MICROCLIMATIC EFFECTS ON WATER RELATIONS, LEAF TEMPERATURES, AND THE DISTRIBUTION OF HERACLEUM LANATUM AT HIGH ELEVATIONS

The small-scale distribution of an understory herb, Heracleum lanatum, was evaluated in terms of leaf temperature and water relations limitations due to a large leaf size (630 cm2). Diurnal variations in transpiration (4 to 60 mg m-2 sl) were influenced by fluctuations in solar irradiance, wind speed, leaf temperature and stomatal conductance. Computer simulations indicated that leaf temperatures in a forest clearing would be > 12 C above air temperature, with maximum transpiration rates of 140 mg m-2 s-l, and daily water loss to be over 200% greater than values at natural understory locations. Simulations of nocturnal temperature relations indicated 100 W m-2 less incident longwave irradiance in the forest clearing as compared to the understory (560 vs. 660 W m-2 at 400 hr). This difference led to predicted leaftemperatures being as low as 6 C below air temperature in the forest clearing while measured leaf temperatures in the forest understory were within 1.5 C of air temperature throughout the night. Furthermore, minimum air temperatures were at or below 6 C on 36% of the nights during the summer growth period indicating that in open areas leaves of H. lanatum would frequently be below 0 C and subject to possible freeze damage. Heracleum lanatum may be more abundant in the shaded understory of the subalpine forest because exposure in open environments would result in high leaf temperatures and increased transpirational water loss during the day, as well as low leaf temperatures with the possibility of freeze damage at night. MANY FACTORS, both biotic and abiotic, have been suggested as possible influences on the small-scale distribution patterns of herbaceous understory species. Several studies found variations in solar irradiance to have a pronounced influence on temperature, water and photosynthetic relations of understory herbs (Bjorkman, Ludlow and Morrow, 1972; Young and Smith, 1980; Smith, 1981; Young and Smith, 1982). In shaded environments, large-leaf species may be well adapted for the conservation of water because of a thick leafboundary air layer which reduces transpiration in comparison to small leaves (Gates and Papian, 1971; Smith and Geller, 1980; Geller and Smith, 1982). Understory herbs in the high elevation coniferous forests of Wyoming are commonly large-leafed, but these species were often found to occupy microsites with a highly variable light regime because of periodic exposure to large sunpatches (Young and Smith, 1979; Smith,1981). As solar irradiance greatly increased in a sunpatch, these species showed 1 Received for publication 5 April 1984; revision accepted 11 September 1984. I thank Dr. William K. Smith for providing the instruments and for his comments on an earlier version of this manuscript. 2 Present address: Department of Biology, Virginia Commonwealth University, Richmond, VA 23284. a marked increase in leaf temperature, stomatal opening, and transpiration (Young and Smith, 1980; Smith, 1981; Young and Smith, 1982). Thus, although water loss may be reduced when shaded throughout much of the day, these large-leaf herbs would lose more water when sunlit than a small-leaf species under identical conditions. The interaction of solar irradiance and a large leaf size may influence the thermal status, water relations, and photosynthetic performance of these species such that their small-scale distributions are limited to selected microsites within the forest understory. In addition to high leaf temperatures and transpiration rates throughout a day, nocturnal microclimate in an open environment may significantly affect the thermal status of large-leaf species. Moen (1968) hypothesized that due to increased exposure to open skies, reductions in nighttime longwave irradiance in forest clearings influenced the distribution patterns of species. This could be especially true for large-leaf species, for radiative cooling and reduced convective heat exchange (Vogel, 1968, 19703 would lead to lower leaf temperatures as compared with those in an understory microsite where the long-wave irradiance would be somewhat greater. At high elevations where air temperatures are lower (Smith and Geller,