09-Aug-03. 11-Aug-03. 13-Aug-03. 15-Aug-03. 17-Aug-03. 19-Aug-03. 21-Aug-03. 23-Aug-03. T r e e t r a n s p ir a tio n. (g. /s. ) Nighttime sap flow. Nighttime ...
TDP Measurements to Determine Nighttime Sap Flow and the Impact of Fog in Tree Transpiration of a Laurisilva Forest C.M. Regalado and A. Ritter Instituto Canario de Investigaciones Agrarias (ICIA) Dep. Suelos y Riegos Apdo. 60 La Laguna 38200 Tenerife Spain
G. Aschan University of Duisburg-Essen Applied Botany Universitätsstr. 5 D-45117 Essen Germany
Keywords: cloud immersion, Granier probes, laurel forest, nighttime sapflow, transpiration Abstract The evergreen ‘laurisilva’ forests of the Canary Islands (Spain) are frequently immersed in fog. However the role of fog precipitation remains yet unanswered, despite the belief that fog is important for the survival and distribution of endemic ‘laurisilva’ tree species. Granier’s sap flow measurements carried out in Erica arborea L. and Myrica faya Ait. trees of the Garajonay National Park (La Gomera), during a ten month period, show that diurnal tree transpiration is greatly reduced, down to nighttime values, during fog occurrence as compared to fog-free periods. Additionally, nighttime sap flow was detected in laurisilva tree species with an ad hoc method which combines evapotranspiration and Granier’s Thermal Dissipation Probes (TDP) measurements. To our knowledge this is the first time that a study shows under natural field conditions and for a long measuring period the effect of fog on whole-tree transpiration using sap flow techniques. INTRODUCTION Laurel forests of the Canary Islands, locally known as ‘laurisilva’, are composed of relict evergreen tree species frequently immersed in fog. Although cloud immersion is believed to be important for the survival of ‘laurisilva’ tree species, this has not yet been verified, and thus remains an open question, such that none of previous studies have confirmed the relevance of fog events in the Canary Islands cloud forests (Kämmer, 1974; Höllermann, 1981; Santana, 1986; Pérez de Paz, 1990; Aboal Viñas, 1998). The role of fog may be manifold. Fog may represent an additional source of water (Bruijnzeel and Proctor, 1995) and nutrients (Eugster, 2007) to the soil due to a process involving fog droplet impaction and gravitational precipitation (Ritter et al., 2008). Additionally, cloud immersion may reduce the incoming solar radiation (Cavelier and Mejia, 1990; Ritter et al., 2008), concomitantly with a lower vapour pressure deficit (VPD) and reduced ambient i.e., leaf temperature. Equally, nighttime fog may be as important in suppressing tree water loss in the dark, as it would be during daylight (Burgess and Dawson, 2004). Nighttime sap flow is widely spread among plant species, and ranges from 5-30% of daily water loss (Snyder et al., 2003; Dawson et al., 2007). This may be relevant in Canarian laurel forests and redwood (Sequoia sempervirens D. Don) Californian fog-immersed forests, where previous studies claim poor stomatal control i.e., a prodigal water use of some tree species (González-Rodríguez et al., 2001; Burgess and Dawson, 2004). Several studies have measured whole tree transpiration in cloud forests using sapflow techniques (McJannet et al., 2007). However, only exceptionally sapflow and fog water measurements have been combined simultaneously (Hutley et al., 1997; Burgess and Dawson, 2004). The objectives of this study were two-fold. Firstly, to illustrate the effect of fog on whole-tree transpiration and concomitant climatic variables. Secondly, to verify the existence of nighttime sapflow in ‘laurisilva’ tree species, using a combination of evapotranspiration and Granier’s Thermal Dissipation Probes (TDP) measurements.
