Leaf functional response to increasing atmospheric CO(2) concentrations over the last century in two northern Amazonian tree species: a historical δ(13) C and δ(18) O approach using herbarium samples

We assessed the extent of recent environmental changes on leaf morphological (stomatal density, stomatal surface, leaf mass per unit area) and physiological traits (carbon isotope composition, δ(13)C(leaf) , and discrimination, Δ(13)C(leaf) , oxygen isotope composition, δ(18)O(leaf) ) of two tropical rainforest species (Dicorynia guianensis; Humiria balsamifera) that are abundant in the Guiana shield (Northern Amazonia). Leaf samples were collected in different international herbariums to cover a 200 year time-period (1790-2004) and the whole Guiana shield. Using models describing carbon and oxygen isotope fractionations during photosynthesis, different scenarios of change in intercellular CO(2) concentrations inside the leaf (C(i)), stomatal conductance (g), and photosynthesis (A) were tested in order to understand leaf physiological response to increasing air CO(2) concentrations (C(a)). Our results confirmed that both species displayed physiological response to changing C(a) . For both species, we observed a decrease of about 1.7‰ in δ(13)C(leaf) since 1950, without significant change in Δ(13)C(leaf) and leaf morphological traits. Furthermore, there was no clear change in δ(18)O(leaf) for Humiria over this period. Our simulation approach revealed that an increase in A, rather than a decrease in g, explained the observed trends for these tropical rainforest species, allowing them to maintain a constant ratio of C(i)/C(a) .     

δ(13) C of leaf-respired CO(2) reflects intrinsic water-use efficiency in barley

Leaf intrinsic water-use efficiency (WUE), the ratio of photosynthetic rate to stomatal conductance (A/g(s) ), is a key plant trait linking terrestrial carbon and water cycles. A rapid, integrative proxy for A/g(s) is of benefit to crop breeding programmes aiming to improve WUE, but also for ecologists interested in plant carbon-water balance in natural systems. We hypothesize that the carbon isotope composition of leaf-respired CO(2) (δ(13) C(Rl) ), two hours after leaves are transferred to the dark, records photosynthetic carbon isotope discrimination and so provides a proxy for A/g(s) . To test this hypothesis, δ(13) C(Rl) was measured in four barley cultivars grown in the field at two levels of water availability and compared to leaf-level gas exchange (the ratio of leaf intercellular to ambient CO(2) partial pressure, C(i) /C(a) , and A/g(s) ). Leaf-respired CO(2) was more (13) C-depleted in plants grown at higher water availability, varied between days as environmental conditions changed, and was significantly different between cultivars. A strong relationship between δ(13) C(Rl) and δ(13) C of sucrose was observed. δ(13) C(Rl) was converted into apparent photosynthetic discrimination (Δ(13) C(Rl) ) revealing strong relationships between Δ(13) C(Rl) and C(i) /C(a) and A/g(s) during the vegetative stage of growth. We therefore conclude that δ(13) C(Rl) may provide a rapid, integrative proxy for A/g(s) in barley.     

The “isohydric trap”: A proposed feedback between water shortage,stomatal regulation,and nutrient acquisition drives differential growth and survival of European pines under climatic dryness

