A Model of Stand Dynamics for Holm Oak-Aleppo Pine Forests
115
From a theoretical point of view, these results can be interpreted in the
context of the mechanistic theory of plant competition and community
structure (Tilman 1982, 1988). According to this view, species distributions
along a soil moisture gradient are controlled by shade tolerance at the upper
limit of the gradient and by drought tolerance at the lower limit. In mesic
sites, and for a fIxed range of disturbance and initial conditions, the model
converges on a monospecifIc equilibrium dominated by the most shadetolerant species (holm oak in this case). As rainfall decreases, however, the
qualitative behaviour of the model changes as a result of the differential responses in seedling mortality to light. Therefore, this result follows Tilman's
(1988) prediction of a generalized trade-off between shade and drought tolerance in plant communities. Plant water balance depends simultaneously on
evaporative demand and supply (Cowan 1986; Schulze et al. 1987), hence
lower soil moisture levels increase the detrimental effect of radiation on
seedling mortality at a rate dependent on species morphology. Accordingly,
this effect is likely to be more severe for shade-tolerant species such as holm
oak that require a larger photosynthetic apparatus and have a higher transpirative demand.
Mediterranean forests have developed according to a variety of ecological
pathways related to disturbances, water balance and ecological strategies
(Ruiz de la Torre 1990; Montoya 1993; Espelta et al. 1995) and their long-term
dynamics under the prevailing conditions of massive land-use change are
uncertain (Romane et al. 1992). There is, therefore, a need for diagnostic
tools that scale up the biological and human aspects that operate in these
systems at different scales. This integration is more likely to be achieved with
the use of simple and biologically interpretable models, rather than by coupling models developed at different levels of biological organization, simply
because complex models are diffIcult to interpret in the light of the mechanisms that bear upon them. The implementation of simple, realistic and yet
predictive models of vegetation dynamics for the Mediterranean region will
require a clear understanding of how competition and environmental factors
regulate species distributions. This understanding seems feasible only under
research programs that consider the natural history of these systems and integrate experimentation, fIeldwork and modelling.
References
Botkin DB (1993) Forest dynamics: an ecological model. Oxford University Press, Oxford
Botkin DB, Janak JF,Wallis JR (1972) Rationale, limitations, and assumptions of a northeastern
forest simulator. IBM J Res Dev 16:106-116
Castell C, Terradas J, Tenhunen JD (1994) Water relations, gas exchange, and growth of resprouts
and mature plant shoots of Arbutus unedo 1. and Quercus ilex 1. Oecologia 98:201-211
Chaparro J (1996) Distribuci6n potencial del bosque y de sus especies arb6reas en zonas mediterraneas semiaridas: modelos y aplicaciones. PhD Thesis, University of Murcia, Murcia
115
From a theoretical point of view, these results can be interpreted in the
context of the mechanistic theory of plant competition and community
structure (Tilman 1982, 1988). According to this view, species distributions
along a soil moisture gradient are controlled by shade tolerance at the upper
limit of the gradient and by drought tolerance at the lower limit. In mesic
sites, and for a fIxed range of disturbance and initial conditions, the model
converges on a monospecifIc equilibrium dominated by the most shadetolerant species (holm oak in this case). As rainfall decreases, however, the
qualitative behaviour of the model changes as a result of the differential responses in seedling mortality to light. Therefore, this result follows Tilman's
(1988) prediction of a generalized trade-off between shade and drought tolerance in plant communities. Plant water balance depends simultaneously on
evaporative demand and supply (Cowan 1986; Schulze et al. 1987), hence
lower soil moisture levels increase the detrimental effect of radiation on
seedling mortality at a rate dependent on species morphology. Accordingly,
this effect is likely to be more severe for shade-tolerant species such as holm
oak that require a larger photosynthetic apparatus and have a higher transpirative demand.
Mediterranean forests have developed according to a variety of ecological
pathways related to disturbances, water balance and ecological strategies
(Ruiz de la Torre 1990; Montoya 1993; Espelta et al. 1995) and their long-term
dynamics under the prevailing conditions of massive land-use change are
uncertain (Romane et al. 1992). There is, therefore, a need for diagnostic
tools that scale up the biological and human aspects that operate in these
systems at different scales. This integration is more likely to be achieved with
the use of simple and biologically interpretable models, rather than by coupling models developed at different levels of biological organization, simply
because complex models are diffIcult to interpret in the light of the mechanisms that bear upon them. The implementation of simple, realistic and yet
predictive models of vegetation dynamics for the Mediterranean region will
require a clear understanding of how competition and environmental factors
regulate species distributions. This understanding seems feasible only under
research programs that consider the natural history of these systems and integrate experimentation, fIeldwork and modelling.
References
Botkin DB (1993) Forest dynamics: an ecological model. Oxford University Press, Oxford
Botkin DB, Janak JF,Wallis JR (1972) Rationale, limitations, and assumptions of a northeastern
forest simulator. IBM J Res Dev 16:106-116
Castell C, Terradas J, Tenhunen JD (1994) Water relations, gas exchange, and growth of resprouts
and mature plant shoots of Arbutus unedo 1. and Quercus ilex 1. Oecologia 98:201-211
Chaparro J (1996) Distribuci6n potencial del bosque y de sus especies arb6reas en zonas mediterraneas semiaridas: modelos y aplicaciones. PhD Thesis, University of Murcia, Murcia