Decreased rainfall interception balances increased transpiration in exotic ponderosa pine plantations compared with native cypress stands in Patagonia, Argentina

Exotic ponderosa pine (Pinus ponderosa Doug. ex. Laws) is being planted for timber production in the natural distribution area of cordilleran cypress [Austrocedrus chilensis (D. Don) Pic. Serm. et Bizzarri] in the forest-steppe ecotone at the base of the Andes Mountains in northwest Patagonia. Our previous studies showed that transpiration of ponderosa pine plantations was between 33% and 64% greater than that of cordilleran cypress under similar growing conditions. Although transpiration is typically the largest term in evapotranspiration (ET) in closed-canopy forests, evaporative losses of rainfall intercepted by the canopy (IL) can also account for a large proportion of ET in forest ecosystems. The objective of this case study was to assess differences in IL between exotic ponderosa pine plantations and cordilleran cypress stands, as well as the combined effect of IL and transpiration. Net and gross precipitation were measured using arrays of manual rain gauges and tipping buckets with troughs in four forest stands of ponderosa pine and cordilleran cypress at two densities each. Additionally, water storage capacity of cut branches was measured in the laboratory. Annual estimates of IL were calculated using the Gash model (Gash, 1979) for two consecutive years with markedly different spring and summer precipitation. In cordilleran cypress stands, IL ranged from 35% to 44% of gross precipitation, whereas IL ranged from 17% to 22% of gross precipitation in pine plantations. Total annual IL fully offset the differences of spring and summer transpiration between ponderosa pine plantations and cordilleran cypress stands of similar densities