Climatic dryness imposes limitations on vascular plant growth by reducing stomatal conductance, thereby decreasing CO₂ uptake and transpiration. Given that transpiration‐driven water flow is required for nutrient uptake, climatic stress‐induced nutrient deficit could be a key mechanism for decreased plant performance under prolonged drought. We propose the existence of an “isohydric trap,” a dryness‐induced detrimental feedback leading to nutrient deficit and stoichiometry imbalance in strict isohydric species. We tested this framework in a common garden experiment with 840 individuals of four ecologically contrasting European pines (Pinus halepensis, P. nigra, P. sylvestris, and P. uncinata) at a site with high temperature and low soil water availability. We measured growth, survival, photochemical efficiency, stem water potentials, leaf isotopic composition (δ¹³C, δ¹⁸O), and nutrient concentrations (C, N, P, K, Zn, Cu). After 2 years, the Mediterranean species Pinus halepensis showed lower δ¹⁸O and higher δ¹³C values than the other species, indicating higher time‐integrated transpiration and water‐use efficiency (WUE), along with lower predawn and midday water potentials, higher photochemical efficiency, higher leaf P, and K concentrations, more balanced N:P and N:K ratios, and much greater dry‐biomass (up to 63‐fold) and survival (100%). Conversely, the more mesic mountain pine species showed higher leaf δ¹⁸O and lower δ¹³C, indicating lower transpiration and WUE, higher water potentials, severe P and K deficiencies and N:P and N:K imbalances, and poorer photochemical efficiency, growth, and survival. These results support our hypothesis that vascular plant species with tight stomatal regulation of transpiration can become trapped in a feedback cycle of nutrient deficit and imbalance that exacerbates the detrimental impacts of climatic dryness on performance. This overlooked feedback mechanism may hamper the ability of isohydric species to respond to ongoing global change, by aggravating the interactive impacts of stoichiometric imbalance and water stress caused by anthropogenic N deposition and hotter droughts, respectively.     

Ontogeny and leaf gas exchange mediate the carbon isotopic signature of herbaceous plants

Values (Δ(i)) predicted by a simplified photosynthetic discrimination model, based only on diffusion through air followed by carboxylation, are often used to infer ecological conditions from the 13C signature of plant organs (δ13C(p)). Recent studies showed that additional isotope discrimination (d that includes mesophyll conductance, photorespiration and day respiration, and post-carboxylation discrimination) can strongly affect δ13C(p); however, little is known about its variability during plant ontogeny for different species. Effect of ontogeny on leaf gas exchange rates, Δ(i) , observed discrimination (Δ(p)) and d in leaf, phloem and root of seven herbaceous species at three ontogenetic stages were investigated under controlled conditions. Functional group identity and ontogeny significantly affected Δ(i) and Δ(p). However, predicted Δ(i) did not match Δ(p). d, strongly affected by functional group identity and ontogeny, varied by up to 14 ‰. d scaled tightly with stomatal conductance, suggesting complex controls including changes in mesophyll conductance. The magnitude of the changes in δ13C(p) due to ontogeny was similar to that due to environmental factors reported in other studies. d and ontogeny should, therefore, be considered in ecosystem studies, integrated in ecosystem models using δ13C(p) and limit the applicability of δ13C(leaf) as a proxy for water-use efficiency in herbaceous plants.     

Foliar and litter needle carbon and oxygen isotope compositions relate to tree growth of an exotic pine plantation under different residue management regimes in subtropical Australia

Background and aim

Significant differences in tree growth were observed in an exotic pine plantation under different harvest residue management regimes at ages 2–10 years. However, the variations in tree growth between residue management treatments could not be explained by soil and foliar nutrient analyses, except by potassium (K) concentration. Therefore, this study determined the carbon isotope composition (δ¹³C) and oxygen isotope composition (δ¹⁸O) of current and archived foliar samples from the exotic pine plantation to establish relationships with foliar K concentration and tree growth indices as a means to determine changes in stomatal conductance (g_s) and photosynthetic rate (A_max) or water use efficiency (WUE), and therefore understand the variations in tree growth across treatments.

Methods

The harvest residue treatments were: (1) residue removal, RR₀; (2) single level residue retention, RR₁; and (3) double level residue retention, RR₂. Foliar δ¹³C and δ¹⁸O were determined for samples at ages 2, 4, 6 and 10 years, and the atmospheric ¹³C discrimination (Δ¹³C), intercellular CO₂ concentration (C_i) and WUE were determined from the δ¹³C data. Litter needle δ¹³C and δ¹⁸O were also determined over 15 months between ages 9 and 10 years. These parameters or variables where correlated to each other as well as to the periodic mean annual increment of basal area (PAIB) and the periodic mean annual increment of tree diameter at breast height (PAID) across the treatments and over time. Foliar δ¹³C and δ¹⁸O were also related to published data of foliar K concentrations of the same trees.

