Leaf-litter nitrogen concentration in hinoki cypress forests in relation to the time of leaf fall under different climatic conditions in Japan

Leaf-litter nitrogen concentration was investigated for 17 hinoki cypress (Chamaecyparis obtusa Endlicher) forests in the Kochi region on the Pacific Ocean side and the Kyoto region on the Sea of Japan side in Japan where both the amount of precipitation and frequency of typhoon attacks differ between regions. Leaf properties were predicted from climatic, stand, and soil properties by multiple regression analysis. Fresh-leaf nitrogen was higher in the Kyoto than Kochi regions and was higher where soil C/N ratio is lower. The time of leaf-fall, i.e., 50% of the annual leaf fall, showed a difference of 86 days among the forests and occurred earlier in forests at higher altitudes. The time of leaf-fall at higher altitudes was earlier due to the higher susceptibility to strong winds from typhoons. Leaf-litter nitrogen concentration of annual leaf-fall or winter leaf-fall was lower when the time of leaf-fall occurred later. The results indicate that nitrogen resorption is proficient when leaf-fall occurs later, leading to lower leaf-litter nitrogen concentration.     

氮添加对中亚热带杜鹃灌丛凋落物生产和叶分解的影响

凋落物的生产和分解是生态系统养分循环的重要过程,受到大气氮沉降的深刻影响。但目前相关研究主要集中于森林和草地生态系统,氮沉降对灌丛生态系统凋落物养分归还的影响规律尚不清楚。因此选择亚热带分布广泛的杜鹃灌丛为研究对象,进行了为期两年的模拟氮沉降试验。试验设置4个处理:对照(CK, 0 g m-2 a-1)、低氮(LN, 2 g m-2 a-1)、中氮(MN, 5 g m-2 a-1)和高氮(HN, 10 g m-2 a-1)。结果显示:CK、LN、MN和HN 4种处理下,群落年平均凋落物量分别为(1936.54±358.9)、(2541.89±112.5)、(2342.97±519.8)、(2087.22±391.8) kg/hm²,LN、MN和HN处理样地的凋落量分别比对照样地高出32.68%、21.16%和7.93%;凋落叶、花果、凋落枝和其他组分占总凋落量的比例分别为75.75%、15.09%、7.70%和1.45%,不同浓度氮处理下各组分的凋落量均高于对照样地;凋落物组分表现出明显的季节动态:凋落叶在10—11月份达到峰值,凋落枝在10月份达到峰值,花果凋落物则在5月份凋落量最高,不同氮处理下凋落物的季节动态基本一致;白檀凋落叶分解速率显著高于杜鹃,二者分解95%所需时间分别为5.08—11.11 a和7.69—17.65 a,施氮使白檀凋落叶分解周期比对照样地缩短18.18%—54.28%;凋落叶分解过程中,N元素表现为富集-释放模式,P元素表现为富集模式。研究表明,氮添加能够促进群落中白檀凋落叶分解及N、P元素的释放,说明施氮可以调节凋落叶养分释放模式,对灌丛生态系统的养分循环具有调控作用。     

Seasonal variation in leaf-litter input and leaf dispersal distances to streams: the effect of converting broadleaf riparian zones to conifer plantations in central Japan

Sugi (Cryptomeria japonica D. Don) is one of the most important evergreen coniferous plantation species in Japan. Much of the riparian forest that was originally dominated by deciduous broadleaf trees has been converted into sugi plantations. The present study investigated the seasonality of leaf-litter input and leaf dispersal to streams to assess the effects of converting riparian forest to sugi plantations. The seasonality of leaf-litter input was assessed at three streams in Nagoya University Forest. At one stream dominated by deciduous broadleaf trees, input was limited to autumn. At two streams in a sugi plantation, input was prolonged from autumn to early spring, and was dominated by sugi needles from winter to early spring. These results suggest that sugi plantations alter the seasonality of leaf-litter input from riparian forests and affect stream ecosystems. Leaf dispersal was assessed by considering the relationship between leaf dispersal distance from three forest layers to the stream and leaf-litter input into two streams. The maximum leaf dispersal distance was 26–28 m for deciduous broadleaf trees from mid-October to November and 10–12 m for sugi needles from December to April. Leaf dispersal distance depended on the tree species. Four species of deciduous broadleaf tree showed greater leaf dispersal than that of sugi. The mean weight of individual sugi needles was higher than that of the broadleaf trees’ leaves, and dispersal depended on strong winds in winter and early spring. Although the leaf dispersal distance from the understory was within 2–4 m, it could be a significant source of leaf-litter input to streams.     

