Reproductive Ecology of a Native Hawaiian Grass (Heteropogon contortus; Poaceae) versus Its Invasive Alien Competitor (Pennisetum setaceum; Poaceae)

Abstract

We compared the reproductive biology of a declining native grass, Heteropogon contortus, and its invasive alien competitor, Pennisetum setaceum, in Hawai'i to identify differences that could explain why the alien has spread so successfully while the native has declined. Both species are drought‐tolerant, perennial, C4 bunchgrasses that rely on apomictic seeds for reproduction. In a series of field observations and greenhouse experiments, we compared the phenology, ovule production, seed production, and seed germination in H. contortus and P. setaceum. In the field, the alien produced two to nine times more seeds per plant than the native. Furthermore, the germination rate for fresh seeds was significantly greater for P. setaceum (45%) than for H. contortus (13%), giving the alien the ability to quickly spread after setting seed. In greenhouse experiments, clipping, burning, and pollen addition did not affect final reproductive output or seed‐set rate in either species. However, P. setaceum recovered from clipping and burning more rapidly both in terms of vegetative regrowth and flowering, which likely gives it an advantage in disturbed environments and in competition with H. contortus. When plants were grown under varying nutrient and water conditions, P. setaceum failed to produce seeds in low‐water and low‐nutrient treatments. In contrast, H. contortus flowered and produced seeds in all treatments, indicating better tolerance of low resources. These findings help explain the current distribution of remnant H. contortus populations in the driest, most nutrient‐poor habitats of the Hawaiian Islands.