Sap Flow Piate and Sap Nutient Content of a Tropical Rain Forest Canopy Species, Dryobalanops Aromatica, in Brunei

We describe a study involving simultaneous measurements of sap flow rate and sap nutrient content in three canopy and four sub-canopy trees of Dryobalanops aromatica Gaertn. f. (Dipterocarpaceae). Sap flow rate was measured using a heat pulse technique. Sap nutrient content was determined for potassium, calcium, total Kjeldahl nitrogen and phosphorus from samples collected diurnally in the tree crowns Sap concentration for all nutrients at each 2-h sampling period was unrelated to tree size, except for K during the 1600-1800 h period. Relative concentrations of the four nutrients followed the general patern of N > K > Ca > P; greatest rates of delivery (mmol h1) to tree crowns tended to occur at mid-day for each of the four nutrients. Distinct diurnal patterns of sap flow and nutrient content were apparent in most trees. Sap nutrient concentration was often positively correlated with the rate of sap flow. We discuss possible explanations for this unexpected correlation in terms of within-tree nutrient cycling, and also the postulated osmotic role of xylem sap solutes. We conclude that combining measurements of sap flow and sap nutrient content, a technique apparently not previously used in a tropical rain forest study, has considerable potential in ecophysiology.