Intertree competition in open-pollinated progeny plantations of Virginia pine (Pinus virginiana Mill.) and white pine (Pinus strobus L.)

Dimensional-spatial relationships among trees were utilized to develop a Dimension-Difference Index (DDI) and Distance-Ratio Index (DRl) for estimating intertree competition in white pine (Pinus strobus L.) and Virginia pine (Pinus virginiana Mill.) open-pollinated test plantations. Additional information gathered from a separate spacing study was required in the DDI. Linear regressions of tree dimension on growing space were used to identify effective adjacent competitors of sampled trees and their levels of influence.

Competition was expressed in terms of six measured characteristics: (1) diameter at breast height (dbh), (2) crown diameter, (3) crown area, (4) height, (5) basal area, and (6) volume, plus a derived statistical factor, TREESIZE. Comparisons were made between values of traits adjusted for competitional variance and their unadjusted values.

There were significant differences among stands and families of both white and Virginia pine in competitive influence on most characteristics. The commercially important characteristics of dbh, basal area, and volume were sensitive to competition, and were among the most highly heritable of the characteristics studied. TREESIZE, although useful as an index of commercially important tree characteristics, failed to yield reliable heritability estimates. The DRI appeared to be better than the DDI in estimating competitive influences, and could be made applicable to forest tree improvement plantations of other species and ages.

The overall variance of each measured trait and distribution of variance components were significantly influenced by intertree competition. Most of the competitionally influenced variance occurred in the component of within—plot variances. Upon adjustment for competition, variance within plots was reduced, and this was accompanied by increased percentage contribution to the total variance by replications and replications X families-within-stand-interaction sources. Overall means and the ranking of stand and family means were not influenced significantly by competition. Narrow-sense heritability estimates were both inflated and deflated by competition, suggesting that competition might significantly influence estimates of gain from selection.