Research on the ecology and life-history of a rare California endemic mint, Monardella venosa

Rare species can play important roles in the maintenance of ecosystem processes and the associated ecosystem goods and services on which humans rely for their well-being. Conservation of a rare species requires a thorough understanding of how it functions at the individual, population, community and ecosystem levels. The presented research asks questions pertaining to the life history and ecological interactions of the Butte County population of the rare California endemic plant, Monardella venosa. Research on the life history of M. venosa found the plant to be capable of self-pollination but likely dependant on cross-pollination for maximum reproductive output. Analysis of the reproductive success of the sampled population of M. venosa found the mean seed-ovule ratio to be less than half of maximum but no mechanism was identified for this reduced reproductive success. Reproductive potential and output varied spatially within the population. Monardella venosa growing in the south canyon area of the study site produced more seeds and had greater seed-ovule ratios when compared to plants in the north canyon. Seed germination rate was high (91.8%) across all treatments in a partial factorial design seeking to identify factors influencing seed germination in the species. Although field observations indicate a strong affinity of M. venosa to the visually distinctive patches of dense clay soil where the plant currently grows, greenhouse and field experiments did not identify any significant mechanisms explaining the observed pattern. Cluster analysis of the vegetation community associated with M. venosa found it to be most closely associated with native versus non-native forbs. To assess the effect of invasive species on the reproduction and growth of M. venosa, a split-plot design consisting of invasive removal and control sub-plots was implemented. Monardella venosa growing in invasive removal sub-plots exhibited a significantly lower relative growth rate but achieved approximately double the stem mass and quadruple the reproductive output than M. venosa growing with invasive plants in control sub-plots. The contrasting pattern of faster growth but lower reproductive success of plants in control sub-plots points to competition for light resources as the mechanism driving this interaction. The results of this research provide quantitative data valuable for the analysis of taxonomic rarity in general, and understanding the response of rare plants to important drivers of global change such as habitat fragmentation and invasive species. ix