Postfire Demography of Thermopsis macrophylla H. A. var. agnina J. T. Howell (Fabaceae), a Rare Perennial Herb in Chaparral

Abstract

-Demography of the rare perennial herb Santa Ynez thermopsis (Thermopsis macrophylla var. agnina) was monitored at two sites for 9 yr after a controlled burn in the Santa Ynez Mountains near Santa Barbara, California. One of the burned sites was located in chaparral and the other in a fuelbreak, an area purposely cleared of chaparral to slow the advance of wildfire. Numerous individuals resprouted in the 1st yr after the fire and thousands of seedlings established from heat-stimulated germination of refractory seed. Vegetation before and after the fire was markedly different between the two sites. Despite this difference, however, seedling production was very similar on the two sites. Losses of recruits-seedlings that survived beyond the 1st yr-also were similar. Nine yr after the fire the density of recruits was equivalent on both sites. Recruits were significantly taller in the fuelbreak than in the chaparral but the average number of stems/individual was not different. Individuals that resprouted after the fire produced more than 106 seeds/900 m2 from 1980-1987. However, only 3.9% of the seed was present in the soil 8 yr after the fire. Seed production by recruits, in contrast, was very low and appeared to be declining. Thus, this species relies heavily on seed production by resprouts to maintain a sizable soil-seed bank. In addition to seed production by resprouts, the survival of recruits during the interfire period is a critical life history stage in the population biology of this species. Recruits that survive both the interfire period and the next fire show marked increases in stem production, seed output and survivorship.