Heart-leaved birch (Betula cordifolia) and yellow birch (B. alleghaniensis) are commonly found growing during early and midsuccession after disturbance in northeastern forests. Betula cordifolia most often occurs at high elevations (above 600 m) and is considered less shade tolerant than B. alleghaniensis. Though shade-tolerance affects distribution of both birches, other physiological differences may also influence the success of each of these species in forest communities. In particular, on boulder-covered depositional zones of landslides common to steep slopes of the Appalachians, water relations may be important for birch survival. In Franconia Notch, NH, a suite of landslides occurred during this century; we investigated water relations of pairs of B. cordifolia and B. alleghaniensis trees rooted within two meters of one another on two of these landslides. Midday water potential of B. alleghaniensis was often significantly more positive (and never more negative) than that of B. cordifolia, whether trees were growing in a soil-rich lower depositional zone of a 1959 slide or on a boulder-covered steep section of an overlapping 1948 slide. Also, the magnitude of differences in water potential between the two birch species varied through the season and between sites. The hydraulic conductivity of stems of the two species was indistinguishable, but predawn water potential measurements indicated that B. cordifolia was rooted in zones with lower water availability. This differential rooting may contribute to differential water stress between young, co-occurring birches in the successional forest environment.
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Vol. 143 • No. 2