Abstract
Intensive transformation of lowland rainforest into oil palm and rubber monocultures is the most common land-use practice in Sumatra (Indonesia), accompanied by invasion of weeds. In the Jambi province, Centotheca lappacea is one of the most abundant alien grass species in plantations and in jungle rubber (an extensively used agroforest), but largely missing in natural rainforests. Here, we investigated putative genetic differentiation and signatures for adaptation in the introduced area. We studied reproductive mode and ploidy level as putative factors for invasiveness of the species. We sampled 19 populations in oil palm and rubber monocultures and in jungle rubber in two regions (Bukit Duabelas and Harapan). Amplified fragment length polymorphisms (AFLP) revealed a high diversity of individual genotypes and only a weak differentiation among populations (FST = 0.173) and between the two regions (FST = 0.065). There was no significant genetic differentiation between the three land-use systems. The metapopulation of C. lappacea consists of five genetic partitions with high levels of admixture; all partitions appeared in both regions, but with different proportions. Within the Bukit Duabelas region we observed significant isolation-by-distance. Nine AFLP loci (5.3% of all loci) were under natural diversifying selection. All studied populations of C. lappacea were diploid, outcrossing and self-incompatible, without any hints of apomixis. The estimated residence time of c. 100 years coincides with the onset of rubber and oil palm planting in Sumatra. In the colonization process, the species is already in a phase of establishment, which may be enhanced by efficient selection acting on a highly diverse gene pool. In the land-use systems, seed dispersal might be enhanced by adhesive spikelets. At present, the abundance of established populations in intensively managed land-use systems might provide opportunities for rapid dispersal of C. lappacea across rural landscapes in Sumatra, while the invasion potential in rainforest ecosystems appears to be moderate as long as they remain undisturbed.
Highlights
Tropical lowland rainforests shrink worldwide rapidly due to intensive ongoing deforestation [1]
The genetic variation was partitioned to 6.53% between Bukit Duabelas and Harapan and to 16.16% among populations within both regions (AMOVA; Table 2)
Populations within land-use systems were moderately differentiated in both Bukit Duabelas (11.72%) and Harapan (19.70%) (AMOVA; Table 2), but no significant differentiation among land-use systems was found within regions
Summary
Tropical lowland rainforests shrink worldwide rapidly due to intensive ongoing deforestation [1]. In Sumatra (Indonesia), lowland rainforest was cut massively in the 1970s and 1980s and transformed into rubber and oil palm plantations, leaving only few remnants of natural forest, which are predominantly located in national parks [2] (Fig 1a and 1b). Beside monocultures of oil palm (Elaeis guineensis), and rubber (Hevea brasiliensis), an extensive management system of rubber trees planted into rainforest, called “jungle rubber”, was established in the early 20th century [2] (Fig 1c, 1d and 1e). Alien species rapidly establish populations and may influence the native flora (“invasiveness” sensu [4]), and native species can colonize novel anthropogeneous habitats in which they were not present before (“colonizers” sensu [4]). Displacement of native biota, change of ecosystems, environmental disturbance and hybridization with native species are the major threats of invasive plants to the maintenance of tropical ecosystems (e.g., [3])
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