Relative growth rate,biomass partitioning and nutrient allocation in seedlings of two threatened trees grown under different light conditions

The present study investigates the variation in the relative growth, biomass and nutrient allocation in two threatened tree species viz. Magnolia punduana Hook.f. & Th. and Elaeocarpus prunifolius Wall. ex Müll. Berol. grown under three different levels of irradiance. The irradiance ranged between 1 and 12 mol m^?2 d^?1. Results showed that the highest relative growth rate (RGR) was achieved under the intermediate light treatment for both the species (mean: 0.005 mg mg^?1 d^?1). The growth response coefficient (GRC) model revealed that net assimilation rate (NAR) was the factor driving the RGR in both species. A significant positive correlation was found between NAR and RGR (R² = 0.33, p = 0.000) whereas specific leaf area (SLA) and leaf mass fraction (LMF) was negatively correlated to RGR. Overall, multiple regression of the studied species based on the independent variables viz. NAR, SLA, and LMF showed a significant relation with RGR (F(3,50,53 = 13.001, p = 0.000, R² = 0.43). The biomass distribution in the studied species is in agreement with the “balanced-growth hypothesis” where high irradiance increased allocation to below ground biomass fraction and decreased irradiance increased allocation to the above ground fraction. The highest nitrogen concentration in leaves was observed under the intermediate light treatment. Overall seedlings growth under intermediate light had a higher mean RGR indicating the species' preference for partial light conditions. Long-term experiments under varied light conditions as in the present study would provide useful insight into plant growth strategies in varied environmental conditions.     

Tree species effects on coupled cycles of carbon, nitrogen, and acidity in mineral soils at a common garden experiment

Forest biogeochemical cycles are shaped by effects of dominant tree species on soils, but the underlying mechanisms are not well understood. We investigated effects of temperate tree species on interactions among carbon (C), nitrogen (N), and acidity in mineral soils from an experiment with replicated monocultures of 14 tree species. To identify how trees affected these soil properties, we evaluated correlations among species-level characteristics (e.g. nutrient concentrations in leaf litter, wood, and roots), stand-level properties (e.g. nutrient fluxes through leaf litterfall, nutrient pools in stemwood), and components of soil C, N, and cation cycles. Total extractable acidity (acidity_tot) was correlated positively with mineral soil C stocks (R ²  = 0.72, P < 0.001), such that a nearly two-fold increase in acidity_tot was associated with a more than two-fold increase of organic C. We attribute this correlation to effects of tree species on soil acidification and subsequent mineral weathering reactions, which make hydrolyzing cations available for stabilization of soil organic matter. The effects of tree species on soil acidity were better understood by measuring multiple components of soil acidity, including pH, the abundance of hydrolyzing cations in soil solutions and on cation exchange sites, and acidity_tot. Soil pH and acidity_tot were correlated with proton-producing components of the soil N cycle (e.g. nitrification), which were positively correlated with species-level variability in fine root N concentrations. Soluble components of soil acidity, such as aluminum in saturated paste extracts, were more strongly related to plant traits associated with calcium cycling, including leaf and root calcium concentrations. Our results suggest conceptual models of plant impacts on soil biogeochemistry should be revised to account for underappreciated plant traits and biogeochemical processes.     

Linkage map construction using InDel and SSR markers and QTL analysis of heading traits in Brassica oleracea var. capitata L.

