Geographic variation in resin canal defenses in seedlings from the Sitka spruce × white spruce introgression zone

Abstract

Seedlings from 18 provenances along a coast-interior transect in the Picea sitchensis (Bong.) Carrière × Picea glauca (Moench) Voss introgression zone in northwestern British Columbia were mechanically wounded at the beginning of their third growing season to simulate natural attack by the white pine weevil, Pissodes strobi (Peck). Constitutive resin canals (CRC) in the cortex and traumatic resin canals (TRC) in the xylem of terminal shoots were characterized microscopically 4 months after wounding. Wounding resulted in a large increase in CRC size and in TRC number and density. Provenances differed significantly in TRC number and in CRC number, size, total area, and the proportion of total bark area occupied by CRC. CRC number and size, TRC number, and provenance weevil resistance (obtained from previously published data) increased with increasing latitude, elevation, and distance from the Pacific Ocean (i.e., towards the P. glauca end of the introgression zone) and decreased with increasing longitude (i.e., towards P. sitchensis). These traits also increased with aridity and continentality and decreased with most temperature, precipitation, and growing season length variables. Statistically significant multiple regression models related variation in some resin canal traits to geographic (r2 = 0.71) and climatic (r2 = 0.62) variables. Provenance mean values for weevil resistance were positively associated with predicted values for TRC number and CRC size. These results indicate that it is possible to predict locations in the introgression zone containing trees that possess desirable resin canal traits using geographic or climatic variables.