Nitrogen Fixation on the Tropical Volcano, La Soufriere (Guadeloupe): I. A Survey of Nitrogen Fixation by Blue-Green Algal Microepiphytes and Lichen Endophytes

Abstract

Results of a survey of nitrogen-fixing blue-green algal epiand endophytes growing in the cloud-shroud climate on the tropical volcano La Soufriere (Guadeloupe, French West Indies) are reported. Microepiphyllae on Clusia mangle leaves increased with leaf age, and rates of C2H2 reduction were 47 times higher during cloud-shroud conditions than during solar shocks. Phyllosphere rates of C2H2 reduction for Norantea were about 0.76-2.22 7) moles/cm2/hr for the oldest leaves, and epicaulous rates of C2H2 reduction for Tibouchina were the highest measured in plants tested. The Hapalosiphon flexuosus rate of C2H2 reduction was 1.26 7) moles/ gdw/hr at pH 4.5 on Sphagnum. Endophytic blue-green algae in the lichens Stereocaulon ramulosum and Cora pavonia were 37.6 and 3966 -j moles/gdw/hr, respectively. THE VEGETATION NEAR MOUNTAIN SUMMITS in the Lesser Antilles is stunted, and the climate is wetter and colder than at lower elevations. In the Greater Antilles, similar plant associations occur at lower altitudes. The dwarf plant associations occurring at high elevations (approximately 1000 m) have been termed dwarf forests, cloud forests, and mossy forests (Holdridge 1967, Howard 1970, Ewell & Whitmore 1973). The predominant aspect of climate is an almost permanent layer of cumulus clouds formed by upsweeping moist air (Odum et al. 1970). Research results reported here were collected on the tropical volcano La Soufriere located in the Parc Naturel on Guadeloupe (16?40'N, 60?10'W), Lesser Antilles, French West Indies. La Soufriere erupted in the years 1590, 1635, 1797, 1836-1837, 1892, and 1976-1977 (Howard et al. 1980). Studies of the vegetation by Henri Stehle (1935, 1979) described pre-eruption plant associations on La Soufriere. The ecotone at 1100 m, between the tropical rain forest below and the stunted vegetation above, is composed of the shrub C. mangle, which encircles the volcano. Above this Clusia ecotone, termed Clusietum guadelupense by Stehle (zone, La foret rabougrie), are cliff faces dominated by Sphagnum and bromeliads (zone, Les savanes d'altitude). The windblown summit (1456 m) has a flora of low-growing mosses, ferns, club mosses, and bromeliads. The flora has been described in detail by Stehle (1935, 1979), Howard et al. (1980), and Sastre (1978). New radial fractures opened on the southeast flank of the dome of La Soufriere in 1956 and reopened in July 1976, emitting H2S, mud, and boulders. Activity continued until 1 March 1977 (Howard et al. 1980). This eruption divided the pre-eruptive flora into discreet posteruption zones (Fig. 1). The south and southwest portions were destroyed, and the southeastern Col de l'Echelle, after receiving phreatic material, was a desert (Howard et al. 1980). The vegetation of the western flank was somewhat damaged by H2S emissions, but the northern flank was untouched (Sastre et al. 1983). Recovery of vegetation after the eruption has been described by Sastre (1978), Sastre et al. (1983), Blanchard et al. (1984), and Blanchard (1985). The cloud-shroud regime (sensu Odum et al. 1970) above 1100 m on La Soufriere is characterized by nearly constant cloud cover, low photon flux densities between 90 and 585 ,uEm2/sec PAR (photosynthetic active radiation, 400-700 nm), heavy rainfall (10 m/yr) and cool temperatures (17?C). This zone may begin below 1000 m as in Puerto Rico, or above 2000 m as in New Guinea (Leigh 1975). The prevalent cloud-shroud climate of La Soufriere was sporadically punctuated by events of intense insolation. Between 18 July 1984 and 20 May 1985, there were 12 completely clear days and 13 partly clear days (for 2-6 hr). These rare insolation shocks had photon flux densities from 1620 to 2070 ,uEm2/sec PAR and air temperatures of approximately 24?C, with higher temperatures inside sunlit plants (pers. obs.). Persistent fog cover lowers photon flux density, which in turn lowers leaf and air temperatures. Low water vapor deficits may depress both growth in some species and promote colonization by the poikilohydric algae, lichens, and mosses (Weaver et al. 1973, Grubb 1977, LaBastille I Received 27 October 1985, revision accepted 18 June 1986. 194 BIOTROPICA 19(3): 194-1-99 1987 This content downloaded from 157.55.39.112 on Wed, 07 Sep 2016 04:59:22 UTC All use subject to http://about.jstor.org/terms