Cabbage heading traits are important quantitative traits that greatly affect both quality and yield of cabbage. However, the genetic control of these traits remains unclear. To detect quantitative trait loci (QTLs) associated with heading traits, a double haploid (DH) population with 196 lines was created from a cabbage hybrid 01–20 × 96–100. A genetic map with insertion–deletion and simple sequence repeat markers was constructed based on the DH population, with a total length of 934.06 cM and average interval length of 2.3 cM between adjacent markers. Field experiments in three seasons were carried out to evaluate the heading traits, including head mature period (Hm), head weight (Hw), core length (Cl), head vertical diameter (Hvd), and the ratio of Cl to Hvd (Cl/Hvd). Using the map and the trait data, 13 reliable QTLs in total were identified and 5 were found in more than one season based on the adjusted means of three seasons. Major QTLs were identified for Hm (R ² = 40.4, LOD = 14.84), Hw (R ² = 28.6, LOD = 9.83), Cl (R ² = 38.8, LOD = 15.73), Hvd (R ² = 19.2, LOD = 9.26), and Cl/Hvd (R ² = 38.8, LOD = 12.75). The most significant QTLs were Hm3.1, Cl3.1, and Cl/Hvd3.1, which were detected in three seasons with the maximum contribution rate of almost 40 %. Six active regions that harbored more than one QTL were identified on five chromosomes, and one of them contained major QTLs associated with five traits. The QTLs obtained in this study should be useful for marker-assisted selection in cabbage breeding and for understanding the genetic control of these traits.     

The distribution of native and exotic plants in a naturally fragmented sagebrush-steppe landscape

We tested two general hypotheses for the diversity of native and exotic plants in an undisturbed, naturally fragmented sagebrush-steppe landscape in SE Idaho, USA, evaluating whether the MacArthur–Wilson hypothesis of island biogeography or a suite of environmental variables explained the distributions of native and exotic plants. We also tested a third hypothesis, which incorporated assumptions about the origin of exotic plants and their interaction with native plants. Of the three hypotheses we tested, the hypothesis that included exotic species best explained the diversity of the native plant community. The MacArthur–Wilson model of island biogeography did not explain the diversity of native (R ² = 0.13) or exotic plants well (R ² = 0.11), and the model fit the data poorly. A model of environmental variables better explained the diversity of native (R ² = 0.48) and exotic plants (R ² = 0.57), but it also fit the data poorly. Instead, proximity to a railroad explained the cover (R ² = 0.59) and richness of exotic plants (R ² = 0.63), which then explained the species richness of native plants (R ² = 0.34), and the model fit was adequate and had the lowest AIC value. This suggests that the transportation corridor had a significant, though indirect, effect on the native plant community, even in this undisturbed area. Moreover, explained variance, model fit, and the AIC model selection criteria favored the model with the railroad and exotic species over the M–W and environmental models. Since the habitat patches we studied were largely undisturbed by people and their activities, our results further suggest that the transportation corridor influenced the distribution of exotic plants by serving as a vector for colonization, rather than as a source of disturbance. Additionally, the results suggest that exotic plant species have had a negative effect on the diversity of the native plant community and have changed its composition. The results also support the inference that the nascent exotic plant community is influenced by source-sink (Pulliam in Am Nat 132:652–661, 1988) and assembly dynamics. In contrast, the native plant community appears to be more strongly influenced by environmental conditions associated with an elevational gradient, but there is evidence that the native community also has undergone directional change in species composition, associated with the invasion by non-native species.     

Sap flow of the southern conifer, <Emphasis Type="Italic">Agathis australis</Emphasis> during wet and dry summers

Key message

Analysis of sap flux density during drought suggests that the large sapwood and rooting volumes of larger trees provide a buffer against drying soil.

Abstract

The southern conifer Agathis australis is amongst the largest and longest-lived trees in the world. We measured sap flux densities (F _d) in kauri trees with a DBH range of 20–176 cm to explore differences in responses of trees of different sizes to seasonal conditions and summer drought. F _d was consistently higher in larger trees than smaller trees. Peak F _d was 20 and 8 g m^?2 s^?1 for trees of diameters of 176 and 20 cm, respectively, during the wet summer. Multiple regression analysis revealed photosynthetically active radiation (PAR) and vapour pressure deficit (D) were the main drivers of F _d. During drought, larger trees were more responsive to D whilst smaller trees were more responsive to soil drying. Our largest tree had a sapwood area of 3,600 cm². Preliminary analysis suggests stem water storage provides a buffer against drying soil in larger trees. Furthermore, F _d of smaller trees had higher R ² values for soil moisture at 30 and 60 cm depth than soil moisture at 10 cm depth (R ² = 0.68–0.97 and 0.55–0.67, respectively) suggesting that deeper soil moisture is more important for these trees. Larger trees did not show a relationship between F _d and soil moisture, suggesting they were accessing soil water deeper than 60 cm. These results suggest that larger trees may be better prepared for increasing frequency and intensity of summer droughts due to deeper roots and/or larger stem water storage capacity.