Controls over leaf and litter calcium concentrations among temperate trees

Four-fold variation in leaf-litter Ca concentration among 14 tree species growing in a common garden in central Poland was linked to variation in soil pH, exchangeable Ca, soil base saturation, forest floor turnover rates, and earthworm abundance. Given the potential importance of tissue Ca to biogeochemical processes, in this study we investigated potential controls on leaf Ca concentrations using studies of both laboratory seedlings and 30-year-old trees in the field. We first assessed whether species differences in Ca concentration of green leaves and leaf litter were due to differences in Ca uptake, plant growth, or Ca translocation to different organs, by measuring seedlings of 6 of the 14 species grown under controlled conditions of varying Ca supply. We also investigated whether trees species with high Ca concentrations in green leaves and leaf litter access soil Ca to a greater extent than low-Ca species by growing more fine roots in high-Ca soil horizons. Root distribution in the field was determined in all 14 tree species by profile wall mapping and soil sampling of excavated pits. There was no correlation between horizon root count density (number of roots m⁻²) and exchangeable soil Ca, nor was there a correlation of stand-level leaf litter Ca with density of roots 45–100 cm deep in the soil, suggesting that a deeper root distribution does not result in greater Ca acquisition among these species. Variation among species in leaf Ca concentration of greenhouse seedlings was positively correlated with leaf Ca concentrations of mature trees, indicating that the same ranking in leaf Ca among species existed under controlled Ca supply. Species also differed in seedling growth response to Ca supply. Tilia, the species with the highest leaf Ca in the field, generated only 10% as much biomass and height at low relative to high Ca supply, whereas the other species exhibited no significant differences. Species exhibited differences in (i) partitioning of whole plant Ca and biomass to leaf, stem and root organs and (ii) the pattern of such partitioning between high and low Ca treatments. Our data support the hypothesis that although soil Ca supply can contribute to variation among trees in leaf and litter Ca concentration, innate physiological differences among species also can be a major cause for species variation.     

Soil micro-habitat effects on fine roots of Chamaecyparis obtusa Endl.: A field experiment using root ingrowth cores

Ingrowth cores in the field were used to compare fine root characteristics of hinoki cypress (Chamaecyparis obtusa) among rooting substrate in the form of needle leaf litter, decomposing organic material, and mineral soil. Fine root growth, morphology, arbuscular mycorrhizal (AM) associations, and tissue C and N concentration were determined. The inorganic N leaching from each soil substrate was taken as a measure of N availability. Although there was no significant difference in total N leaching among substrates, more NH ₊ ⁴ -N leached from the decomposing organic material than other substrates. Rapid fine root production was observed in the organic material, whereas root production in the litter substrate was suppressed. Annual net fine root productions in litter, organic material, and mineral soil were 51, 193, and 132 g m⁻², respectively. In the leaf litter substrate, AM colonization was suppressed and specific root length was higher than in the other substrates, indicating severe nutrient limitation in the litter. These responses of hinoki cypress roots seemed to be a soil exploitation pattern whereby absorptive fine roots were arranged to maximize nutrient acquisition.     

凋落物硫初始含量特征及其影响因素整合分析

硫(S)是植物生长发育不可或缺的营养元素之一,且在凋落物分解过程中起着重要作用,但凋落物硫的初始含量(新鲜凋落物S含量)特征仍不清楚。基于收集自83篇公开发表文献的310个观测值,整合分析了凋落物硫的初始含量特征,同时评估了菌根类型(丛枝菌根、外生菌根与两者都有,即同时含有丛枝菌根和外生菌根)、系统分类(裸子植物与被子植物)、生活型(乔木、灌木与草本植物)、叶型(阔叶与针叶)、土壤性质及气候等因子对其的影响。结果表明:(1)不同类型凋落物初始硫含量差异显著,叶、枝、根、茎、果、树皮和倒木硫含量分别为:2.22 g/kg、0.801 g/kg、0.691 g/kg、1.57 g/kg、1.31 g/kg、0.468 g/kg和0.110 g/kg;(2)灌木植物叶凋落物初始硫含量低于草本植物和禾本植物,而外生菌根植物根凋落物的初始硫含量低于丛枝菌根和同时具有两种菌根的植物;(3)被子植物的叶、枝、根凋落物硫初始含量均高于裸子植物,阔叶树种的叶、枝、根凋落物初始硫含量显著高于针叶树种;(4)最湿月降水量、最暖月最高温以及年均温是枝凋落物初始硫含量的主要调控因子,而最冷月最低温、年均温、土壤总氮是根凋落物的主要驱动因子。研究结果为深入认识硫在凋落物分解及其伴随的物质循环过程中的作用提供了基础数据。     

