Modeling Productivity in Mangrove Forests as Impacted by Effective Soil Water Availability and Its Sensitivity to Climate Change Using Biome-BGC

Ecosystem dynamics and the responses to climate change in mangrove forests are poorly understood. We applied the biogeochemical process model Biome-BGC to simulate the dynamics of net primary productivity (NPP) and leaf area index (LAI) under the present and future climate conditions in mangrove forests in Shenzhen, Zhanjiang, and Qiongshan across the southern coast of China, and in three monocultural mangrove stands of two native species, Avicennia marina and Kandelia obovata, and one exotic species, Sonneratia apetala, in Shenzhen. The soil hydrological process of the model was modified by incorporating a soil water (SW) stress index to account for the impact of the effective SW availability in the coastal wetland. Our modified Biome-BGC well predicted the dynamics of NPP and LAI in the mangrove forests at the study sites. We found that the six mangrove systems differed in sensitivity to variations in the effective SW availability. At the ecosystem level, however, soil salinity alone could not entirely explain the limitation of the effective SW availability on the productivity of mangrove forests. Increasing atmospheric CO₂ concentration differentially affected growth of different mangrove species but only had a small impact on NPP (<7%); whereas a doubling of atmospheric CO₂ concentration associated with a 2°C temperature rise would increase NPP by 14–19% across the three geographically separate mangrove forests and by 12% to as much as 68% across the three monocultural mangrove stands. Our simulation analysis indicates that temperature change is more important than increasing CO₂ concentration in affecting productivity of mangroves at the ecosystem level, and that different mangrove species differ in sensitivity to increases in temperature and CO₂ concentration.     

Modelling the recent historical impacts of atmospheric CO2 and climate change on Mediterranean vegetation

During the past century, annual mean temperature has increased by 0.75°C and precipitation has shown marked variation throughout the Mediterranean basin. These historical climate changes may have had significant, but presently undefined, impacts on the productivity and structure of sclerophyllous shrubland, an important vegetation type in the region. We used a vegetation model for this functional type to examine climate change impacts, and their interaction with the concurrent historical rise in atmospheric CO₂. Using only climate and soil texture as data inputs, model predictions showed good agreement with observations of seasonal and regional variation in leaf and canopy physiology, net primary productivity (NPP), leaf area index (LAI) and soil water. Model simulations for shrubland sites indicated that potential NPP has risen by 25% and LAI by 7% during the past century, although the absolute increase in LAI was small. Sensitivity analysis suggested that the increase in atmospheric CO₂ since 1900 was the primary cause of these changes, and that simulated climate change alone had negative impacts on both NPP and LAI. Effects of rising CO₂ were mediated by significant increases in the efficiency of water‐use in NPP throughout the region, as a consequence of the direct effect of CO₂ on leaf gas exchange. This increase in efficiency compensated for limitation of NPP by drought, except in areas where drought was most severe. However, while water was used more efficiently, total canopy water loss rose slightly or remained unaffected in model simulations, because increases in LAI with CO₂ counteracted the effects of reduced stomatal conductance on transpiration. Model simulations for the Mediterranean region indicate that the recent rise in atmospheric CO₂ may already have had significant impacts on productivity, structure and water relations of sclerophyllous shrub vegetation, which tended to offset the detrimental effects of climate change in the region.     

Leaf area index and net primary productivity along subtropical to alpine gradients in the Tibetan Plateau

