The uptake of potassium by Carex species from swamp habitats varying from oligotrophic to eutrophic

Short-term potassium uptake by excised roots was investigated in Carex species from swamps, ranging from oligotrophic to eutrophic. The range of species from oligotrophic to eutrophic habitat is: C. rostrata Stokes, C. limosa L., C. lasiocarpa Ehrh., C. diandra Schrank, C. hudsonii A. Benn., and C. acutiformis Ehrh. After a short period of potassium starvation the species of the oligotrophic habitat showed a fast potassium uptake rate while the species of the eutrophic habitat showed lower uptake rates. Uptake by all species except C. rostrata and C. diandra was reduced at pH 4.0. Potassium uptake at low potassium concentrations was greatly increased by a period of starvation for potassium especially in the eutrophic species. The results on potassium stress are compared with the ecology of the species and with the reaction of the same Carex species to phosphate stress.     

Growth rate and phosphate utilization of some Carex species from a range of oligotrophic to eutrophic swamp habitats

In a growth experiment at phosphate levels varying between 0,0005 and 0.1 mM phosphate, relative growth rates and other growth parameters were determined in Carex species (C. rostrata Stokes, C. limosa L., C. lasiocarpa Ehrh., C. diandra Schrank, and C. acutiformis Ehrh., listed in order of increasing nutrient availability of their natural habitats). In all species, more efficient utilization of the phosphate was observed with decreasing phosphate levels, together with reduced fresh shoot ratio. In addition, each species shows characteristics which may help it to grow under low phosphate conditions: relatively high phosphate level in the plant (C. rostrata), a low fresh shoot ratio (C. limosa) and regulation of uptake and translocation of phosphate in such a way that relative growth rate during the first two weeks is maintained or even increased for a longer period (C. rostrata, C. limosa, and C. diandra). In the studied Carex species, the contribution of fresh shoot ratio to relative growth rate is much larger than that of net assimilation rate. C. acutiformis, a species of eutrophic areas, had the highest relative growth rate.     

Flooding tolerance of Carex species in relation to field distribution and aerenchyma formation

The flooding tolerance of Carex species was studied in relation to their field distribution and their capacity to form root aerenchyma under controlled conditions. In an alpine meadow, six Carex species were selected which were distributed in a clear zonation correlating with water content of the soil. Carex sempervirens and C. ferruginea were only found on nonflooded soil, the latter species preferring moister conditions. Carex davalliana and C. nigra were both associated with water-saturated soil, whereas C. limosa and C. rostrata preferred partially submerged conditions. Carex davalliana and C. limosa were bound to flooded soils with a relatively high redox potential and horizontally flowing groundwater. Carex rostrata and C. nigra grew in stagnant soil-flooded conditions with low soil redox potentials. The amount of aerenchyma in the roots of all species increased when grown in oxygen-deficient stagnant agar. This increase in root porosity, combined with increased root diameter, presumably improved internal aeration of the roots. Although all species survived experimental soil flooding, partial submergence was lethal to C. sempervirens and, surprisingly, also to the wetland species C. davalliana. Carex ferruginea showed a reduced growth rate during partial submergence. The three other species, all wetland plants, reached highest biomass production under soil-flooded and partially submerged conditions, with slower growth on free-draining soil. It is concluded that aerenchyma is not constitutive in the Carex species under study, and is best developed in Carex species from wetlands. Species with less aerenchyma perform poorly when soil-flooded, but conditions of partial submergence could even affect species with a considerable amount of root aerenchyma.     

Interspecific Variation in RGR and the Underlying Traits among 24 Grass Species Grown in Full Daylight

Abstract: A growth analysis was conducted with 24 central European grass species in full daylight to test whether traits underlying interspecific variation in relative growth rate (RGR) are the same in full daylight as they are at lower light, and whether this depends on the ecological characteristics of the studied species, i.e., their requirements with respect to nutrient and light availability.
In contrast to studies with herbaceous species at lower light, net assimilation rate (NAR) contributed more than leaf area ratio (LAR) or specific leaf area (SLA) to interspecific variation in RGR. This was associated with a larger interspecific variation in NAR than found in experiments with lower light. Without the two most shade-tolerant species, however, the contribution of LAR and its components to interspecific variation in RGR was similar or even higher than that of NAR.
Leaf dry matter content correlated negatively with RGR and was the only component of LAR contributing in a similar manner to variation in LAR and RGR. There was a positive correlation between NAR and biomass allocation to roots, which may be a result of nutrient-limited growth. RGR correlated negatively with biomass allocation to leaves. Leaf thickness did not correlate with RGR, as the positive effect of thin leaves was counterbalanced by their lower NAR.
Low inherent RGR was associated with species from nutrient-poor or shady habitats. Different components constrained growth for these two groups of species, those from nutrient-poor habitats having high leaf dry matter content, while those from shady habitats had thin leaves with low NAR.     

