Nitrogen Fixation on a Tropical Volcano, La Soufriere. II. Nitrogen Fixation by Scytonema sp. and Stereocaulon virgatum Ach. During Colonization of Phreatic Material

Abstract

By 1983, lahar flows deposited by the 1976-1977 eruption of La Soufriere (Guadeloupe, French West Indies) supported a nearly monospecific community of the nitrogen-fixing lichen Stereocaulon virgatum. Diazotrophic, free-living Scytonema (Cyanophyta) lived as a microepiphyte on the lower phyllocladia. The initial colonization of bare rock following deposition and subsequent development of the present community were not recorded. Therefore, succession patterns, nitrogenase activity, and microclimate of disturbed areas within the established lichen community were studied to describe regeneration patterns and postulate from these the initial colonization mechanisms. Scytonema developed a gelatinous mat on bare rock that provided a suitable substrate for the development of S. virgatum propagules. Nitrogenase activity by the mat supporting 2-10 mm high pseudopodetia of S. virgatum exceeded rates by the uncolonized mat and those measured for later stages characterized by the development of a dense lichen canopy over the mat. THE ERUPTION OF THE TROPICAL VOLCANO La Soufriere (Guadeloupe, French West Indies) in 1976-1977 created a desert of phreatic material on the southeast flank of the dome. This natural catastrophe provided an exceptional opportunity to study the physiological ecology of the pioneer flora involved in the successional stages of regeneration. Nitrogen-fixing blue-green algae and lichens initially colonized the lahar flows in this cloud forest vegetational zone. The flora of La Soufriere prior to the 1976-1977 eruption was described by Stehle (1935, 1979), and the successional stages of the posteruption flora by Sastre (1978, 1985), Howard et al. (1980), Sastre et al. (1983), Baudoin (1985), Blanchard (1985), Coute (1985), and Roquebert (1985). The potential contribution of nitrogen by epiphytic and epicaulous blue-green algae to the less damaged zones has been described by Fritz-Sheridan and Portecop (1987). N2-ase activity in the established lichen pioneer community of S. virgatum Ach. and the associated free-living blue-green alga Scytonema in the lahar flow zone has been reported by Fritz-Sheridan and Coxson (1986). The cloud-shroud climate was sporadically punctuated by insolation events (12 clear and 12 partly clear days from 18 July 1984 to 20 May 1985) (Fritz-Sheridan & Portecop 1987). Since no detailed records were kept describing initial colonization of the newly deposited boulders, it was necessary to study recently disturbed sites to reconstruct the pattern of the initial colonization. This paper presents data concerning N2-ase activities for uncolonized mats of Scytonema and for these mats at various stages of development following invasion by S. virgatum. METHODS In the Ravine Matylis, boulders previously scoured to bare rock by mechanical damage and rain, and exhibiting various degrees of recolonization by Scytonema and S. virgatum, were identified. Four regeneration stages were selected for study: (1) sections of the brown, gelatinous, and uncolonized Scytonema mat; (2) algal mats with small pseudopodetia of invading S. virgatum; (3) mats with S. virgatum pseudopodetia 5-7 cm in height; and (4) mats with S. virgatum pseudopodetia 7-10 cm high. Nitrogenase activity was measured using the acetylene reduction technique as described by Fritz-Sheridan (1985). Algal mat strips (0.5 x 5 cm), or S. virgatum pseudopodetia with their basal portions intact, were placed in 12-ml rubber-stoppered blood sample tubes which then received 1 ml of acetylene (C2H2). Incubation was for 1 hr at the collection site. The temperature of the contained experimental material, which was monitored using a 64-,u thermocouple, was within 1?C of the temperature of naturally exposed mat or lichen material. The experimental vials were incubated at the sample site during cloud-shroud conditions or in a water bath during insolation shocks. Ethylene (C2H4) produced was quantified using a GowMac 69-750 gas chromatograph fitted with a stainless steel Poropak? filled column (400C) and N2 as carrier gas. Temperature measurements employed 64-,u thermocouples placed in the mat, mat with small S. virgatum, and at the base of the more developed lichen communities. Temperature readings were taken at 1 0-min intervals. Photon flux density (photosynthetic active radiation-PAR, 400-700 nm) was measured with a cosine-corrected General Electric meter. Readings were reported as microI Received 30 May 1986, revision accepted 16 September 1986. BIOTROPICA 19(4): 297-30