Provenance, life span, and phylogeny do not affect grass species' responses to nitrogen and phosphorus

Successful conservation management requires an understanding of how species respond to intervention. Native and exotic species may respond differently to management interventions due to differences arising directly from their origin (i.e., provenance) or indirectly due to biased representations of different life history types (e.g., annual vs. perennial life span) or phylogenetic lineages among provenance (i.e., native or exotic origin) groups. Thus, selection of a successful management regime requires knowledge of the life history and provenance-bias in the local flora and an understanding of the interplay between species characteristics across existing environmental gradients in the landscape. Here we tested whether provenance, phylogeny, and life span interact to determine species distributions along natural gradients of soil chemistry (e.g., soil nitrogen and phosphorus) in 10 upland prairie sites along a 600-km latitudinal transect running from southern Vancouver Island in British Columbia, Canada, to the Willamette Valley in Oregon, USA. We found that soil nitrate, phosphorus, and pH exerted strong control over community composition. However, species distributions along environmental gradients were unrelated to provenance, life span, or phylogenetic groupings. We then used a greenhouse experiment to more precisely measure the response of common grass species to nitrogen and phosphorus supply. As with the field data, species responses to nutrient additions did not vary as a function of provenance, life span, or phylogeny. Native and exotic species differed strongly in the relationship between greenhouse-measured tolerance of low nutrients and field abundance. Native species with the greatest ability to maintain biomass production at low nutrient supply rates were most abundant in field surveys, as predicted by resource competition theory. In contrast, there was no relationship between exotic-species biomass at low nutrient levels and field abundance. The implications of these findings for management of invasive species are substantial in that they overturn a general belief that reduction of nutrient supplies favors native species. The idiosyncratic nature of species response to nutrients in this study suggests that manipulation of nutrient supplies is unlikely to alter the overall balance between native and exotic species, although it may well be useful to control specific exotic species.     

Environmental and Biological Assessment of Exposure to Benzene in Petroleum Workers

Occupational exposure to benzene was measured in two gasoline marketing terminals and five major refineries in Singapore. A total of 280 workers were monitored over two years. This assessment was carried out with two primary objectives: (1) To find out the extent of occupational exposure to benzene in the petroleum industry in Singapore, (2) To identify suitable biomarkers for monitoring of low levels of benzene exposure. The exposure was measured in five different categories of petroleum and petrochemical workers, i.e., truck drivers, despatch assistant, process operators, oil movements operators and laboratory technicians. The results revealed wide variations in exposure, from 0.01 to 13.6 ppm for personal time weighted average (TWA) exposure over the whole workshift. The exposure of truck drivers appeared to be the highest, with geometric mean (GM) of 1.98 ppm (ranged from 0.25 to 13.6 ppm). The average benzene exposure for process operators was relative low with a GM of 0.04 ppm. Lowest benzene exposure was found in the laboratory technicians, with a GM of 0.02 ppm. As cigarette smoking is known to affect metabolism of benzene, data analyses on the relationships with environmental exposure were conducted only on the 190 nonsmokers. The results showed that urinary trans, trans-muconic acid (ttMA), unmetabolized benzene in urine (UBZ) and benzene in blood (BBZ) were better biomarkers for low level benzene exposure as compared to urinary phenolic metabolites in urine, such as hydroquinone, phenol and catechol.     

Trajectories of change in sagebrush steppe vegetation communities in relation to multiple wildfires

Repeated perturbations, both biotic and abiotic, can lead to fundamental changes in the nature of ecosystems, including changes in state. Sagebrush steppe communities provide important habitat for wildlife and grazing for livestock. Fire is an integral part of these systems, but there is concern that increased ignition frequencies and invasive species are fundamentally altering them. Despite these issues, the majority of studies of fire effects in systems dominated by Artemisia tridentata wyomingensis have focused on the effects of single burns. The Arid Lands Ecology Reserve (ALE), in south-central Washington (U.S.A.), was one of the largest contiguous areas of sagebrush steppe habitat in the state until large wildfires burned the majority of it in 2000 and 2007. We analyzed data from permanent vegetation transects established in 1996 and resampled in 2002 and 2009. Our objective was to describe how the fires, and subsequent postfire restoration efforts, affected communities' successional pathways. Plant communities differed in response to repeated fire and restoration; these differences could largely be ascribed to the functional traits of the dominant species. Low-elevation communities, previously dominated by obligate seeders, moved furthest from their initial composition and were dominated by weedy, early-successional species in 2009. Higher-elevation sites with resprouting shrubs, native bunchgrasses, and few invasive species were generally more resilient to the effects of repeated disturbances. Shrub cover has been almost entirely removed from ALE, although there was some recovery where communities were dominated by resprouters. Bromus tectorum dominance was reduced by herbicide application in areas where it was previously abundant, but it increased significantly in untreated areas. Several resprouting species, notably Phlox longifolia and Poa secunda, expanded remarkably following competitive release from shrub canopies and/or abundant B. tectorum. Our results suggest that community dynamics can be understood through a state and transition model with two axes (shrub/grass and native/invasive abundance), although such models also need to account for differences in plant functional traits and disturbance regimes. We use our results to develop a conceptual model that will be validated with further research.     

Inversion of plant dominance-diversity relationships along a latitudinal stress gradient

Species interactions affect plant diversity through the net effects of competition and facilitation, with the latter more prevalent in physically stressful environments when plant cover ameliorates abiotic stress. One explanation for species loss in invader-dominated systems is a shift in the competition-facilitation balance, with competition intensifying in areas formerly structured by facilitation. We test this possibility with a 10-site prairie meta-experiment along a 500-km latitudinal stress gradient, quantifying the relationships among abiotic stress, exotic dominance, and native plant recruitment over five years. The latitudinal gradient is inversely correlated with abiotic stress, with lower latitudes more moisture- and nutrient-limited. We observed strong negative effects by invasive dominant grasses on plant establishment, but only in northern sites with lower-stress environments. At these locations, disturbance was critical for recruitment by reducing the suppressive dominant (invasive) canopy. In more stressful environments to the south, the impacts of the dominant invaders on plant establishment became facilitative, and diversity was more limited by seed availability. Disturbance prevented recruitment because seedling survival depended on a protective plant canopy, presumably because the canopy reduced temperature or moisture stress. Seed limitation was similarly prevalent in all sites. Our work confirms the importance of facilitation as an organizing process for plants in higher-stress environments, even with transformations of species composition and dominance. It also demonstrates that the mechanisms regulating diversity, including invader impacts, can vary within the same plant community depending on environmental context. Because limits on native plant recruitment are environmentally contingent, management strategies that seek to increase diversity, including invader eradication, must account for site-level variations in the balance between biotic and abiotic constraints.