Limitations to seedling growth and survival by the quantity and quality ofrooting space: Implications for the establishment of Thuja occidentalis oncliff faces

Thuja occidentalis is the dominant tree of ancient, stunted cliff-face fore sts of the Niagara Escarpment, Ontario, Canada, and in Michigan and Wiscons in, U.S.A. Previous field research has shown that nutrient and water limita tions at the sapling stage are absent or small, yet seedling recruitment is relatively rare. To study the relationships between rooting space, nutrien t supply, growth, and survival in very young seedlings, plants were grown f rom seed in three pot volumes (1 mL, 10 mL, and 100 mL) and under three nut rient regimes (watering with water, one-tenth-strength Hoagland's solution, and half-strength Hoagland's solution). Elongation of the main axis and si de branches was measured, and survival also was monitored over 2 yr. All me asures of seedling growth were significantly increased by higher nutrient l evels. Effects of pot size on growth were smaller and less consistent; main axis elongation tended to be greatest in the smallest pots, but branch elo ngation 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 intermed iate 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 thi rd year only 0.1% were alive and all were in large pots. The results indica te that an uncoupling of the relationship between growth and survival is po ssible in an environment where intraspecific competitive interactions are m uted. Survival of trees on cliffs may therefore be unrelated to tree growth and may instead be primarily controlled by the chance of finding safe site s with adequate rooting volume.