Water use in four model tropical plant associations established in the lowlands of Costa Rica

Marco V Gutiérrez Soto,John J Ewel

doi:10.15517/rbt.v56i4.5771

Abstract

We examined soil water use patterns of four model plant associations established in the North Caribbean lowlands of Costa Rica by comparing the stable hydrogen isotope composition, deltaD, in xylem sap and in soil water at different depths, under rainy and dry conditions. Four 5-year-old model plant associations composed of 2 tree species (Hyeronima alchorneoides and Cedrela odorata) having different architecture and phenology were studied. Average tree height was 8.9 and 7.6 m, respectively. Each tree species was grown in monoculture and in polyculture with 2 perennial monocotyledons (Euterpe oleracea and Heliconia imbricata). Maximum rooting depth at the time of 6D determination was approximately 2 m for almost all species. Most roots of all species were concentrated in the upper soil layers. Stomatal conductance to water vapor (gS) was higher in the deciduous C. odorata than in the evergreen H. alchorneoides; within each species, g, did not differ when the trees were grown in mono or in polyculture. During the rainy season, gradients in soil water 6D were not observed. Average rainy season xylem sap deltaD did not differ among members of the plant combinations tested (-30% per thousand), and was more similar to deltaD values of shallow soil water. Under dry conditions, volumetric soil water content declined from 50 to approximately 35%, and modest gradients in soil water deltaD were observed. Xylem sap deltaD obtained during dry conditions was significantly lower than rainy season values. Xylem sap deltaD of plants growing in the four associations varied between -9 and -22% per hundred, indicating that shallow water was predominantly absorbed during the dry period too. Differences in xylem sap deltaD of trees and monocots were also detected, but no significant patterns emerged. The results suggest that: (a) the plant associations examined extracted water predominantly from shallow soil layers (<1 m), (b) the natural isotopic variation in soil and plant water at the study site was low, and (c) the plant mixes obtain water from more than a single soil layer simultaneously. Temporal factors were important in determining the competition and complementary relations observed among the trees and the perennial monocots. Under the prevailing environmental conditions, water use in these plant associations was determined largely by species-specific attributes such as biomass allocation to fine roots, phenology, and canopy architecture, and to a lesser extent by water limitations.

Highlights

Below-ground processes, such as extraction of soil water and competition for water resources, remain the least understood aspects of plant interactions that determine the success of land use systems in the tropics (Sánchez 1976, Gliessman 1986, Ong 1995)
We examined the pattern of soil water utilization by several combinations of trees and perennial monocots, using the relative abundance of natural isotopes of water, and explored the degree of complementary water use displayed by these plant combinations under wet and dry conditions
Rainy season soil water δD values fall within the ranges measured in other tropical forests (Jackson et al 1995, 1999, Meinzer et al 1999, Dawson et al 2002)

Summary

Introduction

Below-ground processes, such as extraction of soil water and competition for water resources, remain the least understood aspects of plant interactions that determine the success of land use systems in the tropics (Sánchez 1976, Gliessman 1986, Ong 1995). These studies have provided new information on competitive interactions and water use patterns of plants in natural and agricultural conditions (Walker and Richardson 1991, Ehleringer and Dawson 1992, Ehleringer and Osmond 1999, Dawson et al 2002) They have revealed the sources of water utilized by different life forms in various environments (Dawson and Ehleringer 1991, Jackson et al 1995, 1999, Meinzer et al 1995, 1999, Pate and Dawson 1999, Stratton et al 2000, Drake and Franks 2003), and the mechanisms involved in the control of water balance (Dawson 1993, Goldstein et al 1996). Further application of these techniques have been conducted at sites where the depth of water uptake varies seasonally (Sternberg et al 2002), and at very wet riparian tropical environments, where soil water partitioning among different life forms may occur only during the dry season (Drake and Franks 2003)

Methods

Results

Conclusion