Does a shift in shade tolerance as suggested by seedling morphology explain differences in regeneration success of northern red oak in native and introduced ranges?

Across North America, forests dominated by Quercus rubra L. (northern red oak), a moderately shade-tolerant tree species, are undergoing successional replacement by shade-tolerant competitors. Under closed canopies, Q. rubra seedlings are unable to compete with these shade-tolerant species and do not recruit to upper forest strata. In Europe, natural regeneration of introduced Q. rubra is often successful despite the absence of fire, which promotes regeneration in the native range. Considering that understorey light availability is a major factor affecting recruitment of seedlings, we hypothesized that Q. rubra seedlings are more shade tolerant in the introduced range than in the native range. Morphological traits and biomass allocation patterns of seedlings indicative of shade tolerance were compared for Q. rubra and three co-occurring native species in two closed-canopy forests in the native range (Ontario, Canada) and introduced range (Baden-Württemburg, Germany). In the native range, Q. rubra allocated a greater proportion of biomass to roots, while in the introduced range, growth and allocation patterns favored the development of leaves. Q. rubra seedlings had greater annual increases in height, diameter and biomass in the introduced range. Q. rubra seedlings in the introduced range were also younger;however, they had a mean area per leaf and a total leaf area per seedling that were five times greater than seedlings in the native range. Such differences in morphological traits and allocation patterns support the hypothesis that Q. rubra expresses greater shade tolerance in the introduced range, and that natural regeneration of Q. rubra is not as limited by shade as in the native range. The ability of Q. rubra seedlings to grow faster under closed canopies in Europe may explain the discrepancy in regeneration success of this species in native and introduced ranges. Future research should confirm findings of this study over a greater geographical range in native and introduced ecosystems, and examine the genetic and environmental bases of observed differences in plant traits.

Environmental variation shifts the relationship between trees and scatterhoarders along the continuum from mutualism to antagonism

The conditional mutualism between scatterhoarders and trees varies on a continuum from mutualism to antagonism and can change across time and space,and among species.We examined 4 tree species(red oak[Quercus rubra],white oak[Quercus alba],American chestnut[Castanea dentata]and hybrid chestnut[C.dentata×Castanea] mollissima)across 5 sites and 3 years to quantify the variability in this conditional mutualism.We used a published model to compare the rates of seed emergence with and without burial to the probability that seeds will be cached and left uneaten by scatterhoarders to quantify variation in the conditional mutualism that can be explained by environmental variation among sites,years,species,and seed provenance within species.All species tested had increased emergence when buried.However,comparing benefits of burial to the probability of caching by scatterhoarders indicated a mutualism in red oak,while white oak was nearly always antagonistic.Chestnut was variable around the boundary between mutualism and antagonism,indicating a high degree of context dependence in the relationship with scatterhoarders.We found that different seed provenances did not vary in their potential for mutualism.Temperature did not explain microsite differences in seed emergence in any of the species tested.In hybrid chestnut only,emergence on the surface declined with soil moisture in the fall.By quantifying the variation in the conditional mutualism that was not caused by changes in scatterhoarder behavior,we show that environmental conditions and seed traits are an important and underappreciated component of the variation in the relationship between trees and scatterhoarders.