Oil Palm and Rubber Tree Transpiration: Topography, Flooding and Tree admixture in Jungle Rubber Stands
von Afik Hardanto
Datum der mündl. Prüfung:2017-03-16
Betreuer:Prof. Dr. Dirk Hölscher
Gutachter:Prof. Dr. Alexander Knohl
Gutachter:Prof. Dr. Hermann Behling
EnglischIn post-forest plantation landscapes of lowland Sumatra, I studied (1) effects of topography and flooding on transpiration by mono-cultural oil palm and rubber tree plantations, and (2) tree water use rates and tree soil water uptake depths in mono-cultural and mixed ‘jungle’ rubber stands. Oil palm and rubber plantations extend over large areas and encompass heterogeneous site conditions. In periods of high rainfall, plants in valleys and at riparian sites are more prone to flooding than plants at elevated topographic positions. I asked to what extent topographic position and flooding affect oil palm and rubber tree water use patterns and thereby influence spatial and temporal heterogeneity of transpiration. In an undulating terrain in the lowlands of Jambi, Indonesia, plantations of the two species were studied in plot pairs consisting of upland and adjacent valley plots. All upland plots were non-flooded, whereas the corresponding valley plots included non-, long-term and short-term flooded conditions. Within each plot pair, sap flux densities in palms or trees were monitored simultaneously with thermal dissipation probes. In plot pairs with non-flooded valleys, sap flux densities of oil palms were only slightly different between the topographic positions, whereas sap flux densities of rubber trees were higher in the valley than at the according upland site. In pairs with long-term flooded valleys, sap flux densities in valleys were lower than at upland plots for both species, but the reduction was far less pronounced in oil palms than in rubber trees (-22% and -45% in maximum sap flux density, respectively). At these long-term flooded valley plots palm and tree water use also responded less sensitively to fluctuations in micrometeorological variables than at upland plots. In short-term flooded valley plots, sap flux densities of oil palm were hardly affected by flooding, but sap flux densities of rubber trees were reduced considerably. Topographic position and flooding thus affected water use patterns in both oil palms and rubber trees, but the changes in rubber trees were much more pronounced: compared to non-flooded upland sites, the different flooding conditions at valley sites amplified the observed heterogeneity of plot mean water use by a factor of 2.4 in oil palm and by a factor of 4.2 in rubber plantations. Rubber tree mono-cultural plantations are expanding and there is also a search for ‘green’ rubber production. Rubber tree cultivation in stands with admixed spontaneously growing native trees, called ‘jungle rubber’, has a long tradition in Sumatra. The objectives of my study were to analyze tree water use rates and tree soil water uptake depths in mono-cultural and mixed ‘jungle’ rubber stands with a focus on the role of tree diameter. Sap flux measurements suggest similar water use rates for rubber trees in the two cultivation systems. The increase in tree water use with tree diameter was however much stronger for rubber trees than for the admixed native tree species. At the stand-level, transpiration of jungle rubber was by 27% higher than of rubber monocultures, which was mainly due to the higher stem density in the jungle rubber stands. In these stands, the share of rubber trees of the total transpiration corresponded with its’ share to total tree basal area. A water stable isotope (δ18O and δD) approach suggests different soil water uptake depths for the rubber trees in the two cultivation systems. In a dry period, the main tree water uptake in the monoculture was relatively close to the soil surface, whereas rubber trees in the jungle rubber mainly took up water from deeper in the soil profile. The admixed native trees in the jungle rubber had their main uptake depth relatively close to the soil surface. This pattern indicates competitive displacement of the rubber tree water uptake. Across rubber trees in both cultivation systems and also among the admixed native trees, there was a clear relationship between tree diameter and soil water uptake depth: bigger trees tended to take up soil water closer to the soil surface. Overall, my study revealed strong differences between oil palms and rubber tree transpiration in response to topography and flooding. Such a pronounced heterogeneity of water use across space and time may be of relevance for eco-hydrological assessments of tropical plantation landscapes. The comparison mono-cultural and mixed ‘jungle’ rubber stands suggests, that rubber trees are relatively weak competitors for water resource uptake, and tree diameter modifies both tree water use rates and tree water uptake depths. The diameter relationship offers opportunities for management interventions in favor of the rubber trees. Mixed species cultivation systems in oil palm with native tree species have only recently been established and are awaiting their evaluation.
Keywords: heterogeneity; Indonesia; sap flux; Sumatra; transpiration; variability; competition; ecohydrology; Hevea brasiliensis; poly-cultures; resource partitioning; stable isotopes