Gas-exchange responses to pulse rain events by native Hawaiian dry forest vine seedlings and the effect of interaction with a non-native grass

The dry forests of Hawaii have experienced reduced seedling recruitment since invasion by Pennisetum setaceum, a C4, perennial bunch grass native to Africa. Precipitation timing and amount are highly variable in these forests. In a common garden experiment, I examined the use of precipitation pulses by seedlings of Canavalia hawaiiensis, a vine native to the Hawaiian dry forest. Canavalia seedlings that were grown alone responded to water pulses of as little as 5 mm, but at least 20 mm was required for a lasting (≥3 day) response. When grown with Pennisetum, the responses of Canavalia seedlings was similar, but their absolute gas exchange rates were never as high as in those grown alone. This suggests that Pennisetum lowers gas exchange overall, but does not inhibit Canavalia's ability to respond to water pulses. These results indicate that Pennisetum alters resource dynamics for Hawaiian dry forest seedlings, specifically, their use of pulse moisture. This impact may contribute to the reduction of native seedling recruitment.