Aim Our aims were to quantify climatic and soil controls on net primary productivity (NPP) and leaf area index (LAI) along subtropical to alpine gradients where the vegetation remains relatively undisturbed, and investigate whether NPP and LAI converge towards threshold‐like logistic patterns associated with climatic and soil variables that would help us to verify and parameterize process models for predicting future ecosystem behaviour under global environmental change. Location Field data were collected from 22 sites along the Tibetan Alpine Vegetation Transects (TAVT) during 1999–2000. The TAVT included the altitudinal transect on the eastern slope of the Gongga Mountains in the Eastern Tibetan Plateau, with altitudes from 1900 m to 3700 m, and the longitudinal‐latitudinal transect in the Central Tibetan Plateau, of approximately 1000 km length and 40 km width. Methods LAI was measured as the product of foliage biomass multiplied by the ratio of specific leaf area. NPP in forests and shrub communities was estimated as the sum of increases in standing crops of live vegetation using recent stem growth rate and leaf lifespan. NPP in grasslands was estimated from the above‐ground maximum live biomass. We measured the soil organic carbon (C) and total and available nitrogen (N) contents and their pool sizes by conventional methods. Mean temperatures for the year, January and July and annual precipitation were estimated from available meteorological stations by interpolation or simulation. The threshold‐like logistic function was used to model the relationships of LAI and NPP with climatic and soil variables. Results Geographically, NPP and LAI both significantly decreased with increasing latitude (P < 0.02), but increased with increasing longitude (P < 0.01). Altitudinal trends in NPP and LAI showed different patterns. NPP generally decreased with increasing altitude in a linear relationship (r² = 0.73, P < 0.001), whereas LAI showed a negative quadratic relationship with altitude (r² = 0.58, P < 0.001). Temperature and precipitation, singly or in combination, explained 60–68% of the NPP variation with logistic relationships, while the soil organic C and total N variables explained only 21–46% of the variation with simple linear regressions of log‐transformed data. LAI showed significant logistic relationships with both climatic and soil variables, but the data from alpine spruce‐fir sites diverged greatly from the modelled patterns associated with temperature and precipitation. Soil organic C storage had the strongest correlation with LAI (r² = 0.68, P < 0.001). Main conclusions In response to climatic gradients along the TAVT, LAI and NPP across diverse vegetation types converged towards threshold‐like logistic patterns consistent with the general distribution patterns of live biomass both above‐ground and below‐ground found in our earlier studies. Our analysis further revealed that climatic factors strongly limited the NPP variations along the TAVT because the precipitation gradient characterized not only the vegetation distribution but also the soil N conditions of the natural ecosystems. LAI generally increased with increasing precipitation and was well correlated with soil organic C and total N variables. The interaction between LAI growth and soil N availability would appear to have important implications for ecosystem structure and function of alpine spruce‐fir forests. Convergence towards logistic patterns in dry matter production of plants in the TAVT suggests that alpine plant growth would increase in a nonlinear response to global warming.     

Leaf traits and associated ecosystem characteristics across subtropical and timberline forests in the Gongga Mountains,Eastern Tibetan Plateau

Knowledge of how leaf characteristics might be used to deduce information on ecosystem functioning and how this scaling task could be done is limited. In this study, we present field data for leaf lifespan, specific leaf area (SLA) and mass and area-based leaf nitrogen concentrations (N_mass, N_area) of dominant tree species and the associated stand foliage N-pool, leaf area index (LAI), root biomass, aboveground biomass, net primary productivity (NPP) and soil available-N content in six undisturbed forest plots along subtropical to timberline gradients on the eastern slope of the Gongga Mountains. We developed a methodology to calculate the whole-canopy mean leaf traits to include all tree species (groups) in each of the six plots through a series of weighted averages scaled up from leaf-level measurements. These defined whole-canopy mean leaf traits were equivalent to the traits of a leaf in regard to their interrelationships and altitudinal trends, but were more useful for large-scale pattern analysis of ecosystem structure and function. The whole-canopy mean leaf lifespan and leaf N_mass mainly showed significant relationships with stand foliage N-pool, NPP, LAI and root biomass. In general, as elevation increased, the whole-canopy mean leaf lifespan and leaf N_area and stand LAI and foliage N-pool increased to their maximum, whereas the whole-canopy mean SLA and leaf N_mass and stand NPP and root biomass decreased from their maximum. The whole-canopy mean leaf lifespan and stand foliage N-pool both converged towards threshold-like logistic relationships with annual mean temperature and soil available-N variables. Our results are further supported by additional literature data in the Americas and eastern China.     

