Needle δ13C and mobile carbohydrates in Pinus koraiensis in relation to decreased temperature and increased moisture along an elevational gradient in NE China

A tree’s crown interacts with atmospheric variables such as CO₂, temperature, and humidity. Physioecology of leaves/needles (e.g. δ¹³C, mobile carbohydrates, and nitrogen) is, therefore, strongly affected by microclimate in and surrounding a tree crown. To understand the physiological responses of leaves to changes in air temperature and moisture, we measured δ¹³C, soluble sugars, starch, and total nitrogen (N) concentrations in current year and 1-yr-old needles of Pinus koraiensis trees, and compared the growing season air temperature and relative humidity within and outside P. koraiensis crowns along an elevational gradient from 760 to 1,420 m a.s.l. on Changbai Mountain, NE China. Our results indicated that needle N and mobile carbohydrates concentrations, as well as needle δ¹³C values changed continuously with increasing elevation, corresponding to a continuous decrease in air temperature and an increase in relative humidity. Needle carbon and nitrogen status is highly significantly negatively correlated with temperature, but positively correlated with relative humidity. These results indicate that increases in air temperature in combination with decreases in relative humidity may result in lower levels of N and mobile carbohydrates in P. koraiensis trees, suggesting that future climate changes such as global warming and changes in precipitation patterns will directly influence the N and carbon physiology at P. koraiensis individual level, and indirectly affect the competitive ability, species composition, productivity and functioning at the stand and ecosystem level in NE China. Due to the relatively limited range of the transect (760–1,420 m) studied, further research is needed to explain whether the present results are applicable to scales across large elevational gradients.     

Variation in woody plant δ13C along a topoedaphic gradient in a subtropical savanna parkland

δ¹³C values of C₃ plants are indicators of plant carbon–water relations that integrate plant responses to environmental conditions. However, few studies have quantified spatial variation in plant δ¹³C at the landscape scale. We determined variation in leaf δ¹³C, leaf nitrogen per leaf area (N_area), and specific leaf area (SLA) in April and August 2005 for all individuals of three common woody species within a 308 × 12-m belt transect spanning an upland–lowland topoedaphic gradient in a subtropical savanna in southern Texas. Clay content, available soil moisture, and soil total N were all negatively correlated with elevation. The δ¹³C values of Prosopis glandulosa (deciduous N₂-fixing tree legume), Condalia hookeri (evergreen shrub), and Zanthoxylum fagara (evergreen shrub) leaves increased 1–4‰ with decreasing elevation, with the δ¹³C value of P. glandulosa leaves being 1–3‰ higher than those of the two shrub species. Contrary to theory and results from previous studies, δ¹³C values were highest where soil water was most available, suggesting that some other variable was overriding or interacting with water availability. Leaf N_area was positively correlated with leaf δ¹³C of all species (p < 0.01) and appeared to exert the strongest control over δ¹³C along this topoedaphic gradient. Since leaf N_area is positively related to photosynthetic capacity, plants with high leaf N_area are likely to have low p _I/p _a ratios and therefore higher δ¹³C values, assuming stomatal conductance is constant. Specific leaf area was not correlated significantly with leaf δ¹³C. Following a progressive growing season drought in July/August, leaf δ¹³C decreased. The lower δ¹³C in August may reflect the accumulation of ¹³C-depleted epicuticular leaf wax. We suggest control of leaf δ¹³C along this topoedaphic gradient is mediated by leaf N_area rather than by stomatal conductance limitations associated with water availability.     

Foliar δ15N values characterize soil N cycling and reflect nitrate or ammonium preference of plants along a temperate grassland gradient

