Why are annual plants rarely spring ephemerals?

The period of active growth for spring ephemeral plants coincides with the period of high light, water and nutrient availability between snow melt and canopy closure in the understorey of deciduous forests in eastern North Am erica. However, low temperatures prevail during this period and might restr ict the performance of these plants. Remarkably, this peculiar phenology is extremely rare among annual plants. To understand better the role of light and water availability and of temperature in the phenology of spring ephem eral plants, we investigated the effects of two temperature regimes (low: 1 6/7 degreesC and high: 21/14 degreesC), three water availability levels (sa turated, control and drought), and three photosynthetically active photon f lux densities: low (85-100 mu mol m-(2) s(-1)); intermediate (182-196 mu mo l m(-2) s(-1)); high (437-454 mu mol m(-2) s(-1)) on the growth and reprodu ction of the annual Floerkea prosperpinacoides. Total biomass, total leaf a rea and flower and seed production increased with increasing temperature, w ater availability and light intensity. Total leaf area and total biomass we re reduced in plants that were stressed under drought. However, at high tem peratures, this reduction was less pronounced when droughted plants were pa rtially shaded. At low temperatures, plants began to senesce after approxim ately 9 wk, whereas at higher temperatures, signs of senescence appeared af ter only 7 wk of growth. Despite shorter longevity, total biomass was appro ximately 1.5 times higher in the control water treatment at higher than at lower temperatures as a result of greater above-ground growth, and plants a llocated a significantly greater proportion of mass gain to seed production . Although F. prosperpinacoides can tolerate low temperatures such as those typical of early spring, higher temperatures such as those of late spring/ early summer are more favorable for growth and reproduction as long as wate r and light are not limiting. Spring ephemeral annuals might be rare becaus e low temperatures reduce growth rate and extend the life cycle. An annual plant might not have time to reproduce before resource availability deterio rates with canopy closure unless reproduction begins early in the life cycl e of the species.