Showing papers by "John M. Kovacs published in 2001"

Mapping disturbances in a mangrove forest using multi-date landsat TM imagery.

Abstract: To evaluate the accounts of local fishermen, Landsat TM images (1986, 1993, 1999) were examined to assess potential losses in the mangrove forests of the Teacapan–Agua Brava lagoon system, Mexico. A binary change mask derived from image differencing of a band 4/3 ratio was employed to calculate any changes within this forested wetland. The results indicate that by 1986 approximately 18% (or 86 km2) of the mangrove area under study was either dead or in poor condition. The majority of this damage had occurred in the eastern section of the Agua Brava basin, which coincides, with the reports of the elderly fishermen. Examination of aerial photographs from 1970 revealed no adverse impacts in this area and would suggest, as postulated by the fishermen and other scientists, that modifications in environmental conditions following the opening of a canal, Cuautla canal, in 1972 may have initiated the large-scale mortality. Although these areas of impact are still developing, the results from the satellite data indicate that the majority of the more recent changes are occurring elsewhere in the system. Obvious in the 1999 satellite data, but not so in the 1993, are large areas of mangrove degradation in the northern section of the Teacapan region. In the Agua Brava basin, the more recent transformations are appearing on the western side of the basin. Since long-term records of environmental conditions are absent, it is difficult to determine why these latest changes are occurring or even if the earlier losses were the result of the canal. Potential agents of change that have recently been observed include a hurricane, a second canal, and the uncontrolled expansion of the Cuautla canal since 1994.

A logistic regression model of hurricane impacts in a mangrove forest of the Mexican Pacific

Abstract: Hurricane Rosa inflicted considerable damage to the mangrove forests of the Teacapan-Agua Brava Lagoon System of Mexico Data collected from five transects indicate an overall reduction in stem density and basal area of approximately 31 and 51 percent respectively Of the 1390 trees examined, only 44% remained well vegetated and 28% were found with their main stem broken or uprooted Rhizophora mangle was the species least affected by the event with 65% of these trees found in a well vegetated condition in contrast to 34% for Laguncularia racemosa and 420/0 for Avicennia germinans A polytomous logistic regression model was developed to further examine the predicted outcome, vegetation condition, by species, main stem condition and diameter at breast height (DBH) Rhizophora was excluded from the model and DBH was reserved as a continuous variable The results from this multivariate approach indicate that the probability of a mangrove being found in a dead condition as compared to a well vegetated one is significantly influenced by the diameter and main stem condition but not by the species As diameter increases, the odds that an intact tree will be classified as dead rather than in a well vegetated condition also increases A broken or uprooted main stem also augments this probability but diameter and the uprooted condition interact to attenuate the odds By comparing a poorly vegetated to a well vegetated outcome, the model again indicates that, separately, increased diameter, the condition of a broken or uprooted main stem all increase the odds of a less favorable outcome More significant interactions were also recorded, including the interaction of species by both diameter and the uprooted condition Although seedling counts suggest considerable recovery following the hurricane, Avicennia and Rhizophora dominated the numbers Changes in light conditions, local topography and substrate conditions following this rare storm event may not currently favour the growth of Laguncularia seedlings