On the Ecology of Mountainous Forests in a Changing Climate: A ...

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178 8 . Conclusions Exploring the mechanisms of forest dynamics with forest gap models The complexity of forest ecosystems together with the large temporal and spatial scales involved in successional processes render experimental approaches to study forest succession extremely difficult (chapter 1; Shugart 1984). Forest gap models (Botkin et al. 1972a,b) have been used successfully to synthesize the existing knowledge on successional dynamics of forests. However, the complexity of these models in turn bears problems, e.g. because their properties are ill-known. The analyses performed in chapter 2 together with the re-implementation of FORECE within RAMSES yielded several systems theoretical, statistical and ecological insights into the structure and functioning of the FORECE gap model (Kienast 1987), which provided a safer basis for improving and interpreting the model. Forest gap models are capable of depicting the successional characteristics of many forest ecosystems in a realistic way (Shugart 1984). The analyses performed in the present study and the subsequent changes to the model, such as the update of state variables and the structural simplification, did still lead to realistic model behaviour for a wide range of sites in the European Alps (chapter 4 & section 5.3) and even in eastern North America (section 5.4). Moreover, the factors that turned out to be most important in the model conform to ecological expectations, e.g. light availability. On the other hand, the factors that contributed little to the simulated dynamics in FORECE and thus could be omitted are those that are also considered to be less important in ecology (Shugart 1984, Ellenberg 1986), or their use in a forest gap model is debatable for principal reasons, e.g. indicator concepts. These results support the hypothesis that forest gap models are powerful tools for exploring the dynamics of forest ecosystems on scales that are not directly observable, and that the models can be used successfully to interface the ecological knowledge from various disciplines (cf. Levin 1992).
Conclusions 179 Ecological factors determining forest dynamics in the European Alps The analysis of the sensitivity of FORECE to structural simplifications (chapter 2) made it possible to derive a hypothesis on the most important factors determining the successional dynamics. According to this hypothesis, forest succession in the European Alps can be portrayed realistically using the following factors: Tree growth is governed by the availability of light, nitrogen, water, and sufficient summer warmth. Major factors influencing sapling establishment are winter minimum temperature, browsing, and again light availability. Tree mortality can be portrayed with two simple functions related to maximum longevity and the occurrence of stress. The point here is not that these factors would not have been identified before (Waring & Schlesinger 1985, Kimmins 1987, Lyr et al. 1992); rather, it is notable how few ecological factors are sufficient to synthesize a realistic picture of successional processes. The systematic simulation studies performed in chapter 5 suggest that the simplification of a complex model like FORECE does not have to hamper its realism. On the contrary, the simplification and the improvements introduced when developing FORCLIM have increased its capability to simulate realistic forest dynamics especially along climate gradients. Moreover, it was possible to simplify some of the remaining equations, such as the formulation of maximum tree growth. Some of these simplified equations even turned out to be biologically more sound. Finally, the parameter space of the model could be reduced drastically, from more than 1300 parameters in FORECE to 540 in FORCLIM. Applicability of FORCLIM to study the impact of climatic change on mountainous forests The theoretical analysis conducted in chapter 2 and the simulation experiments in section 5.3 showed that the FORECE model has not been built and is not apt for impact studies of climatic change. Since most forest gap models share many common features, the same may be surmised for many of these models (Shugart 1984). On the other hand, the validation experiments performed with FORCLIM revealed that this model yields plausible results when it is applied along climate gradients in central Europe (section 5.3) and under the climatic conditions of eastern North America, for which it has not been developed (section 5.4). Moreover, the simulated species composition appears to be reasonably robust to changes of the species parameters (section 5.1). Thus it may be conjectured that FORCLIM yields realistic results also when applied to study the impact of climatic change on forest ecosystems in these areas.
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Diss. ETH No. 10638 On the Ecology
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ii Table of contents A BSTRACT.....
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iv APPENDIX .......................