Proc. VIIth IW on Sap Flow Eds.: E. Fernández and A. Diaz-Espejo Acta Hort. 846, ISHS 2009
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MATERIALS AND METHODS The study was carried out in a 43.7 ha watershed 3 113 873 N, 278 177 E (28R zone) within the Garajonay National Park (La Gomera, Spain) (Fig. 1a). Sap flow measurements were carried out in Erica arborea L. and Myrica faya Ait. trees of different DBH: 15.3, 15.9, 18.5, and 22.3 cm for Erica arborea and 24.2, 30.9, and 31.2 cm for Myrica faya (Fig. 1b). Sapflow was measured with Granier’s heat dissipation probes, using a SFS-2 system from Up GmbH (Cottbus, Germany). Each SFS-2 set-up consisted in two cylindrical needle-like probes (length=20 mm and diameter=2 mm), inserted in two previously drilled holes, one above the other 10 cm distance apart, north-side radially into the most external 20 mm of the active outer ring of conducting xylem, at 1.3 m height from the ground (Fig. 1c). Afterwards the drilled holes were sealed with insulating silicone, and the trunk measuring area protected with an aluminium deflector. As a consequence of the tree’s sap flow, ∆T variations occur such that sap flux density, ν (kg m-2 s-1), may be determined from (Granier, 1985) 1.231 ⎛ ΔT ⎞ (1) ν = 0.119⎜ max − 1⎟ ⎝ ΔT ⎠ where ∆Tmax≥∆T is the temperature difference for zero sap flow, computed as described in Regalado and Ritter (2007). Volumetric flux, QSF, or tree transpiration (kg s-1) was obtained from integration of Error! Reference source not found. across the conducting sapwood radial interval. Additionally, climatic variables such as air temperature, relative humidity, solar radiation, and wind speed, were monitored with a micrometeorological station located at 1270 m (Fig. 1d). Potential evapotranspiration estimations were obtained, as is described in Ritter et al. (2009), from the micrometeorological data using the Penman-Monteith approach, where the canopy surface resistance was considered to vary with the stomata response to environmental factors according to Jarvis (1976) and Stewart (1988). Also fog water collection above the stand, F (L m-2) was registered with an artificial fog catcher (QFC) connected to a spoon tipping Rain-O-Matic raingauge (Pronamic Bekhøi International Trading Engineering Co. Ltd., Denmark) to measure intensity and frequency of the collected fog. The QFC consisted in a 0.5 by 0.5 m screen with a single layer of polypropylene, Raschel-type mesh with 65% shade coefficient, and oriented in the NE direction. RESULTS AND DISCUSSION Figures 2-3 show some examples of fog events affects on tree transpiration and climatic variables for different selected ten-day periods. In general the transpiration trend of all trees was similar in both species investigated independently of tree diameter -cf. panels (b) and (c). Solar radiation (Figs. 2d-3d), VDP (Figs. 2e-3e), and air temperature (Figs. 2f-3f), were reduced when fog water was collected. Wind velocity was higher during fog episodes as compared to fog free periods (Figs. 2g-3g). During fog events, transpiration was greatly reduced in both tree species, down to values registered during nighttime. An ad hoc method which combines Penmann-Monteith-Jarvis evapotranspiration estimates and sap flow measurements (Regalado and Ritter, 2007), permitted us to detect potential periods of nighttime transpiration (Fig. 4). This study shows a significant decrease of tree transpiration as a consequence of the environmental conditions prevailing during cloud immersion. We arrived at this conclusion after monitoring transpiration on individual trees with direct sap flow measurements. Previous studies have shown that although turbulent deposition of fog droplets on tree leaves may be important in wind exposed areas, fog precipitation i.e., soil water refilling, is not an overall relevant phenomena in Garajonay laurel forests (Ritter et al., 2008). Because of the different leave morphology, fog droplet formation is less likely to occur on the broad leaves of M. faya, than on the needle-like leaves of E. arborea (Ritter et al., 2008). This suggests that the effect of fog in reducing transpiration is more likely related to the meteorological conditions prevailing during cloud immersion (lower radiation,
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temperature and VPD) than to a saturation of the boundary layer by merged water droplets sitting on the leaf surface. We have observed that during fog, an associated 75% reduction in median global radiation is accompanied by decreases in both the median ambient temperature range (from 9-21ºC to 7-15ºC) and VDP. Disentangling which of these fog concomitant meteorological conditions is responsible for the reported reduction in tree transpiration deserves further investigation. ACKNOWLEDGEMENTS This work was financed with funds of the INIA-Programa Nacional de Recursos y Tecnologías Agroalimentarias (Projects RTA01-097 and RTA2005-228). A. Ritter acknowledges financial help from the European Social Fund. The authors would like to thank the support from the Garajonay National Park staff and especially by Mr. L.A. Gómez and its director Mr. A. Fernández. Literature Cited Aboal Viñas, J.R. 1998. Los flujos netos hidrológicos y químicos asociados de un bosque de laurisilva en Tenerife. Ph.D. thesis, University of La Laguna, La Laguna, Spain. 