Dormancy, small seed size and low germination rates contribute to low recruitment in Desmodium cuspidatum (Fabaceae)

Abstract

Plant population growth and persistence are strongly influenced by germination and recruitment, which can be dramatically affected by seed dormancy, local site conditions, seed size and seed predation. Understanding factors that limit germination can help to explain low recruitment rates and is particularly important for species of conservation concern. Desmodium cuspidatum has declined dramatically in the northeastern United States and is currently listed as historic, threatened or endangered in all five New England states where it once occurred. Remaining populations exhibit low reproductive success and low recruitment rates, even though seed viability is nearly 100%. Requirements for optimal germination, including the breakdown of physical dormancy, effect of local site characteristics, and seed mass were largely unknown. In addition, while recruitment rates in co-occuring Desmodium species are reportedly higher than those of D. cuspidatum, germination rates in these species were unavailable for comparative purposes. Lastly, the effect of pre-dispersal seed predation, commonly observed in Desmodium species, was unknown. We performed a series of three controlled experiments to assess optimal conditions for germination (D. cuspidatum), the relationship between germination rate and seed size (mass, D. cuspidatum, D. glutinosum and D. paniculatum) and the effect of pre-dispersal seed predation on germination (D. canadense and D. paniculatum). Our results suggest that low recruitment rates observed in New England populations of D. cuspidatum are related to physical dormancy and local site conditions, whereby the highest germination rates are found when seeds are placed on bare soil, regardless of whether they are covered with leaf litter. Germination rates in D. cuspidatum were generally lower than those observed for two more common Desmodium species, D. glutinosum, and D. paniculatum and were positively correlated with seed mass in all three species. Seeds grown with field-collected soil had lower germination rates but higher nodulation rates than those grown in sterilized potting soil. Seed predation by weevils had no detectable difference on germination rates in D. paniculatum and D. canadense.