Substrate type
In a total of 166 plots, tree seedlings were found in 50% of our plots. Of those 166 plots, 52 were on the forest floor and 114 were on nurse logs; 36% of the forest floor plots had tree seedlings whereas 57% of the nurse log plots had tree seedlings. Tsuga heterophylla was the dominant tree species found as seedlings (87% of all seedlings found). On nurse logs, we found a total of 18 different non-vascular species. Of these 18, there were 16 mosses, one liverwort, and one lichen (Table S2). We found only six different bryophyte species on the forest floor (Table S2).
Tree seedling density was significantly influenced by substrate type, as average tree seedling density on nurse logs (20.1 ± 3.6) was 4.6x greater than on the forest floor (4.5 ± 1.8; W = 2027.5, P< 0.001). Substrate type significantly influenced moss depth (F 3,93 = 6.6, P < 0.001, Fig. 1B), and the percent cover of H. splendens(F 3,94 = 17.8, P < 0.001, Fig. 1E). No other factor was significantly influenced by substrate type (Fig. 1).
Decay class was significantly positively associated with moss depth and the percent cover of Hylocomium splendens (Table 1). Moss depth was positively correlated with the percent cover of H. splendensand Rhytidiadelphus loreus and negatively correlated with the percent cover of Rhizomnium glabrescens (Table 1). Percent cover of H. splendens was negatively correlated with the percent cover of Kindbergia praelonga . Finally, the percent canopy cover was positively correlated with the percent cover of Antitrichia curtipendula (Table 1).
The GLM model that best predicted tree seedling density included substrate type, percent canopy cover as a positive factor, and percent cover of Hylocomium splendens as a negative factor (Table 2). Interestingly, moss depth was a negative factor and percent cover ofRhizomnium glabrescens was a positive factor, so their exclusion from the model may be due to their correlative relationships with the percent cover of H. splendens .
Bryophyte composition varied significantly with substrate type (adonis,F 3,97 = 6.8, P = 0.001, Fig. 3). Decay class 1 was significantly different from decay class 3 (P = 0.006) and the forest floor (P = 0.006); there were no other significant differences in bryophyte composition among substrate types (P > 0.01, Fig. 3). These differences were driven by bryophyte species that create thick mats (>4 cm), such asHylocomium splendens (HylSpl), which were associated with the forest floor and later decay classes of nurse logs (class two and three), whereas bryophyte species that don’t create thick mats were associated with early decay classes of nurse logs (one), such asRhizomnium glabrescens (RhiGla) and Scapania bolanderi(ScaBol) (Fig. 3). Bryophyte depth was the only factor that had a significant correlation with bryophyte community composition (r2 = 0.47, p < 0.001). While seedling density was higher on nurse logs that were in the early decay class, it did not significantly correlate with bryophyte composition (r2 = 0.02, p = 0.4). Canopy cover did not significantly correlate with the NMDS ordination but showed a trend towards decay class 2 (r2 = 0.06, p = 0.06).
Light levels were significantly lower under Hylocomium splendensthan beside it (paired t-test, t = -10.3, df = 49, P< 0.001, Fig. S1).
DISCUSSION
Our study found that tree seedlings were largely affected by bryophyte species identity and the thickness of the bryophyte cover. We found overall that tree seedlings were much more abundant on nurse logs than on the forest floor as found previously (Christy & Mack, 1984; Harmon & Franklin, 1989). However, not all nurse logs were created equal; nurse logs at early decay classes supported many tree seedlings and small bryophyte species, such as Rhizomnium glabrescens andScapania bolanderi , while later decay classes had few tree seedlings and bryophyte species with greater depths, such asHylocomium splendens . Moreover, the percent cover of H. splendens was negatively associated with tree seedling density and positively associated with bryophyte depth. Our findings are in accordance with Harmon (1989) who found seedling densities highest when nurse logs were dominated by Rhizomnium spp. and lowest when dominated by H. splendens . Our findings are also supported by another study conducted in old-growth subalpine spruce forests in Japan where tree seedlings on nurse logs were found to be positively associated with a small liverwort, Scapania bolanderi , and negatively associated with H. splendens (Fukasawa & Ando, 2018). As in our study, Fukasawa & Ando (2018) also found that bryophyte community composition shifted from dominance by S. bolanderi toH. splendens as nurse logs decayed. Thus, bryophyte succession on nurse logs influenced tree seedlings differently depending on the stage of succession.
