Van Gunst, K.J. 2012. Forest Mortality in Lake Tahoe Basin from ...

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2ABSTRACTIncreased stand density is commonly cited as a predisposing factor in forest mortality,although evidence over larger landscapes and longer timeframes is lacking. Weinvestigated how the relationship between stand-level mortality and stand density variesby forest type and climatic period in the mixed conifer forests of the Lake Tahoe Basin(LTB) in the central Sierra Nevada. Our analyses used Landsat TM data from 1985-2010to derive annual mortality maps and an annual stocking index. We found that the strengthof density-dependent mortality was variable in the LTB, with no forest showing aubiquitous relationship between mortality and density. However, increases in densitywere weakly associated with increased probability of mortality during major dry periodsand in lower-elevation and pine-dominated forests. In middle- and upper-elevationforests, increased density was associated with increased mortality risk in dry periods,with this effect reversed in wet periods. Years when particular forest types experiencedthe greatest mortality levels did not correspond to years when density-dependentmortality was greatest, suggesting that agents of epidemic mortality may act in a densityindependentmanner.INTRODUCTIONWidespread increases in forest mortality have become prevalent over much of thewestern United States, with mortality rates doubling every 17-29 years in some areas (vanMantgem et al. 2009). Recent increases in tree mortality have been evidenced inwhitebark pine (Pinus albicaulis) in the Greater Yellowstone Ecosystem (Logan et al.2010) and the Sierra Nevada (Millar et al. 2012); limber pine (Pinus flexilis) and mixedconifer stands of the Sierra Nevada (Egan et al. 2011; Ferrell et al. 1994; Guarín and
3Taylor 2005; Maloney et al. 2011; Millar et al. 2007, 2012; van Mantgem andStephenson 2007; van Mantgem et al. 2009); ponderosa pine (Pinus ponderosa), mixedconifer, and pinyon-juniper woodlands of the Southwestern US (Breshears et al. 2005;Floyd et al. 2009; Negrón et al. 2009; Ganey and Vojta 2011); subalpine and middleelevationforests of Utah and Colorado (DeRose and Long 2012; Hebertson and Jenkins2008), lodgepole pine forests of British Columbia, Canada (Aukema et al. 2006); spruceforests of Alaska (Berg et al. 2006), and temperate coniferous forests of Europe (Allen etal. 2010 and references therein; Dobbertin et al. 2005, 2007). These dieback events havebeen caused by a diversity of host-specific mortality agents including mountain pinebeetle (MPB, Dendroctonus ponderosae Hopkins) in lodgepole pine (Pinus contorta),sugar pine (Pinus lambertiana), and whitebark pine stands; spruce beetle (Dendroctonusrufipennis Kirby) in Englemann spruce stands; pinyon ips (Ips confusus Leconte) inpinyon-juniper woodlands, and several insect species including fir engraver (Scolytusventralis) and Jeffrey pine beetle (Dendroctonus jeffreyi) in mixed conifer forests (Eganet al 2011; Ferrell et al. 1994; Ferrell 1996). Forest dieback has been linked to climatewarming and drought (Bentz et al. 2010; Berg et al. 2006; Guarín and Taylor 2005;Hebertsen and Jenkins 2008; Millar et al. 2012; van Mantgem and Stephenson 2007), butalso attributed to denser forests associated with fire exclusion (Dobbertin et al. 2005;Egan et al.2011; Ferrell et al. 1994; Galiano et al. 2010; Guarín and Taylor 2005;Maloney et al. 2011; Millar et al. 2012; Negrón et al. 2009).Forest mortality can be either patchy or widespread, occurring at chronic backgroundlevels with small groups of trees affected, or at outbreak levels with widespread tree dieoffacross much of the landscape. Causes of forest mortality and the conditions that give
Page 1 and 2: University of Nevada, RenoForest Mo
Page 4 and 5: iTHESIS ABSTRACTWidespread and sync
Page 6 and 7: iiimiddle and upper-elevation fores
Page 8 and 9: vpine (Pinus albicaulis) forests of
Page 10 and 11: viiACKNOWLEDGMENTS: Primary financi
Page 12 and 13: ixPlate 2. Photograph of mountain p
Page 14 and 15: xiForest Types and Ecoregions......
