Diurnal variations in the thickness of the inner bark of tree trunk in relation to xylem water potential and phloem turgor

The inner bark plays important roles in tree stems, including radial exchange of water with the xylem and translocation of carbohydrates. Both processes affect the water content and the thickness of the inner bark on a diurnal basis. For the first time, we simultaneously measured the diurnal variations in the inner bark thickness of hinoki cypress (Chamaecyparis obtusa) by using point dendrometers and those of local xylem potential by using stem psychrometers located next to the dendrometers to determine how these variations were related to each other, to phloem turgor and carbohydrate transport. We also estimated the axial hydrostatic pressure gradient by measuring the osmolality of the sap extracted from the inner bark. The inner bark shrunk during the day and swelled during the night with an amplitude related to day-to-day and seasonal variations in climate. The relationship between changes in xylem water potential and inner bark thickness exhibited a hysteresis loop during the day with a median lag of 2 h. A phloem turgor-related signal can be retrieved from the diurnal variations in the inner bark thickness, which was higher at the upper than at the lower position along the trunk. However, a downward hydrostatic pressure gradient was only observed at dawn, suggesting diurnal variations in the phloem sap flow velocity

Methane flux of leaves in a tropical rainforest and a temperate conifer forest

We measured methane (CH[4]) fluxes of leaves growing in a tropical rainforest and a temperate conifer forest using the closed static chamber method. Leaf samples were chosen from the canopy and forest floor of a tropical rainforest at Pasoh Forest Reserve, Peninsular Malaysia and a temperate forest at Kiryu Experimental Watershed (KEW), central Japan. At Pasoh, we sampled four species from the canopy (Xanthophyllum stipitatum, Dipterocarpus sublamellatus, Ptychopyxis caput-medusae, Neobalanocarpus heimii) and four species from the forest floor (Alphonsea maingayi, Rinorea anguifera, Macaranga lowii, Neobalanocarpus heimii). The leaves of Elaeis guineensis were also sampled at an oil palm plantation near the forest reserve area. At KEW, we sampled the leaves of Chamaecyparis obtusa, from both the top (18 m) and middle (16 m) of the canopy as well as leaves of Eurya japonica at the forest floor. Incubation experiments revealed that CH[4] was emitted from leaves of the tested species sampled at two sites, although CH[4] fluxes showed variations on a daily basis. These results suggest that tree leaves both at Pasoh and KEW might function as a small CH[4]source, but their contribution to the CH[4] exchange of the total forest ecosystem is very small compared to the soil CH[4] flu