Seasonal variation of water uptake of a Quercus suber tree in Central Portugal

Hydraulic redistribution (HR) is the phenomenon where plant roots transfer water between soil horizons of different water potential. When dry soil is a stronger sink for water loss from the plant than transpiration, water absorbed by roots in wetter soil horizons is transferred toward, and exuded into dry soil via flow reversals through the roots. Reverse flow is a good marker of HR and can serve as a useful tool to study it over the long-term. Seasonal variation of water uptake of a Quercus suber tree was studied from late winter through autumn 2003 at Rio Frio near Lisbon, Portugal. Sap flow was measured in five small shallow roots (diameter of 3–4 cm), 1 to 2 m from the tree trunk and in four azimuths and at different xylem depths at the trunk base, using the heat field deformation method (HFD). The pattern of sap flow differed among lateral roots as soil dried with constant positive flow in three roots and reverse flow in two other roots during the night when transpiration ceased. Rain modified the pattern of flow in these two roots by eliminating reverse flow and substantially increasing water uptake for transpiration during the day. The increase in water uptake in three other roots following rain was not so substantial. In addition, the flux in individual roots was correlated to different degrees with the flux at different radial depths and azimuthal directions in trunk xylem. The flow in outer trunk xylem seemed to be mostly consistent with water movement from surface soil horizons, whereas deep roots seemed to supply water to the whole cross-section of sapwood. When water flow substantially decreased in shallow lateral roots and the outer stem xylem during drought, water flow in the inner sapwood was maintained, presumably due to its direct connection to deep roots. Results also suggest the importance of the sap flow sensor placement, in relation to sinker roots, as to whether lateral roots might be found to exhibit reverse flow during drought. This study is consistent with the dimorphic rooting habit of Quercus suber trees in which deep roots access groundwater to supply superficial roots and the whole tree, when shallow soil layers were dry

Trophic shift in the diet of the pelagic thresher shark based on stomach contents and stable isotope analyses

The trophic ecology of the pelagic thresher shark (Alopias pelagicus) was analysed using stomach contents and carbon (d13C) and nitrogen (d15N) stable isotope analysis. The sharks were caught in Ecuadorian waters between June and December 2003. Approximately 24 prey species were found in the stomachs of 111 sharks. The most common species were Dosidicus gigas, Benthosema panamense and Sthenoteuthis oualaniensis. Mean (9SD) muscle isotope values of the pelagic thresher shark were 16.090.3 for d13Cand 13.791.1 for d15N; mean (9SD) vertebrae valueswere 16.792.0 for d13Cand 9.491.8 for d15N. There were no significant sex-related differences in muscle d13C and d15N; however, female vertebrae had significantly higher mean (9SD) d13C values ( 16.492.0 ) than males ( 18.890.3 ). Inter-individual variation in d15N values was observed in both tissues and was likely related to foraging in different ecosystems with distinct isotope values. Both stomach content and isotope data suggest that the pelagic thresher shark is a specialist predator in Ecuadorian waters. In general, little is known about the foraging ecology and movement patterns of this globally distributed but elusive top marine predator, and how these fundamental ecological traits vary among populations. By using two independent but complementary approaches, our study addresses these questions for the pelagic thresher sharks in Ecuadorian waters. Such information is critically important for the design of effective management strategies for these highly exploited but poorly understood predators