Differential shading of branches or whole trees: survival, growth, and reproduction.

This study compared the effects of shading individual branches or whole trees on the survival, growth and reproduction of branches of mountain birch [Betula pubescens ssp. czerepanovii (Orlova) Hämet-Ahti]. When a single branch was shaded its performance decreased drastically, while individual branches performed relatively well if the whole tree was shaded. For instance, after two summers 90% of the individually shaded branches were dead, while none of the study branches died following shading of entire trees. These results indicate that the fate of a branch depended more on shading-induced asymmetric competition among branches than on the shading itself. Furthermore, the increased growth of non-shaded branches in the trees where only two branches were shaded suggests that resources were preferentially allocated to branches in more favorable positions. Branch autonomy may promote environmental screening and growth towards favorable directions, optimizing the use of available light resources. In addition, branch autonomy may buffer the genet against environmental hazards, thus increasing the fitness of the genet at the cost of an increased mortality for individual modules.