June-July 2005 CA Grassland Newsletter
LOTS of references this time. I just started checking Web of Science in addition to BIOSIS. Consider it your summer reading for the beach!
Davies KE, Chesson P, Harrison S, Inouye BD, Melbourne BA, Rice KJ. 2005. Spatial heterogeneity explains the scale dependence of the native-exotic diversity relationship Ecology 86 (6): 1602-1610
Abstract: While small-scale studies show that more diverse native communities are less invasible by exotics, studies at large spatial scales often find positive correlations between native and exotic diversity. This large-scale pattern is thought to arise because landscapes with favorable conditions for native species also have favorable conditions for exotic species. From theory, we proposed an alternative hypothesis: the positive relationship at large scales is driven by spatial heterogeneity in species composition, which is driven by spatial heterogeneity in the environment. Landscapes with more spatial heterogeneity in the environment can sustain more native and more exotic species, leading to a positive correlation of native and exotic diversity at large scales. In a nested data set for grassland plants, we detected negative relationships between native and exotic diversity at small spatial scales and positive relationships at large spatial scales. Supporting our hypothesis, the positive relationships, between native and exotic diversity at large scales were driven by positive relationships between native and exotic beta diversity. Further, both native and exotic diversity were positively correlated with spatial heterogeneity in abiotic conditions (variance of soil depth, soil nitrogen, and aspect) but were uncorrelated with average abiotic conditions, supporting the spatial-heterogeneity hypothesis but not the favorable-conditions hypothesis.
Moffatt KC, Crone EE, Holl KD, Schlorff RW, Garrison BA. 2005. Importance of hydrologic and landscape heterogeneity for restoring bank swallow (Riparia riparia) colonies along the Sacramento River, California. RESTORATION ECOLOGY 13 (2): 391-402
Abstract: Human activities have degraded riparian systems in numerous ways, including homogenization of the floodplain landscape and minimization of extreme flows. We analyzed the effects of changes in these and other factors for extinction-colonization dynamics of a threatened Bank Swallow population along the upper Sacramento River, California, U.S.A. We monitored Bank Swallow distributions along a 160-km stretch of the river from 1986-1992 and 1996-2003 and tested whether site extinctions and colonizations corresponded with changes in maximum river discharge, surrounding land cover, estimated colony size, temperature, and precipitation. Colonization probabilities increased with maximum discharge. Extinction probabilities decreased with proximity to the nearest grassland, decreased with colony size, and increased with maximum discharge. To explore the implications for restoration, we incorporated the statistically estimated effects of distance to grassland and maximum discharge into simple metapopulation models. Under current conditions, the Bank Swallow metapopulation appears to be in continued decline, although stable or increasing numbers cannot be ruled out with the existing data. Maximum likelihood parameters from these regression models suggest that the Sacramento River metapopulation could be restored to 45 colonies through moderate amounts of grassland restoration, large increases in discharge, or direct restoration of nesting habitat by removing approximately 10% of existing bank protection (riprap) from suitable areas. Our results highlight the importance of grassland restoration, mixed benefits of restoring high spring discharge, and the importance of within-colony dynamics as areas for future research.
Horz HP, Rich V, Avrahami S, Bohannan BJM. 2005. Methane-oxidizing bacteria in a California upland grassland soil: Diversity and response to simulated global change. APPLIED AND ENVIRONMENTAL MICROBIOLOGY 71 (5): 2642-2652.
Abstract: We investigated the diversity of methane-oxidizing bacteria (i.e., methanotrophs) in an annual upland grassland in northern California, using comparative sequence analysis of the pmoA gene. In addition to identifying type II methanotrophs commonly found in soils, we discovered three novel pmoA lineages for which no cultivated members have been previously reported. These novel pmoA clades clustered together either with clone sequences related to "RA 14" or "VB5FH-A," which both represent clusters of environmentally retrieved sequences of putative atmospheric methane oxidizers. Conservation of amino acid residues and rates of nonsynonymous versus synonymous nucleotide substitution in these novel lineages suggests that the pmoA genes in these clades code for functionally active methane monooxygenases. The novel clades responded to simulated global changes differently than the type II methanotrophs. We observed that the relative abundance of type II methanotrophs declined in response to increased precipitation and increased atmospheric temperature, with a significant antagonistic interaction between these factors such that the effect of both together was less than that expected from their individual effects. Two of the novel clades were not observed to respond significantly to these environmental changes, while one of the novel clades had an opposite response, increasing in relative abundance in response to increased precipitation and atmospheric temperature, with a significant antagonistic interaction between these factors.
