Kevyn Elizabeth Wightman
Read the published paper (in PDF format):
Wightman, K.E., T. Shear, B. Goldfarb, and J. Haggar. 2001. Nursery and field establishment techniques to improve seedling growth of three Costa Rican hardwoods.New Forests 22(1-2):75-96.
Research conducted at the Organization for Tropical Studies (OTS) has lead to the widespread use of native species in reforestation in the Atlantic Lowland region of Costa Rica. To enhance local reforestation efforts, improved nursery production and site establishment techniques must be developed and practiced. The current nursery practices using plastic bags with clay soils and little organic matter incorporation result in planting stock with poorly developed root systems. Planting stock quality could be improved by using other types of containers and organic substrates. The main site constraint is weed competition. Herbicide and fertilizer may speed tree establishment.
The most common seedling production method in non-industrial forest nurseries in Costa Rica involves filling 500 or 750 cm3 perforated plastic (poly) bags. These smooth plastic bags cause root coiling, the spiral growth of roots at the bottom of the bag. This root deformation leads to toppling or basal sweep several years after planting. Planting stock quality is essential to reforestation success. If reforestation with native species is to succeed, farmers must be given the highest quality seedlings. Attributes of planting stock quality include morphological and physiological traits which enable seedlings to become established rapidly and undergo substantial growth during the first year of planting.
Seedlings of five commercially important and widely planted native hardwood species - Cordia alliodora (R.P.) Cham. (Boraginaceae), Hyeronima alchorneoides Fr. Allemao (Euphorbiaceae), Callophyllum brasiliense Cambess (Clusiaceae), Vochysia guatemalensis Donn. Sm., and Vochysia ferruginea (Vochysiaceae) - were grown under 11 nursery treatments to test the effects of container type and substrate quality. Treatments consisted of three container types: root trainers, paper pots, and plastic bags, and five substrates: soil with and without 10 g of N-P-K fertilizer, two composts, and a 50% mixture of soil and compost.
Height and diameter were measured on all trees. Leaf area, root length, dry weight of leaves, stems, and roots, and nutrient concentrations of plant tissues determined for a subset of seedlings. Seedlings were then planted in a 12-month field trial to determine the effects of initial seedling characteristics on final plant size, growth, and survival. The influences of weeding regimes, manual weeding and herbicide treatment, and a one-time application of fertilizer to half of the seedlings, were also tested.
Cordia alliodora responded to increased nutrient availability; total dry weight, height, and diameter were greatest for the plants grown in plastic bags with fertilizer. Hyeronima alchorneoides also responded to increased nutrient availability, but grew better in compost which had better physical characteristics and higher P availability than in soil amended with fertilizer. For both C. alliodora and H. alchorneoides, plant total dry weight, leaf area, and root length were greater for plants grown in root trainers filled with compost than in bags of unamended soil. For C. brasiliense, there were no significant differences between plants grown with and without fertilizer. Total dry weight of plants grown in compost was less than that of plants grown in soil with and without fertilizer. Seedlings of both V. guatemalensis and V. ferruginea were chlorotic and stunted when grown in compost. Vochysia guatemalensis grew better in soil with fertilizer whereas V. ferruginea grew better in unamended soil. Cordia alliodora and C. brasiliense grown in root trainers had fewer deformations than plants grown in plastic bags.
Planting stock characteristics as well as weeding regimes and fertilization influenced final plant size and plant growth during the 12-month period. Each species exhibited unique responses. For C. alliodora, initial plant size did not correlate with final plant size, however it did affect the relative rate of growth. Small plants with a low ratio of above- to below-ground biomass grew fastest. Weed control using herbicide improved growth, but survival was lower than in manually weeded plots. A significant interaction between weeding and fertilizing demonstrated that there was little benefit to fertilizing trees on weedy plots. Plant size at the time of planting determined final plant size of H. alchorneoides. Large trees retained their size advantage because there were few differences between relative growth rates of the different nursery treatments. Weed control with herbicide and fertilization improved growth, but there was an interaction between nursery treatments and weeding. Small, poorly developed plants grown in paper pots and in bags of soil had a larger positive response to the application of herbicide than did the small plants grown in root trainers or bags with compost. Seedling vigor may be a function of plant size and root morphology. Despite initial differences in planting stock characteristics, these differences did not seem to influence the growth and final plant size of C. brasiliense. Field fertilization also did not affect growth. Callophyllum brasiliense may be be suited to poor sites. Further research may be necessary to identify appropriate substrate and target seedling characteristics for this species. These unique responses may reflect differences among species in root morphology and ecological adaptations. Producing high-quality planting stock is essential for successful reforestation strategies and will help ensure tree plantations as a viable land-use option, particularly for small farmers.