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Resistance of stem xylem to water stress-induced cavitation and embolism among chaparral shrub species in California has been extensively studied, providing the opportunity to examine broad patterns in cavitation resistance. We used previously published as well as unpublished vulnerability to cavitation curve data from 16 chaparral shrub species of southern California to examine the variability of cavitation resistance across sites, regions, and seasons. Additionally, these data provided a unique opportunity to address a recent methodological debate within the field of plant hydraulics. We found that different methods, specifically a centrifuge method and a dehydration method, produced similar results (P = 0.184). Vulnerability to cavitation varied seasonally, with species exhibiting greater susceptibility to water-stress induced cavitation during the wet season (P = 0.003). Cavitation resistance did not differ among sites that were less than 10 km apart even though these sites differed in their coastal exposure, precipitation, and temperatures (P = 0.476). However, across larger geographic distances and with increased climatic divergence, cavitation resistance significantly varied (P = 0.005), with populations from a higher rainfall mountain range exhibiting greater susceptibility to cavitation. These data suggest that species may be particularly susceptible to the onset of early summer drought before xylem has hardened. Variation in cavitation resistance may be limited locally, but broadly dispersed species may diverge in cavitation resistance across their range. Maintaining populations that vary in cavitation resistance may be an important component of species conservation planning in an era of increased climatic variability.
Weedy plants may have unique functional traits that distinguish them from other species. We categorized four annual plant species as weedy and five as non-weedy based on their prevalence in disturbed and invaded environments. This designation was tested in a field trial where we scattered equal numbers of viable seeds in 20 different plots and monitored density and cover over three months. The plants a priori designated as weedy had significantly greater cover and densities than species designated as non-weedy. We hypothesized that a suite of functional traits would define the weedy plant habit. We tested this hypothesis by comparing functional traits between weedy and non-weedy plant species. A principal components analysis (PCA) identified three distinct ecological clusters among the analyzed species (weedy forbs, non-weedy forbs, and grasses). The weedy habit was different from the non-weedy habit in several traits (slower growth, heavier diaspores, earlier flowering initiation, and dormant seeds requiring cold-stratification for germination). Weedy annuals in southern California appear to share a suite of traits, suggesting that their success as weeds is linked to adaptive traits. Further understanding of the traits shared among weedy plant species may help screen for native plants that are valuable for ecological restoration of highly invaded landscapes.
Genera in the tribe Cardueae (Asteraceae) include both non-invasive and invasive species widely dispersed in the western U.S. Seed characteristics are important for seed dispersal, seedling growth, and seedling survival. We determined seed characteristics and their variation from natural populations of 22 taxa of Cardueae. We also measured C and N content of seeds for 17 taxa, and conducted a greenhouse growth experiment with five species from this group. We tested the hypothesis that it is possible to distinguish invasive species from non-invasive species based on these characteristics. Seed weight differed significantly (P < 0.0001) among taxa and varied by a factor of 24, from a mean of 1.48 mg for Centaurea solstitialis L. to 35.63 mg for Cynara cardunculus L. subsp. flavescens Wiklund. There were no significant relationships between status as an invasive species and explanatory variables based on logistic regression results (P > 0.05). Seed N, C, and C∶N ratio differed significantly (P < 0.0001) among taxa, but these characteristics were not associated with invasiveness. Relative growth rate (RGR) for greenhouse grown plants ranged from 0.010 to 0.030 g g−1 day−1. Linear regression results indicate that there was no significant relationship between seed weight and RGR or other measures of plant growth and condition and seed weight. For many of the taxa analyzed in this study, the information on seed weight, nutrient content, and growth measures (RGR, net photosynthesis rate) have not been previously reported.
