Maintenance of Root Function in Tropical Woody Species During Droughts: Hydraulic Redistribution, Refilling of Embolized Vessels, and Facilitation Between Plants

Abstract

Most woody dominated tropical ecosystems are subjected to droughtperiods of different lengths, from few days or weeks in wet forests to severalmonths in drought deciduous forests and savanna ecosystems. The roots duringthese low soil water availability periods may experience hydraulic and metabolicdysfunctions resulting not only in a substantial decrease in root growth rates, butalso in reduced water and nutrient uptake activity. We discuss three groups ofprocesses: (1) hydraulic redistribution, (2) refilling of embolized vessels, and(3) ecological facilitation, which can contribute together or in isolation to continuousroot activities despite low soil water availability. Furthermore they can helpavoid physiological uncoupling between above and belowground plant parts andthus help to preserve the integration of the root, stem and leaf hydraulic continuumwithin trees. The two mechanisms for refilling embolized vessels that help repairthe functionality of the water transport system discussed are the transient pressureimbalance between the xylem and surrounding tissues, and positive root pressures.Tree root systems not only obtain water from the soil to compensate for the waterlost through transpiration, but they can also move water from moist to dry layerswithin the soil profile along gradients in soil water potential. This process known ashydraulic lift or hydraulic redistribution provides water for reabsorption by roots ofthe same plant or by neighboring individuals with active roots in the soil wherewater is released through the process of ecological facilitation.