Optimizing fertilization and root symbioses to improve seedling performance in abandoned pastures of Hawaiʻi

Abstract

The starting of cattle ranching in the middle of the 19th century has led to the conversion of most of Hawaiian mesic and even drier montane forest into pasture. The decline of that industry has created opportunities to restore these sites with native species. However, outplanted seedlings of such species as māmane (Sophora chrysophyla) and koa (Acacia koa) suffer low survival due to competition with pasture grasses, periodic drought, and cold night-time temperatures. Greenhouse management techniques such as exponential fertilization and mycorrhizal inoculation have separately been promoted to improve seedling vigor and establishment after outplanting. Therefore, we investigated the compatibility and benefits of combining these two treatments for koa and māmane seedling production. We determine the optimal level of fertilization to produce seedlings with large size, high nutrient concentration, and high levels of mycorrhizal colonization. Higher levels of fertilization reduced mycorrhizal colonizationand the benefits the mycorrhizae conferred for seedling growth and nutrient uptake. After outplanting, seedlings produced this way had greater survival and growth after 1-2 years under harsh field conditions, namely pasture grass competition, prolonged drought, and night-time freezing temperatures. A controlled seedling-grass competition experiment suggested water stress is the dominant limitation for seedling establishment, while grass competition in the absence of water stress can limit root development and to a lesser extent nutrient uptake. Under these controlled conditions, exponentially fertilized seedlings had a much greater capacity to respond to removal of water limitation or grass competition. Given these results, the combination of exponential fertilization and mycorrhizal inoculation are highly recommended for nursery-grown seedlings. These treatments produce larger, more vigorous seedlings that perform better under harsh field conditions and that have a greater capacity to respond to removal of competition or resource limitation. These techniques are strongly recommended for outplanting into remote field sites where grass competition or environmental conditions create significant stressors to seedling establishment and early growth.