Download The Intricacies of a forest ecosystem or Nurse Logs and the Plants

The Intricacies of a forest ecosystem or Nurse Logs and the Plants that Love Them
Eboni Summer Cooper
Education, Environment and Community Graduate Residency, IslandWood/University of Washington, Bainbridge Island, Seattle, WA 98110, U.S.A
Received February 15, 2012
Abstract
The purpose of this investigation was to find patterns (if any) that exist when comparing the dominant
plant organisms atop nurse logs in two 12’x13’ quadrats along the Marsh Loop and Suspension Bridge
trails at IslandWood. The intent of this investigation was to elucidate why certain plants grow on
decaying wood. Two locations were chosen in the temperate second-growth forest at IslandWood. Two
quadrats were designated for repeated observations. An investigator observed, recorded and researched
the organisms found atop the nurse logs within each quadrat. Observations, data recording and research
on plant communities within and around each quadrat also occurred. The investigator inferred throughout
the process, while immersing herself in the landscape. Comparisons were made, two patterns emerged
and many inferences were debunked by subsequent research on the following: bark fragmentation, decay
classification, bryophyte/nurse log relationships, tree seedling growth, and plant regeneration. Two major
learnings are a result of this investigation. 1) Fallen log decomposition requires quite a few years before
tree seedling establishment can occur. 2) Succession happens on nurse logs as well, which is largely
determined by decay class and chance.
Introduction
Myriad field guides, books and studies discuss the importance of decaying wood in forest ecosystems
and some even point out distinct relationships between nurse logs and particular plant species. Numerous
studies indicate that nurse logs are nutrient-rich substrates for a variety of plant species (Kennedy and
Quinn, 2000). It is established that certain plants thrive on nurse logs. For example in the field guide,
Plants of the Pacific Northwest Coast, co-authors and editors, Jim Pojar and Andy MacKinnon included
the following in their description of the liverwort, Lepidozia Reptans (Little hands liverwort). It reads,
“The stage of decay of the rotting wood is important to Lepidozia. “Kickable” stumps—those with a
pleasant soft sound when kicked—almost always have Lepidozia and Tetraphis; stumps in early stages of
decay have harder substrates and often have species of…”(441). Readers are left to wonder why
Lepodozia react differently than other bryophytes to harder decayed wood. The same occurs here in
Xuan Guo’s paper, “Natural Regeneration on Course Woody Debris,” he writes, “Although logs provide
advantageous habitats for some species, they are inferior seedbeds for other species; for example maple
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and red alder prefer the forest floor over rotten wood for regeneration” (page 5). Again, readers are left to
wonder why.
Do patterns exist that help clarify why nurse logs and other course woody debris are suitable
substrates for particular plants? Is there a dominant factor in the relationship between certain plants and
nurse logs, or is the role of a nurse log in plant growth just one indiscriminate variable within a complex,
multi-faceted and somewhat capricious process? Having limited scientific experience, experimental skills
and knowledge to procure or use high tech, mathematical formula-based tools; observation and analysis
seemed to best match the investigator. This investigation design exemplifies inquiry-based learning.
Observation, Hypothesis, Experiment, Conclusion and Knowledge (O.H.E.C.K.) —this memorable
acronym used in schools to teach the scientific method was modified. Instead, this model used Inference
rather than Hypothesis and Investigation rather than Experiment. The scientific merits of this
investigation stem from uncovering information through observation, making inferences, and executing
additional research to uncover new information and new questions; while spending hours in the woods.
Methods & Materials
Two plant communities, consisting of several bryophytes, shrubs, one tree seedling and one
unidentified vascular plant, along with nine nurse logs were observed. Partial temporary excavation
occurred; where gamephytes, fronds and leaves obscured rhizoids, the tops of rhizomes and roots.
Organisms were not permanently unearthed and no samples were taken. Photographs of all the organisms
found growing on each nurse log were taken and catalogued. Observations took place in quadrats that
were demarcated by colorful plastic strips tied to trees and other hearty plants. The nurse logs within these
quadrats were observed, poked and knocked to discern decay class.