     

Fine Root Morphology,Biochemistry and Litter Quality Indices of Fast- and Slow-growing Woody Species in Ethiopian Highland Forest

Fine root turnover of trees is a major C input to soil. However, the quality of litter input is influenced by root morphological traits and tissue chemical composition. In this study, fine roots of ten tropical woody species were collected from an Afromontane forest in the northern highlands of Ethiopia. The fine roots were analysed for root morphological traits and tissue chemistry measured as proxy carbon fractionations. Based on stem increment, the 10 species were divided into faster- and slower-growing species. Faster-growing species exhibited higher specific root length (1362 cm g^?1) than slower-growing species (923 cm g^?1). Similarly specific root area was higher in faster-growing species (223 cm² g^?1) than in slower-growing species (167 cm² g^?1). Among the carbon fractions, the acid-insoluble fraction (AIF) was the highest (44–51%). The carbon content, AIF, and the lignocellulose index were higher for slower-growing species. Root tissue density was lower in faster-growing species (0.33 g cm^?3) than slower-growing species (0.40 g cm^?3) and showed a strong positive correlation with carbon content (r ² = 0.84) and the AIF (r _pearson = 0.93). The morphological traits of fine roots between faster- and slower-growing species reflect the ecological strategy they employ. Slower-growing species have a higher tissue density which may reflect a greater longevity.     

Morphology, sociality, and ecology: can morphology predict pairing behavior in coral reef fishes?

Morphology can contain valuable information about the ecological performance of reef fishes, but it has rarely been used in combination with social traits. Social behavior is known to influence the ecological role of fishes; however, the ecological basis for pairing in reef fishes is not well understood. Field observations of 2,753 individuals, in 47 species in six families of biting reef fishes (Acanthuridae, Chaetodontidae, Kyphosidae, Labridae, Pomacanthidae, Siganidae), were used in combination with six morphological measurements, to examine the morphology of fishes in different social systems. A principal components analysis of morphological traits segregated species with high proportions of pairing individuals from non-pairing species along principal component 1, explaining 40.8 % of the variation. Pairing species were characterized by large eyes, concave foreheads, pointed snouts, deep bodies, and small maximum sizes. There was a significant positive relationship between these morphological traits (i.e., scores on PC1) and the prevalence of pairing within the Chaetodontidae (r ² = 0.59; P = 0.026), Siganidae (r ² = 0.72; P = 0.004), and Acanthuridae (r ² = 0.82; P < 0.001). This was consistent when traits were corrected for phylogenetic effects. No pattern was evident in the scarine Labridae (r ² = 0.15; P = 0.17). The morphological characteristics found among pairing species suggest that pairing species share common ecological traits, including foraging for small prey items in micro-topographically complex environments such as reef crevices. These ecological traits may have played a role in the evolution of pairing behavior and subsequently led to the development of reproductive patterns based on monogamy.     

Environmental variables controlling soil respiration on diurnal, seasonal and annual time-scales in a mixed mountain forest in Switzerland

