It is likely that a number of the factors influencing tree
density may co-vary in ways that confound their effects,
making it difficult to determine how strong an influence
each factor exerts independently. For example, it is difficult
to separate the effects of topographical position on tree
density from the effects of the history of land development.
That is because upper paddy fields, which tend to have
more trees than lower fields, not only offer more favorable
conditions for tree survival but also were usually cleared
much more recently than the lower paddy fields, whereas
older fields are usually located in the lowest elevation land
in the mini-watersheds, which have the most favorable soil
and moisture conditions for rice growing but are an unfavorable habitat for trees (Prachaiyo 2000). Because the
pioneering farmers preferred to first clear the forest from
lower-lying fields, and only later expanded rice cultivation
onto less desirable higher areas (Takaya and Tomosugi
1972; Vityakon 1993, 2001), the older lower paddies have
been cultivated for a longer period of time than the newly
reclaimed upper paddy fields and also provide a less
favorable habitat for forest trees.
It is likely that a number of the factors influencing treedensity may co-vary in ways that confound their effects,making it difficult to determine how strong an influenceeach factor exerts independently. For example, it is difficultto separate the effects of topographical position on treedensity from the effects of the history of land development.That is because upper paddy fields, which tend to havemore trees than lower fields, not only offer more favorableconditions for tree survival but also were usually clearedmuch more recently than the lower paddy fields, whereasolder fields are usually located in the lowest elevation landin the mini-watersheds, which have the most favorable soiland moisture conditions for rice growing but are an unfavorable habitat for trees (Prachaiyo 2000). Because thepioneering farmers preferred to first clear the forest fromlower-lying fields, and only later expanded rice cultivationonto less desirable higher areas (Takaya and Tomosugi1972; Vityakon 1993, 2001), the older lower paddies havebeen cultivated for a longer period of time than the newlyreclaimed upper paddy fields and also provide a lessfavorable habitat for forest trees.
การแปล กรุณารอสักครู่..
