Adaptation of Trees to Low-Light Environments: Effect on Branching Pattern of Fraxinus americana

Abstract

Newly planted trees are often moved from the full sun environment of the nursery to shady locations around buildings or in urban canyons. Trees must adapt in order to survive. An adaptive strategy for trees exposed to changing irradiance levels in forest settings is the ability to change branching habit to maximize whole tree photosynthesis. The genetic ability to alter branching habit has potential adaptive value for survival in the low-light environment found in many urban landscapes. The ability to alter branching habit when growing in shady versed bright habitats was tested on Fraxinus amerícana (white ash) trees by measuring branching habit, as expressed by bifurcation ratio, on native stands of trees growing in a sunny old-field habitat and an adjacent shady understory habitat. Bifurcation ratios were similar for trees growing in both habitats, indicating an inability of white ash to change basic branching pattern when exposed to different irradiance environments. Other possible adaptive mechanisms and implications for planting in shady environments are discussed.