Empirically estimating the lateral transport of riparian leaf litter to lakes

1. An experimental study found the lateral transport of riparian leaf litter to increase in relation to surface slope, exposure to precipitation, and absence of ground flora. Deciduous leaves displayed greater lateral redistribution than did coniferous needles. 2. When these results were combined with the proportional representation of different slopes, canopy exposures and ground roughness measured from transects throughout the riparian zones of sixteen north-western Ontario lakes, the average annual lateral transport distance of coniferous needles was estimated to be 0.3 m and that of deciduous leaves to be 0.5 m. 3. Data on the annual riparian litterfall and distributional proximity of trees to the shorelines suggested that the lateral input of riparian litter was 2 g m^?1 forested shoreline yr^?1, an amount which represents 6% of the total annual allochthonous input of leaf-fall to these study lakes.     

Impacts of the exotic,nitrogen-fixing black locust (Robinia pseudoacacia) on nitrogen-cycling in a pine–oak ecosystem

We investigated the influence of the exotic nitrogen-fixing black locust (Robinia pseudoacacia) on nitrogen cycling in a pitch pine (Pinus rigida) −scrub oak (Quercus ilicifolia, Q. prinoides) ecosystem. Within paired pine-oak and adjacent black locust stands that were the result of a 20-35 year-old invasion, we evaluated soil nutrient contents, soil nitrogen transformation rates, and annual litterfall biomass and nitrogen concentrations. In the A horizon, black locust soils had 1.3-3.2 times greater nitrogen concentration relative to soils within pine-oak stands. Black locust soils also had elevated levels of P and Ca, net nitrification rates and total net N-mineralization rates. Net nitrification rates were 25-120 times greater in black locust than in pine-oak stands. Elevated net N-mineralization rates in black locust stands were associated with an abundance of high nitrogen, low lignin leaf litter, with 86 kg N ha^–1 yr^–1 in leaf litter returned compared with 19 kg N ha^–1 yr^–1 in pine-oak stands. This difference resulted from a two-fold greater litterfall mass combined with increased litter nitrogen concentration in black locust stands (1.1% and 2.6% N for scrub oak and black locust litter, respectively). Thus, black locust supplements soil nitrogen pools, increases nitrogen return in litterfall, and enhances soil nitrogen mineralization rates when it invades nutrient poor, pine-oak ecosystems. This revised version was published online in June 2006 with corrections to the Cover Date.     

Dependence of the aboveground CO2 exchange rate on tree size in field-grown hinoki cypress (Chamaecyparis obtusa)

The CO₂ exchange of the aboveground parts for five different-sized 17-year-old (as of 1991) hinoki cypress (Chamaecyparis obtusa) trees growing in the field was non-destructively measured over one year, using an open CO₂ exchange system. The CO₂ exchange of individual trees decreased with decreasing tree sizes, such as aboveground phytomass, leaf mass and leaf area. However, the CO₂ exchange abruptly decreased near the smallest-suppressed sample tree. The size dependence was well described by a generalized power function. The annual gross photosynthesis of individual trees was proportional to the square root of leaf mass or leaf area. The dependence of CO₂ exchange on annual phytomass increment was described by a simple power function with an exponent value less than unity, suggesting that CO₂ exchange per unit of phytomass increment was lower in larger-sized trees than in smaller-sized trees. The mean photosynthetic activity of a tree, i.e., gross photosynthesis per unit of leaf area, slightly increased to its highest value with decreasing leaf area and then decreased abruptly near the smallest sample tree. The maximum value of mean photosynthetic activity was estimated to be 2.85 kg CO₂ m⁻² year⁻¹ for a leaf area of 1.56 m² tree⁻¹. The ratio of mean photosynthetic activity to the maximum photosynthetic activity was the highest in an intermediate tree and decreased gradually toward larger-sized trees by ca. 60% and also decreased toward the smallest suppressed tree by ca. 35%.     

