Claus Holzapfel

BIDIRECTIONAL FACILITATION AND INTERFERENCE BETWEEN SHRUBS AND ANNUALS IN THE MOJAVE DESERT

Composition and structure of plant communities can be strongly influenced by plant interactions. Interactions among plants commonly comprise positive and negative effects operating simultaneously and bidirectionally. Thus, a thorough understanding of plant interactions requires experimental separation and quantitative assessment of the bidirectional positive and negative effects that add up to the net effects of plant interactions. Using the close spatial association of annual plants with a desert shrub (Ambrosia dumosa) in a sandy area of the Mojave Desert of California as a test system, we separated and quantified negative and positive effects of annuals on shrubs and of shrubs on annuals. We achieved the separation of negative and positive effects with an experimental design that included reciprocal removals of neighbors and simulations of physical effects of neighbors using artificial structures. All experimental manipulations were conducted on space originally occupied by Ambrosia shrubs to focus on immediate effects of neighbors on water availability rather than on long-term microenvironmental effects (e.g., nutrient accumulation). We quantified positive effects by calculating the difference between performance parameters of neighbors growing with artificial structures (thatch to mimic the physical effects of the presence of annuals and artificial canopies to mimic the physical effects of shrubs) and those of neighbors growing alone (removals). We estimated negative effects by calculating the difference between plant performance on control plots (shrubs and annuals growing together) and performances of plants growing with the artificial structures. Annuals had simultaneously strong negative and weak positive effects on shrub water status, growth, and reproductive output. Annuals also had an impact on the sex expression of shrubs by inducing shifts toward a higher male proportion in inflorescences of monoecious Ambrosia. In contrast, we found strong positive and weak or no negative effects of the shrubs on survival, biomass production, and seed production of the entire annual community and of selected annual species (the abundant native Chaenactis fremontii and the two dominant introduced annual species Bromus madritensis ssp. rubens and Schismus barbatus). Overall, in net effect, the interaction between shrubs and annuals can be described as facilitation or positive net effects of shrubs on annuals, and interference or negative net effects of annuals on shrubs. However, during the growing season, the ratios between positive and negative effects shifted. Annual plants benefited from the presence of shrubs to the greatest extent early in the growing season, and initial negative effects of annuals on shrubs declined as annuals senesced later in the season. Results of this study support the view that an experimental resolution of bidirectional positive and negative effects is necessary to achieve an accurate, mechanistic understanding of species interactions.

Middle-Eastern plant communities tolerate 9 years of drought in a multi-site climate manipulation experiment.

For evaluating climate change impacts on biodiversity, extensive experiments are urgently needed to complement popular non-mechanistic models which map future ecosystem properties onto their current climatic niche. Here, we experimentally test the main prediction of these models by means of a novel multi-site approach. We implement rainfall manipulations—irrigation and drought—to dryland plant communities situated along a steep climatic gradient in a global biodiversity hotspot containing many wild progenitors of crops. Despite the large extent of our study, spanning nine plant generations and many species, very few differences between treatments were observed in the vegetation response variables: biomass, species composition, species richness and density. The lack of a clear drought effect challenges studies classifying dryland ecosystems as most vulnerable to global change. We attribute this resistance to the tremendous temporal and spatial heterogeneity under which the plants have evolved, concluding that this should be accounted for when predicting future biodiversity change.

Metal accumulation and performance of nestlings of passerine bird species at an urban brownfield site.

The use of passerine species as bioindicators of metal bioaccumulation is often underutilized when examining the wildlife habitat value of polluted sites. In this study we tested feathers of nestlings of two common bird species (house wren and American robin) for accumulation of Pb, Zn, As, Cr, Cu, Fe in comparison of a polluted, urban brownfield with a rural, unpolluted site. House wren nestlings at the study site accumulated significantly greater concentrations of all target metals except Zn. At the polluted site we found significant species differences of metal concentrations in feathers, with house wrens accumulating greater concentrations of Pb, Fe, and Zn but slightly lesser accumulations of Cr and Cu than American robins. Although house wren nestlings demonstrated significant accumulation of metals, these concentrations showed little effect on size metrics or fledge rates during the breeding season compared to nestlings from the control site.

