Impacts of dwarf mistletoe on the physiology of host Tsuga heterophylla trees as recorded in tree-ring C and O stable isotopes

Abstract

Dwarf mistletoes, obligate, parasitic plants with diminutive aerial shoots, have long-term effects on host tree water relations, hydraulic architecture and photosynthetic gas exchange and can eventually induce tree death. To investigate the long-term (1886–2010) impacts of dwarf mistletoe on the growth and gas exchange characteristics of host western hemlock, we compared the diameter growth and tree-ring cellulose stable carbon (C) and oxygen (O) isotope ratios (δ¹³C_cell, ¹⁸O_cell) of heavily infected and uninfected trees. The relative basal area growth of infected trees was significantly greater than that of uninfected trees in 1886–90, but declined more rapidly in infected than uninfected trees through time and became significantly lower in infected than uninfected trees in 2006–10. Infected trees had significantly lower δ¹³C_cell and δ¹⁸O_cell than uninfected trees. Differences in ä¹⁸O_cell between infected and uninfected trees were unexpected given that stomatal conductance and environmental variables that were expected to influence the δ¹⁸O values of leaf water were similar for both groups. However, estimates of mesophyll conductance (gm) were significantly lower and estimates of effective path length for water movement (L) were significantly higher in leaves of infected trees, consistent with their lower values of δ¹⁸O_cell. This study reconstructs the long-term physiological responses of western hemlock to dwarf mistletoe infection. The long-term diameter growth and δ¹³C_cell trajectories suggested that infected trees were growing faster than uninfected trees prior to becoming infected and subsequently declined in growth and leaf-level photosynthetic capacity compared with uninfected trees as the dwarf mistletoe infection became severe. This study further points to limitations of the dual-isotope approach for identifying sources of variation in δ¹³C_cell and indicates that changes in leaf internal properties such as gm and L that affect δ¹⁸O_cell must be considered.

Citation

Marias, D.E.; Meinzer, F.C.; Woodruff, D.R.; Shaw, D.C.; Voelker, S.L.; Brooks, J.R.; Lachenbruch, B.; Falk, K.; McKay, J. 2014. Impacts of dwarf mistletoe on the physiology of host Tsuga heterophylla trees as recorded in tree-ring C and O stable isotopes. Tree Physiology. 34(6): 595-607.