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

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142 Chapter 5 this phenomenon: (1) Most simply, the parameters denoting the drought tolerance of the tree species may have been estimated erroneously; (2) the soil water balance submodel is not capable of tracking soil moisture content under warm-dry conditions, especially where soils are sandy; (3) the feedbacks between vegetation properties (e.g. LAI) and soil water balance (e.g. evapotranspiration) that have been neglected in the model formulation may become important under these conditions; (4) neither the “dry days” approach (FORECE) nor the evapotranspiration deficit approach (FORCLIM) are appropriate indices for expressing drought stress as experienced by trees; (5) the indices are appropriate, but the relationship between the index and the annual growth increment is wrong. Further research is required to address these issues. Transition to insubrian and mediterranean forests: None of the three models is capable of simulating the transition from beech forests to insubrian and mediterranean oak-chestnut forests (Fig. 5.9): When the precipitation sum is above 1000 mm/yr, the models do not simulate any drought. However, the large precipitation sum in these areas (e.g. Ticino, Switzerland) does not mean that there is no drought: Often there are extreme precipitation events where a large fraction of the monthly precipitation falls within a few days. The monthly averages used in all three models do not capture the properties of such distributions, and this may allow beech to dominate although it should be outcompeted due to summer drought. Moreover, the moderate standard deviations of precipitation used in the present analysis are not characteristic of these areas (cf. the omission of Locarno for the derivation of climatic input data); this may also prevent the occurrence of dry months with concomitant drought. Finally, it should also be taken into account that under mediterranean conditions other species become abundant that were not included in the species pool for European conditions, such as Quercus ilex. Also for this reason, the present model approaches the limits of applicability in this area. 5.3.3 Conclusion From the analysis of the steady-state species compositions of FORECE, FORCLIM-E/P and FORCLIM-E/P/S in a space spanned by the annual mean temperature and the annual precipitation sum, we may conclude that the two FORCLIM model variants produce steady state species compositions that conform well to field-based empirical expectations in large parts of this (T,P) space (e.g. Ellenberg & Klötzli 1972, Ellenberg 1986; Fig. 5.1). On the other hand, the FORECE model contains several unrealistic thresholds (Fig. 5.4, 5.5, 5.6).
Parameter sensitivity & model validation 143 The FORECE model fails to simulate the occurrence of oak (Quercus spp.) except under warm-dry conditions close to the dry timberline. In the FORCLIM-E/P model, oak is a codominant species at higher temperatures, which conforms better to the expectations by Rehder (1965) and Ellenberg (1986). In the FORCLIM-E/P/S model, the soil submodel (FORCLIM-S) produces periods with low nitrogen concentrations, and oak gets a competitive advantage, thus increasing its abundance (Fig. 5.7). According to the FORCLIM model, oak requires this heterogeneity of nutrient availability to be competitive; this constitutes an interesting hypothesis that requires further testing. The problems encountered with all three models along drought gradients deserve to be studied in more detail: Simulation experiments performed with the FORECE, FORSKA, and FORCLIM models in the warm, dry area extending from Germany through Poland into Byelorussia suggest that each model fails in different ways, also affecting the behaviour of species such as Tilia spp. and Carpinus betulus (M. Lindner & P. Lasch, pers. comm.). They found vast differences between the models e.g. concerning the amount of simulated evapotranspiration and drought stress. Probably several of the hypotheses listed above are involved in causing the failure of forest gap models in this area. Excluding the areas where both FORECE and FORCLIM fail to produce plausible results, we may conclude that FORCLIM simulates more plausible species compositions and more realistic gradients, whereas FORECE contains many threshold effects. Especially the latter renders the application of FORECE for impact studies of climatic change questionable. On the other hand, FORCLIM may be considered to be a valid tool for simulating forest dynamics for a large part of the range of temperature and precipitation explored in this experiment. 5.4 Behaviour of FORCLIM in eastern North America In a well-known application of the forest gap model FORENA, Solomon (1986) studied forest dynamics at 21 locations along a latitudinal gradient in eastern North America, extending from the Canadian tundra to the temperate-subtropical forests of southern Georgia. The application of FORCLIM to perform simulation experiments along this same gradient appears to be interesting, but is faced with two problems: First, the near-natural forests of Central Europe and eastern North America have no species in common; thus it is necessary to change the species pool and to derive the FORCLIM species parameters for
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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
Page 85 and 86: The forest model FORCLIM 91 The mas
Page 87 and 88: The forest model FORCLIM 93 FORCLIM
Page 89 and 90: Behaviour of FORCLIM along a transe
Page 115 and 116: Parameter sensitivity & model valid
Page 135: 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 and 172: Discussion 177 end of the 21st cent
Page 173 and 174: Conclusions 179 Ecological factors
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
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References 193 Olson, J.S., 1963. E
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References 195 Rudloff, W., 1981. W
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References 197 Smith, T.M., Leemans
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References 199 Whittaker, R.H., 195
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Appendix 201 II. Derivation of para
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Appendix 203 Tab. A-4: Tree species
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Appendix 205 Tab. A-7: Values for m
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Appendix 207 The minimum winter tem
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Appendix 209 Tab. A-11: Shade toler
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Appendix 211 Tab. A-14: Climatic pa
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Appendix 213 IV. Source code of the
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Appendix 215 FROM SimBase IMPORT De
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Appendix 217 END; (* IF *) prevDay
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Appendix 219 PROCEDURE InitializeFW
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Appendix 221 InstallSeparator( fMen
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Appendix 223 FROM FCPFileIO IMPORT
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Appendix 225 modelSp^.p.kHm := sens
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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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