The natural abundance of stable (15)N isotopes in soils and plants is potentially a simple tool to assess ecosystem N dynamics. Several open questions remain, however, in particular regarding the mechanisms driving the variability of foliar delta(15)N values of non-N(2) fixing plants within and across ecosystems. The goal of the work presented here was therefore to: (1) characterize the relationship between soil net mineralization and variability of foliar Deltadelta(15)N (delta(15)Nleaf - delta(15)Nsoil) values from 20 different plant species within and across 18 grassland sites; (2) to determine in situ if a plant's preference for NO (3) (-) or NH (4) (+) uptake explains variability in foliar Deltadelta(15)N among different plant species within an ecosystem; and (3) test if variability in foliar Deltadelta(15)N among species or functional group is consistent across 18 grassland sites. Deltadelta(15)N values of the 20 different plant species were positively related to soil net mineralization rates across the 18 sites. We found that within a site, foliar Deltadelta(15)N values increased with the species' NO (3) (-) to NH (4) (+) uptake ratios. Interestingly, the slope of this relationship differed in direction from previously published studies. Finally, the variability in foliar Deltadelta(15)N values among species was not consistent across 18 grassland sites but was significantly influenced by N mineralization rates and the abundance of a particular species in a site. Our findings improve the mechanistic understanding of the commonly observed variability in foliar Deltadelta(15)N among different plant species. In particular we were able to show that within a site, foliar delta(15)N values nicely reflect a plant's N source but that the direction of the relationship between NO (3) (-) to NH (4) (+) uptake and foliar Deltadelta(15)N values is not universal. Using a large set of data, our study highlights that foliar Deltadelta(15)N values are valuable tools to assess plant N uptake patterns and to characterize the soil N cycle across different ecosystems.     

Foliar δ15N is affected by foliar nitrogen uptake, soil nitrogen, and mycorrhizae along a nitrogen deposition gradient

Foliar nitrogen isotope (δ¹⁵N) composition patterns have been linked to soil N, mycorrhizal fractionation, and within-plant fractionations. However, few studies have examined the potential importance of the direct foliar uptake of gaseous reactive N on foliar δ¹⁵N. Using an experimental set-up in which the rate of mycorrhizal infection was reduced using a fungicide, we examined the influence of mycorrhizae on foliar δ¹⁵N in potted red maple (Acer rubrum) seedlings along a regional N deposition gradient in New York State. Mycorrhizal associations altered foliar δ¹⁵N values in red maple seedlings from 0.06 to 0.74 ‰ across sites. At the same sites, we explored the predictive roles of direct foliar N uptake, soil δ¹⁵N, and mycorrhizae on foliar δ¹⁵N in adult stands of A. rubrum, American beech (Fagus grandifolia), black birch (Betula lenta), and red oak (Quercus rubra). Multiple regression analysis indicated that ambient atmospheric nitrogen dioxide (NO₂) concentration explained 0, 69, 23, and 45 % of the variation in foliar δ¹⁵N in American beech, red maple, red oak, and black birch, respectively, after accounting for the influence of soil δ¹⁵N. There was no correlation between foliar δ¹³C and foliar %N with increasing atmospheric NO₂ concentration in most species. Our findings suggest that total canopy uptake, and likely direct foliar N uptake, of pollution-derived atmospheric N deposition may significantly impact foliar δ¹⁵N in several dominant species occurring in temperate forest ecosystems.     

Needle-age related variability in nitrogen, mobile carbohydrates, and δ13C within Pinus koraiensis tree crowns

For both ecologists and physiologists, foliar physioecology as a function of spatially and temporally variable environmental factors such as sunlight exposure within a tree crown is important for understanding whole tree physiology and for predicting ecosystem carbon balance and productivity. Hence, we studied concentrations of nitrogen (N), non-structural carbohydrates (NSC = soluble sugars + starch), and δ¹³C in different-aged needles within Pinus koraiensis tree crowns, to understand the needle age- and crown position-related physiology, in order to test the hypothesis that concentrations of N, NSC, and δ¹³C are needle-age and crown position dependent (more light, more photosynthesis affecting N, NSC, and δ¹³C), and to develop an accurate sampling strategy. The present study indicated that the 1-yr-old needles had significantly higher concentration levels of mobile carbohydrates (both on a mass and an area basis) and N_area (on an area basis), as well as NSC-N ratios, but significantly lower levels of N_mass (on a mass basis) concentration and specific leaf area (SLA), compared to the current-year needles. Azimuthal (south-facing vs. north-facing crown side) effects were found to be significant on starch [both on a mass (ST_mass) and an area basis (ST_area)], δ¹³C values, and N_area, with higher levels in needles on the S-facing crown side than the N-facing crown side. Needle N_mass concentrations significantly decreased but needle ST_mass, ST_area, and δ¹³C values significantly increased with increasing vertical crown levels. Our results suggest that the sun-exposed crown position related to photosynthetic activity and water availability affects starch accumulation and carbon isotope discrimination. Needle age associated with physiological activity plays an important role in determining carbon and nitrogen physiology. The present study indicates that across-scale sampling needs to carefully select tissue samples with equal age from a comparable crown position.     