Survival curves and longevity of the leaves of Alnus japonica var. arguta in Kushiro Marsh

The seasonal changes in leaf emergence and leaf-fall in a Japanese alder stand of the fen in Kushiro Marsh were studied, and survival curves for the leaves were drawn. Leaves collected in litter traps were dried and weighed to study the seasonal changes, peaks in mid-August and late September to October suggested a bimodal annual curve. Study of the seasonal changes in the number of emerged and fallen leaves per shoot revealed a third peak about one month before the August peak, showing a trimodal annual curve. First and second leaves had a longevity of about 40 and 50–60 days, respectively. The longevity increased until the fifth leaf. With the sixth and following leaves, longevity decreased. Leaf size increased with leaf rank, with the first leaf being the smallest. The first leaf had only about 10% and the second leaf only 20% of the area of the fifth leaf. On this basis, the early to mid-July peak in number of fallen leaves was composed of first and second leaves which were smaller and short-lived. The early August and September/October peaks were high in both number and mass of fallen leaves. Compared to reports on Japanese alder of other mountainous districts in Hokkaido, the alder trees of Kushiro Marsh had about the same number of leaves per shoot, but had a season of leaf emergence which was about 6 weeks shorter. In addition, the longevity of the longest-lived fifth leaf was about 30–40 days shorter. The short life span of the leaves could be considered as an adaptive strategy of this species to environmental constraints of its habitat.     

Vegetation and litter production in Southern Guinea savanna,Nigeria

Summary This study examines the woody vegetation, annual leaf-fall and annual wood-fall in Southern Guinea savanna near Mokwa, Nigeria. There were 1425±402 (95% limits) trees ha^-1 of which Caesalpiniaceous legumes made up 53%. Annual leaf-fall was 2.387 t ha^-1 a^-1 equivalent to 11,238,932 kcal ha^-1 a^-1 and was highly seasonal, peaking from November to February, with a maximum in January after the annual bush fires. Annual wood-fall was 1.391 t ha^-1 a^-1 equivalent to 7,598,256 kcal ha^-1 a^-1 and was less seasonal but with two peaks, one from January to March after the fires and another from May to July in the rainy season.The litter-fall data currently available from West Africa are reviewed and indicate a negative correlation between litterfall and latitude (P<0.001). Litter production is found to be positively correlated with rainfall (P<0.001) and it is suggested that seasonal distribution of rainfall may also be a factor contributing to the limits of litter production.     

Leaf Size and Leaf Area Index in <Emphasis Type="Italic">Fagus sylvatica</Emphasis> Forests: Competing Effects of Precipitation,Temperature, and Nitrogen Availability

Plants across diverse biomes tend to produce smaller leaves and a reduced total leaf area when exposed to drought. For mature trees of a single species, however, the leaf area–water supply relationship is not well understood. We tested the paradigm of leaf area reduction upon drought by a transect study with 14 mature Fagus sylvatica forests along a steep precipitation gradient (970–520 mm y⁻¹) by applying two independent methods of leaf size determination. Contrary to expectation, average leaf size in dry stands (520–550 mm y⁻¹) was about 40% larger and SLA was higher than in moist stands (910–970 mm y⁻¹). As a result of increased leaf sizes, leaf area index significantly increased from the high- to the low-precipitation stands. Multiple regression analyses suggested that average leaf size was primarily controlled by temperature, whereas the influence of soil moisture and soil C/N ratio was low. Summer rainfall of the preceding year was the most significant predictor of total leaf number. We assume that leaf expansion of beech was independent of water supply, because it takes place in May with ample soil water reserves along the entire transect. In contrast, bud formation, which determines total leaf number, occurs in mid-summer, when droughts are severest. We conclude that leaf expansion and stand leaf area of beech along this precipitation gradient are not a simple function of water availability, but are controlled by several abiotic factors including spring temperature and possibly also nitrogen supply, which both tend to increase toward drier sites, thus overlaying any negative effect of water shortage on leaf development.     

