Effects of nutrient and light limitation on mountain hemlock : susceptibility to laminated root rot Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/q811kn31c

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  • Mountain hemlock forests in the Oregon Cascades exhibit wave-form dieback resulting from infection by laminated root rot (Phellinus weirii). Although Phellinus remains viable in dead roots after the wave of dieback passes, many regenerating mountain hemlock forests do not become immediately reinfected. We measured at least a doubling of nitrogen availability in the dieback and regrowth zones, and thought that this increased availability could improve tree resistance to the fungus. To test this hypothesis, we grew small mountain hemlocks under nutrient and light limitations in a growth-room, and then inoculated with the fungus. Trees growing without added nutrients had significantly greater foliage damage and mortality after Phellinus inoculation than did trees growing with nutrients. Shading significantly increased susceptibility whether or not nutrients were added. We believe that increased nitrogen availability and possibly increased light levels after dieback in the field act similarly to increase resistance and prevent reinfection of the regrowing stands. Foliage damage and susceptibilty to infection were related to pool sizes of total nitrogen, phosphorus, and non-structural carbohydrates. Plants with very low nitrogen reserves ( < 10 mg N per plant) , or very low energy reserves ( < 20 mg starch per plant), were more susceptible. It appears that resistance to Phellinus occurs via a defensive pathway that requires resources of both nutrients and carbohydrates.
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