Drought responses of conifers in ecotone forests of northern Arizona: tree ring growth and leaf δ<Superscript>13</Superscript>C

We sought to understand differences in tree response to meteorological drought among species and soil types at two ecotone forests in northern Arizona, the pinyon-juniper woodland/ponderosa pine ecotone, and the higher elevation, wetter, ponderosa pine/mixed conifer ecotone. We used two approaches that provide different information about drought response: the ratio of standardized radial growth in wet years to dry years (W:D) for the period between years 1950 and 2000 as a measure of growth response to drought, and ¹³C in leaves formed in non-drought (2001) and drought (2002) years as a measure of change in water use efficiency (WUE) in response to drought. W:D and leaf ¹³C response to drought for Pinus edulis and P. ponderosa did not differ for trees growing on coarse-texture soils derived from cinders compared with finer textured soils derived from flow basalts or sedimentary rocks. P. ponderosa growing near its low elevation range limit at the pinyon-juniper woodland/ponderosa pine ecotone had a greater growth response to drought (higher W:D) and a larger increase in WUE in response to drought than co-occurring P. edulis growing near its high elevation range limit. P. flexilis and Pseudotsuga menziesii growing near their low elevation range limit at the ponderosa pine/mixed conifer ecotone had a larger growth response to drought than co-occurring P. ponderosa growing near its high elevation range limit. Increases in WUE in response to drought were similar for all species at the ponderosa pine/mixed conifer ecotone. Low elevation populations of P. ponderosa had greater growth response to drought than high-elevation populations, whereas populations had a similar increase in WUE in response to drought. Our findings of different responses to drought among co-occurring tree species and between low- and high-elevation populations are interpreted in the context of drought impacts on montane coniferous forests of the southwestern USA.     

Stipa tenacissima Does not Affect the Foliar δ13C and δ15N of Introduced Shrub Seedlings in a Mediterranean Semi-arid Steppe

Recent studies have shown that the tussock grass Stipa tenacissima L. facilitates the establishment of late-successional shrubs, in what constitutes the first documented case of facilitation of woody plants by grasses. With the aim of increasing our knowledge of this interaction, in the present study we investigated the effects of S. tenacissima on the foliar δ13C, δ15N, nitrogen concentration, and carbon: nitrogen ratio of introduced seedlings of Pistacia lentiscus L., Quercus coccifera L., and Medicago arborea L. in a semi-arid Mediterranean steppe. Six months after planting, the values of δ13C ranged between -26.9‰ and -29.6‰,whereas those of δ15N ranged between -1.9‰ and 2.7‰. The foliar C: N ratio ranged between 10.7 and 53.5,and the nitrogen concentration ranged between 1.0% and 4.4%. We found no significant effect of the microsite provided by S. tenacissima on these variables in any of the species evaluated. The values of δ13C were negatively correlated with predawn water potentials in M. arborea and were positively correlated with relative growth rate in Q. coccifera. The values of δ15N were positively correlated with the biomass allocation to roots in the latter species. The present results suggest that the modification of environmental conditions in the are surrounding S. tenacissima was not strong enough to modify the foliar isotopic and nitrogen concentration of shrubs during the early stages after planting.     

Diversity,metabolic types and δ13C carbon isotope ratios in the grass flora of Namibia in relation to growth form,precipitation and habitat conditions

The grass flora of Namibia (374 species in 110 genera) shows surprisingly little variation in ¹³C values along a rainfall gradient (50–600 mm) and in different habitat conditions. However, there are significant differences in the ¹³C values between the metabolic types of the C4 photosynthetic pathway. NADP-ME-type C4 species exhibit the highest ¹³C values (–11.7 ) and occur mainly in regions with high rainfall. NAD-ME-type C4 species have significantly lower ¹³C values (–13.4 ) and dominate in the most arid part of the precipitation regime. PCK-type C4 species play an intermediate role (–12.5 ) and reach a maximum abundance in areas of intermediate precipitation. This pattern is also evident in genera containing species of different metabolic types. Within the same genus NAD species reach more negative ¹³C values than PCK species and ¹³C values decreased with rainfall. Also in Aristida, with NADP-ME-type photosynthesis, ¹³C values decreased from –11 in the inland region (600 mm precipitation) to –15 near the coast (150 mm precipitation), which is a change in discrimination which is otherwise associated by a change in metabolism. The exceptional C3 species Eragrostis walteri and Panicum heterostachyum are coastal species experiencing 50 mm precipitation only. Many of the rare species and monotypic genera grow in moist habitats rather than in the desert, and they are not different in their carbon isotope ratios from the more common flora. The role of species diversity with respect to habitat occupation and carbon metabolism is discussed.     

