Susan E. Franson

Wind Dispersal of Artifical Fruits Varying in Mass, Area, and Morphology

We used a field experiment to quantify the independent effects of variation in several fruit characteristics and in wind speed on dispersal of the windblown samaras of the tropical tree, Tachigalia versicolor. On Barro Colorado Island, Panama, we simultaneously released 15 different types of artificial models of these fruits, varying in seed mass, fruit area, and basic morphology, from the top of a 40—m tower, dropping them under five different wind conditions. In general, the horizontal distance a model fruit moves from the base of the tower is proportional to the average wind speed to which it was subject during the whole of its descent, divided by the square root of its wing—loading (wing—loading is the force of gravity on the fruit divided by its projected surface area). This relation was observed for fruits with masses from 1.0 to 1.9 g, with areas from 9 to 76 cm², and with (wing—loading)¹ / ² from 15.3 to 30.6 mPa. Two separate changes in the design of the fruit, but with no change in the wing—loading value from that of T. versicolor, caused the models to have either a more rapid rate of descent despite the appropriate aerodynamic motion, or an improper aerodynamic motion resulting in a pronounced increase in rate of descent. Model fruits that had the lowest wing—loading (achieved by decreasing seed mass or increasing fruit area) or that were blown by the strongest mean winds were dispersed the greatest mean distances and were scattered most evenly and over the widest area. A change in wing—loading or wind speed changed dispersal distance the least and the evenness of the distribution the most. Variation in wind speed has the potential to override the effect of variation in mass or area on dispersal, unless the differences in mass or area among models are quite large.

Bird Species Assemblages as Indicators of Biological Integrity in Great Basin Rangeland

The study evaluates the potential for bird species assemblages to serve as indicators of biological integrity of rangelands in the Great Basin in much the same way that fish and invertebrate assemblages have been used as indicators in aquatic environments. Our approach was to identify metrics of the bird community using relatively simple sampling methods that reflect the degree of rangeland degradation and are consistent over a variety of vegetation types and geographic areas. We conducted the study in three range types (i.e., potential natural plant community types) in each of two widely separated areas of the Great Basin: south-eastern Idaho (sagebrush steppe range types) and west-central Utah (salt-desert shrub range types). Sites were selected in each range type to represent three levels of grazing impact, and in Idaho included sites modified for crested wheatgrass production. Birds were sampled by point counts on 9 100-m radius plots at 250-m spacing on each of 20 sites in each area during the breeding season. In sagebrush-steppe, 964 individuals in 8 species of passerine birds were used in analyses. Five metrics were significantly related to impact class, both when analyzed within range type and when analyzed with all range types combined. Species richness, relative abundance of shrub obligate species, and relative abundance of Brewers sparrow were generally lower for the higher impact classes, whereas the reverse was true for dominance by a single species and for relative abundance of horned larks. In contrast, total number of individuals did not differ significantly as a function of impact class. In salt-desert shrub, a total of 843 birds in 4 species were included in analyses, 98% of which were horned larks. None of the metrics identified above was significantly related to impact class. Two metrics for breeding birds in sagebrush steppe (species richness and dominance) showed little overlap between values for the extremes of impact class, and thus they have potential as indicators of biological integrity. However, the sensitivity of these metrics appears to be greatest at the high impact end of the spectrum, which suggests they may have limited utility in distinguishing between sites having light and moderate impact.

Input of wind–dispersed seeds into light–gaps and forest sites in a Neotropical forest

A total of 52,467 wind-dispersed seeds from 14 tree and 32 liana species fell into 1720 seed traps in 43 paired light-gap and adjacent forest sites on Barro Colorado Island, Panama. Summed at the community level, many more wind-dispersed seeds were collected from light-gaps (61%) than from forest sites (39%). They accumulated from March through May, 1984 to a density of 328 m -2 in gaps and 207 m -2 in forest sites. In contrast, only 33% of the total of 2782 non-wind-dispersed seeds were collected in gaps. Due to the extreme heterogeneity of the seed rain, these differences between gap and forest sites were not statistically significant at the community-level. Gap sites received more wind-dispersed seeds than adjacent forest sites in only 20 of 43 locations and in 13 of 20 species, especially those with individuals of high fecundity near gap sites. Of the estimated 105 million wind-dispersed seeds contributing to the seed rain of the 50 ha study plot, only 4.1% were dispersed to the rare gap sites that enhance the establishment and growth of seedlings for many of these species.