Growth characteristics,nutrient allocation and photosynthesis ofCarex species from floating fens

Summary The purpose of this study was to investigate various growth parameters, dry matter and nitrogen, phosphorus and potassium allocation and photosynthesis ofCarex acutiformis, C. rostrata andC. diandra growing in fens with, in this order, decreasing nutrient availability and decreasing aboveground productivity. Plants were grown from cuttings at optimum nutrient conditions in a growth chamber. Growth analysis at sequential harvests revealed that the species had no inherently different relative growth rates which could explain their different productivity, but that their LAR (LWR and SLA) decreased in the orderC. acutiformis, C. rostrata, C. diandra and their NAR increased in this order. All growth parameters decreased during plant growth even under the controlled conditions of the experiment.C. acutiformis allocated relatively much dry matter to the leaves,C. rostrata to the rhizomes andC. diandra to the roots. This may, in part, explain the higher aboveground biomass production ofC. acutiformis in the field. Nitrogen, but not phosphorus and potassium, allocation patterns were different for the three species.C. diandra, the species from the nitrogen-poorest site, had the highest leaf N content of the three species and also a higher chlorophyll content. Related to this, this species had the highest photosynthetic activity of whole plants both when collected from the field and when grown in the growth chamber. The nitrogen productivity was similar for the three species and the photosynthetic nitrogen use efficiency, determined forC. acutiformis andC. diandra, was similar for these two species.C. diandra had the most finely branched root system, i.e., the highest specific root length of the three species and its root surface area to leaf surface area ratio was also the highest. All three species showed higher nitrate reductase activity in the leaves than in the roots when grown on nutrient solution. The growth ofC. diandra at a relatively nutrient-poor site and a rather open low vegetation is assumed to be adapted to its habitat by a relatively high NAR made possible by a high rate of photosynthesis concurrent with a high leaf N content. The growth ofC. acutiformis at a relatively nutrient-rich site and a more dense and higher vegetation is adapted to its habitat by a high LAR.     

Clonal structure and hybrid susceptibility to a smut pathogen in microscale hybrid zones of northern wetland Carex (Cyperaceae)

Interspecific hybrid taxa, especially those with the potential for clonal spread, may play important roles in community dynamics and plant-pathogen interactions. This study combines the mapping of clonal structure for two rhizomatous sedges (Carex limosa, C. rariflora) and their nearly sterile interspecific hybrid with an investigation of the relationship between these taxa and a nonsystemic floral smut pathogen (Anthracoidea limosa) in six subarctic fens in Nouveau-Québec, Canada. We used allozyme polymorphisms in 14 of 18 putative loci to confirm hybrid identification and to distinguish among genotypes for mapping. The incidence of A. limosa was 5-20 times greater on hybrids than on parental taxa across all sites at two spatial scales (intensive extent = 10.5 m(2), extensive extent = entire fens). Spatial autocorrelation was detected in smut incidence; however, its statistical removal did not alter the strong association between hybrids and smut infection. Smut incidence on both C. limosa and hybrids was greater when they were growing in areas of high hybrid density. Our study provides evidence that disease can help maintain boundaries between species. We suggest explanations for hybrid susceptibility and provide evidence for a model in which hybrids act as a source for reinfection for all three taxa during subsequent years.     