No increase in alpine snowbed productivity in response to experimental lengthening of the growing season

Climate change effects on snow cover and thermic regime in alpine tundra might lead to a longer growing season, but could also increase risks to plants from spring frost events. Alpine snowbeds, i.e. alpine tundra from late snowmelt sites, might be particularly susceptible to such climatic changes. Snowbed communities were grown in large monoliths for two consecutive years, under different manipulated snow cover treatments, to test for effects of early (E) and late (L) snowmelt on dominant species growth, plant functional traits, leaf area index (LAI) and aboveground productivity. Spring snow cover was reduced to assess the sensitivity of snowbed alpine species to severe early frost events, and dominant species freezing temperatures were measured. Aboveground biomass, productivity, LAI and dominant species growth did not increase significantly in E compared to L treatments, indicating inability to respond to an extended growing season. Edapho‐climatic conditions could not account for these results, suggesting that developmental constraints are important in controlling snowbed plant growth. Impaired productivity was only detected when harsher and more frequent frost events were experimentally induced by early snowmelt. These conditions exposed plants to spring frosts, reaching temperatures consistent with the estimated freezing points of the dominant species (～?10 °C). We conclude that weak plasticity in phenological response and potential detrimental effects of early frosts explain why alpine tundra from snowbeds is not expected to benefit from increased growing season length.     

The relationships between net primary productivity, human population density and species conservation

Aim In this study, I determine the relationships between net primary productivity (NPP), human population density, species richness and land use. I also examine the implications of human settlement patterns for species conservation. Location Australia. Methods I document the associations between NPP, human population density and the species richness of birds, butterflies and mammals using correlations and spatial regressions. I also assess changes in land‐use with NPP and population density, focussing particularly on protected areas. An initial exploration into the implications of the NPP‐population density relationship for regional conservation strategies is provided. Results Human population density increases with NPP suggesting that available energy may be a key driving force of human settlement patterns. The species richness of each taxonomic group and geographically restricted species also increases with NPP leading to substantial overlap between species diversity and populated regions. The percentage of land designated as minimal use decreases considerably with increasing human population density and NPP, while intensive agriculture is confined entirely to areas of high NPP. There are strong negative relationships between the size of Australia's National Parks and human population density and NPP. Small parks are often surrounded by relatively dense settlements, but have high average NPP, while large parks are mostly isolated and characterized by low productivity. There are no areas in the highest quartile of NPP that also occur in the most sparsely populated regions, presenting challenges for conservation strategies wanting to protect productive areas under the least threat of human development. Main conclusions Human population density and species richness respond similarly to variation in NPP, leading to spatial congruence between human settlements and productive, species rich regions. Planning strategies are required that minimize the potential threat posed by human development to diverse ecosystems and maximize the underlying productivity of protected areas. Reducing the level of threat may require stabilizing the size of the human population, while capturing larger areas of relatively high productivity in the conservation reserve system would lead to greater protection of local diversity.     

Nitrogen response efficiency increased monotonically with decreasing soil resource availability: a case study from a semiarid grassland in northern China

The concept of nutrient use efficiency is central to understanding ecosystem functioning because it is the step in which plants can influence the return of nutrients to the soil pool and the quality of the litter. Theory suggests that nutrient efficiency increases unimodally with declining soil resources, but this has not been tested empirically for N and water in grassland ecosystems, where plant growth in these ecosystems is generally thought to be limited by soil N and moisture. In this paper, we tested the N uptake and the N use efficiency (NUE) of two Stipa species (S. grandis and S. krylovii) from 20 sites in the Inner Mongolia grassland by measuring the N content of net primary productivity (NPP). NUE is defined as the total net primary production per unit N absorbed. We further distinguished NUE from N response efficiency (NRE; production per unit N available). We found that NPP increased with soil N and water availability. Efficiency of whole-plant N use, uptake, and response increased monotonically with decreasing soil N and water, being higher on infertile (dry) habitats than on fertile (wet) habitats. We further considered NUE as the product of the N productivity (NP the rate of biomass increase per unit N in the plant) and the mean residence time (MRT; the ratio between the average N pool and the annual N uptake or loss). The NP and NUE of S. grandis growing usually in dry and N-poor habitats exceeded those of S. krylovii abundant in wet and N-rich habitats. NUE differed among sites, and was often affected by the evolutionary trade-off between NP and MRT, where plants and communities had adapted in a way to maximize either NP or MRT, but not both concurrently. Soil N availability and moisture influenced the community-level N uptake efficiency and ultimately the NRE, though the response to N was dependent on the plant community examined. These results show that soil N and water had exerted a great impact on the N efficiency in Stipa species. The intraspecific differences in N efficiency within both Stipa species along soil resource availability gradient may explain the differences in plant productivity on various soils, which will be conducive to our general understanding of the N cycling and vegetation dynamics in northern Chinese grasslands.     

