Does Rubus canadensis Interfere with the Growth of Fraser Fir Seedlings?

Vegetation change in the Great Smoky Mountains following balsam woolly adelgid-caused mortality of Fraser fir has included development of a dense Rubus canadensis shrub layer. Many fir seedlings have persisted in the forest understory, but the possible effects of Rubus on their annul growth have not been intensively studied.

This study had two objectives: (1) to determine if significant associations exist among density and shoot growth of Fraser fir seedling, density/biomass of Rubus canadensis, canopy closure, and soil chemical parameters, and (2) to determine the effects of removal of aboveground Rubus stems on fir seedling shoot growth.

Eighty 1 x 1 m plots were used on Mount Collins, Great Smoky Mountains National Park, to characterize the understory habitat of Fraser fir seedlings. Terminal and lateral shoot lengths of the 1983-1987 growth were measured on fir seedlings. Seedlings were classified according to substrate type, surface type, substrate form, presence of adelgid damage, and age. Soil samples were taken for analyses of pH, potassium, phosphorus, and calcium. Overstory composition was characterized with prism plots. Fourteen 2 x 2 m plots were established to determine effects of Rubus removal on fir seedling growth, herbaceous cover, shrub/seedling counts, and soil parameters.

Rubus canadensis density was highest on the southwest-facing slope and lowest on the northeast-facing slope of Mount Collins. Fraser fir seedling density was highest on theortheast-facing slope and lowest on the southwest -facing slope. The spearman rank correlation between Rubus density and fir seedling density was -0.376 (P < 0.01); a graph of these data showed variability in fir density to decrease with increasing Rubus density. Most years of fir terminal shoot growth showed positive associations with Rubus density (0.309-0.396, P < 0.01); a graph of these data showed no pronounced relationship. Rubus density and biomass were significantly correlated with soil phosphorus and potassium concentrations. All years of terminal shoot growth of fir seedlings measured showed positive associations with soil pH and potassium concentration.

Most fir seedlings were ≤25 cm tall. Only 38% were <5 years old. More seedling than expected occupied dead wood substrates, and more than expected were found on bryophyte-covered surfaces. More than 25% of seedlings showed adelgid damage; adelgid damage was more prevalent among seedlings ≥5 years old. Most seedlings showed a trend of increasing gains of terminal shoot growth over previous years of growth.

Removal of Rubus stems produced no significant effect of fir seedling shoot growth over one season. No Rubus-removal effects were found on any other understory variable measured; Rubus itself responded to removal by rapid appearance and growth of new stems. Large seasonal changes in bryophyte cover and red spruce seedling density (from germination) occurred, but these changes were not affected by Rubus removal.

Germinal Fraser fir seedlings are scarce and are not likely to appear in large numbers unless existing understory firs reach reproductive age. Rubus appears to inhibit the establishment of fir seedlings. Fir shoot growth does not appear to be associated with Rubus density or biomass. Fir Shoot growth trends are probably consequences of normal growth patterns and recovery from adelgid infestation.