Effect of precipitation variability on net primary production and soil respiration in a Chihuahuan Desert grassland

Precipitation regimes are predicted to become more variable with more extreme rainfall events punctuated by longer intervening dry periods. Water‐limited ecosystems are likely to be highly responsive to altered precipitation regimes. The bucket model predicts that increased precipitation variability will reduce soil moisture stress and increase primary productivity and soil respiration in aridland ecosystems. To test this hypothesis, we experimentally altered the size and frequency of precipitation events during the summer monsoon (July through September) in 2007 and 2008 in a northern Chihuahuan Desert grassland in central New Mexico, USA. Treatments included (1) ambient rain, (2) ambient rain plus one 20 mm rain event each month, and (3) ambient rain plus four 5 mm rain events each month. Throughout two monsoon seasons, we measured soil temperature, soil moisture content (θ), soil respiration (R_s), along with leaf‐level photosynthesis (A_net), predawn leaf water potential (Ψ_pd), and seasonal aboveground net primary productivity (ANPP) of the dominant C₄ grass, Bouteloua eriopoda. Treatment plots receiving a single large rainfall event each month maintained significantly higher seasonal soil θ which corresponded with a significant increase in R_s and ANPP of B. eriopoda when compared with plots receiving multiple small events. Because the strength of these patterns differed between years, we propose a modification of the bucket model in which both the mean and variance of soil water change as a consequence of interannual variability from 1 year to the next. Our results demonstrate that aridland ecosystems are highly sensitive to increased precipitation variability, and that more extreme precipitation events will likely have a positive impact on some aridland ecosystem processes important for the carbon cycle.     

A new precipitation technique provides evidence for the permeability of Casparian bands to ions in young roots of corn (Zea mays L.) and rice (Oryza sativa L.)

Using an insoluble inorganic salt precipitation technique, the permeability of cell walls and especially of endodermal Casparian bands (CBs) for ions was tested in young roots of corn (Zea mays) and rice (Oryza sativa). The test was based on suction of either 100 µm CuSO₄ or 200 µm K₄[Fe(CN)₆] into the root from its medium using a pump (excised roots) or transpirational stream (intact seedlings), and subsequent perfusion of xylem of those root segments with the opposite salt component, which resulted in precipitation of insoluble brown crystals of copper ferrocyanide. Under suction, Cu²⁺ could cross the endodermis apoplastically in both plant species (although at low rates) developing brown salt precipitates in cell walls of early metaxylem and in the region between CBs and functioning metaxylem vessels. Hence, at least Cu²⁺ did cross the endodermis dragged along with the water. The results suggested that CBs were not perfect barriers to apoplastic ion fluxes. In contrast, ferrocyanide ions failed to cross the mature endodermis of both corn and rice at detectable amounts. The concentration limit of apoplastic copper was 0.8 µm at a perfusion with 200 µm K₄[Fe(CN)₆]. Asymmetric development of precipitates suggested that the cation, Cu²⁺, moved faster than the anion, [Fe(CN)₆]^4–, through cell walls including CBs. Using Chara cell wall preparations (‘ghosts’) as a model system, it was observed that, different from Cu²⁺, ferrocyanide ions remained inside wall-tubes suggesting a substantially lower permeability of the latter which agreed with the finding of an asymmetric development of precipitates. In both corn and rice roots, there was a significant apoplastic flux of ions in regions where laterals penetrated the endodermis. Overall, the results show that the permeability of CBs to ions is not zero. CBs do not represent a perfect barrier for ions, as is usually thought. The permeability of CBs may vary depending on growth conditions which are known to affect the intensity of formation of bands.     

Comparative life‐history traits in a fig wasp community: implications for community structure

1. Whether life‐history traits can determine community composition and structure is an important question that has been well explored theoretically, but has received scant empirical attention. Life‐history traits of a seven‐member community of galler and parasitoid fig wasp species (Chalcidoidea), developing within the inflorescences (syconia) of Ficus racemosa (Moraceae) in India, were determined and used to examine community structure and ecology. 2. Gallers were pro‐ovigenic (all eggs are mature upon adult emergence) whereas parasitoids were synovigenic (eggs mature progressively during adult lifespan). Initial egg load was correlated with body size for some species, and there was a trade‐off between egg number and egg size across all species. Although all species completed their development and left the syconium concurrently, they differed in their adult and pre‐adult lifespans. Providing sucrose solutions increased parasitoid lifespan but had no effect on the longevity of some galler species. While feeding regimes and body size affected longevity in most species, an interaction effect between these variables was detected for only one species. 3. Life‐history traits of wasp species exhibited a continuum in relation to their arrival sequence at syconia for oviposition during syconium development, and therefore reflected their ecology. The largest number of eggs, smallest egg sizes, and shortest longevities were characteristic of the earliest‐arriving galling wasps at the smallest, immature syconia; the converse characterised the later‐arriving parasitoids at the larger, already parasitised syconia. Thus life history is an important correlate of community resource partitioning and can be used to understand community structure. 4. This is the first comprehensive study of life‐history traits in a fig wasp community. The comparative approach revealed constraints and flexibility in trait evolution.     

