Climatic factors of geographic variation in Yucca whipplei Torrey

Cursory comparisons of geographic distributions fail to reveal any obvious causes for the differences of morphology found in various parts of the California range of Yucca whipplei Torrey. The purpose of this study therefore is to determine from statistical, experimental, evolutionary, and spatial relationships the most probable connections between climatological factors and development of three major contrasting morphological lines or Series of Y. whipplei. Geographically, Series 1 represents large parts of Ssps. typica and intermedia; Series 2A is in sestern portions of Ssps. percursa and Parishii; Series 2B corresponds with Ssp. caespitosa and eastern portions of Ssp. percursa. Apparent hybrids between Series 1 and 2A, i.e., Series 3A, or between Series 2A and 2B, i.e., Series 3B, account for large remaining populations. Several numerical and statistical methods were used to establish the most likely genealogical relationships among these Series, using the sample data means of 285 field studies and of an additional 1,000 potgrown seedlings transplanted to a uniform garden for comparisons. Computer studies utilizing plotted morphology diagrams, numerical taxonomy, and statistical analyses were employed to establish the most consistent ordination, or morphological ranking of types, as an hypothetical genealogy of leaf rosette morphology in Y. whipplei. Size of fruiting panicle was directly correlated with size of leaf rosette, but leaf area and volume of the largest rosette of a plant were inversely related to annual effective sunlight energy available in wet growing seasons, and were positively related to rainfall. The same factors that influenced seedling success and different growth habits determined size and duration of leaf rosettes in an independent way. Plant size in turn determined inversely the rate and positively the amount of reproduction sufficient to maintain populations in each of three fundamental types of climate: (1) Relatively Optimal climates having longest and most favorable growing seasons and maximal effective insolation; (2) Adverse climates having shortest favorable growing seasons with minimal effective insolation and most deleterious non-growing seasons often with maximal irradiation; (3) Intermediate climates with moderately long, humid or mesic growing seasons, but with variable seasonal temperatures and limitation of effective insolation due to persistent light screens such as snow, fog, cloud cover, and shade. In Optimal climates primitive pre-series and Series 1 yuccas with thin, narrow leaves of varying length predominated while in Adverse climates only Series 2B or 3B occurred, having leaves of maximal thickness, moderate or large width, respectively, and varying length. Thin- but wide-leaved Series 3A and 2A occurred mostly in Intermediate climates, but in some areas of Intermediate climate, populations of other Series were located instead. Series 3A, 2A, and 3B were in comparatively mesic regions. Rainfall in areas of Series 1, pre-series, and especially Series 2B was sparse, air humidity being crucial to existence of the first two. (See more in text)