Morphological and physiological responses of fine hardwood tree species to plantation thinning.

摘要

Hardwood tree species such as black walnut (Juglans nigra L.), northern red oak (Quercus rubra L.), and white oak ( Q. alba L.) found in the Central Hardwood Forest Region (CHFR) of the United States can produce trees of considerable economic value. Forest stands or plantations often require intermediate silvicultural treatments such as thinning to ensure an appropriate density of residual trees are produced. To increase our understanding of thinning response, there is a need to investigate the biological means by which trees respond to a change in environmental conditions (light, water, nutrients) created by thinning. A series of experiments demonstrated that the light environment was most responsive to thinning. Increases in light near thinned trees generated higher net photosynthesis when compared to unthinned trees. Black walnut showed great photosynthetic adaptation to changes in the light environment created by thinning (chapters 3 and 4). The influence of thinning on water relations was variable. A pilot study (chapter 2) showed higher soil water content in thinned plots and higher predawn leaf water potential in thinned northern red oak during the first growing season after tree removal.;Short-term physiological responses of black walnut (chapter 3) showed higher midday leaf water potential one year after thinning, but no change in stomatal conductance or transpiration. Photosynthetic responses of black walnut along a vertical canopy gradient (chapter 4) showed increases in stomatal conductance and transpiration in thinned trees, but no changes in soil water content and midday leaf water potential. Modifications in soil or leaf nutrients were found (chapter 3), with thinned black walnut showing increased leaf nitrogen content per unit area one year after thinning. Changes in environmental conditions generated diameter growth increases in all three species. Relative diameter growth increases were also found in northern red oak and white oak. Overall, the magnitude of the short-term (two or three years) response was influenced by: (1) crown class of measurement trees at time of thinning, (2) degree of competition for light, water, and nutrients, (3) thinning intensity, and (4) annual precipitation.