Environmental distribution of four Carex species (Cyperaceae) in an old-growth forest

We conducted an in-depth characterization of the range of micro-environments (1 m) in which four Carex species (C. backii, C. communis, C. plantaginea, and C. platyphylla) grow in the understory of an old-growth, deciduous forest in southern Québec, Canada. All four species occurred in significantly different micro-environments. Carex plantaginea was found at the wet end of a moisture gradient, in soils with high nitrate availability. Carex backii and C. platyphylla were found at the dry end of the moisture gradient, with C. backii occupying soils with higher phosphorus availability than C. platyphylla. Carex communis, the only ant-dispersed species studied, was found in the broadest range of environmental conditions. Our results suggest that environmental heterogeneity and interspecific microhabitat preferences are important for the maintenance of local species diversity in the forest understory, not only for common species as demonstrated in previous studies, but for infrequent species, and those within a functional group (upland Carex species). However, there was some evidence that the distributions of C. backii and C. communis were not in equilibrium with current environmental conditions, indicating that historical factors, such as dispersal and colonization events, may also have important effects on the distributions of these species and the maintenance of species diversity in old-growth forest.     

Ecological significance of leaf life span among Central European grass species

Interspecific variation in leaf life span reflects the variation in nutrient conservation ability among different plant species and is considered to be associated with nutrient availability in the characteristic habitat. As defoliation interferes with nutrient conservation by the long-lived leaves, we hypothesized that disturbance rate is another important environmental factor working as a selective force on interspecific variation in leaf life span. In order to investigate this, we measured leaf life span of 32 grass species in mature garden-grown individuals. Variation in leaf life span was compared to measured leaf traits, to available data on species occurrence along gradients of nutrient availability and disturbance, and to published relative growth rates of the species. Leaf life span was associated positively with leaf tissue mass density and negatively with specific leaf area. Leaf life span correlated negatively with the disturbance rate in the characteristic habitat of a species, but not with nutrient availability. The latter relationship did not come about due to the long leaf life spans of species from nutrient-rich habitats with a relatively low disturbance rate, and to some extent also due to the short leaf life spans of annual species from relatively nutrient-poor sites. We conclude that although leaf longevity is an important means of reducing nutrient losses, this is a selective advantage only if the plant is not subjected to frequent defoliation. The frequently postulated association between leaf life span of a species and nutrient availability in its characteristic habitat may occur among species of habitats with positively correlated nutrient availability and disturbance rate. Leaf life span is negatively associated with seedling RGR, but there may be deviations in this relationship due to species with contrasting characteristics at seedling stage and at maturity.     

Exsertion, elongation, and senescence of leaves of Eriophorum vaginatum and Carex bigelowii in Northern Alaska

The seasonal patterns of leaf exsertion, elongation, and senescence were described and compared for two of the most abundant graminoid species of Alaskan moist tussock tundra, Eriophorum vaginatum and Carex bigelowii . In addition the responses of both species to NPK fertilizer and to variation in site fertility (water track vs. non-track areas) were also assayed and compared. The research was done over two full growing seasons at two sites near Toolik Lake, Alaska, where other aspects of the ecology of both species have been the subject of intensive and ongoing research.
Both species showed the typical graminoid pattern of sequential leaf growth, in which the exsertion and elongation of new leaves is coincident with the senescence of old leaves. However, the rates of these processes were much slower and steadier in Eriophorum than in Carex , with much greater overlap in the life histories of individual leaf cohorts. The total and green leaf lengths of whole tillers in Eriophorum were also less variable over the entire year than in Carex . The conclusion is that leaf growth in Carex should depend more on external storage of carbon and nutrients than Eriophorum , with a much greater seasonal variation in demands on storage and retranslocation to and from leaves.
The effects of fertilizer and the water track on leaf growth dynamics and turnover rates were largely nonsignificant, despite major effects on total tiller size and productivity. This is in contrast to previous research on evergreen leaf dynamics, but similar to results of previous research on overall production and biomass regulation in Eriophorum . It is concluded that the graminoid response to increased nutrient availability in the Arctic is to dilute the greater amounts of nutrient uptake by greater growth, so that nearly the same metabolic homeostasis is achieved as under low nutrient availability, but at a higher biomass.     

