Tree species and diversity effects on soil water seepage in a tropical plantation

Highlights

• Seepage rates under six-species stands were lower than the average monoculture stand at a tropical plantation.

• A simple water balance approach was superior to a physical based model (Hydrus-1D) during the wet season.

• More productive stands have lower groundwater tables and soil moisture.

Abstract

Plant diversity has been shown to influence the water cycle of forest ecosystems by differences in water consumption and the associated effects on groundwater recharge. However, the effects of biodiversity on soil water fluxes remain poorly understood for native tree species plantations in the tropics. Therefore, we estimated soil water fluxes and assessed the effects of tree species and diversity on these fluxes in an experimental native tree species plantation in Sardinilla (Panama). The study was conducted during the wet season 2008 on plots of monocultures and mixtures of three or six tree species. Rainfall and soil water content were measured and evapotranspiration was estimated with the Penman-Monteith equation. Soil water fluxes were estimated using a simple soil water budget model considering water input, output, and soil water and groundwater storage changes and in addition, were simulated using the physically based one-dimensional water flow model Hydrus-1D.

In general, the Hydrus simulation did not reflect the observed pressure heads, in that modeled pressure heads were higher compared to measured ones. On the other hand, the results of the water balance equation (WBE) reproduced observed water use patterns well. In monocultures, the downward fluxes through the 200 cm-depth plane were highest below Hura crepitans (6.13 mm day⁻¹) and lowest below Luehea seemannii (5.18 mm day⁻¹). The average seepage rate in monocultures (±SE) was 5.66 ± 0.18 mm day⁻¹, and therefore, significantly higher than below six-species mixtures (5.49 ± 0.04 mm day⁻¹) according to overyielding analyses. The three-species mixtures had an average seepage rate of 5.63 ± 0.12 mm day⁻¹ and their values did not differ significantly from the average values of the corresponding species in monocultures. Seepage rates were driven by the transpiration of the varying biomass among the plots (r = 0.61, p = 0.017). Thus, a mixture of trees with different growth rates resulted in moderate seepage rates compared to monocultures of either fast growing or slow growing tree species. Our results demonstrate that tree-species specific biomass production and tree diversity are important controls of seepage rates in the Sardinilla plantation during the wet season.

Introduction

The area of planted forest increased globally by about 5 million ha per year between 2005 and 2010 and accounts for an estimated 7% (264 million ha) of the total forested area today (FAO, 2010). In Panama, the area of planted forests in 2010 was 79,000 ha, six times larger than in 1990, with an annual growth of 5% over the last 5 years. Today, 44% of the land is still forested and the deforestation rate is low (−0.36% yr⁻¹) (FAO, 2010). It can be expected that with the expanding carbon markets (FAO, 2011) and increasing demands for bioenergy (UN Energy, 2007) the total area of forest plantation will further grow in the near future. Monocultures, for example, of the exotic hardwood tree Tectona grandis L.f. (teak tree) that accounted for 76% of plantations established in Panama between 1992 and 2000, may be very productive, but appear to have drawbacks like promotion of soil erosion and reduction of soil quality (Wishnie et al., 2007). In general, the wide-spread monocultures of exotic species threaten the local biodiversity (Healey and Gara, 2003). Furthermore, it is increasingly reported that plant diversity is crucial for maintaining the function and stability of ecosystems and biogeochemical cycles calling for increased establishment of mixed-species plantations (Loreau et al., 2001).

Global analysis by Jackson et al. (2005) over 504 catchments showed that stream flow decreased dramatically within a few years after setting up plantations, which can impact the water cycle up to 20 years. In the watershed of the Panama Canal, reduction of the water supply due to enhanced evapotranspiration (ET) losses by large scale afforestation might cause limited trafficability of the canal, because the water budget for the canal is tight (Ibanez et al., 2002). However, forests are seen to secure the water supply of the canal by reducing sediment loads and ensuring sufficient dry season flow (Condit et al., 2001, Harmon, 2005).

Because of the growing concern that tree plantations might affect base flow conditions (Malmer et al., 2010, Scott and Prinsloo, 2008), the effect of mixed-species plantations as compared to the traditional monoculture plantations needs to be assessed thoroughly. Surveys over the last decade showed that tree diversity has an impact on ecosystem functioning such as the control of nutrient cycles in a mixed-species tree plantation in Panama (Oelmann et al., 2010, Potvin et al., 2011, Zeugin et al., 2010). From grassland experiments it is further known that plants in mixtures of species are often more productive and have greater water use than in monocultures, which might be related to complementary effects (Caldeira et al., 2001, Verheyen et al., 2008). Recently, Forrester et al. (2010) found higher transpiration rates in mixtures of Eucalyptus globulus Labill. and Acacia mearnsii De Wild, but the mixture also used the water more efficiently. In line with these findings, Kunert et al. (2012) showed for an experimental plantation in Panama that individual tree transpiration is enhanced in mixed-tree stands compared to monocultures, the water use efficiency of five-species mixtures being half that of either monocultures, two- or three-species mixtures. Consequently, mixed-tree stands might increase evaporative water losses following afforestation and thus, pose problems to water supply of the Panama Canal. Because of controversial results on water use of mixed-tree stands, it remains unclear whether tree diversity influences seepage rates during the wet season.

To answer this question, our objectives were to (1) calculate the seepage rates at plots of one, three, and six tree species with two approaches of different complexity and (2), to analyze the effect of the different species and the number of tree species on the seepage rates. Two approaches of water balance assessment were used to test if a simple water balance calculation can predict species specific water fluxes at a tropical plantation for the wet season in 2008 similarly well as a physically based soil water transport model (Hydrus-1D).