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| The Effects of Introduced Exotic Grasses, African Star Grass Hyparrhenia rufa and Anglinton Dichanthium annulatum, on Tropical Dry Forest Regeneration |
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To the Classroom Rubric References Data Table 1
An unfortunate artifact of human travel around the globe has been the transport of non-native species into new environments. These introductions may have little effect or a profound effect depending on the particular community into which they are introduced. Hawaii (rats and mongoose) and Australia (cats and rabbits) have suffered greatly from intentional and accidental introductions. In the United States zebra mussels, purple lusestrife and kudzu are but a few examples. This study will attempt to shed light on the increasing problem of invasive, exotic species by focusing on the regeneration of tropical dry forest in the presence of the introduced grasses; African Star Grass, Hyparrhenia rufa and Anglinton, Dichanthium annulatum. These two grasses were chosen because they are common, introduced species that have become established in disturbed areas in the tropical dry forests throughout Costa Rica. African Star Grass was introduced in the 1940’s as pasture grass for cattle. Anglinton was introduced accidentally by the cattle that were brought into Costa Rica (pers. com. Chiavarrķa). Burning to control these grasses has been attempted, but previous studies here at Palo Verde National Park have shown that burning may enhance growth of these grasses. The burns may have additional negative impacts on forest regeneration by killing younger seedlings directly or destroying the mycorrhizae and other soil flora that support their growth. The native tropical dry forest was replaced with grasses. This conversion may have important implications since important native tree seed dispersers like peccaries and monkeys are absent in grassland habitats. These reasons, as well as simple proximity, make grassy areas further from the forest less likely to be colonized by new seedlings. The above information supports our general contention that these invasive grasses are impeding regeneration of the forest. Hypothesis The regeneration of native tree seedlings may be affected permanently as a result of the introduction of grasses. Areas with increased grass coverage will contain fewer non-grass species and woody stems/seedlings. In addition, we expect that transect two, which is nearer the forest will have an increased number of woody stems because of proximity to parent trees, seed-dispersing animals such as birds and mammals, and more favorable abiotic conditions.
Methodology Two transects, each 100 meters in length, were sampled. Transect One was 15 meters from the road. Transect Two was 10 meters from Transect One and 15 meters from the edge of the forest. Both transects were parallel to the rock/dirt road located by traveling on the main road between the entrance and Catalina Station, approximately 2.5 km north of the station and off the main road to the Catalina Station. At each 10 meter point (10m, 20m, 30m, etc.), a one meter square grid made of flagging tape was placed on the ground, with the sampling point in the middle. For each sampling point, soil temperature and the percent light was measured. Inside the grid, each new species of plant encountered was placed in a zip lock bag and given an identifying mark. The percent coverage of the grid area by each plant species (herbaceous and woody) was estimated. The percent cover of matted grass and grasses (Hyparrhenia rufa and Dichanthium annulatum) was also determined. Site markers were moved to the next 10 meter interval. All observations and samples were repeated at each successive 10 meter mark through 100 meters. Plant specimens were brought back to the lab and identified by Dr. Evan Notman and Ulises Chavarria. The Flowering Plants of Palo Verde National Park dichotomous key and books with illustrations were used to verify the plant specimens.
Materials
Results There was a small but significant negative correlation between the number of woody stems and the percent coverage of plots by grasses and matted grass. (F = 5.609, P < 0.05) The adjusted R2 value was 0.19 indicating that the grass and matted grass coverage explains about 19% variation in the number of woody stems. (Figure 1) There was no statistically significant relationship between plot coverage and the numbers of non-grass species. (F = 0.267, P > 0.05) (Figure 2) Comparisons of the physical conditions, cover, and numbers of woody stems between the two transects did not yield significant differences. (Figure 3, Figure 4, Figure 5, and Figure 6) Overall, there were 13 woody species and 16 herbaceous species. There were 4 unique woody species in Transect One and 5 unique woody species in Transect Two. There were 4 woody species common to both transects. (Data Table 1)
Discussion We expected that areas with increased grass coverage would contain fewer non-grass species and woody stems/seedlings. The data did not show significant differences in the numbers of non-grass species. The two grass species did seem to decrease the overall abundance of woody plants. The mechanism by which the grasses prevent the growth of woody stems was not clear and could be the subject of future studies. Much of the grass was matted possibly preventing many seeds from reaching the ground and preventing germinated seeds from growing due to insufficient sunlight. We also expected that the transect closer to the forest would have an increased number of woody stems when compared with the second transect. While the data indicated a difference, it was not statistically significant. While it still seems possible that these introduced species are slowing regeneration of the forest, their effects could not be identified in this study. Repeated studies in future years could indicate that the forest is regenerating at a slow rate. Acknowledgements. We thank Dr. Evan Notman and Ulises Chavarria for kindly providing assistance. | ||||||||||||||||||||||||||||||||
