The initial responses of some tropical rain forest tree seedlings to a large gap environment.
The responses of dipterocarp seedlings grown in deep shade to a dramatic increase in insolation are reported and briefly discussed. Experiments were carried out during November 1986 at the Muka Head Field Station, Penang, Malaysia, using Shorea macroptera (Dipterocarpaceae) and Trema tomentosa (Ulmaceae), a local pioneer tree species, as a comparison. Three month old seedings were transferred either to an open area 15 m diameter, or to a fully shaded forest site, and leaf temperature and stomatal conductance were measured for 3 days following simulation of gap creation. By the end of the second day, one S. macroptera seedling exhibited bleached areas on expanded leaves (which became necrotic), whereas T. tomentosa seedlings showed no discernible ill effects from exposure to high irradiances. Leaf temperature in S. macroptera was up to 9.8°C above ambient (= 40°C), and hotter than that in T. tomentosa. Stomatal resistances of T. tomentosa seedlings in the open site were generally lower than those in the shade (allowing greater cooling); in contrast, S. macroptera stomatal resistances were often higher in the open site, or fluctuated more.