Using phytosociological methods to generate hypotheses about the habitat preferences of Listera australis Lindl
Previous study of Listera australis in New Jersey indicated a greater degree of variability in plant community associations than was described in the literature. L. australis, or Southern Twayblade, is a terrestrial orchid known to inhabit moist lowland forests and shaded bogs throughout a range that extends from Quebec and Ontario, into the Northeast and Mid-Atlantic, and south along the Atlantic and Gulf coasts into Texas. The plant is listed as “vulnerable” to “critically imperiled” in most of these states/provinces, and its conservation would benefit from increased understanding of its community ecology. In order to develop hypotheses about species interactions – and the effect of management activities on those interactions – the present study utilized phytosociological methods to describe vegetation types and reveal spatial patterns in ecological communities where the orchid is present. Study sites in three forest types in NJ were selected – an Atlantic white cedar (AWC) swamp, a pitch pine lowland, and a successional red maple swamp. Data regarding vertical structure, dominant tree species, tree density and canopy cover, plant community composition, and ground surface attributes were collected and analyzed for a 400 m2 plot and 6 – 4 m2 subplots at each site.
The AWC swamp is 45.1% Chamaecyparis thyoides and 31.6% Pinus rigida and has 1175 trees/ha. The lowland has 775 trees/ha and is 46.6% P. rigida and 26.0% Nyssa sylvatica. Maple swamp density is intermediate – 1000 trees/ha. 36.7% of standing stems are AWC snags and 31.7% are young Acer rubrum, indicating recent disturbance. Canopy cover is greatest in the AWC swamp (155%), less in the pine lowland (110%), and lowest in the maple swamp (83%). Microhabitat shading follows the opposite pattern (28, 60, and 75%, respectively). Sphagnum sp. are a major contributor to surface cover in all sites. Plant litter covers >70% of the ground surface at the AWC site and 30% in the pine lowland. L. australis abundance is greatest in the AWC swamp, intermediate in the lowland, and lowest in the maple swamp. All sites have similar species richness but significant differences in understory plant community composition. Preliminary analysis suggests that L. australis may prefer older forests with a closed canopy, a moderately developed shrub layer, and ample leaf litter. Orchid-fungi nutritional mutualisms and plant associations can be further explored in this context. Phytosociological data serve as a useful starting point for a more refined approach to data collection.