Results

Significant variations of foliar δ¹³C, and therefore WUE and Δ¹³C, across treatments were only observed at ages 4 and 10 years old, and foliar δ¹⁸O at age 4 years old only. The results showed increasing foliar δ¹³C, δ¹⁸O and WUE, and decreasing Δ¹³C and Ci, from RR₀ to RR₂ treatments. However, while the WUE was positively related to the PAID and PAIB at age 4 years, it was negatively related to PAID and PAIB at age 10 years old. Litter needle δ¹³C, indicative of WUE, was also negatively related to the PAID at age 10 years old. . At age 4 years, foliar δ¹³C and δ¹⁸O were positively related with a steep slope of 7.70 ‰ across treatments, and that both isotopes were positively related to foliar K concentrations. Similarly, δ¹⁸O was negatively related to the Δ¹³C. No significant relationship can be determined between foliar δ¹³C, or Δ¹³C, and δ¹⁸O at age 10 years old. In addition, WUE was increasing (p?<?0.001) and Δ¹³C and C_i decreasing (p?<?0.001) with decreasing PAID over time.

Conclusions

The variations at age 4 years in foliar δ¹³C or Δ¹³C and δ¹⁸O and increasing WUE with increasing growth rate suggest growth induced water-stress with increasing residue-loading rate as a result of the nutritional effect of the harvest residues on tree growth. At age 10 years, the negative relationships between WUE and PAID indicate nutrient limitation has an over-riding effect on δ¹³C variations rather than g_s. This was due to the lack of a significant relationship between foliar Δ¹³C and δ¹⁸O at this age, as well as over time.     

Comparison of seasonal variations in water-use efficiency calculated from the carbon isotope composition of tree rings and flux data in a temperate forest

Tree-ring δ(13) C is often interpreted in terms of intrinsic water-use efficiency (WUE) using a carbon isotope discrimination model established at the leaf level. We examined whether intra-ring δ(13) C could be used to assess variations in intrinsic WUE (W(g), the ratio of carbon assimilation and stomatal conductance to water) and variations in ecosystem WUE (W(t) , the ratio of C assimilation and transpiration) at a seasonal scale. Intra-ring δ(13) C was measured in 30- to 60-μm-thick slices in eight oak trees (Quercus petraea). Canopy W(g) was simulated using a physiologically process-based model. High between-tree variability was observed in the seasonal variations of intra-ring δ(13) C. Six trees showed significant positive correlations between W(g) calculated from intra-ring δ(13) C and canopy W(g) averaged over several days during latewood formation. These results suggest that latewood is a seasonal recorder of W(g) trends, with a temporal lag corresponding to the mixing time of sugars in the phloem. These six trees also showed significant negative correlations between photosynthetic discrimination Δ calculated from intra-ring δ(13) C, and ecosystem W(t), during latewood formation. Despite the observed between-tree variability, these results indicate that intra-ring δ(13) C can be used to access seasonal variations in past W(t).     

Stand structure modulates the long-term vulnerability of Pinus halepensis to climatic drought in a semiarid Mediterranean ecosystem

We investigated whether stand structure modulates the long-term physiological performance and growth of Pinus halepensis Mill. in a semiarid Mediterranean ecosystem. Tree radial growth and carbon and oxygen stable isotope composition of latewood (δ(13)C(LW) and δ(18)O(LW), respectively) from 1967 to 2007 were measured in P. halepensis trees from two sharply contrasting stand types: open woodlands with widely scattered trees versus dense afforested stands. In both stand types, tree radial growth, δ(13)C(LW) and δ(18)O(LW) were strongly correlated with annual rainfall, thus indicating that tree performance in this semiarid environment is largely determined by inter-annual changes in water availability. However, trees in dense afforested stands showed consistently higher δ(18)O(LW) and similar δ(13)C(LW) values compared with those in neighbouring open woodlands, indicating lower stomatal conductance and photosynthesis rates in the former, but little difference in water use efficiency between stand types. Trees in dense afforested stands were more water stressed and showed lower radial growth, overall suggesting greater vulnerability to drought and climate aridification compared with trees in open woodlands. In this semiarid ecosystem, the negative impacts of intense inter-tree competition for water on P. halepensis performance clearly outweigh potential benefits derived from enhanced infiltration and reduced run-off losses in dense afforested stands.     

