Agroforestation of Grasslands in Southeast Asia: WaNuLCAS Model Scenarios for Shade-Based Imperata Control During Tree Establishment - Smallholder Tree Growing for Rural Development and Environmental Services - page 137

6.2 Model Scenario Shade-Based Imperata Control

6.2.1 Model Scenario for the Indonesia Site

Four tree-based systems were simulated for 10 years with a similar annual rainfall
pattern of 2,641 mm per year throughout the simulation. In these systems, Imperata
grew simultaneously with crops and trees, unless specifically excluded from any or
all spatial zones around trees by weeding. Management factors influencing the
growth of Imperata were tested based on the improvement of tree growth through
weeding and tree spacing (Table 6.1). Three tree species were selected: fast grow-
ing trees represented by Paraserianthes falcataria (albizia) and Acacia mangium
(black wattle, mangium) and slow growing trees were represented by Swietenia
macrophylla (mahogany) and Hevea brasiliensis (rubber).
Using the above scenarios, simulation results were focussed on:

1. Tree growth in the presence of Imperata (using the WaNuLCAS option of con-
tinuous regrowth of Imperata as soon as conditions allow).
2. Light available at the crop or Imperata level as a function of tree basal area,
which reflect how long it will take for the Imperata in the shade based systems
to die naturally.
3. The effect of trees and tree spacing on Imperata growth and crop growth oppor-
tunities, including the duration of the initial cropping phase.

6.2.1.1 Model Validity Test for Hevea Brasiliensis

The results of simulation of rubber stem diameter growth with the WaNulCAS
model with strip level weeding (zone 1, i.e., the spatial zone immediately around
the tree - up to 1 m distance - is kept weed-free) showed a good agreement with
actual tree diameter measured in farmers field in West Kalimantan as shown in
Fig. 6.2 (Mulyoutami et al. 2005).

Table 6.1 Systems and management scheme applied in the shade-based Imperata control
Tree Species

Spacing Tree density Weeding

(m × m) (ha −1 )

Paraserianthes falcataria 4 × 2 1,250 • Total (clean weeding: all zones)

Acacia mangium 4 × 4 650 • Partial (Zone 1: 1 m around tree)

Swietenia macrophylla 3 × 3 1,111 • No weeding

4 × 8 313

Hevea brasiliensis 8 × 8 156 • Total (clean weeding: all zones)

6 × 3 556 • Partial (Zone 1: 1 m around tree)

5 × 3 666 • No weeding

4 × 4 625

6 × 6 278

6 4× 12 139