The mechanistic basis underpinning forest succession is the gap-phase paradigm in which overstory distur- bance interacts with seedling and sapling shade tolerance to determine successional trajectories. The theory, and ensu- ing simulation models, typically assume that understory plants have little impact on the advance regeneration layer's composition. We challenge that assumption by reviewing over 125 papers on 38 species worldwide that form dense and persistent understory canopies. Once established, this layer strongly diminishes tree regeneration, thus altering the rate and direction of forest succession. We term these dense strata recalcitrant understory layers. Over half of the cases re- viewed were linked to increases in canopy disturbance and either altered herbivory or fire regimes. Nearly 75% of the studies declared that competition and allelopathy were the likely interference mechanisms decreasing tree regeneration, yet only 25% of the studies used manipulative field experiments to test these putative mechanisms. We present a con- ceptual model that links the factors predisposing the formation of recalcitrant understory layers with their interference mechanisms and subsequent impacts on succession. We propose that their presence constricts floristic diversity and ar- gue for their explicit inclusion in forest dynamics theory and models. Finally, we offer management suggestions to limit their establishment and mitigate their impacts.

https://www.fs.fed.us/pnw/pubs/gtr802/Vol2/pnw_gtr802vol2_royo.pdf

On the formation of dense understory layers in forests worldwide: consequences and implications for forest dynamics, biodiversity, and succession

Aim We set out to test the hypothesis that rates of pre- and post-dispersal seed predation would be higher towards the tropics, across a broad range of species from around the world. We also aimed to quantify the slope and predictive power of the relationship between seed mass and latitude both within and across species.

Methods  Seed mass, pre-dispersal seed predation and post-dispersal seed removal data were compiled from the literature. Wherever possible, these data were combined with information regarding the latitude at which the data were collected. Analyses were performed using both cross-species and phylogenetic regressions.

Results  Contrary to expectations, we found no significant relationship between seed predation and latitude (log10 proportion of seeds surviving predispersal seed predation vs. latitude, P = 0.63; R2 = 0.02; n = 122 species: log10 proportion of seeds remaining after postdispersal seed removal vs. latitude, P = 0.54; R2 = 0.02; n = 205 species). These relationships remained non-significant after variation because of seed mass was accounted for. We also found a very substantial (R2 = 0.21) relationship between seed mass and latitude across 2706 species, with seed mass being significantly higher towards the tropics. Within-species seed mass decline with latitude was significant, but only about two-sevenths, as rapid as the cross-species decline with latitude. Results of phylogenetic analyses were very similar to cross-species analyses. We also demonstrated a positive relationship between seed mass and development time across ten species from dry sclerophyll woodland in Sydney (P < 0.001; R2 = 0.77; Standardized Major Axis slope = 0.14). These data lend support to the hypothesis that growing period might affect the maximum attainable seed mass in a given environment.

Main conclusions  There was no evidence that seed predation is higher towards the tropics. The strong relationship between seed mass and latitude shown here had been observed in previous studies, but had not previously been quantified at a global scale. There was a tenfold reduction in mean seed mass for every c. 23° moved towards the poles, despite a wide range of seed mass within each latitude.

http://bio.mq.edu.au/research/groups/comparative/reprints/Mo03JB.pdf

Latitude, seed predation and seed mass

Seed predation by rodents affects plant population dynamics and it mayrespond to changes in vegetation structure at forest edges. This studyinvestigates the magnitude and direction of a potential edge effect intemperatedeciduous forests, and it seeks possible explanations based on predatorabundance and vegetation structure. The study was conducted at twelve forestedges in northern Switzerland; in six sites all shrubs at the forest edge wereremoved, whereas the other sites maintained a structurally rich shrub layer. Ineach site three transects were laid out from the edge towards the forestinterior. In six plots along these transects (at 1, 3, 6, 12, 25, 50m) we studied seed removal from experimental dishes, rodentabundance in live-traps and four characteristics of the vegetation structure.For both woody seed species (Prunus avium,Viburnum lantana) predation was significantly higher nearthe forest edge in the structurally rich sites; in the other sites no suchgradient was found. Selectively accessible dishes revealed that rodents werethemain predators, whereas predation by insects or molluscs was not observed.Abundance of rodents (Apodemus flavicollis, A.sylvaticus, Clethrionomys glareolus) washighestunder dense shrubs close to the forest edge. In the structurally rich sitesthere was a clear gradient of decreasing shrub cover from the edge towards theforest interior; a weaker gradient was observed in the structurally poor sites.We conclude that high shrub cover near the forest edge is the main determinantfor edge effects in seed predation, and edges without a shrub belt show no sucheffect.

