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 s tages and on overall population growth. To determine the state of our under standing of recruitment limitation we examined how well we can estimate par ameters corresponding to these vital rates. Our two-part analysis consists of (1) a survey of published literature to determine the spatial and tempor al scale of sampling that is basis for parameter estimates, and (2) an anal ysis of extensive data sets to evaluate sampling intensity found in the lit erature. We find that published studies focus on fine spatial scales, empha sizing large numbers of small samples within a single stand, and tend not t o sample multiple stands or variability across landscapes. Where multiple s tands are sampled, sampling is often inconsistent. Sampling of seed rain, s eed banks, and seedlings typically span <1 yr and rarely last 5 yr. Most st udies of seeding establishment and growth consider effects of a single vari able and a single life history stage. By examining how parameter estimates are affected by the spatial and temporal extent of sampling we find that fe w published studies are sufficiently extensive to capture the variability i n 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 tin time) than are early life history stages. Because variabl es that affect recruitment rates interact, studies of these interactions ar e needed to assess their full impacts. We conclude that greater attention t o spatially extensive and longer duration sampling for early life history s tages is needed to assess the role of recruitment limitation in forests.