Effect of galls induced by Endocronartium harknessii on stem hydraulic conductivity and growth of lodgepole pine

To test the hypothesis that galls resulting from infection by western gall rust [Endocronartium harknessii (J. P. Moore) Y. Hiratsuka] on lodgepole pine [Pinus contorta Dougl. ex Loud.] create hydraulic constrictions that reduce tree growth, the hydraulic conductivity (K_ψ; m² s^?1 Pa^?1), leaf area (A_L) and sapwood area (A_S) were measured on 6‐ to 15‐year‐old trees having galls encircling 0–100% of the stem. Increases in gall encirclement were accompanied by decreases in K_ψ, foliar nitrogen concentration, A_L and A_S, although the A_L:A_S ratio remained constant. Dye flow patterns through galled stem sections indicated that water moved primarily through the non‐galled portions of the stem. In a second field study, xylem and phloem wounds were applied to 13‐year‐old galled and non‐galled trees on either the galled or non‐galled side of the tree. Although both galls and xylem wounds reduced K_ψ, radial and height growth were unaffected. In aggregate the two experiments showed that although trees are quite tolerant to small galls and short term reductions in K_ψ, the disruption of water movement by larger galls may be sufficient to initiate a chain of events leading to tree death, especially in very dense stands with high levels of intraspecific competition.