Limitations to Seedling Growth and Survival by the Quantity and Quality of Rooting Space: Implications for the Establishment ofThuja occidentalison Cliff Faces

Abstract

Thuja occidentalis is the dominant tree of ancient, stunted cliff‐face forests of the Niagara Escarpment, Ontario, Canada, and in Michigan and Wisconsin, U.S.A. Previous field research has shown that nutrient and water limitations at the sapling stage are absent or small, yet seedling recruitment is relatively rare. To study the relationships between rooting space, nutrient supply, growth, and survival in very young seedlings, plants were grown from seed in three pot volumes (1 mL, 10 mL, and 100 mL) and under three nutrient regimes (watering with water, one‐tenth‐strength Hoagland's solution, and half‐strength Hoagland's solution). Elongation of the main axis and side branches was measured, and survival also was monitored over 2 yr. All measures of seedling growth were significantly increased by higher nutrient levels. Effects of pot size on growth were smaller and less consistent; main axis elongation tended to be greatest in the smallest pots, but branch elongation was not affected. Survival results showed trends opposite to those for growth; higher levels of nutrients did not influence seedling survival, but survival was slightly better in larger pots than in small and intermediate pots. When averaged over all treatments, 61% of seedlings survived to the end of year 1 and 14% to the end of year 2. By the beginning of the third year only 0.1% were alive and all were in large pots. The results indicate that an uncoupling of the relationship between growth and survival is possible in an environment where intraspecific competitive interactions are muted. Survival of trees on cliffs may therefore be unrelated to tree growth and may instead be primarily controlled by the chance of finding safe sites with adequate rooting volume.