The role of native species plantations in recovery of understory woody diversity in degraded pasturelands of Costa Rica

Abstract

Tropical timber plantations provide a variety of environmental services, including recovery of biodiversity on degraded lands. For example, plantations can speed forest successional processes by improving microsite conditions and attracting seed dispersers, thus promoting woody regeneration. Timber species have been hypothesized to differ in understory recruitment success. In the present research, understory regeneration of woody plants was compared for six native timber species on tropical plantations in the Atlantic humid lowlands of Costa Rica. Timber species compared were: Calophyllum brasiliense Cambess, Hieronyma alchorneoides Allemao, Terminalia amazonia (J.F. Gmel.) Exell, Virola koschnyi Warb., Vochysia ferruginea Mart., and Vochysia guatemalensis Sprague. The six species were present at each of the three sites: one experimental plantation and two small-scale plantations belonging to farmers of the region. All plantations were 9–10 years. The experimental plantation was 100 m from continuous forest (i.e., seed source), and the farmers’ plantations were 1.3 and 2.5 km from continuous forest. Four plots were sampled for each timber species at each site using a randomized block design. All understory woody species were counted, identified, and separated by height class. Canopy openness and leaf litter biomass on the plantation floor were also evaluated. All of the plantations studied showed significantly higher levels of understory regeneration than control plots on abandoned pastures ( P<0.05). In this study, plantation site was the most significant factor affecting understory woody species diversity ( P<0.0001). Different timber species were most successful at recruiting understory regeneration in each of the three sites. On the experimental plantations at site 1, V. guatemalensis and C. brasiliense had the greatest recruitment success, with 75,581 and 69,219 regenerating individuals/ha, respectively. In the commercial plantations, T. amazonia (16,250 regenerating individuals/ha) had the greatest recruitment success at site 2, and V. ferruginea (29,219 regenerating individuals/ha) had the greatest recruitment success at site 3. Across sites, plots with intermediate canopy openness had greater abundance of understory regeneration than plots with low or high percentages of canopy openness ( P=0.02). There was no relationship between understory regeneration and leaf litter biomass. Of the planted species most successful at restoring understory diversity, V. guatemalensis, T. amazonia, and V. ferruginea have demonstrated good form and growth for timber, making them important species for reforestation in the region.