Influence of Microclimate and Growth Form on Plant Temperatures of Early Spring Species in a High-elevation Prairie

Abstract

The influence of microhabitat, growth form and leaf morphology on plant temperatures was investigated for nine species (five families) that were first to appear in early spring at a high-elevation, shortgrass prairie site in SE Wyoming. Aboveground plant growth prior to the measurement period was restricted to S-facing slopes, and all species, except one, had leaf temperatures that were substantially higher than air temperature during the daytime. Maximum increases (> 15 C) above ambient air temperature occurred in a cushion plant (Haplopappus acaulis) and a broadleaf species (Penstamon eriantherus) that had relatively large leaves in close contact with the soil surface. Leaf temperatures over 10 C above air temperature also occurred for two species (Hymenoxys acaulis and Gutierrezia sarothrae) whose current-year growth was sheltered by old vegetative growth. Artificially disturbing microsite and growth characteristics resulted in major reductions in plant temperatures to within about + 2 C of ambient air temperature. Only one (Zigadenus venenosus) of the nine species that first appeared in early spring did not have elevated leaf temperatures or any apparent growth characteristics that would contribute to such elevations. Thus, variation in microsite, growth form and leaf morphology in these early spring species appears to contribute substantially to the avoidance of low temperatures during a portion of the season when growth may be limited by temperature. INTRODUCTION Plants can potentially avoid the effects of extreme environments by occupying favorable microsites or by adaptations in growth form or leaf morphology which affect their energy balance. An alternative to avoidance is physiological adaptation or tolerance (Levitt, 1980). Little is known regarding the relative numbers of avoider vs. tolerator species within communities and, thus, the apparent prevalence of one adaptive strategy over the other. Furthermore, avoidance mechanisms of species in low temperature environments have received little attention compared to tolerance mechanisms (Levitt, 1980) or in comparison to high temperature avoidance mechanisms (Turner and Kramer, 1980). The purpose of this study was to compare the influence of microhabitat or microsite preference, growth form and leaf morphology on leaf temperatures of the species appearing first in early spring in a high-elevation prairie of southeastern Wyoming. Comparisons were made when aboveground vegetative growth was just beginning on southern aspects and was entirely absent on northern aspects. Nine species from five families, representing a variety of growth forms and leaf morphologies, were monitored. Microclimatic measurements were compared for a representative northern and southern aspect, as well as for numerous microsites on the southern aspect. The number of species occupying favorable microsites was evaluated by comparing soil surface temperatures immediately adjacent to individual plants with mean surface temperatures. Also, the relative number of avoiders and tolerators was evaluated by assessing the number of species with growth forms and leaf morphologies that contributed to substantial elevations in leaf temperature above air temperature. The observed avoidance mechanisms are compared according to their ability to elevate leaf temperature above air temperature.