The effect of substrate type and mycorrhizal fungi on northern hardwood seedlings

Background/Question/Methods

Gaining access to resources is a challenging task for young seedlings established on the forest floor. Due to their high moisture holding capacity and elevated position, nurse logs provide seedlings with stable moisture and potential refuge from a variety of factors which threaten seedling survival on the forest floor. Despite these seemingly favorable attributes, evidence indicates that not all species of seedlings successfully develop on nurse logs, and not all species of nurse logs appear capable of supporting seedling development. In order to develop a mechanistic explanation for these patterns, we conducted a 10x7x2 factorial potted plant experiment investigating the influence of substrate nutrient content and mycorrhizal colonization on seedling growth and survival across a suite of forest substrates. From seed, we established ten northern hardwood tree species across seven different substrate types (six nurse log species and mineral soil) that were either unsterilized or sterilized with gamma irradiation (30-60kGy). Seedlings were grown for one year under covered hoop houses (PAR 254-344 µmol m^-2s^-1) before being measured for height growth and survival. At the end of the experiment, a subset of seedlings from each unique combination of factors was harvested and investigated for mycorrhizal colonization. Substrate nitrate (NO₃^-) and ammonium (NH₄⁺) content was measured from a subset of unseeded plots in each unique substrate treatment combination.  

Results/Conclusions

Significant differences were detected in height growth and survival among seedling species. Shade intolerant and mid-tolerant species demonstrated greater growth rates than shade tolerant species. Seedling survival was high for all conifer species (>90%) but varied greatly among deciduous species, with Betula alleghaniensis surviving at the highest rate (89%) and Fraxinus americana at the lowest rate (28%).  Substrate type also had significant effects on seedling growth.  In general, mineral soil and B. papyrifera, Tsuga canadensis, and Thuja occidentalis nurse logs supported greater height growth than nurse logs of A. saccharum, B. alleghaniensis, and Abies balsamea. This pattern corresponded positively with trends in substrate inorganic N content. Mycorrhizal colonization positively influenced A. rubrum, T. occidentalis, P. strobus, and B. alleghaniensis seedling growth, and was positively associated with changes in species performance rankings across different substrate types. Collectively, these findings suggest that substrate nutrient content and mycorrhizal colonization are important factors contributing to differing patterns of seedling development across different substrate types. It also suggests that management efforts should focus on preserving individual species of nurse logs rather than considering them a generic entity.