Seabloom EW, Bjornstad ON, Bolker BM, Reichman OJ. 2005. Spatial signature of environmental heterogeneity, dispersal, and competition in successional grasslands. ECOLOGICAL MONOGRAPHS 75 (2): 199-214.
Abstract: The fate of individual plants is strongly influenced by their spatial context (e.g., their location relative to competitors or environmental gradients). Recent theoretical developments indicate that spatial structure in plant communities arises as an outcome of environmental heterogeneity, intra- and interspecific competition, and localized dispersal. Furthermore, each of these forces is predicted to leave a signature on the spatial and temporal pattern within the community. We track the development of spatial structure in successional grasslands in southern California for five years, using a series of large-scale field experiments. We compare the observed changes in spatial structure to the predictions of a spatially explicit model of plant competition in heterogeneous landscapes. We conclude that environmental heterogeneity led to the development of aggregation in the first year at scales of 1-2 in. Over time, aggregation was detectable at distances greater than 4 m, and species become segregated as a result of competition and localized dispersal. Large-scale disturbances (fire) homogenized the community by decreasing the level of aggregation and segregation. In contrast, disturbance by pocket gophers and ground squirrels increased local-scale heterogeneity by creating annual-dominated aggregates. The speed with which spatial structure develops in seemingly homogeneous environments illustrates the importance of incorporating spatial context in empirical studies and models of plant community dynamics.
Moore LA, Field CB. 2005. A technique for identifying the roots of different species in mixed samples using nuclear ribosomal DNA. JOURNAL OF VEGETATION SCIENCE 16 (1): 131-134.
Abstract: Question: Is it possible to determine the species composition of root samples containing multiple species, without first disentangling individual roots?
Methods: The internal transcribed spacer (ITS) region of nuclear ribosomal DNA was amplified and sequenced from four California annual grassland species (two Poaceae and two Asteraceae). Restriction enzymes that cut the ITS region of each species into uniquely sized fragments were identified based on DNA :sequence variation of the ITS regions. Mixed root samples were analysed to test the ability of the method to identify the presence or absence of each species in multi-species samples.
Results: The technique successfully identified species present in multi-species samples. ITS regions were shorter in Poaceae than in Asteraceae, so size differences alone were sufficient to distinguish these taxonomic groups. At the species level, digestion of ITS regions with the appropriate restriction enzymes yielded at least one uniquely sized fragment for each species.
Conclusions: This method is the first to identify the species composition of mixed root samples. It should be applicable to most plant species because the ITS region is flanked by universal primers and most species have unique ITS sequences. The ability to determine species-specific rooting distributions has broad applications in vegetation science.
Ingels CA, Scow KM, Whisson DA, Drenovsky RE. 2005. Effects of cover crops on grapevines, yield, juice composition, soil microbial ecology, and gopher activity. AMERICAN JOURNAL OF ENOLOGY AND VITICULTURE 56 (1): 19-29.
Abstract: Several cover crop mixes were planted in a winegrape vineyard in Sacramento County to test their effects on vine growth, production, juice composition, soil microbial ecology, and gopher activity over a three-year period (1998 to 2000). The trial was conducted in a Vitis vinifera L. cv. Merlot vineyard on a silt loam soil. Vines were planted in 1993 on 5BB rootstock, spaced 2.1 x 3.4 m. The mixes used were: California native perennial grass (no-till), annual clover (no-till), green manure (disked), cereals (disked), and disked control. Cover crops were planted on either side of entire rows, with a disked alley separating treatment replicates. A 1.2-m herbicide strip was maintained under the vines. Drip irrigation and fertigation were applied uniformly across all treatments, but additional nitrogen fertilizer was applied to the grass mixes. Weed biomass increased in the clover mix but decreased in the native grass mix. Grapevine petiole nitrogen content was highest in the bell bean mix and very low in the native grass mix. There were very few differences in leaf water potential or pruning weights of the vines, and in yields or juice Brix, pH, or titratable acidity in any year. Cover-cropped soils had greater microbial biomass than disked or berm soils, and the no-till mixes had greater microbial biomass than the disked mixes. Gophers were very numerous in 1999 only, with nearly all activity exclusively in the clover mix.
