Precipitation pulse use by an invasive woody legume: the role of soil texture and pulse size

Plant metabolic activity in arid and semi-arid environments is largely tied to episodic precipitation events or “pulses”. The ability of plants to take up and utilize rain pulses during the growing season in these water-limited ecosystems is determined in part by pulse timing, intensity and amount, and by hydrological properties of the soil that translate precipitation into plant-available soil moisture. We assessed the sensitivity of an invasive woody plant, velvet mesquite (Prosopis velutina Woot.), to large (35 mm) and small (10 mm) isotopically labeled irrigation pulses on two contrasting soil textures (sandy-loam vs. loamy-clay) in semi-desert grassland in southeastern Arizona, USA. Predawn leaf water potential (Ψ_pd), the isotopic abundance of deuterium in stem water (δD), the abundance of ¹³C in soluble leaf sugar (δ¹³C), and percent volumetric soil water content (θ_v) were measured prior to irrigation and repeatedly for 2 weeks following irrigation. Plant water potential and the percent of pulse water present in the stem xylem indicated that although mesquite trees on both coarse- and fine-textured soils quickly responded to the large irrigation pulse, the magnitude and duration of this response substantially differed between soil textures. After reaching a maximum 4 days after the irrigation, the fraction of pulse water in stem xylem decreased more rapidly on the loamy-clay soil than the sandy-loam soil. Similarly, on both soil textures mesquite significantly responded to the 10-mm pulse. However, the magnitude of this response was substantially greater for mesquite on the sandy-loam soil compared to loamy-clay soil. The relationship between Ψ_pd and δ¹³C of leaf-soluble carbohydrates over the pulse period did not differ between plants at the two sites, indicating that differences in photosynthetic response of mesquite trees to the moisture pulses was a function of soil water availability within the rooting zone rather than differences in plant biochemical or physiological constraints. Patterns of resource acquisition by mesquite during the dynamic wetting–drying cycle following rainfall pulses is controlled by a complex interaction between pulse size and soil hydraulic properties. A better understanding of how this interaction affects plant water availability and photosynthetic response is needed to predict how grassland structure and function will respond to climate change.     

Temporal and spatial partitioning of water resources among eight woody species in a Hawaiian dry forest

Lowland dry forests are unique in Hawaii for their high diversity of tree species compared with wet forests. We characterized spatial and temporal partitioning of soil water resources among seven indigenous and one invasive dry forest species to determine whether the degree of partitioning was consistent with the relatively high species richness in these forests. Patterns of water utilization were inferred from stable hydrogen isotope ratios (δD) of soil and xylem water, zones of soil water depletion, plant water status, leaf phenology, and spatial patterns of species distribution. Soil water δD values ranged from –20‰ near the surface to –48‰ at 130 cm depth. Metrosideros polymorpha, an evergreen species, and Reynoldsia sandwicensis, a drought-deciduous species, had xylem sap δD values of about –52‰, and appeared to obtain their water largely from deeper soil layers. The remaining six species had xylem δD values ranging from –33 to –42‰, and apparently obtained water from shallower soil layers. Xylem water δD values were negatively correlated with minimum annual leaf water potential and positively correlated with leaf solute content, an integrated measure of leaf water deficit. Seasonal patterns of leaf production ranged from dry season deciduous at one extreme to evergreen with near constant leaf expansion rates at the other. Species tapping water more actively from deeper soil layers tended to exhibit larger seasonality of leaf production than species relying on shallower soil water sources. Individuals of Myoporum sandwicense were more spatially isolated than would be expected by chance. Even though this species apparently extracted water primarily from shallow soil layers, as indicated by its xylem δD values, its nearly constant growth rates across all seasons may have been the result of a larger volume of soil water available per individual. The two dominant species, Diospyros sandwicensis and Nestegis sandwicensis, exhibited low leaf water potentials during the dry season and apparently drew water mostly from the upper portion of the soil profile, which may have allowed them to exploit light precipitation events more effectively than the more deeply rooted species. Character displacement in spatial and temporal patterns of soil water uptake was consistent with the relatively high diversity of woody species in Hawaiian dry forests. Received: 20 May 1999 / Accepted: 2 March 2000     

Silicon Isotopic Fractionation by Banana (Musa spp.) Grown in a Continuous Nutrient Flow Device

