Nitrogen Fixation on the Tropical Volcano, La Soufrière (Guadeloupe): Nitrogen Fixation, Photosynthesis and Respiration During the Prevailing Cloud/Shroud Climate By Stereocaulon Virgatum

Abstract

Stereocaulon virgatum Ach. has colonized lava flows deposited on the west flank of the volcano La Soufrière. The mean annual rate of acetylene reduction was 43·4 nmol C₂H₄ gdw⁻¹ h⁻¹ with maximum rates during the prevalent cloud/shroud meteorology of 101 and minimum rates during rare high insolation events of 0·63 nmol gdw⁻¹ h⁻¹. Percentage thallus moisture was the major variable controlling nitrogenase activity. During cloud/shroud conditions the upper 90% of the lichen canopy reduced 85% of the acetylene. Canopy shading reduced intra-canopy temperatures allowing the basal 10% of the canopy to fix nitrogen during insolation shocks. Basal portions of pseudopodetia exhibited reduced rates of ethylene production when exposed to canopy surface light intensities during cloud/shroud conditions. The recovery pattern of nitrogenase following desiccation during an insolation shock is presented. Rates of photosynthesis during cloud/shroud conditions were high, reaching 50% of those attained during saturating light intensities.