Plant-plant interactions change during succession on nurse logs in a northern temperate rainforest

Plant-plant interactions can vary depending on the severity of the environment. Positive interactions, such as facilitation, are important in early life stages while negative interactions, such as competition, predominate in later stages. Through succession, plant-plant interactions often change from facilitative to competitive. In northern temperate rainforests, gap dynamics result in tree falls that facilitate tree regeneration (nurse logs) as well as bryophyte succession. While the importance of nurse logs for tree seedlings is known, how the interactions of bryophyte communities and tree seedlings vary through succession of the log remains unclear. We examined the relationships of tree seedlings, bryophyte community composition, bryophyte depth, and percent canopy cover in 166 plots on nurse logs and the forest floor in the Hoh rainforest in Washington, USA. Tree seedling density was highest on young logs with early-colonizing bryophyte species (e.g., Rhizomnium glabrescens), and lowest on decayed logs with Hylocomium splendens, a long-lived moss that reaches depths >20 cm. As a result, bryophyte depth increased with nurse log decay and was negatively associated with tree seedling density. Tree seedling density was 4.6x higher on nurse logs than on the forest floor, which was likely due to competitive exclusion by H. splendens. Nurse logs had 17 species of bryophytes while the forest floor had six, indicating that nurse logs contribute to maintaining bryophyte diversity. Nurse logs are essential for forest dynamics as they enable both tree seedlings and smaller bryophyte species to avoid competition with the dominant forest floor bryophyte, H. splendens. Given that H. splendens has a global distribution and is often dominant in forested systems across the northern hemisphere, it is likely a widespread driver of plant community structure. Our findings indicate that plant-plant interactions shift with succession on nurse logs from facilitative to competitive and, thus, influence forest community structure and dynamics.

Importance of nurse logs as a substrate for the regeneration of pioneer tree species on Barro Colorado Island, Panama

Fallen tree trunks (‘nurse logs’) are important recruitment sites for trees in temperate forest, however nurse log use is seldom reported in tropical forests. We predicted that logs should be important for the regeneration of small-seeded tropical pioneer species because surface leaf litter and competition with established vegetation reduces the establishment success of these species from soil seed banks. In a survey on Barro Colorado Island, Panama, we found that pioneer seedlings were present on logs in 40 of 95 recent treefall gaps. In gaps where seedlings were present on logs, seedling density was not significantly different from adjacent areas of soil. However, species composition was significantly different; logs were disproportionately colonized by smaller-seeded and wind-dispersed species. In growing-house experiments using 12 species, we found that wood substrate had little effect on seed germination. In contrast, seedling growth was 50% lower on decayed wood than soil. Furthermore, species growth rates on wood were not significantly correlated with growth rates in soil (df = 10, r = 0.48). If establishment on logs eventually leads to recruitment to the forest canopy, then logs may promote the maintenance of diversity by favouring a different group of species from those that recruit in soil.

Regeneration of Picea engelmannii and Abies lasiocarpa in high-elevation forests of south-central British Columbia depends on nurse logs

We studied the regeneration niche of Picea engelmannii and Abies lasiocarpa in wet sub-zones of the Engelmann spruce-subalpine fir (ESSF) zone of southern British Columbia. We characterized the niche on transects from clearcuts into adjacent old-growth stands, in terms of aspect, elevation, micro-relief, light, seedbed, canopy cover, and competing vegetation. Fourteen transects were sampled at four watersheds. The sampling design ensured similar representation of small (1–30 cm), medium (31–100 cm) and tall (101–300 cm) saplings, and of both species. In total, 465 Picea and 623 Abies saplings were sampled. For every third sapling, an adjacent microsite with no sapling was selected using a random procedure and correspondingly characterized, enabling a pairwise comparison of microsites with and without saplings. Recruitment of both species was related to rotten wood seedbeds, i.e., mainly logs, but also stumps and root plates. This effect was stronger for Picea saplings, and it decreased with increasing sapling size, suggesting lower survival on rotten wood seedbeds. A higher occurrence of tall Abies saplings, in comparison to small ones, under the cover of canopy trees suggests higher survival under canopy than in gaps. Levels of direct light did not generally differ either between microsites with and without regeneration, or between Picea and Abies saplings. The microsite effects on sapling occurrence were quite consistent across gentle south- and north-facing slopes, upper and lower ESSF forests, sites, and along clearcut – old-growth gradients. The results suggest a strong dependence of Picea and Abies establishment on logs in ESSF forests. Key words: seedling establishment, microsite, seedbed, nurse logs, direct light, advance regeneration, ecological niche

Pathology of conifer seed and timing of germination in high-elevation subalpine fir and Engelmann spruce forests of the southern interior of British Columbia

Unstratified seed of Engelmann spruce (Picea engelmannii Parry) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) in nylon mesh bags was placed on various natural and disturbed forest floor seed beds in the Engelmann Spruce - Subalpine Fir Zone in the southern interior of British Columbia in September 1995 and recovered just before snow melt in June 1996. Fifty-two and 86% of the viable spruce and fir seed, respectively, had germinated before snowmelt. Germination under snow may be an adaptation of these high-elevation species to short cool growing seasons. Seed viability at recovery was significantly lower on undisturbed forest floor seed beds (spruce, 13%; fir, 12%) than on exposed mineral soil (spruce, 57%; fir, 42%). Viability of seed placed on nurse logs was 38 and 23% for spruce and fir, respectively. Isolation from ungerminated seed yielded a Rhizoctonia sp., an as yet unidentified black mold at high frequencies, and several other seed pathogens at low frequency. Multiple linear regression of the frequency of isolation of Rhizoctonia and black mold on seed viability was highly significant for both tree species. Seed pathogens appear to cause a major loss of seed and seedlings in these forests, and this may explain both the common occurrence of regeneration on nurse logs and the requirement of mineral soil seed beds for adequate regeneration.

Hurricane damage to old-growth forest in Congaree Swamp National Monument, South Carolina, U.S.A.

Hurricane Hugo caused much damage to the old-growth forests of the Congaree Swamp National Monument, but most of the damage to trees >20 cm dbh consisted of crown breakage and defoliation. Serious damage (>25% of crown lost, snapped trunk, or uprooted) was more common in mixed bottomland forest (49% of trees seriously damaged) than in adjacent sloughs dominated by Taxodiumdistichum (L.) L.C. Rich, and Nyssaaquatica L. (19% of trees seriously damaged). Of the trees >20 cm dbh, about 12% were uprooted in the bottomland forest, whereas only 2% were uprooted in sloughs. The storm reduced diversity in sloughs because most trees of species characteristic of better drained sites, and especially those rooted on nurse logs and other unstable elevated microsites, were uprooted. Dynamics of the entire forest were greatly influenced by the capacity of most tree species to recover vegetatively after suffering even severe crown and stem damage. Trees with resprouted crowns, however, were particularly likely to be broken, presumably owing to the presence of stem rots and architecturally unsound branching patterns.