This statistical method employs a combination of the Relative Frequency and the Relative Abundance of species in the plots in each habitat in order to calculate an Indicator Value. The Monte Carlo randomization estimates the probability that the Indicator Value of a given species would differ from the expected value if plots were randomly distributed among habitats.
No significant differences in mean number of individuals per plot of any height class were observed among the four different habitats table 1. The largest variation among plots was seen in the understorey near the burned area UB , principally among the smallest height classes. This variation was associated with the large number of individuals found in one of the plots UB5 , a value significantly above the mean of individuals plot -1 for all habitats combined table 1. The estimated number of species S est for a sample size of individuals was significantly lower in the burned area B when compared to gaps figure 1.
Understorey plots UG and UB did not differ among themselves, nor in relation to the other habitats table 2. The first axis of the DCA showed a clear separation between habitats. Burned plots B and a group formed by plots at the treefall gaps G and the understorey UG were ordered at opposite ends of the 1 st axis, while the understorey plots near the burned area UB were at intermediate position figure 2.
Second axis expressed idiosyncrasies of some plots.
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At this axis, one plot of the understorey near the burned area UB4 , and the gap plot G1 were separated from the other plots of the same habitat. This separation was due in large part to the absence of Merostachys sp. Rubiaceae , and to the presence of a larger number of individuals of Philodendron surinamense Miq. These two axes accounted for When the other habitats G, UG, and UB were compared as pairs, no significant differences were observed in their species composition. The principal indicator species were Pteridium aquilinum L.
Kuhn, a sub-shrub fern, Scleria secans L. Urban, a scandent Cyperaceae, and Miconia mirabilis Aubl. Williams, an arboreal Melastomataceae frequently associated with human-disturbed environments table 3.
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Marchand , and one hemi-epiphyte Cyclanthaceae Evodianthus funifer Poit. In the understorey adjacent to the treefall gaps, two hemi-epiphyte species of Araceae Philodendron surinamense and Rhodospatha latifolia Poepp. In the understorey near the burned area, only the sub-shrub palm Geonoma pauciflora Mart. A total of individuals of tree species including treelets between 0. These represented approximately one third of all plants sampled in this study within this height class.
However, the other habitats UG and UB did not differ from either the burned area or treefall gaps table 4. The estimated species richness for samples of 60 individuals was smaller in the burned area in relation to treefall gaps and to the understorey near the gaps, but did not differ from the understorey near the burned area table 4. The estimated tree species richness did not differ among treefall gaps and the two understorey areas. The understorey areas had no tree species with significant indicator species indices.
This is the first study that sampled small and medium-sized plants of all growth habits in tropical forest environments subjected to different degrees of disturbance. Most previous studies have focused on specific growth habits in disturbed areas, usually tree or shrub species Brokaw , Uhl et al. A great effort was done to have the plant taxa as fully-resolved as possible. These values indicate a high level of identification for tropical plant communities, especially when including all growth habits.
For the sake of comparison of all vascular plants among the four habitats we were not able to have a large number of plots in each habitat, and this should be considered in the following discussion. Large differences among habitats could be properly detected with this sampling effort, but if small differences eventually do exist, they would require a larger sample size to become apparent.
Among the habitats analysed here, the burned area had the clearest differentiation from the others in terms of species composition. Somewhat surprisingly however, it did not demonstrate further differences in other aspects of community structure. A majority of the studies on post-fire regeneration have evaluated areas that had previously been cut slash and burn agriculture , a situation that produces a cleaner post-burning environment.
The present study, however, evaluated an area in which numerous trees had survived fire disturbance and had re-sprouted thus continuing to utilize soil nutrients. Shading by these trees may have inhibited to some extent plants that require high light levels for optimal growth. This seems to be the case for Scleria secans Cyperaceae , Miconia mirabilis Melastomataceae , and Pteridium aquilinum Dennstaeditiaceae , which dominated the taller height classes in the burned area.
In the burned area, the sample-corrected mean number of species of all growth habits was equivalent to that encountered in the two understorey areas. These models predict a rapid recovery of species richness in areas that have suffered only moderate disturbance, although with a different species composition, dominated at first by fast-growing, short-lifespan species. Also, the lower number of species in the burned area compared to the treefall gaps could be related to the strong dominance of these species in the burned area.
The situation observed in the burned area is quite similar to that prediction, principally in light of the sharp separation of the burned area plots in both the DCA and the MRPP analyses. Considering only tree species, there was no difference in the estimated number of species richness between the burned area and the adjacent understorey.
Hence, species richness of trees have been reestablished very early during succession. Although the number of species did not differ, the tree species composition differed significantly between the burned area and the adjacent understorey, mainly because pioneer tree species such as Miconia mirabilis Melastomataceae , Cecropia pachystachya Cecropiaceae and Henriettea succosa Melastomataceae , have occurred exclusively in the burned area.
Otherwise, some species common in the forest understorey, such as Euterpe edulis Mart. Arecaceae , Licania hoehnei Pilg. Chrysobalanaceae and Ecclinusa ramiflora Mart. In fact, the species composition in the burned area was different from the other three habitats, while the treefall gaps did not differ from the understorey areas.
