A persistent cloud field provides humidity via fog interception to a chain of diversely vegetated patches occurring on mountaintops along the arid coastal margin of western South America. The fog-inundated relict forest of Fray Jorge National Park (30 S) is the southernmost member of this chain. The relict forest is composed of hundreds of patches receiving 120 mm annual rainfall on average. Because fog water influx is critical for the persistence of forest patches, we monitored for 12 years trends in fog and rainwater inputs. We investigated the constancy of fog inputs and the relationship of fog influx with current declining rainfall trends in the area. We measured water inputs derived from passive cloud interception, stem-flow, throughfall, and direct precipitation in six patches varying in size and location.
Results/Conclusions
Extreme fluctuations in rainfall were closely associated with negative or positive phases of El Niño Southern Oscillation (ENSO), with dry extremes of <10 mm per year, and wet extremes with 200-300 mm per year. Overall, rainfall declined by 12% in the decade of study. Peak rainfall in the austral winter months (June-August) was followed by 8-10 rainless months. Fog remained the only source of moisture to forest patches during the rainless period. We estimate that both fog interception and stemflow contributed together an additional ~300 mm of water. Persistent clouds and lower temperatures on hilltops reduce water losses thru evaporation. Only small differences in fog interception were recorded between small (<1 ha) and large (20-30 ha) patches, suggesting that all fragments have the same potential for water capture. Smaller patches are drier because of high evaporation caused by pronounced edge effects. Although cloud interception showed no significant trends for the decadal period, smaller patches are increasingly susceptible to declines because of larger edge effects. This is critical, because small patches outnumber large patches.