Potthoff M, Jackson LE, Steenwerth KL, Ramirez I, Stromberg MR, Rolston DE. 2005. Soil biological and chemical properties in restored perennial grassland in California. RESTORATION ECOLOGY 13 (1): 61-73.
Abstract: Restoration of California native perennial grassland is often initiated with cultivation to reduce the density and cover of non-native annual grasses before seeding with native perennials. Tillage is known to adversely impact agriculturally cultivated land; thus changes in soil biological functions, as indicated by carbon (C) turnover and C retention, may also be negatively affected by these restoration techniques. We investigated a restored perennial grassland in the fourth year after planting Nassella pulchra, Elymus glaucus, and Hordeum brachyantherum ssp. californicum for total soil C and nitrogen (N), microbial biomass C, microbial respiration, CO2 concentrations in the soil atmosphere, surface efflux of CO2, and root distribution (0- to 15-, 15- to 30-, 30- to 60-, and 60- to 80-cm depths). A comparison was made between untreated annual grassland and plots without plant cover still maintained by tillage and herbicide. In the uppermost layer (0- to 15-cm depth), total C, microbial biomass C, and respiration were lower in the tilled, bare soil than in the grassland soils, as was CO2 efflux from the soil surface. Root length near perennial bunchgrasses was lower at the surface and greater at lower depths than in the annual grass-dominated areas; a similar but less pronounced trend was observed for root biomass. Few differences in soil biological or chemical properties occurred below 15-cm depth, except that at lower depths, the CO2 concentration in the soil atmosphere was lower in the plots without vegetation, possibly from reduced production of CO2 due to the lack of root respiration. Similar microbiological properties in soil layers below 15-cm depth suggest that deeper microbiota rely on more recalcitrant C sources and are less affected by plant removal than in the surface layer, even after 6 years. Without primary production, restoration procedures with extended periods of tillage and herbicide applications led to net losses of C during the plant-free periods. However, at 4 years after planting native grasses, soil microbial biomass and activity were nearly the same as the former conditions represented by annual grassland, suggesting high resilience to the temporary disturbance caused by tillage.
Peters HA, Chiariello NR, Mooney HA, Levin SA, Hartley AE. 2005. Native harvester ants threatened with widespread displacement exert localized effects on serpentine grassland plant community composition. OIKOS 109 (2): 351-359.
Abstract: Seed-harvesting ants can influence the abundance and distribution of plant species through both the selective harvesting of seeds and the construction of nutrient-rich nest mounds, but the relative contributions of these two mechanisms have not been addressed by previous studies. Furthermore, the impact of ant seed harvesting in California serpentine grasslands remains unresolved because of divergent results from several previous experiments. This study investigates the influence of harvester ants on serpentine grassland plant species composition by examining two potential signatures of seed harvesting ants on plant community composition: species composition on versus off ant nest mounds, and species abundance as a function of distance from nest mounds. Of the 28 plant species identified in this study, 22 exhibited spatial patterns consistent with effects of seed harvesting, nest construction, or both. Although most species showed significant gradients in abundance with distance from a nest, there were no clear relationships between plant species distributions and previously reported harvester ant seed foraging patterns. Harvester ant nest mounds supported plant communities that were distinct from the surrounding serpentine grassland, with notably higher densities of legumes and invasive annual grasses. Comparison of our results with those of previous studies indicates that the patterns we observed are generally consistent over time, but affect more species and a larger fraction of the grassland than previously reported. Unaffected areas of the grassland seem likely to serve as important refuges for some plant species.
Espeland EK, Carlsen TM, Macqueen D. 2005. Fire and dynamics of granivory on a California grassland forb. BIODIVERSITY AND CONSERVATION 14 (2): 267-280.
