Development and Parameterization of a General Forest Gap Dynamics Simulator for the North-Eastern Mediterranean Basin (Greek Forest Species)
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ecological modelling 204 (2007) 439–456 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/ecolmodel Development and parameterization of a general forest gap dynamics simulator for the North-eastern Mediterranean Basin (GREek FOrest Species) Nikolaos M. Fyllas a,∗, Oliver L. Phillips b, William E. Kunin c, Yiannis G. Matsinos a, Andreas I. Troumbis a a Biodiversity Conservation Laboratory, Department of Environment, University of the Aegean, Lesvos, Greece b Center for Biodiversity and Conservation, School of Geography, University of Leeds, UK c Center for Biodiversity and Conservation, School of Biology, University of Leeds, UK article info abstract Article history: This paper describes the development of a model, able to simulate the dynamics of typi- Received 23 February 2006 cal mountainous Mediterranean ecosystems, following the forest gap dynamics framework. Received in revised form The model has been adapted to the bioclimatic conditions and species traits of the North- 26 January 2007 eastern part of the Mediterranean Basin, based on forest inventories and climate data from Accepted 1 February 2007 Greece. With GREFOS (GREek FOrest Species), we tried to develop a generalized forest sim- Published on line 26 March 2007 ulator able to both perform realistically in the mountainous Mediterranean climatic zone, and to identify transitional zones with the lower elevation Mediterranean vegetation pro- Keywords: file. GREFOS follows the structure and “evolution” of the ForClim model, which was initially Forest gap model developed for Temperate Central European forests. A life history strategy parameter, which Mountainous Mediterranean forests affects (under a functional group type approach) the regeneration and mortality pathways Plant functional types of the species included in the model, has been incorporated. In addition, a simplified fire Greece submodel was also embodied. For all species included in the model, we have computed the whole set of essential parameters used in forest gap models. Simulation exercises were car- ried out in two geographical areas with district site characteristics (Krania and Parnassos), where quantitative and qualitative field data were available, respectively. In both cases an altitudinal gradient exists and vegetation changes from a Mediterranean to a mountainous Mediterranean profile. The model produces realistic outputs despite its generality, while areas dominated by Mediterranean sclerophyllous species are successfully identified. As a final simulation exercise, for the second area of study, which comprises a natural reserve, we used GREFOS to explore scenarios of changes in the fire frequency. Following these scenarios pioneer pine species seem able to enhance their abundance, at both the upper distributional limit of typical Mediterranean forest communities and the lower limit of more Temperate oriented vegetative patterns. © 2007 Elsevier B.V. All rights reserved. 1. Introduction Over the last decades, many forest growth models, based on the gap-phase dynamics hypothesis (Watt, 1947)have ∗ Corresponding author. been developed to analyze sustainable forest management E-mail address: [email protected] (N.M. Fyllas). practices (Botkin et al., 1972; Shugart, 1984) and to predict for- 0304-3800/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2007.02.006 440 ecological modelling 204 (2007) 439–456 est vegetation dynamics across climatic gradients (Bugmann, dicting the general forest patterns found at mountainous 1996) and under the influence of changing climatic patterns areas of the Eastern part of the Mediterranean Basin. GRE- (Solomon, 1986; Prentice et al., 1991, 1993; Miller and Urban, FOS (GREek FOrest Species) follows the structure of Bugmann’s 1999). (1994, 1996a) ForClim model. Although the gap dynamics Forest gap models simulate long-term vegetation dynam- framework corresponds to typical forest structure (but see ics, by mimicking physiological mechanisms that express the Coffin and Laurenroth, 1990; Peters, 2002), one of our main effect of abiotic and biotic conditions on growth, through concerns is to develop a generalized simulator, able to iden- simple response functions. These simplifying assumptions, tify Mediterranean to mountainous Mediterranean transition increased the applicability of forest gap models and led to the zones, derived by bioclimatic limitations and/or fire recur- development of a significant number of simulators for differ- rence patterns. The geomorphology of Greece presents such ent ecosystems (Shugart et al., 1992). The trade-off between types of bioclimatic gradients and vegetation patterns, which simplicity and modeling