The community composition of Californian coastal sage scrub

Summary Coastal sage scrub is a community found extensively throughout cismontane California south of San Francisco, but has been surprisingly little studied. In the study area, which extends from Santa Barbara to the San Gorgonio Pass, two major floristic groupings can be found. In the basin bounded coastwards by a line drawn along the axis of the Santa Ana Mountains a large number of native and introduced annual herbs and a few shrubs (e.g.Encelia farinosa), rare or absent in the remainder of the study area, characterize one floristic group. In the coastal region the variety of shrub species increases, and the herbs are predominantly native and more restricted in number. Eleven groups defined by physiognomy, structure and species dominance, and arbitrarily called associations, are recognized. These associations can be grouped into four physiognomic-structural types which transgress the boundaries of the floristic groups. The results of this study and the limited previous literature suggest that Californian coastal sage scrub could be divided, mainly on floristic criteria, into Venturan, San Diegan and Riversidian sage.Plant nomenclature follows Munz & Keek (1968).We gratefully acknowledge the financial help provided by the Department of Earth Sciences, University of California, Riverside, and the aid in plant identification provided by Mr. Oscar Clarke, Museum Scientist, Department of Biology, University of California, Riverside.     

Foraging ecology of the California gnatcatcher deduced from fecal samples

The California gnatcatcher is a threatened species essentially restricted to coastal sage scrub habitat in southern California. Its distribution and population dynamics have been studied intensely, but little is known about its diet. We identified arthropod fragments in 33 fecal samples of the California gnatcatcher to gain insight into its foraging ecology and diet. Fecal samples were collected from adult males, adult females, fledglings, and nestlings. Leaf- and planthoppers (Homoptera) and spiders (Araneae) predominated numerically in samples. Spider prey was most diverse, with eight families represented. True bugs (Hemiptera) and wasps, bees, and ants (Hymenoptera) were only minor components of the gnatcatcher diet. Gnatcatcher adults selected prey to feed their young that was larger than expected given the distribution of arthropod size available in their environment, and chicks were provisioned with larger prey items and significantly more grasshoppers and crickets (Orthoptera) and spiders than adults consumed themselves. Both adults and young consumed more sessile than active prey. Further studies are needed to determine whether arthropods sampled in coastal sage scrub that are common in fecal samples are good indicators of California gnatcatcher habitat. Received: 30 December 1998 / Accepted: 28 April 1999     

Fire history and patterns of Venturan subassociations of Californian coastal sage scrub

Californian coastal sage scrub has floristically distinct subassociations with sharp boundaries in the Santa Monica Mountains. This mesoscale biogeographic pattern has been variously attributed to the timing and pattern of fire and to differences in the moisture availability on sites. An examination of the actual fire history of sites reveals that recent fire events are unlikely to have caused the observed patterns. Sites with similar fire histories are not as similar in vegetation as sites with different fire histories but similar aspect. Single short fire intervals do not result in dissimilar communities; fires are unlikely to recur with the same boundaries as these two subassociations. A simulation of shrub response to fire interval and intensity indicates that long-term fire trends are also unlikely to have caused the distinction or the sharp boundaries of the two subassociations. Both subassociations include species with life history characteristics adapted to short and to long fire intervals. The subassociations exist without regard to fire history.     

Bridging the gap between micro - and macro-scale perspectives on the role of microbial communities in global change ecology

In order to understand the role microbial communities play in mediating ecosystem response to disturbances it is essential to address the methodological and conceptual gap that exists between micro- and macro-scale perspectives in ecology. While there is little doubt microorganisms play a central role in ecosystem functioning and therefore in ecosystem response to global change-induced disturbance, our ability to investigate the exact nature of that role is limited by disciplinary and methodological differences among microbial and ecosystem ecologists. In this paper we present results from an interdisciplinary graduate-level seminar class focused on this topic. Through the medium of case studies in global change ecology (soil respiration, nitrogen cycling, plant species invasion and land use/cover change) we highlight differences in our respective approach to ecology and give examples where disciplinary perspective influences our interpretation of the system under study. Finally, we suggest a model for integrating perspectives that may lead to greater interdisciplinary collaboration and enhanced conceptual and mechanistic modeling of ecosystem response to disturbance.     