Nitrous oxide emissions from a cropped soil in a semi-arid climate

Understanding nitrous oxide (N₂O) emissions from agricultural soils in semi‐arid regions is required to better understand global terrestrial N₂O losses. Nitrous oxide emissions were measured from a rain‐fed, cropped soil in a semi‐arid region of south‐western Australia for one year on a sub‐daily basis. The site included N‐fertilized (100 kg N ha^?1 yr^?1) and nonfertilized plots. Emissions were measured using soil chambers connected to a fully automated system that measured N₂O using gas chromatography. Daily N₂O emissions were low (?1.8 to 7.3 g N₂O‐N ha^?1 day^?1) and culminated in an annual loss of 0.11 kg N₂O‐N ha^?1 from N‐fertilized soil and 0.09 kg N₂O‐N ha^?1 from nonfertilized soil. Over half (55%) the annual N₂O emission occurred from both N treatments when the soil was fallow, following a series of summer rainfall events. At this time of the year, conditions were conducive for soil microbial N₂O production: elevated soil water content, available N, soil temperatures generally >25 °C and no active plant growth. The proportion of N fertilizer emitted as N₂O in 1 year, after correction for the ‘background’ emission (no N fertilizer applied), was 0.02%. The emission factor reported in this study was 60 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.25%), suggesting that the default may not be suitable for cropped soils in semi‐arid regions. Applying N fertilizer did not significantly increase the annual N₂O emission, demonstrating that a proportion of N₂O emitted from agricultural soils may not be directly derived from the application of N fertilizer. ‘Background’ emissions, resulting from other agricultural practices, need to be accounted for if we are to fully assess the impact of agriculture in semi‐arid regions on global terrestrial N₂O emissions.     

Geographical variation in an ant–plant interaction correlates with domatia occupancy,local ant diversity,and interlopers

Interactions between potentially mutualistic partners can vary over geographic areas. Myrmecophytes, which are plants harbouring ants, often do not exhibit sufficient intraspecific variability to permit comparative studies of myrmecophytic traits over space or time. Humboldtia brunonis (Fabaceae), a dominant, endemic myrmecophyte of the Indian Western Ghats, is unique in exhibiting considerable variability in myrmecophytic traits, e.g. domatia presence, as well as domatia occupancy and associated ant diversity throughout its geographic range. Although its caulinary domatia are occupied by at least 16 ant species throughout its distribution, young leaves and floral buds producing extrafloral nectar (EFN) are protected by ants from herbivory only in the southernmost region, where Technomyrmex albipes (Dolichoderinae) is the most abundant ant species. The extent of protection by ants was positively related to local species richness of ants and their occupancy of domatia. On the other hand, the highest abundance of interlopers in the domatia, including non‐protective ants, the arboreal earthworm Perionyx pullus, and other invertebrates, occurred in sites with the least protection from herbivory by ants. Whereas domatia morphometry did not vary between sites, domatia occupied by protective ants and invertebrate interlopers were longer and broader than empty ones at all sites. The lowest percentage of empty domatia was found at the southernmost site. There was a progressive decline in ant species richness from that found at the sites, to that feeding on H. brunonis EFN, to that occupying domatia, possibly indicating constraints in the interactions with the plants at various levels. Our study of this dominant myrmecophyte emphasizes the impact of local factors such as the availability of suitable ant partners, domatia occupancy, and the presence of interlopers on the emergence of a protection mutualism between ants and plants. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 538–551.     

Exposure to preindustrial, current and future atmospheric CO2 and temperature differentially affects growth and photosynthesis in Eucalyptus

To investigate if Eucalyptus species have responded to industrial-age climate change, and how they may respond to a future climate, we measured growth and physiology of fast- ( E. saligna ) and slow-growing ( E. sideroxylon ) seedlings exposed to preindustrial (290), current (400) or projected (650 μL L⁻¹) CO₂ concentration ([CO₂]) and to current or projected (current +4 °C) temperature. To evaluate maximum potential treatment responses, plants were grown with nonlimiting soil moisture. We found that: (1) E. sideroxylon responded more strongly to elevated [CO₂] than to elevated temperature, while E. saligna responded similarly to elevated [CO₂] and elevated temperature; (2) the transition from preindustrial to current [CO₂] did not enhance eucalypt plant growth under ambient temperature, despite enhancing photosynthesis; (3) the transition from current to future [CO₂] stimulated both photosynthesis and growth of eucalypts, independent of temperature; and (4) warming enhanced eucalypt growth, independent of future [CO₂], despite not affecting photosynthesis. These results suggest large potential carbon sequestration by eucalypts in a future world, and highlight the need to evaluate how future water availability may affect such responses.     

