Norway spruce sapling plasticity in their responses of architecture and growth to light gradient decreases with altitude in subalpine stands

Abstract

In their natural environments, trees are able to act adaptively by changing their morphology, functional traits and growth while facing different factors and stimuli. Yet, there is insufficient information on how intraspecific growth and architectural properties vary along elevation gradients, especially for young saplings of tree species regarded as highly important in forestry. We measured the height, diameter, crown growth and several architectural properties of Norway spruce (Picea abies (L.) H. Karst.) saplings along the light gradient in three elevational transects encompassing natural distributions of subalpine spruce forest in two different locations over a 3-year span. We employed generalized linear and mixed models to determine intraspecific differences in spruce architecture and growth responses to light at different elevations. Light was a better predictor of sapling growth compared to architecture properties. At higher altitudes, we observed limited abilities in the spruce saplings to change their crown architecture and enhance their height, crown length and crown width growth with the improvement in light conditions. However, the response of diameter growth to light was well pronounced in all elevational transects. Our results suggested that spruce sapling plasticity in shaping their crown morphology and growth decreased with altitude. Yet, we reported a similar improvement in diameter growth with an improvement in light conditions regardless of altitude. These results raised the question, to what extent do current climatic conditions at upper elevations inhibit Norway spruce diameter growth. We recommend that forest managers increase their efforts to obtain a desirable stem quality and crown shape by regulating light conditions mainly in the middle and lower parts of subalpine forests rather than at the elevational limit of this species.