No abstract

A large proportion of rainfall in dryland ecosystems is intercepted by plant foliage and is generally assumed to evaporate to the atmosphere or drip onto the soil surface without being absorbed. We demonstrate foliar absorption of intercepted rainfall in a widely distributed, continental dryland, woody-plant genus: Juniperus. We observed substantial improvement in plant water status, exceeding 1.0 MPa water potential for drought-stressed plants, following precipitation on an experimental plot that excluded soil water infiltration. Experiments that wetted shoots with unlabeled and with isotopically labeled water confirmed that water potential responded substantially to foliar wetting, that these responses were not attributable to re-equilibration with other portions of the xylem, and that magnitude of response increased with water stress. Foliar absorption is not included in most ecological, hydrological, and atmospheric models; has implications for interpreting plant isotopic signatures; and not only supplements water acquisition associated with increases in soil moisture that follow large or repeated precipitation events, but also enables plants to bypass soil water uptake and benefit from the majority of precipitation events, which wet foliage but do not increase soil moisture substantially. Foliar absorption of intercepted water could be more important than previously appreciated, especially during drought when water stress is greatest.

Nickel hyperaccumulating plants have more than 1000 mg Ni kg dry weight when grown on nickel-bearing soils. We hypothesized that Ni hyperaccumulation could serve as a chemical defense against herbivores In feeding experiments with potential insect herbivores and Ni hyperaccumulating plants, only those inseets fed leaves from plants grown on non-nickel-bearing soil survived or showed a weight gain. Among chemical parameters measured, only Ni content of plants was sufficient to explain this result. When subjected to herbivory by lepidopteran larvae, plants grown on Ni-amended soil showed greater survival and yield than plants on unamended soil. Ni hyperaccumulation may be an effective plant chemical defense against herbivores because of its high lethality, apparent low cost, and broad spectrum of toxicity.

Metal hyperaccumulating plants contain very high metal contents. Because of the general toxicity of metals, chemically-mediated biotic interactions involving hyperaccumulating plants may differ greatly from those of non-hyperaccumulators. Recent research has demonstrated a defensive function for hyperaccumulated metals against herbivores and pathogens. We predict that some herbivore/pathogen species have evolved metal tolerance, and suggest that resulting high metal levels in herbivores/pathogens may defend them against their own predators. Little is known regarding interference and commensal interactions involving hyperaccumulating plants. Decreased competition may occur through an interference interaction similar to allelopathy, in which enrichment of metal in the soil under a hyperaccumulator plant's canopy may inhibit another plant species, thus resulting in ''elemental allelopathy''. Metal enrichment of soil under hyperaccumulators also may result in commensalism if another plant species (possibly another hyperaccumulator) derives a benefit from growing in the metal-enriched soil under the canopy of a hyperaccumulating overstory plant. It seems likely that high-metal plant litter will host a specialized microflora of decomposers and may affect nutrient cycling rates. Mutualist biotic interactions also may be affected by the elevated metal contents of hyperaccumulating species. Mycorrhizal fungi may form mutualisms with hyperaccumulators, but the phenomenon is poorly-explored. The few cases investigated to date have not detected mycorrhizae. Pollination and seed dispersal mechanisms may require biotic vectors that might be affected by plant metal content. Hyperaccumulating plants may have solved this dilemma in three ways. First, some may rely on abiotic vectors for pollen or seed dispersal. Second, biotic vectors used by these species may have varied diets and thus dilute metal intake to non-toxic levels. Finally, biotic vectors may have evolved tolerance of elevated dietary levels of metals, and perhaps have become specialists on hyperaccumulator species.

No abstract

Abstract. Semi‐arid woodlands are two‐phase mosaics of canopy and inter‐canopy patches. We hypothesized that both aboveground competition (within canopy patches), and below‐ground competition (between canopy patches), would be important structuring processes in these communities. We investigated the spatial pattern of trees in a Pinus edulis‐Juniperus monosperma woodland in New Mexico using Ripley's K‐function. We found strong aggregation of trees at scales of 2 to 4 m, which indicates the scale of canopy patches. Canopy patches were composed of individuals of both species. Crown centers of both species were always less aggregated than stem centers at scales less than canopy patch size, indicating morphological plasticity of competing crowns. In the smallest size classes of both species, aggregation was most intense, and occurred over a larger range of scales; aggregation decreased with increasing size as is consistent with density‐dependent mortality from intraspecific competition. Within canopy patches, younger trees were associated with older trees of the other species. At scales larger than canopy patches, younger trees showed repulsion from older conspecifics, indicating below‐ground competition. Hence, intraspecific competition was stronger than interspecific competition, probably because the species differ in rooting depth. Woodland dynamics depend on the scale and composition of canopy patches, aggregated seed deposition and facilitation, above‐ and below‐ground competition, and temporal changes in the spatial scale of interactions. This woodland is intermediate in a grassland‐forest continuum (a gradient of increasing woody canopy cover) and hence we expected, and were able to detect, the effects of both above‐ and below‐ground competition.

No abstract

Summary1. The pioneering work of Peter Eagleson has been proposed as a theoretical basis for ecohydrology. Central to the theory are three 'ecological optimality hypotheses' which represent ecologically important aspects of atmosphere-soil-vegetation interactions. 2. The model and its underlying assumptions have never been evaluated in an explicitly ecological context. We examine each of the three optimality hypotheses in turn and test the ability of the model to make ecologically relevant predictions using climate, soil and vegetation data from a semi-arid woodland site in central New Mexico, USA. 3. We find that all three of the optimality hypotheses are ecologically flawed. While we could qualitatively reproduce previously published results, model behaviour under novel conditions was highly variable and frequently unrealistic. 4. The poor performance of the model was probably due to the inadequate treatment of water-limited transpiration in its original derivation. The theory thus requires redevelopment for ecological application, and we suggest several strands of research that could contribute to its improvement.