Establishment patterns in a secondary tree line ecotone
Authored by Alexander Peringer, Gert Rosenthal
Date Published: 2011
DOI: 10.1016/j.ecolmodel.2011.05.025
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Platforms:
Visual C#
Model Documentation:
ODD
Flow charts
Mathematical description
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Abstract
On semi-open pre-alpine fen pastures Alder encroachment creates a
dynamic mosaic of grassland and woodland, which is rich in ecotones from
fen to Carr. The structural diversity in colonisation patterns of Alder
on fens suggests a dependency on multiple environmental drivers.
Unidirectional progressive ecotone development provides an opportunity
to address a current deficit in understanding successional patterns, i.e. process-pattern relationships in a multiple factor regime.
We developed an individual-based model of Alder establishment on fen
grassland to investigate the dependency of encroachment patterns upon
seed production, dispersal distances and safe site availability. The
purpose of the model is to provide a causal understanding of
establishment patterns of Alder. In the model, all life processes of
Alder individuals were parameterised with field data. This allowed us to
strictly perform bottom-up simulations and successfully check
plausibility by comparing simulated establishment patterns of cohorts
with observed ecotone structures.
Simulation results show that establishment patterns strongly depend on
environmental drivers. Spatial progression of Alder encroachment and
width of ecotones, respectively, mainly depend on wind speed during seed
dispersal. Dense establishment of Alder leading to community change from
fen grassland to Carr, requires windows of opportunity, which are
defined by the rare coincidence of widespread dispersal, high seed
production and favorable establishment conditions. Life-history traits
of Alder (mast year cycle, high seed weight, weak establishment in fen)
spatially and temporarily constrain the encroachment process. The
structural diversity of long-term encroachment patterns is explained by
the event-driven character of encroachment.
Modelling individual establishment pathways of seedlings starting from
germination revealed an endogenous stochasticity in establishment
patterns emerging from low seed densities in the tail of the dispersal
function. We conclude an inherent stochastic structure of
dispersal-limited tree line ecotones, which limits reconstruction of
processes from patterns.
In order to describe long-term successional patterns of Alder
encroachment at landscape scale, we propose the combination of two
concepts: deterministic ``patch-movement{''} of Alder woodland driven by
continuous ecotone migration together with rare and stochastic
``infiltration{''} of single Alder trees into open fen grasslands.
Conservation management can control predictable ``patch-movement{''} by
cutting off maturing saplings around existing Alder woods. But the
preservation of the actual large proportion of open grassland in fen
pastures from infiltrating Alder seedlings and from the subsequent shift
of the pasture to a densely wooded state would require mowing
additionally to extensive grazing. (C) 2011 Elsevier B.V. All rights
reserved.
Tags
Dynamics
Light
ecology
Succession
Vegetation
Plants
Gap models
Glutinosa l gaertn
Wooded pastures
Grazing pressure