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Journal of Coastal Research 30-37 West Palm Beach, Florida Winter 2001 A Logistic Regression Model of Hurricane Impacts .In a Mangrove Forest of the Mexican Pacific! John M. Kovacs] , Manuel Blanco-Correa*, and Francisco Flores-Verdugo** [Department of Geography *Direccion de Investigacion **Instituto del Ciencias del Nipissing University Cientifica Mar y Limnologia North Bay, Ontario Universidad Autonoma de Universidad Naciona l PIE 8L7, Cana da Nayarit Autonorna de Mexico 63190 Tepic, Nayarit, Mexico Estacion Mazatlan, 82000 Mazatlan, Sinaloa, Mexico ABSTRACT _ KOVACS, J .M.; BLANCO-CORREA, M., and FLORES-VERDUGO, F., 2001. A Logistic Regression Modelof Hurricane ,tflllllll:. Impacts in a Mangrove Forest of the Mexican Pacific. Journal of Coastal Research, 17(1), 30-37. West Palm Beach (Florida), ISSN 0749-0208. ~.~ ~ -;;J Hurricane Rosa inflicted considerab le 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 approx =+ +-- imately 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 42% 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 resu lts 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. ADDITIONAL IND EX WORDS: Coastal forested wetland, multinomiallogit model, Pacific coast, Mexico. INTRODUCTION cularia racemosa. In contrast, SMITH et al. (1994) and Mcoov et al. (1996) noted Laguncularia racemosa as equally if not Recent investigations are suggesting possible re lationships more prone to hurricane impacts than Rhizophora mangle. between pa rticular characteristics of mangrove forests a nd the extent of damage incurred from hurricanes. For example, Each study has provided valuable insight into how h urri in their study of Hurricane Gilbert's impact on mangrove for ca nes affect these forested wetlands but attempts at compar ests in Jamaica, WUNDERLE et al. (1992) indicated that the isons between the results ofthese investigations are difficu lt. most severe structural damage occurred amongst the larger Reasons for t hese limitations include unique criteria for mea diameter trees. This relationship h as since been supported suring damage, numerous classification methods and multi by others (ROTH, 1992; SMITH et al., 199 4; IMBERT et al., tudes of statistical tests. For example, although tree diameter 1996; McCoy et al., 1996) who h ave also suggested that the measurem ent s represent a continuous data set, researchers degree of impact may also be linked to the species. In their tend to transform this variable into discrete diameter classes investigations , ROTH (1992) and IMBERT et al. (1996) both that vary from one study to another. More notable are the reported Rhizophora mangle as more readily damaged t han individual methods of assessing tree da mage. SMITH et al. Avicennia germinans. Moreover , ROTH (1992) indicated that (1994) and IMBERT et al. (1996) both employ a single measure Rhizophora mangle was also more susceptible than Lagun- of damage assessme nt (scale) that combines both t he main stem and vegetation con dition. However, the number of cat 'Field work was financially supported by a research grant (YCRA egories a nd the ir descriptors va ry cons iderably from one an 97-0800-12) from the International Development Research Centre of other. McCoy et al. (1996) adopt a criterion of assessment, Canada. 99068 received 13 December 1999; accepted in revision 10 August simila r to ROTH (1992), that considers separate evaluations 2000. for vegetation and stem condition . Although not assessed in Logistic Regression Model of Hurricane Impacts 31 a complete multivariate context, their study indicates various witnessing any other hurricane of this magnitude. Research associations between diameter, species, main stem and veg ers have also noted the lack of recurrent hurricane activity etation condition, suggesting a potential dependency of veg for this region. For example, ROLLET (1974) reported that he etation condition on all three other variables. could not identify any natural phenomena of destruction in Building upon the success of these investigations, this the mangroves of the Teacapan-Agua Brava Lagoon System. study will employ a multiple logistic regression model to fur POOL et al. (1977) stated that there was no visible evidence ther examine the dependency of vegetation condition on the of the region ever being subject to a hurricane event. More other independent variables and to explore potential inter over, they commented that, unlike other mangrove forests actions between the predictor variables. Using this approach, that are periodically influenced by strong winds and hurri the diameter variable can be maintained as continuous. The canes, this region exhibited a taller canopy, a less dense use of the odds ratios, the output of this particular statistical stand and a larger number of trees with larger diameters. model, may facilitate interpretability of hurricane impact More recently, during their examination of productivity, FLO data and, consequently, provide a platform that will allow for RES-VERDUGO et al. (1992) suggested that, relative to other unconstrained comparison between future research. sites in Mexico, the low leaf fall for this area is indicative of an absence of major hurricanes. STUDY AREA METHODS Located on an alluvial plain that extends the Mexican Pa cific coast (Figure 1), the Teacapan-Agua Brava Lagoon Sys During the months of November and December 1997 five tem is recognized as one of the largest mangrove systems on strip transects were conducted perpendicular to the water's the western coast of the Americas (FLORES-VERDUGO et al., edge. These transects were situated far inland from the coast, 1990). With the vast majority of the forest located far inland, with three located in the northern section and two in the the basin mangrove forest type dominates the landscape (see southern section of the 'I'eacapan-Agua Brava Lagoon Sys LUGO and SNEDAKER (1974) for classification types). Large tem. The northern and southern transects measured 0.1 ha expanses of mono specific Avicennia germinans and Lagun (5 m X 200 m) and 0.05 ha (5 m X 100 m), respectively. cularia racemosa are located in the northern and southern Transportation by boat was supplied by the Tecuala branch sections of this system, respectively (FLORES-VERDUGO et al., of the Secretaria del Medio Ambiente, Recursos Naturales y 1992). Rhizophora mangle can be found along various inlets Pesca of Nayarit and by the villages of Francisco Villa, El and channels as well as interspersed amongst the two more Pescadero and Pericos. The location of each transect was dominant species. Conocarpus erectus flourishes further in identified using maps and aerial photographs and confirmed land in the drier reaches of the coastal plain. SNEDAKER using a Ground Positioning System. Within each transect, all (1982) has suggested that more research is needed to explain trees measuring 2.5 em in diameter or greater were recorded the unique dominance of Laguncularia racemosa in this re for their diameter at breast height (DBH), species, height, gion. Possible reasons for this anomaly include very high location, condition of main stem and vegetation condition. fresh water inputs (ROLLET, 1974; POOLet al., 1977) and the Similar to McCoy et al. (1996), stem condition categories in regional development of the land-forms (SNEDAKER, 1982). cluded "broken", "uprooted" and "intact" and vegetation con However, a lack of detailed botanical maps, soil data and an dition categories included "well vegetated", "poorly vegetat nual hydrologic data (e.g., salinity, tides) limit any ecological ed" and "dead". Trees classified as "uprooted" were those explanation to this dominance. whose main stem was found parallel to the ground. As with In addition to the mangroves, the Teacapan-Agua Brava McCov et al. (1996), the dichotomy between the "well" and Lagoon System contains a complex of tidal channels, coastal "poorly vegetated" cases was clearly visible in the field, with lagoons, seasonal floodplains, three seasonal rivers and one the 50% leaf loss as the criterion for separation. In addition perennial river. For an extensive review of the geomorphic to trees, seedlings were also recorded from quadrates (2 m X history of the region, the reader is referred to CURRAy et al. 2 m) selected at random from within each ten meter length (1969). The system is located in a tropical sub-humid climate of transect. zone with a mean annual temperature of 27° C.