Miguel F.Acevedo
ABSTRACTS
Growth dynamics of tropical savanna grass
species using projection
matrices
Ecological Modelling
special
issue.
V. Krivtsov, Guest Editor In press.
J. Raventós1, J.
Segarra1 and M. F. Acevedo2
1Departamento de Ecología,
Universidad de Alicante, Alicante, España (Jraventos@ua.es)
2Institute
of Applied Sciences and Department of Geography, University of North
Texas,
Denton, Texas 76203 (acevedo@unt.edu)
Abstract
Savannas are very important tropical ecosystems characterized by
co-dominance of herbaceous vegetation and less abundant trees and
shrubs. Models of the dynamics of the spatial distribution of
aboveground components of grass plants can be very helpful to
understand the dynamics of savanna ecosystems. We developed a model of
grass plant growth as a collection of the individual dynamic behavior
of shoots inspired in data for plants of three species of common
grasses in the Venezuelan savannas. The species are: Elyonurus adustus,
(Trin.) E. Ekman., Leptocoryphium lanatum (Kunth) Nees and Andropogon
semiberbis (Nees) Kunth. These species represent various types of
architecture and regeneration response to fire. The individual-shoot
model is based on shoot emergence, mortality, and elongation given by
Richards’ equation, plus a few simple geometric considerations. Model
output is shoot density in each cell of a square grid at several
vertical levels. Differences in patterns of shoot density among species
are explained by changing a set of parameter values related to growth
form and phenology. Vertical distribution of shoot density was
calculated from the simulation results and the field data with the
purpose of deriving a simpler lumped shoot-population model. This
simpler demographic model is based on a projection matrix that predicts
the essential dynamics of growth in the vertical dimension. The
transient and final behavior of vertical distribution of shoot density
are calculated with the matrix model and compared to field data
yielding good fit as assessed by the root mean square (RMS) error or
difference between the field data and the model results. This error is
of the same magnitude as the RMS difference among the three field data
replicates. Sensitivity analysis of the matrix model for the three
species was used to identify those vertical levels for which the
damping ratio is more sensitive to a perturbation. For early and
precocius species (E. adustus and L. lanatum), the top level is more
sensitive whereas the late blooming species (A. semiberbis) is more
sensitive at lower levels. Although developed for these three species,
the matrix model is generic and therefore can be applied to plants with
the basic growth form of perennial grasses of seasonal savanna
ecosystems. Applications include scaling-up the individual-shoot model
to larger areas and analysis of savanna dynamics subject to fire.
Keywords: grass plants, savanna, individual-based
models, matrix
models, vertical distribution, sensitivity analysis, triangular matrix.
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