Nitrogen and phosphorus economy of the riparian shrub <Emphasis Type="Italic">Salix gracilistyla</Emphasis> in western Japan

Salix gracilistyla is one of the dominant plants in the riparian vegetation of the upper-middle reaches of rivers in western Japan. This species colonizes mainly sandy habitats, where soil nutrient levels are low, but shows high potential for production. We hypothesized that S.␣gracilistyla uses nutrients conservatively within stands, showing a high resorption efficiency during leaf senescence. To test this hypothesis, we examined seasonal changes in nitrogen (N) and phosphorus (P) concentrations in aboveground organs of S. gracilistyla stands on a fluvial bar in the Ohtagawa River, western Japan. The concentrations in leaves decreased from April to May as leaves expanded. Thereafter, the concentrations showed little fluctuation until September. They declined considerably in autumn, possibly owing to nutrient resorption. We converted the nutrient concentrations in each organ to nutrient amounts per stand area on the basis of the biomass of each organ. The resorption efficiency of N and P in leaves during senescence were estimated to be 44 and 46%, respectively. Annual net increments of N and P in aboveground organs, calculated by adding the amounts in inflorescences and leaf litter to the annual increments in perennial organs, were estimated to be 9.9 g and 0.83 g m⁻² year⁻¹, respectively. The amounts released in leaf litter were 6.7 g N and 0.44 g P m⁻². These values are comparable to or larger than those of other deciduous trees. We conclude that S. gracilistyla stands acquire large amounts of nutrients and release a large proportion in leaf litter.     

Environmental gradients across four rainforest-open forest boundaries in northeastern Queensland

Abstract This paper examines gradients and correlations among nine environmental factors across four contrasting rainforest-open forest boundaries at Skull Pocket in northeastern Queensland. With few exceptions, the environmental factors formed significant gradients across all four boundaries. Gradients were most pronounced for canopy openness, depth of leaf litter, soil pH, soil temperature and understorey solar radiation, and least pronounced or non-existent for total nitrogen and organic carbon. Boundary site location was shown to have some influence on the degree of change in environmental factors across the rainforest-open forest boundary, with depth of leaf litter, organic carbon, soil temperatures in March and June, and understorey solar radiation in June being significantly different among the four boundary sites. The number of significant correlations among the nine environmental factors was also affected by boundary site location.     

Solar radiation transmission in and around canopy gaps in an uneven-aged <Emphasis Type="Italic">Nothofagus betuloides</Emphasis> forest

The transmission of direct, diffuse and global solar radiation in and around canopy gaps occurring in an uneven-aged, evergreen Nothofagus betuloides forest during the growing season (October 2006–March 2007) was estimated by means of hemispherical photographs. The transmission of solar radiation into the forest was affected not only by a high level of horizontal and vertical heterogeneity of the forest canopy, but also by low angles of the sun’s path. The below-canopy direct solar radiation appeared to be variable in space and time. On average, the highest amount of transmitted direct solar radiation was estimated below the undisturbed canopy at the southeast of the gap centre. The transmitted diffuse and global solar radiation above the forest floor exhibited lower variability and, on average, both were higher at the centre of the canopy gaps. Canopy structure and stand parameters were also measured to explain the variation in the below-canopy solar radiation in the forest. The model that best fit the transmitted below-canopy direct solar radiation was a growth model, using plant area index with an ellipsoidal angle distribution as the independent variable (R ² = 0.263). Both diffuse and global solar radiation were very sensitive to canopy openness, and for both cases a quadratic model provided the best fit for these data (R ² = 0.963 and 0.833, respectively). As much as 75% and 73% of the variation in the diffuse and global solar radiation, respectively, were explained by a combination of stand parameters, namely basal area, crown projection, crown volume, stem volume, and average equivalent crown radius.     