accuracy of physiological mecha- enable the calibration and validation of such type of mod- nisms, still remains a point of discussion among ecosystem els. Taking into account the critiques concerning the forest modelers (Pitelka et al., 2001; Reynolds et al., 2001). The incor- gap dynamics scheme, we have replaced and added ecosys- poration of simplified techniques proved to be essential for the tem processes considered significant for the representation of acceptability of the gap dynamics models as (1) they became a such systems. Thus, we have (a) parametrized a wider species starting point for many ecosystem modeling approaches and pool including both typical Temperate and typical Mediter- (2) dealt with physiological problems that had not been widely ranean forest tree species (52 species), (b) fitted a modified investigated (e.g. precise effects of some abiotic conditions on drought response function, (c) based the mortality and regen- growth) through species classification into common response eration process on a life-history strategies classification and groups. The latter assumption hints at a functional view of (d) added a simple fire submodel. The outputs of the model ecosystem structure and processes (Shugart, 1997). have been validated for two regions of interest (Krania and Par- Gap models are characterized by a “Gleasonian” view nassos), while we used GREFOS to explore scenarios of altered of succession, thus presenting a “cyclic vegetation change” fire occurrence regime at the second study area, which com- (Bugmann, 1996a). They simulate the succession of mixed- prise a natural reserve. aged and mixed-species forest patches, by following the life of each individual (Shugart, 1984). The individualistic approach to succession harmonizes with the individual-based nature 2. Materials and methods of this group of models. The structure of gap dynamics models are described elsewhere (Shugart, 1984; Prentice and 2.1. Forest dynamics of the mountainous Leemans, 1990; Botkin, 1993). Traditional gap models simulate Mediterranean zone the processes of forest succession in unitary forest patches. Basic ecological processes such as recruitment of new indi- A lot of work has been done, regarding the complex vegeta- viduals, growth and mortality are considered explicitly, and tion mosaic found at areas surrounding the Mediterranean critical reviews have been presented regarding the advan- Basin, with focus on the classification of the established for- tages and disadvantages of the assumptions undertaken at est types (Quezel, 1977; Debazac, 1983). The erratic distribution each life stage (Bugmann, 2001). Such reviews (Fischlin et al., of the abundant species and their assemblages emerges from 1995; Loehle and LeBlanc, 1996) provide important critiques the climatic and topographic heterogeneity (Bradbury, 1981), about their ability to predict realistic forest composition, espe- in addition with the long-standing human management (Le cially under varying climatic patterns. These critiques point Houerou, 1981; Barbero et al., 1990) of the region. Phytosoci- out the significance of the climatic inputs and the role of ological (Quezel, 1981) and bioclimatic studies (Daget, 1977; realized niche as driving forces of the early gap dynamics Nahal, 1981), have underlined the ecological value of Mediter- simulators. Furthermore, they underline issues regarding the ranean vegetation, in the context of both enhanced species implementation of these simulators for different types of diversity (Cowling et al., 1996) and “model ecosystems”, where forests ecosystems. However, the relative simple rationale the interaction of natural and anthropogenic factors could be of gap dynamics models and the continuant re-examination explored (Lavorel et al., 1998). of their basic compartments—submodels, provide obvious Forests found above the Mediterranean sclerophyllous advantages regarding the diffusion of controlled scientific evergreen zone, present a significant species diversity information to policy makers (Pace, 2003). (Debazac, 1983). The diversity of species’ structural and Despite the wide acceptance and applicability of this type functional traits, in addition with the heterogenous micro- of models, a gap simulator has not yet been developed for environmental conditions could lead to discrete forest the mountainous Mediterranean forests of the Eastern part dominance patterns. Frequently in such regions, the bound- of the Basin. We will hereafter use the term “mountainous aries between vegetation types are not sharp (Blasi et al., 1999). Mediterranean zone” to refer to the Upper Mediterranean Most of the deciduous broadleaved species found in this belt and the Mediterranean mountain forests (sensu Quezel, 1977). (e.g. Quercus frainetto, Quercus cerris, Quercus pubescens, Castanea We believe that a gap