Studies on soil respiration in mountain forests are rather scarce compared to their broad distribution. Therefore, we investigated daily, seasonal and annual soil respiration rates in a mixed forest (Lägeren), located at about 700 m in the Swiss Jura mountains, during 2 years (2006 and 2007). Soil respiration (SR) was measured continuously with high temporal resolution (half-hourly) at one single point (SR_automated) and periodically with high spatial resolution (SR_manual) at 16 plots within the study site. Both, SR_automated and SR_manual showed a similar seasonal cycle. SR strongly depended on soil temperature in 2007 (R ² = 0.82–0.92), but less so in 2006 (R ² = 0.56–0.76) when SR was water limited during a summer drought. Including soil moisture improved the fit of the 2006 model significantly (R ² = 0.78–0.97). Total annual SR for the study site was estimated as 869 g C m^?2 year^?1 for 2006 and as 907 g C m^?2 year^?1 for 2007 (uncertainty <10% at the 95% confidence interval, determined by bootstrapping). Selected environmental conditions were assessed in more detail: (1) Rapid, but contrasting changes of SR were found after summer rainfall. Depending on soil moisture at pre-rain conditions, summer rain could either cause a pulse of CO₂ from the soil or an abrupt decrease of SR_automated due to water logging of soil pores. (2) Two contrasting winter seasons resulted in SR being about 60–70% (31.2–44.6 g C m^?2) higher during a mild winter (2007) compared to a harsh winter (2006). (3) Analysing SR for selected periods on a diurnal scale revealed a counter-clockwise hysteresis with soil surface temperatures. This indication of a time-lagged response of SR to temperature was further supported by a very strong relationship (R ² = 0.86–0.90) of SR to soil temperature with a time-lag of 2–4 h.     

Congruence and the Biomonitoring of Aquatic Ecosystems: Are Odonate Larvae or Adults the Most Effective for the Evaluation of Impacts

Odonata have been widely used as indicators for the biomonitoring of terrestrial and aquatic habitats due to their sensitivity to environmental impacts. We aimed to determine whether the larval or adult phases of these insects were the best predictors of variation in habitat parameters and the loss of environmental integrity. Specimens were collected during three seasons (dry, rainy, and ebb) from 12 points in the Suiá-missu River basin, at the headwaters of the Xingu River in Mato Grosso, Brazil. The Protest analysis indicated a high degree of congruence between the assemblages of larvae and adults in streams with varying degrees of habitat integrity (R = 0.832, p < 0.001, m ² = 0.307). When the congruence with environmental factors was analyzed, a significant association was found only for the larval phase (R = 0.318, p = 0.03, m ² = 0.888). When the suborders were analyzed separately, congruence was confirmed for anisopteran adults (R = 0.338, p = 0.031, m ² = 0.885) and larvae (R = 0.417, p = 0.003, m ² = 0.826) and for the zygopteran adults (R = 0.345, p = 0.027, m ² = 0.881) and larvae (R = 0.405, p = 0.011, m ² = 0.836). These results indicate that both larvae and adults respond systematically to environmental impacts. We suggest that either life phase can be used for biomonitoring, given their effectiveness for the interpretation of disturbance in terrestrial and aquatic habitats. These findings further reinforce the effectiveness of this insect order for the detection of modifications to the environment, showing that they are good indicators of environmental conditions.     

Large-tree growth and mortality rates in forests of the central North Island, New Zealand

Large trees are a significant structural component of old-growth forests and are important as habitat for epiphytic biodiversity; as substantial stores of biomass, carbon and nutrient; as seed trees; and as engineers of large gap sites for regeneration. Their low density across the landscape is an impediment to accurately measuring growth and mortality, especially as infrequent tree deaths are rarely captured without long periods of monitoring. Here we present large-tree (≥ 30 cm in diameter at breast height) growth and mortality rates for six common New Zealand tree species over a 42-year period from 28 large permanent plots (0.4?0.8?ha) in the central North Island. Our goal was to examine how rates of growth and mortality varied with tree size and species. In total we sampled 1933 large trees across 11.6 ha, corresponding to a large-tree density of 167 trees?ha^?1, of which we used 1542 as our six study species. Mean annual mortality rates varied more than 10-fold among species being least in Dacrydium cupressinum (0.16%) and greatest in Weinmannia racemosa (2.21%). Diameter growth rates were less variable among species and ranged from 1.8 mm?yr^?1 in Ixerba brexioides to 3.3?mm?yr^?1 in D.?cupressinum. Tree size influenced the rate of mortality in Beilschmiedia tawa, I. brexioides and W.?racemosa but there was no support for including tree size in models of the remaining three species. Likewise, tree size influenced growth rates in I.?brexioides and Nothofagus menziesii but not the remaining four species. These data provide robust size- and species-specific estimates of large-tree demographic rates that can be used as baselines for monitoring the impacts of management and global change in old-growth forests.     

