Field performance of Pinus caribaea var. Hondurensis seedlings with specific ectomycorrhizae and fertilizer after three years on a savanna site in Liberia

Abstract

Seedlings of Pinus caribaea var. hondurensis were grown in containers for 4 months in a bush nursery in Gbarnga, Liberia. Vegetative inocula of Pisolithus tinctorius (Pt), Thelephora terrestris (Tt), and soil inoculum, called Monrovia mixture (MM), collected from under 16-year-old trees of the same species, were used to form specific ectomycorrhizae on the seedlings. Seedlings in these three treatments received the same fertility in the nursery as seedlings in a noninoculated control treatment; in a fifth nursery treatment noninoculated control seedlings received 25% more fertilizer. Seedlings inoculated with Pt, Tt, and MM had ectomycorrhizae on 61, 18, and 6% of their short roots, respectively. These inoculated seedlings were significantly smaller, with lower top/root ratios, and contained less foliar N than seedlings in either control treatment. A trace of ectomycorrhizae was detected microscopically on seedlings in both control treatments. It was concluded that this contamination came from the introduced Tt which formed abundant fruit bodies in the nursery. Seedlings were outplanted on a prepared savanna site near Gbarnga; half of the seedlings in each treatment were fertilized. Seedlings were measured at the end of each 7.5-month rainy season and each 4.5-month dry season for 3 years. At the end of the first rainy season, seedlings inoculated with Tt in the nursery and the two controls had an average of 49% Tt ectomycorrhizae, regardless of field fertilization. Over 2000 Tt fruit bodies were found during each rainy season in each of these plots. Seedlings in the Pt and MM treatments had 82 and 86% ectomycorrhizae, of which only 2–11% were formed by Tt. The fungi were reisolated from their respective ectomycorrhizae. Rhizosphere soil from seedlings with Pt ectomycorrhizae contained more total N than soil from seedlings in other treatments. These samples, however, did not differ in N 2-fixation rate (acetylene reduction technique) from other soil samples containing less total N collected at the beginning of the third dry season. After the first rainy season, seedlings with Pt ectomycorrhizae with and without field fertilization had significantly more N and other elements in needles than most other treatments. These differences were much less after the third rainy season. After 3 years, seedling survival (96.0-83.6%) was not seriously affected by treatment. Seedlings with Pt ectomycorrhizae in fertilized plots had significantly greater seedling and plot volumes than those in other treatments. Only seedlings with Pt ectomycorrhizae responded to field fertilization. Seedlings with MM ectomycorrhizae had greater volumes than seedlings with Tt ectomycorrhizae or seedlings in either control treatment. Seedlings intentionally inoculated with Tt were not significantly different in any field measurement from those in the two control treatments contaminated with Tt. Seedlings with Pt and MM ectomycorrhizae grew as much or more during the short dry season than seedlings of other treatments grew during the longer rainy season. Physical properties of stem tissue sampled at the end of the third rainy season did not vary by treatment. Bark averaged 35.5% of stem tissue. Moisture content of wood was 192.5% and specific gravity of wood and bark was 0.293. Fruit bodies of Alpova pachyphloeus were found in 4- and 5-year old pine plantations and those of Thelephora sp. were found on pine seedlings left in the nursery prior to fungal introductions. Since all of these seedlings had been inoculated with MM in the Gbarnga nursery, these fungi coild be components of the MM inoculum or they could be indigenous on native trees in the Gbarnga area. Pt fruit bodies were observed 2 km downwind from the study site in young plantations of Eucalyptus grandis and E. camaldulensis. Apparently basidiospores of Pt were windblown from the study site to these plantations. It was concluded that differences in seedling growth caused by specific ectomycorrhizal and field fertilization treatments were attributed to differences in amounts of ectomycorrhizae formed during the growing season and in efficacy of specific ectomycorrhizae in nutrient and water absorption.