Effects of solar ultraviolet radiation on photochemical efficiency of <Emphasis Type="Italic">Chaetoceros curvisetus</Emphasis> (Bacillariophyceae)

To assess the short- and long-term impacts of Ultraviolet radiation (UVR, 280–400 nm) on the red tide alga Chaetoceros curvisetus, we exposed cells to three different solar radiation treatments–PAB:280–700 nm, PA:320–700 nm, and P:400–700 nm under 20°C incubated temperature. Short-term exposures were investigated: the photochemical efficiency (Φ_PSII) versus irradiance curves under six levels of solar radiation by covering the incubators with a variable number of neutral density screens (the irradiance thus varied from 100 to 3%) lasting 1 h, and long-term exposures were designed to assess how the cells acclimate to solar radiation (the growth, UV_abc and ratio of repair to damage rates of D1 protein were detected). A significant decrease in the photochemical efficiency (Φ_PSII) at high irradiance (100% of incident solar radiation, 261.6 Wm⁻²) was observed in short-term exposure (1 h). UVR-induced photoinhibition was reduced to 7% in 3% solar radiation (4.08 Wm⁻²), compared with 66% in 100% solar radiation (261.6 Wm⁻²). In long-term experiments (11 days) using batch cultures, cell densities during the first 6 days were relatively constant for treatment P, and decreased slightly under PA and PAB treaments, reflecting a change in the irradiance experienced in the laboratory to that of incident solar irradiance. Thereafter, cell density increased and UV-induced photoinhibition decreased with the following days, indicating acclimation to solar UV. At the end of experiment, cells were found to exhibit both higher ratios of repair to UV-related damage and increased concentrations of UV-absorbing compounds, whose maximum absorption was found to be at 329 nm. Our data indicate that C. curvisetus is sensitive to ultraviolet radiation, but was able to acclimate relatively rapidly (ca. 6 days) by synthesizing UV-absorbing compounds and by increasing the rates of repair processes of D1 protein in PSII.     

Intraspecific competition and water use efficiency in <Emphasis Type="Italic">Quercus suber</Emphasis>: evidence of an optimum tree density?

The dehesa and montado agroecosystems seem to be water-stress induced structures and some authors have found evidence of a relationship between stand tree density and mean annual precipitation. In order to assess the ecophysiological responses of Quercus suber to increasing tree density and to evaluate if there was evidence of an “optimum tree density” with respect to mean annual rainfall in our study area, we established a tree-density gradient of 20, 40, and 60 trees ha⁻¹, coded D20, D40, and D60, respectively. Trees in D40 plots had higher specific leaf area (SLA) values and the highest water content. Both D20 and D60 trees had leaves with higher dry matter content (LDMC). The trade-off between SLA and LDMC was very strong and placed D40 trees as the least stressed, in terms of leaf anatomy. We also found differences in mean carbon isotope discrimination (Δ), between different density plots, as big as 1.25‰. Again, D40 trees discriminated more against ¹³C than the trees from D20 and D60 plots, which suggests a less conservative water use strategy, possibly deriving from greater water availability. The lower water retention in the D20 areas and the increased competition for water resources in D60 plots placed trees, in both areas, in a more stressful situation, regarding water availability. The D40 areas thus seemed to represent a more favourable environment for Q. suber in our montado. Therefore, the areas with 40 trees ha⁻¹ suggest the existence of an optimum tree density with respect to annual rainfall in our site, and may represent a compromise between an increased retention of water resources and intraspecific competition.     

Do leaf-litter decomposers control biofilm primary production and benthic algal community structure in forest streams? Insights from an outdoor mesocosm experiment

• 1. Forested headwater streams are generally considered to be light-limited ecosystems where primary production is reduced, and the main source of energy and nutrients is composed of allochthonous detritus. We hypothesised that in these ecosystems, the development of primary producers might also be limited by (1) competition for nutrients with leaf-litter decomposers (e.g. bacteria and fungi), and (2) leaf-litter leachates or allelopathic compounds produced by aquatic fungi.
• 2. To test these hypotheses, a 48-day mesocosm experiment was performed in 12 artificial streams containing stream water inoculated with epilithic biofilm suspensions collected from a forested headwater stream. Three different treatments were applied: control without leaf litter (C), microbially conditioned leaf litter added at the beginning of the experiment and left to decompose throughout the experiment (L), or leaf litter renewed three times during the experiment (RL).
• 3. We predicted that (1) the presence of litter, through microbial nutrient immobilisation and allelopathy, would reduce primary production and that (2) this effect would be amplified by litter renewal. We also predicted that nutrient competition would mean that (3) leaf-litter decomposers will alter primary producer community composition and physiology. These predictions were tested by analysing biofilm development, physiology, stoichiometry, and benthic algal community structure. To distinguish between the effects of nutrient immobilisation and allelopathy, the biofilm responses to leaf-litter leachates collected after different microbial conditioning durations were also measured in a parallel laboratory experiment.
• 4. Contrary to our expectations, by day 28, primary producer growth was higher in the mesocosms containing leaf litter (L and RL) despite the rapid decrease in dissolved nutrients when leaf litter was present. After 48 days, the lowest phototrophic biofilm development was observed when leaf litter was renewed (RL), whereas phototrophic biofilm development was similar in the C and L treatments. Biofilm stoichiometry indicated that this effect was most probably related to greater nitrogen limitation in the RL treatment. The presence of leaf litter also affected primary producers' photophysiology, which could be attributed to changes in taxonomic composition and to physiological adjustments of primary producers.
• 5. Laboratory measurements showed that despite a strong inhibition of primary producer growth by unconditioned leaf-litter leachates, microbially conditioned leaf litter had either low or no effects on the development of primary producers.
• 6. These results reveal that leaf-litter decomposers can have both positive and negative effects on primary producers underlining the need to consider microbial interactions when investigating the functioning of forested headwater streams.

