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

Published source details Goergen E. & Daehler C.C. (2002) Factors affecting seedling recruitment in an invasive grass (Pennisetum setaceum) and a native grass (Heteropogon contortus) in the Hawaiian Islands. Plant Ecology (formerly Vegetatio 1948-1996), 161, 147-156.
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Summary

In the Hawaiian Archipelago, many introduced African grasses have invaded disturbed habitats. One of the most aggressive is fountain grass Pennisetum setaceum, which in recent decades has replaced the native pili grass, Heteropogon contortus in many arid habitats on O'ahu and Hawai'i. Both species are perennial bunchgrasses that rely on recruitment by seeds. In addition to faster vegetative growth of P.setaceum reported in earlier studies, differences in conditions favoring seed germination and establishment, may at least partly explain the invasion by P.setaceum into former H.contortus-dominated grasslands. To help elucidate factors promoting the spread of feather grass and decline of pili grass, the effects of seedling survival under three drought regimes (water every 5, 7 and 10 days) was investigated.

Greenhouse drought tolerance trials: H.contortus and P.setaceum seedlings were exposed to three different drought treatments in a greenhouse (under natural light; temperature range from 34ºC during the day to 24ºC at night) at the University of Hawaii, Manoa, USA.

Seeds were sown (1 seed/0.25 m-length of dibble tube) in a mix of local topsoil and peat moss (3:1 ratio) and watered every other day for the first two weeks to promote germination. When approx. 5 cm tall, seedlings were randomly assigned to one of three drought treatments: watered with 10 ml of water every 5 days, every 7 days, or every 10 days. A set of seedlings watered to field capacity each day served as a control.

Mortality was recorded weekly. At the end of the 12 week experimental period, all seedlings that had been classified as dead were watered to field capacity daily for 2 weeks to ensure that they were indeed dead (i.e. that they did not resprout).

In all three drought treatments, seedling survival was greater in H.contortus than P.setaceum: For example at three, 4-weekly intervals through the 12 week study period, percentage survival was:

After 4 weeks:
H.contortus - 5 day treatment - 100%; 7 day treatment - 94%; 10 day treatment -100%
P.setaceum - 5 day treatment - 87%; 7 day treatment - 60%; 10 day treatment -15%

After 8 weeks:
H.contortus - 5 day treatment - 100%; 7 day treatment - 80%; 10 day treatment - 9%
P.setaceum - 5 day treatment - 42%; 7 day treatment - 0%; 10 day treatment - 0%

After 12 weeks:
H.contortus - 5 day treatment - 100%; 7 day treatment - 36%; 10 day treatment - 9%
P.setaceum - 5 day treatment - 22%; 7 day treatment - 0%; 10 day treatment - 0%

The well-watered control seedlings suffered no mortality.

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