Nonstructural carbohydrate dynamics of dying lodgepole pines following mountain pine beetle attack

Background/Question/Methods

The mechanisms underlying tree mortality remain poorly understood, and the role of nonstructural carbohydrates (NSC) in these processes is highly debated. Recent work has documented changes in NSC levels during the mortality process but has focused mostly on droughted seedlings, often in only a subset of tree organs. We took advantage of a natural outbreak of mountain pine beetle (MPB) in British Columbia to explore how NSC dynamics in mature lodgepole pine (Pinus contortaDouglas) change throughout the tree following a lethal attack. Prior to outbreak, the bottom 4 m of boles of protected trees were wrapped to prohibit attack. Attacked trees were also wrapped, but had a 1-m long section of wrap cut out to expose trees to attack. We measured the NSC concentrations of needles, branches, and roots as well as bole sapwood and bark above, below, and within the attack region of attacked and protected trees from before beetle attack (May 2011) until death (October 2012). Trees were considered dead when canopies no longer retained green foliage (canopy 100% red or orange).

Results/Conclusions

Following the onset of beetle attack in summer 2011, NSC concentrations were first reduced in the bole bark and sapwood in and directly below the attack region. Needle NSCs were the first distant reserves to be significantly reduced in October 2011, while roots and branch NSC concentrations remained unaffected through July 2012. By October 2012, 11 out of 15 attacked trees were dead and had lower NSC concentrations than protected trees in all organs. The remaining attacked trees were dead by September 2013. Comparisons between attacked trees that were dead or were still living in October 2012 indicated that trees surviving longer had sustained less damage by MPB. The reduced damage was associated with slightly higher bark sugar concentrations prior to attack, suggesting that greater reserves may have aided defense. In October 2012, surviving attacked trees maintained higher NSC levels than dead trees in needles, branches, roots, and the base of the bole. We discuss the implications of these findings for carbon partitioning, the role of NSCs in defense, and mortality mechanisms.