The determination of the plant-induced Si-isotopic fractionation is a promising tool to better quantify their role in the continental Si cycle. Si-isotopic signatures of the different banana plant parts and Si source were measured, providing the isotopic fractionation factor between plant and source. Banana plantlets (Musa acuminata Colla, cv Grande Naine) were grown in hydroponics at variable Si supplies (0.08, 0.42, 0.83 and 1.66 mM Si). Si-isotopic compositions were determined on a multicollector plasma source mass spectrometer (MC-ICP-MS) operating in dry plasma mode. Results are expressed as δ²⁹Si relative to the NBS28 standard, with an average precision of ± 0.08‰ (±2σ_D). The fractionation factor ²⁹ε between bulk banana plantlets and source solution is −0.40 ± 0.11‰. This confirms that plants fractionate Si isotopes by depleting the source solution in ²⁸Si. The intra-plant fractionation Δ²⁹Si between roots and shoots amounts to −0.21 ± 0.08‰. Si-isotopic compositions of the various plant parts indicate that heavy isotopes discrimination occurs at three levels in the plant (at the root epidermis, for xylem loading and for xylem unloading). At each step, preferential crossing of light isotopes leaves a heavier solution, and produces a lighter solution. Si-isotopic fractionation processes are further discussed in relation with Si uptake and transport in plants. These findings have important implications on the study of continental Si cycle.     

Interspecific variability of δ13C among trees in rainforests of French Guiana: functional groups and canopy integration

The interspecific variability of sunlit leaf carbon isotope composition (δ¹³C), an indicator of leaf intrinsic water-use efficiency (WUE, CO₂ assimilation rate/leaf conductance for water vapour), was investigated in canopy trees of three lowland rainforest stands in French Guiana, differing in floristic composition and in soil drainage characteristics, but subjected to similar climatic conditions. We sampled leaves with a rifle from 406 trees in total, representing 102 species. Eighteen species were common to the three stands. Mean species δ¹³C varied over a 6.0‰ range within each stand, corresponding to WUE varying over about a threefold range. Species occurring in at least two stands displayed remarkably stable δ¹³C values, suggesting a close genetic control of species δ¹³C. Marked differences in species δ¹³C values were found with respect to: (1) the leaf phenology pattern (average δ¹³C=–29.7‰ and –31.0‰ in deciduous-leaved and evergreen-leaved species, respectively), and (2) different types of shade tolerance defined by features reflecting the plasticity of growth dynamics with respect to contrasting light conditions. Heliophilic species exhibited more negative δ¹³C values (average δ¹³C=–30.5‰) (i.e. lower WUE) than hemitolerant species (–29.3‰). However, tolerant species (–31.4‰) displayed even more negative δ¹³C values than heliophilic ones. We could not provide a straightforward ecophysiological interpretation of this result. The negative relationship found between species δ¹³C and midday leaf water potential (Ψ_wm) suggests that low δ¹³C is associated with high whole tree leaf specific hydraulic conductance. Canopy carbon isotope discrimination (Δ _A ) calculated from the basal area-weighed integral of the species δ¹³C values was similar in the three stands (average Δ _A =23.1‰), despite differences in stand species composition and soil drainage type, reflecting the similar proportions of the three different shade-tolerance types among stands. Received: 30 November 1999 / Accepted: 23 March 2000     