Components of relative growth rate and sensitivity to nitrogen availability in annual and perennial species of Bromus

Summary Two grass species, the annual Bromus sterilis and the perennial Bromus erectus, were grown from seeds for 28 days in a hydroponic culture system at 1 and 100 M NO₃ ^- in the nutrient solution. At 100 M NO₃ ^-, the relative growth rate (RGR) of the perennial was 30% lower than that of the annual. This was only the consequence of the higher specific mass of its leaves, since its leaf mass ratio was higher than that of the annual and the unit leaf rates (ULR), calculated on an area basis, were similar for the two species. At 1 M, the RGR of the annual was 50% lower than at 100 M, while that of the perennial was not significantly lower. This was due mainly to a lower ULR for the annual. while for the perennial ULR was the same in both treatments. These differences between the two species were all the more striking in that the differences in total nitrogen concentrations and nitrate reductase activities between the two treatments were very similar for both species. These different responses together with differences in the nitrogen productivity of the two species suggest that the level of nutrient availability may play an important role in the distribution of these Bromus species in natural habitats. Scope: Components of growth and response to nitrate availability in annual and perennial grasses     

Decomposition of Carex and Sphagnum litter in two mesotrophic fens differing in dominant plant species

Peatlands can be classified into fens and bogs based on their hydrology. Development of fens to bogs is accompanied by the invasion of Sphagnum species. The purpose of this study was to determine how the decomposition process in fens is influenced by the transition from a vascular plant-dominated system to a Sphagnum -dominated system. We carried out a reciprocal litter bag experiment, using litter of Carex diandra , C. lasiocarpa , Sphagnum papillosum and S. squarrosum in a fen dominated by Sphagnum species and a fen without Sphagnum . Decomposition rate and nitrogen and phosphorus dynamics of the plant litter were measured in a field experiment for two years. Decomposition rate was highest for the Carex litter types and lowest for the Sphagnum litter types. Surprisingly, decomposition rates hardly differed between the two sites. Nutrient dynamics, however, showed a clear site-effect: In the Sphagnum site net mineralization was observed for all litter types whereas in the Carex site net immobilization was observed. These results show that carbon and nutrient cycles were coupled in a different way in a Sphagnum -dominated and a Carex -dominated site, respectively. Nutrient availability and adaptation of the microbial community to nutritional and other environmental conditions may be the main regulators of carbon and nutrient cycles in these peatlands.     

Respiratory energy costs for the maintenance of biomass, for growth and for ion uptake in roots of Carex diandra and Carex acudformis

van der Werf, A., Kooijman, A., Welschen, R. and Lambers, H. 1988. Respiratory energy costs for the maintenance of biomass, for growth and for ion uptake in roots of Carex diandra and Carex acutiformis. - Physiol. Plant. 72: 483–491. The respiratory characteristics of the roots of Carex diandra Schrank and Carex acutiformis Ehrh. were investigated. The aims were, firstly to determine the respiratory energy costs for the maintenance of root biomass, for root growth and for ion uptake, and secondly to explain the higher rate of root respiration and ATP production in C. diandra. The three respiratory energy components were derived from a multiple regression analysis, using the relative growth rate and the net rate of nitrate uptake as independent variables and the rate of ATP production as a dependent variable. Although the rate of root respiration and ATP production was significantly higher in C. diandra than in C. acutiformis, the two species showed no significant difference in their rate of ATP production for the maintenance of biomass, in the respiratory energy coefficient for growth (the amount of ATP production per unit of biomass produced) and the respiratory energy coefficient for ion uptake (amount of ATP production per unit of ions absorbed). It is concluded that the higher rate of root respiration of C. diandra is caused by a higher rate of nitrate uptake. At relatively high rates of growth and nitrate uptake, the contribution of the rate of ATP production for ion uptake to the total rate of ATP production amounted to 38 and 25% for C. diandra and C. acutiformis, respectively. At this growth rate, the respiratory energy production for growth contributed 37 and 50%, respectively, to the total rate of ATP production. The relative contribution of the rate of ATP production for the maintenance of biomass increased from 25 to 70% with increasing plant age for both species. The results suggest that ion uptake is one of the major sinks for respiratory energy in roots. These experimentally derived values for the rate of ATP production for the maintenance of biomass, the respiratory energy coefficient for growth and the respiratory energy coefficient for ion uptake are discussed in relation to other experimentally and theoretically derived values.     

