Vertical root distribution in Himalayan trees: about half of roots occur below 30 cm, the generally sampled depth

Almost nothing is known about the vertical root distribution for Himalayan forest trees. By providing an analysis of vertical distribution of root density of ten important central Himalayan tree species (two evergreen conifers, five evergreen broadleaf species and three deciduous broadleaf species), this paper attempts to address this gap. We used trench profile method to measure tree root density, both of fine (≤ 3 mm diameter) and coarse (> 3 mm diameter) roots. Principal Component Analysis was performed to identify the major sources of variation among species in root distribution. In ten study tree species (eg., sal, oaks, pine and Cupressus torulosa) the rooting depth varied from 90 to 150 cm. On average across the study species, about 49% total roots (all diameter classes) occurred below 30 cm depth and 20.6% below 60 cm depth. These percentages are almost identical to that of tropical rain forest of Amazon. The tree species varied in root density (in entire soil column) from 26.04 ± 2.66 root/100 cm2in Q. floribundato 52.7 ± 4.96 root/100 cm2in Machilus duthei. The ordination graph indicated that tree species of a growth form and even a genus did not form groups and differed markedly in root characters. Tree water status and roots were related only partially and in a complex way. The study has shown that (1) root density measured by digging trenches gives a reasonable estimate of proportional root distribution of trees, and (2) studies which consider only top 30 cm depth can grossly misrepresent root distribution and its consequence.