The legacy of disturbance on individual tree and stand-level aboveground biomass accumulation and stocks in primary mountain Picea abies forests

Abstract

Disturbances, both natural and human induced, influence forest dynamics, ecosystem functioning, and ecosystem services. Here, we aim to evaluate the consequences of natural disturbances on the magnitude and dynamics of tree- and stand-level biomass accumulation from decadal to centennial scales. We use tree-ring data from 2301 trees and biometric data from 4909 trees sampled in 96 plots (each 1000m2) to quantify the influence of mixed severity disturbance regimes on annual aboveground biomass increment (AGBI) and total aboveground biomass accumulation (AGB) across a mountainous monotypic Norway spruce (Picea abies (L.) Karst.) primary forest. We hypothesise that the multiple internal and external factors constraining tree growth will cause differences in tree and stand-level biomass trajectories in these natural forests.Although we found that tree-level AGB growth increases with tree size, we also found that tree age and disturbance legacies plays a crucial role for AGB in the investigated Norway spruce forests. Importantly, while younger trees of the same diameter class have an average current AGBI rate that is ∼225% higher than older trees (300–400years), we find trees that have been suppressed for up to 120years can respond vigorously when competition is reduced. On average, post disturbance AGBI was ∼400% greater than pre-disturbance AGBI. Growth of suppressed trees, independent of their age, followed similar trajectories after canopy accession. While aboveground biomass generally increased through time, the time since disturbance and disturbance severity are important co-predictors for stand-level AGBI and AGB. These forests regained most of the above ground living biomass over short interval (∼50years) after low intensity disturbances. The highest stand-level living AGB was observed on plots that experienced >40% canopy removal 160–190years ago, whereas the highest AGBI occurred in plots disturbed recently within the past 40–50years.Our results emphasize the importance of including both individual tree age and disturbance legacies to accurately characterize biomass dynamics and trajectories in forest ecosystems. Importantly, the period of time that a tree is in the canopy, and not tree age, modulates the trajectory of tree level AGBI. Growth rates begin to decline after ∼30years (tree-rings width) and ∼100years (AGBI) in the canopy. We demonstrate that even late-seral forests can rapidly regain biomass lost to low intensity disturbance.