SOIL MOISTURE AS A PROXY FOR GROUNDWATER FLOW: INFLUENCE ON THE DISTRIBUTION OF THREE SPECIES OF SPIRANTHES (ORCHIDACEAE)

Botanical literature often mentions “moisture” conditions as part of the habitat description for native orchids (family Orchidaceae). Examples include qualitative (e.g., wet, moist, dry, etc.) or semi-qualitative (e.g., wetness index, coefficient of wetness, etc.) descriptors. Many years of field observations of the type, and abundance, of native orchid populations have revealed a striking (albeit qualitative) link to groundwater flow systems. There appears to be a correlation between orchid species and the ecosystem position within the “Unit Basin” of regional groundwater flow systems. Previously, a fourfold “classification” groundwater scheme for the native orchids of Alberta was proposed: those found in discharge; recharge; and midline areas; and a group that shows no general preference. In order to test the hypothesis that position in the groundwater flow system controls the distribution of (certain) orchid species, soil moisture conditions and topography were measured at flowering sites for three closely-related species of orchids.

Fieldwork was conducted in southern Nova Scotia, Canada where the range and flowering times of Spiranthes cernua; Spiranthes ochroleuca, and Spiranthes casei variety novaescotiae are known to overlap. Soil moisture values were measured for the three species at 21 different locations, including one site that hosted all three species along one highway embankment/road cut. Results show: Spiranthes cernua was only found in wet and slope-bottom sites; S. casei variety novaescotiae was only found in dry sites; and S. ochroleuca was found in intermediate to dry sites. At the one site were all three species were found, there was a clear spatial demarcation based on soil moisture and topography. Using soil moisture conditions as a proxy for groundwater flow, these three species would be located in the discharge, recharge, and midline portions respectively of the Unit Basin of regional groundwater flow. Thus, there appears to be semi-quantitative evidence that regional groundwater flow can control the type and abundance of native orchids.