Partitioning the Factors of Spatial Variation in Regeneration Density of Shade-Tolerant Tree Species

Abstract

Abstract. Understanding coexistence of highly shade-tolerant tree species is a longstanding challenge for forest ecologists. A conceptual model for the coexistence of sugar maple (Acer saccharum) and American beech (Fagus grandifolia) has been proposed, based on a lowlight survival/high-light growth trade-off, which interacts with soil fertility and small-scale spatiotemporal variation in the environment. In this study, we first tested whether the spatial distribution of seedlings and saplings can be predicted by the spatiotemporal variability of light availability and soil fertility, and second, the manner in which the process of environmental filtering changes with regeneration size. We evaluate the support for this hypothesis relative to the one for a neutral model, i.e., for seed rain density predicted from the distribution of adult trees. To do so, we performed intensive sampling over 86 quadrats (5 3 5 m) in a 0.24-ha plot in a mature maple-beech community in Quebec, Canada. Maple and beech abundance, soil characteristics, light availability, and growth history (used as a proxy for spatiotemporal variation in light availability) were finely measured to model variation in sapling composition across different size classes. Results indicate that the variables selected to model species distribution do effectively change with size, but not as predicted by the conceptual model. Our results show that variability in the environment is not sufficient to differentiate these species' distributions in space. Although species differ in their spatial distribution in the small size classes, they tend to correlate at the larger size class in which recruitment occurs. Overall, the results are not supportive of a model of coexistence based on small-scale variations in the environment. We propose that, at the scale of a local stand, the lack of fit of the model could result from the high similarity of species in the range of environmental conditions encountered, and we suggest that coexistence would be stable only at larger spatial scales at which variability in the environment is greater.

Large-scale synchrony of gap dynamics and the distribution of understory tree species in maple–beech forests

“…That variations in release frequency (likely a function of small-scale disturbance frequency) were the main factor aVecting the relative abundance of ASA and FGR among sites would probably not be particularly revealing at the local scale (Poulson and Platt 1996;Gravel et al 2008); however, this is a surprising result when considered at the regional scale. Forest communities are never at equilibrium at the scale of a single stand (Woods 2000); this nonequilibrium could also be true at the regional scale.…”

“…(FGR) as a function of past suppression and release history. These two shade-tolerant species have been widely studied in attempts to understand the mechanisms allowing their coexistence (e.g., Woods 1979;Poulson and Platt 1996;Beaudet et al 1999Beaudet et al , 2007aGravel et al 2008), making them good candidates for this analysis. Although our focus was primarily on understory trees, we also performed this analysis on all of the understory woody species found in our forests.…”

Sapling age structure and growth series reveal a shift in recruitment dynamics of sugar maple and American beech over the last 40 years

“…Growth and survival of maple seedlings are especially affected by variation in soil pH and calcium availability (Kobe et al 2002), while beech is more sensitive than maple to drought (Caspersen and Kobe 2001). Besides the effects from light and soil properties (Gravel et al 2008), spatiotemporal variations in the abundance of maple and beech are also affected by factors such as preferential browsing on maple by white-tailed deer (Long et al 2007), climatic conditions (e.g., through their effects on phenology of fruit production; Fang and Lechowicz 2006), and variation in the importance of root sprouting in beech (Kitamura and Kawano 2001).…”

Comment on “Present-day expansion of American beech in northeastern hardwood forests: Does soil base status matter?”Appears in Can. J. For. Res. 39: 2273–2282 (2009).

“…Concluding that the relative abundance of saplings determines the future relative abundance of trees assumes that transition rates are the same for all species and over all life history stages, which is unsupported by empirical data (Pacala et al 1994;Kobe et al 1995;Kneeshaw et al 2006;Gravel et al 2008).…”

“…D&O also overlooked the fact that there was no correlation between SM absolute basal area in the sapling stratum and base cations (Spearman's r of 0.017 and 0.018 for Ca and Mg, respectively; Table 3). Finally, D&O failed to mention the many contradictory observations on the relationships between SM-AB abundance and soil properties (e.g., van Breemen et al 1997;Beaudet et al 1999;Schwarz et al 2003;Arii et al 2005;Gravel et al 2008). In our opinion, D&O did not have sufficient quantitative and statistical evidence to support their conclusion of a SM regeneration failure associated with base-poor sites, nor therefore their inferences attributing AB increases to a negative impact of atmospheric acid deposition on soil base cation status.…”