The Past Matters: Previous Management Strategies Modulate Current Growth and Drought Responses of Norway Spruce (Picea abies H. Karst.)

Abstract

Due to high productivity and past management approaches, the forests of Central Europe are heavily dominated by conifers, even on sites where they do not occur naturally at all. One prominent example is Norway spruce (Picea abies H. Karst.), a species considered particularly vulnerable to severe droughts, especially outside of its ecological niche where it has been widely planted over the past centuries. In the face of global change, it is a major task for foresters to increase these forests’ ability to cope with the impacts of increasing climatic extremes. Therefore, gaining more knowledge about how different management strategies affect the drought responses of trees is crucial. However, we still know little about the influence of the individual treatment history of a tree on its growth. We used a dendroecological approach to address this issue and to assess how initial spacing, structural diversity, tree size, and density regulation approaches modulate annual growth, especially in drought years. We hypothesized that stand establishment and past silvicultural treatment codetermine tree growth and drought resilience. Our study took place at the combined spacing-thinning trial Fürstenfeldbruck 612 (FFB 612) in Southern Germany, since it delivered precise long-term data covering a broad range of treatments. Based on linear mixed effect models, we showed that the individual treatment history of a tree affects its annual growth and drought responses considerably. In more detail, we found that (i) high structural diversity in the vicinity of each tree favored growth and improved a tree’s performance under drought; (ii) larger trees were more climate-sensitive; (iii) previous high variations in thinning intensity, and consequently strong fluctuations in growth, affected current growth negatively and reduced recovery from droughts. Furthermore, we sought to understand the underlying mechanisms and to draw potential implications for refining silvicultural guidelines.