Sulfometuron was applied at 0.1, 0.15, and 0.2 kg ai/ha over the top of oak seedlings. Three separate field studies were completed. In 1997, 0.1 and 0.2 kg ai/ha was applied PRE over six oak species. In 1998 and 1999, 0.1 and 0.15 kg ai/ha were applied PRE and 0.1 kg ai/ha was applied POST over two species of oaks. Results demonstrate that first-year survival of oak seedlings is greater in areas which receive competition control. Survival was 21 to 44% greater in treated areas as compared to nontreated areas. Observations indicate that survival differences are increased in droughty years. Competition control appears essential to obtaining desirable levels of survival when oak seedlings are planted in areas with established herbaceous competition.

The types and causes of death of canopy and subcanopy trees were characterized for the tropical montane cloud forest of Tamaulipas, Mexico. Field work was carried out at the Biosphere Reserve ‘El Cielo’, located on the eastern slope of the Sierra Madre Oriental. Three study plots, each of 2 ha, were selected within the forest differing in their degree of exposure to wind. Results indicate that the tropical montane cloud forest presents a differential treefall regime derived from the great hurricane activity to which this mountain range is exposed. This effect is illustrated by the great number of fallen logs (545 trees in 6 ha) and the high percentage of uprooted (48.3%) and snapped off (37.6%) trees. Other treefall factors included the rocky and shallow soils and weakness of the dominant trees from insect attack. A differential disturbance level was obtained between zones. The disturbance was higher for the exposed zone, where the crown cover of emergent trees was not closed. Species composition of the dead material was related to the abundance of the overstorey species as standing vegetation. Ten of the 20 recorded species were able to sprout and just 7.5% of the 545 fallen trees were sprouting. Quercus germana, Q. sartorii, Clethra pringlei, Liquidambar styraciflua and Ceris canadensis, accounted for 78.5% of the gap-forming species. The type of treefall differed between species of the same genera: Q. germana was generally found uprooted, while Q. sartorii died standing probably because of previous weakening from dipterous insects.

Recent research in whole-plant stomatal physiology, conducted largely with potted plants in controlled environments, suggests that stomatal conductance (gs) might be more closely linked to plant chemical variables than to hydraulic variables. To test this in a field situation, seasonal gs was examined in relation to a number of plant and environmental variables in 11 temperate, deciduous forest tree species. Stomatal conductance was generally better correlated with environmental variables (air temperature, vapor pressure deficit, PPFD) than with plant variables, and slightly better correlated with plant hydraulic variables (shoot water and osmotic potentials) than with plant chemical variables (xylem sap ABA concentration, xylem sap pH). We examined a model, developed previously for maize, which describes regulation of gs by xylem sap ABA concentration with leaf water status acting to modify stomatal sensitivity to the ABA signal. This model explained slightly more variation in seasonal gs in the forest trees than did single plant variables but not more variation than most single environmental variables. Response surface models, especially those incorporating environmental variables, were more consistently successful at explaining gs across species.