In order to determine the relative importance of the history
of land development and topographical position we compared
the density of trees in all of the cells with older paddy fields
with that of more recently developed cells occupying a lower
lying topographical position within the landscape. Out of 73
cells assigned to the flood plain/low terrace landform groups,
56 were categorized as older paddy fields and only five were
categorized as newer paddy fields (12 cells could not be
assigned to either category). The mean density of the 56 older
paddy field grid cells was 9.4 trees/ha and that of five newer
paddy field grid cells was 15.9 trees/ha. Although the difference in density is not statistically significant (|p| B 0.1)
because of the small sample size of newer paddy fields, the fact
that newly cleared fields have higher tree densities than older
fields within the same land form suggests that the history of
land development may be a more important factor than
topographical position in determining tree density. However,
the retention of more trees in higher level fields than in lower
level fields may also be a reflection of their relative benefits
and costs to the farmers. The upper paddies generally have
much lower rice yields than the lower paddies because their
soils are less fertile and have lower clay content, less organic
matter, and poorer water retention capabilities (Craig 1988).
The farmers are well aware that addition of tree litter can help
improve infertile soil (Pendleton 1943), so may recognize that
retaining trees in the upper paddies is more essential than it is
in the case of the better endowed lower paddies. It is also the
case that the upper paddies have much less stable yields than
the lower paddies, indeed fail to produce a harvest at all in 2
out of 3 years (Rigg 1985). Keeping trees in the upper paddy
fields may to some extent help to stabilize fluctuations in rice
yields because, in years with abundant rainfall, yields will be
relatively high, despite the lower productivity of the shaded
rice plants growing close to the trees, whereas in drought
years, when the unshaded rice plants perform poorly, the rice
plants growing in the shade of the trees will still have some
yield (Craig 1988; Vityakon 1993). Moreover, since, in any