Quantifying distance‐independent tree competition for predicting Norway spruce mortality in unmanaged forests

The competition environments of trees in pristine forests and managed single‐cohort systems differ substantially. Unconventional forest management or conservation planning for Norway spruce (Picea abies (L.) Karst) forests is currently impaired due to a lack of quantitative data on what factors drive its mortality. We collected data from 57 unmanaged Norway spruce stands in Finland to estimate factors influencing the competition‐induced mortality of spruce. We investigated how a simple competition index (CI) should be formulated in order to predict spruce mortality in these forests. According to our measurements and the competition index, competition among spruce was clearly one‐sided, meaning that smaller trees did not affect the mortality probabilities of larger trees. In the best performing CI, which was based on competitor $\sqrt{dbh}$ , competitors were approximately 4 cm larger in dbh than the subject trees before they began to affect subject tree mortality. A nearly size‐invariant threshold suggests that smaller spruces tolerate their larger competitors relatively better, when competitor effect is proportional to their $\sqrt{dbh}$ . In contrast, the CI based on competitor basal areas (∝dbh ²) predicted tree mortality poorly, and the threshold was size‐dependent. Our study suggests that a simple competition index based on a constant dbh‐threshold and a sum of the square roots of competitors’dbh is used to predict spruce mortality in unmanaged spruce forests. This index seems to capture both the competition asymmetry and the slow mortality of suppressed trees frequently observed in forests. A simple competition index for predicting spruce mortality in unmanaged forests can easily be integrated to many forest growth models to support modeling of mortality and conservation value of Norway spruce stands.