The influence of seasonality and species effects on surface fine roots and nodulation in tropical legume tree plantations

Background: Fine roots comprise a dynamic carbon pool in forests. Legumes, widespread in the tropics, have a specialized strategy of nitrogen acquisition. However, the belowground dynamics of this group are poorly understood. Scope: We studied the seasonal and spatial variation in surface fine root mass (FRM) and nodulation over 2 years in plantations of four legume species (Dalbergia retusa, Enterolobium cyclocarpum, Gliricidia sepium, Leptolobium panamense) in a dry forest in Costa Rica. We measured soil moisture, FRM, and nodule mass at 2 soil depths (0–15 and 15–30 cm) and at 2 distances from the tree bole (1 and 2 m). Mean FRM per species ranged from 10 to 17 g m⁻² during the dry season to 86–116 g m⁻² the following wet season. Species differed in belowground foraging strategies: G. sepium and L. panamense had ~41 % more roots in the surface layer, but in D. retusa plantations, 44.3 % more roots were in the deeper layer. In G. sepium and L. panamense, nodulation fluctuated seasonally, while the other species did not nodulate. Conclusions: FRM varied in synchrony with rainfall and responded to interannual precipitation anomalies. Thus, FRM is a sensitive component of the forest carbon pool, vulnerable to shifts in species composition and climate regimes.