Graduate Thesis Or Dissertation

Interactive influences of wildfire and nonnative species on plant community succession in Hawaii Volcanoes National Park

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  • The role of fire as a natural disturbance, its interactions with nonnative species and effects of repeated fires in the Hawaiian Islands have received little investigation. We are unsure of the role fire played in shaping forest structure and composition as well as affecting evolutionary processes of the native biota. Yet, many species do have adaptations that facilitate their capacity to establish, grow, reproduce, and persist on either the individual or the population level when fire occurs. The objectives of this study were to document individual survival and colonization of native Hawaiian species after fire and to examine the potential interactions of nonnative species and fire. Specifically, I hypothesized that (1) many native Hawaiian species would survive and or colonize the postfire environment because they are adapted to a wide array of disturbance events, (2) the interaction of fire and nonnative species would alter native plant community succession because fire would facilitate nonnative species invasions, and the presence of nonnative species would limit native species recovery, and (3) the occurrence of a second fire within one year would result in a more impoverished native flora because sprouts from native surviving trees would be killed by the second fire. To understand the role of fire in tropical forests of Hawaii and how forest species respond to fire, I established replicate plots (n=5) in burned and unburned areas in five vegetation communities along an elevation/community gradient in Hawaii Volcanoes National Park. At lower elevations the sampled plant communities were two shrubdominated communities (Dodonaea viscosa/ Andropogon virginicus and Dodonaea/ Nephrolepis multiflora) and at higher elevations three forest communities (Metrosideros polymorpha/ Nephrolepis multiflora, Metrosideros/ Dicranopteris linearis, and Metrosideros/ Cibotium glaucum). Fires in all community types were stand-replacing, where >95% of the dominant native woody species were top-killed. Results from this study indicate that many native Hawaiian species had the capacity to survive fire vegetatively and/or established from seed in the postfire environment. Nineteen native tree, shrub and tree fern species survived fire primarily by sprouting from the base. Many of these species also established from seeds or spores postfire. Metrosideros, in particular, both exhibited widespread survival (>50%) primarily via basal sprouting and established from seed postfire. In addition, the effects of fire differed across species, populations and vegetation communities along the elevation gradient. Fire differentially affected the communities with greater differences in composition and structure observed in the three forest communities than the shrubdominated communities. In the forested communities, fire dramatically altered structure from a closed-canopy Metrosideros forest to shrub, fern and herb dominated sites. Understory cover differed between unburned and burned forest sites with reduced cover in the Nephrolepis and Dicranopteris forests and greater cover in the Cibotium forest. In the previously native-dominated Dicranopteris and Cibotium forest communities, nonnative species became increasingly abundant following fire suggesting that fire facilitated nonnative species invasion in these communities. The native fern Dicranopteris linearis was the most abundant understory species in the unburned sites, but nonnative ferns and vines dominated the understory in the burned sites postfire. Species richness, percent nonnative, and understory diversity were greater in the burned sites two years postfire than the unburned sites for each community. In contrast, in the Nephrolepis forest community the nonnative fern Nephrolepis multiflora dominated the understory (>50% cover) in both the unburned and burned sites. Metrosideros survival and recovery, quantified as basal sprout height, elliptical crown area and volume, differed among forest communities. Measures of sprout vigor were greatest two years following fire in the native Dicranopteris forest, where understory recovery was slowest presumably due to the thick litter layer that remained following fire acting as a barrier to understory colonization. Postfire vegetation composition and cover of the understory in the Nephrolepis and Cibotium forests was due largely to vigorous Nephrolepis multiflora sprouting and Paspalum conjugatum grass invasion, respectively. In addition, Cibotium glaucum tree ferns in the subcanopy tier had very high survival rates (>85%) and constitute a large portion of cover in the Cibotium forest community. Lower Metrosideros sprout growth rates in the Nephrolepis and Cibotium forest communities suggest that the high survival of tree ferns (Cibotium forest) and the rapid establishment of a nonnative-dominated understory (Nephrolepis and Cibotium forests) may be limiting Metrosideros tree recovery during early postfire succession. The occurrence of two fires in two years in some Dicranopteris and Cibotium forest communities dramatically increased mortality of Metrosideros. In the Dicranopteris community, 71% of Metrosideros trees survived a single fire, but only 22% survived repeated fires. Similarly in the Cibotium community, Metrosideros survival was reduced from 48% to 6% following repeated fires. Vegetative survival of the native tree fern Cibotium glaucum was also significantly reduced from 93% following a single fire to 56% following a second fire. Metrosideros seedling recruitment did not differ between forests that burned once and forests that burned twice. The composition of the understory in both of the sampled communities following repeated fires differed from that of forests that burned once and unburned control forests. Interestingly, the most abundant species in the understories following repeated fires were native sedges (Cyperus polystachyos) and shrubs (Pipturus albidus). However, these species are typically disturbance oriented short-lived species. Repeated fires resulted in lower Metrosideros survival, no significant increase in native tree seedling establishment, and rapid occupation native herbaceous and shrub species, all of which may delay, or even prevent, recovery to native forest dominance. Fire in the shrub-dominated communities, which were already heavily invaded by nonnative species, had little effect on vegetation composition and structure. These communities were previously modified by past fires (1972 and 1992) and nonnative grass (Andropogon virginicus) and fern (Nephrolepis multiflora) invasions. Notably absent from these communities were young native tree species suggesting that native forest recovery was not occurring. These communities demonstrate how nonnative species invasions coupled with repeated fires may alter successional trajectories such that native forest recovery is less likely.
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