植物叶片水分利用效率研究综述

植物能否适应当地的极限环境条件,最主要的看它们能否很好地协调碳同化和水分耗散之间的关系,即植物水分利用效率(WUE)是其生存的关键因子.就近来研究最多的叶片水平上的WUE,从叶片WUE的定义,方法,进展等方面对其进行总结概括,并就今后植物叶片水分利用效率的研究提出了几点看法:方法上,叶片碳同位素方法是目前植物叶片长期水分利用效率研究的最佳方法,而δ13C的替代指标将继续是方法研究中的一个方向,前景乐观;研究内容上,要加强极端干旱区河岸林木的δ13C和WUE的研究;结合植物生理生态学,生物学和稳定同位素技术,探究植物叶片长期水分利用效率的机理,特别是要加强运用双重同位素模型加深和理解植物叶片长期水分利用效率变化规律和内在机制的研究;要结合多种方法,加强多时空尺度植物叶片WUE及其之间的转换研究.     

Correlating δ13C and δ18O in cellulose of trees

We measured the carbon and oxygen isotopic composition of stem cellulose of Pinus sylvestris, Picea abies, Fagus sylvatica and Fraxinus excelsior. Several sites along a transect of a small valley in Switzerland were selected which differ in soil moisture conditions. At every site, six trees per species were sampled, and a sample representing a mean value for the period from 1940 to 1990 was analysed. For all species, the mean site δ¹³C and δ¹⁸O of stem cellulose are related to the soil moisture availability, whereby higher isotope ratios are found at drier sites. This result is consistent with isotope fractionation models when assuming enhanced stomatal resistance (thus higher δ¹³C of incorporated carbon) and increased oxygen isotope enrichment in the leaf water (thus higher δ¹⁸O) at the dry sites. δ¹⁸ O-δ¹³C plots reveal a linear relationship between the carbon and oxygen isotopes in cellulose. To interpret this relationship we developed an equation which combines the above-mentioned fractionation models. An important new parameter is the degree to which the leaf water enrichment is reflected in the stem cellulose. In the combined model the slope of the δ¹⁸O-δ¹³C plot is related to the sensitivity of the p_i/p_a of a plant to changing relative humidity.     

Photosynthesis, water use efficiency and stable carbon isotope composition are associated with anatomical properties of leaf and xylem in six poplar species

Although fast‐growing Populus species consume a large amount of water for biomass production, there are considerable variations in water use efficiency (WUE) across different poplar species. To compare differences in growth, WUE and anatomical properties of leaf and xylem and to examine the relationship between photosynthesis/WUE and anatomical properties of leaf and xylem, cuttings of six poplar species were grown in a botanical garden. The growth performance, photosynthesis, intrinsic WUE (WUE_i), stable carbon isotope composition (δ¹³C) and anatomical properties of leaf and xylem were analysed in these poplar plants. Significant differences were found in growth, photosynthesis, WUE_i and anatomical properties among the examined species. Populus cathayana was the clone with the fastest growth and the lowest WUE_i/δ¹³C, whereas P. × euramericana had a considerable growth increment and the highest WUE_i/δ¹³C. Among the analysed poplar species, the highest total stomatal density in P. cathayana was correlated with its highest stomatal conductance (g_s) and lowest WUE_i/δ¹³C. Moreover, significant correlations were observed between WUE_i and abaxial stomatal density and stem vessel lumen area. These data suggest that photosynthesis, WUE_i and δ¹³C are associated with leaf and xylem anatomy and there are tradeoffs between growth and WUE_i. It is anticipated that some poplar species, e.g. P. × euramericana, are better candidates for water‐limited regions and others, e.g. P. cathayana, may be better for water‐abundant areas.     

