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

This study compared the effects of shading individual branches or whole tre es on the survival, growth and reproduction of branches of mountain birch [ Betula pubescens ssp. czerepanovii (Orlova) Hamet-Ahti]. When a single bran ch was shaded its performance decreased drastically, while individual branc hes performed relatively well if the whole tree was shaded. For instance, a fter two summers 90% of the individually shaded branches were dead, while n one of the study branches died following shading of entire trees. These res ults indicate that the fate of a branch depended more on shading-induced as ymmetric competition among branches than on the shading itself. Furthermore , the increased growth of non-shaded branches in the trees where only two b ranches were shaded suggests that resources were preferentially allocated t o branches in more favorable positions. Branch autonomy may promote environ mental screening and growth towards favorable directions, optimizing the us e of available light resources. In addition, branch autonomy may buffer the genet against environmental hazards, thus increasing the fitness of the ge net at the cost of an increased mortality for individual modules.