Abstract
Insect herbivory constitutes an important constraint in the viability and management of targeted reforestation sites. Focusing on young experimental stands at about 2000 m elevation in southern Ecuador, we examined foliar damage over one season as a function of tree species and habitat. Native tree species (Successional hardwood: Cedrela montana and Tabebuia chrysantha; fast-growing pioneer: Heliocarpus americanus) have been planted among prevailing local landcover types (abandoned pasture, secondary shrub vegetation, and a Pinus patula plantation) in 2003/4. Plantation trees were compared to conspecifics in the spontaneous undergrowth of adjacent undisturbed rainforest matched for height and foliar volume. Specifically, we tested the hypotheses that H. americanus as a pioneer species suffers more herbivory compared to the two successional tree species, and that damage is inversely related to habitat complexity. Overall leaf damage caused by folivorous insects (excluding leafcutter ants) was low. Average leaf loss was highest among T. chrysantha (7.50% ± 0.19 SE of leaf area), followed by H. americanus (4.67% ± 0.18 SE) and C. montana (3.18% ± 0.15 SE). Contrary to expectations, leaf area loss was highest among trees in closed-canopy natural rainforest, followed by pine plantation, pasture, and secondary shrub sites. Harvesting activity of leafcutter ants (Acromyrmex sp.) was strongly biased towards T. chrysantha growing in open habitat (mean pasture: 2.5%; shrub: 10.5%) where it could result in considerable damage (> 90.0%). Insect folivory is unlikely to pose a barrier for reforestation in the tropical Andean mountain forest zone at present, but leafcutter ants may become problematic if local temperatures increase in the wake of global warming.
Highlights
The global rate of deforestation appears to be slowing down, increases in the Earth's forest cover are mostly restricted to higher latitudes and offset by net losses still taking place in the tropics [1, 2]
Leaf area loss was significantly higher in T. chrysantha (7.7% ± 0.4 standard error), compared to H. americanus (4.8% ± 0.3 SE), which in turn suffered more damage than C. montana (3.4% ± 0.2 SE; Fig 2A)
Relative to the ‘Forest’ and ‘Pasture’ sites, trees planted beneath P. patula were characterized by higher proportional leaf damage in T. chrysantha, but lower herbivory in both C. montana and H. americanus. (Fig 2D)
Summary
The global rate of deforestation appears to be slowing down, increases in the Earth's forest cover are mostly restricted to higher latitudes and offset by net losses still taking place in the tropics [1, 2]. Many governments are in the process of abandoning or revising policies that actively promote the conversion of forest to agricultural land (Cost Rica: [3], Brazil: [4]), but market forces, population growth, and unsustainable management practices continue to foster. In the equatorial Andean highlands, comparatively poor soil conditions and the intrusion of aggressive weeds in the wake of slash-and-burn management (e.g. Pteridium arachnoideum; [7]) often render the sustained use of pastures uneconomic, forcing the tenants to periodically clear new land for grazing. Competition by the robust successional shrub/fern community [8] and reduced seed deposition [9] severely impede the course of natural forest regeneration on former pasture land
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