Page 16 and 17: xiiiAPPENDIX A. 25 years of annual
Page 18 and 19: xvefficiency measures agreement bet
Page 20 and 21: xviimortality; Negative DDM: decrea
Page 22 and 23: xixratios greater than one indicate
Page 26 and 27: 4rise to outbreaks of widespread mo
Page 28 and 29: 6of compounds and structures necess
Page 30 and 31: 8landscapes (Logan et al. 1998; Nel
Page 33 and 34: 11(Sierra Nevada) of the LTB with m
Page 35 and 36: 13treatment areas were also exclude
Page 37 and 38: 15We evaluated the accuracy of mode
Page 39 and 40: 17field data. In early fall of 2010
Page 41 and 42: 19gray canopies, a result of the co
Page 43 and 44: 21established five climatic periods
Page 45 and 46: 23far greater during the first drou
Page 47 and 48: 25slightly more frequent during the
Page 49 and 50: 27but persistent factor for forest
Page 51 and 52: 29For sugar pine, mortality risk in
Page 53 and 54: 31treatments to be variable, incons
Page 55 and 56: 33Baret, F. and G. Guyot. 1991. Pot
Page 57 and 58: 35Das, Adrian, John Battles, Nathan
Page 59 and 60: 37Fettig, Christopher J., Robert R.
Page 61 and 62: 39Huete, A.R., H.Q. Liu, K. Batchil
Page 63 and 64: 41Loomis W E. 1932. Growth-differen
Page 65 and 66: 43Moritz, Max A., Paul F. Hessburg,
Page 67 and 68: 45Scholl, Andrew E. and Alan H. Tay
Page 69 and 70: 47van Mantgem, P.J., Stephenson, N.
Page 71 and 72: 49Table 1-1. Description of LTB for
Page 73 and 74: 51TABLE 1-3a,b. Validation of remot
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53TABLE 1-5. Summary of odds-ratios
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FIGURE 1-1. Location of Lake Tahoe
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57a.b.FIGURE 1-3a, b. LAI Validatio
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60b1.b2.d. c.FIGURE 1-6. Pictorial
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62FIGURE 1-8. Mortality levels and
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64FIGURE 1-10. Direction and magnit
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66FIGURE 1-12. Number of models whe
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Ratio: Positive DDM:Negative DDM681
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70FIGURE 1-16. Average odds ratio f
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72APPENDIX A. 95 th Percentile of L
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19801982198419861988199019921994199
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76APPENDIX C. Final models in five
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78ForestType Year Model LRT ModelTy
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80Chapter 2 - Spatial and Temporal
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83mountainous regions of snow-domin
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85McDowell et al. 2008; Rice et al.
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87periods in the same forest may re
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89Forest Types and EcoregionsWe exa
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91Environmental VariablesIn additio
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93(Figure 2-6). Forest mortality ma
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95In JP forests, decreases in eleva
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97that abrupt declines in growth ar
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99and d2) and increased temperature
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101concentrated at lower elevations
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103responses may be due to the grea
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105Berg, Edward E, J. David Henry,
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107from 1991-1996 near Spooner Lake
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109Hansen, E., Bentz, B. and Turner
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111reference to historical disturba
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113Ogle, Kiona, Thomas G. Whitham,
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115SNEP [Sierra Nevada Ecosystem Pr
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117Vygodskaya, N.N., E.D. Schulze,
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119ForestType Coefficients Standar
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121a.b.c.d.FIGURE 2-1. Ecoregions (
Page 145 and 146:
123FIGURE 2-3. Solar radiation (WH/
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125FIGURE 2-5. Boxplots of annual m
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127FIGURE 2-7. Effect plots for Ann
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129a: JPb: MFc: RFFIGURE 2-9a-c. Le
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131FIGURE 2-11. Location of increas
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133FIGURE 2-13. Location of increas
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1351993-94 1994-95 1995-96 1996-971
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137THESIS SUMMARYForest mortality i
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Van Gunst, K.J. 2012. Forest Mortality in Lake Tahoe Basin from ...

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