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the inheritance patterns of secondary chemicals, found that 13 of 32 compounds had dominant inheritance. Given dominant inheritance, if the parent possessing the high levels of defensive chemical is resistant, then F1 hybrids should also be resistant. Paige and Capman (1993) showed that hybrid breakdown in resistance to Pemphigus betae occurs only in backcrosses of Populus hybrids to the susceptible parent, and F1 hybrids are as resistant as the resistant P. fremomii. This illustrates the Dominance hypothesis in the cottonwood-aphid system. There was also considerable support for the Hybrid Susceptibility hypothesis. Three herbivore species consistently supported the Hybrid Susceptibility hypothesis, and over three years 9 of 33 cases supported this hypothesis. The Hybrid Susceptibility hypothesis was also supported by the analysis of total herbivore densities in 1993 (Table 1, Fig. 2). Mosseler (pers. comm.) reports that in his garden of F1 hybrids and parents of seven Salix species, hybrids were highly susceptible to Melampsora rust, but parents were immune. Fritz et al. (1994) has also found for Melampsora rust that hybrids were much more susceptible than either parental species. This observation shows that Fl's appear to show breakdown of resistance. The Hybrid Resistance hypothesis was supported only by the Aculops gall mite in 1992. Density of mites was lower on hybrids and on either parent. Boecklen and Spellenberg (1990) found some evidence for this hypothesis in their study of leaf mining and leaf galling herbivores on oak. A problem with the support for the Hybrid Resistance hypothesis from that study is that many herbivore species were combined into feeding guilds for analysis and they did not explicitly test for a significant departure from the Dominance hypothesis. The results of this study suggest that interspecific hybridization of plants has a variety of effects on resistance of hybrids and it does not lead only to a pattern of breakdown of resistance (i. e., support of the Hybrid Susceptibility hypothesis). The various herbivore responses suggest that different resistance traits of plants have different mechanisms of inheritance and/or that different resistance factors could affect herbivores differently. An important question is: How can the hypothesized herbivore responses be explained by the known patterns of inheritance of resistance traits? The Additive and Dominance hypotheses both have fairly straightforward explanations. If resistance is dosage-dependent, then an Additively inherited trait should lead to support for the Additive hypothesis. A dominantly inherited trait would be fully expressed in hybrids and should result in a pattern of herbivore resistance that supported the Dominance hypothesis. Backcrossing to the susceptible parent, as seems to have occurred in cottonwoods, could lead to the loss of one or more dominant resistance genes, thereby resulting in Hybrid Susceptibility of the backcross progeny. Paige and Capman (1993) suggest that there is more than one dominant gene involved in resistance to aphids in the cottonwood system. Additively inherited traits could also result in herbivore densities that support the Hybrid Susceptibility hypothesis. Herbivores that require a threshold amount of a chemical defense could have higher densities on hybrids (Hybrid Susceptibility) if the dosage in hybrids was less than the threshold amount. Hybrid Resistance could result from a mechanism of dominance inheritance. Hybrids with dominant inheritance of two different resistance traits from each parent would possess complete expression of two different resistance mechanisms. If particular herbivores are negatively affected by both resistance mechanisms, hybrids could be more resistant than either parent (i.e., Hybrid Resistance). It also seems possible that if two additively inherited resistance mechanisms interacted synergistically to affect herbivores Hybrid Resistance could also result. These alternative mechanisms suggest that various outcomes of interspecific hybridization on resistance are likely and that mechanisms of inheritance of resistance and their effect on herbivores need to be investigated in hybrid systems. The year-to-year variation in support for particular hypotheses was an unexpected result of this analysis. Seven of the 11 species had at least one change in which hypothesis was supported between adjacent years. These changes could be considered either: (1) random fluctuations in herbivore numbers on hybrids and parents or accidents of sampling, (2) consequences of temporarily lower population sizes that made differences difficult to detect, or (3) real year-to-year changes in the expression of resistance of hybrids and/or parent species due to variation in environmental factors. Point 1 is always possible, but it is impossible to discount using the census data. A number of shifts in