Root Architecture of Shrubs and Saplings in the Understory of a Tropical Moist Forest in Lowland Panama

Abstract

Entire root systems of 2-3 m tall plants in tropical moist forest near Barro Colorado Island, Panama were excavated and measured. We studied 3-4 plants, each, of the canopy and subcanopy species Trichilia tuberculata (Meliaceae), Prioria copaifera (Caesalpinoideae), Hirtella triandra (Chrysobalanaceae), and Tetragastris panamensis (Burseraceae); the immature treelet Faramea occidentalis (Rubiaceae); and the mature shrubs Piper cordulatum (Piperaceae), Psychotria horizontalis (Rubiaceae), and Hybanthusprunifolius (Violaceae). Root and crown architecture of intermediate specimens was illustrated. Shrubs had shallower root systems with a higher proportion of total surface area at 0-20 cm depth than tree saplings. Shrubs also had greater root surface area, both absolutely and relative to leaf area. An analysis of Coster's data for 6-month old plants of 44 species grown in a Javanese garden confirmed that shrubs tend to have relatively shallower root systems than tree species. We examine several explanations of differing root architecture for these life forms based on adult requirements for water, support, and nutrients. We speculate that reproduction creates greater nutrient demands in shrubs than in saplings. By analogy with community patterns on very poor soils, a well developed, superficial root system increases nutrient absorption from the relatively richer upper stratum. The tradeoff appears to be reduced resistance to water stress in shrubs compared with tree species.