Carbon isotope composition of boreal plants: functional grouping of life forms

 We tested the hypothesis that life forms (trees, shrubs, forbs, and mosses; deciduous or evergreen) can be used to group plants with similar physiological characteristics. Carbon isotope ratios (δ¹³C) and carbon isotope discrimination (Δ) were used as functional characteristics because δ¹³C and Δ integrate information about CO₂ and water fluxes, and so are useful in global change and scaling studies. We examined δ¹³C values of the dominant species in three boreal forest ecosystems: wet Picea mariana stands, mesic Populus tremuloides stands, and dry Pinus banksiana stands. Life form groups explained a significant fraction of the variation in leaf carbon isotope composition; seven life-form categories explained 50% of the variation in δ¹³C and 42% of the variation in Δ and 52% of the variance not due to intraspecific genetic differences (n=335). The life forms were ranked in the following order based on their values: evergreen trees<deciduous trees=evergreen and deciduous shrubs=evergreen forbs<deciduous forbs=mosses. This ranking of the life forms differed between deciduous (Populus) and evergreen (Pinus and Picea) ecosystems. Furthermore, life forms in the Populus ecosystem had higher discrimination values than life forms in the dry Pinus ecosystem; the Picea ecosystem had intermediate Δ values. These correlations between Δ and life form were related to differences in plant stature and leaf longevity. Shorter plants had lower Δ values than taller plants, resulting from reduced light intensity at lower levels in the forest. After height differences were accounted for, deciduous leaves had higher discrimination values than evergreen leaves, indicating that deciduous leaves maintained higher ratios of intracellular to ambient CO₂ (c _i/c _a) than did evergreen leaves in a similar environment within these boreal ecosystems. We found the same pattern of carbon isotope discrimination in a year with above-average precipitation as in a year with below-average precipitation, indicating that environmental fluctuations did not affect the ranking of life forms. Furthermore, plants from sites near the northern and southern boundaries of the boreal forest had similar patterns of discrimination. We concluded that life forms are robust indicators of functional groups that are related to carbon and water fluxes within boreal ecosystems. Received: 15 April 1996 / Accepted: 16 November 1996     

Nocturnal and seasonal patterns of carbon isotope composition of leaf dark-respired carbon dioxide differ among dominant species in a semiarid savanna

The C isotope composition of leaf dark-respired CO₂ (δ¹³C_l) integrates short-term metabolic responses to environmental change and is potentially recorded in the isotopic signature of ecosystem-level respiration. Species differences in photosynthetic pathway, resource acquisition and allocation patterns, and associated isotopic fractionations at metabolic branch points can influence δ¹³C_l, and differences are likely to be modified by seasonal variation in drought intensity. We measured δ¹³C_l in two deep-rooted C₃ trees (Prosopis velutina and Celtis reticulata), and two relatively shallow-rooted perennial herbs (a C₃ dicot Viguiera dentata and a C₄ grass Sporobolus wrightii) in a floodplain savanna ecosystem in southeastern Arizona, USA during the dry pre-monsoon and wet monsoon seasons. δ¹³C_l decreased during the nighttime and reached minimum values at pre-dawn in all species. The magnitude of nocturnal shift in δ¹³C_l differed among species and between pre-monsoon and monsoon seasons. During the pre-monsoon season, the magnitude of the nocturnal shift in δ¹³C_l in the deep-rooted C₃ trees P. velutina (2.8 ± 0.4‰) and C. reticulata (2.9 ± 0.2‰) was greater than in the C₃ herb V. dentata (1.8 ± 0.4‰) and C₄ grass S. wrightii (2.2 ± 0.4‰). The nocturnal shift in δ¹³C_l in V. dentata and S. wrightii increased to 3.2 ± 0.1‰ and 4.6 ± 0.6‰, respectively, during the monsoon season, but in C₃ trees did not change significantly from pre-monsoon values. Cumulative daytime net CO₂ uptake was positively correlated with the magnitude of the nocturnal decline in δ¹³C_l across all species, suggesting that nocturnal δ¹³C_l may be controlled by ¹³C/¹²C fractionations associated with C substrate availability and C metabolite partitioning. Nocturnal patterns of δ¹³C_l in dominant plant species in the semiarid savanna apparently have predictable responses to seasonal changes in water availability, which is important for interpreting and modeling the C isotope signature of ecosystem-respired CO₂.     

Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions

Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δ¹⁵N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δ¹⁵N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δ¹⁵N varied widely, between 0 and –2‰ for nitrogen-fixing species, between 0 and –7‰ for deciduous non-fixing species, and between 0 and –11‰ for coniferous species. Relatively constant δ¹⁵N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δ¹⁵N differences among species at individual sites. In fact, the largest variation among plant δ¹⁵N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δ¹⁵N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δ¹⁵N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T _r). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δ¹⁵N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δ¹⁵N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T _r. Received: 14 December 1998 / Accepted: 16 August 1999     

Indications of Hydraulic Lift by Pinus halepensis and Its Effects on the Water Relations of Neighbour Shrubs