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vi Harald BUGMANN, 1994: On the eco
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viii Harald BUGMANN, 1994: Aspekte
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1 1 . Introduction 1.1 Climatic cha
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Introduction 3 1.2 Methods for the
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Introduction 5 in a changing climat
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Introduction 7 Their integrative ca
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Introduction 9 (1984) provides a mo
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Introduction 11 The main advantage
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13 2 . Analysis of existing forest
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Analysis of existing forest gap mod
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The forest model FORCLIM 45 carbon
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The forest model FORCLIM 47 TREE GR
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The forest model FORCLIM 49 gBFlag
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The forest model FORCLIM 51 Disturb
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The forest model FORCLIM 53 decay o
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The forest model FORCLIM 55 the est
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The forest model FORCLIM 57 3.3 Mod
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The forest model FORCLIM 59 Light a
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The forest model FORCLIM 61 Overall
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The forest model FORCLIM 63 D (cm)
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The forest model FORCLIM 65 a) b) c
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The forest model FORCLIM 67 Growth
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The forest model FORCLIM 69 Stress-
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The forest model FORCLIM 71 Tab. 3.
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The forest model FORCLIM 73 3.3.2 F
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The forest model FORCLIM 75 where k
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The forest model FORCLIM 77 NITROGE
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The forest model FORCLIM 79 peratur
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The forest model FORCLIM 81 of degr
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The forest model FORCLIM 83 microcl
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The forest model FORCLIM 85 Tab. 3.
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The forest model FORCLIM 87 3.4.3 F
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The forest model FORCLIM 89 All the
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The forest model FORCLIM 91 The mas
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The forest model FORCLIM 93 FORCLIM
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Behaviour of FORCLIM along a transe
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Parameter sensitivity & model valid
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Page 121 and 122: Parameter sensitivity & model valid
Page 147 and 148: 153 6 . Model applications Climatic
Page 149 and 150: Model applications 155 The simulate
Page 151 and 152: Model applications 157 6.2 Possible
Page 153 and 154: Model applications 159 simulation s
Page 155 and 156: Model applications 161 Simulation e
Page 157 and 158: Model applications 163 At the site
Page 159 and 160: Model applications 165 Bever Biomas
Page 161 and 162: Model applications 167 tainty inher
Page 163 and 164: Model applications 169 scenario cho
Page 165 and 166: Discussion 171 Finally, the analysi
Page 167 and 168: Discussion 173 bitrarily chosen par
Page 169 and 170: Discussion 175 comparably small imp
Page 171: Discussion 177 end of the 21st cent
Page 175 and 176: Conclusions 181 species composition
Page 177 and 178: References 183 Begon, M., Harper, J
Page 179 and 180: References 185 Burger, H., 1951. Ho
Page 181 and 182: References 187 Faber, P.J., 1991. A
Page 183 and 184: References 189 Huntley, B. & Birks,
Page 185 and 186: References 191 Leemans, R. & Prenti
Page 187 and 188: References 193 Olson, J.S., 1963. E
Page 189 and 190: References 195 Rudloff, W., 1981. W
Page 191 and 192: References 197 Smith, T.M., Leemans
Page 193 and 194: References 199 Whittaker, R.H., 195
Page 195 and 196: Appendix 201 II. Derivation of para
Page 197 and 198: Appendix 203 Tab. A-4: Tree species
Page 199 and 200: Appendix 205 Tab. A-7: Values for m
Page 201 and 202: Appendix 207 The minimum winter tem
Page 203 and 204: Appendix 209 Tab. A-11: Shade toler
Page 205 and 206: Appendix 211 Tab. A-14: Climatic pa
Page 207 and 208: Appendix 213 IV. Source code of the
Page 209 and 210: Appendix 215 FROM SimBase IMPORT De
Page 211 and 212: Appendix 217 END; (* IF *) prevDay
Page 213 and 214: Appendix 219 PROCEDURE InitializeFW
Page 215 and 216: Appendix 221 InstallSeparator( fMen
Page 217 and 218: Appendix 223 FROM FCPFileIO IMPORT
Page 219 and 220: Appendix 225 modelSp^.p.kHm := sens
Page 221 and 222: Appendix 227 DeclMV( meanLitt[leafS
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Appendix 229 Swiss Federal Institut
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Appendix 231 (*********************
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Appendix 233 BEGIN WITH sp^ DO d :=
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Appendix 235 Definition module FCPB
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Appendix 237 Purpose Simulation mod
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Appendix 239 END Immobilization; PR
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Appendix 241 CONST lem = 3; VAR ef:
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Appendix 243 Purpose Provides the b
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Appendix 245 Example of a text file
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Appendix 247 Tab. A-15: Lower end o
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Appendix 249 Tab. A-17: Percentage
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Appendix 251 Tab. A-19: Significant
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Appendix 253 VI. Derivation of para
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Appendix 255 Tab. A-21: Species-spe
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Appendix 257 Tab. A-22 (continued)
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