D. F. Greene

A Model of Wind Dispersal of Winged or Plumed Seeds

We derived a micrometeorological model for the dispersal of winged or plumed seeds from a point source. The model is based on six measurable parameters: mean release height, mean and standard deviation of the terminal velocities of seeds, standard deviation of vertical wind velocities, and the mean and standard deviation of the natural logarithms of horizontal wind velocities. Predictions of the model include (1) the distri- bution of the dispersal curve (defined as numbers vs. distance from source) is right skewed; (2) the median and long-range dispersal distances need not be well correlated; and (3) increased variance in flight trajectories for a seed population will place the mode of the dispersal curve closer to the source. Empirical tests of the model showed that it adequately characterizes the observed dispersal curves for experimental releases. It is shown that a simple ballistic equation provides a good estimate of the median dispersal distance (and mean distance if the skew of the dispersal curve is slight) for experimental releases. Tests of the model using natural releases from isolated trees indicated that winged seeds do not detach from the parent randomly with respect to horizontal wind velocity. The need to understand the relationship between the probability of detachment and the frequency distribution of horizontal wind velocities is stressed.

Wind Dispersal of Seeds from a Forest Into a Clearing

Although there are many studies of wind dispersal of seeds from a forest into an adjacent clearing, no physical model has yet been advanced. The model constructed here calculates the trajectories of seeds from individual trees in the area source to a line of seed traps (in the clearing) oriented perpendicular to the forest edge. The model uses a log-normal distribution of horizontal wind velocities at a nearby reference station to evaluate wind velocities at any point in the forest and clearing as a function of both height above the ground and distance from the forest edge. The model predicts that (1) the slope of the area source dispersal curve (seed density vs. leeward distance) approximates a negative exponential ; and (2) the great majority of seed deposition in the clearing is contributed by source trees lying within a few tree heights of the forest edge. An evaluation of previously published empirical data shows that the area source model adequately characterizes the decline in seed density with leeward distance.

Estimating the Mean Annual Seed Production of Trees

Intraspecifically, plant seed production is a function of both seed size (mean mass per seed) and plant size. In this paper we examined the interspecific relationship between the size of seeds and plants and the mean long-term annual seed production per tree. For canopy trees, we show that seed production is highly (inversely) correlated with the mean seed mass as a power law argument. Tree size (basal area or leaf mass) is directly proportional to seed production over a limited range. Analysis of seed production for herbaceous plants indicates a relationship similar to that for trees. As the exponent relating seed size to seed production is >-1.0, it follows that large-seeded plants produce more total annual crop mass than do small-seeded species. However, this is balanced by the greater investment in ancillary reproductive tissue by smaller seeded species. The results obtained here ought to be of theoretical and applied interest in, for example, stand-level simulations of population dynamics or for planning the optimal size and shape of clear- cuts intended to be regenerated naturally.

Seed mass and dispersal capacity in wind-dispersed diaspores

Is there necessarily a trade-off between seed size (mass) and dispersal capacity for wind-dispersed diaspores? Within three families (Pinaceae, Aceraceae, and Leguminosae) with asymmetric samaras, shape is maintained (isometry) despite size change. Consequently, within these three families, equilibrium descent velocity is proportional to samara mass raised to the 1/6 power and, necessarily, larger samaras are more poorly dispersed

The reduction of organic-layer depth by wildfire in the North American boreal forest and its effect on tree recruitment by seed

We compared prefire and postfire organic-layer depths in boreal forest types (14 fires) across Canada, and examined tree recruitment as a function of depth. There was extensive within-stand variation in depth, much of it due to clustering of thinner organic layers around boles. There were no significant differences in postfire organic-layer depth among sites with different prefire forest species composition, but sites in the eastern boreal region had thicker postfire organic layers than those in the western boreal region. Mean organic-layer depth was much greater in intact stands than after fires; overall, fire reduced organic-layer depth by 60%, largely because of increases in the area of thin (<3 cm) organic layers (1% in intact stands vs. 40% in postfire stands). There was more variation in organic-layer depth within postfire than within prefire stands; notably, some areas in postfire stands were deeply combusted, while adjacent parts were only lightly combusted. We speculate that the diminished role of energy loss to latent heat around tree boles increased organic-layer consumption around tree boles. Seedlings were clustered around burned tree bases, where organic layers were thinner, and the dependence of a species on thin organic layers was an inverse function of seed size.