Edges effects on seed predation by rodents in deciduous forests of northern Switzerland

In forests characterized by a dense woody and herbaceous understory layer, seedling recruitment is often directly suppressed via interspecific competition. Alternatively, these dense layers may indirectly lower tree recruitment by provid- ing a haven for seed and seedling predators that prey on neighboring plant species. To simultaneously test for resource competition and indirect, habitat-mediated effects, we factorially manipulated understory plant cover (removed versus in- tact) and predation (exclosures versus controls) at three forested sites. We found that vegetation cover created privileged foraging areas that increased seed removal and seedling predation rates. Predator preference was directly related to seed size with larger seeded species including Prunus serotina Ehrh. and Fagus grandifolia Ehrh. removed more readily than smaller seeded species such as Fraxinus americana L. We found strong species-specific evidence for habitat-mediated indi- rect effects; establishment of P. serotina and Acer saccharum Marsh.was significantly lower under an intact hay-scented fern (Dennstaedtia punctilobula (Michx.) T. Moore) canopy when small mammals were present. Competition also played a strong role; both P. serotina and Acer rubrum L. survival as well as A. rubrum emergence were reduced under a fern can- opy with or without seed predators. The impact of habitat-mediated indirect effects and resource competition appear to vary predictably based upon predator preferences and differences in the timing of woody seed dispersal and germination relative to vegetation cover phenology. Overall, our results suggest that habitat-mediated indirect effects may be common and occur wherever vegetation provides the potential for creating privileged foraging areas. Resume´ : Dans les forets caracterisees par un sous-etage dense composed'especes ligneuses et herbacees, la competition interspecifique empeche souvent de facon directe le recrutement de semis. De facon indirecte, ces strates denses peuvent aussi diminuer le recrutement de semis en fournissant un refuge aux predateurs de graines et de semis qui s'attaquent aux especes environnantes. Dans le but de tester simultanement les effets directs de la competition pour les ressources et les effets indirects par l'intermediaire de l'habitat, nous avons etabli un dispositif factoriel en manipulant la couverture vegetale en sous-etage (enlevee et intacte) et la predation (exclos et temoins) dans trois stations forestieres. Nous avons observeque le couvert vegetal creait des aires d'alimentation privilegiees dans lesquelles les taux de prelevement de graines et de predation des semis etaient plus eleves. La preference des predateurs etait directement relieeala taille des graines puisque les graines des especes qui produisent de grosses semences, comme Prunus serotina Ehrh. et Fagus grandifolia Ehrh., etaient prelevees plus rapidement que celles des especes qui produisent de petites semences, comme Fraxinus americana L. Nous avons trouvede serieux indices specifiques que l'habitat a des effets indirects : l'eta- blissement de P. serotina et d'Acer saccharum Marsh. etait significativement reduit sous un couvert intact de Dennstaed- tia punctilobula (Michx.) T. Moore lorsque de petits mammiferes etaient presents. La competition a aussi joueun role important : la survie de P. serotina et d'A. rubrum L. ainsi que l'emergence d'A. rubrum etaient reduites sous un couvert de fougere que des predateurs soient presents ou non. L'impact de la competition indirecte par l'intermediaire de l'habitat et de la competition directe pour les ressources semble varier de facon previsible en fonction des preferences des predateurs et des differences dans la periode de dispersion et de germination des graines des especes ligneuses par rapport ala phenologie du couvert vegetal. Dans l'ensemble, nos resultats indiquent que les effets indirects par l'interme´- diaire de l'habitat peuvent etre frequents et peuvent survenir partout oula vegetation offre la possibilitede creer des aires d'alimentation privilegiees. (Traduit par la Redaction)

https://www.nrs.fs.fed.us/pubs/jrnl/2008/nrs_2008_royo_003.pdf

Direct and indirect effects of a dense understory on tree seedling recruitment in temperate forests: habitat-mediated predation versus competition

The influence of the spatial pattern of trees on forest floor vegetation and silver fir (Abies alba Mill.) regeneration in uneven-aged forests

Reducing predation of conifer seeds by clear-cutting Rubus fruticosus agg. in two montane forest stands

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Reducing predation of conifer seeds by clear-cutting Rubus fruticosus agg. in two montane forest stands