Vegetation and Disturbance - Environmental Modelling: Finding Simplicity in Complexity - page 226

14

Vegetation and Disturbance

Stefano Mazzoleni 1 , Francisco Rego 2 , Francesco Giannino 3 ,
Christian Ernest Vincenot 4 , Gian Boris Pezzatti 5 and Colin Legg 6
1 Dipartimento di Arboricoltura, Universita di Napoli 'Federico II', Italy
2 Centro de Ecologia Aplicada 'Prof. Baeta Neves' (CEABN), Lisboa, Portugal
3 Dipartimento di Ingeneria Agraria e Agronomia del Territorio, Universita degli Studi di
Napoli Federico II, Italy
4 Biosphere Informatics Laboratory, Kyoto University, Japan
5 Insubric Ecosystem Research Group, Bellizona, Switzerland
6 School of Earth, Environmental and Geographical Sciences, University of Edinburgh,
Scotland

chosen by Waring and Running (1998), for example, who
defined disturbance in a forest as any factor that brings
about a significant reduction in the overstory leaf area
index for a period of more than one year. Grubb (1985)
reviewed the concept of disturbance and distinguished
vegetation resilience in situ and 'by migration' (Grubb
and Hopkins, 1986), referring to the regeneration capacity
of surviving individuals in the disturbed area and re-
colonization from neighbouring areas respectively.
In the simplest case of one type of disturbance at a
homogeneous site, the disturbance regime can be gener-
ally described by its frequency and its intensity. However,
if intensity is associated with accumulated biomass, as in
the case of fire, there is an inverse relationship between
intensity and frequency. These considerations were very
well established for fire regimes in north America (see
for instance Heinselman, 1978) and in Europe (e.g.
Angelstam, 1997). Therefore the simplest way to describe
a fire regime is to determine the average time between
disturbances (years) or conversely, the mean frequency
of the disturbance (expressed in year -1 ).
This paper focuses only on fire and grazing with
an emphasis on work in Mediterranean environments.
Henderson and Keit (2002) performed a canonical cor-
respondence analysis on the sources of variation in shrub

14.1 The system complexity: effects of
disturbance on vegetation dynamics

Vegetation is continuously changing in time and space.
The changes may be autogenic, driven by the properties
of the component plant species, or allogenic, driven
by external factors. Allogenic change may be a result
of gradual changes in environment, but more often
results from disturbance events, and the way vegetation
responds will be very dependent upon the disturbance
history (Miles, 1979).
The term 'disturbance' is used in ecology to refer to
the partial or complete reduction of biomass due to some
external factors and does not include natural mortality
or decay processes. Typical causes of disturbance are
grazing, cutting, fire, and frost. Disturbance can also
be defined as an event that causes a significant change
from the normal pattern in an ecological system (Forman
and Godron, 1986), but the problem with this relative
definition of disturbance stems from the difficulty of
defining the 'normal' range of conditions for an ecosystem
(White and Harrod, 1997). Thus an absolute definition
requiring measures of real change is more appropriate for
a mechanistic modelling approach. This was the option