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Natural hybridization is a relatively common feature of vascular plant species and has been demonstrated to have played an important role in their evolution. Nonetheless, it is not clear whether spontaneous hybridization occurs as a general feature of all plant families and genera or whether certain groups are especially prone to spontaneous hybridization. Therefore, we inspected five modern biosystematic floras to survey the frequency and taxonomic distribution of spontaneous hybrids. We found spontaneous hybridization to be nonrandomly distributed among taxa, concentrated in certain families and certain genera, often at a frequency out of proportion to the size of the family or genus. Most of these groups were primarily outcrossing perennials with reproductive modes that stabilized hybridity such as agamospermy, vegetative spread, or permanent odd polyploidy. These data suggest that certain phylogenetic groups are biologically predisposed for the formation and maintenance of hybrids.

Few empirical data exist to examine the influence of regional scale environmental gradients on productivity patterns of plant species. In this paper we analyzed the productivity of several dominant grass species along two climatic gradients, mean annual precipitation (MAP) and mean annual temperature (MAT), in the Great Plains of the United States. We used climatic data from 296 weather stations, species production data from Natural Resource Conservation Service rangeland surveys and a geographic information system to spatially integrate the data. Both MAP and MAT were significantly related to annual above-ground net primary production (ANPP). MAP explained 54 % to 89 % of the variation in ANPP of two C4 short-grasses, Bouteloua gracilis and Buchloë dactyloides, and two C4 tall-grasses, Andropogon gerardii and Schizachyrium scoparium (= Andropogon scoparius). MAT explained 19 % to 41 % of the variation in ANPP of two C4 grasses, B. gracilis and B. dactyloides, and 41 % to 66 % of the variation in ANPP of two C3 grasses, Agropyron smithii and Stipa comata. ANPP patterns for species along both gradients were described by either linear, negative exponential, logistic, normal or skewed curves. Patterns of absolute ANPP (g/m2) for species differed from those of relative ANPP (%) along the MAP gradient. Responses were similar for species with common functional characteristics (e.g. short-grasses, tall-grasses, C3, C4). Our empirical results support asymmetric responses of species to environmental gradients. Results demonstrate the importance of species attributes, type of environmental gradient and measure of species importance (relative or absolute productivity) in evaluating ecological response patterns.

The concept of magnitude and frequency in geomorphology maintains that the amount of work done on a landscape (the amount of sediment moved) by geomorphic events is determined by the product of the magnitude of the event and the frequency with which that sized event occurs. For many geomorphic systems, it has been found that events of moderate size produce the most sediment transport. This seems not to be the case for aeolian processes on the Southern High Plains. Using meteorological records on visibility reductions caused by blowing dust-a record of the relative amount of dust transported by the wind-at Lubbock, Texas on the Southern High Plains for the period 1947-1989, we find that small magnitude and high frequency events move the most material; that rare, large magnitude events account for considerable transport as well; and, contrary to findings for many other geomorphic systems, moderate events move relatively little sediment. Magnitude and frequency analysis conventionally assumes an identifiable relationship between force applied and the amount of sediment moved. On the Southern High Plains, the soil's resistance to erosion and the vegetation's modification of wind energy are both highly variable in time and space making it unlikely that the sediment transported by two winds of the same speed will be identical. In cases such as these where the resistance to erosion and the effectiveness of the force vary in space and time, magnitude and frequency relations cannot be easily predicted.

The comparative financial performance of Southern Plains and US farm businesses was analyzed for the 1987–1989 period. The results showed no significant differences between the profitability ratios of the two groups of farms, except in the$40,000 to $ 249,999 sales class, where US farms performed at a higher level in two of the three years studied. In both groups, farms with gross sales of $250,000 or more had significantly higher profitability ratios among the three sales classes analyzed. The intergroup or interclass differences in performance were due primarily to the differences in efficiency and/or the leverage position of the farms. © 1993 John Wiley & Sons, Inc.