Changes in physiological attributes of ponderosa pine from seedling to mature tree

Plant physiological models are generally parameterized from many different sources of data, including chamber experiments and plantations, from seedli ngs to mature trees. We obtained a comprehensive data set fora natural stan d of ponderosa pine (Pinus ponderosa Laws.) and used these data to paramete rize the physiologically based model, TREGRO. Representative trees of each of five tree age classes were selected based on population means of morphol ogical, physiological, and nearest neighbor attributes. Differences in key physiological attributes (gas exchange, needle chemistry, elongation growth , needle retention) among the tree age classes were tested. Whole-tree biom ass and allocation were determined for seedlings, saplings, and pole-sized trees. Seasonal maxima and minima of gas exchange were similar across all t ree age classes. Seasonal minima and a shift to more efficient water use we re reached one month earlier in seedlings than in older trees because of de creased soil water availability in the rooting zone of the seedlings. Howev er, carbon isotopic discrimination of needle cellulose indicated increased water-use efficiency with increasing tree age. Seedlings had the lowest nee dle and branch elongation biomass growth. The amount of needle elongation g rowth was highest for mature trees and amount of branch elongation growth w as highest for saplings. Seedlings had the highest biomass allocation to ro ots, saplings had the highest allocation to foliage, and pole-sized trees h ad the highest allocation to woody tissues. Seedlings differed significantl y from pole-sized and older trees in most of the physiological traits teste d. Predicted changes in biomass with tree age, simulated with the model TRE GRO, closely matched those of trees in a natural stand to 30 years of age.