Variation of biomass and morphology of the cushion plant Androsace tapete along an elevational gradient in the Tibetan Plateau

Alpine ecosystems are among those biomes that are most vulnerable to climate change. Cushion plants are an important life form of alpine ecosystems and will likely play a critical role for the resilience of these habitats to climate change. We studied cushion size distribution and different measures of the compactness of cushions (biomass and rosette density, leaf area index) of the cushion plant, Androsace tapete along an elevational gradient from 4500 to 5200 m a.s.l. in the Nyainqentanglha Mountains of the central Tibetan Plateau. Cushion size distribution, total cover, and compactness of cushions varied substantially along the elevational gradient. At the driest site at low elevation we found the lowest total cushion cover, a particularly high proportion of very small cushions, and the most compact cushions (highest rosette and biomass densities, and leaf area index (LAI) per cushion). Our results indicate that in the semi‐arid Tibetan Plateau water availability is the more important climate factor than temperature affecting cushion plant traits and morphology.     

A comparison of specific leaf area, chemical composition and leaf construction costs of field plants from 15 habitats differing in productivity

Laboratory experiments have shown a large difference in specific leaf area (SLA, leaf area: leaf mass) between species from nutrient-poor and nutrient-rich habitats, but no systematic difference in the construction costs (the amount of glucose required to construct 1 g biomass). We examined how far these patterns are congruent with those from field-grown plants. An analysis was made of the vegetation in a range of grasslands and heathlands differing in productivity. The SLA of the dominant species in 15 different habitats was determined, as well as chemical composition and construction costs of bulk samples of leaves. SLA in the field was generally lower than in the laboratory, but showed consistency in that the ranking across species remained the same. Species from highly productive habitats had higher SLA than those from sites of low productivity, although individual species sometimes deviated substantially from the general trend. Construction costs were similar for plants from different habitats. This was mainly due to the positive correlation between an expensive class of compounds (proteins) and a cheap one (minerals).     

The influence of Triticum aestivum density, sowing pattern and nitrogen fertilization on leaf area index and its spatial variation

We investigated leaf area index (LAI) and its spatial variation early in the growing season in Triticum aestivum (spring wheat) sown in two spatial patterns (standard rows and a uniform pattern), at three densities (204, 449 and 721 seeds m⁻²), and two nitrogen fertilization levels (0 and 80 kg nitrogen ha⁻¹). Our main hypothesis was that a more uniform distribution of individual plants does not affect overall LAI but reduces its spatial variation. We used the number of leaves touching a vertical pin (LAI*) as a measure of LAI. LAI* increased with sowing density, nitrogen fertilization and, contrary to our hypothesis, spatial uniformity. The coefficient of variation of LAI* was higher (1) at lower sowing density, (2) without nitrogen fertilizer and (3) in the row pattern. Both the increase in LAI and the decrease in its variation in more spatially uniform crops may contribute to increased weed suppression and increased yield.     

Effects of CO2 elevation on canopy development in the stands of two co-occurring annuals

Elevated CO₂ may increase dry mass production of canopies directly through increasing net assimilation rate of leaves and also indirectly through increasing leaf area index (LAI). We studied the effects of CO₂ elevation on canopy productivity and development in monospecific and mixed (1:1) stands of two co-occurring C₃ annual species, Abutilon theophrasti, and Ambrosia artemisiifolia. The stands were established in the glasshouse with two CO₂ levels (360 and 700 l/l) under natural light conditions. The planting density was 100 per m² and LAI increased up to 2.6 in 53 days of growth. Root competition was excluded by growing each plant in an individual pot. However, interference was apparent in the amount of photons absorbed by the plants and in photon absorption per unit leaf area. Greater photon absorption by Abutilon in the mixed stand was due to different canopy structures: Abutilon distributed leaves in the upper layers in the canopy while Ambrosia distributed leaves more to the lower layers. CO₂ elevation did not affect the relative performance and light interception of the two species in mixed stands. Total aboveground dry mass was significantly increased with CO₂ elevation, while no significant effects on leaf area development were observed. CO₂ elevation increased dry mass production by 30–50%, which was mediated by 35–38% increase in the net assimilation rate (NAR) and 37–60% increase in the nitrogen use efficiency (NUE, net assimilation rate per unit leaf nitrogen). Since there was a strong overall correlation between LAI and aboveground nitrogen and no significant difference was found in the regression of LAI against aboveground nitrogen between the two CO₂ levels, we hypothesized that leaf area development was controlled by the amount of nitrogen taken up from the soil. This hypothesis suggests that the increased LAI with CO₂ elevation observed by several authors might be due to increased uptake of nitrogen with increased root growth.     

