The objective of the study was to elucidate the patterns of phenology of growth initiation and cessation among diverse sources of yellow birch (Betula alleghaniensis Britton) and sweet birch (B. lenta L.) along latitudinal and elevational gradients in the Appalachian Mountains. Fruit was collected from parent trees of both species, encompassing a range of 9° of latitude and 3000 ft (914 m) of elevation, and was sown in the greenhouse. The resulting seedling populations were grown in a uniform garden located in southeastern Michigan, where they were analyzed for differences in leaf flushing and growth cessation associated with latitude and elevation of the seed source. Under the conditions of the experiment, northern populations flushed earlier than southern populations. The absence of differences in leaf flushing among populations from different elevations suggests that environmental factors in addition to temperature regulate the initiation of growth. Populations of high latitudes and high elevations ceased growth earlier than populations from low latitudes and low elevations, indicating that populations from shorter growing seasons adapted by ceasing growth earlier. Sixty-seven percent of the variation in the phenology of shoot growth among seedling populations of both species was associated with latitude and elevation of the seed source; the rest was attributed to local variations in topography and resulting microclimates.Observations of parent trees in situ and of half-sib seedling progeny in the uniform garden revealed that individuals of yellow birch exhibit an earlier phenology than individuals of sweet birch from the same field sites. This appears to be an important feature of niche differentiation and a function of the adaptation of yellow birch to colder climates.
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