The uptake of phosphate by Carex species from oligotrophic to eutrophic swamp habitats

Abstract Phosphate uptake by excised roots of Carex species from a range of oligotrophic to eutrophic swamps was investigated. The range of species from oligotrophic to eutrophic is: C.rostrata Stokes, C.limosa L., C.lasiocarpa Ehrh., C.diandra Schrank, C.hudsonii A.Benn. and C.acutiformis Ehrh. All species showed two phases for P_i uptake in the P_i concentration range of 0.01 – 50 μM. In phase 1, C.rostrata and C.lasiocarpa had relatively high Vmax:s and Km:s, whereas the species from richer areas had intermediate values. The lowest Vmax and Km values were found in C.limosa and C.hudsonii. In phase 2, apart from the high Vmax and Km values found for C.lasiocarpa, the kinetic constants showed little variation, indicating a similar P_i carrier mechanism for all the species. Results on phosphate uptake and leakage are discussed against the phosphate requirement of each species in its specific habitat, and against the literature data of agricultural crops, which generally show a much lower affinity for phosphate uptake.     

Responses to mineral nutrient availability and heterogeneity in physiologically integrated sedges from contrasting habitats

Clonal plants from poor habitats benefit less from morphologically plastic responses to heterogeneity than plants from more productive sites. In addition, physiological integration has been suggested to either increase or decrease the foraging efficiency of clonal plants. We tested the capacity for biomass production and morphological response in two closely related, rhizomatous species from habitats that differ in resource availability, Carex arenaria (from poor sand dunes) and C. disticha (from nutrient-richer, moister habitats). We expected lower total biomass production and reduced morphological plasticity in C. arenaria, and that both species would produce more ramets in high nutrient patches, either in response to signals transported through physiological integration, or by locally determined responses to nutrient availability. To investigate mineral nutrient heterogeneity, plants were grown in boxes divided into two compartments with homogeneous or heterogeneous supply of high (H) or low (L) nutrient levels, resulting in four treatments, H-H, H-L, L-H and L-L. Both C. arenaria and C. disticha produced similar biomass in high nutrient treatments. C. disticha responded to high nutrients by increased biomass production and branching of the young parts and by altering root:shoot ratio and rhizome lengths, while C. arenaria showed localised responses to high nutrients in terms of local biomass and branch production in high nutrient patches. The results demonstrated that although it has a conservative morphology, C. arenaria responded to nutrient heterogeneity through morphological plasticity. An analysis of costs and benefits of integration on biomass production showed that young ramets of both species benefited significantly from physiological integration, but no corresponding costs were found. This suggests that plants from resource-poor but dynamic habitats like sand dunes respond morphologically to high nutrient patches. The two species responded to nutrient heterogeneity in different traits, and this is discussed in terms of local and distant signalling of plant status.     

高寒草甸不同生境粗喙薹草补偿生长研究

补偿生长反映了植物的耐牧性, 受采食和土壤养分资源获得性的共同影响。但何种条件易于引起植物发生超补偿是长期以来争论的问题。通过一项野外调查试验, 研究了高寒矮嵩草(Kobresia humilis)草甸3种生境(I. 畜圈, 富养, 放牧; II. 牧道, 低养, 放牧; III. 封育草地, 低养, 不放牧)中粗喙薹草(Carex scabrirostris)的补偿生长模式与机制, 以及采食率、土壤养分和水分因子对补偿生长的相对贡献。粗喙薹草补偿生长量和分株盖度、密度及高度在生境I显著高于生境II和生境III, 在生境I和生境II均发生超补偿生长, 在生境III为低补偿。分株生物量对生长器官的分配在生境I和生境II相同, 并都高于生境II; 生物量对储藏器官的分配在生境II最高, 生境III次之, 生境I最低。储藏分配与生长分配和克隆繁殖分配间, 生长分配与有性繁殖分配间均存在负偶联(trade-offs)关系。对补偿生长贡献最大的因子是相对生长率和6月份土壤有机质含量, 其次是8月份土壤氮素营养和采食率。研究结果表明, 粗喙薹草的补偿生长和生物量分配具有可塑性, 并决定着补偿生长模式随生境条件而变化。不论在富养环境或低养环境, 储藏分配高的粗喙薹草都容易发生超补偿, 富养生境条件能降低采食的负面效应, 增加植物的耐牧性。     

