Abstract
Forests of Nanjenshan, located in southern Taiwan, are chronically disturbed by strong winds of the northeasterly monsoon for as long as 6 mo each year. However, not much is known about the effects of wind on stomatal activities of trees that are found there. This research chose seedlings of 18 species representing tree species either growing specifically at a windward site, a leeward site, or ubiquitously at both sites. We measured variations in stomatal conductance of these species in a walk-in growth chamber, where the wind speeds were adjusted to 1~4 m s-1. Results showed that at wind speeds of 1 and 2 m s-1, 9 and 17 species, respectively, of the 18 tested species showed significantly lower stomatal conductance than that of the control. Under wind speeds of 3 and 4 m s-1, reductions in the stomatal conductance of windward species were significantly less than those of species from the other 2 habitat types. At a wind speed of 4 m s-1, stomatal conductance of leeward species and ubiquitous species had decreased 43 and 34%, respectively, but had only decreased by 27% in windward species. The slope obtained from a linear regression analysis of variation in the percentages of stomatal conductance vs. increasing wind speeds of each species was also significantly steeper in leeward species than in windward species. In another experiment, we exposed seedlings of all 18 tested species to a wind speed of 6 m s-1 for 10 min and monitored variations in leaf temperatures. The leaf temperature of windward species decreased 3.4℃ on average, while it decreased 4.8℃ (significantly differing from the former) in leeward species. Species with a larger leaf mass per unit area or smaller leaf area were less affected by wind, in terms of both lowered leaf temperatures and decreased stomatal conductance. Windward species of the Nanjenshan forest, in contrast to leeward species, had larger leaf masses per unit area and smaller leaf areas; they did not exhibit substantial reductions in stomatal conductance or leaf temperatures under windy conditions. Possessing the aforementioned morphological and physiological characteristics, the windward species of the Nanjenshan forest clearly have adapted to the chronic wind stress of the northeasterly monsoon.