Abstract
Clove oil extracted from the leaves of clove trees (Syzygium aromaticum) is an important cash crop in East Java, Indonesia. The standard monoculture that has been recently adopted for clove oil production can reduce biodiversity. We investigated the plant species occurrence in a typical mixed-culture (MIX) stand growing clove trees and other crops, and compared it with that in a clove monoculture (MON) stand in East Java, Indonesia, in order to examine the advantage of MIX in conserving plant species diversity. We recorded the occurrence of vascular plants in the understory at 20 quadrats (1 m × 1 m) each systematically placed in a 20 m × 100 m plots established for both MIX and MON. Altogether, 46 plant species were found in the two plots. MIX and MON included 40 and 17 species, respectively, indicating far greater species richness in MIX with a diverse life form composition and large numbers of woodland and/or native species. The α- and the β-diversities were higher in MIX than MON, which is associated with a high dissimilarity of plant occurrence among the quadrats within the plot. These results suggest the superiority of MIX for conserving plant species diversity compared to MON. Among the microsite conditions examined, no significant differences between MIX and MON were observed for soil water content (SWC) and sky factor (SF). The species number per quadrat was positively correlated with SF in both MIX and MON, but the correlation was weaker in MON than in MIX. The understory vegetation cover (UVC) and the litter cover (LC) showed wider ranges in MIX compared to those in MON. UVC was correlated positively with the species number in both stands. LC showed negative and positive correlations to species number in MIX and MON, respectively. From these results, we concluded that in MON, the human disturbances to the soil surface associated with frequent clove litter collection from the whole stand reduced the plant species richness by inhibiting plant establishment and cancelling the positive effects of the variability in physical environment, and that the higher species richness of MIX was due to alleviating the effects of litter-collection disturbances, which facilitated the effects of the heterogeneous physical environment within the stand.
