Spatial variation in soil available water holding capacity alters carbon mobilization and allocation to chemical defenses along jack pine stems

Abstract

Carbohydrate metabolism in trees has received considerable attention due to its role in global forest decline. However, mobilization, storage, and allocation of carbohydrates show large variation along tree stems due to various source-sink relationships. Currently, we have a limited understanding of the causes or consequences of carbohydrate variation on plant chemical defenses. We measured non-structural carbohydrates (NSC), monoterpenes, and diterpene resin acids along jack pine (Pinus banksiana) stems before inoculating with a phytopathogenic fungus in multiple sites with different soil available water holding capacities (AWHC). We then assessed induced defense responses in trees by measuring lesion areas (necrotic tissues), NSC mobilization, and terpene concentrations along different stem heights. Before inoculation, NSC concentrations increased with stem height, diterpene resin acid concentrations decreased, and monoterpene concentrations did not differ. However, with increasing AWHC, NSC concentrations decreased, monoterpene concentrations increased, and diterpene resin acid concentrations did not differ. After inoculation, trees on the high AWHC sites developed smaller lesions, mobilized local NSC, and induced higher monoterpene concentrations in the phloem and lesions. Diterpene resin acid concentrations did not differ with AWHC but differed at each stem height. Inoculation density showed no difference. We conclude that gradients in soil AWHC can affect stem vulnerability in jack pine trees by influencing carbon mobilization, allocation, and utilization.