Measures of resilience: the response of coastal sage scrub to fire

Measures of four components of resilience are developed and used to quantify the response of coastal sage scrub to fire in southern California: (1) elasticity (rate of recovery following disturbance), (2) amplitude (threshold of disturbance beyond which recovery to the original state no longer occurs), (3) malleability (extent of alteration of the new stable-state from the original) and (4) damping (extent and duration of oscillation in an ecosystem parameter following disturbance). Vegetation and soil properties measured before fire, and for the first 5–6 yr after fire on four coastal (Venturan association) and four inland (Riversidian association) sites of coastal sage were used to follow changes. In addition, results from a simulation model of post-fire succession in Venturan coastal sage scrub (the FINICS model of Malanson) were used to examine resilience behavior over a 200 yr period. Resilience behavior of coastal sage scrub is critically influenced by the presence of a competitive mix of inherently strongly and weakly resprouting species. Sites dominated by weak resprouters exhibit lower elasticity and less damping of year-to-year fluctuations in composition in the early post-fire years. Sites with a mixture of weak and strong resprouters have a lower threshold of disturbance (amplitude) before species extirpation occurs, a result intensified by a higher frequency of disturbance. Malleability is also greater in these systems under higher disturbance frequency.Nomenclature follows. P.A. Munz, 1974. A Flora of Southern California. Univ. California Press.     

Xerie Mediterranean-type shrubland associations of Alta and Baja California and the community/continuum debate

A survey of the xeric shrublands of Pacific coastal North America from San Francisco to El Rosario (Mexico), including the inner Channel Islands, was conducted using 99 sample sites of 0.063 ha size. TWINSPAN classification and DECORANA ordination confirmed the existence of two plant formations, distinguishable physiognomically: coastal sage scrub and coastal succulent scrub. Within coastal sage scrub, four floristic associations were recognized: Diablan, Venturan, Riversidian and Diegan. Within coastal succulent scrub, two floristic associations were defined: Martirian and Vizcainan. These associations occur in distinct geographical regions following the coastline, with the Riversidian association occurring in the basin inland from Venturan and Diegan regions. Their locations are strongly correlated with differences in evapotranspirative stress regimes. The Channel Island sites show affinities to several of the mainland associations. The Venturan association can be further subdivided floristically into two subassociations, dominated by Salvia mellifera and S. leucophylla respectively. These subassociations which are coextensive geographically at a regional scale, typically do not intermingle at a local scale but often meet along sharp boundaries in the landscape. The dominant species segregate by moisture preference, S. mellifera preferring coarser-texture soils and more southerly aspects than S. leucophylla. Richness and equitability of these sites are depressed relative to other xeric shrubland sites, reflecting the fact that the two subassociations partition the Venturan flora into substantially non-overlapping subsets of species. This segregation of associates between the two Salvia dominance types strongly suggests biotic influence of the dominants on subordinate species, perhaps mediated by allelopathy. This biotic interaction, leading to relatively strong floristic subassociations segregating independently in the landscape, would provide an example of the holistic community structure referred to by Clements and his followers, embedded within a larger pattern of continuity in species distributions.Nomenclature follows Munz & Keck (1959), Munz (1974) and Wiggins (1980).I am grateful to the following for research assistance: S. Coon, E. Hobbs, S. Lavinger, J. F. O'Leary, K. R. Preston, B. Rich and A. Troeger. I also thank the numerous public and private land owners who permitted access to the study sites. This research is based upon work supported by the National Science Foundation under grant DEB 76-81712.     

Influence of N2-fixing Trifolium on plant species composition and biomass production in alpine tundra

Alpine Trifolium species have high rates of symbiotic N₂-fixation which may influence the abundance and growth of plant species growing near them. The potential for facilitative effects on plant abundance and growth in dry meadow alpine tundra of Niwot Ridge, Colo., characterized by low resource availability, was investigated by measuring soil N, aboveground biomass production, and plant species composition in patches of Trifolium dasyphyllum and surrounding tundra. Extractable inorganic N was more than twofold greater and extractable P was 27% lower in Trifolium patches than in surrounding tundra. Aboveground production was twofold greater in Trifolium patches than in surrounding tundra. However, the difference was largely due to the production of T. dasyphyllum relative to the non-Trifolium component of biomass, which was not different between the Trifolium patches and surrounding tundra. In the Trifolium patches, the proportion of graminoid biomass was lower while the proportion of forb biomass was higher relative to surrounding tundra. Although the abundance of some species was positively associated with the presence of Trifolium, other species were less abundant, possibly due to increased competition for P and differential abilities of alpine species to respond to increased N availability. Trifolium may exert both facilitative and inhibitive effects on dry meadow alpine species and, in the process, substantially influence the spatial heterogeneity in community structure and primary production. Received: 14 October 1997 / Accepted: 2 February 1998     