青藏高原草地地下生物量与环境因子的关系

青藏高原草地生物量大部分分布于地下,地下生物量在其碳循环研究中起着重要的作用.基于大规模野外样地调查数据,分析比较了青藏高原南北和东西样带上草地地下生物量与环境因子的相关关系,探讨了环境因子对地下生物量控制作用的区域差异.研究结果表明:对于所有采样点而言,青藏高原草地地下生物量的环境控制因素主要有土壤含水量、表层土壤有机碳和全氮含量.通过比较南北和东西样带研究结果发现,草地地下生物量与土壤含水量、土壤表层有机碳和全氮含量相关的显著性水平,在东西样带上明显高于南北样带.同时,东西样带上草地地下生物量与降水量有显著正相关关系,这种关系在南北样带上不显著,表明水分对东西样带草地地下生物量的控制作用较强.气温与南北样带草地地下生物量呈显著负相关,但与东西样带草地地下生物量相关不显著,由此说明环境因子对青藏高原草地地下生物量的控制存在显著区域差异.     

Environmental Correlates of Tree and Seedling–Sapling Distributions in a Mexican Tropical Dry Forest

Bray and Curtis ordination was used to explore which environmental variables explained importance values and the presence–absence of tropical tree seedlings, saplings and adults in La Escondida-La Cabaña, Sierra de Manantlán, Jalisco, Mexico. The diameters of trees ≥2.5 cm DBH and the presence and height of seedlings and saplings were measured in nine 0.1 ha sites. Four matrices including presence–absence data and importance value indices for trees and seedlings and saplings were analyzed through Bray and Curtis ordination. The matrices were based on density, frequency, and dominance of adult trees as well as seedlings and saplings. The environmental matrix consisted of 18 variables, including elevation, slope, canopy gaps, disturbance, and soil variables. We recorded 63 tree species and 38 seedling and sapling species in the nine sites. The ordination explained 70.9% of the variation in importance value data for trees and 62.6% for seedlings and saplings. The variation explained in presence–absence data for trees was 67.1 and 77.4% for seedlings and saplings. The variance in the ordination axes of seedlings and sapling presence–absence data was poorly explained by the number of gaps in the tree, shrub, or herb layer, suggesting little light specialization by seedlings and saplings. Habitat specialization for soil nutrients appears to be important in explaining the presence–absence of seedlings and saplings. Seedling and sapling specialization along different soil microsites could promote species coexistence in this forest, while heterogeneity in light conditions may instead determine differences in growth and, thus, importance value of trees. We hypothesize that in tropical dry forest in Jalisco, Mexico, a habitat specialization for soil resources is likely more importance at early stages in tree life histories than in later life history.     

Annual variability in the life-history characteristics of brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus) in a subalpine Norwegian lake

The annual variability in growth and life history traits of brown trout (Salmo trutta L.) and Arctic charr (Salvelinus alpinus (L.)) in Lake Atnsjøen, a Norwegian subalpine lake, was studied over a period of 13 years (1985–1997). The extent to which life-history characteristics recorded on one occasion can be regarded as representative for the population was explored. We found inter-cohort variation in growth for both species; estimates of asymptotic length (L _∞) in ten cohorts ranged between 225–305 mm (CV = 10.5%) for brown trout and 273–301 mm (CV = 4.1%) for Arctic charr. However, this variation was much lower than inter-population variation for brown trout based on single samples from 169 populations (CV = 24.6%). In Lake Atnsjøen, annual growth increment correlated highly with the number of days warmer than 7?°C (R ²=0.60–0.89) for brown trout, and days warmer than 10?°C (R ²=0.40–0.58) for Arctic charr. Females of Arctic charr were younger at sexual maturity than males, while no such difference was found in brown trout. Generally speaking, early maturing individuals of both species grew faster, particularly from age-2 and onwards, than immature individuals. Early maturing individuals, however, were smaller at maturity than those maturing one year older. Age and size at maturity were significantly correlated with asymptotic lengths only in Arctic charr females.     

