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Impacts of dwarf mistletoe on the physiology of host Tsuga heterophylla trees as recorded in tree ring C and O stable isotopes
Marias, D., F. Meinzer, D. Woodruff, D. Shaw, S. Voelker, J. Renee Brooks, B. Lachenbruch, K. Falk, AND J. McKay. Impacts of dwarf mistletoe on the physiology of host Tsuga heterophylla trees as recorded in tree ring C and O stable isotopes. TREE PHYSIOLOGY. Heron Publishing, Victoria, B.C, Canada, 34(6):595-607, (2014).
Climate change can have large impacts on the spread and severity of infectious diseases. Dwarf mistletoe is an infectious pathogen that has been increasing in occurances within the Pacific Northwest. Trees infected with dwarf mistletoe eventually die, but they also increase fire danger through the extensive “witches brooms” which the infected branches form, creating fire ladders and increasing fire severity. Understanding the dynamics of dwarf mistletoe over time is important for predicting how this important pathogen may be interacting with climate change. In this paper, we compared diameter growth and tree-ring cellulose stable carbon and oxygen isotope ratios (δ13Ccell, δ18Ocell) of heavily infected and uninfected trees to investigate long-term impacts of dwarf mistletoe on growth and gas exchange characteristics of host western hemlock. Long-term negative impacts of dwarf mistletoes on tree growth and carbon balance are likely to be indirect and related to disruption of water transport and consequent alterations in hydraulic architecture and photosynthetic gas exchange rather than directly related to interception of water and fixed carbon. Because dwarf mistletoe infections are initially localized and increase in severity over several decades, it is difficult to estimate when trees were initially infected and evaluate host physiological responses to the infection through time. This study reconstructs long-term physiological responses of western hemlock to dwarf mistletoe infection.
• 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 long-term impacts of dwarf mistletoe on growth and gas exchange characteristics of host western hemlock, we compared diameter growth and tree-ring cellulose stable carbon and oxygen isotope ratios (δ13Ccell, δ18Ocell) of heavily infected and uninfected trees. • Relative basal area growth was initially greater, but declined more rapidly in infected than uninfected trees. δ13Ccell and δ18Ocell were significantly lower in infected trees. Lower δ18Ocell in infected trees was unexpected given that stomatal conductance and external variables expected to influence δ18O values of leaf water were similar for both groups. Estimates of mesophyll conductance (gm) were significantly lower and effective path length for water movement (L) significantly higher in leaves of infected trees, consistent with their lower values of δ18Ocell. • This study reconstructs long-term physiological responses of western hemlock to dwarf mistletoe infection. It further points to limitations of the dual isotope approach for identifying sources of variation in δ13Ccell and indicates that changes in leaf internal properties such as gm and L that affect δ18Ocell must be considered.