Decaying deadwood as a substrate for tree seedling recruitment

Unless the substrate on which a seed resides is preferable, the germination and successful recruitment of the seedling will likely fail. Compiled with the fact that different species’ seeds require different substrates and conditions to germinate and establish, recognising exactly what constitutes preferable is important. For smaller-sized seeds (such as birch), which will lack nutrients that are internally available with larger seeds (such as acorns), it is typically accepted that a more nutrient-rich and external substrate is necessary. Whilst this nutrient-rich substrate may very well be the soil itself, it may even more often not be the soil but decaying deadwood that, courtesy of decomposers (bacteria, fungi, insects), has had its locked-away nutrient mineralised (made freely available for uptake by other organisms). In this sense, for smaller-sized seeds, a change in seedbed (substrate) and microsite conditions may very likely signal a change in the rate at which such seeds germinate.

The focus of this post is a study that invesitgated how different seedbed conditions impacted upon the rate of seedling recruitment, performance, and morphology of Betula alleghaniensis (yellow birch). The study was undertaken in 2010 within Quebec, Canada, which is part of the species’ native range, and the seedbeds were all located in forest stands dominated by Acer saccharum (sugar maple). As a general rule of thumb, yellow birch prefers nutrient-rich substrates (notably decaying deadwood, though also soil) that have good light availability and are largely leaf litter-free; the same can most probably be said for many other species of the genus Betula, too. Therefore, the main aim of the study was to ascertain whether there was a difference in suitability between these preferable substrates available in such an area of the species’ native range.

Within the overall study location, four areas were selected. All had ‘recently’ been harvested with a selective cutting method, with two areas having been cut in 2004 (new cut) and another two having been cut in 1994 and 1995 (old cut). Sugar maple was the most dominant tree species on all sites, with yellow birch also featuring prominently. Other tree species were present, including (but not exclusively) Acer rubrum (red maple), American beech (Fagus grandiflora), and Thuja occidentalis (eastern white cedar). In each area, yellow birch presence (a total of 1,015 individuals) within the understorey was measured, and all that ranged from 15-330cm in height were recorded. Where each yellow birch was recorded, the seedbed (mineral soils, mosses, deadwood, moss-covered deadwood) and microsite types (pits, stumps, fallen trunks, etc) were also noted, as was the level of canopy ‘openness’. Where a seedling was found upon deadwood, the level of decay (from 1-5) was also recorded, as was from what species the deadwood came. To ascertain performance and morphology, a total of 274 of the yellow birch were also removed from the site, and assessed for their growth increment, their number of branches, and their age. These individuals were also dissected into their roots, stem, branches, and leaves, and each part was weighed after being dried to measure dry-weight biomass. Roots were also measured and segmented into categories associated with their diameter.

ybirchsmaplecanada
A view of the understorey of a stand of predominantly sugar maple and yellow birch in Quebec, Canada. Source: Wikimedia Commons.

Explanations relating to the methodology now over, we can begin to assess the results.

Firstly, it was found that seedbed and microsite types were different between the old and newly cut stands. From this, we can perhaps recognise that harvesting of a site can have an effect upon recruitment of yellow birch seedlings. Looking more into this this difference, the authors found that moss-covered deadwood was the principal substrate upon which yellow birch seedling would grow (46% and 29% of all seedlings in old and newly-cut stands, respectively), though mineral soils would support more seedlings in newly-cut stands (at 41% of all seedlings found, compared to 34% in older cuts) – probably because of less deadwood being available. Curiously, deadwood lacking moss was less often used as a substrate, with only around 20% of seedlings in each stand type growing upon it (below, the upper graph demonstrates these differences). In terms of what microsite was preferred, skid trails were most frequently found to accomodate seedlings in newly-cut stands, and in older stands skid trails were second-best to stumps (below, the lower graph demonstrates microsite differences). However, across both site types, skid trails supported almost equal amounts of seedlings (of which most grew directly within the soil), as did trunks. Perhaps, the suitability of skid trails is clear, as they may very well generally have higher light levels reaching the forest undercanopy.

ybirchseedbed
How the seedbed type (DW – deadwood; MDW – moss-covered deadwood; MS – mineral soil) impacted upon seedling recruitment of yellow birch in newly-cut stands, and those cut some time ago in 1994 and 1995.
ybirchmicrosite
How different microsite types supported different amounts of yellow birch (PM – pits and mounds; S – stump; ST – skid trail; T – trunk; WD – woody debris).

In terms of the age distribution of seedlings across both stands, seedlings in the stands cut in 1994 and 1995 (15-16 years prior to the study) were, on average, 12.6 years old, and for newly-cut standsĀ (cut six years prior to the study) they were 9 years old on average. This in itself is interesting, as it suggests some of the seedlings in the newly-cut stands pre-existed the change in site conditions when harvesting took place (though, of course, where seedlings were found in skid trails they surely could not have pre-existed harvesting). As for the average height of seedlings and how open the canopy was above their location, no marked differences were found between both stand types, though mineral soil did support seedlings of greater height (on average) when compared to deadwood and moss-covered deadwood.