Airborne LiDAR derived canopy height model reveals a significant difference in radiata pine (Pinus radiata D. Don) heights based on slope and aspect of sites

Key message

Significant relationship between tree height and ALS-derived topography was shown. Taller trees were found on slopes <10° and southerly aspects. Potential value of ALS in forest management applications was defined.

Abstract

Accurate information on tree height distribution can provide a better understanding of forest productivity and biomass estimation. Airborne light detection and ranging remote sensing, also known as airborne laser scanning (ALS), has proven to be an effective tool for deriving tree height information in forests. While tree height has been reported to vary in response to many environmental factors, few researchers have demonstrated the effect of topography on tree height variation using ALS data. This study investigated the relationship between tree height variation and ALS-derived topographic aspect and slope factors within two even-aged radiata pine (Pinus radiata D. Don) plantation sites in Nundle, New South Wales, Australia. A total of 447 trees was sampled from 77 plots in two plantation age classes: 193 trees from a 34-year-old site and 254 trees from a 9-year-old site. ALS height estimates were highly correlated with field heights (R ² = 0.90 and RMSE = 0.66 for 2002 and R ² = 0.87 and RMSE = 1.49 for 1977 sites). ALS-derived slope and aspect metrics were shown to have a significant relationship with height variation across the stands. The slope (P < 0.01) and aspect (P < 0.001) were significant in the mixed linear models. Overall taller trees were found on slopes below 10° and on southerly aspects, while shorter trees dominated steeper slopes (>20°) and on northerly aspects. However, aspect gradient appeared to have more significant effect on tree heights than slope classes. These results were further verified using an additional 2,000 randomly located trees sampled across the plantations. The study demonstrates a significant relationship between tree height variation and ALS-derived ground aspect and slope categories which may have potential benefits for improving current wood resource inventories and future productivity models.     

Shade tolerance and herbivory are associated with RGR of tree species via different functional traits

• Relative growth rate (RGR) plays an important role in plant adaptation to the light environment through the growth potential/survival trade‐off. RGR is a complex trait with physiological and biomass allocation components. It has been argued that herbivory may influence the evolution of plant strategies to cope with the light environment, but little is known about the relation between susceptibility to herbivores and growth‐related functional traits.
• Here, we examined in 11 evergreen tree species from a temperate rainforest the association between growth‐related functional traits and (i) species’ shade‐tolerance, and (ii) herbivory rate in the field. We aimed at elucidating the differential linkage of shade and herbivory with RGR via growth‐related functional traits.
• We found that RGR was associated negatively with shade‐tolerance and positively with herbivory rate. However, herbivory rate and shade‐tolerance were not significantly related. RGR was determined mainly by photosynthetic rate (A_max) and specific leaf area (SLA). Results suggest that shade tolerance and herbivore resistance do not covary with the same functional traits. Whereas shade‐tolerance was strongly related to A_max and to a lesser extent to leaf mass ratio (LMR) and dark respiration (R_d), herbivory rate was closely related to allocation traits (SLA and LMR) and slightly associated with protein content.
• The effects of low light on RGR would be mediated by A_max, while the effects of herbivory on RGR would be mediated by SLA. Our findings suggest that shade and herbivores may differentially contribute to shape RGR of tree species through their effects on different resource‐uptake functional traits.

