WATER-BALANCE AND NUTRIENT INPUTS IN BULK PRECIPITATION IN TROPICAL MONTANE CLOUD FOREST IN PANAMA

The hydrological balance and nutrient inputs in water were measured in a lower montane rain forest (1200 m) in Panama. Rainfall and throughf all were measured daily between July 1988 and July 1989 using a standa rd rain gauge and 50 longitudinal PVC collectors, respectively. Stemfl ow was measured daily between November 1990 and December 1991 in nine trees varying from 16 to 54 cm diameter at breast height (dbh), using tygon collars. Rainfall and throughfall water samples were collected e very 15 days between January and October 1989 for chemical analyses (p H, conductivity, NO3-, NO2-, NH4+, PO43-, SO42-, Ca2+, Cl-, Mg2+, K+, Mn, Fe and Si). Bulk precipitation was 3510 mm. Interception was 37.2% , net precipitation was 62.8%, of which, throughfall was 62.4% and ste mflow was 0.4%. The probability distribution of throughfall as a propo rtion of bulk precipitation, showed that 82% of the total variability is lower than a bulk precipitation value of 1.0 and that there are hig her values in this mountain forest than in a lowland rain forest in Br azil were this value was also calculated, suggesting that the canopy o f this montane forest is more heterogeneous, In only 6 of 140 of the r ain events, was net precipitation higher than bulk precipitation, sugg esting that direct inputs of cloud water by fog interception, is not a n important water source for this forest. Interception at Fortuna is t he highest value yet reported for a tropical montane rain forest; it m aybe attributed to a very tall canopy (20-30 m) and abundant epiphyte load. Nutrient inputs in water varied from 0.70 kg ha(-1) per year of PO4-P to 34.5 kg ha(-1) per year of Cl. Leaching varied from 1.45 kg h a(-1) per year for PO4-P to 49.71 kg ha(-1) per year of K. Net absorpt ion by the canopy was measured for S (-7.0 kg ha(-1) per year SO4-S) w hile for N the total annual input was about the same as the annual thr oughfall. Annual fluxes of calcium, sodium and potassium in bulk preci pitation in the Fortuna forest were in the upper range of values repor ted for other tropical montane sites, perhaps owing to the proximity t o the Caribbean Sea, Nutrient inputs of N, P and K in throughfall are consistent with inputs of throughfall relative to other nutrient pathw ays in tropical and temperate forests. (C) 1997 Elsevier Science B.V.