Abstract:
The amount and quality of competition among
individuals in developing plant communities is an important factor determining community structure, composition and development. In perennial plant communities, this competition is reflected by shifts in carbon allocation among plant parts and can result in long-term structural differences between neighboring individuals. This study was designed to provide information on the mechanics of the competitive process and the effect of competition on aboveground tree growth. Plots with a wide range of spacings of red alder seedlings were planted 4 years ago in the Cascade Head Experimental
Forest on the central Oregon Coast Range of mountains. Biweekly growth measurements of stem and branch components and predawn plant water potential were made throughtout the growing season. Branch samples taken from the plot were destructively ana1yed to determine biomass and leaf area. Combining these measurements with regression equations previously developed from the site, parameter
values from regression models are used to compare differences in growth and carbon allocation among trees grown at different levels of intraspecific competition.
Increased density produced significant reductions in leaf area, height, basal diameter and biomass of the red alder
trees. Increased density also produced significant reductions in the relative biomass growth rate. A gradient of plant water potential was present on several
measurement days during the growing season. Growth analysis of these dates showed that predawn plant water potential was a significant factor determining overall
tree relative growth rate as well as branch and leaf area relative growth rate. A negative exponential relationship described the relationship between overall relative
aboveground biomass growth rate and predawn water stress. Relative growth rate declined sharply at a water stress level between -0.3 and -0.4 MPa. Above this water stress level, yearly relative growth rates were fairly uniform at a value of 1. The relationship between water stress and branch and leaf area relative growth rate also approached a negative exponential relationship as the intensity of the water stress gradient increased. Examination of the tallest and shortest trees at the two lowest spacings
found significant differences in leaf area, percent biomass allocated to branches, and crown depth. This study showed that the effects of competition on growth and
allocation of biomass, and leaf area can be significantly affected by periods of water stress lasting only a few weeks.