As nurse logs decayed, the relationship of bryophyte communities and tree seedlings changed from facilitation to competition. We found support for this in our study as the pattern of seedling density mirrored that of the percent cover of Rhizomnium glabrescens(Figure 1A and 1D) while opposite that of the percent cover ofHylocomium splendens (Figure 1A and 1E), and H. splendenscame out in the model as the only negative driver of seedling density. These patterns are consistent with other studies. In subalpine forests of Japan, Tsuga seedling survival was restricted to nurse logs covered with small bryophyte species (Nakamura, 1992), and in the Hoh Rainforest, seedling establishment was highest on logs with thin mosses as seed retention on bare logs was low, and thick mosses excluded seedlings (Harmon & Franklin, 1989). While our study and others found correlative support of these relationships, findings from manipulative field experiments provide the strongest evidence. In old-growth forests in western Oregon, for example, seedling emergence was higher on nurse logs of decay class 3 (our class 2) when bryophytes were present than when they were removed (Christy & Mack, 1984), and in a field experiment in the Hoh Rainforest testing the influence of moss depth on seedling density, seedling density of Tsuga was higher in a moss mat of 3.7 cm than in 1.4 cm or 7.8 cm (Harmon & Franklin, 1989). The positive effect of bryophytes on tree seedlings could be due to protection from herbivores, increasing moisture or nutrient availability, protecting seeds from being blown off by the wind, or providing a humus substrate into which seedling roots can grow (Graham & Cromack Jr., 1982; Harmon & Franklin, 1989; Nakamura, 1992). Many studies have found competitive interactions between bryophytes and tree seedlings (Christy & Mack, 1984; Fukasawa & Ando, 2018; Harmon & Franklin, 1989; Nakamura, 1992). The negative effect of thick bryophytes on tree seedlings could be due to preventing seeds from reaching the humus or soil layer, which can inhibit germination (Iijima & Shibuya, 2010), the reduction of light levels below the threshold needed for tree seedling germination and survival as found in our study and others (Harmon, 1986; Iijima & Shibuya, 2010), or reducing nitrogen availability to vascular plants as found in the boreal forest (Gornall et al., 2011). The “window of time” for tree seedling establishment on logs of a particular stage of decomposition seems to be driven by bryophyte succession on logs (Fukasawa & Ando, 2018), and suggests that the available substrate for successful seedling regeneration is even more limited than the area covered by nurse logs would suggest.
The benefits of nurse logs to tree seedlings could be due to a release from competition with forest floor plants. In western Oregon old-growth forests dominated by Pseudotsuga menziesii and Tsuga heterophylla , 98% of the seedlings in the plot were on nurse logs despite the fact that nurse logs only covered 6% of the plot, andT. heterophylla emergence was higher on mineral soil when litter, bryophytes (such as Hylocomium splendens ) and herbaceous plants were removed than in the forest floor control where nothing was removed (Christy & Mack, 1984). In the Hoh Rainforest where our study took place, tree seedling survival was significantly higher on the forest floor when bryophytes and vascular plants were removed (Harmon & Franklin, 1989). Our findings corroborate this, as we found lower seedling densities on the forest floor where H. splendens was in high abundance. Our finding that H. splendens reduces light levels below the threshold for seedling survival (< 12.5 µmol/m2/sec) suggests that the mechanism by whichH. splendens out-competes tree seedlings is through light competition as suggested previously (Harmon & Franklin, 1989). Christy & Mack (1984) also suggest that nurse logs enable seedlings to avoid being covered by litter that dominates the forest floor in western Oregon forests. Interestingly, the small tree seedlings (<15 cm) that were on the forest floor in our study were on areas covered by litter where bryophytes were not abundant. The tree seedlings we did find on the forest floor or mounds of very decayed nurse logs that were covered in H. splendens were quite tall (>15 cm), which suggests that seedlings that establish on nurse logs prior toH. splendens could grow tall enough to surpass the height of the moss. This could add bryophyte cover as a contributor to the decrease in tree seedling survival after the first 1-2 years of growth (Christy & Mack, 1984; Friesner & Potzger, 1944; Haig, Davis, & Weidman, 1941). While litter may negatively impact tree seedlings, manipulative field experiments with long-term seedling survival data are further required to disentangle the relative influence of bryophytes and litter on tree seedlings both on the forest floor and on nurse logs.