Focussing on those individuals found on deadwood, over 60% were found growing on conifer logs. Because the dominant species in the forest stands were not conifers, this suggests that coniferous species occupying a smaller total population of no more than 20% of the total amount of trees (including eastern white cedar) more markedly supported the regeneration of a species dominant in the stand (yellow birch). This alone is important for forest managers and conservationists. The inverse was actually found with sugar maple, where its deadwood only supported around 10% of all seedlings – this is in spite of it occupying 50% of total the total number of individuals in the stand. Away from the species of deadwood, more heavily-decayed deadwood was found to support more yellow birch seedlings (see below graph).

decayclassybirch
How different decay classes of deadwood (1 being the lowest) supported different numbers of yellow birch seedlings.

Looking at morphological traits of the seedlings, it was found that those seedlings growing within the soil had the greatest growth increments. However, those growing upon deadwood (both covered and not covered in moss) had a higher fine mass root ratio, meaning that their root systems were more developed than those growing in mineral soil of an identical height. Not only this, but leaf area ratio (total leaf area compared to overall seedling dry mass) was highest in seedlings growing on moss-covered deadwood. This suggests that, whilst height growth is reduced, the seedlings have better root systems and more leaf area (aiding in resource uptake and photosythesis, respectively), and may therefore be in a better ‘condition’. After all, height is not the only determinant in the quality of a seedling.

From these results, what can we conclude? Principally, the benefits of heavily-decayed deadwood cannot go ignored. Evidently, deadwood, and particularly moss-covered deadwood (which will typically be more decayed, anyway – higher moisture levels), is critical for the recruitment of yellow birch, and even in spite of it occupying only small to moderate levels of the forest floor. This is likely because decayed deadwood is more readily penetrable by roots, has more nutrients available for uptake, reduced competitin for resources with other plants, and retains higher levels of moisture which seedlings require. In this sense, much akin to how the less plentiful coniferous deadwood is more important for yellow birch recruitment than broadleaved deciduous deadwood (probably because coniferous deadwood decays more slowly, thereby providing a viable substrate for a longer period of time, which enables a seedling to develop a better rooting system and anchor into the mineral soils beneath), deadwood is more important than the more spatially abundant mineral soils. This clearly demonstrates how more ‘niche’ substrates need to be conserved in forest ecosystems, in an attempot to retain ecological integrity. Just because something is found in the greatest abundance does not mean it is the most important thing for the ecosystem – perhaps, even the opposite!

ybirchstump
Here, a yellow birch that started its life growing on the now very heavily-decayed stump, has developed ‘stilts’ from where the roots anchored into the mineral soil beneath and the stump then rotted away. Source: Dr John’s Blog.

In terms of whether harvesting impacts upon seedling recruitment – yes, and no. Of course, the presence of seedlings growing upon stumps was notable (particularly in older stands, where stumps were of course likely to be more significantly decayed), and such stumps would not exist to any marked degree if harvesting operations did not occur. Granted, one must recognise that stumps may have been more routinely used as a substrate because trunks were likely less available, and therefore seedlings may only be using what is available to them. That aside, the authors comment that yellow birch may be less reliant upon harvesting to free-up the canopy and allow more light to penetrate through, and instead grow successfully and retain a constant seedling cohort if suitable substrates and microsites persist (largely, deadwood that is well-decayed – canopy openness seems not to be highly significant). This is because many seedlings in the newly-harvested stands pre-existed the harvesting date (asides from those growing in skid trails, directly within the soil), suggesting it wasn’t the harvest operations that most actively facilitated seedling recruitment levels.

Therefore, the benefits of (well-decayed) deadwood are yet further accentuated. As if there wasn’t already enough justification to retain deadwood, we can now add this study to the ever-expanding arsenal that is amassing at the gates of the forest manager. Without such deadwood accumulation (and notably of coniferous species), the recruitment and performance of yellow birch seedlings may begin to suffer. In time, the entire composition of the forest may begin to alter, if such a suitable substrate becomes lacking. Without doubt, one must allow for a stand to succeed at all times, though there is also a need to conserve and manage with responsibility. For yellow birch, reducing the presence of coniferous species within the stand and not allowing deadwood to accumulate and decay in situ would be an ecological travesty.

Source: Lambert, J., Ameztegui, A., Delagrange, S., & Messier, C. (2015) Birch and conifer deadwood favour early establishment and shade tolerance in yellow birch juveniles growing in sugar maple dominated stands. Canadian Journal of Forest Research. 46 (1). p114-121.

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Decaying deadwood as a substrate for tree seedling recruitment

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