Research Article |
Corresponding author: Pei-Luen Lu ( peiluen@mail.dyu.edu.tw ) Academic editor: Pavel Stoev
© 2016 Pei-Luen Lu, John DeLay.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Lu P-L, DeLay JK (2016) Vegetation and fire in lowland dry forest at Wa’ahila Ridge on O’ahu, Hawai’i. PhytoKeys 68: 51-64. https://doi.org/10.3897/phytokeys.68.7130
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Long-term ecological studies are critical for providing key insights in ecology, environmental change, natural resource management and biodiversity conservation. However, island fire ecology is poorly understood. No previous studies are available that analyze vegetative changes in burned and unburned dry forest remnants on Wa’ahila Ridge, Hawai’i. This study investigates vegetation succession from 2008 to 2015, following a fire in 2007 which caused significant differences in species richness, plant density, and the frequency of woody, herb, grass, and lichens between burned and unburned sites. These findings infer that introduced plants have better competitive ability to occupy open canopy lands than native plants after fire. This study also illustrates the essential management need to prevent alien plant invasion, and to restore the native vegetation in lowland areas of the Hawaiian Islands by removing invasive species out-planting native plants after fire.
Fire ecology, island ecology, O’ahu, restoration, vegetation ecology
Fire has a significant influence on global ecosystems (
Islands are good locations to study the influence of biological mechanisms on ecosystem-level properties (
The mesic and dry forests of the Hawaiian Islands have been reduced due to habitat loss, including development, and the introduction and spread of invasive plants and animals (
While agriculture and alien plant invasion are responsible for significant landscape transformations in the Hawai’i, fires cause dramatic and immediate changes to the original vegetation (
In this study, the impacts of fire were examined by comparing the difference between burned and unburned plots. Plot level studies can provide an understanding of general post-fire sequences in forest ecosystems, landscape-scale monitoring, and analysis of post-fire recovery trajectories (
The study site is in Wa’ahila Ridge State Recreation Area on O’ahu in Hawai’i, at 21°18'1"N and 157°48'41"W. The average elevation of burned and unburned sites is about 100 m. Mean annual precipitation is 1039-2400 mm, with 64-80% falling October through March (
The date of fire was July 5, 2007. It burned for at least 6 hours and covered about 20 hectares (State of Hawai’i 2007). Seven years of data were collected on the following occasions: Oct. 2, 2008; Oct. 12, 2009; Nov. 1, 2010; Oct. 1, 2011; Oct. 7, 2012; Oct. 5, 2013; and Oct. 4, 2014. Fifty unburned and 50 burned 1 m2 plots were examined. The plots were randomly distributed within unburned and burned sites, but at the same site from year to year. Species richness, plant density, species dominance (based on frequency), and species diversity were examined on unburned area and burned areas.
Sampling followed the protocol of The Nature Conservancy of Hawai’i for long-term vegetation monitoring (
One year after the fire, the proportion of alien species on unburned and burned sites was similar (Table
Species re-sprouting and seed regenerating on unburned and burned sites one year after the fire. Heteropogon contortus and Waltheria indica are native species in Hawai’i. Heteropogon contortus, Chloris barbata, and Urochloa maxima are grass species. T is tree. G is grass. F is forb.
Unburned site | Type | Burned site | Type |
---|---|---|---|
Haematoxylum campechianum | T | Stapelia gigantea | F |
Leucaena leucocephala | T | Urochloa maxima | G |
Chloris barbata | G | Leucaena leucocephala | T |
Grevillea robusta | T | Chamaecrista nictitans | F |
Heteropogon contortus | G | Agave attenuata | F |
Pimenta dioica | T | Chloris barbata | G |
Urochloa maxima | G | Waltheria indica | F |
Lichens (additional information) | Fucraea foetida | F | |
Kalanchoe pinnata | F | ||
Hyptis pectinata | F | ||
Senna septemtrionalis | F | ||
Murraya paniculata | F |
Unburned site | Burned site |
---|---|
Woody plant: Leucaena leucocephala | Grass plant: Urochloa maxima |
Individuals: 427 | Individuals: 333 |
48/50 plots | 49/50 plots |
Percentage (%) Frequency of species on unburned and burned sites one year after the fire.
