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The severity and spatial extent of bark-beetle outbreaks substantially increased in recent decades worldwide. The ongoing controversy about natural forest recovery after these outbreaks highlights the need for individual-based long-term studies, which disentangle processes driving forest regeneration. However, such studies have been lacking. To fill this gap, we followed the fates of 2,552 individual seedlings for 12 years after a large-scale bark-beetle outbreak that caused complete canopy dieback in mountain Norway spruce (Picea abies) forests in southeast Germany. We explore the contribution of advance, disturbance-related, and post-disturbance regeneration to forest recovery. Most seedlings originated directly within the three-year dieback of canopy trees induced by bark-beetle outbreak. After complete canopy dieback, the establishment of new seedlings was minimal. Surprisingly, advance regeneration formed only a minor part of all regeneration. However, because it had the highest survival rate, its importance increased over time. The most important factor influencing the survival of seedlings after disturbance was their height. Survival was further modified by microsite: seedlings established on dead wood survived best, whereas almost all seedlings surrounded by graminoids died. For 5 cm tall seedlings, annual mortality ranged from 20 to 50% according to the rooting microsite. However, for seedlings taller than 50 cm, annual mortality was below 5% at all microsites. While microsite modified seedling mortality, it did not affect seedling height growth. A model of regeneration dynamics based on short-term observations accurately predicts regeneration height growth, but substantially underestimates mortality rate, thus predicting more surviving seedlings than were observed. We found that P. abies forests were able to regenerate naturally even after severe bark-beetle outbreaks owing to advance and particularly disturbance-related regeneration. This, together with microsite-specific mortality, yields structurally and spatially diverse forests. Our study thus highlights the so far unrecognized importance of disturbance-related regeneration for stand recovery after bark-beetle outbreaks.
The severity and spatial extent of bark-beetle outbreaks substantially increased in recent decades worldwide. The ongoing controversy about natural forest recovery after these outbreaks highlights the need for individual-based long-term studies, which disentangle processes driving forest regeneration. However, such studies have been lacking. To fill this gap, we followed the fates of 2,552 individual seedlings for 12 years after a large-scale bark-beetle outbreak that caused complete canopy dieback in mountain Norway spruce (Picea abies) forests in southeast Germany. We explore the contribution of advance, disturbance-related, and post-disturbance regeneration to forest recovery. Most seedlings originated directly within the three-year dieback of canopy trees induced by bark-beetle outbreak. After complete canopy dieback, the establishment of new seedlings was minimal. Surprisingly, advance regeneration formed only a minor part of all regeneration. However, because it had the highest survival rate, its importance increased over time. The most important factor influencing the survival of seedlings after disturbance was their height. Survival was further modified by microsite: seedlings established on dead wood survived best, whereas almost all seedlings surrounded by graminoids died. For 5 cm tall seedlings, annual mortality ranged from 20 to 50% according to the rooting microsite. However, for seedlings taller than 50 cm, annual mortality was below 5% at all microsites. While microsite modified seedling mortality, it did not affect seedling height growth. A model of regeneration dynamics based on short-term observations accurately predicts regeneration height growth, but substantially underestimates mortality rate, thus predicting more surviving seedlings than were observed. We found that P. abies forests were able to regenerate naturally even after severe bark-beetle outbreaks owing to advance and particularly disturbance-related regeneration. This, together with microsite-specific mortality, yields structurally and spatially diverse forests. Our study thus highlights the so far unrecognized importance of disturbance-related regeneration for stand recovery after bark-beetle outbreaks.
Wood decay fungi alter the abiotic and biotic properties of deadwood, which are important as nurse logs for seedling regeneration. However, the relationship between fungal decay type and seedling performance has not been evaluated experimentally. In this study, we examined the germination, growth, and survival of six arbuscular mycorrhizal (AM) and six ectomycorrhizal (ECM) tree species on three substrates (pine logs with brown and white rot and soil) by conducting seed‐sowing experiments in a mixed forest dominated by Pinus densiflora and Quercus serrata. Analysis using ribosomal DNA internal transcribed spacer 1 (rDNA ITS1) sequencing revealed that the fungal community was significantly different across three substrates. The richness of operational taxonomic units (OTUs) of AM and ECM fungi was the largest on brown rot logs and soil, respectively. The substrate significantly affected the seedling performance when comparing wood decay types, but these were not consistent across the mycorrhizal status of the seedlings. Nevertheless, seedlings of some AM trees showed better growth and enhanced mycorrhizal colonization on brown rot logs than on white rot logs. The wood decay type influenced fungal communities in the logs and the performance of some seedling species partly by different mycorrhizal colonization rates. However, the effect was seedling species dependent and showed no apparent difference between AM and ECM trees.
Smaller‐seeded tree species often require mineral soil and coarse woody debris (CWD) to establish. Limited data suggest that species‐based variation in tree seedling–CWD relations exists; however, the generalizability of this pattern and its mechanistic basis remain unknown. We investigated interspecific substrate–seedling relations and the potential that differences in mycorrhizal status and substrate nutrition underlie these patterns with a potted plant experiment. Eight temperate tree species were grown in a lath shade house on six species of CWD and mineral soil that had either been sterilized with gamma irradiation or left untreated to separate the effects of mycorrhizae from substrate nutrition. We assessed the impact of substrate identity on height growth and investigated possible mechanisms via relating growth to substrate inorganic N concentration, seedling foliar N (%), and mycorrhizal colonization. Seedling height varied across substrates consistently among species (i.e., substrate effects were strong, but seedling species x substrate interactions were weak). Seedlings were generally tallest on mineral soil followed by Betula papyrifera and Thuja occidentalis CWD, and shortest on B. alleghaniensis or Acer saccharum CWD. Sterilization main effects on height were not significant; however, the response of species to sterilization varied, as Acer rubrum and T. occidentalis seedlings were significantly shorter on sterilized substrate. For all seedling species, growth correlated positively with substrate [N] and foliar N. Among the four species examined for mycorrhizae, root colonization density was highest on mineral soil and lowest on A. saccharum and B. alleghaniensis CWD). For these species, mean colonization density was more strongly associated with mean seedling height than substrate inorganic [N]. Beneficial mycorrhizal effects were suggested by positive height–mycorrhizal root density relationships for B. alleghaniensis, A. rubrum, and T. occidentalis. Collectively, for the group of northern hardwood tree species examined in this study, these results demonstrate that in a controlled environment, most species grow best on mineral soil, B. alleghaniensis and Picea glauca attained maximum growth on CWD, species‐specific growth responses occur on CWD, and that substrate nutrient availability and mycorrhizal fungi contribute to the variation observed in seedling growth response across substrates.
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