BIOMASS ALLOCATION BY BROAD-LEAF MAHOGANY SEEDLINGS, SWIETENIA-MACROPHYLLA (KING), IN ABANDONED PASTURE AND SECONDARY DRY FOREST IN GUANACASTE, COSTA-RICA

Root system development of planted and sown seedlings of Swietenia macrophylla King (broad-leaf mahogany) was studied in abandoned pasture and secondary forest in former tropical dry forest in Guanacaste Conservation Area, Costa Rica. Differences between and within secondary dry forest habitats were assumed to cause differences in biomass allocation. Biomass allocation, root/shoot (R/S) ratio, root surface area/leaf area, rooting depth and the root specialization (tap root/total root system), were measured on 5, 13, 29 and 32 month-old seedlings. The youngest seedling groups harvested had been sown directly in the field, while the two oldest age groups had been grown in a nursery and transplanted to the field at three to four months of age. The field conditions were abandoned (Hyparrhenia rufa Stapf) pasture with mown and unmown grass and semi-evergreen secondary forest with normal and with reduced root competition under both thinned and unthinned forest canopy. Seedling biomass was lower in the secondary forest than in the pasture. Biomass was higher in mown than in unmown pasture. In the secondary forest, light availability rather than reduced root competition promoted biomass investment. R/S ratio did not increase by increasing light levels in the forest. A tendency of an increase in R/S ratio and root specialization was found with increasing seedling age, suggesting the importance of uptake and storage organs.

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