The Fern Understory as an Ecological Filter: Emergence and Establishment of Canopy-Tree Seedlings

George, Lisa O.; Bazzaz, F. A.

doi:10.2307/177021

Cited by 104 publications

(157 citation statements)

References 29 publications

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“…However, many field surveys contradict these expectations when they report a similar performance among seedlings of different oak species along environmental gradients (e.g. Ashton and Larson 1996;Long and Jones 1996;George and Bazzaz 1999, and for Mediterranean oaks (Bacilieri et al 1993;Lookingbill and Zavala 2000). Our experimental study and modelling approach show that pre-dispersal predation may be a key process to explain inter-specific differences in the recruitment of co-occurring oaks.…”

Section: Discussionmentioning

confidence: 66%

Acorn crop size and pre-dispersal predation determine inter-specific differences in the recruitment of co-occurring oaks

et al. 2009

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The contribution of pre-dispersal seed predation to inter-specific differences in recruitment remains elusive. In species with no resistance mechanisms, differences in pre-dispersal predation may arise from differences in seed abundance (plant satiation) or in the ability of seeds to survive insect infestation (seed satiation). This study aimed to analyse the impact of pre-dispersal acorn predation by weevils in two co-occurring Mediterranean oaks (Quercus ilex and Quercus humilis) and to compare its relevance with other processes involved in recruitment. We monitored the patterns of acorn production and acorn infestation by weevils and we conducted experimental tests of acorn germination after weevil infestation, post-dispersal predation and seedling establishment in mixed forests. Monitoring and experimental data were integrated in a simulation model to test for the effects of pre-dispersal predation in recruitment. In both oaks pre-dispersal acorn infestation decreased with increasing acorn crop size (plant satiation). This benefited Q. ilex which exhibited stronger masting behaviour than Q. humilis, with almost a single and outstanding reproductive event in 6 years. Acorn infestation was more than twice as high in Q. humilis (47.0%) as in Q. ilex (20.0%) irrespective of the number of seeds produced by each species. Although germination of infested acorns (seed satiation) was higher in Q. humilis (60%) than in Q. ilex (21%), this could barely mitigate the higher infestation rate in the former species, to reduce seed loss. Conversely to pre-dispersal predation, no inter-specific differences were observed either in post-dispersal predation or seedling establishment. Our results indicate that pre-dispersal predation may contribute to differences in seed supply, and ultimately in recruitment, between co-existing oaks. Moreover, they suggest that seed satiation can barely offset differences in seed infestation rates. This serves as a warning against overemphasising seed satiation as a mechanism to overcome seed predation by insects.

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Section: Discussionmentioning

confidence: 66%

Acorn crop size and pre-dispersal predation determine inter-specific differences in the recruitment of co-occurring oaks

et al. 2009

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“…Despite the potential for understorey vegetation to act as a major Wlter to recruitment (George and Bazzaz 1999;Royo and Carson 2006), few experiments have tested its multiple eVects on tree seedling establishment through resource and consumer-mediated interactions (Royo and Carson 2008). We found that bamboo presence determined spatial patterns of seedling survival through the Wrst post-emergence summer in a closed-canopy Patagonian forest.…”

Section: Discussionmentioning

confidence: 86%

“…As a result, newly emerged seedlings face a variety of hazards, which operate as ecological Wlters to establishment (Harper 1977;George and Bazzaz 1999), and may ultimately inXuence plant community structure (Silvertown 2004). Vegetation cover plays a major role in shaping the nature of recruitment microsites, by modifying microhabitat conditions and the balance of positive (facilitation) and negative (competition, predation) interactions for emerging seedlings (McAuliVe 1986;Gill and Marks 1991;Callaway and Walker 1997;Holmgren et al 1997;Rousset and Lepart 2000).…”

Section: Introductionmentioning

confidence: 99%

“…Yet, few studies have focused on the multiple mechanisms underlying species-speciWc eVects of understorey plants on tree seedling survival (George and Bazzaz 1999;Clark 2003, 2005;Royo and Carson 2008). By contrast, much work has been devoted to comparing recruitment patterns between closed understorey and treefall gaps (e.g.…”

Section: Introductionmentioning

confidence: 99%

“…Beckage et al 2000;Chacón and Armesto 2006;Norghauer et al 2008) or among diVerent gap sizes (Pearson et al 2003). Spatial heterogeneity in understorey cover creates a forest-Xoor mosaic of recruitment opportunities through its inXuence on abiotic conditions (George and Bazzaz 1999;Beckage et al 2000;Beckage and Clark 2003;Giordano et al 2009), litter deposition (Gray and Spies 1997;Christie and Armesto 2003), and habitat use by resident consumers (Wada 1993;Abe et al 2001;Pearson et al 2003;Caccia et al 2006;Royo and Carson 2008). Understorey vegetation may constrain tree seedling growth and survival by reducing light availability (Beckage et al 2000;Taylor et al 2004) and belowground resources (Lewis and Tanner 2000;Beckage and Clark 2003).…”

