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Aims: Evergreen and deciduous forests can coexist at different spatial scales in the seasonally dry tropics of Southeast Asia. At present, the main drivers for their differences in soil carbon (C) stock remains unclear. We aim to characterize the patterns of C and nitrogen (N) isotope ratios in soil profiles of two forest types to get a better understanding of how soil organic matter (SOM) pools are maintained under different types of forest ecosystem. Methods: We analyzed the coupled trends in the natural abundances of C and N isotopes (δ13C and δ15N) in leaf litter, and mineral soil and its density fractions, collected from eight sites in Cambodian tropical forests. Results: The mean value of the beta coefficient, which was calculated as the slope of the regression line for δ13C values against the logarithm of the C concentration and can serve as a natural indicator for SOM turnover, was − 1.9, while the Δ15N, which was the difference in δ15N between the leaf litter and mineral soil at 15 − 30 cm depth, was 6.2‰. Contrary to our expectations, these indexes did not differ significantly between evergreen and deciduous forest types. Conclusions: The absence of clear difference in beta coefficients and Δ15N values between forest types was considered as derived from counteracting effects of several factors, e.g., tree biomass, precipitation and soil mineralogy, associated with multiple processes of C and N cycles. We especially highlighted the significant role of soil acidity in characterizing SOM cycles in seasonally dry tropical forests. [ABSTRACT FROM AUTHOR]

Black spruce (Picea mariana (Mill.) B.S.P.) is a dominant species on north-facing slopes located on the permafrost in the Alaskan Interior, where tree growth rates vary significantly across slope gradients. To better understand the effects of nitrogen (N) and mycorrhizal associations on plant growth, we investigated the relationships between tree growth and isotope ratios of nitrogen (δ[sup.15]N) in foliage and roots, because plant δ[sup.15]N values reflect tree dependencies on N uptake by mycorrhizae. We established two transects (one along a slope gradient, and a second on a north-to-east axis at a fixed elevation) and four plots on the slope gradient transect. We measured current shoot growth of saplings on transects and aboveground biomass (AGB) in the plots. We collected foliage and fine root samples and measured their δ[sup.15]N signatures and N concentrations. The AGB and shoot growth varied widely across the gradient: values were higher at high elevations and at the east-northeast aspect. Foliage δ[sup.15]N values and N concentrations were positively correlated with tree growth, but growth rate was negatively correlated with the dependency on mycorrhizae for N uptake. Thus, black spruce had a reduced dependency on mycorrhizae for N uptake on the upper slope, where conditions were N-rich. On the lower slope, where the soil active layer was shallow, black spruce growth was limited and had an elevated dependency on mycorrhizae for N uptake. Key words: ectomycorrhizal, ericoid mycorrhizal fungi, nitrogen isotope, nitrogen uptake, Picea mariana. L'epinette noire (Picea mariana (Mill.) B.S.P.) est une espece dominante sur les pentes exposees au nord situees sur le pergelisol a l'interieur de l'Alaska, lal ou le taux de croissance des arbres varie de facon significative suivant l'inclinaison de la pente. Pour mieux comprendre les effets de l'azote (N) et des associations mycorhiziennes sur la croissance des plantes, nous avons etudies les relations entre la croissance des arbres et les rapports isotopiques de l'azote (δ[sup.15]N) dans le feuillage et les racines etant donne que les valeurs de δ[sup.15]N indiquent dans quelle mesure les arbres dependent de l'absorption de N par les mycorhizes. Nous avons etabli deux transects (le premier suivant l'inclinaison de la pente et l'autre suivant un axe est-nord-est a une altitude fixe) et quatre places echantillons sur le transect le long l'inclinaison de la pente. Nous avons mesure la croissance courante des pousses des gaules le long des transects ainsi que la biomasse aerienne (BA) dans les places echantillons. Nous avons preleve des echantillons du feuillage et des racines fines et mesure leur signature isotopique δ[sup.15]N ainsi que leur concentration en N. La croissance des pousses et de la BA variait beaucoup le long de la pente : les valeurs etaient plus elevees a haute altitude et pour l'exposition est-nord-est. Les concentrations de N et les valeurs de δ[sup.15]N etaient positivement correlees avec la croissance des arbres mais le taux de croissance etait negativement correle avec la dependance envers les mycorhizes pour l'absorption de N. Par consequent, l'epinette noire avait une moins forte dependance envers les mycorhizes pour l'absorption de N dans le haut de la pente ou N etait abondant. Dans le bas de la pente ou la couche active de sol etait peu profonde, la croissance de l'epinette noire etait limitee et fortement dependante des mycorhizes pour l'absorption de N. [Traduit par la Redaction] Mots-cles: ectomycorhize, champignons mycorhiziens ericoides, isotope d'azote, absorption de l'azote, Picea mariana., Introduction Carbon (C) stock accumulation and tree growth rates in boreal forests are generally limited by nitrogen (N) availability (Shaver and Chapin 1980). Low temperatures, water logging, and high soil [...]