Over the course of a month and a half, nine visits took place at each site. Site 1 on the Marsh Loop
trail (GPS coordinates: N47 36.369 – W122.31.893) is a section of the temperate forest with high sunlight
penetration. Canopy coverage is minimal—the overstory consists of only five trees. One Bigleaf maple
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grows just outside the quadrat, but litters the quadrat floor with its leaves. Sword fern and Evergreen
huckleberry abound on the forest floor, which is uneven, springy in certain places and hummock covered.
Site 2 on the Suspension Bridge trail (GPS coordinates: N47 36.228 – W122 31.575), just a bit off path
toward the canopy tower is a section of temperate forest with mostly evergreen trees and Red Alder trees
in the distance. Scouting a second site that did not include large amounts of maple shed was important to
study the possible effects of the leaves in Site 1. The canopy coverage density in Site 2 is high just above
the quadrat. Openings in the canopy are to the west of the site, where the presence of Sword fern and Red
Alder increases. A slope exists from west to east with a two-foot difference in elevation (196 feet to 194
feet, respectively). An 11-point observation sheet was used to record data (See Appendix A). Each sheet
correlates to one nurse log within a quadrat. Data about the nurse log, the organisms growing on the nurse
log and any other factors that might influence the microhabitat of the quadrat were compared. The data
was also compared to information found while researching related forest ecology topics.
Results
The purpose of this investigation was two-fold: a) to immerse oneself in the natural landscape and b)
to learn more about the natural history of the landscape through an investigative question. My concrete
learnings are as follows:
1.
Fallen log decomposition requires quite a few years before tree seedling establishment can occur.
2.
Succession takes place on nurse logs as well, which is largely determined by decay class and
chance.
3. Basic distinctions between common moss and vascular plants.
4. Basic distinctions between two unique bryophytes: common moss and liverworts.
5. The interconnectedness of biotic and abiotic factors within a forest ecosystem are vast and highly
variable. Many of the whys behind natural phenomena are still unknown and continue to be
explored.
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The two patterns that emerged were the dominance of bryophytes and the lack of tree seedling
establishment on nurse logs in both sites.
Discussion
I chose my first spot pretty randomly. I wanted a 12’x13’ quadrat that offered a variety of course
woody debris. I sought a “young” spot that would not have towering thick trees and mature shrub
thickets, which would reflect the history of the area. I wanted “recent” growth that reflects the
environment’s current ecosystem. I now recognize my ignorance in regard to the relevance of time in a
forest ecosystem and how words like “young” and “recent” are relative. The overstory of the first site had
one Bigleaf maple, which generously littered the quadrat floor. I questioned how much of a role this
played on the lack of species diversity on the nurse logs within the quadrat. Therefore, I purposefully,
sought out a second site without the “Bigleaf maple-influence”. I tried to find an area on Islandwood’s
campus that was predominantly coniferous. The second site had some leaves from the deciduous Red
alder, but unlike the Marsh Loop site, the forest floor was not covered by a carpet of leaves. Subsequent
research revealed that some plants thrive on organic materials, as long as the material remains moist;
while others are prone to dying if the organic matter dries out because of small seed size and short
radicals. Different variables began to emerge. I quickly realized that this study of the whys behind plant
regeneration on nurse logs is really a study of the relationships between all of the organisms within, above
and around the quadrat; along with abiotic factors as well. Plant reproduction methods (seed dispersal,
clonal colonization, and vegetation), canopy coverage, water saturation and wood decay (among others)
are all factors in the regeneration conversation. The following are pertinent parts of the surrounding
ecosystem I thought might affect the possibility of nurse log-plant growth patterns:
Site 1 – Marsh Loop Trail
Elements Outside the Quadrat – A discussion of the Bigleaf maple and its inferred effect was mentioned
earlier. However, a second plant not within the confines of my study could have also impacted what grew
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on the nurse logs. The map in Appendix B shows the existence of a 7’-8’ tree stump with a massive
cluster of Evergreen huckleberry growing from it. About four separate Evergreen huckleberry shrubs, a
few feet apart, made their way toward the quadrat. The last cluster was actually within it. This is
important because log #3 had a small vascular plant, which I mistook for Salal until just recently. While
cataloguing the photos of this investigation, I realized the immature plant was actually an Evergreen
huckleberry. Out of all the logs within the quadrat, only two logs had non-bryophytes growing on them. I
wondered what caused this shrub to grow in the middle of log 3 (see its observation sheet for details).