Understanding causes of tree growth response to gap formation: ∆13C-values in tree rings reveal a predominant effect of light

Key message

Carbon isotope ratios in growth rings of a tropical tree species show that treefall gaps stimulate diameter growth mainly through changes in the availability of light and not water. The formation of treefall gaps in closed canopy forests usually entails considerable increases in light and nutrient availability for remaining trees, as well as altered plant water availability, and is considered to play a key role in tree demography. The effects of gaps on tree growth are highly variable and while usually stimulatory they may also include growth reductions. In most studies, the causes of changes in tree growth rates after gap formation remain unknown. We used changes in carbon isotope ¹³C discrimination (Δ¹³C) in annual growth rings to understand growth responses after gap formation of Peltogyne cf. heterophylla, in a moist forest of Northern Bolivia. We compared growth and Δ¹³C of the 7 years before and after gap formation. Forty-two trees of different sizes were studied, half of which grew close (<10 m) to single treefall gaps (gap trees), the other half more than 40 m away from gaps (controls). We found variable responses among gap trees in growth and Δ¹³C. Increased growth was mainly associated with decreased Δ¹³C, suggesting that the growth response was driven by increased light availability, possibly in combination with improved nutrient availability. Most trees showing zero or negative growth change after gap formation had increased Δ¹³C, suggesting that increased water stress did not play a role, but rather that light conditions had not changed much or nutrient availability was insufficient to support increased growth. Combining growth rates with Δ¹³C proved to be a valuable tool to identify the causes of temporal variation in tree growth.     

Foliar δ13C within a temperate deciduous forest: spatial,temporal, and species sources of variation

Summary Foliar ¹³C-abundance (¹³C) was analyzed in the dominant trees of a temperate deciduous forest in east Tennessee (Walker Branch Watershed) to investigate the variation in foliar ¹³C as a function of time (within-year and between years), space (canopy height, watershed topography and habitat) and species (deciduous and coniferous taxa). Various hypotheses were tested by analyzing (i) samples collected from the field during the growing season and (ii) foliar tissues maintained in an archived collection. The ¹³C-value for leaves from the tops of trees was 2 to 3%. more positive than for leaves sampled at lower heights in the canopy. Quercus prinus leaves sampled just prior to autumn leaf fall had significantly more negative ¹³C-values than those sampled during midsummer. On the more xeric ridges, needles of Pinus spp. had more positive ¹³C-values than leaves from deciduous species. Foliar ¹³C-values differed significantly as a function of topography. Deciduous leaves from xeric sites (ridges and slopes) had more positive ¹³C-values than those from mesic (riparian and cove) environments. On the more xeric sites, foliar ¹³C was significantly more positive in 1988 (a dry year) relative to that in 1989 (a year with above-normal precipitation). In contrast, leaf ¹³C in trees from mesic valley bottoms did not differ significantly among years with disparate precipitation. Patterns in foliar ¹³C indicated a higher ratio of net CO₂ assimilation to transpiration (A/E) for trees in more xeric versus mesic habitats, and for trees in xeric habitats during years of drought versus years of normal precipitation. However, A/E (units of mmol CO₂ fixed/mol H₂O transpired) calculated on the basis of ¹³C-values for leaves from the more xeric sites was higher in a wet year (6.6±1.2) versus a dry year (3.4±0.4). This difference was attributed to higher transpiration (and therefore lower A/E) in the year with lower relative humidity and higher average daily temperature. The calculated A/E values for the forest in 1988–89, based on ¹³C, were within ±55% of estimates made over a 17 day period at this site in 1984 using micrometeorological methods.     