Abstract: This study examines the effects of burning and granivory on the reproductive success of the rare plant Amsinckia grandiflora ( Boraginaceae). Fire is often used in California grasslands as a means of exotic species control, but the indirect effects these controls have on the reproductive ecology of native plants are rarely assessed. The interaction of fire with granivory of A. grandiflora seeds was examined in California grasslands over five years ( 1998 - 2002). In 1998 and 1999, both burned and unburned plots had bird-exclusion ( netted) and no-exclusion ( open) treatments. Predation rates were high ( 51 - 99%) and final predation rates did not differ among treatments. In 2000, granivory rates in the unburned, open plots were lower than in previous years (14%), and rodent trapping yielded only a single animal. Low granivory rates were observed in 2001 for unburned, open plots (47%). In 2001, burned/open plots experienced significantly more granivory (87%) than either burned/netted plots (37%) or unburned/open plots ( 47%). In 2002, every seed was taken from burned, open plots. Granivory was highly variable, ranging from 4 to 100% per plot over a 3-week period. Nearly all plots were discovered (> 10% predation) by granivores in all trials in all years. When data from all treatments were combined, significant differences in granivory rates occurred among years, indicating stronger inter-year effects than within-year effects due to burning or bird exclusion. Fire affects granivory when overall predation rates are low, but when predation levels are high ( as they were in 1998 and 1999), fire may not affect granivory occurring within the same year. Models extending seed survivorship through the dry summer indicate that most seeds are eaten, even when granivory rates are low.
Augustine DJ, McNaughton SJ. 2004. Temporal asynchrony in soil nutrient dynamics and plant production in a semiarid ecosystem. ECOSYSTEMS 7 (8): 829-840
Abstract: A central goal of ecosystem ecology is to understand how the cycling of nutrients and the growth of organisms are linked. Ecologists have repeatedly observed that nutrient mineralization and plant production are closely coupled in time in many terrestrial ecosystems. Typically, mineralization rates of limiting nutrients, particularly of nitrogen, during the growing season determine nutrient availability while pools of mineral nutrients remain low and relatively constant. Although several previous reports suggest nitrogen mineralization has the potential to vary seasonally and out of phase with plant production, such a phenomenon has been poorly documented. Here we report results from a semiarid savanna ecosystem characterized by distinct temporal asynchrony in rates of soil nitrogen cycling and plant production. Periods of positive plant growth following the onset of rains coincide with periods of low N turnover rates, whereas higher rates occur late in the wet season following plant senescence and throughout dry seasons. Plant uptake from the substantial mineral N pool present early in the growing season is sufficient to explain most of the N allocation to aboveground plant biomass during the growing season, even in the absence of any wet-season mineralization. The mineral N pool is subsequently recharged by late wet- and dry-season mineralization, plus urine inputs at sites with high levels of ungulate activity. These findings suggest fundamental changes in the quality of substrates available to decomposers over a seasonal cycle, with significant implications for the partitioning of limiting nutrients by plant species, the seasonal pattern of nutrient limitations of aboveground production, and the effective use of N fertilizers in semiarid ecosystems.
Gillespie, Ian G. 2005. Habitat characteristics and distribution of Erodium macrophyllum (Geraniaceae). Madrono. 52(1).
Ecological data are lacking for many species covered in multi-species habitat conservation plans and other regional conservation plans. Erodium macrophyllum (Geraniaceae) is a rare native annual forb found throughout California, southern Oregon, and northern Baja California (Mexico) for which very little ecology is known. I report on the habitat characteristics and current and historical distribution of E. macrophyllum by collecting data on populations in Riverside and San Louis Obispo Counties and by conducting an herbaria search and electronic-mail survey. Erodium macrophyllum population sizes range from a few individuals to over 1000 and are restricted to heavy clay soils. Most populations are found on the eastern side of the coast ranges in California. The clay soils on which it is found typically have low cover of native and exotic species but often have other rare species. They are also generally low in nitrogen and phosphorus, but there is considerable variation. Based on herbaria records and the e-mail survey, E. macrophyllum was probably once more common and may have occurred on other soil types.
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