Diego Dierick

Comparison of tree water use characteristics in reforestation and agroforestry stands across the tropics

In the tropics, reforestations and agroforestry become increasingly important and may help mitigate climate change. However, high water use by trees may deplete water resources for associated crops or other purposes. Choice of tree species might reduce water use rates to acceptable levels, but available information on species-specific water use characteristics is scarce. We addressed the following questions: 1) do species differ in xylem sap flux response to fluctuating environmental conditions, 2) are there species-specific differences in quantities of water used, and specifically 3) do universal rules relating tree size to water use apply? This chapter combines data on tree sap flux and water use gathered in Indonesia, Panama and the Philippines. These studies applied the same methods and were conducted in recently established stands (5–12 years old when studied) characterised by small diameter trees and relatively simple stand structure. We analyse data from more than 100 trees belonging to 17 species using a simple sap flux model. Model application suggested species-specific differences in parameters such as maximal sap flux velocity and responses to radiation and vapour pressure deficit. With respect to the quantity of water used per tree, we observed a strong correlation between tree diameter and tree water use, which confirms earlier publications. However, e.g. in the stands in the Philippines where tree diameter explained 65% of observed variation, some species clearly followed distinct trajectories. For a given diameter, up to twofold differences in tree water use among species were observed. Our findings thus support the idea that species selection can be used to control tree water use of future reforestations and within agroforestry Teja Tscharntke, Christoph Leuschner, Edzo Veldkamp, Heiko Faust, Edi Guhardja, Arifuddin Bidin (editors): Tropical rainforests and agroforests under global change: Ecological and socio-economic valuations. Springer Berlin 2010, pp systems. This will be especially relevant in areas where water resources are limited already or where climate scenarios predict decreasing precipitation.

Can deuterium tracing be used for reliably estimating water use of tropical trees and bamboo

Reliable estimates of water use by trees and other woody plants are crucial for an improved understanding of plant physiology and for water resource management. Since the 1980s, the thermal dissipation probe (TDP) method has been widely applied in trees, proved to be fairly accurate but is challenging in remote areas. Also in the 1980s, the deuterium (D(2)O or deuterium oxide) tracing method was proposed, which so far has less often been applied. However, deuterium tracing requires less sophisticated equipment in the field and new analytical methods reduce costs and increase sample throughput. The objectives of this study were (i) to compare plant water use estimates of the TDP and D(2)O method and (ii) to determine whether the D(2)O method is appropriate for assessing absolute magnitudes of plant water use. The two methods were employed on five tropical tree species and a bamboo species growing in a reforestation stand in the Philippines and an agroforestry system in Indonesia. For bamboo, an increase in D(2)O values in neighbouring, non-labelled culms suggests that injected D(2)O was partly redistributed among culms, which would seriously limit the accurate estimation of water use for the target culm. For trees, water use estimates resulting from the D(2)O tracing method were proportional to the TDP results (r(2) = 0.85, P < 0.001), but absolute values were, on average, about seven times higher. This overestimation may be due to the assumptions underlying the D(2)O tracing method, such as the conservation of tracer mass, not being met. Further, it cannot be excluded that underestimation of water use by the TDP method contributed partly to the observed difference. However, when considering known sources of error, a large part of the observed difference remains unexplained. Based on our results, the use of the D(2)O tracing method cannot be recommended without further experimental testing if absolute values of whole-plant water use are a major goal. However, the D(2)O tracing method appears suitable for answering other questions, such as relative differences in water use among trees, water redistribution among neighbours and internal water transport and storage processes in plants.

The effect of regional groundwater on carbon dioxide and methane emissions from a lowland rainforest stream in Costa Rica

In the tropical rainforest at La Selva Biological Station in Costa Rica, regional bedrock groundwater high in dissolved carbon discharges into some streams and wetlands, with the potential for multiple cascading effects on ecosystem carbon pools and fluxes. We investigated carbon dioxide (CO2) and methane (CH4) degassing from two streams at La Selva: the Arboleda, where approximately one third of the streamflow is from regional groundwater, and the Taconazo, fed exclusively by local groundwater recharged within the catchment. The regional groundwater inflow to the Arboleda had no measurable effect on stream gas exchange velocity, dissolved CH4 concentration, or CH4 emissions but significantly increased stream CO2 concentration and degassing. CO2 evasion from the reach of the Arboleda receiving regional groundwater (lower Arboleda) averaged 5.5 mol C m−2 d−1, ~7.5 times higher than the average (0.7 mol C m−2 d−1) from the stream reaches with no regional groundwater inflow (the Taconazo and upper Arboleda). Carbon emissions from both streams were dominated by CO2; CH4 accounted for only 0.06–1.70% of the total (average of both streams: 5 × 10−3 mol C m−2 d−1). Annual stream degassing fluxes normalized by watershed area were 48 and 299 g C m−2 for the Taconazo and Arboleda, respectively. CO2 degassing from the Arboleda is a significant carbon flux, similar in magnitude to the average net ecosystem exchange estimated by eddy covariance. Examining the effects of catchment connections to underlying hydrogeological systems can help avoid overestimation of ecosystem respiration and advance our understanding of carbon source/sink status and overall terrestrial ecosystem carbon budgets.