Altered vegetative assemblage trajectories within an urban brownfield.

Recognizing the growing importance of both structure (maintenance of biodiversity) and function (fostering natural cycles) of urban ecologies, we examine coarse scale (herbaceous, shrub and forest) beta guild trajectory in an urban brownfield. The distribution of the pioneer forest assemblage dominated by Betula populifolia Marsh. and Populus spp. could be correlated positively with total soil metal load (arsenic, cadmium, chromium, copper, lead, zinc, lead and vanadium),whereas herbaceous and shrub guilds were negatively correlated. Distinct assemblage development trajectories above and below a critical soil metal threshold are demonstrated. In addition, we postulate that the translocation of metals into the plant tissue of several dominant species may provide a positive feedback loop, maintaining relatively high concentrations of metals in the litter and soil. Therefore assembly theory, which allows for the development of alternate stable states, may provide a better model for the establishment of restoration objectives on degraded urban sites.

The Use and Misuse of Climatic Gradients for Evaluating Climate Impact on Dryland Ecosystems - an Example for the Solution of Conceptual Problems

Current trends of emissions of greenhouse gases are expected to cause the global temperature to rise faster over the present and next century than during any previous period (Houghton et al., 1996, 2001; Zweirs, 2002). Climate models for the Middle East predict an increase in winter temperatures combined with changes in rainfall amounts and distribution (Ben-Gai et al., 1998; Black, 2009, Klafe and Bruins, 2009). These changes may alter ecosystem functioning, with direct effects on ecosystem, community and population processes such as plant litter decomposition, nutrient cycling, primary productivity, biodiversity, plant recruitment and survival (e.g., Aronson et al., 1993; Hobbie, 1996; Robinson et al., 1998; Sternberg et al., 1999; Chapin et al., 2000; Hughes, 2000; Sarah, 2004). Considerable research has been directed at understanding the responses of terrestrial ecosystems to global environmental change. This topic is of great societal concern in the light of the potential impacts on the natural resources on which the human population depends on (Vitousek, 1994). Nevertheless, the challenge to predict ecosystem response to climate change is based on the multi-dimensional and multi-scale nature of the problem (Osmond et al., 2004). Complex ecological interactions make it difficult to extrapolate from individuals to communities and to predict the ecosystem response when only few levels of ecosystem organization are targeted. In addition, the lack of realistic climatic scenarios at relevant scales adds further complexity to the up-scaling process (Harvey, 2000). Vast experimental research efforts have been invested in understanding the effects of global warming and CO2 atmospheric enrichment on ecosystem functioning. These processes are considered key drivers of environmental change, particularly in northern latitudes.

ROADSIDE VEGETATION ALONG TRANSECTS IN THE JUDEAN DESERT

In a rainshadow desert, ranging from sub-Mediterranean areas to extremely arid environments, the vegetation was studied on comparable plots along roadsides and in the surrounding area. The uniqueness of roadside vegetation and flora is shown by using indices and measurements which allow a comparison along the climatic gradient. Biomass and species diversity are distinctively higher and the chorological composition is different near roads, this at least under an arid regime. The reasons for these differences are discussed based on investigations of site conditions. Increased water runoff and more favorable soil conditions seem to be decisive in this respect.

Effects of Human-Caused Disturbances on the Flora along a Mediterranean-Desert Gradient

Summary The species composition of the flora in disturbed and natural sites along a Mediterranean to desert aridity gradient was studied. The chorological and ecological data (i.e. phytogeographical composition, growth form spectra, primary production) show that the main impact of man is the levelling of zonal differences. Thus, while local species diversity increased in most regions, an overall decrease in diversity occurred along the gradient, with the sharply distinct natural vegetation of the different zones giving way to a similar ruderal flora in the disturbed sites. These generally lacked endemic species and showed a rise in annuals, in mesic, Mediterranean elements as well as in introduced and wind-pollinated species.

