Density-dependent variation in reproductive success in a terrestrial orchid

Within population variation in plant density can affect reproductive succes s and breeding systems. We examined such effects in Listera cordata R. Br., a north temperate terrestrial orchid, in a subalpine population in Colorad o, USA. Hand pollinations showed that L. cordata was self-compatible as all pollinations produced fruits. Cross-pollinations differed significantly fr om self-pollinations, and had a higher number of seeds per capsule and high er % of fertilized ovules. Together they could compensate for the transmiss ion advantage of self-pollinations. Average number of flowers per infloresc ence was 4.2-4.4 and average fruit set was 20.7-23.7%. By establishing 36 p lots with varying plant densities (1, 4, 8 and 16) and emasculating all flo wers in target plants of a set of 20 plots, we estimated density effects on fruit and seed production. Plant density had no effect on the amount of re production except in reduced seed production of highly dispersed plants. Ou r results suggest that inbreeding depression may be an important selective factor in L. cordata. Further studies are needed to define its role in the evolution of mating systems in this species.