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1 Smoke-stimulated germination in the post-fire flora of California chaparral does not appear to be triggered by nitrate. Application of freshly prepared unbuffered KNO3 solutions (pH c. 6.2) failed to enhance germination of five populations of Emmenanthe penduliflora or one Phacelia grandiflora population, regardless of light or stratification conditions. 2 KNO3 buffered at acidic pH (or unbuffered solutions equilibrated with atmospheric CO2) did induce germination, but KNO3 solutions at pH 7 failed to induce germination. Induction of germination is therefore not due to the nitrate ion per se, but rather to high [H+], although buffered controls gave weak germination at low pH, suggesting a role for H+ plus nitrate. However, other anions such as sulphate were equally as effective as nitrate at breaking dormancy. 3 The germination response to KNO3 was affected by the type of filter paper used and this may be linked to differences in pH. 4 NO2, at concentrations present in biomass smoke, was highly effective at inducing germination, and other oxidizing agents also induced germination. 5 Several growth regulators, including nitrite and gibberellin, were stimulatory only at acidic pH, but KCN was stimulatory across a broad pH range. 6 Germination decreased at smoke exposures longer than a few minutes. Also, smoked water samples effective at breaking dormancy were acidic and were less effective when buffered to pH >7. 7 Physical scarification of the seed coat induced germination but the effect was not due to penetration of a water barrier, or to enhanced oxygen uptake or to wound responses such as CO2 or ethylene production. 8 Different effects of the gibberellin inhibitor CCC (chlorocholine chloride) suggested that the mechanisms of scarification-induced and smoke-induced germination may differ. 9 We conclude that either oxidizing gases in smoke and/or acids generated on burnt sites play a role in germination of post-fire annuals in chaparral. [ABSTRACT FROM AUTHOR]

Examines the species interactions in a successional grassland in California. Species distributions; Seed rain and seedling recruitment; Estimation of seed bank contribution to recruitment using seed exclosures.

(1) Trichostema lanceolatum, a common herb in the California annual grassland, contains 2-9% volatile essential oil in its fresh foliage. Vapours and aqueous extracts from its leaves have been shown to inhibit germination and growth of other plants in vitro. The present study evaluated the potential of T. lanceolatum to restrict plant growth in the field. (2) Dry soil charged with T. lanceolatum volatiles in the laboratory reduced radicle elongation of seeds, but dry seeds and dry soil exposed to volatiles under T. lanceeolatum plants in the field did not show appreciable inhibition. (3) Trichostema lanceolatum throughfall from early-season rains was at most, only weakly toxic. Soil at the base of T. lanceotatum plants was not rendered toxic by throughfall and stemflow from the first autumn rain. (4) Water and ether extracts of dead, brown T. lanceolatum leaves were very toxic in bioassays in Petri dishes, but similar leaves mixed with soil had little alletopathic effect. (5) Trichostema lanceolatum recallings were no more resistant to volatiles of T. lanceotatum essential oil than seedlings of other annual grassland species. (6) The allelopathic potential of T. lanceolatum under natural conditions appears to be low. This conclusion is surprising since both green and dead, brown leaves from it were much more inhibitory in laboraory bioassays than were leaves and litter from Salvia leucophylla, a species previously reported to be strongly allelopathic. [ABSTRACT FROM AUTHOR]

1 Resprouting capacity (R) and propagule-persistence (P) are traits that are often considered to have evolved where there are predictable crown fires. Because several indicators suggest a stronger selective pressure for such traits in California than in the Mediterranean Basin, we hypothesize that plant species should have evolved to become R+ and P+ more frequently in California than in the Mediterranean Basin. 2 To test this hypothesis we studied the phylogenetic association between R and P states in both California and the Mediterranean Basin using published molecular phylogenies. 3 The results suggest that R and P evolved differently in the two regions. The occurrence of the states differs significantly between regions for trait P, but not for trait R. The different patterns (towards R+ and P+ in California and towards R+ and P– in the Mediterranean Basin) are reflected in the higher abundance and the wider taxonomic distribution of species with both persistence traits (R+P+ species) in California. 4 The differential acquisition of fire persistence mechanisms at the propagule level (P+) supports the idea that fire selective pressures has been higher in California than in the Mediterranean Basin. 5 Our comparative phylogenetic-informed analysis contributes to an understanding of the differential role of the Quaternary climate in determining fire persistence traits in different Mediterranean-type ecosystems and, thus, to the debate on the evolutionary convergence of traits. [ABSTRACT FROM AUTHOR]

Summary 1 Spatial pattern analyses were used to generate hypotheses about the processes that shape the structure of a plant community in the Mojave Desert of North America, with a focus on the semi-shrub Ambrosia dumosa . We analysed spatial distributions and sizes of this species relative to other semi-shrubs, shrubs and annuals, and the relationships between spatial patterns and abiotic and biotic habitat characteristics. 2 The analyses were based on maps of sample plots placed along a transect spanning two adjacent geological substrates: aeolian sand and gravelly, sandy to loamy alluvium. Of these two substrates, sand supported higher total biomasses of Ambrosia and of all woody perennials, while alluvium supported on average higher biomasses of winter annuals. 3 Annuals and seedlings of Ambrosia were much more strongly aggregated with Ambrosia canopies on sand than on alluvium, suggesting that these small plants were more strongly facilitated by Ambrosia on sand than on alluvium. 4 Ambrosia semi-shrubs were spatially segregated on sand but aggregated on alluvium, and the degree of segregation on sand increased with the total above-ground biomass of Ambrosia per unit area, indicating that negative interactions between Ambrosia plants were stronger in more productive habitats. Canopy sizes of Ambrosia in all mapped plots increased with distance to the nearest conspecific neighbour, which suggests that neighbour interactions negatively affected plant sizes. 5 Ambrosia plants on sand were spatially aggregated with Acamptopappus sphaerocephalus semi-shrubs, suggesting that at least one of these species may benefit from the association. Ambrosia plants were spatially segregated from Larrea tridentata shrubs on both substrates, possibly due to negative effects of Larrea roots on Ambrosia roots reported in previous studies. 6 Subtle differences in substrate characteristics were correlated with strong differences in the spatial distribution of Ambrosia plants... [ABSTRACT FROM AUTHOR]

Makes observations on the species composition and species population densities of herbaceous plant communities between Quercus agrifolia woodland and adjacent grasslands in California. Seedling mortality in areas of reduced radiation under evergreen oak canopy; Reduced plant density under shaded areas; Survival of plant species with a persistent seedbank.

Studies the demography of annual and perennial species of asteraceae in a coastal grassland in California. Measurement of net productive rates and finite rates of population growth for perennials and annuals; Role of different demographic and life-history traits on the relative fitness of each species; Reproductive maturity of perennials.

Investigates the balance betwen vegetative and sexual reproduction of mimulus primuloides at different altitudes in California. Relationship between biomass and vegetative reproduction; Comparison of vegetative and sexual reproductive allocation; Seed dispersal and establishment; Types of plant reproduction.

Studies the germination and seedling establishment in California annual grassland. Significance of the differences in autumn patterns of establishment; Numbers of germinable seed in the soil prior to the start of the growing season; Comparison of depletion of the seed-bank in the soil and increase of plant density.