herbivore response occurred between 1991 and 1992 and then back to 1991 patterns in 1993. This may have something to do with the decreased abundance of herbivores in 1992 (unpublished results). Lower numbers could have made differences difficult to detect. While point 2 is possible, it would be wrong to attribute the shifts in support for hypotheses to this factor alone. Shifts in support for hypotheses may be real changes in the relative expression of resistance between hybrid and parental plants. Environmental variation (e. g., solar radiation, temperature, nutrients, etc.) is known to affect the expression of resistance among genotypes of many plant species. It therefbre seems likely that changes in environment could affect resistance of hybrid and parent species of plants in nature. There were some substantial shifts in the weather among the years of this study. The summer of 1992 was cool and rainy compared to normal in the northeast, whereas in 1993 there were some long periods of high temperatures and 139
drought (pers. obs.). These factors might have contributed to the variation in resistance of hybrids. However, experiments wilt be required to test if tile relative resistance of hybrids shifts with manipulated changes in specific environmental factors. Guild membership was not a reliable indicator of similar responses of herbivores to hybrids and parents. Among the leaf galling guild, all four hypotheses were supported in at least 1 year. Leaf miners supported two different hypotheses (Additive and Hybrid Susceptibility) considering the 3 years together (Table 2). These results indicate that combining the densities of two or more species in the same guild for hybrid-parent comparisons will obscure the underlying patterns of herbivore densities on hybrids and parents. In this analysis, numbers of Phyllocolpa nigrita and Phyllocolpa sp. nov. were combined in 1992 and 1993, because densities of these species had not been distinguished on S. sericea of the willow species in 1991. Therefore, the conclusions drawn from these species should be considered with caution. SUMMARY We studied herbivory of two species of willows (Salix sericea and S. eriocephala) and their interspecific hybrids to test four alternative hypotheses concerning the effects of hybridization on plant resistance. Individually marked plants were identified using morphological traits in the field, and RAPD band analysis was used to verify the genetic status of some parental and hybrid plants. The densities of 11herbivore species were compared between 2 parents and their hybrids in the field. We found the most support for the Additive hypothesis and the Hybrid Susceptibility hypothesis over the three years. We found some evidence for the Dominance hypothesis, and one species in one year supported the Hybrid Resistance hypothesis. Guild membership was not a good predictor of similar responses of species to hybrid versus parental plants. This study demonstrates the diversity of responses of phytophages in response to interspecific hybridization, and indicates the presence of year-to-year variation, which might indicate the influence of environmental variation on the expression of hybrid resistance. ACKNOWLEDGMENTS We would like to thank Len and Ellie Sosnowski for allowing us to work on their property. We thank S. Kaufman, N. Murphy, P. Zee, M. Momot, A. Samsel, A. Dao, J. Hama, A. Samsel, and G. Sylvan for help in the field. This research was supported by the National Science Foundation grants BSR 89-17752 to R. S. Fritz and DEB 92-07363 to R. S. Fritz and C. M. Orians, and by the Vassar College General Research Fund. LITERATURE CITED AGUILAR, J.M. and BOECKLEN, W.J. 1992. Patterns of herbivory in the Quercus grisea x Quercus gambelii species complex. Oikos 64: 498-504. ARGUS, G.W. 1974. An experimental study of hybridization and pollination in Salix (willows). Can. J. Bot. 52: 1613-1619. ARGUS, G.W. 1986. The genus Salix (Salicaceae) in the southeastern United States. Syst. Bot. Monogr. 9: 1-170. BOECKLEN, W.J.and SPELLENBERG, R. 1990. Structure of herbivore communities in two oak (Quercus spp.) hybrid zones. Oecologia 85: 92-100. DRAKE, D.W. 1981. Reproductive success of two Eucalyptus hybrid populations. II. Comparison of predispersal seed parameters. Aust. J. Bot. 29: 37-48. ERICSON, L., BURDON, J.J., and WENNSTROM, A. 1993. Inter-specific host hybrids and phalacrid beetles implicated in the local survival of smut pathogens. Oikos 68: 393-400. FLOATE, K.D., KEARSLEY, M.J.C., and WHITHAM, T.G. 1993. Elevated herbivory in plant hybrid zones: Chrysomela cot_uens, Populus and phenological sinks. Ecology 74: 2056-2065. 140
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DYNAMICS OF FOREST HERBIVORY: QUEST
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TABLE OF CONTENTS Seeking The Rules
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32. Companion planting of the nitro
