NUTRIENT AND MYCORRHIZAL EFFECTS ON THE ROOT-SHOOT RATIO OF CONTAINERIZED PONDEROSA PINE SEEDLINGS.

Abstract

Attempts at reforestation of many sites in the southwestern United States have repeatedly failed. Experience and research show that moisture stress is the primary cause of seedling mortality. Therefore, it is of utmost importance to produce planting stock for these areas that are as drought tolerant as possible. Seedlings with high ratios of root mass to shoot mass and adequate mycorrhizal infections should be more resistant to harsh conditions than most seedlings currently produced in nurseries and greenhouses. Inferences from other research indicated that the root-shoot ratios of seedlings could be increased solely by decreasing the amount of nitrogen supplied to them. Mycorrhizal formation would also be enhanced by minimal nitrogen levels. Containerized ponderosa pine seedlings (Pinus ponderosa Lawson var. scopulorum Engelm.) were grown in commercial greenhouses at various levels of nitrogen fertilization and treated with several mycorrhizal inoculums. Mycorrhizal treatments significantly increased shoot height and diameter, but no other seedling parameters. The inoculums did not affect the root-shoot ratio or the percent of short roots that became infected. Nitrogen levels significantly affected all parameters measured. As nitrogen concentration increased, shoot height, diameter, and weight increased, while root weight, root-shoot ratios, and mycorrhizal infections decreased. The inverse relationship between fertilizer nitrogen concentration and the resulting root-shoot ratios of the seedlings was linear and highly correlated. Nitrogen and percent mycorrhizal infection was nearly linear and also inversely correlated. The results of this research are immediately applicable to current greenhouse and nursery operations. When stock is to be planted on sites where seedling survival may be compromised by harsh environmental conditions, production methods should be modified to yield seedlings with maximum root-shoot ratios and heavily infected with mycorrhizae.