Flooding tolerance of Carex species. I. Root structure

Young Carex extensa Good., C. remota L. and C. pseudocyperus L. plants were subjected to aerobic or anaerobic growth conditions in nutrient solution for 40 d. Root anatomy was studied by serial transsections and longitudinal sections of the root tip. Under both growth conditions, the flooding-intolerant C. extensa developed the typical Carex root pattern, i.e. an intact cortex in the youngest part of the root, but lysogenous aerenchyma in maturing parts. In contrast, flooding-tolerant C. remota from periodically flooded habitats showed a similar root anatomy to C. extensa under aerobic conditions, but a cortex with fine intercellular spaces throughout most of the root under anaerobic conditions. The flooding-tolerant C. pseudocyperus from permanently flooded stands developed an intact cortex over most of the root length under both growth conditions. Fine intercellular spaces on four sides of each cortical cell penetrated into the very tip of the root in this species, connecting the whole root with the lacunae of the leaves and the atmosphere. In both flooding-tolerant species, morphometry showed that even under anaerobic growth conditions and despite the maintenance of a juvenile growth habit, i.e. intact cortical cells, the average root porosities were more than 20% and there was an increase in the contact area between intercellular gas spaces and the surface of cortical cells. While C. remota showed radial oxygen loss along the whole root length, C. pseudocyperus released oxygen to an oxygen-free medium only at the root tip. It is concluded that the maintenance of a juvenile root structure in combination with a system of fine intercellular spaces allows efficient nutrient uptake and plant growth during anaerobiosis in flooding-tolerant Carex species, in contrast to those species which may tolerate periods of anaerobiosis by forming aerenchyma at the cost of decreased nutrient uptake and growth. Received: 10th February 1998 / Accepted: 2nd July 1998     

Modelling of the responses to nitrogen availability of two Plantago species grown at a range of exponential nutrient addition rates

Abstract. Plantago major ssp. major and P. lanceolata were grown in solution culture with exponential nutrient addition rates. Compared with P. lanceolata, P. major major showed a higher shoot weight ratio (SWR, fraction of plant dry weight in the shoot) and a higher net assimilation rate (NAR, expressed on a leaf dry weight basis) at equal plant (PNC) and shoot (SNC) nitrogen concentration, respectively. No difference was observed in shoot nitrogen ratio (SNR, fraction of plant nitrogen in the shoot) against PNC between the two species. The effect of these differences in matter partitioning and NAR on plant growth was examined by using a growth model. The model assumed (1) that the SWR and SNR are a linear function of PNC and (2) that the NAR is a rectangular hyperbolic function of SNC. Curvilinear relationships were observed between relative growth rate (RGR) and PNC. P. major major had a higher RGR at equal PNC and, thus, a higher nitrogen productivity (NP) than P. lanceolata. Steady-stale exponential growth was simulated for different nitrogen availability in the environment. P. major major had a higher RGR over the whole range of nitrogen availability but the difference attenuated with decreasing availability of nitrogen. The simulation also showed P. lanccolata having higher plasticity in the shoot/root ratio, which resulted from its higher variability in PNC.     

Growth of tree seedlings in a tropical dry forest in relation to soil moisture and leaf traits

"AimsThe growth of plant species in tropical dry forest (TDF) is expected to be largely governed by the availability of soil moisture. In this study we attempt to identify mechanisms by which seedlings of dry tropical trees cope with water stress by adjusting their leaf characteristics to water availability and micro environments, and address following questions: How are leaf traits and relative growth rate (RGR) of the dominant seedling species of TDF affected by seasonal changes in soil moisture content (SMC)? What is the relationship of functional traits with each other? Can leaf traits singly or in combination predict the growth rate of seedling species of TDF? The study was conducted in situ on four sites (viz., Hathinala, Gaighat, Harnakachar and Ranitali, listed in order of decreasing SMC) within the tropical dry deciduous forest in northern India. Methods Five leaf traits viz., specific leaf area (SLA), leaf dry matter content (LDMC), concentrations of leaf nitrogen (leaf N), phosphorus (leaf P) and chlorophyll (Chl) and two physiological processes, viz., stomatal conductance (Gs net) and photosynthetic rate (A net), and RGR, of four dominant tree seedling species of a TDF (viz., Buchanania lanzan, Diospyros melanoxylon, Shorea robusta and Terminalia tomentosa) on four sites were analysed for species, site and season effects over a 2-year period. Step-wise multiple regression was performed to predict RGR from mean values of SMC, leaf traits and physiological processes. Principal component analysis (PCA) was performed to observe the extent of intra- vs. inter-specific variability in the leaf traits and physiological rates.Important findings All the traits and physiological rates were interrelated and showed significant positive relationship with RGR except for the correlation of LDMC with RGR which was not significant. Further, relationships of SMC with all leaf traits, physiological rates and RGR were significant, except for that between SMC and SLA for B. lanzan and D. melanoxylon. The slope of seedling trait:SMC relationship, a measure of phenotypic plasticity in response to soil moisture gradient, varied among species. Among the four species, T. tomentosa was the most plastic and S. robusta the least. In conclusion, leaf traits and physiological processes were strongly related to soil water availability on the one hand and seedling growth on the other. Gs net is the most important variable which accounted for the greatest amount of variability (62%) in RGR, emphasizing the role of stomatal conductance in shaping growth patterns across spatial and temporal gradients of soil water availability. Gs net and SMC together explained 64% variability in RGR, indicating that other traits/factors, not studied by us are also important in modulating the growth of tropical tree seedlings.     