Species richness,relative abundances and habitat use in local assemblages ofSorex shrews in Eurasian boreal forests

Using a large body of observational data on the occurrence ofSorex shrews in boreal forests, we test two models that predict the structure of small mammal communities along a gradient of increasing habitat productivity. Tilman’s (1982) model predicts a humped curve of species richness along productivity gradients. In contrast, we found a linear increase in species richness with increasing logarithm of the pooled density of shrews, which we use as a measure of habitat productivity for shrews. The model of Hanski and Kaikusalo (1989) assumes a trade-off between exploitative and interference competitive abilities, and it predicts that the size structure of small mammal communities should shift from the dominance of small species (superior in exploitative competition) in unproductive habitats to the dominance of large species (superior in interference competition) in productive habitats. Shrew assemblages show such a shift. Though it is not possible to draw definite conclusions about the role of interspecific competition from our observational data, the changing size structure of local shrew assemblages with increasing habitat productivity is a predictable feature of their community structure.     

基于景观尺度过程模型的长白山净初级生产力空间分布影响因素分析

对长白山自然保护区的净初级生产力(NPP)的空间分布格局进行了模拟,对它们与环境因子和植被因子间的相互关系进行了分析．结果表明,1995年NPP的模拟值平均为0．680kgC·m^-2·年^-1,变幅为0．105-1．241kgC·m^-2·年^-1(82．1％),其中阔叶红松林的NPP最高(1．084kgC·m^-2·年^-1)．环境条件决定了长白山植被年NPP空间分布的大趋势．土壤含水量对NPP的限制最大,呈负相关关系(R=-0.65),长白山植物生长一般不存在水分不足的问题．植被的NPP与LAI高度正相关(R＝0．81),当LAI增大到4-5m^2·m^-2时,NPP出现饱和．植被的NPP与冠层蒸腾量呈显著的正相关关系(R＝0．77)．岳桦林和阔叶林对环境因子、LAI和冠层蒸腾的响应与其它植被有较大差异。     

Seasonal leaf dynamics across a tree density gradient in a Brazilian savanna

Interactions between trees and grasses that influence leaf area index (LAI) have important consequences for savanna ecosystem processes through their controls on water, carbon, and energy fluxes as well as fire regimes. We measured LAI, of the groundlayer (herbaceous and woody plants <1-m tall) and shrub and tree layer (woody plants >1-m tall), in the Brazilian cerrado over a range of tree densities from open shrub savanna to closed woodland through the annual cycle. During the dry season, soil water potential was strongly and positively correlated with grass LAI, and less strongly with tree and shrub LAI. By the end of the dry season, LAI of grasses, groundlayer dicots and trees declined to 28, 60, and 68% of mean wet-season values, respectively. We compared the data to remotely sensed vegetation indices, finding that field measurements were more strongly correlated to the enhanced vegetation index (EVI, r ²=0.71) than to the normalized difference vegetation index (NDVI, r ²=0.49). Although the latter has been more widely used in quantifying leaf dynamics of tropical savannas, EVI appears better suited for this purpose. Our ground-based measurements demonstrate that groundlayer LAI declines with increasing tree density across sites, with savanna grasses being excluded at a tree LAI of approximately 3.3. LAI averaged 4.2 in nearby gallery (riparian) forest, so savanna grasses were absent, thereby greatly reducing fire risk and permitting survival of fire-sensitive forest tree species. Although edaphic conditions may partly explain the larger tree LAI of forests, relative to savanna, biological differences between savanna and forest tree species play an important role. Overall, forest tree species had 48% greater LAI than congeneric savanna trees under similar growing conditions. Savanna and forest species play distinct roles in the structure and dynamics of savanna–forest boundaries, contributing to the differences in fire regimes, microclimate, and nutrient cycling between savanna and forest ecosystems.     