The morphological and physicochemical traits of leaves are important in terms of plant adaptation to various growth environments, because such traits play central roles in various functions including photosynthesis. We measured the toughness, mass per unit area (LMA), nitrogen content, and S15N levels of the leaves of different life-forms of 39 broadleaf woody plants in interior Alaska. The plants were divided into three life-forms based on the maximum height of adult plants: understory ( Key words: δ[sup.15]N, heterobaric leaf, LMA, nitrogen, leaf toughness. Les traits morphologiques et physicochimiques des feuilles sont importants pour l'adaptation des plantes a divers environnements de croissance parce qu'ils jouent un role central dans diverses fonctions, incluant la photosynthese. Nous avons mesure la coriacite, la masse par unite de surface (MFS), la teneur en azote et le niveau de δ[sup.15]N des feuilles de differents types biologiques de 39 plantes ligneuses feuillues du centre de l'Alaska. Les plantes ont ete divisees en trois types biologiques en fonction de la hauteur maximale des plantes adultes : sous- etage ( Mots-cles: δ[sup.15]N, feuille heterobarique, masse foliaire par unite de surface foliaire, azote, coriacite., Introduction The morphological and physicochemical traits of leaves are important in terms of plant adaptation to growth environments, because such traits play central roles in various functions including photosynthesis and [...]

Black spruce ( Picea mariana (Mill.) B.S.P.) is a dominant species on north-facing slopes located on the permafrost in the Alaskan Interior, where tree growth rates vary significantly across slope gradients. To better understand the effects of nitrogen (N) and mycorrhizal associations on plant growth, we investigated the relationships between tree growth and isotope ratios of nitrogen (δ15N) in foliage and roots, because plant δ15N values reflect tree dependencies on N uptake by mycorrhizae. We established two transects (one along a slope gradient, and a second on a north-to-east axis at a fixed elevation) and four plots on the slope gradient transect. We measured current shoot growth of saplings on transects and aboveground biomass (AGB) in the plots. We collected foliage and fine root samples and measured their δ15N signatures and N concentrations. The AGB and shoot growth varied widely across the gradient: values were higher at high elevations and at the east-northeast aspect. Foliage δ15N values and N concentrations were positively correlated with tree growth, but growth rate was negatively correlated with the dependency on mycorrhizae for N uptake. Thus, black spruce had a reduced dependency on mycorrhizae for N uptake on the upper slope, where conditions were N-rich. On the lower slope, where the soil active layer was shallow, black spruce growth was limited and had an elevated dependency on mycorrhizae for N uptake. [ABSTRACT FROM AUTHOR]

The morphological and physicochemical traits of leaves are important in terms of plant adaptation to various growth environments, because such traits play central roles in various functions including photosynthesis. We measured the toughness, mass per unit area (LMA), nitrogen content, and δ15N levels of the leaves of different life-forms of 39 broadleaf woody plants in interior Alaska. The plants were divided into three life-forms based on the maximum height of adult plants: understory (<1 m), small (≥1 m to <5 m), and canopy (≥5 m). Evergreen species accounted for a large proportion of understory woody plants (44%), whereas most small (87%) and canopy (100%) trees were deciduous. Higher LMA, increased toughness, and higher C-to-N ratios prolonged leaf life-span in evergreen species. We also evaluated the presence or absence of bundle-sheath extensions (BSEs), which contribute to the mechanical support of leaves and enhance photosynthetic ability by improving hydraulic properties. In total, 11 species (28%) had homobaric leaves (BSEs were absent) and 28 species (72%) had heterobaric leaves; these proportions are similar to those of woody plants of other temperate deciduous forests. The δ15N values reflected the presence of root symbionts such as ericoid mycorrhiza or Frankia sp. Our results suggest that leaf traits may affect both the life-form of the host plants and leaf longevity in both evergreen and deciduous species. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Forest Research is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Canadian Journal of Forest Research is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: On the Loess Plateau, China, several planted tree species such as Populus hopeinsis and Robinia pseudoacacia suffer occasional diebacks in the top shoots, reducing growth rates after maturation. However, this does not usually occur in other species, e.g., Ulmus pumila and Zizyphus jujuba. We compared stable carbon isotope ratios (δ 13C) as indicators of leaf water-use efficiency (WUE), leaf mass per unit area (LMA), and leaf area- and mass-based nitrogen content (Narea and Nmass) by tree height (1, 5, and 10m) in P. hopeinsis, R. pseudoacacia, U. pumila, and Z. jujuba. In P. hopeinsis and R. pseudoacacia, leaf δ 13C and Narea were significantly lower in 10-m trees than in 1-m saplings, indicating that leaf WUE and photosynthetic rates of both species decreased with tree height. In contrast, δ 13C in Z. jujuba varied little with tree height. The δ 13C of 10-m-tall U. pumila trees was significantly higher than that of 1-m plants, demonstrating an increase in WUE with tree height. Decreasing WUE, leaf Narea, and Nmass with height increases in P. hopeinsis and R. pseudoacacia may be related to water and nutrient limitations for these species in semiarid regions. In contrast, stable or increasing δ 13C with tree height in U. pumila and Z. jujuba may account (to some extent) for successful production of these species under identical environmental conditions. Diameter growth rate also decreased with maturity in P. hopeinsis and R. pseudoacacia, but increased or was stable for U. pumila and Z. jujuba. The differences in leaf WUE and LMA among species with tree maturity may be related to species’ growth patterns and susceptibility to drought stress, and are likely to be important new criteria for plantation species selection on the Loess Plateau. [Copyright &y& Elsevier]