Does rainfall explain variation in leaf morphology and physiology among populations of red ironbark (Eucalyptus sideroxylon subsp. tricarpa) grown in a common garden?

We investigated adaptation of leaf morphology and physiology of red ironbark (Eucalyptus sideroxylon Cunn. Ex. Wools subsp. tricarpa L.A.S. Johnson) in a common garden experiment. Fifteen populations, representing a rainfall range of 500 to 1055 mm per annum at the sites of seed collection, were grown at the same site. Because environmental variables other than rainfall did not vary significantly among populations, we were able to test if leaf morphology and physiology were related to seed-source rainfall. There were large differences among and within populations in all measured variables. Most univariate relationships with seed-source rainfall were not significant. Notable exceptions were the weak positive correlation of specific leaf area with seed-source rainfall-consistent with expectations-and the weak negative correlation of photosynthesis and stomatal conductance with seed-source rainfall-the opposite of what we predicted. In many cases, populations collected from sites of similar rainfall differed greatly in leaf morphology and physiology. Principal component analysis (PCA) reduced the 13 input variables to five principal components (PC) explaining 73.0% of the total variance in the original data. Some of the PC axes could be interpreted in terms of adaptation to drought (i.e., to seed-source rainfall), but relationships of accumulated variables (the PC axes) with seed-source rainfall were significant for only one PC axis. Hence, among red ironbark populations grown in a common garden, there was significant genetic variation in leaf morphology and physiology, but for most traits, this variation was unrelated to rainfall at the site of seed collection. This study adds to a growing body of common garden literature showing weak within-species relationships of leaf morphology and physiology with seed-source rainfall, in contrast to the consistently stronger relationships among species growing at different points along broad environmental gradients.

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