A clear association between the length of time that has
passed since the initial construction of paddy fields and the
density of trees remaining in the fields was found
(Table 4). The mean density of trees in paddy fields of the
newer paddy field group (15.9 trees/ha) is higher than that
of older paddy field group (10.7 trees/ha). The difference is
highly significant (|p| B 0.01) according to the Mann–
Whitney U test so the result clearly supports the hypothesis
that density is influenced by the history of land development in each area. This finding is in accord with the
observations about the relationship between time of land
clearance and density of tress in paddy fields made by
earlier researchers including Grandstaff et al. (1986), Vityakon (1993, 2001), and Prachaiyo (2000).