Consequences for seed distributions of intra-crop variation in wing-loading of wind-dispersed species

We used a computer simulation to quantify how intra-crop variation in wing-loading in a wind-dispersed species affects the seed distribution around a parent plant. We used a data set of seed distributions generated from a previous field study using artificial fruits varying in seed mass or fruit area dispersed from a tower into a tropical forest. For this study, the spatial distribution from each hypothetical parents fruit crop of 1000 was calculated by randomly drawing locations of dispersed fruits from the previous data base. Three parents with contrasting fruit crops were used to test two hypotheses: 1) Increasing within-parent variance in wing-loading (= weight/area), while maintaining the mean, will lead to an increase in the area and uniformity of the seed distribution, without changing the mean dispersal distance. 2) Decreasing within-parent mean wing-loading, (which also decreases variance), will lead to an increase in mean dispersal distance, area, and uniformity of the seed distribution. The hypotheses were tested under four wind speeds. Increasing variance in wing-loading resulted in increasing the area and uniformity of density of the seed distribution without changing mean dispersal distance. Decreasing mean and variance in wing-loading resulted in increasing the area and uniformity of density of the seed distribution, as well as increasing the mean dispersal distance. Similar results occurred whether the differences in wing-loading arose by altering seed mass or fruit area. The effect of wind speed was consistently greater than the effect of parent. Generally, the same pattern of parent effects on seed distributions occurred, regardless of wind condition. The effects on seed distributions differed for alterations in mean versus variance, specifically in whether mean dispersal distance was increased. How selection may act on intra-crop mean and variance in wing-loading will depend on additional factors, e.g. the relative importance of distance, area, and density on seedling recruitment and the relative costs for crop size and seedling establishment of making fruit crops of a given mean and variance.

Seed predation and patterns of fruit production in Asclepias syriaca L.

Seed predation in relation to patterns of pod (and seed) distribution was examined in five samples of the common milkweed (Asclepias syriaca L.), in tests of the “resource-concentration” effect both within- and beween-plants. The proportion of pods in a cluster that were attacked, particularly by the weevil Rhyssomatus lineaticollis, was not consistently related to the number of pods contained in a cluster, the frequency of cluster sizes in a population, or the variance of cluster sizes on a stem. Proportions of pods attacked per stem were not correlated with stem density in one sample and were not associated with stem height in any sample. Large clusters of pods produced more undamaged pods and, by this measure, were more successful than smaller clusters, but there was no tendency in any population for stems to produce the more successful cluster sizes more frequently. Stems producing more clusters of pods also produced more undamaged pods. We found no evidence that weevil predation contributed to the regulation of the distribution of pods among clusters on a stem.

Intraspecific variation in seed dispersal of a Neotropical tree and its relationship to fruit and tree traits.

Abstract The distribution of wind‐dispersed seeds around a parent tree depends on diaspore and tree traits, as well as wind conditions and surrounding vegetation. This study of a neotropical canopy tree, Platypodium elegans, explored the extent to which parental variation in diaspore and tree traits explained (1) rate of diaspore descent in still air, (2) distributions of diaspores dispersed from a 40‐m tower in the forest, and (3) natural diaspore distributions around the parent tree. The geometric mean rate of descent in still air among 20 parents was highly correlated with geometric mean wing loading1/2 (r = 0.84). However, diaspore traits and rate of descent predicted less variation in dispersal distance from the tower, although descent rate−1 consistently correlated with dispersal distance. Measured seed shadows, particularly their distribution edges, differed significantly among six parents (DBH range 62–181 cm) and were best fit by six separate anisotropic dispersal kernels and surveyed fecundities. Measured rate of descent and tree traits, combined in a mechanistic seed dispersal model, did not significantly explain variation among parents in natural seed dispersal distances, perhaps due to the limited power to detect effects with only six trees. Seedling and sapling distributions were at a greater mean distance from the parents than seed distributions; saplings were heavily concentrated at far distances. Variation among parents in the distribution tails so critical for recruitment could not be explained by measured diaspore or tree traits with this sample size, and may be determined more by wind patterns and the timing of abscission in relation to wind conditions. Studies of wind dispersal need to devote greater field efforts at recording the “rare” dispersal events that contribute to far dispersal distances, following their consequences, and in understanding the mechanisms that generate them.

Quality assurance considerations for use of the FluorImager SI∖R and FragmeNT Analysis software

The FluorImager SI® (FSI) from Molecular Dynamics is one of several scanning instruments available for the detection of fluorescent emissions associated with DNA samples in a variety of matrices (agarose and polyacrylamide gels, membranes and microplates). In our laboratory, we measured the electrophoretic mobility of randomly amplified polymorphic DNA (RAPD) fragments stained with ethidium bromide in agarose using the FSI to scan gels and the associated Molecular Dynamics software (ImageQuaNT®, and FragmeNT Analysis) for analysis. Initial scans and analyses resulted in inconsistent band detection across the same gel and across several scans of the same gel. To determine the best types of calibration for the instrument, several factors were considered and then evaluated. Tests of calibration acceptability were also evaluated. Band detection by FragmeNT Analysis was improved following optimization of matrices and parameters used in calibration and experimental scans. In addition, use of software templates for analysis and modifications in the staining procedure, which have resulted in decreased instrument associated variance, are discussed.