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 drought periods of different lengths, from few days or weeks in wet forests to several months in drought deciduous forests and savanna ecosystems. The roots during these low soil water availability periods may experience hydraulic and metabolic dysfunctions resulting not only in a substantial decrease in root growth rates, but also in reduced water and nutrient uptake activity. We discuss three groups of processes: (1) hydraulic redistribution, (2) refilling of embolized vessels, and (3) ecological facilitation, which can contribute together or in isolation to contin- uous root activities despite low soil water availability. Furthermore they can help avoid physiological uncoupling between above and belowground plant parts and thus help to preserve the integration of the root, stem and leaf hydraulic continuum within trees. The two mechanisms for refilling embolized vessels that help repair the functionality of the water transport system discussed are the transient pressure imbalance between the xylem and surrounding tissues, and positive root pressures. Tree root systems not only obtain water from the soil to compensate for the water lost through transpiration, but they can also move water from moist to dry layers within the soil profile along gradients in soil water potential. This process known as hydraulic lift or hydraulic redistribution provides water for reabsorption by roots of the same plant or by neighboring individuals with active roots in the soil where water is released through the process of ecological facilitation.