Study of Structural, Successional and Spatial Patterns in Tropical Rain Forests Using TROLL, a Spatially Explicit Forest Model
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Ecological Modelling 124 (1999) 233–254 www.elsevier.com/locate/ecolmodel Study of structural, successional and spatial patterns in tropical rain forests using TROLL, a spatially explicit forest model Je´roˆme Chave * Ser6ice de Physique de l’Etat Condense´, DRECAM CEN Saclay, l’Orme des Merisiers, F-91191 Gif sur Y6ette, France Accepted 15 June 1999 Abstract Competition for light, treefall gap formation and recruitment are the critical phenomena in the genesis of tropical rain forests. These aspects are taken into account to build a new spatially explicit forest growth model called TROLL. Competition for light is modelled by calculating exactly the three-dimensional field of photosynthetically active radiation in the forest understorey. Typically, 106 light intensities are computed per hectare at each time step. This light field controls the growth of each tree and establishment/death events. Seed dispersal, dormancy and establish- ment success as well as a model of treefalls are also included. A special care is paid to the justification and to the validation of each of these modules. The TROLL model is parameterized for a Neotropical rain forest in French Guiana using 12 functional groups of species. Using this model the vertical canopy structure and the tree diametric distribution are investigated. The role of treefalls in maintaining pioneer species is also evidenced. A forest succession scenario is simulated and compared to field data. The model is then used to simulate the recolonization of a previously sterilized area by a rain forest plant community. This scenario is interpreted using information available from palaeorecords over the Holocene period and their validity is discussed. It is suggested that this model could be used to study mosaic-like patterns in rain forests, installation of slowly dispersing species, speciation hypotheses and landscape scale dynamics of rain forests. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Tropical rain forest; Simulation; Individual-based model; Parallel processing; Canopy gaps; Spatial patterns 1. Introduction ham et al., 1990), treefall gap dynamics (van der Meer et al., 1994) and seed dispersal patterns (van A tropical forest naturally regenerates under der Pijl, 1982; Ribbens et al., 1994), all of them several constraints such as light interception (Can- being crucial for the sylvigenesis (Halle´ et al., 1978; Richards, 1996; Whitmore, 1998). A firm * Present address: Department of Ecology and Evolutionary understanding of all these processes and of their Biology, Princeton University, Princeton, NJ 08544, USA. Fax: +33-1-6908-8786. coupling is a challenge for ecology because tropi- E-mail address: [email protected] (J. Chave) cal ecosystems are currently under threat of 0304-3800/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0304-3800(99)00171-4 234 J. Cha6e / Ecological Modelling 124 (1999) 233–254 strong anthropogenic disturbances. This rapid de- patches interactions. A direct consequence of this forestation could in turn have a strong influence lack of spatialization is that gap models must on the global climate of the next century (Phillips resort to a sampling over several simulations et al., 1998). A related issue is to understand how (Monte-Carlo average) to get statistically signifi- tropical trees communities can have recolonized cant results. In the presence of spatial correla- large areas after the end of the last glacial stage tions, averaging over a statistical sample of (Servant et al., 1993; Bush, 1994; Schwartz et al., independent plots smaller than the correlation 1996; Charles-Dominique et al., 1998) in order to length of the system under study is not equivalent better predict how rain forests could recover if to taking one large plot. Here, seed dispersal conservation policies were adopted. Such issues limitation implies a correlation length greater can be addressed through modelling techniques as than 100 m. The two-dimensional model of it is shown in the present article. canopy crown due to Moravie et al. (1997) should It has long been recognized that the most cru- be mentioned as an interesting direction to over- cial step in the sylvigenetic cycle is the formation come this problem. of natural canopy openings. This observation has Due to the increasing power of computers it is motivated the ‘gap model’ approach (Botkin et now possible to model directly the trees and their al., 1972; Shugart, 1984; Prentice and Leemans, interactions, avoiding aggregation procedures. A 1990; Schenk, 1996) where forest patches of size good example of this approach is the SORTIE 0.1–0.4 ha, (or eco-unit, after Oldeman, 1990) model (Pacala et al., 1996; Deutschman et al., have their own dynamics. KIAMBARAM 1997) in which many important aspects of dy- (Shugart et al., 1980; Shugart, 1984) and FOR- namic processes in a temperate forest are individ- MOSAIC (Liu and Ashton, 1998) are two exam- ually modelled