These results suggest that the fire is a factor of higher importance than light in determining changes in species composition after disturbances. Additionally, the community that has been established in the burned area will determine the composition and abundance of seeds in the seed rain and thus will affect the assemblage of the plant community and of the seed bank in the future Young et al. The natural treefall gaps and the two areas of understorey were consistently similar in regards to their species composition, species richness, and density of individuals in the different size classes.
The remarkable similarity observed between the two understorey areas adjacent to the gaps or adjacent to the burned area in all variables analysed in the present study suggests that the forest structure and composition was the same before the disturbances in the whole studied area. The absence of differences in plant density among the treefall gaps and the two understorey areas goes against the idea that natural disturbances such as those formed by treefalls provides a local increase in plant density that is stimulated by the liberation of resources such as light and by diminished competition from the fallen tree itself Brokaw , Denslow An initial increase in the number of individuals that become established in treefall gaps Brokaw , Hubbell et al.
The present study generated similar results, in terms of both the whole plant community, including all growth habits and tree species alone. The lack of any difference in species richness between treefall gaps and the understorey was associated with a similar lack of difference in species composition. There was no large number of species occurring specifically in treefall gaps, as might have been expected from the hypothesis that treefall gaps play an essential role in promoting and maintaining species diversity Denslow , , Only three species occurred preferentially in treefall gaps in this survey Evodianthus funifer, Pouroma mollis , and Protium heptaphyllum.
In general, the treefall gaps in the present study did not show larger numbers of individuals or species than the adjacent understorey, nor a distinct plant community, as would be expected by the niche-based hypotheses. These results seems to be in agreement with the current hypotheses that species composition in treefall gaps is strongly determined by the composition of the understorey before gap formation Uhl et al. Additionally, the absence of differences in species composition between treefall gaps and the understorey could be attributed to the relatively small size of the gaps sampled in this study.
Some authors had suggested that large gaps - greater than m 2 Denslow or 1. However, in previous studies carried on sites that are not usually exposed to large disturbances such as hurricanes or cyclones, the largest gaps sampled were less than m 2 Abe et al. In the present study, all gaps were less than m 2 , but they represent the largest gaps in this forest and all but one were greater than m 2.
In summary, the very high richness of tree species observed in the studied forest Thomas et al. Under such disturbance regime dominated by small gaps, it could be expected an increase in the growth rate of the plants established in the area before the gap formation, instead of changes in the species composition Putz et al. The analysis of growth and survival rates of the established plants in treefall gaps and understorey areas might help to elucidate the role of this mode of disturbance in the forest dynamics.
In the lower stratum analysed, for each arboreal individual surveyed, 2. Likewise, for every tree species, 2 non-tree species were observed Martini These values highlight the importance of sampling non-tree individuals because they could be potential competitors of the arboreal individuals in the lower stratum. In the present study, the same pattern was observed when all growth habits were considered together, and with tree species alone. Hence, the observed patterns of species richness for either tree species or all vascular plants in this forest could not be attributed to gap-niche partitioning.
The relations described here were observed in a forest formation located in an area with poor soils, high rainfall, and no marked dry period. As such, it would be interesting to conduct similar studies including species of all growth habits in areas with a strong seasonal component, in order to verify if the same processes are involved. Acknowledgements - This work is part of the PhD dissertation of A. The authors wish to thank Sr. Saturnino Neto F. ABE, S. Factors influencing sapling composition in canopy gaps of a temperate deciduous forest.
Treefall gap characteristics and regeneration in the laurel forest of Tenerife. Journal of Vegetation Science Neotropical pioneer and shade tolerant tree species: do they partition treefall gaps? Tropical Ecology Gap-phase regeneration in a tropical forest. Ecology Cody, M.
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The Biology of Disturbed Habitats
Pluralism in ecology. Annual Review of Ecology and Systematics — Middleton, G. The Biology of Polar Regions D. The Biology of Deserts David Ward. Table of contents Preface ; 1. Introduction ; 2. Terrestrial habitats ; 3. Aquatic habitats ; 4. Anthropogenic habitats ; 5.
Ecosystem processes ; 6. Biodiversity and invasive species ; 7. Spatial patterns ; 8. Temporal dynamics ; 9. Management ; Global concerns and future scenarios ; Glossary ; References ; Index show more. About Lawrence R. Walker Lawrence R. Walker is a professor of plant ecology at the University of Nevada, Las Vegas. His research focuses on the mechanisms that drive plant succession after natural and anthropogenic disturbances including cyclones, volcanoes, landslides, mine tailings, and abandoned roads.
These disturbances are located in tropical, temperate, boreal, and arctic biomes. He also studies the management of disturbances through restoration. Walker emphasizes comparisons across environmental gradients and many different types of disturbances, searching for useful generalizations about disturbance ecology, succession, and restoration. He has over scientific publications, including seven books, has won several research awards, is co-editor of a book series, and is an ISI highly cited researcher. Rating details.
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