Microsite occupancy and the spatial structure of understorey regeneration in three late-successional Norway spruce forests in northern Europe

Abstract

We compared microsite occupancy and three spatial structure of regeneration in three primeval late-successional Norway spruce dominated forests. One area lay in the middle boreal zone in Russia (Dvina-Pinega) where larger-scale disturbance from bark beetles and drought had occurred; the other areas lay in the northern boreal zone, one in Finland (Pallas-Ylläs) had encountered only small-scale disturbance, and one in Russia (Kazkim) had been influenced by fire. We mapped all spruce (Picea abies) and birch (Betula pendula and Betula pubescens) trees with diameter at breast height (DBH) ≥ 10 cm on 40 m × 400 m plots, and those with DBH < 10 cm on 2 m or 4 m × 400 m subplots. On the subplots we also recorded microsite occupancy and estimated microsite availability. At all study areas small seedlings (h < 0.3 m) of both spruce and birch were found largely on disturbance-related microsites. Birch saplings (h ≥ 1.3 m, DBH < 10 cm) disproportionately occupied deadwood-related microsites at Dvina-Pinega. In contrast, spruce saplings at all study areas, and birch saplings at Kazkim and Pallas-Ylläs, showed less, or no, preference. Our results thus confirm the importance of disturbance-related microsites for regeneration establishment, but not necessarily for longterm survival. No spatial segregation between the overstorey (DBH ≥ 10 cm) and saplings (h ≥ 1.3 m, DBH < 10 cm) or seedlings (h < 1.3 m) was found at Pallas-Ylläs or Kazkim, and only three instances of very weak segregation were found at Dvina-Pinega. This suggests that the regeneration gap concept may not be useful for describing the regeneration dynamics of primeval boreal forests.

Spatial tree community structure in three stands across a forest succession gradient in northern boreal Fennoscandia

“…This clumping of young trees was similar to results of Grenfell et al (2011) in the same forest type in northwestern Russia and northwestern Finland. A potential explanation for these findings is the availability of regeneration microsites: as the thick humus layer outside the disturbed microsites is a poor seedbed (Johnstone and Chapin 2006), sites with a disturbed raw humus layer, highly decayed logs as well as the crown periphery around large trees can function as important regeneration microsites (Aaltonen 1919;Sirén 1955;Grenfell et al 2011). High regeneration rates per unit area on these disturbed microsites results in clumped patterns in regeneration (Manabe et al 2000).…”

“…regeneration from seeds in these forests tends to require disturbed microsites, such as exposed mineral soil or decayed logs. Although the two species appear to have some separation in their regeneration niches (Grenfell et al 2011), Betula spp. regeneration in these forests faces an additional constraint in the form of intensive browsing by reindeer.…”

Natural tree regeneration and vegetation dynamics across harvest gaps in Norway spruce dominated forests in southern Finland

“…From small to moderate covers, 367 microsites of covered stones promoted higher than average Norway spruce seedling densities, 368 but this effect became negative with greater coverage. Both young and decaying stumps/logs 369 were favorable microsites for birch seedlings, as found in previous studies by Liu and Hytteborn 370 (1991), Grenfell et al (2011), andRobert et al (2012). However, in this study the effect was 371 unimodal, peaking at 10-15 % cover, then waning with greater coverage.…”

Life and death of Picea abies after bark‐beetle outbreak: ecological processes driving seedling recruitment

“…However, the regeneration process after bark-beetle outbreaks has been investigated only through studies that did not follow individual seedlings over time (Jonášová and Prach 2004, DeRose and Long 2010, Diskin et al 2011, Zeppenfeld et al 2015. For instance, clumped spatial pattern of spruce seedlings was repeatedly observed, but the processes responsible for the formation of such a pattern remain unclear (Grenfell et al 2011, Wild et al 2014. The resulting snapshot data covering only a subset of regeneration can easily provide biased results, as it is extremely difficult to infer actual processes behind the observed static patterns (Wiegand et al 2003).…”