and precisely parameterized ples of gap models applied to rain forests. The (growth, mortality, dissemination). Nonetheless gap model approach turns out to give reliable SORTIE requires a large number of data to be results for assessing short term scenarios. Never- fully parameterized and, except for a few well theless several problems result from this documented plots such as Barro Colorado Island approach. (Panama), Mudumalai (India) or Pasoh Firstly, it often avoids the mechanistic mod- (Malaysia), this program is difficult to apply in elling of tree interactions by incorporating com- rain forests. As Vanclay (1995) remarks, ‘The plicated descriptions of tree growth with challenge is to provide sufficient physiological and interaction indexes (however, see Ko¨hler and ecological basis to ensure realistic predictions un- Huth, 1998). The gap size is fixed and therefore der a variety of sites and stand conditions, even small-scale light fluctuations (sun flecks) cannot when empirical data for calibration are limited’. be modelled, although they are of crucial impor- Hence we are in the need of simple, mechanistic- tance in dense forest understoreys. Moreover based approaches for tropical forests. Moreover treefalls are not modelled mechanistically but existing models cannot account for large-scale patches are usually cleared randomly. Secondly, spatial patterns such as the migrations of plant seed dispersal is generally modelled only by as- communities observed in palaeorecords since the suming that seeds are always present. Recent field last glacial maximum. work has shown that seed dispersal limitation is In the present work we describe and we use crucial for maintaining species diversity (Hubbell TROLL, a new succession model which attempts et al., 1999) thus confirming the winning-by-for- to overcome both difficulties. The sub-models of feit scenario (Hurtt and Pacala, 1995). Thirdly, competition for light, dispersion and treefall gaps spatial effects are difficult to reproduce since are all built on simple mechanistic and physio- neighbouring patches are not interacting in most logic hypotheses (Section 2). The number of gap models. Bossel and Krieger (1994) and Lis- parameters is reduced as much as possible to chke et al. (1996) have attempted to resolve this make the parameterization easier (Section 3). difficulty by introducing nearest neighbour Thanks to a powerful parallel computer, large J. Cha6e / Ecological Modelling 124 (1999) 233–254 235 forest stands are simulated and the Monte-Carlo period), therefore a time discretisation into steps sampling is avoided. TROLL is shown to repro- of 1 year provides a detailed enough description duce structural (diametric distributions) and suc- of the dynamics. cessional patterns of a tropical rain forest with a Only woody plants of more than 1 cm diameter good agreement. In addition a rain forest invasion at breast height (dbh) are considered. Due to is simulated and it is compared to available infor- space exclusion of stems and root systems, a mation of the forest migrations in the Holocene minimal surface is necessary for a tree to survive. period. Discussions and perspectives are gathered This fixes a minimal nearest neighbour distance in Section 5. which is called l. In this study the value l=1m was taken. The maximal stem number is therefore 10 000 stems per hectare, which is roughly twice more than those counted in field studies (Hubbell 2. Description of the model and Foster, 1990; Condit, 1995b). The study area is discretised into squares of edge l in which only TROLL simulates the growth of trees located one tree can grow. This assumption saves com- on a study plot of variable size. In one time step puter memory and avoids population blow-ups. a discrete three-dimensional field (voxel field) of leaf density is computed, trees are grown or killed in function of the local light availability (or by 2.1. Tree description treefalls), and the seed bank is updated by the dispersion of seeds produced by fruiting trees. The One tree is spatially defined by several geomet- dynamics is discrete in time and the time step is ric variables: dbh, D, height H, crown radius R fixed to 1 year. This choice is easily generalized to and crown depth h (Fig. 1). The age A is recorded shorter time steps which should be taken when for each tree, as well as a species label. In the modelling seasonal effects such as rainfall fluctua- present version, a set of parameters is defined for tions. In the present work, we assume a constant each species (see section parameterization and climate (temperature, rainfall, duration of the dry Table 1). Fig. 1. Geometric description of a tree and corresponding voxel space. 236 J. Cha6e / Ecological Modelling 124 (1999) 233–254 Table 1 1983; Moravie et al., 1997). Neither ecological nor Average species parameters theoretical arguments can provide a firm justifica- Value tion for the choice of one of these models. Fur- thermore, the quantitative difference between Geometric parameters these curves is much smaller than the uncertainty D0 1cm Initial dbh on field measurements.