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Regressions of leaf width as a function of year with latitude as covariate: (a) bootstrapped coefficients (points) and 95% CI (bars): ‘pre’ (1880–1950), ‘post’ (1950–2011), ‘all’ (1880–2011), ‘north’ (1950–2011, northernmost degree of latitude only), ‘mid’ (1920–1980), ‘other’ (1920–2011); (b) pre-1950 (filled circles and solid line) versus post-1990 (open circles and dashed line) as categorical predictor. Crosses indicate new data.