We measured the stable deuterium isotopic composition of xylem sap, the shoot predawn and midday water potentials, and the leaf δ¹³C of Mediterranean shrubs Pistacia lentiscus, Globularia alypum and Rosmarinus officinalis in a south-oriented transect from a large (12 m tall) Aleppo pine tree, Pinus halepensis. We aimed to study the possibility of hydraulic lift from the deep roots of this pine tree to the shallow soil layers and its influence on these neighbour shrubs. These same traits were also studied in several individuals of the shrub Pistacia lentiscus growing with different types of neighbours: just shrubs, a small (3 – 4 m) pine tree, and the above mentioned large pine tree. The greater the distance from P. halepensis the plants grew, the higher xylem water δD, the lower the soil water content, and, the lower the predawn and midday water potentials were found. These results suggest the existence of an hydraulic lift from deep roots to shallow soil in this big tree. Further indication of this existence is provided by the improved water status of P. lentiscus (higher water potentials and δD, and lower δ¹³C and, therefore, lower water use efficiencies) when growing close to the big pine in comparison with the same shrub species growing close to small pines or just surrounded by other shrubs. Moreover, all these trends occurred in the dry summer season, but disappeared in the wet spring season. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stable isotope characteristics across narrow savanna/woodland ecotones in Wolfe Creek Meteorite Crater,Western Australia

The stable isotopic composition (δ¹³C) of sediments from lakes are frequently analyzed to reconstruct the proportion of the regional vegetation that used either the C₃ or C₄ photosynthetic pathways, often without conducting a detailed survey of the current local vegetation. We performed a study on the modern vegetation composition within the Wolfe Creek Meteorite Crater to complement our future paleoecological investigation of the crater. A bull’s-eye pattern exists where C₄ grasses dominate an outer ring and salt tolerant species, including shrubs, herbs, chenopods, and halophytic algae, dominate the inner pan of the crater. The ecotone between the inner and outer zones is narrow and occupied by tall (>7 m) Acacia ampliceps, with some C₄ grasses in the understory. Along with the highest water table and most saline soils the center of the crater has C₃ plants present with the highest δ¹³C and δ¹⁵N values. The range of δ¹³C and δ¹⁵N values from the analysis of surface soil organic matter (OM) was much smaller compared with the range of values from plant materials implying that either: (1) the current plant OM has not yet been integrated into the soils, or (2) processes within the soil have acted to homogenize isotopic variability within the crater. The application of a two end member mixing model to calculate %C₄ and %C₃ biomass from the δ¹³C of surface soil OM was complicated by: (1) the crater containing both a dry habitat with C₄ grasses and a central pan with C₄ halophytic plants and, (2) the large variation in the δ¹³C of the plants and soil OM.     

Partitioning of soil water among canopy trees in a seasonally dry tropical forest

Little is known about partitioning of soil water resources in species-rich, seasonally dry tropical forests. We assessed spatial and temporal patterns of soil water utilization in several canopy tree species on Barro Colorado Island, Panama, during the 1997 dry season. Stable hydrogen isotope composition (δD) of xylem and soil water, soil volumetric water content (θ_v), and sap flow were measured concurrently. Evaporative fractionation near the soil surface caused soil water δD to decrease from about –15‰ at 0.1 m to –50 to –55‰ at 1.2 m depth. Groundwater sampled at the sources of nearby springs during this period yielded an average δD value of –60‰. θ_v increased sharply and nearly linearly with depth to 0.7 m, then increased more slowly between 0.7 and 1.05 m. Based on xylem δD values, water uptake in some individual plants appeared to be restricted largely to the upper 20 cm of the soil profile where θ_v dropped below 20% during the dry season. In contrast, other individuals appeared to have access to water at depths greater than 1 m where θ_v remained above 45% throughout the dry season. The depths of water sources for trees with intermediate xylem δD values were less certain because variation in soil water δD between 20 and 70 cm was relatively small. Xylem water δD was also strongly dependent on tree size (diameter at breast height), with smaller trees appearing to preferentially tap deeper sources of soil water than larger trees. This relationship appeared to be species independent. Trees able to exploit progressively deeper sources of soil water during the dry season, as indicated by increasingly negative xylem δD values, were also able to maintain constant or even increase rates of water use. Seasonal courses of water use and soil water partitioning were associated with leaf phenology. Species with the smallest seasonal variability in leaf fall were also able to tap increasingly deep sources of soil water as the dry season progressed. Comparison of xylem, soil, and groundwater δD values thus pointed to spatial and temporal partitioning of water resources among several tropical forest canopy tree species during the dry season. Received: 5 October 1998 / Accepted: 23 June 1999