Differential roles of seed and sprout regeneration in forest diversity and productivity after disturbance

Natural regeneration after disturbances is a key phase of forest development, which determines the trajectory of successional changes in tree species composition and diversity. Regenerating trees can originate from either seeds or sprouts produced by disturbed trees with sprouting ability. Although both regeneration strategies often develop and co-occur after a disturbance, they tend to affect forest development differently due to significant functional differences. However, the origin of tree regeneration is rarely distinguished in post-disturbance forest surveys and ecological studies, and the differential roles of seed and sprout regeneration in forest productivity and diversity remain poorly understood. To address these research gaps, we explored the role of sprout and seed regeneration in the formation of woody species diversity and above-ground biomass (AGB) productivity in early-stage forest development. Data were collected in two experimental forest stands in the Czech Republic, where trees were cut with varying intensities with the density of residual (uncut) trees ranging from 0 to 275 trees per hectare. All trees were mapped and their sizes were measured before cutting and then, either as a stump with sprouts or a residual tree, remeasured 11 years later. In addition, all tree saplings were mapped and measured 11 years after logging, and their origin (sprout or seed) was identified. To assess abundances and productivity, we estimated AGB of all 2,685 sprouting stumps of 19 woody species and 504 generative (i.e., seed origin) individuals of 16 woody species, using allometric equations. Mixed-effects models were used to analyze the effects of each regeneration strategy on woody species diversity and the total AGB under varying densities of residual trees. Nonmetric multidimensional scaling was used to evaluate the effect of regeneration strategies on species composition. AGB and diversity of sprouts were significantly higher than those of seed regeneration. Sprouts formed on average 97.1% of the total regeneration AGB in Hády and 98.6% in Soběšice. The average species richness of sprouts was 4.7 in Hády and 2.2 in Soběšice, while the species richness of seed regeneration averaged 2.1 and 1.1 in Hády and Soběšice, respectively. Increasing density of residual trees reduced AGB and diversity of both sprouts and seed regeneration, but seed regeneration was affected to a greater extent. Residual trees had an especially strong inhibitory effect on the establishment of seed regeneration. Consequently, seed-originated saplings were nearly absent in plots with high residual tree density, and abundant sprouts accounted for most of the AGB and diversity. However, unlike sprouts whose species composition resembled that of the original stand, seed regeneration brought in new species, enriching the stand's overall species pool and beta diversity. Our results demonstrated differential roles of sprout and seed regeneration in the early stage of forest succession. Sprout regeneration was the main source of woody AGB productivity as well as species diversity, and its importance increased with the increasing density of standing mature trees. The results indicate the crucial yet previously underestimated role of sprout regeneration in post-disturbance forest dynamics. They suggest that the presence of residual mature trees, whether retained after partial cutting or undisturbed, can substantially suppress seed regeneration while the role of sprout regeneration in early succession becomes more distinctly evident.