Negative effects of habitat degradation and fragmentation on the declining grassland plant Trifolium montanum

Changes in land use have resulted in a strong decline in the plant diversity of nutrient-poor grasslands, but little is known about the combined effects of habitat degradation and fragmentation on populations of individual species. We studied these effects on stage structure, recruitment, reproduction and offspring fitness in populations of the declining perennial grassland plant Trifolium montanum in central Germany. Density and survival probability of juvenile plants decreased with light competition, measured as leaf area index (LAI) above T. montanum plants, resulting in aged populations with few juvenile plants at unmanaged sites with higher LAI. Reproduction of T. montanum was not related to LAI, but increased strongly with local density, suggesting pollinator limitation in fragmented populations with a low density of flowering plants. In the common garden, the survival of sown offspring increased with mean seed size, whereas seed production of offspring decreased with isolation, and in strong contrast to previous studies, also decreased with size and density of the population of origin. This could be due to increased inbreeding because of pollination between closely related neighbouring plants in dense and large populations. Our results indicate that both habitat degradation and fragmentation have negative effects on populations of T. montanum, but affect different phases of the life cycle. In the short term, the effects of habitat degradation are more important than those of fragmentation, and populations of T. montanum are primarily threatened by an increase in light competition in unmanaged sites, which rapidly affects the dynamics of the populations. The observed opposite effects of habitat fragmentation on reproduction and offspring fitness indicate that the effects of population size, density and isolation on plant fitness and population viability may be complex.     

Changes in eco-morphological parameters of alpine plants' leaves as an effect of fertilization

Plants growing on rich soil usually have thin leaves with large specific leaf area. On the other hand, at intraspecific level; soil fertilization results in leaves size increasing which, in turn, can lead to reduction in specific leaf area. To what extent soil fertilization implies only leaves increasing in size and does not affect other eco-morphological characteristics is a question that is still open. To assess coherence between plants intraspecific reactions to changes in soil richness and general tendencies in changes of leaves parameters in communities with different productivity, an experiment has been conducted in alpine plant communities of the north-western Caucasus. Changes in leaf traits are studied in four types of alpine plant communities after long term application of mineral nutrients (NP and lime treatment). It is shown that in all species, except legume Hedysarum caucasicum, fertilization results in size leaf characteristics (leaf area, wet and dry mass) increase. Specific leaf area appears to decrease in plants inhabiting alpine heathlands and increase in plants inhabiting alpine snow beds and in dominant species of Geranium-Hedysarum meadows, Geranium gymnocaulon. After correction of specific leaf area that accounts for changes in leaf size, it becomes discernable that in most species the increase in leaf area per se results in specific leaf area reduction while changes in leaf structure under influence of fertilization leads to this trait increasing. Those species demonstrating the increase in specific leaf area as an effect of fertilization, also gain more in terms of biomass.     

Net primary production, carbon storage and climate change in Chinese biomes

Net primary production (NPP) and leaf area index (LAI) of Chinese biomes were simulated by BIOME3 under the present climate, and their responses to climate change and doubled CO₂ under a future climatic scenario using output from Hadley Center coupled ocean‐atmosphere general circulation model with CO₂ modelled at 340 and 500 ppmv. The model estimated annual mean NPP of the biomes in China to be between 0 and 1270.7 gC m^‐2 yr^‐1 at present. The highest productivity was found in tropical seasonal and rain forests while temperate forests had an intermediate NPP, which is higher than a lower NPP of temperate savannas, grasslands and steppes. The lowest NPP occurred in desert, alpine tundra and ice/polar desert in cold or arid regions, especially on the Tibetan Plateau. The lowest monthly NPP of each biome occurred generally in February and the highest monthly NPP occurred during the summer (June to August). The annual mean NPP and LAI of most of biomes at changed climate with CO₂ at 340 and 500 ppmv (direct effects on physiology) would be greater than present. The direct effects of carbon dioxide on plant physiology result in significant increase of LAI and NPP. The carbon storage of Chinese biomes at present and changed climates was calculated by the carbon density and vegetation area method. The present estimates of carbon storage are totally 175.83 × 10¹² gC (57.57 × 10¹² gC in vegetation and 118.28 × 10¹² gC in soils). Changed climate without and with the CO₂ direct physiological effects will result in an increase of carbon storage of 5.1 and 16.33 × 10¹², gC compared to present, respectively. The interaction between elevated CO₂ and climate change plays an important role in the overall responses of NPP and carbon to climate change.     

