Abstract
Conversion of tropical forests into intensely managed plantations is a threat to ecosystem functions. On Sumatra, Indonesia, oil palm (Elaeis guineensis) plantations are rapidly expanding, displacing rain forests and extensively used rubber (Hevea brasiliensis) agro-forests. Here, we tested the influence of land use systems on root traits including chemical traits (carbon, nitrogen, mineral nutrients, potentially toxic elements [aluminium, iron] and performance traits (root mass, vitality, mycorrhizal colonization). Traits were measured as root community-weighed traits (RCWTs) in lowland rain forests, in rubber agro-forests mixed with rain forest trees, in rubber and oil palm plantations in two landscapes (Bukit Duabelas and Harapan, Sumatra). We hypothesized that RCWTs vary with land use system indicating increasing transformation intensity and loss of ecosystem functions. The main factors found to be related to increasing transformation intensity were declining root vitality and root sulfur, nitrogen, carbon, manganese concentrations and increasing root aluminium and iron concentrations as well as increasing spore densities of arbuscular mycorrhizas. Mycorrhizal abundance was high for arbuscular and low for ectomycorrhizas and unrelated to changes in RCWTs. The decline in RCWTs showed significant correlations with soil nitrogen, soil pH and litter carbon. Thus, our study uncovered a relationship between deteriorating root community traits and loss of ecosystem functionality and showed that increasing transformation intensity resulted in decreasing root nutrition and health. Based on these results we suggest that land management that improves root vitality may enhance the ecological functions of intense tropical production systems.
Highlights
Tropical rain forests are rapidly converted to plantation agriculture [1]
Root community-weighed traits are massively affected by the land use system
We further determined root community-weighed traits” (RCWTs) that are related to root vitality and mycorrhizal association (Fig 3)
Summary
Tropical rain forests are rapidly converted to plantation agriculture [1]. In Indonesia, which is together with Malaysia the worlds largest producer of palm oil [2], 40% of the forest (64 Mio ha) was lost since the countriesindependence in 1945 [3]. In the 1950s rubber (Hevea brasiliensis) was introduced as a crop tree and is currently cultivated in two systems, in intense monocultures often with high yielding clones (rubber plantation) or as jungle rubber. Tree species richness is lower but the forest structure of jungle rubber is similar to that of unmanaged lowland rain forests [4,5,6]. Pristine lowland rain forests exist only in fragments and most unmanaged forests, even in protected areas, are secondary forests. Since the 1990s with the introduction of oil palms (Elaeis guineensis), expansion of plantation area at the expense of primary and secondary forests has drastically increased [7], with high rates (> 2% per year) on Sumatra [8]. While the alterations of above-ground ecosystem properties and processes have been intensively studied, much less is known about the below-ground plant responses to these massive changes
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