Analysis of δ13C of CO2 distinguishes between microbial respiration of added C4-sucrose and other soil respiration in a C3-ecosystem

The main aim of this study was to test various hypotheses regarding the changes in ¹³C of emitted CO₂ that follow the addition of C₄-sucrose to the soil of a C₃-ecosystem. It forms part of an experimental series designed to assess whether or not the contributions from C₃-respiration (root and microbial) and C₄-respiration (microbial) to total soil respiration can be calculated from such changes. A series of five experiments, three on sieved (root-free) mor-layer material, and two in the field with intact mor-layer (and consequently with active roots), were performed. Both in the experiments on sieved mor-layer and the field experiments, we found a C₄-sucrose-induced increase in C₃-respiration that accounted for between 30% and 40% of the respiration increase 1 h after sucrose addition. When the course of C₃-, C₄- and total respiration was followed in sieved material over four days following addition of C₄-sucrose, the initially increased respiration of C₃-C was transient, passing within less than 24 h. In a separate pot experiment, neither ectomycorrhizal Pinus sylvestrisL. roots nor non-mycorrhizal roots of this species showed respiratory changes in response to exogenous sucrose. No shift in the ¹³C of the evolved CO₂ after adding C₃-sucrose to sieved mor-layer material was found, confirming that the sucrose-induced increase in respiration of endogenous C was not an artefact of discrimination against ¹³C during respiration. Furthermore, we conclude that the C₄-sucrose induced transient increase in C₃-respiration is most likely the result of accelerated turnover of C in the microbial biomass. Thus, neither respiration of mycorrhizal roots, nor processes discriminating against ¹³C were likely sources of error in the field. The estimated ¹³C of evolved soil CO₂ in three field experiments lay between –25.2 and –23.6. The study shows that we can distinguish between CO₂ evolved from microbial mineralisation of added C₄-sucrose, and CO₂ evolved from endogenous carbon sources (roots and microbial respiration).     

Relationship between δ13C and photosynthetic parameters and their responses to leaf nitrogen content in six broadleaf tree species

Stable carbon isotope composition (δ¹³C), net photosynthetic rate (P _N), actual quantum yield of photosystem 2 (PS2) electron transport (ΦPS2), nitrogen content (N_c), and photosynthetic nitrogen use efficiency (PNUE) in the leaves of six broadleaf tree species were determined under field environmental conditions. The six tree species were Magnolia liliflora Desr., M. grandiflora Linn., M. denudata Desr., Prunus mume (Sieb.) Sieb. et Zucc. cv. Meiren Men, P. mume (Sieb.) Sieb. et Zucc. f. alphandii (Carr.) Rehd., and P. persica (L.) Batsch. var. rubro-plena. The relationships among δ¹³C, Φ_PS2, P _N, and PNUE, as well as their responses to N_c in the six species were also studied. Both P _N and δ¹³C negatively correlated with N_c, but Φ_PS2 positively correlated with N_c. This indicated that with N_c increase, P _N and δ¹³C decreased, while Φ_PS2 increased. There were weak negative correlations between δ¹³C and PNUE, and strong negative correlations (p<0.01) between Φ_PS2 and PNUE. According to the variance analysis of parameters, there existed significant interspecific differences (p<0.001) of δ¹³C, P _N, Φ_PS2, PNUE, and N_c among the tree seedlings of the six tree species, which suggests that the potential photosynthetic capacities depend on plant species, irradiance, and water use capacity under field conditions.     

Stipa tenacissima Does not Affect the Foliar δ13C and δ15N of Introduced Shrub Seedlings in a Mediterranean Semi-arid Steppe

Recent studies have shown that the tussock grass Stipa tenacissima L. facilitates the establishment of late-successional shrubs, in what constitutes the first documented case of facilitation of woody plants by grasses. With the aim of increasing our knowledge of this interaction, in the present study we investigated the effects of S. tenacissima on the foliar δ13C, δ15N, nitrogen concentration, and carbon ： nitrogen ratio of introduced seedlings of Pistacia lentiscus L., Quercus coccifera L., and Medicago arborea L. in a semi-arid Mediterranean steppe. Six months after planting, the values of δ13C ranged between -26.9‰ and -29.6‰, whereas those of δ15N ranged between -1.9‰ and 2.7‰. The foliar C ： N ratio ranged between 10.7 and 53.5, and the nitrogen concentration ranged between 1.0% and 4.4%. We found no significant effect of the microsite provided by S. tenacissima on these variables in any of the species evaluated. The values of δ13C were negatively correlated with predawn water potentials in M. arborea and were positively correlated with relative growth rate in Q. coccifera. The values of δ15N were positively correlated with the biomass allocation to roots in the latter species. The present results suggest that the modification of environmental conditions in the are surrounding S. tenacissima was not strong enough to modify the foliar isotopic and nitrogen concentration of shrubs during the early stages after planting.     