Variation in exotic and native seed arrival and recruitment of bird dispersed species in subtropical forest restoration and regrowth

Invasive bird-dispersed plants often share the same suite of dispersers as co-occurring native species, resulting in a complex management issue. Integrated management strategies could incorporate manipulation of dispersal or establishment processes. To improve our understanding of these processes, we quantified seed rain, recruit and seed bank density, and species richness for bird-dispersed invasive and native species in three early successional subtropical habitats in eastern Australia: tree regrowth, shrub regrowth and native restoration plantings. We investigated the effects of environmental factors (leaf area index (LAI), distance to edge, herbaceous ground cover and distance to nearest neighbour) on seed rain, seed bank and recruit abundance. Propagule availability was not always a good predictor of recruitment. For instance, although native tree seed rain density was similar, and species richness was higher, in native plantings, compared with tree regrowth, recruit density and species richness were lower. Native plantings also received lower densities of invasive tree seed rain than did tree regrowth habitats, but supported a similar density of invasive tree recruits. Invasive shrub seed rain was recorded in highest densities in shrub regrowth sites, but recruit density was similar between habitats. We discuss the role of microsite characteristics in influencing post-dispersal processes and recruit composition, and suggest ways of manipulating these processes as part of an integrated management strategy for bird-dispersed weeds in natural areas.     

Seasonal variations in leaf δ<Superscript>13</Superscript>C and nitrogen associated with foliage turnover and carbon gain for a wet subalpine fir forest in the Gongga Mountains,eastern Tibetan Plateau

It is still unclear to what extent variations in foliar δ¹³C and nitrogen can be used to detect seasonal changes in canopy productivity. We hypothesize that in a wet and cloudy fir forest, seasonally higher litterfall and lower leaf area index (LAI) are correlated with higher mass-based leaf nitrogen (N _mass) and net primary productivity (NPP), while foliar δ¹³C may change with specific leaf area (SLA), area-based leaf nitrogen (N _area), and/or starch concentration. In order to test our hypotheses, stand-level litterfall and the means of δ¹³C, N _mass, N _area, SLA, and starch concentration of canopy needles for a wet and cloudy Abies fabri forest in the Gongga Mountains were monthly measured during the growing season. Seasonal estimates of LAI were obtained from our previous work. A conceptual model was used to predict seasonal NPP of the fir forest. Seasonal mean δ¹³C and N _mass and climatic variables were used as inputs. The δ¹³C across 1–7-year-old needles increased from May to September associated with decreasing SLA and increasing N _area. There were no significant differences in seasonal starch concentration. With increasing litterfall and decreasing LAI, seasonal mean N _mass increased, while the δ¹³C varied little. The simulated NPP increased with increasing litterfall and related traits of N _mass and N _area. Our data generally supported the hypotheses. The results also suggest that in the forest with relatively moist and cloudy environment, the largest fraction of annual carbon gain may occur in the early part of the growing season when higher litterfall results in higher N _mass of canopy leaves.     

Leaf area index, canopy structure and photosynthesis of red clover (Trifolium pratense L.)

Abstract. The influence of leaf age, total leaf area and its dispersion in space on canopy photosynthesis were studied using microswards of red clover ( Trifolium pratense L.) which were established in the greenhouse. Two varieties, Renova (flowering) and Molstad (non-flowering), were sown in separate plastic boxes at densities of 225, 400 and 625 plants per m².
Vertical distribution of photosynthetically active radiation (PAR), leaf area, leaf age and ¹⁴CO₂-fixation were determined periodically. Net photosynthesis and dark respiration of canopies were measured. Maximum photosynthetic capacity of individual leaves was measured on plants taken from the intact canopy or from plants where shading of the growing leaves had been prevented.
Net photosynthetic rate of canopies increased linearly with leaf area index (LAI) up to an LAI of 3.5 and then declined at higher LAI, independent of variety and sowing density. Below the optimum LAI, net photosynthesis depended mainly on interception of PAR. Decrease in canopy photosynthesis above the optimum LAI was due to a higher proportion of old leaves with decreased photosynthetic capacity, and not to an increase in respiring plant parts. It is concluded that LAI and position of leaf age categories in the canopy are more important than vertical distribution of leaf area in determining canopy photosynthesis of red clover.