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GRIMALSKY, ¥.I., Research Institut
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POUTTU, ANTTI, Finnish Forest Resea
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persist in mature leaves and affect
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THE SINK-SOURCE HYPOTHESIS: TYPE AN
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late season defoliation decreases t
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BLANCHE, C.A., LORIO, EL., Jr., SOM
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NIEMELA, R, TUOMI, J,, and LOJANDER
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induced reactions have been studied
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summer, but the other sawfly specie
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LARSSON, S., BJ(_RKMAN, C., and GRE
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STAND AND LANDSCAPE DIVERSITY AS A
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not related to resistance to the se
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In contrast to the situation in eas
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ROOT, R.B. 1973. Organization of a
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We tlhus neglect two potential sour
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c__ c__ 1T1 Ill a) a) 0 0 0 e 1 o e
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c_ >l.s S S S m N < DN DN < D c DN
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:hen k > 0 suggest that the cue or
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oth as protective weapons and as po
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DEFENSE THEORIES AND BIRCH RESISTAN
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P 45 A L 40 A T 35 A B 30 I L 25 I
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However, hares showed strong prefer
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DUGLE, J.A. 1966. A taxonomic study
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accommodate findings related to mic
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Table l.--Mean ±S.E. area eaten by
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Sulfhydryl-Disulfide Mechanisms In
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Iraa prior experimental analysis of
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FRAZIER, J.L. and HEITZ, J.R. 1975.
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REICHARD, R 1993. From RNA to DNA,
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; trees, and between and within ind
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Encapsulated objects are data struc
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_re3.--The Lignum model can be cont
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SEVERE DEFOLIATION Tree Fine Root +
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RUMBAUGH, J., BLAHA, M., PREMERLANI
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oo0 a b ol ¢' _achiman_i ()® 1,eA
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1000 Conspicuous Defo i at ion 100
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2.oo 2.oo o t 4 t ¢._ e'- 1.80 1.8
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Qualitative Changes in [,eaves and
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t00 2 Yr 1 Yr o_ 80 Treatment Treat
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Table 2.--Body size of Q. purzctate
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06 b AddedBSA < 04 _ 2rag -'=' i:3
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Severe Defol iat ion Site-A 1993 _
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KAMATA, N. and IGARASHI, M. 1995b.
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Page 100 and 101: 2,500 F 000 F NF NF b _7 a _;_ a _7
Page 102 and 103: 8O D 09 i 4,'.) F AS4L a Z___7 2o !
Page 104 and 105: 2,500 _ st2-st4 El st5 0 st6 [_ pup
Page 106 and 107: 96 SLANSKY, E, Jr. 1990. Insect nut
Page 108 and 109: later, we selected one more floweri
Page 110 and 111: Table 2.---Mean spruce budworm perf
Page 112 and 113: Table 3..--Mean spruce budworm perf
Page 114 and 115: FOLIVORE FEEDING ON MALE CONIFER FL
Page 116 and 117: Table 3.--Lymantria monacha prefere
Page 118 and 119: 500- 400- i 300 v ¢ m a 200- I00 L
Page 120 and 121: From the point of view of trees, th
Page 122 and 123: THE BLACKMARGINED APHID AS A KEYSTO
Page 124 and 125: Wood et al. (1987) report that M. c
Page 126 and 127: MIZELL, R.E 1991. Pesticides and be
Page 128 and 129: METHODS Study Site The study took p
Page 130 and 131: the samples. Fertilization had subs
Page 132 and 133: Table 2.--Percent nutrient content
Page 134 and 135: Some tree responses to fertilizatio
Page 136 and 137: PHYTOCHEMICAL PROTECTION AND NATURA
Page 138 and 139: Indirect effects of abiotic factors
Page 140 and 141: HUNTER, M.D. and PRICE, P.W. 1992.