The Characteristics of Foliar δ 13C Values of Plants and Plant Water Use Efficiency Indicated by δ13C Values in Two Fragmented Rainforests in Xishuangbanna,Yunnan

The foliar δ13 C values of 226 plant species sampled from 7 transects belonging to 2 rainforests (named Chengzi and Bubeng, respectively) in Xishuangbanna of Yunnan Province, China were analyzed. The characteristics of foliar δ13 C values and water use efficiency (WUE) of plants indicated by δ13 C values in this region were studied. In the mean time, the average foliar δ13 C values of plants in different transects and with different growth forms (i.e., tree, shrub, herb and liana) were compared. The results showed that: 1. the foliarδ13 C value in Xishuangbanna, ranging from -3.865%--2.760%, was similar to those of other typical rainforests in the world (Amazon and Puerto Rico, for instance), which indicated that the rainforests in this region had the typical characteristics of rainforests of the world; 2. there were significant differences between Chengzi and Bubeng in average foliarδ13 C values, with the latter being lower than that of the former; significant differences also existed between different transects in the same forest, which indicated that water conditions and orientation of transects might have resulted in these differences; 3. the average foliarδ13 C value of evergreen plants was significantly lower than that of deciduous plants, and there also existed significant differences between different growth forms, implying that the WUE of these plant species was also quite different.      

White spruce foliar δ13C and δ15N indicate changed soil N availability by understory removal and N fertilization in a 13-year-old boreal plantation

Background and Aims

Phosphorus (P) mineralisation from crop residues is usually predicted from total P or carbon: phosphorus (C: P) ratios. However, these measures have limited accuracy as they do not take into account the presence of different P species that may be mineralised at different rates. In this study P speciation was determined using solution ³¹P nuclear magnetic resonance (NMR) spectroscopy to understand the potential fate of residue P in soils.

Methods

Mature above-ground biomass of eight different crops sampled from the field was portioned into stem, chaff and seed.

Results

The main forms of P detected in stem and chaff were orthophosphate (25–75 %), phospholipids (10–40 %) and RNA (5–30 %). Phytate was the dominant P species in seeds, and constituted up to 45 % of total P in chaff but was only detected in minor amounts (<1 %) in stem residue. The majority (65–95 %) of P in stems was water-extractable, and most of this was detected as orthophosphate. However, this includes organic P that may have been hydrolysed during the water extraction.

Conclusions

This study indicates that the majority of residue P in aboveground plant residues has the potential to be delivered to soil in a form readily available to plants and soil microorganisms.

     

Climatic implications of δ13C and δ18O ratios from C3 and C4 plants growing in a tropical montane habitat in southern India

The stable carbon and oxygen isotope ratios in cellulose of C3 and C4 plants growing on the surface of a montane peat bog in the Nilgiri hills, southern India, were measured. The mean monthly δ¹³C values in cellulose of both C3 and C4 plants are found to be significantly related to rainfall, while the δ¹⁸O values are sensitive to changes in maximum temperature and relative humidity of the region. Further, higher δ¹⁸O values were observed in C4 plants compared to C3 plants, suggesting that C4 plants are probably less sensitive to relative humidity as compared to C3 plants and are able to photosynthesize even during drier conditions. The plant isotope-climate correlations thus established can be used for reconstructing the past temperature and rainfall conditions of the tropics from the isotopic ratios of peat deposits, derived from a mixture of C3 and C4 plants in the region.     

N:P ratios, δN fractionation and nutrient resorption along a nitrogen to phosphorus limitation gradient in an oligotrophic wetland complex