Sex-specific responses of Populus cathayana to drought and elevated temperatures

Dioecious plant species represent an important component of terrestrial ecosystems. Yet, little is known about sex-specific responses to drought and elevated temperatures. Populus cathayana Rehd, which is a dioecious, deciduous tree species, widely distributed in the northern, central and southwestern regions of China, was employed as a model species in our study. In closed-top chamber experiments, sex-specific morphological, physiological and biochemical responses of P. cathayana to drought and different elevated temperatures were investigated. Compared with the controls, drought significantly decreased the growth and the net photosynthesis rate (A), and increased the intrinsic water use efficiency (WUE(i)), carbon isotope composition (delta13C), and the malondialdehyde (MDA) and abscisic acid (ABA) contents in droughted plants. In contrast, elevated temperatures significantly promoted the growth and the A, but decreased the WUE(i), delta13C, MDA and ABA contents in well-watered individuals. When compared with males, elevated temperatures induced well-watered females to express a greater increase in the height growth (HG), basal diameter (BD), leaf area (LA), total number of leaves (TNL), dry matter accumulation (DMA) and specific leaf area (SLA), and a lower decrease in the A value, transpiration (E), stomatal conductance (g(s)), MDA and ABA contents, while elevated temperatures induced drought-stressed females to exhibit lower values of HG, BD, LA, TNL, DMA, A, E, g(s) and the intercellular CO2 concentration (C(i)), and higher levels of SLA, WUE(i), delta13C, MDA and ABA contents. Our results indicated that the female individuals of P. cathayana are more responsive and suffer from greater negative effects than do males when grown under environments with increased drought stress and elevated temperature.     

沿土壤水分梯度黄囊苔草碳同位素组成及其适应策略的变化

 选取了内蒙古锡林河流域6个水分条件不同的典型植物群落,测定了各群落中黄囊苔草 (Carex korshinskyi) 叶片δ13C值、叶片含水量（LWC）及其种群特征的变化。结果表明：1）不同生境下,黄囊苔草叶片的碳同位素组成发生明显变化（变幅为1.8‰）。沿土壤水分梯度,随着土壤含水量的降低,黄囊苔草叶片δ13C值显著增大,水分利用方式更加保守。2）虽然不同生境下,黄囊苔草叶片含水量变化不大,但其叶片δ13C值与LWC表现出显著的负相关关系(p=0.051)。这表明黄囊苔草水分利用效率对其叶片水分状况变化的反应非常敏感。3）在不同生境下,黄囊苔草种群的植株高度、密度、地上生物量及其在群落中的出现频度明显不同。具有较高δ13C值的黄囊苔草种群在群落中出现的频度和地上生物量所占比例都显著增加。以上结果表明,生长在不同生境下的黄囊苔草种群能够通过改变其水分利用效率适应不同的土壤水分状况,使其在植物群落中表现出更强的竞争能力和生态适应性。     

Leaf carbon isotope ratio and water use efficiency of urban roadside trees in summer in Kyoto city

The effect of summer climate on leaf carbon isotope composition (δ¹³C) of the major roadside tree species Prunus × yedoensis (P. yedoensis) was investigated in Kyoto city, Japan, to explore the implications for alterations in urban environments. Temperature and the vapor pressure deficit were higher at sites of higher traffic volumes, possibly affected by a heat island effect. The leaf δ¹³C of P. yedoensis trees was affected strongly by leaf carbon isotope discrimination (Δ), with much less effect of δ¹³C on atmospheric CO₂. Leaf Δ values in the summer were smaller at sites of higher traffic volumes with high atmospheric temperatures, suggesting a higher long-term water use efficiency (WUE) at these sites. Gas exchange measurements of P. yedoensis leaves indeed suggested a higher intrinsic WUE at sites of higher traffic volumes with high atmospheric temperatures. These results suggest that leaf Δ is related to the response of WUE to summer climates, and that leaf δ¹³C in urban areas is a useful tracer for understanding the influences of urban environments on plant photosynthetic processes.     