Unburned site % | Burned Site % | ||
---|---|---|---|
Haematoxylum campechianum | 12 | Stapelia gigantea | 6 |
Leucaena leucocephala | 96 | Urochloa maxima | 98 |
Chloris barbata | 2 | Leuceana leucocephala | 10 |
Grevillea robusta | 18 | Chamaecrista nictitans | 4 |
Heteropogon contortus | 52 | Agave attenuata | 4 |
Pimenta dioica | 4 | Chloris barbata | 2 |
Urochloa maxima | 40 | Waltheria indica | 26 |
Lichens (additional information) | 94 | Fucraea foetida | 22 |
Kalanchoe pinnata | 6 | ||
Hyptis pectinata | 2 | ||
Senna septemtrionalis | 2 | ||
Murraya paniculata | 8 |
Species richness and plant density are greater on unburned sites than burned sites (Figs
The frequency of lichens, woody plants, forb plants, grass on the unburned site and the burned site per 1 m2. Error bars are ± SD (n=50). Lichens: T-value = 27.71; P-value < 0.001. Woody plants: T-value = 18.96; P-value < 0.001. Forb plants: T-value = -7.90; P-value < 0.001. Grass: T-value = -3.72; P-value =0.001.
Understanding long-term succession and fire ecology is essential interpreting ecosystem fire responses and planning vegetation restoration. The results indeed supported the three hypotheses. The vegetation structure indicates that invasive plants were favored by the fire disturbance on Wa’ahila Ridge which created open spaces for pioneer species to establish seedlings more easily. Additionally, the environment surrounding the burned area was already dominated by invasive species which dominate the seed bank and few native species existed in the vicinity to contribute to the seed bank. Alien plants invasions in native ecosystems have become a topic of great concern in recent years, particularly in isolated island ecosystems such as the Hawaiian Islands (Loop and Mueller-Dombois 1989,
The invasive grass Urochloa maxima was the dominant species on burned sites, whereas the alien woody plant Leuceana leucocephala was the dominant species on unburned sites. However, a high proportion of the native grass Heteropogon contortus, and U. maxima was also present on unburned sites. This indicates that after the fire U. maxima has a better competitive ability than the woody alien species Leuceana leucocephala and the other alien grass species Agave attenuata to colonize quickly burned sites. Notably, many species occurred on burned sites that were not present on unburned sites. That suggests that the invasive plants on Wa’ahila Ridge have greater opportunity and ability to replace native plants in the short term after a fire.
Leucaena leucocephala is dominant in, and has highest frequency on, unburned sites. It indicates that this species has the ability to establish a large population on Wa’ahila Ridge. In addition, the native grass, Heteropogon contortus coexists equally with the invasive grass, Urochloa maxima on unburned sites, which infers that Leuceana leucocephala may play a critical role in the coexistence. In contrast, on burned sites, the invasive grass, Urochloa maxima has the highest frequency and other species have low frequency, which shows that the burned area is primarily occupied by the single species of U. maxima. Year by year, the burned site became dominated this single invasive species, with scattered Agave attenuata, another dryland invasive species recently occurring in Hawai’i.
Although total species numbers on burned sites are higher than on unburned sites, the species numbers per plot are higher on unburned sites than burned sites. The results indicates that the species tends to be concentrated more in plots on the burned sites, but are more equally distributed over all plots on unburned sites. A possible explanation is that burned sites are dominated by grass and unburned sites are dominated by trees.
Plant density is greater on unburned sites. This indicates that a one-year recovery time is not enough for plant species to reach their maximum population density and burned materials may not provide enough nutrients to seedlings of the other types of plants, or after the fire disturbance invasive plants quickly occupy those open canopy and do not allow other species to dominate those areas. However, the plant density is trending towards similarity on burned and unburned sites year by year.
The results of this study support the findings of previous research on vegetation in the lowlands, which have a similar dynamic structure. For example, herbaceous species dominate the immediate post-burn environment, but most generally disappear after three to four years because they were shaded out by the recovering shrubs and trees in California (
In conclusion, this study provides primary data but fundamental information for policy makers develop appropriate conservation strategies to mitigate the serious loss of native habitats on O’ahu, Hawai’i and also provides useful information for island ecosystems and tropical vegetation ecology. Seven years of field data show the vegetation changes over time. It suggests the vegetation will not be recovering to the pre-fire state and even worse. Other studies have shown that post-fire vegetative structure does tend to return to its pre-fire state (
Professor Donald R. Drake and Lara S. Reynolds gave helpful suggestions in this study. Neeva Shrestha, Professor Chalita Bundhuwong, and Asheshwor Man Shrestha assisted with the fieldwork. Thanks also to Professor Clifford W. Morden for identifying species and providing commentary.