184 p. (in Spanish) Bruijnzeel, L.A. and Proctor, J. 1995. Hydrology and biochemistry of tropical montane cloud forests: What do we really know? In: L.S. Hamilton, J.O. Juvik and F.N. Scatena (eds.), Tropical montane cloud forests. Ecological Studies no. 110. Springer– Verlag, New York, 38–78. Burgess, S.S.O. and Dawson, T.E. 2004. The contribution of fog to the water relations of Sequoia sempervirens (D. Don): foliar uptake and prevention of dehydration. Plant, Cell Environ. 27:1023–1034. Cavelier, J. and Mejia, C.A. 1990. Climatic factors and tree stature in the elfin cloud forest of Serrania–de–Macuira, Colombia. Agric. Forest Meteorol. 53:105–123. Dawson, T.E., Burgess, S.S.O., Tu, K.P. Oliveira, R.S. Santiago, L.S. Fisher, J.B. Simonin, K.A. and Ambrose, A.R. 2007. Nighttime transpiration in woody plants from contrasting ecosystems. Tree Physiol. 27:561-575. Eugster, W. 2007. The relevance of fog for the vegetation: is it the water or the nutrients that matter? In: A. Biggs and P. Cereceda (eds.), Proceedings of the Fourth International Conference on Fog, Fog Collection and Dew. La Serena, Chile, p. 359– 362. González-Rodríguez, A.M., Morales, D. and Jiménez, M.S. 2001. Gas exchange characteristics of a Canarian laurel forest tree species (Laurus azorica) in relation to environmental conditions and leaf canopy position. Tree Physiol. 21:1039–1045. Granier, A. 1985. Une nouvelle méthode pour la mesure des flux de sève dans le tronc des arbres. Ann. Sci. For. 42:193–200. Höllermann, P. 1981. Microenvironmental studies in the laurel forest of the Canary Islands. Mountain Res. Develop. 3–4:193–207. Hutley, L.B., Doley, D., Yates, D.J. and Boonsaner, A. 1997. Water balance of an Australian subtropical rainforest at altitude: the ecological and physiological significance of intercepted cloud and fog. Aust. J. Bot. 45:311–329. Jarvis, P.G. 1976. The interpretation of leaf water potential and stomatal conductance found in canopies in the field. Phil. Trans. R. Soc. London, Ser. B 273:593-610. Kämmer, F. 1974. Klima und Vegetation auf Tenerife, besonders in Hinblick auf den Nebelniederschlag. Scripta Geobotanica 7:1–78. (in German) McJannet, D., Fitch, P., Disher, M. and Wallace, J. 2007. Measurements of transpiration in four tropical rainforest types of north Queensland, Australia. Hydrol. Process. 21:3549–3564. Pérez de Paz, P.L. 1990. Parque Nacional de Garajonay, Patrimonio Mundial. Excmo. Cabildo Insular de La Gomera, Instituto Nacional para la Conservación de la Naturaleza, Spain (in Spanish). Regalado, C.M. and Ritter, A. 2007. An alternative method to estimate zero flow
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temperature differences for Granier’s thermal dissipation technique. Tree Physiol. 27:1093–1102. Ritter, A., Regalado, C.M. and Aschan, G. 2008. Fog water collection in a subtropical elfin laurel forest of the Garajonay National Park (Canary Islands): a combined approach using artificial fog catchers and a physically based model. J. Hydrometeorol. 9: 920–935. Ritter, A., Regalado, C.M. and Muñoz Carpena, R. 2009. Temporal water dynamics in the top-soil of a forest watershed: common patterns and contribution of explanatory hydrological fluxes. Vadose Zone J. (in press). Santana, L. 1986. Estudio de la precipitación de niebla en Tenerife. Instituto Nacional para la Conservación de la Naturaleza, 97 p. (in Spanish). Snyder, K.A., Richards, J.H. and Donovan, L.A. 2003. Nighttime conductance in C3 and C4 species: do plant lose water at night? J. Exp. Bot. 54:861–865. Stewart, J.B. 1988. Modelling surface conductance of pine forest. Agric. For. Meteorol. 43:19-37.
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Figures
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Fig. 1. Location of the selected watershed (a), and detail of the experimental plot (b) where sapflow sensors (c) and the micrometeorological station (d) were installed.
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a) Fog event b)
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Fig. 2. Ten day period (8/July/03-18/July/03) illustrating the effect of fog on tree transpiration and concomitant micrometeorological conditions (a) QFC collected fog water, F (L m-2), (b) M. faya and (c) E. arborea tree transpiration, QSF (kg s-1), (d) global radiation, Rad (kW m-2), (e) vapor pressure deficit, VPD (kPa), (f) air temperature, T (ºC), and (g) wind velocity, u (m s-1).
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a) Fog event
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Fig. 3. Ten day period (6/September/03-16/September/03) illustrating the effect of fog on tree transpiration and concomitant micrometeorological conditions (a) QFC collected fog water, F (L m-2), (b) M. faya and (c) E. arborea tree transpiration, QSF (kg s-1), (d) global radiation, Rad (kW m-2), (e) vapor pressure deficit, VPD (kPa), (f) air temperature, T (ºC), and (g) wind velocity, u (m s-1).
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Tree transpiration (g/s)
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Fig. 4. An example showing estimated Penmann-Monteith-Jarvis evapotranspiration and TDP measured tree transpiration, indicating periods of potential nighttime sap flow.
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