Testing masting mechanisms of boreal forest species at different stand densities

Abstract

Mast seeding is considered a reproductive trait resulting from several potential evolutionary forces. Although the mechanisms driving reproduction have been thoroughly investigated and discussed, their relative importance and possible coexistence remain an open question. Seed rain abundance and viability of balsam fir Abies balsamea, white spruce Picea glauca, and white birch Betula papyrifera were monitored during 1994-2007 along a chronosequence of developmental stages at different densities represented by four stands in the boreal forest of Quebec, Canada. The aim of the study was to verify whether seeding temporal dynamics and abundance were affected by stand density, and to test the causal relationships between the hypotheses of pollination efficiency and predator satiation. Seed rain abundance varied substantially among years and species, with the highest values being observed in 1994 and 1996, and attaining up to 28.2  10 3 seeds m 2 year 1 . However, the annual dynamics of seed production was similar in the three sites with the higher densities. The greater proportions of germinating and dead seeds were observed in white birch, while balsam fir showed an average of 11.2% of damaged seeds, which were infected by larvae. The conifer species had the higher proportions of empty seeds. The causal models tested on balsam fir demonstrated that seed rain abundance influenced the amount of both viable and larval-infected seeds, although no causal relationship existed between these last two variables. The comparable and synchronous seed rain abundances observed among sites demonstrated that stand density can significantly affect the reproduction of trees, probably by limiting the access to resources, but not the annual dynamics of seeding. Pollination efficiency and predator satiation are important forces in the reproductive effort of trees and could be not mutually exclusive because both these mechanisms of natural selection synergistically coexist in driving the seeding dynamics of balsam fir.

Inter- and intra-annual patterns of seed rain in the black spruce stands of Quebec, Canada

“…In all sites, seed rain abundance varied by more than one order of magnitude among years, and included some unusually large reproductive efforts. The coefficients of variation, representing the variability in seed rain among years, were close to or higher than 1 in most cases in balsam fir and white birch, and confirmed their mast seeding habit (Messaoud et al 2007, Rossi et al 2012b, Roland et al 2014. In contrast, the coefficients of variation in black spruce ranged between 0.24 and 0.54, indicating a more homogeneous inter-annual amount of seed dispersal.…”

“…The germination tests ran for a period of 21 days with weekly counts of germinated seeds. In previous tests, 85% of seeds germinated during the first 3 weeks while beyond this period germination was negligible (Rossi et al 2012b). At the end of this period, the ungerminated seeds were cut open and analyzed.…”

“…By standardizing the measure of dispersal (standard deviation) to the mean, CV allowed the variability within and between species to be efficiently compared and the intensity of the masting habit to be measured, the species showing CV>1 being considered as exhibiting mast-seeding (Silvertown 1980, Kelly 1994. The seeds collected from the traps in spring and summer were referenced to the seed production of the previous calendar year (Rossi et al 2012b). CV were compared between species and sites using a two-way analysis of variance (ANOVA).…”

“…The characteristics of the cones of these sympatric species are different, and related to the specific dynamics of reproduction . Balsam fir and white birch release the seeds quickly at the end of their maturation (Rossi et al 2012b), exhibiting a pattern of seed rain clearly divergent from that of black spruce. We monitored the spatial and temporal variability of seed rain abundance and viability during the period 2000-2007 in four black spruce stands of the boreal forest in Quebec, Canada.…”

“…The question of whether masting is a response of the physiological constraints of reserve accumulation and consumption, a reproductive strategy evolved to assure optimal fertilization of flowers and predator satiation, or the consequence of favorable environmental events remains an intriguing but still unresolved issue (Koenig & Knops 2000, Kelly & Sork 2002. Despite the lost opportunities for reproduction and density-dependent seedling mortality, there is a generalized convergence of tree species towards a marked periodicity in seed production (Silvertown 1980, Rossi et al 2012b. Species with seeds of short longevity, characterizing ecosystems where seed production is usually low or sporadic, germinate promptly after dispersal, thus reducing seed mortality and increasing the probability of recruitment of new individuals (Zasada et al 1992, Rossi et al 2012a.…”