Hydrology and Biogeochemistry of Tropical Montane Cloud Forests: What Do We Really Know?

Abstract

overview: least understood of humid forests as far as water and nutrient dynamics; headwater areas supply water to downstream settlements during rainless periods what is the most imp factor determining stature of forest? hydrological aspects: quantification of cloud is difficult; fog precipitation (HP) is "movement of water onto vegetation surfaces via condensation or by direct contact of cloud droplets"; quantity depends on vegetational factors and climatic factors (see stands review for further details) measurements: fog -- use of fog catchers (fog catchers r sensitive to height of catcher with respect to canopy or ground), comparison of canopy drip measured inside a stand with rainfall measured in the open (this method is better) as long as water intercepted and evaporated from canopy also measured; HP ranges from 70 mm in an elfin forest to 940 in eastern Mexico; without weather stations better to separate daily throughfall totals for fog free and fog bound days; throughfall msmts vary from 70-80% to 130-180%; must also measure stemflow(many stems may enhance nutrient availability) stemflow in LMF is usually low (5-10%) evaporation: expected to decrease with elevation as radiation, temp and VPC decline; average values of Eo at low to intermediate elevations =2.0-3.75 mm per day (lowlands = 5 mm per day) total evapotranspiration: = 1) evaporation of precip intercepted by veg (Ei); 2)transpiration; 3) evaporation from ground layer; latter is insignificant Ei depends on climatic and vegetational factors (duration & intensity of precip, w speed, available E, VPD, ht, roughness, size arrangement, biomass estimates rely on water balance technique to evaluate total evapotranspiration (ET); forests with much HP, ET = 300-400 mm per year, where in LMF they are 1200 mm per year -- estimates must be adjusted for HP (added rainfall, higher ET rates) leaving 570-695 mm per year streamflow and rainfall records could help with estimates evaporation of intercepted precipitation- INCIDENT RAINFALL - THROUGHFALL - STEMFLOW; this method cannot be used in cloud forest experimental design -- temporal and spatial variations in throughfall and stemflow, observations on occurrence and intensity of fog or rainfall, duration of canopy wetness ecophysiological research- importance of moisture for bryophitic growth (see Hosokawa, Odani, and Tagawa 1964) ** found a positive relationship between photosynthetic rate and relative humidity under conditions of equal light transpiration-reliable estimates scarce; ET - Ei gives Et. Bruijnzeel (1990) found values of 510-830 mm per year; values for MCF are 300 mm per year; low rates of water uptake ?? hypotheses range from soil and nutrient deficiency, leaching, reduced radiation, and high atmospheric humidity, or halting under conditions of a fully wetted canopy, BUT evidence that water uptake is slow in full sunshine -reduction in Et/Eo is not caused by difference in climatic conditions?? in indonesia since climate integrated into Eo msmts. -enhanced stomatal resistance in response to increase in atmospheric VPD for many MCFS -BUT when stomatal resistance was measured in conjunction with leaf water potential or transpiration, resistance was not accompanied by decrease in leaf water potential & increase in resistance did not prevent increase in transpiration under increased evaporative demand -kapos and tanner (85) stomatal response to changing atmospheric conditions not pronounced -- stomatal control not aimed at preventing water stress?? -EVIDENCE THAT TRANSPIRATION I SUPPRESSED - xeromorphic leaves transpiration- heat pulse velocity method, estimates sensitive to size of wounds by probes -- half hourly observations of saplow rates over a 3 week period with observation of soil water tension and climate above canopy soil water regime-- observations of soil water and leaf water potentials suggest trees do not experience severe soil water deficits (see pg 52 for references) when annual streamflow total is expressed as a ratio of incident rainfall, highest values reported are for TMCF (due to extra moisture and low ET); unsure whether total water yield will go up or down after clearing; hydrological consequences of clearing depend on amt of HP and ET associated with new vegetation cover, and infiltration rates; possible that water yield changes will be small. total water use of crops or grass (ET) in montane tropics almost equals water transpired by forest total (Et)