TITLE: Plant-plant interactions change during succession on nurse logs
in a northern temperate rainforest
Carrie L. Woods1, Katy Maleta, and Kimmy Ortmann
Biology Department, University of Puget Sound, 1500 N. Warner Ave.,
Tacoma, WA 98416
Correspondence
Carrie L. Woods, Biology Department, University of Puget Sound,
Tacoma, WA, USA. Email:
cwoods@pugetsound.edu,
ORCID id 0000-0002-2156-3078
ABSTRACT
1. Plant-plant interactions can vary depending on the severity of the
environment. Positive interactions, such as facilitation, are often
important in early life stages of plants whereas negative interactions,
such as competition, predominate in later stages of growth. Thus,
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.
2. We surveyed 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. We examined the
relationship of bryophyte community composition, bryophyte depth,
percent canopy cover, and tree seedling density on nurse logs of three
decay classes and the forest floor.
3. Tree seedling density was highest on young logs with early-colonizing
bryophyte species, such as Rhizomnium glabrescens , and lowest on
decayed logs with Hylocomium splendens a long-lived moss that can
reach depths of >20 cm. As a result, bryophyte depth
increased with nurse log decay stage 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 only six, indicating that nurse
logs contribute to maintaining bryophyte diversity.
4. Nurse logs are, therefore, 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.
Synthesis: 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.
KEY WORDS: determinants of plant community structure; moss; nurse log;
Olympic rainforest; plant-plant interactions; Stress Gradient
Hypothesis; structural diversity; temperate rainforest
INTRODUCTION
Plant-plant interactions are complex and can vary from facilitative to
competitive depending on the severity of the external environment
(Bertness & Callaway, 1994; Brooker & Callaghan, 1998; Callaway &
Walker, 1997). Most positive plant interactions have been found to occur
in severe environments (e.g., arid, salt marshes, polar tundra), and
happen through the amelioration of those stressful conditions (Brooker
& Callaghan, 1998; Callaway et al., 2002). Facilitation, in particular,
appears to be important in colonization and early community development
as environments in early succession are typically severe (Callaway &
Walker, 1997; Clements, 1916; Connell & Slatyer, 1977; Egler, 1954;
Gómez-Aparicio, 2009). As the community develops, the amelioration of
disturbance and stress by the physical presence of plants (e.g.,
stabilizing slopes, herbivory protection, microclimate), results in
competitive interactions becoming more important over time (Brooker &
Callaghan, 1998; Egler, 1954; Ricklefs, 1977; Walker & Chapin, 1987;
but see Maestre, Valladares, & Reynolds, 2005). While vascular plants
have largely been the focus of plant-plant interaction research,
bryophytes can have facilitative or competitive interactions with
vascular plants depending on multiple abiotic and biotic factors
(Doxford, Ooi, & Freckleton, 2013; Gornall, Woodin, Jónsdóttir, & van
der Wal, 2011; Gough, 2006; Sedia & Ehrenfeld, 2003; Soudzilovskaia et
al., 2011; Staunch, Redlecki, Wooten, Sleeper, & Titus, 2012). In
locations where bryophytes dominate the ground cover, such as in the
arctic and boreal forest, bryophytes can structure the composition of
vascular plant communities (Gavini, Suárez, Ezcurra, & Aizen, 2019;
Gornall et al., 2011; Gough, 2006). However, how various stages of
bryophyte succession influence vascular plants remains largely
unexplored.
In old-growth forests, forest regeneration is often triggered by
disturbances, such as tree falls, that create gaps into which early
colonizing species proliferate (i.e. , small-scale gap dynamics,
McCarthy, 2001; Ricklefs, 1977; Yamamoto, 2000). These gaps often result
in a change in light levels that facilitate regeneration of
early-colonizing shade-intolerant plants if the gaps are large, and
shade-tolerant climax species if the gaps are small (< 200
m2); colonization of small gaps by the canopy
dominants can perpetuate the current canopy species composition
(Denslow, Schultz, Vitousek, & Strain, 1990; McCarthy, 2001; Runkle,
1981). In Appalachian forests in Tennessee, for example,
shade-intolerant trees were only able to establish in gaps created by
multiple tree falls whereas small gaps were colonized by shade-tolerant
trees (Barden, 1981). In contrast, in subalpine fir forests of coastal
British Columbia, Pacific silver fir (Abies amabilis )
preferentially colonized all gaps regardless of gap size (Lertzman,
1992). However, western hemlock (Tsuga heterophylla ) was more
dominant on stumps in gaps (Lertzman, 1992), suggesting that substrate
changes during gap formation could also influence forest dynamics. In
temperate coniferous forests of the Pacific Northwest, gap regeneration
after tree falls can be quite slow (>25 yr, Spies,
Franklin, & Klopsch, 1990), which results in large areas occupied by
canopy gaps (13.1%, Spies et al., 1990). The trees that fall to create
these gaps become essential sites of forest regeneration (i.e.nurse logs, Franklin et al., 2002) that cover more area in these forests
(up to 25%) than in other forests (<4%) (Harmon et al.,
1986). Similar to the subalpine fir forests of British Columbia, these
nurse logs provide safe germination sites for late succession tree
species’, such as shade-tolerant Tsuga heterophylla (Christie &
Armesto, 2003; Harmon & Franklin, 1989). Nurse logs are thought to
facilitate seedlings through several means, such as by becoming a
physical barrier between plants and terrestrial fungal pathogens,
limiting competition between seedlings, herbaceous species, and
bryophytes on the forest floor, and providing nutrients for potential
seedling growth (Franklin, Shugart, & Harmon, 1987; Graham & Cromack
Jr., 1982; Harmon, 1986; Harmon & Franklin, 1989). They may also be key
to maintaining bryophyte diversity.
