Leaf area, light transmission, roots and leaf damage in nine tropical plant communities

Abstract

The vertical distribution of leaf area by species; transmission of photosynthetically active radiation; root biomass and fine-root surface area; and leaf damage were measured in nine tropical ecosystems: six in Costa Rica and three in Mexico. Ecosystems studied included monocultures of maize (young and mature) and sweet potato; year-old natural succession and vegetation designed to mimic succession; a 2.5-year-old mixture of three arborescent perennials (cacao, plantain, Cordia alliodora); 2.7-year-old plantation of Gmelina arborea; coffee shaded by Erythrina poeppigiana; and an old, diverse wooded garden.

Leaf area index ranged from 1.0 in young maize to 5.1 in natural succession and the gmelina plantation. The vertical distribution of leaves was most uniform in diverse ecosystems, and most clumped in species-poor ecosystems. Light transmission was inversely proportional to leaf area, and two dense-canopied monocultures (sweet potato and gmelina) were nearly as effective at light capture as were some of the more diverse ecosystems. Optical density of the canopy ranged from < 0.5 (35% transmission) in the young maize to > 2.0 (< 1% transmission) in the natural succession.

Large roots (> 5 mm diameter) accounted for most root biomass in the older ecosystems at a soil dept of 5–25 cm, and fine roots (< 5 mm diameter) were most important in the surface 5 cm in all ecosystems. The range of values for root biomass (39 to 422 g m⁻² to a depth of 25 cm) were similar to the range of values for leaf biomass (33 to 345 g m⁻², and, with the exception of two monocultures, ecosystems with high leaf biomass also had high root biomass. The surface area of the fine roots was lower than leaf area, and ranged from 0.5 to > 2.0 m² m⁻² of ground. Total root surface area increased with age and diversity, and the monocultures — even those effective at light capture — had low root surface area.

Herbivore damage on leaves of 35 species ranged from < 2 to > 16% of leaf area. Heavily damaged species contributed less to total ecosystem leaf area than did species damaged less than average. Ecosystem-level damage was not well correlated with age or diversity. Leaf damage in all ecosystems ranged from about 2 to 10% of leaf area, or < 2 to > 25 g m⁻² of ecosystem.

Young monocultures do not necessarily capture less light, provide less soil cover, and experience more herbivory than older, more diverse ecosystems. However, root surface area (and therefore possible nutrient-capture ability) is high only in ecosystems that are diverse or old, and this is an important design consideration for agroecosystems appropriate for the humid tropical lowlands.