Influence of Rainfall and Tree Traits on Stem Flow Production of Selected Urban Tropical Trees

Abstract

Stem flow is small but important fraction of intercepted precipitation channeled through a tree canopy
to the base of the trunk. Given increased impervious covers in cities, quantification of stem flow is
necessary for a holistic understanding of hydrology in urban ecosystems. The values of urban trees
are increasingly being recognized over the decades and authorities have invested in urban green
infrastructure including urban forestry. However, knowledge related to rainfall redistribution by
urban trees and reduce storm water are yet to be discovered for topical tree species. Therefore, this
study aimed study the effect of canopy traits on stem flow percent and funneling ratios for isolated
urban trees in a tropical wet climate. The stem flow of 21 individual trees (7 species with 3 replicates)
located in University of Sri Jayewardenepura, Sri Lanka were measured from August 2019 to
November 2019 by installing SF collars at diameter at breast height (dbh). The stem flow was
measured for 20 individual storm events started with dry canopy condition. Gross precipitation was
recorded using an automated weather station located in an open area in University premises. All trees
produced stemflow for storm events larger than 4 mm. Felicium decipiens generated the highest stem
flow and Pterocarpus marsupium Roxburgh generated the lowest among studied species. While F.
decipiens showing negative correlation between rainfall and stem flow others showed positive
correlation. In all species, crown volume, dbh and bark thickness showed negative relationship with
stem flow. However, individual-tree stem flow percent and funneling ratio values were variable, even
for similar rain depths, suggesting that meteorological factors play a role. Our findings highlight the
need of careful species selection for urban forestry and importance of infiltration capacity at the base
of urban trees when integrating storm water management with vegetation and soils in cities.

Keywords: Stem flow percent, Funneling ratio, Urban hydrology, Urban forestry, Storm water management