The impacts of ploidy level changes on plant physiology and ecology present interesting avenues of research, and many questions remain unanswered. Here, we examine the connections between cytotype, taxon, stomata characteristics, and environmental variables in black-fruited hawthorns of the Pacific Northwest (Crataegus ser. Douglasianae; Maleae, Amygdaloideae, Rosaceae). We explore the extent to which stomatal measurements can be used to predict ploidy level and how differences in ploidy level and stomata characteristics relate to geographic distributions. We sampled trees from across the geographic ranges of the putative sister taxa Crataegus suksdorfii (Sarg.) Kruschke (diploids and autotriploids) and C. douglasii Lindl. (tetraploids). We found that stomata differed between the two species, with tetraploid C. douglasii having larger average stomata sizes than diploid and triploid C. suksdorfii. We also obtained climatological and elevation data for the sites at which these samples were collected, and examined the associations between taxon, ploidy level, stomatal size and density, elevation, and environmental parameters. Our analyses indicate positive associations between stomatal size and latitude, and between ploidy level and elevation. Negative associations were found between temperature and precipitation variables and both ploidy level and stomatal size, particularly for the fall and winter quarters. There appeared to be no significant association between stomatal density and any of the environmental variables. Tetraploid C. douglasii occupied a wider range of environmental conditions than did either the diploids or the autotriploids.
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