Abstract
Stand dynamics and the gap initiation prior to gap formation are not well‐understood because of its long‐term nature and the scarcity of late‐successional stands. Reconstruction of such disturbance is normally based on historical records and dendroecological methods. We investigated gap initiation and formation at the fine‐scale stand level in the old‐growth reserve of Karlshaugen in Norway. Given its long‐term conservation history, and thorough mapping in permanent marked plots with spatially referenced trees, it provides an opportunity to present stand development before, during, and after gap formation. Late‐successional decline in biomass was recorded after more than 50 years of close to steady state. Gaps in the canopy were mainly created by large old trees that had been killed by spruce bark beetles. Snapping by wind was the main reason for treefall. Long‐term dominance of Norway spruce excluded downy birch and Scots pine from the stand. Comparisons of the forest floor soil properties between the gap and nongap area showed significantly higher concentrations of plant available Ca within the gap area. Plant root simulator (PRS™) probes showed significantly higher supply rates for Ca and Mg, but significantly lower K for the gap compared to the nongap area. Soil water from the gap area had significantly higher C:N ratios compared to the nongap area. Fine‐scale variation with increasing distance to logs indicated that CWD is important for leaking of DOC and Ca. Our long‐term study from Karlshaugen documents gap dynamics after more than 50 years of steady state and a multiscale disturbance regime in an old‐growth forest. The observed disturbance dynamic caused higher aboveground and belowground heterogeneity in plots, coarse woody debris, and nutrients. Our study of the nutrient levels of the forest floor suggest that natural gaps of old‐growth forest provide a long‐lasting biogeochemical feedback system particularly with respect to Ca and probably also N. Norway spruce trees near the gap edge responded with high plasticity to reduced competition, showing the importance of the edge zone as hot spots for establishing heterogeneity, but also the potential for carbon sequestration in old‐growth forest.
Highlights
Old-growth forests are characterized by the presence of large trees and complex horizontal and vertical structures and large amounts of woody detritus (Harmon et al, 1986; Lewis & Lindgren, 2000; Oliver & Larson, 1996; Wirth et al, 2009; Messier, Bergeron, Frank, & Fankhanel, 2009)
This indication of steady state is supported by the complete inventory of diameter classes of Norway spruce from the stand in 1930, 1955, 1978, and 1997
We found no significant differences between the gap and nongap area with respect to inorganic nitrogen; neither in soil, soil water, or the PRSTM-probes
Summary
Old-growth forests are characterized by the presence of large trees and complex horizontal and vertical structures and large amounts of woody detritus (Harmon et al, 1986; Lewis & Lindgren, 2000; Oliver & Larson, 1996; Wirth et al, 2009; Messier, Bergeron, Frank, & Fankhanel, 2009). The importance of small-scale gap or patch dynamics in old-growth boreal forests has probably been underestimated compared to large-scale disturbances (Kuuluvainen, Syrjanen, & Kalliola, 1998; McCarthy, 2001). A study of old-growth Norway spruce (Picea abies) on a landscape scale from Archangelsk demonstrated a multiscaled disturbance regime with a combination of small stand-scale gap dynamics, and landscape scale intermediate severity mortality episodes (Kuuluvainen et al, 2014). The presented study is a small-scale case study describing the stand dynamics and patterns along gap formation, but we believe this study reveals generality, as the forest vegetation of Karlshaugen represents the most common vegetation type over large portions of the European boreal forests. We discuss the observed disturbance regime in light of ecological theory and implications for forest management
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