     

Spatial patterns of foliar stable carbon isotope compositions of C<Subscript>3</Subscript> plant species in the Loess Plateau of China

The spatial pattern of foliar stable carbon isotope compositions (δ¹³C) of dominant species and their relationships with environmental factors in seven sites, Yangling, Yongshou, Tongchuan, Fuxian, Ansai, Mizhi and Shenmu, standing from south to north in the Loess Plateau of China, was studied. The results showed that in the 121 C₃ plant samples collected from the Loess Plateau, the foliar δ¹³C value ranged from −22.66‰ to −30.70‰, averaging −27.04‰. The foliar δ¹³C value varied significantly (P<0.01) among the seven sites, and the average δ¹³C value increased by about 1.69‰ from Yangling in the south to Shenmu in the north as climatic drought increased. There was a significant difference in foliar δ¹³C value among three life-forms categorized from all the plant samples in the Loess Plateau (P<0.001). The trees (−26.74‰) and shrubs (−26.68‰) had similar mean δ¹³C values, both significantly (P<0.05) higher than the mean δ¹³C value of herbages (−27.69‰). It was shown that the trees and shrubs had higher WUEs and employed more conservative water-use patterns to survive drier habitats in the Loess Plateau. Of all the C₃ species in the Loess Plateau, the foliar δ¹³C values were significantly and negatively correlated with the mean annual rainfall (P<0.001) and mean annual temperature (P<0.05), while being significantly and positively correlated with the latitude (P<0.001) and the annual solar radiation (P<0.01). In general, the foliar δ¹³C values increased as the latitude and solar radiation increased and the rainfall and temperature decreased. The annual rainfall as the main influencing factor could explain 13.3% of the spatial variations in foliar δ¹³C value. A 100 mm increment in annual rainfall would result in a decrease by 0.88‰ in foliar δ¹³C values.     

Effects of experimental warming and nitrogen enrichment on leaf and litter chemistry of a wetland grass,Phragmites australis

As global climate is warming and the nitrogen cycle accelerates, plants are likely to respond not only by shifting community composition, but also by adjusting traits such as tissue chemistry. We subjected a widespread wetland plant, Phragmites australis, to increased nitrate supply and elevated temperature in enclosures that were established in a littoral permanently submerged freshwater marsh. The nitrogen (N) and phosphorus (P) concentrations in green leaves ranged from 11.4 to 13.8 mg N and from 1.5 to 2.0 mg P g⁻¹ dry mass. While the N concentration changed little in brown litter, the P concentration decreased to 0.53–0.65 mg P g⁻¹ litter dry mass. Neither experimental warming of the water and sediment surface, nor nitrate enrichment during the growing season affected nitrogen or phosphorus concentrations in green leaves. Concentrations of the two major structural carbon compounds in plant litter, cellulose and lignin, were also unaffected, ranging from 32.1 to 34.2% of dry mass for cellulose and from 16.3 to 17.7% of dry mass for lignin. Warming, however, significantly increased the nitrogen concentration of fully brown leaf litter. Thus, temperature appears to be more important than the supply of dissolved N in the water, especially in affecting leaf litter N concentrations in P. australis, even when only water but not air temperature is increased. This result may have implications for decomposition processes and decomposer food webs, which both depend on the quality of plant litter.