 in the air. The Ci/Ca ratios of plants, however, change according to the long-term water stress. For example, the ratios vary from 0.64 to 0.40 for Zelkowa serrata TABLE 1. Carbon isotope ratios, and the carbon and nitrogen content of Zelkowa leaves along Sakuradori Steet in Toyama City during September 1996