Interpreting recruitment limitation in forests.

Studies of tree recruitment are many, but they provide few general insights into the role of recruitment limitation for population dynamics. That role depends on the vital rates (transitions) from seed production to sapling stages and on overall population growth. To determine the state of our understanding of recruitment limitation we examined how well we can estimate parameters corresponding to these vital rates. Our two-part analysis consists of (1) a survey of published literature to determine the spatial and temporal scale of sampling that is basis for parameter estimates, and (2) an analysis of extensive data sets to evaluate sampling intensity found in the literature. We find that published studies focus on fine spatial scales, emphasizing large numbers of small samples within a single stand, and tend not to sample multiple stands or variability across landscapes. Where multiple stands are sampled, sampling is often inconsistent. Sampling of seed rain, seed banks, and seedlings typically span <1 yr and rarely last 5 yr. Most studies of seeding establishment and growth consider effects of a single variable and a single life history stage. By examining how parameter estimates are affected by the spatial and temporal extent of sampling we find that few published studies are sufficiently extensive to capture the variability in recruitment stages. Early recruitment stages are especially variable and require samples across multiple years and multiple stands. Ironically, the longest duration data sets are used to estimate mortality rates, which are less variable (in time) than are early life history stages. Because variables that affect recruitment rates interact, studies of these interactions are needed to assess their full impacts. We conclude that greater attention to spatially extensive and longer duration sampling for early life history stages is needed to assess the role of recruitment limitation in forests.

Climate change disequilibrium of boreal permafrost peatlands caused by local processes.

Boreal forest and tundra are the biomes expected to experience the greatest warming during the course of the next century. The transient responses of boreal peatlands to climate change could be more complex than a simple large release of carbon and rapid migrations of vegetation and permafrost. Here we used alternative models to demonstrate that local processes typical of permafrost peatlands control carbon and vegetation dynamics in ways that strongly mediate effects of regional temperature gradients. Regional temperature affected stability and thaw rate. Thaw rate increased with mean annual temperature, and rates have accelerated within the last 50 yr. Local factors exerted a strong influence on stability, the levels of which were highest in three of the four temperature zones studied along the shaded south-southwest edges of collapse scars. The presence of Sphagnum fuscum cover increased stability. In all zones, survey points with S. fuscum showed more than twice the stability of points with feather moss, lichen, or no vegetation. In a direct model comparison between regional and local control, local factors were more important. Our results suggest that local processes mediate the effects of regional climate, and an accurate representation of ecosystem dynamics benefits from both local and regional processes.