     

Analysis of cylindrospermopsin- and microcystin-producing genotypes and cyanotoxin concentrations in the Macau storage reservoir

The Macau storage reservoir (MSR) has experienced algal blooms in recent years, with high levels of Cylindrospermopsis and Microcystis and detectable concentrations of cyanotoxins. To analyze the cyanotoxin-producing genotypes and relate the corresponding cyanotoxins to the water quality parameters, a quantitative real-time polymerase chain reaction was developed and applied to the water samples in three locations of MSR. Cylindrospermopsin polyketide synthetase (pks) gene and a series of microcystin synthetase (mcy) genes were used for identifying and quantifying cylindrospermopsin- and microcystin-producing genes, and the corresponding water parameters were measured accordingly. Our results showed that high concentrations of cylindrospermopsin and low concentrations of microcystin were measured during the study period. There was a strong correlation between the pks gene numbers and cylindrospermopsin concentrations (R ² = 0.95), while weak correlations were obtained between the mcy genes numbers and microcystin concentrations. Furthermore, the pks gene numbers were strongly related to Cylindrospermopsis (R ² = 0.88), cyanobacterial cell numbers (R ² = 0.96), total algae numbers (R ² = 0.95), and chlorophyll-a concentrations (R ² = 0.83), consistent with the dominant species of Cylindrospermopsis among the cyanobacteria existing in MSR. NH₄–N (R ² = 0.68) and pH (R ² = 0.89) were the water quality parameters most highly correlated with the pks gene numbers. These results contribute to monitoring for potential cyanotoxins in raw water.     

Plant species richness in mountain forests of the Bavarian Alps

Abstract

Based on a stratified random sample of 93 vegetation plots (144 m²) from montane and subalpine climax forests in a representative section through the Bavarian Alps, spatial pattern and environmental correlates of species density of trees, vascular understorey and epigeic bryophytes were analysed. Detecting landscape scale patterns in beta- and gamma-diversity based on interpretation of rarefaction curves proved to be difficult in a sample that had been stratified by ecological criteria. In 144 m² plots tree species density (5 ± 2.0, max. 10) declined with elevation and increased with stand age (multiple R ² = 0.557). The latter effect can be attributed to the secular history of game management and browsing pressure, which has hindered the regeneration of species-rich tree stands since ca. 150 yr. Species density of the forest undergrowth reached remarkably high levels for vascular plants (42 ± 12.8, max. 69) and bryophytes (14 ± 6.0, max. 30) and strongly depended on cover of the respective layer in a unimodal pattern, suggesting to separate direct and indirect effects, mediated through the mass effect, in the subsequent construction of regression models. Multiple regression (R ² = 0.47) revealed that vascular species density is limited chiefly through low plant cover, which in turn decreases with tree cover, elevation and soil quality, and secondly by species pools that contain larger numbers of species requiring high pH and ample light. Cover and direct effects had roughly equal weight in controlling bryophyte species density (R ² = 0.57). Biomass depended on the proportion of conifers in the tree layer and on site quality, less fertile sites tending to have higher bryophyte cover. The increase of bryophyte species density with elevation was interpreted as an effect of a pool of largely boreal-subalpine species. The increase of species density with stand age suggests dispersal limitation and deserves further study.     

Soil CO2 efflux and soil carbon balance of a tropical rubber plantation

Natural rubber is a valuable source of income in many tropical countries and rubber trees are increasingly planted in tropical areas, where they contribute to land-use changes that impact the global carbon cycle. However, little is known about the carbon balance of these plantations. We studied the soil carbon balance of a 15-year-old rubber plantation in Thailand and we specifically explored the seasonal dynamic of soil CO₂ efflux (F _S) in relation to seasonal changes in soil water content (W _S) and soil temperature (T _S), assessed the partitioning of F _S between autotrophic (R _A) and heterotrophic (R _H) sources in a root trenching experiment and estimated the contribution of aboveground and belowground carbon inputs to the soil carbon budget. A multiplicative model combining both T _S and W _S explained 58 % of the seasonal variation of F _S. Annual soil CO₂ efflux averaged 1.88 kg C m^?2 year^?1 between May 2009 and April 2011 and R _A and R _H accounted for respectively 63 and 37 % of F _S, after corrections of F _S measured on trenched plots for root decomposition and for difference in soil water content. The 4-year average annual aboveground litterfall was 0.53 kg C m^?2 year^?1 while a conservative estimate of belowground carbon input into the soil was much lower (0.17 kg C m^?2 year^?1). Our results highlighted that belowground processes (root and rhizomicrobial respiration and the heterotrophic respiration related to belowground carbon input into the soil) have a larger contribution to soil CO₂ efflux (72 %) than aboveground litter decomposition.     

