Population dynamics and ecological characteristics of Cleome multicaulis, a rare annual wetland halophyte of the San Luis Valley, Colorado

Abstract

A three-year study characterized the San Luis Valley, Colorado habitat occupied by Cleome multicaulis, an annual wetland halophyte. Soils occupied by C. multicaulis were moist and salinesodic, with mean pH of 8.9 (0.58) and mean electrolytic conductivity of 8.5 (8.6) dS/m. Common co-associates were Distichlis stricta var. spicata and Juncus balticus. The response of Cleome multicaulis to soil pH is distinct from that of soil conductivity, and increases in pH are associated with enhanced reproductive output of C. multicaulis. This study provided no evidence that the germination of C. multicaulis seeds coincided with periods of seasonally low soil salinity. A multi-year field experiment (Kalisz 1991) revealed that the conditional probability of seed emergence (std.error = 1.4) declined with seed age from 6% to 1.6%. Seed bank persistence probabilities were influenced by microsite characteristics and did not decline with seed age. Seeds of C. multicaulis may float and move laterally during seasonal inundation and were observed to endure multi-year flooding. The population structure of Cleome multicaulis is best described as a series of remnant populations persisting temporally as seeds in the soil. Estimates of population growth rates highlight the vulnerability of local populations to above-ground (stem) extinction and the importance of understanding mechanisms of seed bank extinction. Periodic matrix analysis provided the intra-annual detail necessary to distinguish the persistent (1-3 years) and transient (0-1 years) components of seed bank elasticity. Rapidly growing populations of C. multicaulis were characterized by large (> 15cm), widely-spaced plants and greater elasticities of growth rate from the transient seed bank, whereas declining populations (λ < 1) had comparatively greater reliance on the persistent seed bank. Small (< 15cm) reproductive plants appearing late in the growing season contributed little to population growth. Niche dimension in this species is expanded by the presence of a persistence seed bank and management efforts should seek to identify environmental conditions favorable for both above-ground and seed bank growth. Further research is needed to identify pollinator relationships. Management recommendations are made in hopes of increasing the likelihood of C. multicaulis population persistence in the San Luis Valley of Colorado and in Wyoming.