Microclimatic Effects on Water Relations, Leaf Temperatures, and the Distribution of Heracleum lanatum at High Elevations

Abstract

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 s−1) 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−1, 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 leaf temperatures 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.