On nurse logs, what drives the initial colonization and subsequent turnover in species composition is largely unknown. Rodents and slugs have been found to disperse lichen and bryophyte propagules (Asplund, Larsson, Vatne, & Gauslaa, 2010; Barbé et al., 2016; Kimmerer, 1994; Kimmerer & Young, 1995), and mammals use fallen logs as trails through the forest, so it could be a combination of differences in propagule dispersal ability, germination requirements, and biotic interactions that determine these changes. Alternatively, different decomposer fungi could influence bryophyte community structure on nurse logs as found in alpine forests in Japan (Ando, Fukasawa, & Oishi, 2017). Thus, the influence of bryophytes on tree seedlings could be mediated by other biotic interactions through a trophic cascade. Regardless, we did find that nurse logs are not only essential for forest regeneration but for the maintenance of bryophyte diversity in late stage forests. We found 17 bryophyte species on nurse logs and only six on the forest floor, and there were significant differences in species composition between our nurse log decay class 1 and the forest floor. Without gap-phase dynamics driven by large tree falls, these early nurse log bryophyte species would eventually be competitively excluded by the late stage bryophytes.
Our results support the predictions of the Stress Gradient Hypothesis that plant-plant interactions are complex and can vary from facilitative to competitive depending on the severity of the external environment (Bertness & Callaway, 1994). For tree seedlings, some level of bryophyte cover on nurse logs seems to be needed to facilitate germination and seedling establishment, while too much bryophyte cover inhibits germination and seedling survival. Hylocomium splendens , in particular, has been found to consistently exclude seedlings (Fukasawa & Ando, 2018; Harmon & Franklin, 1989). This is likely due to H. splenden ’s distinctive “stairstep” architecture, which lends it unusual height for a bryophyte. The average lifespan ofHylocomium splendens is 8 years (Binkley & Graham, 1981), and it dominates nurse logs that are >30 years old (Harmon, 1989a). Forest floor bryophytes may have an almost unlimited life span given their continued upward growth (Fenton & Bergeron, 2006). Thus, unless there is a disturbance that disrupts the dominance by these forest floor bryophytes, the regeneration of the dominant trees will be limited to the small window of opportunity found in early successional stages of bryophytes communities on nurse logs, which is less than 30 years. Interestingly, above the alpine tree line in subarctic tundra in Sweden, spruce tree establishment was improved when planted in bryophyte mats of Hylocomium splendens (Lett, Nilsson, Wardle, & Dorrepaal, 2017) further supporting that species interactions fall along a spectrum depending on site conditions. Bryophytes, though often overlooked in forested ecosystems, could be an important driver of plant community composition and dynamics.
AUTHOR CONTRIBUTIONS
CLW, KM and KO conceived the ideas and designed methodology; CLW, KM and KO collected the data; CLW and KM analysed the data; CLW led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.
ACKNOWLEDGEMENTS
We would like to thank undergraduate students G. Dolkas, E. Didier, J. Wood, and A. Marchand for help with field work. We would also like to thank the Olympic Natural Resources Center in Forks, WA for logistical support. We are grateful to the Olympic National Park for access to the park for research (permits: OLYM-2016-SCI-0013; OLYM-2017-SCI-0025; OLYM-2018-SCI-0046) and the Pacific Northwest Permanent Sample Plot Program for access to their plots. Funding was provided by the University of Puget Sound Summer Research Grants for undergraduate students in the Sciences and Math to KM and KO, and the University of Puget Sound Biology department. Authors declare no conflict of interest.
DATA ACCESSIBILITY
Variables used as predictors of tree seedling density in the GLM, a list of non-vascular species found on all substrate types, and light irradiance under and beside Hylocomium splendens are uploaded as online supporting information. A list of tree seedling density, percent canopy cover, bryophyte depth, and the percent cover of the most abundant bryophyte species in each plot are deposited in the Dryad depository:
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