Leaf attributes and tree growth in a tropical dry forest

Questions: How are leaf attributes and relative growth rate (RGR) of the dominant tree species of tropical deciduous forest (TDF) affected by seasonal changes in soil moisture content (SMC)? What is the relationship of functional attributes with each other? Can leaf attributes singly or in combination predict the growth rate of tree species of TDF? Location: Sonebhadra district of Uttar Pradesh, India. Methods: Eight leaf attributes, specific leaf area (SLA); leaf carbon concentration (LCC); leaf nitrogen concentration (LNC); leaf phosphorus concentration (LPC); chlorophyll concentration (Chl), mass‐based stomatal conductance (Gs_mass); mass based photosynthetic rate (A_mass); intrinsic water use efficiency (WUEi); and relative growth rate (RGR), of six dominant tree species of a dry tropical forest on four sites were analysed for species, site and season effects over a 2‐year period. Step‐wise multiple regression was performed for predicting RGR from mean values of SMC and leaf attributes. Path analysis was used to determine which leaf attributes influence RGR directly and which indirectly. Results: Species differed significantly in terms of all leaf attributes and RGR. The response of species varied across sites and seasons. The attributes were positively interrelated, except for WUEi, which was negatively related to all other attributes. The positive correlation was strongest between Gs_mass and A_mass and the negative correlation was strongest between Gs_mass and WUEi. Differences in RGR due to site were not significant when soil moisture was controlled, but differences due to season remained significant. The attributes showed plasticity across moisture gradients, which differed among attributes and species. Gs_mass was the most plastic attribute. Among the six species, Terminalia tomentosa exhibited the greatest plasticity in six functional attributes. In the step‐wise multiple regression, A_mass, SLA and Chl among leaf attributes and SMC among environmental factors influenced the RGR of tree species. Path analysis indicated the importance of SLA, LNC, Chl and A_mass in determining RGR. Conclusion: A _mass, SMC, SLA and Chl in combination can be used to predict RGR but could explain only three‐quarters of the variability in RGR, indicating that other traits/factors, not studied here, are also important in modulating growth of tropical trees. RGR of tree species in the dry tropical environment is determined by soil moisture, whereas the response of mature trees of different species is modulated by alterations in key functional attributes such as SLA, LNC and Chl.     

Response of floodplain understorey species to environmental gradients and tree invasion: a functional trait perspective

Plants are connected to habitats by functional traits which are filtered by environmental gradients. Since tree species composition in the forest canopy can influence ecosystem processes by changing resource availability, litter accumulation, and soil nutrient content, we hypothesised that non-native invasive trees can establish new environmental filters on the understorey communities. In the hardwood floodplain forests in Northern Italy, the invasive trees Robinia pseudoacacia L. and Prunus serotina Ehrh. are the dominant canopy species. We used trait data assembled from databases and iterative RLQ analysis to identify a parsimonious set of functional traits responding to environmental variables (soil, light availability, disturbance, and stand structure) and the dominant native and invasive canopy species. Then, RLQ and fourth-corner analysis was conducted to investigate the joint structure between macro-environmental variables and species traits and functional groups were identified. The trait composition of the herb-layer was significantly related to the main environmental gradients and the presence of the invaders in the canopy showed significant relationships with several traits. In particular, the presence of P. serotina may mitigate or even erase the effect of disturbances, maintaining a stable forest microclimate and thus favouring ‘true’ forest species, while R. pseudoacacia may slow down forest succession and regeneration by establishing new stable associations with a graminoid-dominated understorey. The impact of the two invasive trees on herb layer composition appears to differ, indicating that different management and control strategies may be needed.