Systematics of dusky salamanders, Desmognathus (Caudata: Plethodontidae), in the mountain and Piedmont regions of Virginia and North Carolina, USA

We analysed mitochondrial (cytochrome  b ) nucleotide sequences, nuclear allozyme markers, and morphometric characters to investigate species boundaries and phylogenetic relationships among dusky salamanders ( Desmognathus ) in the southern Blue Ridge and adjacent Piedmont Physiographic Provinces of Virginia and North Carolina. Our results revealed four distinct mitochondrial DNA clades that are also characterized by distinct allozyme markers. One clade consists of sequences derived from populations distributed from New England to south-western Virginia that are referable to Desmognathus fuscus Rafinesque, 1820, although there is considerable sequence and allozyme divergence within this clade. A second clade consists of sequences derived from populations referable to Desmognathus planiceps Newman, 1955, a form that we resurrect from its long synonymy under D. fuscus . Desmognathus planiceps and D. fuscus also differ in mandibular tooth shape. Two other cytochrome  b sequences recovered from populations along the Blue Ridge escarpment in southern Virginia are quite divergent from those of the previous two clades, and these populations may represent yet another distinct species. Sequences from a population in the Brushy Mountains in the Piedmont of northern North Carolina are similar to those of Desmognathus carolinensis . Population groupings indicated by allozyme data generally correspond to the cytochrome  b clades. Cryptic diversity in Appalachian desmognathan salamanders clearly requires further study. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society , 2008, 152 , 115–130.     

A nutritional analysis of foraging in the Malabar giant squirrel (Ratufa indica)

The effect of nutrients and their availability on the diet of the herbivorous Malabar giant squirrel Ratufa indica (Sciuridae) was investigated at Magod and Bhimashankar in western India. The daily consumption of food items (percent wet diet) and the intake rates of these items and the contained nutrients (wet g s^-1) were determined by continuous observation of focal animals. Water content was a significant positive predictor of relative food item consumption while mineral contents in general had the opposite effect. The intake rates of water and more digestible nutrients such as nonstructural carbohydrates, as well as the intake rate of minerals such as zinc (at Magod), were significant positive predictors of the relative contribution of an item to the daily diet, thus indicating considerations of feeding costs versus benefits. Tannins, some alkaloids, and other secondary metabolites may negatively influence food choice. Protein content, relative to digestibility reducers, influenced food item consumption only at Bhimashankar. There was seasonal variation in daily biomass consumption. At Bhimashankar, daily biomass consumption increased with the proportion of fruit in the diet while this did not occur at Magod. This is perhaps a result of the higher water content and the lower content of some soluble nutrients within fruit at Bhimashankar relative to Magod. There was also intra-month variation between individual squirrels in daily biomass of food consumed. Squirrels consumed ephemeral food items opportunistically and non-ephemeral items such as mature leaves and bark on a regular daily basis. Squirrels probably obtained minerals and nitrogen from both fruit (especially seeds) and non-fruit sources (mature leaves and bark in the case of minerals, young and mature leaves in the case of nitrogen). However, they obtained lipid and non-structural carbohydrates mostly from fruit. Squirrels were selective in their utilization of tree species on a monthly basis, but this selectivity was not evident over a longer time period because individuals were constrained to use different phenological stages of tree species present within their territories. Large body size in this squirrel permits dietary flexibility and enables an overall generalist feeding strategy.     

Cell wall polysaccharidase activities and growth processes: a possible relationship

The turions of Potamogeton crispus L. develop in early summer and function in propagation and dispersal. Under natural conditions during longday periods, an average minimum air temperature of more than 12°C was found to be important for turion formation. Experiments with controlled environments indicate that both temperature and photoperiod regulate turion formation. Turions can be induced at 13°C or above but not at 8 or 10°C. At a temperature range of 13–24°C turions form in both 12- and 16-h days, but not in 8-h days. By increasing temperature from 24 to 30 or 35°C turions can be induced under 8-h days. Light intensity was found to be important in the formation of turions.