Page 142 and 143: A° ADDITIVE C. HYBRID SUSCEPTIBILI
Page 144 and 145: Galls, leaf miners, or leaf folds w
Page 146 and 147: Table 2.--Summary of the hypotheses
Page 150 and 151: FLOATE, K.D. and WHITHAM, T.G. 1993
Page 152 and 153: Elements of the Suitability/Defense
Page 154 and 155: Table l.--Comparing the mean number
Page 156 and 157: ACKNOWLEDGEMENTS The authors sincer
Page 158 and 159: SINGH, D.R 1986_ Breeding for resis
Page 160 and 161: The objectives of this presentation
Page 162 and 163: Table 1.--Average weevil attack on
Page 164 and 165: 1977, Kline and Mitchell 1979, Wood
Page 166 and 167: Further research is underway to cla
Page 168 and 169: A SUGI CLONE HIGHLY PALATABLE TO HA
Page 170 and 171: RESULTS Seasonal Stability of Palat
Page 172 and 173: -
Page 174 and 175: OGAWA, A., NAGATA, Y., and SUKENO,
Page 176 and 177: MATERIALS AND METHODS Field Work In
Page 178 and 179: Laboratory Procedures The week afte
Page 180 and 181: E 120 Nogent-sur-Vernisson Remnings
Page 182 and 183: Interdependence Between Reaction Zo
Page 184 and 185: alance hypothesis (Lorio 1986) cann
Page 186 and 187: SOLHEIM, H., LANGSTROM, B., and HEL
Page 188 and 189: In a second experiment, three 30-ye
Page 190 and 191: Considering biochemical pathways, i
Page 192 and 193: - = 120 N /'_" =40 1/ _ j Days 0 _
Page 194 and 195: BIGGS, A.R. 1985. Suberized boundar
Page 196 and 197: DIFFERENTIAL SUSCEPTIBILITY OF WHIT
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Table 2.--Mortality of white fir pr
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Results from the geographic range p
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esulting seedlings were then rooted
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-0.2 0 5 82. _j -0,4 e a a a D4 E I
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1960, Kalkstein 1976). Increased su
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RYAN, B.F., JOtNER, B.L., and RYAN,
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It is seldom feasible to control wa
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15 15 u. 10 |O O N tal 5 5 g ao o 4
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Only recently have we conducted stu
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esistance to beetle attack. Another
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LORIO, EL., Jr'. and HODGES, J.D. 1
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EFFECTS OF ROOT INHABITING INSECT-F
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OCCURRENCE OF ROOT AND STEM SUBCORT
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]Predisposition of Root-infected Tr
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chemistry associated with root colo
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Implications to Natural Resource Ma
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KLEPZIG, K.D., RAFFA, K.F., and SMA
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RAFFA, K.F., PHILLIPS, T., and SALO
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METHODS The study was installed in
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FURNISS, M.M., LIVINGSTON, R.L., an
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METHODS In the spring and summer of
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The importance of a compound as a r
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WERNER, R.A. 1995. Toxicity and rep
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41 40 42 \ 4t 42 41 42 ",,,\ 42 40
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Table 1.--Summary data for the 26 p
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One weakness of using data from gen
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Lower Peninsula (populations 14-26)
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AUCLAIR, A.ND., WORREST, R.C., LACH
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KELLOMAKI, S., HANNINEN, H., and KO
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WARGO, RM. and HAACK, R.A. 1991. Un
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investigation. Sampling was done on
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Figure 2.--Changes in protein preci
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q) J 400 T J ! i i i i o control tr
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attacked trees than in control tree
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ACIDIC DEPOSITION, DROUGHT, AND INS
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Table 1.--Impact of acidic fog upon
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MENGEL, K., BREININGER, T., and LUT
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THE RESISTANCE OF SCOTCH PINE TO DE
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EXPERIMENTAL METHODS Experiments we
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C w Q. 1,200 1,000 o 800 C_ --- 600
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0 .................................
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FONTAtNE, R.G. 1985. Forty years of
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Host Defenses An important componen
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Functional Heterogeneity of Forest
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The Connectivity Index (CI). The CI
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FUNCTIONAL HETEROGENEITYANALYSIS :
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The Modified Hazard Map. The next l
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systems facilitate mapping of fores
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KOLASA, J. and ROLLO, C.D. 1991. In
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) i,i_i)i_)i_i_ U.S. Departme_]t of
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