Foliage shedding in deciduous forests lifts up long-distance seed dispersal by wind

Abstract

Seed terminal velocity and release height are recognized as key biotic determinants of long-distance dispersal (LDD) of seeds by wind. Yet, potential determinants at the ecosystem level, such as seasonal dynamics in foliage density characterizing many deciduous forests, have received much less attention. We integrated detailed field observations and experiments with a mechanistic wind dispersal model to assess how seasonal variation in foliage density, estimated by leaf-area index (LAI), affects LDD in deciduous forests. We found that the model, previously shown to accurately predict seed dispersal by wind, also reliably describes the effects of LAI variation on wind statistics for a wide range of canopy types. Sparser canopies are characterized by more organized vertical eddy motion that promotes LDD by uplifting seeds to higher elevations where winds are stronger. Yet, sparser canopies are also characterized by reduced mean windspeed aloft. We showed that former effect more than compensates for the latter, i.e., conditions of low LAI are favorable for LDD. This may account for the tendency of many temperate tree species to restrict seed release to either early spring or late fall, when LAI is relatively low. Sensitivity analysis reveals that the typical seasonal variation in LAI can be more important to LDD of seeds by wind than the natural variation in seed terminal velocity. Because our model accurately describes the effects of LAI variation for distinctly different sites, species, and life forms, we suggest that its results reflect a general association between LDD and foliage density dynamics.canopy turbulence ͉ leaf-area index ͉ mechanistic models ͉ phenology D eciduous trees, which shed leaves seasonally, dominate forest communities in many temperate regions and are also common in tropical and subtropical climates (1). The seasonal phenomenon of leaf shedding has been studied intensively (2-4) but seldom in relation to seed dispersal, another striking seasonal phenomenon of organ abscission in plants (5). The study of seed dispersal, on the other hand, has encompassed multiple aspects related mostly to seed and tree attributes (6-10), but the influence of forest canopy foliage dynamics has rarely been investigated. The two phenomena, however, are not independent, especially for species adapted for wind dispersal. Seasonal changes in forest foliage density, through their effects on wind flow patterns, affect seed dispersal by wind, particularly longdistance dispersal (LDD), and thus bear significant implications for plant population spread, species survival in fragmented landscapes, and gene flow patterns (10, 11).Foliage density is often characterized by the leaf-area index (LAI), the one-sided leaf area per unit ground area (m 2 ⅐m Ϫ2 ). The annual course of LAI variation in deciduous temperate forests typically exhibits a square-wave-like pattern, increasing from near zero values in the late winter to some site-specific (12) peak values in the midst of the growing season during summer, and rapidly drop...

How fragmentation and corridors affect wind dynamics and seed dispersal in open habitats

“…2A). The strongest enhancement of wind speed is at the center of wings and corridors because wind in the patches experiences less drag relative to wind at the same height in the surrounding forest matrix due to the lack of tree-canopy obstacles (25). Although the bellowing effect occurs in both winged and connected patches, it only promotes between-patch movement for connected patches ( Fig.…”

Environmental variability and the initiation of dispersal: turbulence strongly increases seed release

“…On the other hand, detailed modelling studies of seed flight have shown that turbulence may also increase dispersal distances because structured turbulent eddy motions tend to lift seeds to higher elevations with higher horizontal wind speeds (Horn et al 2001;Nathan et al 2002;Soons et al 2004) There is also an interaction between turbulence and wind speed during seed flight as there is a tendency to higher uplifting by autocorrelated eddies in stronger winds and sparser canopies (Nathan et al 2002;Soons et al 2004;Nathan & Katul 2005), and by thermal updrafts that tend to develop only in relatively weak winds (Tackenberg 2003). However, the effect of turbulence on the seed abscission phase was not included in any of these studies.…”

Increases in air temperature can promote wind-driven dispersal and spread of plants

“…The LDD of wind-dispersed seeds and pollen is typically promoted by turbulent vertical fluctuations in wind and by coherency in vertical eddy motion that uplifts seeds well above the vegetation canopy (Nathan et al 2002;Tackenberg 2003;Soons et al 2004;Nathan & Katul 2005;. Turbulent wind conditions can be induced by atmospheric instability that, among other factors, depends on temperature differences between the surface and the atmosphere (Monin & Obukhov 1954), which generates the socalled convective (thermal) turbulence (e.g.…”