Dichelostemma lacuna-vernalis L.W. Lenz was proposed in 1974 for populations of a diminutive Dichelostemma associated with vernal pool terrain. The author of Dichelostemma in the 1993 edition of The Jepson Manual did not accept the new species, reporting that the morphological and ecological characteristics of D. lacuna-vernalis were within the ranges for D. capitatum (Benth.) Alph.Wood. The purpose of this paper was to test the validity of D. lacuna-vernalis using a morphometric analysis of eighteen morphological characters in populations of D. capitatum and D. lacuna-vernalis sampled in the field and by comparing plants of both taxa grown under common garden conditions. The data were subjected to cluster analysis, principal components analysis, and discriminant analysis. The results of the analysis confirm the morphological distinctness of D. lacuna-vernalis and support its recognition as a separate taxon. Based on current taxonomic concepts in the Brodiaeoideae, this taxon is best recognized at subspecies rank, as D. capitatum subsp. lacuna-vernalis (L. W. Lenz) D.W. Taylor. Populations of D. capitatum subsp. lacuna-vernalis are distinguished by their short scapes (ca. 15 cm), inflorescences with one or two flowers, short (≤4 mm) perianth tubes, and outer perianth lobes that are ovate, decurrent at the base, and wider than the inner perianth lobes.
Devils Postpile National Monument is 319 ha in size, located in Madera County, California, on the western slope of the Sierra Nevada at an average elevation of 2280 m. Though small in size, the monument supports diverse habitats, including forests, chaparral, riparian corridors, meadows, seeps, and ponds. In 2001, the National Park Service conducted a survey to 1) inventory the vascular flora and document with vouchered specimens; and 2) describe the distribution and abundance of species of special management concern (rare and/or non-native). Methods coupled a species-level inventory with vegetation mapping, covered an estimated 70 percent of the monument's surface area, and combined broad and targeted search strategies. Survey results yielded a 121 percent increase (from 169 to 373) in the number of documented plant taxa, representing 60 families and 199 distinct genera. Forty-five percent of species were clustered within six families: Asteraceae, Poaceae, Cyperaceae, Brassicaceae, Onagraceae, and Boraginaceae. The survey found three rare and eight non-native taxa previously unknown from the monument and documented nine potential range extensions. Rare species included Cinna bolanderi (new county record), Hulsea brevifolia, and Mimulus laciniatus. Localized infestations of the non-native and invasive Cirsium vulgare were discovered. Control measures for this species were initiated during the field season and continued in subsequent years.
Willows (Salix spp.) produce many small seeds that are dispersed primarily through the air by winds (anemochory) and sometimes secondarily by flowing water (hydrochory). In this paper I identify another way willow seeds are dispersed – being blown by winds while sailing or floating on the surface of standing water, here termed pleustochory. In experiments, seeds of Salix gooddingii C.R. Ball floated on water for four days and sailed swiftly on the surface of pools when light winds blew, reaching speeds in excess of five meters per minute. Field observations showed that S. gooddingii seeds sailing on water were blown downwind and soon came to rest at the edge of the pool, in their preferred safe sites. This dispersal via pleustochory from unsuitable sites (middle of a pool) into safe sites (edge of pool) is therefore a new example of directed dispersal. Salix gooddingii seed dispersal and seedling densities were studied at a large, remote pool in the Tijuana River Valley, San Diego Co., to examine the influence of pleustochory on S. gooddingii seedling densities. The study focused on a 15-day period when the pool was slowly drying and S. gooddingii seeds were dispersing from a distant, isolated stand. The S. gooddingii seedlings that established during the 15-day period formed a band that encircled the entire pool with the highest densities (3140 seedlings per m2) occurring along the On Shore. The numbers of S. gooddingii seeds arriving in the seedling sites via three dispersal routes – hydrochory, anemochory, and pleustochory – were measured or estimated. A model combining all three dispersal routes accurately predicted the pattern of seedling densities around the pool and estimated that pleustochory accounted for more than 99% of the seedlings along the On Shore. Together these results showed that pleustochory played a vital role in the transport of S. gooddingii seeds to safe sites and was the underlying mechanism producing the pattern of seedling densities around a large pool.
Ceanothus decornutus V. T. Parker is a newly described species found on serpentine outcrops in western Marin County. This taxon has been considered as part of C. jepsonii Greene in recent treatments. Ceanothus decornutus differs in a number of ways from C. jepsonii and suggests relationships to other northern San Francisco Bay species. Beyond the species description and a key to similar taxa north of San Francisco Bay, an analysis of leaf characters is used to indicate possible relationships.