Polyphenol oxidase activity in the roots of seedlings of Bromus (Poaceae) and other grass genera

UNLABELLED PREMISE OF THE STUDY Phenolic compounds exuded by roots have been implicated in allelopathic interactions among plants. Root enzymes that destroy phenolics may protect plants against allelopathic inhibition and thus may aid in invasiveness. Phenolic-degrading enzymes are chiefly found in aboveground plant parts, but have also been previously reported in root tissues where the enzymes function is unknown. We explored phenolic oxidase activity in emerging roots of grasses in a survey across different grass genera; in particular, we aimed to test whether grasses of the genus Bromus, known for their large invasion potential, differ in this respect from other grass taxa. • METHODS We assayed a range of grass genera commonly found in the United States for root enzyme activity with spectrophotometric assays of phenol oxidase activity using l-DOPA as the main substrate. • KEY RESULTS In the survey of a grass genera, we discovered that roots of the genus Bromus contain large amounts of polyphenol oxidase (PPO) activity, while all other tested grass genera, even ones closely related to Bromus, did not. PPO was found to be present at germination and remained active throughout the life of the plant. Compared to other PPOs, the enzyme present in Bromus appears to have a narrow substrate range. • CONCLUSIONS The specific functions of the root PPO and the ecological ramifications of the special status of Bromus are not yet clear. The possibility that the enzyme plays a role in plant species interaction for bromes, a genus of grasses known to have high invasive potential, is raised.

Vegetation Pattern in Northern Tibet in Relation to Environmental and Geo-spatial Factors

Abstract: Environmental and Geo-spatial factors have long been considered as crucial determinants of species composition and distributions. However, quantifying the relative contributions of these factors for the alpine ecosystems is lacking. The Tibetan Plateau has a unique ecological environment and vegetation types. Our objectives are to quantify the spatial distributions of plant communities on the Northern Tibetan Alpine grasslands and to explore the relationships between vegetation composition, Geo-spatial factors and environmental factors. We established 63 field plots along a 1200-km gradient on the Northern Tibetan Plateau Alpine Grassland and employed the two-way indicator species analysis (TWINSPAN) and the detrended canonical correspondence analysis (DCCA). Fourteen communities of alpine grassland were identifiable along the transect and consisted of three vegetation types: Alpine meadow, Alpine steppe, and desert steppe. Vegetation composition and spatial distribution appeared to be largely determined by mean annual precipitation and less influenced by temperature. A large fraction (73.5%) of the variation in vegetation distribution was explained by environmental variables along this transect, somewhat less by Geo-spatial factors (56.3%). The environmental and Geo-spatial factors explained 29.6% and 12.3% of the total variation, respectively, while their interaction explained 43.9%. Our findings provide strong empirical evidence for explaining biological and environmental synergetic relationships in Northern Tibet.

Population structure of Ailanthus altissima (Simaroubaceae): The role of land-use history and management1

Abstract The population structure of invasive species is determined both by species-specific breeding strategies and by region-specific conditions and events during the history of invasion. In this context, we investigate the role of human land-use policy on the gene flow within population of the widespread invasive tree, Ailanthus altissima. We predicted that genetic diversity would be elevated in urban environments and reduced in exurban, and that human mediated dispersal would allow propagules to break geographic isolation. Six populations of A. altissima, divided evenly among urban, exurban and suburban sites, were surveyed using a set of eight microsatellite loci. A total of 276 individuals were sampled. Populations were assessed for partitioning of genetic variation, gene flow between sites, and genetic cluster estimation. Effective population size based on genetic variation was also modeled using Bayesian Markov chain Monte Carlo simulations. Despite a strong propensity for clonal growth, the microsatellite data revealed no evidence of clonal reproduction at the population level. Gene flow between sites was found to be independent of geographic distance; instead, gene flow was correlated with the level of human traffic at a site. Genetic diversity was found to generally increase in correlation to human development; however, reduced admixture at managed sites suggests that land management practices were effective at inhibiting gene flow into managed sites.