Forest Gap Modelling

1. GAP MODELS
Mir Faizan Anwar

2. Gap formation
• Gaps: Small openings in a forest canopy
occupying less <1 ha area
• Larger gaps raise the probability of
regeneration of shade intolerant species.
• Maximum gap size: 370- 730 m3
• Average rate of gap formation: 0.5- 2.0 % per
year

3. Gap Dynamics Theory
• Shade intolerant species can
maintain their population by
regenerating within gaps in the
mature or old growth forest.
• Assumption: breakage of a forest
canopy by natural disturbances
certainly occurs in any closed forest
stand
• The gaps provide micro
environmental conditions favoring
establishment of shade intolerant
species.

5. GAP MODELS
• Gap models simulate the changes in a
forest gap.
• Project the annual growth of each
individual tree on the patch as well as
the death of each tree and
regeneration of new individual trees.
• The first model of this genre was the
JABOWA model developed by Botkin
et al. (1972)

6. Structure of gap models
• Model structure emphasizes 2 features:
- The response of the individual tree to the prevailing environmental
conditions
- How the individual tree modifies those environmental conditions
Parameters in the models
• The individual tree parameters are used in species-specific equations for
the growth, relation between diameter and height, and other biological
and ecological descriptors.
• Parameters characterizing the interactions among trees or that simulate
the overall condition on the plot.

7. Growth
• Calculated using a species specific equation to predict the expected
diameter increase for each tree under optimal conditions
• Optimum increment then modified by the environmental response
functions and the realized increment is added to the tree
• Growth equations - Assumption : growth in trees is the consequence of
two opposite processes:
- A positive: as trees get larger, the leaf area of the tree increases, which
increases potential growth and its magnitude depends on the net
photosynthetic rate of the tree per unit of leaves.
- A negative rate that is associated with the energetic cost of maintaining
a given volume of living tissue. As a tree increases in size, these negative
costs increase.
• The difference in these two rates is the actual growth, which becomes
smaller as trees become larger
• JABOWA Model and FORCLIM Model

8. Growth equation:
• Tree volume increment = d[D2 H]/ dt = rLa ( 1- DH/ Dmax Hmax)
(D: diameter of a tree, H: Height of a tree, r: Growth rate parameter, La:
Tree’s leaf area)

9. Environmental Constraints and Resource Competition among Trees
• Models simulate individual tree’s response to light availability at height
intervals on the plot
• An equation for the extinction of light through the canopy is computed to
capture the effect of trees shading one another
• Other resources are incorporated :
- Soil moisture, fertility, temperature, as well as disturbances such as fires,
hurricanes, floods, and wind throw.
• The environmental responses are modelled via a constrained potential
paradigm.
• Competition depends on the relative performance of different trees under
the environmental conditions on the model plot.
• Competitive success depends on the environmental conditions on the plot,
species present, and the relative sizes of the trees.

10. Mortality
• Death modelled as a stochastic process
• Two components of mortality:
- Age related and stress-induced
• Stress defined with respect to a minimal diameter increment (typically
10% of optimum growth for a given size of tree) and individuals failing to
meet this minimal condition are subjected to an elevated mortality rate.
Establishment
• Constrained by environmental factors

11. Applicability
• Examination of vegetation
response to changing
environmental conditions
• Prediction of forest yield
tables and forest
composition

12. ForClim
• Sensitive forest succession (“gap”) model, developed to simulate forest
stand dynamics
• Establishment, growth, competition and mortality of trees are simulated
on small areas (‘patches’)
• Assumption: no interaction between patches ----forest succession on a
patch is independent of its neighbourhood
• Simulation results from many patches averaged to obtain the general
succession pattern at the stand scale

13. Structure of the model
• PLANT submodel
• WEATHER submodel;
• WATER submodel;
• MANAGEMENT submodel

14. Model applications
• Stand structure and species composition across three biogeographic
regions of the northern Hemisphere
• Implications of early senescence of CO2-fertilized trees in an ecosystem
context
• Testing the relationship between species richness and productivity in the
long term
• Examination of the combined effects of climate change and ungulate
browsing along elevation transects

15. THANK YOU