Factors affecting seedling recruitment in an invasive grass (Pennisetum setaceum) and a native grass (Heteropogon contortus) in the Hawaiian Islands

In the Hawaiian Islands, native Heteropogon contortus (pili grass) is being replaced by alien grasses, one of which is Pennisetum setaceum (fountain grass). Both grasses depend on seeds for population growth. To help understand factors promoting the spread of the alien and decline of the native, we investigated the effects of physical disturbance, nutrient addition, and seed supplementation on seedling recruitment in experimental field plots. In the first year, our field site experienced an unusual drought, and seedling recruitment was greater for H. contortus than for P. setaceum under all treatments. Disturbance increased recruitment of H. contortus seedlings during some sampling periods. Recruitment was not significantly increased by seed additions for either species despite our finding of only 49 and 4 seeds m^–2 in the seed bank for H. contortus and P. setaceum, respectively. In the first year, most P. setaceum seedlings died between monthly surveys. We resurveyed our field plots in a second, wetter year and found the pattern was reversed: recruitment of P. setaceum seedlings was greater than H. contortus seedlings in most treatments. Greenhouse comparisons of seedling survival under three drought regimes (water every 5,7 and 10 days) revealed that H. contortus seedlings tolerate drought better than P. setaceum seedlings. Seedling recruitment for these species in the leeward Hawaiian Islands appears to be primarily dependent on water availability, with the alien having the advantage in wetter years. Once seedlings of the long-lived alien become established, the alien seems capable of maintaining its dominance over H. contortus, even during periods of drought.