Section: Introductionmentioning

confidence: 99%

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Direct and indirect effects of understorey bamboo shape tree regeneration niches in a mixed temperate forest

2009

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Plant cover plays a major role in shaping the nature of recruitment microsites through direct (resource mediated) and indirect (consumer mediated) interactions. Understorey plants may differentially affect seedling establishment, thus contributing to regeneration-niche separation among canopy tree species. We examined patterns of early tree seedling survival resulting from interactive effects of understorey bamboo (Chusquea culeou) and resident consumers in a mixed temperate Patagonian forest, Argentina. Newly germinated seedlings of Nothofagus dombeyi and Austrocedrus chilensis were planted in bamboo thickets and non-bamboo patches, with or without small-vertebrate exclosures. We found species-specific patterns of seedling survival in relation to bamboo cover. Nothofagus survival was generally low but increased under bamboo, irrespective of cage treatment. Desiccation stress accounted for most Nothofagus mortality in open, non-bamboo areas. In contrast, Austrocedrus survival was highest in non-bamboo microsites, as most seedlings beneath bamboo were killed by small vertebrates through direct consumption or non-trophic physical damage. There was little evidence for a negative impact of bamboo on tree seedling survival attributable to resource competition. The balance of simultaneous positive and negative interactions implied that bamboo presence facilitated Nothofagus early establishment but inhibited Austrocedrus recruitment via apparent competition. These results illustrate the potential for dominant understorey plants to promote microsite segregation during early stages of recruitment between tree seedlings having different susceptibilities to water stress and herbivory. We recognise, however, that patterns of bamboo-seedling interactions may be conditional on moisture levels and consumer activity during establishment. Hence, both biotic and abiotic heterogeneity in understorey environments should be incorporated into conceptual models of regeneration dynamics and tree coexistence in forest communities.

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Stasis in forest regeneration following deer exclusion and understory gap creation: A 10‐year experiment

Royo

Carson

2022

Ecological Applications

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Chronically elevated ungulate browse pressure in temperate forests worldwide often generates ecological legacies characterized by low plant diversity and contributes to the formation of dense, nearly monodominant, and highly recalcitrant layers of understory vegetation. Once established, these recalcitrant layers combined with continued browsing may jointly constrain tree establishment and diversity so completely that understory recovery may be unattainable without mitigating browse pressure, the recalcitrant layer, or both. Here, we investigate the independent and synergistic effects of both white‐tailed deer (Odocoileus virginianus) browsing and hay‐scented fern (Dennstaedtia punctilobula) competition on tree regeneration in a 10‐year experiment. Specifically, we examine how tree seedling establishment, growth, and composition are filtered by fern cover versus fern removal (gaps), browser presence versus absence (exclosures), and their combined effects during 10 years at three hardwood forest sites in Pennsylvania, USA. Fern gaps enhanced establishment for multiple tree species, increasing seedling density and diversity, particularly in the first 3 years post‐treatment, and enhancing richness (≤1 species) over the course of the experiment. Excluding deer for a decade increased the height growth of other regeneration and altered species composition, but had no effect on diversity, richness, and density. Notably, we observed higher Prunus serotina seedling densities outside exclosures, possibly due to greater secondary dispersal. We argue that browsing legacies in second growth forests established at the turn of the last century created two conditions inimical to diverse forest regeneration: an overstory dominated by two species, P. serotina and Acer rubrum (86% of basal area), and a dense recalcitrant understory layer dominated by a native fern. The first condition limits propagule supply, the second strongly filters seedling establishment, and both create impoverished forest understories composed of few individuals and species. In undisturbed forest understories, the inertia toward impoverishment was sustained across the decade even where browsing was eliminated. Consequently, stand replacing disturbances (whether natural or anthropogenic) that disrupt the understory layer and reinitiate succession may be necessary to propel forests out of their current stasis and down a pathway leading to greater diversity.

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The Fern Understory as an Ecological Filter: Emergence and Establishment of Canopy-Tree Seedlings

Cited by 104 publications

References 29 publications

Acorn crop size and pre-dispersal predation determine inter-specific differences in the recruitment of co-occurring oaks

Acorn crop size and pre-dispersal predation determine inter-specific differences in the recruitment of co-occurring oaks

Direct and indirect effects of understorey bamboo shape tree regeneration niches in a mixed temperate forest

Stasis in forest regeneration following deer exclusion and understory gap creation: A 10‐year experiment

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