Because of the special importance of old growth red pine at Itasca, we explored in detail the patterns of variation in red pine defenses against bark beetles. There was extensive variation among 12 old growth stands of red pine in resin flow and all measurements of tree growth, tree morphology, and stand attributes (Figs. 2.7-2.9). The old growth forest at Itasca is a mosaic of red pine stands that differ in their constitutive defenses against bark beetles and many other attributes as well. We used regression analyses to identify the simplest combinations of parameters that could predict spatial patterns in constitutive resin flow (Fig. 2.10, Tables 2.1-2.2). Average resin flow at a site can be predicted as Resin = 0.031•DBH - 0.013•BA, where DBH = diameter at breast height in cm and BA = basal area of the stand in m² / ha. This relationship explained 74% of the variation among sites in average resin flow. Thus, old growth stands of red pine that are least defended against bark beetles are those with relatively small trees and a high basal area. Such sites deserve the most careful consideration for management strategies to minimize risk from bark beetles (e.g., by removing, debarking, or burning the trunks of large pine trees that have blown down before they can produce bark beetles).

In addition to variation among sites, there were strong differences within sites between trees that were classified a priori as belonging to codominant or intermediate crown classes (resin flow averaged 43% higher in codominant vs. intermediate trees; Fig. 2.7). Thus, within a stand, trees of intermediate crown class are least defended against bark beetles.