dynamics of seeds and seedlings
Department of Biological Sciences
Hanover, NH 03755
phone: (603) 646-2847
fax: (603) 646-1347
ECOLOGICAL PROCESSES in the early life history stages of tree species may have profound implications for the dynamics of forest communities. In our research, we analyze the population dynamics in the early life stages of two of the dominant canopy species in a northern hardwood forest, sugar maple (Acer saccharum) and American beech (Fagus grandifolia). With a series of experiments and field measurements we quantify the influences of microhabitat and natural enemies (predators, herbivores and pathogens) affecting the survival of individuals at the seed and seedling stages. At the seed stage, we include two other common species, white ash (Fraxinus americana) and red maple (A. rubrum). We synthesize the findings for seeds and seedlings to assess the emergent patterns of spatial and temporal dynamics, with emphasis on density dependent death rates and their relation to the absolute and relative densities of conspecific seeds, seedlings and adult trees. These density dependent trends may strongly influence the spatial structure and relative abundances of canopy trees in this forest.
Our 9 ha plot is located west of Watershed 101 in the Hubbard Brook Experimental Forest. All trees with DBH above 5 cm were mapped and tagged in 1999. We monitored seed input from June 2000 to June 2001, with 100 seed traps (each 0.25 m2). Surviving seeds were sampled in December 2001 and new germinates were tagged in 64 subplots (each 1 m2) in May 2001. The fates of seedlings were monitored through Summer 2003. During Summer and Fall 2000, we introduced seeds to field plots and continue to monitor their fates at two spatial scales (900 m2 and 9 ha) and three densities (high, medium and single seed).
The mortality of seeds and seedlings was analyzed with individual hazard
models (Zens and Peart, 2003), with conspecific seed or adult density, herbivore
level, pathogen and light index as covariates.
Overall transition rates from seed fall to 1 year-old seedlings were very low; 2% for sugar maple and 1% for beech. Dynamics in the early life stages altered the relative abundances of the two species, and thus altered the species composition of potential recruits into the canopy. The mean proportions of seeds that survived over the season were 21±2% for sugar maple, 12±4% for beech, 8±4% for red maple and 19±10% for ash. Thus the relative abundance of sugar maple increased during the seed stage. The pattern was reversed at the seedling stage. Overall proportions of seedlings surviving at the end of the first growth season were 13.9% for sugar maple and 19.0% for beech.
Seed survival was negatively dependent on both local conspecific seed density and local adult density for several species, across spatial scales. The negative density dependence we detected in the seed and seedling stages may dissociate local seedling density from local adult density in maple and reduce the spatial associations in beech. Although we detected strong spatial association between adjacent stages of the life cycle in sugar maple and beech, the association of adult and juvenile abundance became weaker with age in both species, and disappeared with by the 1 year-old seedling stage in sugar maple.
Zens, MS and Peart, DR (2003). Dealing with death data: individual hazards, mortality and bias. Trends Ecol Evol 18 (7): 366-373.
Date Prepared: February 2004