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 141

of the tree spacing selected (but the tree stem diameter did depend on spacing, as
can be seen by comparing the various Y-axes in Fig. 6.4). The total weeding in tree
monocultures (lower two rows in Fig. 6.4) had only a small further improvement of
tree growth in most species and planting patterns. Growing crops in the early phase
of agroforestation (upper two rows in Fig. 6.4) had no negative effect on predicted
tree growth when compared to 'total weeding', and was superior to 'partial weed-
ing' in tree monoculture.

6.3.1.2 Light Received at Crop or Imperata Level as a Function
of Basal Area

Relative light intensity reached at Imperata or crop level was derived from the sim-
ulations to allow direct comparison with the data collected by Purnomosidhi et al.
2005. Figure 6.5 shows that increasing stem basal area of all tree species is fol-
lowed by decreasing relative light intensity at Imperata or crop level. The observa-
tion points tend to indicate that a slightly larger stem basal area is needed to achieve
a certain level of shading than the simulation results show, probably linked to inter-
tree variability which is underrepresented in the model.
Predicted light intensity reduces more quickly, not only in time but also with
respect to increasing stem basal area, for the fast growing P. falcataria and A. man-
gium than for the slower growing trees. For controlling Imperata growth under slow
growing trees, weeding around the trees is essential and the trees need to be
arranged in narrow spacing to have a realistic chance of achieving shade-based
control.
When the simulation results across the various plant spacings are compared to
the survey results of the smallholder agroforestry systems of P. falcataria , A. man-
gium and H. brasiliensis conducted in North Lampung by Purnomosidhi et al.
(2005) (Fig. 6.6), an acceptable relationship (R 2 > 0.80) was found between basal
area and relative light intensity for all species (Table 6.3).

6.3.1.3 Imperata Regrowth Window

Indications so far are that a light level of about 15 percent of full sunlight is the
threshold for Imperata growth, while a level of about 50 percent is needed for
acceptable yields of light-demanding crops such as maize (Purnomosidhi et al.
2005). Beans and cassava might tolerate light levels down to 30 percent and still
be grown profitably. The Imperata regrowth window (stage B in Fig. 6.1) is
thus operationally defined as the period between 50 percent and 15 percent light
availability at crop or Imperata level, with the first value as limit to realistic
opportunities for intercropping light demanding crops, and the second an esti-
mate of critical light levels for vigourous growth of Imperata . The age of trees
at which Imperata will be controlled depends on initial weeding regime and tree
spacing (Fig. 6.7).