Transpiration in an oil palm landscape: effects of palm age

[摘要] Oil palm (Elaeis guineensis Jacq.) plantations cover large and continuously increasing areasof humid tropical lowlands. Landscapes dominated by oil palms usuallyconsist of a mosaic of mono-cultural, homogeneous stands of varying age,which may be heterogeneous in their water use characteristics. However,studies on the water use characteristics of oil palms are still at an earlystage and there is a lack of knowledge on how oil palm expansion will affectthe major components of the hydrological cycle. To provide first insightsinto hydrological landscape-level consequences of oil palm cultivation, wederived transpiration rates of oil palms in stands of varying age, estimatedthe contribution of palm transpiration to evapotranspiration, and analyzedthe influence of fluctuations in environmental variables on oil palm wateruse. We studied 15 two- to 25-year old stands in the lowlands of Jambi,Indonesia. A sap flux technique with an oil palm specific calibration andsampling scheme was used to derive leaf-, palm- and stand-level water userates in all stands under comparable environmental conditions. Additionally,in a two- and a 12-year old stand, eddy covariance measurements wereconducted to derive evapotranspiration rates. Water use rates per leaf andpalm increased 5-fold from an age of 2 years to a stand age of approx. 10 years and then remained relatively constant. A similar trend was visible,but less pronounced, for estimated stand transpiration rates of oil palms;they varied 12-fold, from 0.2 mm day⁻¹ in a 2-year old to2.5 mm day⁻¹ in a 12-year old stand, showing particularly high variability intranspiration rates among medium-aged stands. Comparing sap flux andeddy-covariance derived water fluxes suggests that transpiration contributed8 % to evapotranspiration in the 2-year old stand and 53 % in the12-year old stand, indicating variable and substantial additional sources ofevaporation, e.g., from the soil, the ground vegetation and from trunkepiphytes. Diurnally, oil palm transpiration rates were characterized by anearly peak between 10 and 11 a.m.; there was a pronounced hysteresis in theleaf water use response to changes in vapor pressure deficit for all palmsof advanced age. On the day-to-day basis this resulted in a relatively lowvariability of oil palm water use regardless of fluctuations in vaporpressure deficit and radiation. We conclude that oil palm dominatedlandscapes show some spatial variations in (evapo)transpiration rates, e.g.,due to varying age-structures, but that the temporal variability of oil palmtranspiration is rather low. The stand transpiration of some of the studiedoil palm stands was as high or even higher than values reported fordifferent tropical forests, indicating a high water use of oil palms underyet to be explained site or management conditions. Our study provides firstinsights into the eco-hydrological characteristics of oil palms as well as afirst estimate of oil palm water use across a gradient of plantation age. Itsheds first light on some of the hydrological consequences of the continuingexpansion of oil palm plantations.

[学科分类] 地球化学与岩石