Shrubland formations and associations in mediterranean-desert transitional zones of northwestern Baja California

The area between Ensenada and EI Rosario (Baja California, Mexico) has long been considered as a transitional zone in which two great ecoclimatic regions (Mediterranean and Tropical-Desert) overlap. The floristic and biotypical diversity of this area was evaluated by analyzing its shrubland formations from a phytosociological point of view. This phytosociological study, carried out according to the Braun-Blanquet method and supported by cluster analysis, describes sixteen shrubland associations from Northwestern Baja California.Floristic diversity of the transitional zone was evaluated using two indices, endemic value (EV) and endemic community value (ECV), which are related to the degree of endemism in the flora and plant associations. The phytosociological analysis showed that the high number of shrubland associations found in this area reflected its transitional character. The closer the associations are to the transitional zone, the higher their biotypical and floristic diversity.Abbreviations ADE Adenostoma fasciculatum - AES Fraxinus trifoliata-Aesculus parryi - BER Bergerocactus emoryi-Agave shawii - DES Echinocereus engelmannii-Agave deserti - ENC Viguiera deltoidea-Encelia asperifolia - EUR Eurotia lanata-Yucca schidigera - FOU Agave cerulata-Fouquieria columnaris - FRA Atriplex julacea-Frankenia palmeri - HYM Baccharis glutinosa-Hymenoclea monogyra - KEC Clematis lasiantha-Keckiella antirrhinoides - LAR Ambrosia chenopodifolia-Larrea tridentata - LYC Ephedra californica-Lycium brevipes - MAH Malosma laurina-Heteromeles arbutifolia - MUN Salvia munzii-Artemisia californica - ROS Rosa minutifolia-Aesculus parryi - SAL Salvia apiana-Viguiera laciniata     

Elevated [CO2] and increased N supply reduce leaf disease and related photosynthetic impacts on Solidago rigida

To evaluate whether leaf spot disease and related effects on photosynthesis are influenced by increased nitrogen (N) input and elevated atmospheric CO₂ concentration ([CO₂]), we examined disease incidence and photosynthetic rate of Solidago rigida grown in monoculture under ambient or elevated (560 μmol mol⁻¹) [CO₂] and ambient or elevated (+4 g N m⁻² year⁻¹) N conditions in a field experiment in Minnesota, USA. Disease incidence was lower in plots with either elevated [CO₂] or enriched N (−57 and −37%, respectively) than in plots with ambient conditions. Elevated [CO₂] had no significant effect on total plant biomass, or on photosynthetic rate, but reduced tissue%N by 13%. In contrast, N fertilization increased both biomass and total plant N by 70%, and as a consequence tissue%N was unaffected and photosynthetic rate was lower on N fertilized plants than on unfertilized plants. Regardless of treatment, photosynthetic rate was reduced on leaves with disease symptoms. On average across all treatments, asymptomatic leaf tissue on diseased leaves had 53% lower photosynthetic rate than non-diseased leaves, indicating that the negative effect from the disease extended beyond the visual lesion area. Our results show that, in this instance, indirect effects from elevated [CO₂], i.e., lower disease incidence, had a stronger effect on realized photosynthetic rate than the direct effect of higher [CO₂].     

Earthworm invasions of ecosystems devoid of earthworms: effects on soil microbes

Recent studies document North American earthworm invasions and their profound effects on the structure of the soil profile, which is the habitat for soil microorganisms (mainly fungi and bacteria). Dramatic alterations made to these layers during earthworm invasion significantly change microbial community structure and therefore microbial activities such as C transformations. Understanding the impacts of earthworm invasion on the microbes themselves will give insight into earthworm effects on microbial activities. Bacterial and actinomycete communities in earthworm guts and casts have not been studied in environments recently invaded by earthworms. Earthworm invasion tended to decrease fungal species density and fungal species diversity and richness. The presence of earthworms decreased zygomycete species abundance probably due to disruption of fungal hyphae. Physical disruption of hyphae may also explain decreased mycorrhizal colonization rates, decreased mycorrhizal abundance and altered mycorrhizal morphology in the presence of earthworms. Mixing of organic layers into mineral soil during earthworm invasion tended to decrease microbial biomass in forest floor materials while increasing it in mineral soil. In newly invaded forest soils, microbial respiration and the metabolic quotient tended to decline. In forests where either the microbial community has had time to adapt to earthworm activities, or where the destruction of the forest floor is complete, as in invasions by the Asian Amynthas hawayanus, the presence of earthworms tends to increase the metabolic quotient indicating a shift to a smaller, more active microbial community.