Oxygen isotope enrichment (delta18O) as a measure of time-averaged transpiration rate

Experimental evidence is presented to show that the 18O enrichment in the leaf biomass and the mean (time-averaged) transpiration rate are positively correlated in groundnut and rice genotypes. The relationship between oxygen isotope enrichment and stomatal conductance (g(s)) was determined by altering g(s) through ABA and subsequently using contrasting genotypes of cowpea and groundnut. The Peclet model for the 18O enrichment of leaf water relative to the source water is able to predict the mean observed values well, while it cannot reproduce the full range of measured isotopic values. Further, it fails to explain the observed positive correlation between transpiration rate and 18O enrichment in leaf biomass. Transpiration rate is influenced by the prevailing environmental conditions besides the intrinsic genetic variability. As all the genotypes of both species experienced similar environmental conditions, the differences in transpiration rate could mostly be dependent on intrinsic g(s). Therefore, it appears that the delta18O of leaf biomass can be used as an effective surrogate for mean transpiration rate. Further, at a given vapour pressure difference, delta18O can serve as a measure of stomatal conductance as well.     

The combined effect of constant water deficit and nitrogen supply on WUE, NUE and Δ13C in durum wheat potted plants

Water scarcity and nitrogen shortage are the main constraints on durum wheat productivity. This paper examines the combined effects of a constant water deficit and nitrogen supply (NS) on growth, photosynthesis, stomatal conductance (g_s) and transpiration, instantaneous and time‐integrated water use efficiency (WUE) and nitrogen use efficiency (NUE) and carbon isotope discrimination (Δ¹³C) in durum wheat genotypes grown in pots under greenhouse conditions. Three water levels (40%, 70% and 100% container capacity), two nitrogen doses (high and low N) and four genotypes were assayed in a total of 24 experimental treatments. Water and nitrogen treatments were imposed 2 weeks after plant emergence. The growth, nitrogen content and Δ¹³C of the shoot and the gas exchange in the flag leaf were determined about 2 weeks after anthesis. As expected, both water and NS had a strong positive effect on growth. However, a reduction in water supply had low effect decreasing photosynthesis and transpiration, Δ¹³C and NUE and increasing WUE. On the contrary, increasing the level of nitrogen supplied had a significant negative effect on g_s, which decreased significantly the ratio of intercellular to ambient CO₂ concentrations and Δ¹³C, and increased both instantaneous and time‐integrated WUE. In addition, a higher N level also negatively affected the instantaneous and time‐integrated NUE. The Δ¹³C of shoots correlated significantly and negatively with either instantaneous or time‐integrated measurements of WUE. Moreover, within each NS, Δ¹³C also correlated negatively with the integrated NUE. We concluded that under our experimental conditions, Δ¹³C gives information about the efficiency with which not just water but also nitrogen are used by the plant. In addition, this study illustrates that a steady water limitation may strongly affect biomass without consistent changes in WUE. The lack of effect of the different water regimes on gas exchange, WUE and Δ¹³C illustrate the importance of how stress is imposed during growth.     

毛白杨杂种无性系叶片δ13C差异与气体交换参数

在苗木生长的不同时期对13个毛白杨(Populus tomentosa)杂种无性系叶片碳同位素δ¹³C和气体交换参数(净光合速率P_n、蒸腾速率T_r、瞬时水分利用效率WUE_i、气孔导度G_s和胞间CO₂浓度C_i)的差异进行研究, 分析不同无性系间δ¹³C与气体交换参数的相互关系, 目的在于探求δ¹³C在筛选高光合及高水分利用效率毛白杨杂种无性系中的应用价值。结果表明: 不同生长时期和不同无性系间δ¹³C、T_r、WUE_i、G_s和C_i的差异均显著, δ¹³C和WUE_i表现为9月>7月, T_r、G_s和C_i表现为7月>9月, P_n在不同生长时期差异不显著。季节变化是引起毛白杨杂种无性系叶片δ¹³C差异的主要原因。同一时期, 无性系间δ¹³C和WUE_i表现出较好的一致性, 即WUE_i较高的无性系30、42、46、83、BL2和BL5, 其δ¹³C值也较高, WUE_i较低的无性系B331和TG34, 其δ¹³C值也较低, 且不同时期(7月和9月) δ¹³C和WUE_i呈较强的正相关, 相关系数r分别为0.739 0和0.545 8, 高δ¹³C可以作为筛选高WUE_i毛白杨的有效指标, 且在苗木生长旺盛时期选育能得到更为可靠的结果。对毛白杨而言, 高WUE_i的无性系, 一般具有适中或较低的G_s和C_i, 但不一定具有很高的P_n, 气孔调节使得毛白杨在不影响光合作用的同时保持较高的WUE。     

