Root functioning in tropical high-elevation forests: Environmental vs. biological control of root water absorption

Lowered temperatures may reduce the root water uptake of tropical trees at high elevations through several mechanisms; however, field studies to test their relevance are lacking. We measured sap flux density (J) in small-diameter tree roots across a 2000-m elevation transect in a tropical mountain forest for quantifying the effects of temperature (Ta), VPD and soil moisture (θ) on root water flow and uptake at different elevations. Recently developed miniature heat balance-sap flow gauges were applied to roots of about 10 mm in diameter in mountain forest stands at 1050, 1890 and 3060 m a.s.l. in the Ecuadorian Andes and the measured flow was related to anatomical properties of the root xylem. Between 1050 and 3060 m, mean J decreased to about a third. VPD was the most influential environmental factor controlling J at 1050 and 1890 m, while Ta was the key determinant at 3060 m. Large vessels were absent in the root xylem of high-elevation trees which resulted in a 10-fold decrease of theoretical hydraulic conductivity (khtheor) between 1050 and 3060 m. We conclude that both physical limitations (reduced VPD, increased viscosity of water) and biological constraints (large decrease of khtheor) result in a significantly reduced J and root water uptake of the trees in high-elevation tropical forests.

Research highlights
► Sap flow was measured in situ in very small roots (ca. 10 mm diameter) of tropical high-elevation trees.
► Sap flow density decreased to about a third with increasing elevation from 1000 to 3000 m.
► Theoretical hydraulic conductivity of the roots decreased by a factor of 10 from 1000 to 3000 m due to the disappearance of large vessels with increasing elevation.
► The large decrease in root water flow at high elevations is a consequence of both increasing physical limitation and biological constraints (root anatomy) and may contribute to the explanation of the alpine timberline.