Influence of light and nutrient conditions on seedling growth of native and invasive trees in the Seychelles

Several recent studies have shown that plant invasions can occur in resource-poor and relatively undisturbed habitats. It is, therefore, important to investigate whether and how life-history traits of species invasive in such habitats differ from those of species that are only invasive in disturbed and resource rich habitats. We compared the growth of seedlings of native and invasive tree species from nutrient-poor secondary forests in the tropical Seychelles. We hypothesised that the relative performance of the two groups would change predictably along resource gradients, with native species performing better at low levels of resource availability and invasive species performing better at higher levels. To test this hypothesis, we performed a common garden experiment using seedlings of six invasive and seven native tree species grown under three levels of light (65, 11 and 3.5% of ambient light) and two of nutrients (low and high). Due to large variation among species, differences in growth rates (RGR) were not significant among seedlings of the native and the invasive species. However, seedlings of the invasive species showed higher specific leaf areas (SLA) and higher leaf nutrient contents than seedlings of the native species. They also exhibited greater plasticity in biomass and nutrient allocation (i.e., greater plasticity in LAR, RSR and leaf nutrient contents) in response to varying resource availability. However, differences between the mean values of these parameters were generally small compared with variation within groups. We conclude that successful invaders on nutrient-poor soils in the Seychelles are either stress-tolerant, possessing growth traits similar to those of the native species, or fast-growing but adapted to nutrient-poor soils. In contrast, the more typical, fast-growing alien species with no particular adaptations to nutrient-poor soils seem to be restricted to relative nutrient-rich sites in the lowlands. The finding—that some introduced species thrive in resource-poor habitats—suggests that undisturbed habitats with low resource availability may be less resistant to plant invasions than was previously supposed.     

Factors Affecting Revegetation of Carex lacustris and Carex stricta from Rhizomes

The revegetation of sedge meadows has been problematic because natural recolonization does not occur under many circumstances and because planted propagules often fail to reestablish successfully. In this study, detached rhizomes of Carex lacustris Willd. and Carex stricta Lam. were transplanted in both fall (September) and spring (May) into three experimental wetlands to determine the effects of both planting season and hydrology on survival and establishment. Each experimental wetland had the same mean water depth across 5% slopes, but one had a constant water depth (0.5 m) throughout the growing season, another fell from a mean depth of 0.75 m to 0.25 m, and a third rose from a mean depth of 0.25 m to 0.75 m. Initial rhizome survival, shoot growth, and soil characteristics were recorded over 2 years. Neither planting proved successful (6.9% versus 0.5%) for C. stricta, a tussock-forming sedge. For C. lacustris, a sedge with spreading rhizomes, spring planting had greater rhizome survival (53.2% survival) than fall planting (0.7%). Since both species initiate new shoots in the fall, they are susceptible to transplant failure during this season. The highest survival rates (71–100%) and plant production (736.0 and 494.5 g/m²) for C. lacustris occurred near the water’s edge in both the constant and falling basins. In the rising basin, establishment and growth of this species was high at all water depths (71–96%; 399 g/m²). C. lacustris grew optimally at the same elevations where rhizome survival was greatest, suggesting that shoots are more sensitive to early-season than late-season water levels.