Abstract. Plant species differ in degrees of silicon (Si) uptake and accumulation, and may differentially influence biogeochemical cycles of Si, possibly in interaction with other environmental factors. Here, we report how patterns of Si cycling by vegetation differ with species composition, elevation and bedrock types for species-rich tropical forests on Mt. Kinabalu, Borneo. We used eight forest monitoring plots established in 1995 at four altitudes (700, 1700, 2700 and 3100 m above sea level) on two geological substrates (Si-rich acidic sedimentary and less Si-rich ultrabasic igneous rock), where tree species composition, abundance, biomass and litterfall had been monitored. For live leaves of 71 dominant tree species (total relative basal area > 60 % in each plot) and leaf litter collected in traps, Si concentration was determined after alkaline extraction. Si availability in the upper 10 cm of mineral soil was determined as Si dissolved to water after shaking overnight. Tree species with high leaf Si concentrations occurred mostly in the lowest elevation plots. The community-mean Si concentration in live leaves, as well as Si concentration in leaf litter, decreased with increasing elevation. The estimated annual flux of leaf litter mass and Si also decreased with increasing elevation. Leaf and litter Si concentrations showed no difference between the two bedrock types without interaction with elevation. Due to large turnovers of species composition with elevation and bedrock types, most species occurred only in one plot. For 11 species that occurred at two or three plots, only one species showed a weakly significant difference in leaf Si concentration between bedrock types. Surface soil Si availability was greater at lower elevation plots and differed with bedrock types only at 1700 m. This pattern was consistent with a hypothesis that Si input via litter in the form of plant opal, rather than bedrock types, should influence the soluble Si in the upper soil horizons. These results suggest that Si cycling by vegetation is more active in lower elevation forests regardless of bedrock types, most likely because Si accumulating species are more abundant in lowland tropical forests.
Shifting tree species composition of upland oak forests alters leaf litter structure, moisture, and flammability
