- 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
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.