Interpreting recruitment limitation in forests

Abstract

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.

Spatial patterns of seed dispersal, their determinants and consequences for recruitment

“…Whatever their methods, studies of seed dispersal are most often conducted in single populations and single seasons, over short distances and without consideration of the microsite (Box 1) upon which seeds are deposited. Such studies necessarily fail to capture important variation at larger spatial scales and longer temporal scales 13 . We refer the reader to Clark et al 13 for an excellent discussion of population-level variation, and focus here on patterns of long-distance dispersal, microsite-dependent deposition and temporal variation.…”

“…Such studies necessarily fail to capture important variation at larger spatial scales and longer temporal scales 13 . We refer the reader to Clark et al 13 for an excellent discussion of population-level variation, and focus here on patterns of long-distance dispersal, microsite-dependent deposition and temporal variation.…”

“…These studies not only provide more information on variation among populations and individuals, across microsites, in time and at long distances from sources, but also link these to their consequences for recruitment. Extensive natural history data have been collected on seed-dispersion patterns, dispersal-agent behavior and establishment success in many systems [5][6][7]9,12 ; however, most of these data are on small spatial scales and short temporal scales 13 , and are not closely linked to each other. Herrera 49 provides a rare example of a long-term study on seed dispersal by birds 37 ; we know of no comparable studies on seed-dispersion patterns themselves.…”

Comparisons of recruitment, survival, and growth in invasive and native saplings on a volcano

“…However, there have been few log-term studies on sapling recruitment, survival and growth for native vs. invasive plants (Yurkonis and Meiners 2004), although this is a prerequisite for clarifying mechanisms how biological invasions affect plant community dynamics (Clark et al 1999;Rooney et al 2000). Overstory trees suppress sapling regeneration via above-and below-ground competition (Levine et al 2003;Yurkonis and Meiners 2004;Kueffer et al 2007), and the suppression acts differently between species (Dullinger et al 2005).…”

“…Since the sapling stage is decisive to determine forest dynamics, this stage should be a focus for study of forest ecosystem invasion (Clark et al 1999;Rooney et al 2000).…”