Citation. Davis, M. A.; Ritchie, B.; Graf, N.; Gregg, K. 1995. An experimental study of the effects of shade, conspecific crowding, pocket gophers and surrounding vegetation on survivorship, growth and reproduction in Penstemon grandiflorus. American Midland Naturalist 134:237-243.   [1036  LTER]

Abstract. A variety of mircohabitat changes occur as oak savannas are transformed into woodlands, including an increase in woody canopy shade and a decrease in patches of bare soil, the latter dues to the decline of grassland dwelling and mound building animals such as badgers and pocket gophers. In an oak savanna and woodland in east central Minnesota, Penstemon grandiflorus (Scrophulariaceae) is associated with bare soil patches created by the northern plains pocket gopher, Geomys bursarius, and both species are confined mostly to openings in the woodland and savanna. The purpose of this study was to experimentally manipulate shade and bare soil in order to determine the relative effects of woody canopy shade and surrounding herbaceous vegetation on the survival, growth and reproduction of Penstemon grandiflorus in an experimental garden. During the three year experiment, unshaded plants growing in bare soil had the highest rates of survivorship, growth and reproduction, whereas shaded plants surrounded by herbaceous vegetation had the lowest respective rates. The results indicate a relatively high shade tolerance by P. grandiflorus, and thus the absence of this species from the closed canopy of the oak woodland must be due to factors other than shade. The absence of pocket gophers, G. bursarius, and hence bare soil, in woodlands is likely one of these factors.