Comparison of leaf gas exchange and stable isotope signature of water-soluble compounds along canopy gradients of co-occurring Douglas-fir and European beech

Combined δ(13) C and δ(18) O analyses of water-soluble leaf and twig phloem material were used to determine intrinsic water-use efficiency (iWUE) and variability of stomatal conductance at different crown positions in adult European beech (Fagus sylvatica) and Douglas-fir (Pseudotsuga menziesii) trees. Simultaneous gas exchange measurements allowed evaluation of the differences in calculating iWUE from leaf or phloem water-soluble compounds, and comparison with a semi-quantitative dual isotope model to infer variability of net photosynthesis (A(n) ) between the investigated crown positions. Estimates of iWUE from δ(13) C of leaf water-soluble organic matter (WSOM) outperformed the estimates from phloem compounds. In the beech crown, δ(13) C of leaf WSOM coincided clearly with gas exchange measurements. The relationship was not as reliable in the Douglas-fir. The differences in δ(18) O between leaf and phloem material were found to correlate with stomatal conductance. The semi-quantitative model approach was applicable for comparisons of daily average A(n) between different crown positions and trees. Intracanopy gradients were more pronounced in the beech than in the Douglas-fir, which reached higher values of iWUE at the respective positions, particularly under dry air conditions.     

Response of photosynthetic carbon gain to ecosystem retrogression of vascular plants and mosses in the boreal forest

In the long-term absence of rejuvenating disturbances, forest succession frequently proceeds from a maximal biomass phase to a retrogressive phase characterized by reduced nutrient availability [notably nitrogen (N) and phosphorus (P)] and net primary productivity. Few studies have considered how retrogression induces changes in ecophysiological responses associated with photosynthetic carbon (C) gain, and only for trees. We tested the hypothesis that retrogression would negatively impact photosynthetic C gain of four contrasting species, and that this impact would be greater for vascular plants (i.e., trees and shrubs) than for non-vascular plants (i.e., mosses). We used a 5,000-year-old chronosequence of forested islands in Sweden, where retrogression occurs in the long-term absence of lightning-ignited wildfires. Despite fundamental differences in plant form and ecological niche among species, vascular plants and mosses showed similar ecophysiological responses to retrogression. The most common effects of retrogression were reductions in photosynthesis and respiration per unit foliar N, increases in foliar N, δ(13)C and δ(15)N, and decreases in specific leaf areas. In contrast, photosynthesis per unit mass or area generally did not change along the chronosequence, but did vary many-fold between vascular plants and mosses. The consistent increases in foliar N without corresponding increases in mass- or area-based photosynthesis suggest that other factor(s), such as P co-limitation, light conditions or water availability, may co-regulate C gain in retrogressive boreal forests. Against our predictions, traits of mosses associated with C and N were generally highly responsive to retrogression, which has implications for how mosses influence ecosystem processes in boreal forests.     

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

For 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.     

Variation in the carbon and oxygen isotope composition of plant biomass and its relationship to water‐use efficiency at the leaf‐ and ecosystem‐scales in a northern Great Plains grassland

Measurements of the carbon (δ¹³C_m) and oxygen (δ¹⁸O_m) isotope composition of C₃ plant tissue provide important insights into controls on water‐use efficiency. We investigated the causes of seasonal and inter‐annual variability in water‐use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf‐scale) and eddy covariance measurements (ecosystem‐scale). The positive relationship between δ¹³C_m and δ¹⁸O_m values for samples collected during 1998–2001 indicated that variation in stomatal conductance and water stress‐induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ¹³C_m and biomass production during this time. By comparison, the lack of a significant relationship between δ¹³C_m and δ¹⁸O_m values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water‐use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf c_i/ca during the drier conditions of 2000. Calculated values of leaf‐scale water‐use efficiency were 2–3 times higher than ecosystem‐scale water‐use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.