Nurse logs are sites for bryophyte colonization and succession. In
coniferous forests in Colorado, bryophytes were found to colonize logs
after lichen establishment (McCullough, 1948), and in coniferous forests
of the Pacific Northwest, epiphytic plants from standing trees were
replaced by bryophytes more commonly found on the forest floor (Sharpe,
1956). In the Hoh Rainforest in Washington, bryophyte cover on fallen
logs reached approximately 90% in 11-19 y, and succession followed
dominance by Dicranum spp. and Hypnum circinale in early
succession, Rhizomnium spp. in mid succession, and feather mossesKindbergia oregano and Hylocomium splendens in late
succession (Harmon, 1989b). Given that late successional bryophytes are
tall and dense enough to prevent tree seedling establishment (Harmon,
1986), they may also out-compete early- and mid-successional bryophyte
species. Thus, nurse logs with moderate decay may be a refuge for early-
and mid-successional terrestrial bryophytes that have difficulty
establishing on the forest floor either due to insufficient light or
competition with late successional bryophyte species. Thus, gap dynamics
may also be important for the maintenance of bryophyte diversity.
These changes in bryophyte communities may have facilitative or
competitive interactions with tree seedlings. In the Hoh rainforest,
seedling density was highest when logs were dominated by the mid
successional bryophytes Rhizomnium spp. and lowest when logs were
dominated by late succession bryophytes (Harmon, 1989b). Harmon &
Franklin (1989) argue through their controlled experiments that the
interaction of mosses with seedlings changes from facilitative to
competitive once the moss layer exceeds 5 cm in depth. Previous research
on bryophyte-vascular plant interactions found that thicker mosses can
hinder vascular plants by reducing temperatures and nutrient
availability (Gornall et al., 2011; Pearce, Woodin, & Van Der Wal,
2003). However, they could also hinder vascular plant seedlings by
reducing light levels if the bryophytes are thick and dense enough. In
coniferous forests, removal of forest floor mosses showed positive
effects on tree seedlings (Wardle, Lagerström, & Nilsson, 2008;
Zackrisson, Nilsson, Dahlberg, Jäderlund, & Jaderlund, 1997). Thus, the
importance of nurse logs for tree regeneration may be mitigated by
bryophyte communities.
Here we build on the findings of previous studies (Harmon, 1986, 1989b;
Harmon & Franklin, 1989) by diving deeper into the role of bryophyte
species on tree seedling density. Over the course of three years, we
conducted separate studies in northern temperate rainforests on the
Olympic peninsula in Washington State (Hoh rainforest) to examine the
importance of nurse logs for tree seedlings focusing on the effects of
the nurse log bryophyte community, how that could change with nurse log
decay class, and how it may differ from the forest floor. We tested the
following predictions: 1) tree seedling density would be higher on nurse
logs than on the forest floor, as shown previously (Christie & Armesto,
2003; Harmon & Franklin, 1989); 2) bryophyte community composition and
the depth of bryophyte mats on nurse logs would influence tree seedling
densities (Fukasawa & Ando, 2018; Harmon & Franklin, 1989); and 3) the
decay class of the nurse log would influence both bryophyte communities
and tree seedling densities. Because bryophytes vary in their growth
patterns, we also examined if particular bryophyte species were more
abundant on the forest floor or nurse logs and were associated with high
and low tree seedling density as found previously (Harmon, 1989b). We
focused on the dominant feather moss, Hylocomium splendens , given
its abundance in these forests and its impact on tree seedling growth in
other studies (Fukasawa & Ando, 2018; Harmon & Franklin, 1989). We
tested our hypothesis that the effect of H. splendens on seeding
growth was due to competition for light by measuring light under and
beside this moss. If bryophyte communities on nurse logs change
predictably through succession and influence tree seedlings differently
at different successional stages, it would further limit the degree of
safe regeneration sites in these forests for trees, and highlight the
essential role of nurse logs and plant-plant interactions in forest
dynamics and diversity maintenance.
MATERIALS AND METHODS