Secondary Dispersal of Tree Seeds on Snow

1 We studied secondary seed dispersal on snow using a wind tunnel, experimental releases in clearings, and observations of the dispersal curve of an isolated Betula alleghaniensis in a large clearing. We derived a model for wind dispersal which couples the primary and secondary movement for a point source, and then used this to modify an area source model dispersal curve. 2 The wind tunnel results showed that the probability of entrainment of the seed was directly proportional to wind speed and inversely proportional to the square root of wing loading. Field releases showed that secondary distance traveled was, in turn, proportional to the probability of entrainment and to travel time. 3 The point source model overestimated seed density in the distance interval of rising entrainment probability and underestimated seed density at subsequent distances. The area source model adequately predicted dispersal for autumn vs. winter fractions of a B. alleghaniensis stand dispersing seeds into a clearing. The isolated B. alleghaniensis indicated that travel time was 10 h. 4 According to the model, secondary dispersal for trees on snow is inconsequential when the fraction of seeds that abscise in winter is < 0.15 (conifers and most angiosperms). However, it will have an important role for genera such as Betula which have large winter-abscising crop fractions. 5 Further, we note that the relative importance of secondary dispersal is almost solely an inverse function of release height, and therefore for very small statured plants (e.g. herbs), secondary dispersal should display a dramatic effect even if only a small portion of the crop is involved. Finally, we argue that wind speeds near the surface within a forest are so low that the effect of secondary dispersal within forests will be minor.

Can the Variation in Samara Mass and Terminal Velocity on an Individual Plant Affect the Distribution of Dispersal Distances

We examined the contribution of seed mass variation to (1) variation in terminal velocity and (2) variation in dispersal distances for species with winged seeds. Coefficients of variation of terminal velocity were usually on the order of 0.13. These values were too low to have a significant effect on distance variation because of the much larger magnitude of the turbulent intensities of the wind in the vertical and horizontal directions. This assertion was tested with a micrometeorological model of dispersal from an elevated point source within a forest canopy. The coefficient of variation of terminal velocity was necessarily low because of (1) the aerodynamic specification that terminal velocity be proportional to the square root of mass and (2) the design problems inherent in a negative correlation between mass and planform area. We conclude that there is unlikely to be selection for mass variation to reduce sibling densities and therefore intersib competition.

The aerodynamics of plumed seeds.

The aerodynamics of plumed seeds are examined using four species of Compositae and Asclepias syriaca. The hairs comprising the plume of the seeds are modelled as a single long cylinder experiencing an ambient wind velocity equivalent to the measured terminal velocity in still air

Particulate diffusion models and the dispersal of seeds by the wind

David F. Greene and Edward A. Johnson SEED DISPERSAL has long been of interest in biogeography, forestry, succession studies and population genetics. More recently, dispersal has figured in analytical models of plant species coexistence’ and in a renewed interest in immigration in population ecology (‘supply-side ecology’)2. Even so, we still know little about the process of seed dispersal. The long tradition of descriptive studies has sought merely to measure the dispersal curve dQ/dx (where Q is the number of seeds shed and x is the distance from the source). At best, this descriptive tradition has spawned empirical formulations, such as power-law functions, valid only for a given place and time3. A concurrent tradition was initiated by the use of mustard gas in World War I: the study of the diffusion of particles and aerosols in turbulent flow. Micrometeorological models have repeatedly demonstrated both experimentally and analytically that the dispersal curve for an elevated point source emitting particles with appreciable terminal velocities will be skewed to the right4. Thus, x,,, < xo.5 < X, where the subscripts m and 0.5 denote the modal and median deposition distance respectively, and X is the mean deposition distance. Remarkably, the descriptive tradition in dispersal ecology has almost completely ignored the analytical models. The one aspect of the micrometeorological models that ecologists have seized upon is the simple ballistic expectation for the median dispersal distance from a point source:

Non-serotinous woody plants behave as aerial seed bank species when a late-summer wildfire coincides with a mast year

Abstract Trees which lack obvious fire-adaptive traits such as serotinous seed-bearing structures or vegetative resprouting are assumed to be at a dramatic disadvantage in recolonization via sexual recruitment after fire, because seed dispersal is invariably quite constrained. We propose an alternative strategy in masting tree species with woody cones or cone-like structures: that the large clusters of woody tissue in a mast year will sufficiently impede heat transfer that a small fraction of seeds can survive the flaming front passage; in a mast year, this small fraction would be a very large absolute number. In Kootenay National Park in British Columbia, we examined regeneration by Engelmann spruce (Picea engelmannii), a non-serotinous conifer, after two fires, both of which coincided with mast years. Coupling models of seed survivorship within cones and seed maturation schedule to a spatially realistic recruitment model, we show that (1) the spatial pattern of seedlings on a 630 m transect from the forest edge into the burn was best explained if there was in situ seed dissemination by burnt trees; (2) in areas several hundred meters from any living trees, recruitment density was well correlated with local prefire cone density; and (3) spruce was responding exactly like its serotinous codominant, lodgepole pine (Pinus contorta). We conclude that non-serotinous species can indeed behave like aerial seed bank species in mast years if the fire takes place late in the seed maturation period. Using the example of the circumpolar boreal forest, while the joint probability of a mast year and a late-season fire will make this type of event rare (we estimate P = 0.1), nonetheless, it would permit a species lacking obvious fire-adapted traits to occasionally establish a widespread and abundant cohort on a large part of the landscape.