Effects of coarse woody debris on plant and lichen species composition in boreal forests

Abstract

Question Although the importance of coarse woody debris (CWD) for understorey species diversity has been recognized, the relative effects of coarse woody debris decay class and substrate species on understorey species composition have received little attention. We examined how the species composition of understorey vegetation change with CWD decay class and substrate species. Location Boreal mixed‐wood forests, Ontario, Canada. Methods To cover a wide range of CWD decay classes and substrate species, we sampled fire‐origin boreal forest stands that varied in stand age and canopy tree species composition. Vegetation on CWD was sampled by visually estimating percentage cover of each species within a 0.1 m × 0.5 m quadrat, randomly laid lengthwise on top of each sampled CWD log. We also recorded the forest floor vegetation by establishing an adjacent plot of the same size at a distance of 1.0 m in a random direction from the CWD vegetation sample. Results Multivariate analysis showed that understorey species composition differed among decay classes and substrate species. A NMDS ordination of understorey species composition revealed a clear separation of decay classes 1 and 2 from higher decay classes, and that decay classes 4 and 5 shared several species with the forest floor. The species composition on the forest floor was completely different from the species composition on CWD decay classes 1, 2 and 3. Two distinct groupings of substrates according to CWD species composition were found: conifer species (Pinus banksiana and Picea spp.) and broad‐leaf species (Betula papyrifera and Populus spp.), with Abies balsamea taking an intermediate position. Indicator species analysis showed distinct understorey species affiliations to substrate species at advanced decay classes. Understorey species composition on the CWD of P. banksiana showed particularly pronounced changes from the dominance of lichens on decay classes 2 and 3 to dominance by mosses and vascular species on decay classes 4 and 5. Conclusions Understorey species composition on CWD not only differed with decay class, but also with CWD substrate species. Conservation strategies should aim at retaining diversity of CWD in terms of both decay classes and species composition in boreal forests.

Linking resource availability and heterogeneity to understorey species diversity through succession in boreal forest of Canada

“…We found that substrate diversity was positively associated with stand age, which indirectly increased the overall understorey species richness but not cover. This result is consistent with increases in microsite heterogeneity in the form of tip-up mounds and coarse woody debris with increasing stand age (Bartels & Chen, 2010;Brassard & Chen, 2006;Kumar et al, 2017b;Mills & Macdonald, 2004) Messier, & Canham, 2006;Macdonald & Fenniak, 2007), and are generally more productive than conifer or broadleaf stands (Zhang, Chen, & Reich, 2012;Zhang et al, 2017). Enhanced productivity is expected to result in accelerated stand development and self-thinning; this is expected to enhanced substrate diversity in the form of coarse woody debris and exposed mineral soil in productive or mixed-wood stands (e.g.…”

“…Coarse woody debris is the most abundant source of substrate heterogeneity in this system; in related work, we found that early decayclass woody debris showed much lower total vegetation cover than did the forest floor (Kumar et al, 2017b).…”

“…Here, SEM models detected a positive effect of variation in light levels for vascular and non‐vascular plant cover, but microsite variability was excluded as a predictor. Coarse woody debris is the most abundant source of substrate heterogeneity in this system; in related work, we found that early decay‐class woody debris showed much lower total vegetation cover than did the forest floor (Kumar et al., ).…”

“…The decrease in light availability is most rapid after closure of the initial post-fire cohort of shadeintolerant hardwoods and pines, but commonly continues as shadetolerant conifers become increasingly dominant in the overstorey at late successional stages. On the other hand, as stands further mature, substrate heterogeneity, in the form of micro-topographic tip-up mounds and coarse woody debris (CWD) of different decay stages increases (Bartels & Chen, 2010;Beatty, 1984;Chen & Popadiouk, 2002;Kumar, Chen, Thomas, & Shahi, 2017b), which might result in higher cover and species diversity of non-vascular plants. Similarly, soil nitrogen, a key limiting nutrient in fire-driven boreal forests, may also increase with stand development ages due to increased nitrogen fixation (Akaike, 1974;Hume, Chen, Taylor, Kayahara, & Man, 2016;Zackrisson, DeLuca, Nilsson, Sellstedt, & Berglund, 2004), and litter production (Chen, Brant, Seedre, Brassard, & Taylor, 2017), with the predominant form of nitrogen changing from NO 3 − to NH 4 + (DeLuca, Nilsson, & Zackrisson, 2002).…”

Rapid legacy‐dependent succession of lichen assemblages after forest fires: Insights from two boreal regions

“…Indeed, shade‐preferring species and species with crustose growth form have been characterized as fire‐vulnerable (Johansson et al., ), and at the tree scale, microlichens have a slightly slower initial post‐fire recovery compared to macrolichens (Hämäläinen, Kouki, & Lõhmus, ). The positive impact of logs in old burns can be linked with the supply of specific microhabitats (Kumar, Chen, Thomas, & Shahi, ). Macrolichens benefit most strongly, in particular this is seen for Cladonia species that are typical inhabitants of downed, decayed wood (Botting & DeLong, ; Söderström, ).…”