1. DO AMAZONIAN TREES LOOSE WATER AT NIGHT? Rafael S. Oliveira 1, Todd E. Dawson 1, Stephen O.O. Burgess 2, Scott S. Saleska 3, Steven C. Wofsy 3, Daniel C. Nepstad 4 1 Department of Integrative Biology, Valley Life Sciences Building, University of California – Berkeley, CA 94720 USA 2 School of Plant Biology, University of Western Australia, 35 Stirling Highway, Nedlands WA 6009 AUSTRALIA 3 Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138 4 Woods Hole Research Center, Woods Hole, MA Introduction A common assumption among biologists and hydrologists is that plant transpiration does not occur in the dark (at night). Here, we show that this assumption is incorrect and provide evidence that nighttime transpiration can constitute a significant part of daily water loss for trees inhabiting the Tapajós National Forest, Amazônia. Methods To quantify nighttime water fluxes at the tree and ecosystem scales we used heat ratio sapflow (HRM, Fig.1) on trees and eddy covariance methods (Fig.2) during 2 years in the “km 67” plot of the Tapajós forest. The HRM technique is described in detail in Burgess (1998), but its principle is basically to measure the increase in temperature following a heat pulse at two points, above and below a heater (Fig 1). One heater and a pair of copper-constantan thermocouples were inserted radially into the xylem tissue of the trunk at 70 cm height of 10 trees of 3 species (Coussarea racemosa, Manilkara huberi, Protium robustum). Each thermocouple had two junctions to measure sap velocity at two depths in the xylem tissue. At the end of the study we cut all stems to stop sap flow. This procedure was suggested by Burgess et al. (1998) as the most accurate way to determine the reference velocity (zero) flow line. Figure 1. Schematic representation of the heat ratio sap flow method. Yellow sensors represent thermocouples above and below a central heater. Figure 2. Eddy flux tower at the Km 67 of the Tapajós forest Methodological implications Most, but not all, sapflow methods estimate canopy water loss based on the assumption that water flux at night through trees is zero. In fact, some sapflow methods (Granier) not only assume zero flow at night but apply an algorithm that resets the drift known to occur each day with the sensors back to zero each night; this procedure of course must assume that no flow occurs at night. Using the HRM sapflow system that empirically resolves zero flow our data shows, for trees in the Tapajós forest, that NT E occurs and therefore flow is never zero. Under conditions where NT E occurs but zero flow is assumed the data obtained would be invalidated. Theoretical implications A long standing assumption in plant water relations research for both wild and crop plant studies is that the plant water status (as water potential) measured just before dawn is an excellent surrogate of soil water potential at the root-soil interface and therefore an index of the water availability for plants. This assumption is based on two further assumptions: (a) that the water in plants (or at least plant roots) and in the soil are in equilibrium and (b) that nighttime water loss via plant transpiration is zero thereby permitting the plant-soil water potential equilibrium to be achieved. Our data show that water loss can occur at night and therefore violates these assumptions. The implication here is that for species with nighttime transpiration or under conditions where nighttime transpiration may occur pre-dawn measurement of plant water potential cannot be assumed to be an accurate proxy for soil water potential since it is clear that root-soil water potential equilibrium is not achieved. Burgess, S.S.O., Adams, M.A., Turner N.C. and Ong C.K. (1998). The redistribution of soil water by tree root systems. Oecologia 115:306-311. Results and Discussion We found that tree crowns do loose water at night (Fig. 3, 4 and 5) and that nighttime transpiration (NT E) can constitutes 5-32% of the total daily tree water use (Fig.4). On an ecosystem scale, NT E can represent from 2-16% of the total forest evapotranspiration (ET) (Fig 5). A relationship between night time sap flow and vapour pressure deficit (VPD) was evident for the 3 species measured and data is shown for Coussarea racemosa (caferana) (Fig. 3 and 4). Longitudinal Cross Section of the stem Figure 3. Sap flow in the stem of C. racemosa during a representative period of the dry season at FLONA-Tapajós. Note sap flow values well above zero during the night (shaded bars). Figure 4. Relationship between night time sap flow (measured between 12-6 am) and vapour pressure deficit (kPa) for C. racemosa. NT flow rates in response to VPD often exceed 10% of maximum tranpiration and on a few occasions rates over 25% were recorded. Figure 5. Night time water fluxes (measured between 19:00 and 5:00 and expressed as percent of total weekly ET) derived from eddy flux measurements at the Floresta Nacional do Tapajós.