Aim
Old‐growth, mesic temperate forests are often assumed to be structured by gap‐phase processes, resulting in quasi‐equilibrial long‐term dynamics. This assumption influences management focused on simulation of natural disturbance dynamics and is embedded in most models of forest successional dynamics. We use multi‐decade monitoring of permanent plots in old‐growth forests to assess demographic assumptions directly with respect to tree mortality rates.
Location
Sixteen sites in mesic, temperate old‐growth forests in eastern North America and Europe with multi‐decade monitoring.
Time period
Variable across sites, spanning c. 20–78 years from 1936 to 2014.
Major taxa studied
Tree species of late‐successional, cool‐temperate forests of Europe and eastern North America.
Methods
We calculated and compared the annualized mortality rates (m), with confidence intervals, by species, size class and measurement interval, for tree species of sufficient abundance.
Results
Retrospective analysis shows dynamic and diverse demographic properties across populations and sites. Stand‐scale mortality rates of 0.7–2.5%/year average higher than previous estimates for old‐growth temperate forests. Variations among species, over time and among size classes, suggest that gap‐phase models are inadequate to explain stand dynamics, implying instead that rare disturbance events of moderate severity have long‐lasting effects in old‐growth forests and that indirect anthropogenic influences affect old‐growth, unlogged forests.
Main conclusions
Multi‐decade baseline data, essential for understanding community assembly and long‐term dynamics in these “slow systems,” are rare and poorly integrated. Our analysis demonstrates the value of the few long‐term, “legacy” data sets. Results suggest that differences in life history interact with complex disturbance histories, resulting in non‐equilibrial dynamics in old‐growth temperate tree communities, and that changes in disturbance patterns through anthropogenic climate change might, therefore, be an important driver of ecosystem change.
& Key message The disturbance of a research plot by a windstorm allowed us to study the role of the seedling bank in the regeneration processes. The released advance regeneration dominated among the saplings; taller individuals retained their position until the end of the study. Pioneer species occurred sporadically. Seven years after the disturbance, the windthrow was covered by a dense thicket of young trees. & Context The dominant role played by advance regeneration in natural regeneration processes after intense wind disturbances is still a matter of dispute. & Aims We took advantage of a windstorm in one of our research plots to study the role of the seedling bank released by the disturbance in the regeneration processes. & Methods We collected data in 70 plots, recording the survivorship of seedlings, annual height growth, and signs of browsing. The height ranking was analyzed with Kendall's concordance coefficient, and the height growth rates were compared using Dunn's test. & Results The density of seedlings increased from 6.7/m 2 in 2008 to 8.1/m 2 in 2010 and then decreased to 1.2/m 2 in 2015. The density of saplings increased continuously from 0.14 to 1.9/m 2. The highest size differentiation occurred in sycamore maple; the individuals which were taller before the windstorm retained their position until the year 2015. The only species that was recruited mainly from germinants was European hornbeam. & Conclusion The advance regeneration released by the windstorm played a major role in the regeneration process, while pioneer species occurred only sporadically. Seven years after the disturbance, the windthrow was already covered by a dense thicket of young trees.
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