The vegetation N:P ratio is thought to be a diagnostic indicator of the nature of nutrient limitation in wetland vegetation. It should therefore be closely linked to other indicators of nutrient acquisition and conservation, such as nitrogen stable isotope fractionation (δ¹⁵N), nutrient resorption efficiency (RE) and resorption proficiency (RP). However, the interrelationships among these traits and the N:P ratio remain unclear. We compared tissue nutrient concentrations, N:P ratios, δ¹⁵N fractionation, RE, and RP along an N to P limitation gradient in an oligotrophic wetland valley in the South Island of New Zealand. Within the valley, the soil TN:TP ratio increased from 1.3 to 18.0 in three discrete wetlands along the gradient. In pooled data from all vegetation communities within each site, the mass-based vegetation N:P ratio correlated significantly (r² = 0.35, P < 0.01) to soil TN:TP ratios and increased from 10.2 ± 2.7 to 13.5 ± 3.6 along the N to P limitation gradient. This was accompanied by an increase in tissue δ¹⁵N enrichment from 2.05 ± 1.12‰ to 6.27 ± 1.70‰, consistent with more open N cycling and lower N demand. These trends held within all vegetation types, but were particularly strong in a Typha orientalis (C-strategist) community (soil TN:TP vs vegetation N:P correlation r² = 0.78, P < 0.001; δ¹⁵N increase from 1.81 ± 0.44‰ to 7.73 ± 1.79‰). The individual N and P concentrations and retention patterns were more species-specific and less responsive to the nutrient limitation gradient. T. orientalis maximised N resorption as N limitation increased (increasing NRE from 50.8 ± 3.3% to 71.7 ± 7.4%; reducing NRP from 0.70 ± 0.12% to 0.36 ± 0.13%) but did not alter PRE or PRP, whereas the S-strategist Schoenus pauciflorus maximised P resorption as P limitation increased (increasing PRE from 48.0 ± 5.6% to 73.5 ± 10.1%; reducing PRP from 0.053 ± 0.008% to 0.015 ± 0.004%) but did not alter NRE or NRP. These results show that the tissue N:P ratio and its associated δ¹⁵N enrichment are highly responsive indicators of the relative availability of N and P at the site and community level. However, they are not indicators of species-specific physiological requirements for N and P, or of likely responses of individual species to N or P enrichment, which are better interpreted from indicators such as RE and RP that describe nutrient retention behaviour.     

Mull and mor (müller-hesselman) in relation to the soil water regime of a forest

Summary The distribution of mull and mor, in an area which is believed to have borne forest for a number of centuries at least, led to investigations into the reasons for their development.Although the term forest-floor type has been substituted for humus type, mull and mor are used in the Müller-Hesselman sense.There appeared to be no obvious relationship between the distribution of the various forest-floor subtypes and the results of chemical analyses and pH determinations on soil samples.Where free CaCO₃ occurred in the A₁-horizon, a mull forest floor had developed even on coarse sand and the associated herb flora differed only slightly from that occurring on very acid mull. As the manner in which lime influences most soil processes is still unknown, the areas in which free CaCO₃ occurs were excluded from the investigations.Assessments of various other soil characteristics were made and the frequency of association of the various categories of these characteristics ascertained.It is concluded that the colour of the upper part of the A₁-horizon and its degree of leaching are related to its texture, and that the forest-floor subtypes are more closely related to soil texture than to soil colour or degree of leaching.     

Effects of climate, tree age, dominance and growth on δ15N in young pinewoods

Needles, annual rings from basal stem discs and bark of three dominant and three suppressed Pinus pinaster from a 12-year-old pine stand (naturally regenerated after a wildfire) were analysed to study the effects of climate, tree age, dominance, and growth on tree δ¹⁵N. Foliar-N concentration in dominant pines (0.780–1.474% N) suggested that soil N availability was sufficient, a circumstance that allowed isotopic discrimination by plants and (greater) differences in δ¹⁵N among trees. The δ¹⁵N decreases in the order wood (−0.20 to +6.12‰), bark (−1.84 to +1.85‰) and needles (−2.13 to +0.77‰). In all trees, before dominance establishment (years 1–8), the N stored in each ring displayed a decreasing δ¹⁵N tendency as the tree grows, which is mainly due to a more “closed” N cycle or an increasing importance of N sources with lower δ¹⁵N. After dominance establishment (years 9–12), wood δ¹⁵N values were higher in suppressed than in dominant trees (2.62 and 1.46‰, respectively; P < 0.01) while the reverse was true for needles and bark; simultaneously, the absolute amount of N stored by suppressed pines in successive rings decreased, suggesting a lower soil N assimilation. These results could be explained by lignification acting as major N source for needles in suppressed pines because products released and reallocated during lignification are ¹⁵N-depleted compared with the source. According to principal component analysis, wood δ¹⁵N appears associated with wood N concentration and precipitation during the growing season, but clearly opposed to age, basal area increment and mean temperature in spring and summer.