Sites in need of restoration typically have one or more environmental factors that limit seedling establishment; identifying ecophysiological responses to environmental stressors can be advantageous in growing seedlings able to overcome such constraints on survival. To maximize survival after outplanting, seedlings should be grown in a manner that considers both the natural development of the species as well as the potential outplanting conditions. Black locust (Robinia pseudoacacia L.) and koa (Acacia koa A. Gray) are nitrogen-fixing trees that are used in restoring degraded ecosystems; this thesis focuses on projects aimed at providing a stronger understanding of the seedling behavior of those species in response to environmental conditions.
The first study examined how koa seedlings responded to reduced water (W), reduced phosphorus (P), and combined reduced W and P conditions. It was found that after 17 weeks, seedlings subjected to reduced W or reduced P treatments accumulated less biomass, had smaller root-collar-diameters, narrower root structures, developed less nodules, contained lower C and N contents, and lower foliar P concentrations. Combined reductions in W and P interacted such that seedlings increased their root to shoot dry biomass and developed shorter shoots. Seedlings treated with reduced W had reduced instantaneous rates of CO2 assimilation, but higher instantaneous water-use efficiency. Seedlings under reduced P treatments had a similar rate of CO2 assimilation relative to those grown with adequate P, suggesting that koa is able to employ strategies to avoid physiological impairment during photosynthesis. Water stressing seedlings in an artificial environment before planting on sites low in soil moisture may physiologically cue seedlings to develop greater resistance to drought stress. Similarly, field P fertilization may enhance seedling growth rates, especially on sites with low plant-available water.
The second study evaluated if the amount of fertilizer used to grow black locust seedlings can be reduced without compromising seedling morphology through inoculating seeds with rhizobium. Seedlings were grown under different fertilizer application rates (0, 2, or 4 mg Applied Fertilizer·seedling-1·week-1) and either inoculated with rhizobium or left uninoculated to determine the nursery growing regime that produced the largest seedlings with the greatest nodule formation. It was found that seedlings grown under 4 mg Applied Fertilizer·seedling1·week-1 were the largest regardless of inoculation. However, seedlings grown under 2 mg Applied Fertilizer·seedling-1·week-1 had a comparable height and root dry mass.
Inoculation and fertilizer rate impacted nodule formation such that seedlings inoculated or grown under the 2 mg Applied Fertilizer·seedling-1·week-1 had the greatest nodule formation; this treatment combination interacted such that seedlings had comparable relative height growth and shoot dry mass compared to the seedlings grown under 4 mg Applied Fertilizer·seedling-1·week-1. Seedlings grown under 0 mg Applied Fertilizer·seedling-1·week-1 were stunted in growth and formed less nodules regardless of inoculation. The result indicates that nurseries should fertilize black locust seedlings, but may be able to drastically reduce the amount of fertilizer used, reducing the possibility of local surface and groundwater pollution, if seeds are inoculated with rhizobium while sowing.