Subsequent research showed that Evergreen huckleberry typically regenerate through rhizome sprouts,
similarly to Salal. In a study about the regeneration of Salal, scientist reported their longest rhizome
extension reaching 282 cm (Huffman, Tappiener and Zasada, 1993). Could that explain the one “vegling”
amidst a sea of bryophtytes on log 3? It is a possibility—the sapling and the shrub cluster were within
9’.4” of each other. Browsing by deer and small mammals could have carried it there as well.
The Forest Floor – The forest floor was somewhat of a mystery to me. Maple leaves covered almost
everything, save for the many Swordferns that splayed their 4’ fronds. The hummocks made of possible
soil or decay class 5 nurse logs were also covered. The shade from the fronds was obviously too strong to
bear for most other plants. However, frond cover and litter also occurred on logs one, two and five. No
patterns were found to indicate that frond cover and litter made a significant difference on nurse logs.
Canopy Coverage – The canopy consisted of five surrounding trees: one Bigleaf maple, one Red alder,
two Western hemlocks and one Doulgas-Fir. More light hit the ground through the fall/winter months
because of the large opening the Bigleaf maple left. Although shade tolerance is discussed often regarding
succession, various studies have various findings on the effects of light on understory vegetation. Again,
sun exposure is simply one variable that I do not believe plays a large role in vegetation on the nurse logs
in site 1 at this time.
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Site 2 – The Suspension Bridge Trail
Elements outside the quadrat – Hemlock trees are within and just outside the perimeter of the quadrat. I
observed quite a lot of litterfall; however, I now know that vegetative reproduction does not happen as
often with Western hemlock trees (Packee, website). Swordfern live just to the west of the quadrat on the
higher end of the slope. Log 3 had a fern that I could not identify for quite some time. I later realized it
was a fern sapling. The leaves were in such an in between phase it was hard to identify. However, I
should have realized earlier that the Swordfern to the west would sooner or later have their spores settle
on a log within the quadrat.
Both sites and their surroundings offered interesting relationship possibilities. However, the possibilities
seemed almost endless. I needed to carefully study what was right in front of me—bryophytes!
Bryophytes are non-vascular plants consisting of common moss, liverworts and hornworts. They
absorb water through their bodies. Root-like structures called rhizoids absorb water a little, but their main
function is typically to root the leafy-green part of the plant down. At this point, I thought perhaps the
common moss and liverworts observed were formidable competitors against other plants due to their
dominance of every nurse log within the quadrat. I was surprised to learn later that bryophytes are
essentially the second tier of life on nurse logs. Lichen typically emerges first, then mosses and
liverworts, understory shrubs and then tree seedlings (Jitjay, 2007). Up until then, I thought succession
was a process of tree life cycles in relation to other trees in the forest. My understanding of this cycle did
not transfer to other plant communities. In The Hidden Forest: The Biography of an Ecosystem, Jon R.
Luoma wrote,
Cowles’s classic paper ‘Ecological Relationships of the Vegetation of the Sand Dunes of Lake Michigan’
published in 1899, stunned biologists around the world, for it established, once and for all, that
communities of species move through a sort of community evolution, at each successional stage laying a
biological foundation for the next, and simultaneously for their own demise (page 30).
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I was stunned to know this applied to the colonization of nurse logs as well and further perplexed that I
had come across nurse logs in both sites with little growing on it. I wondered at what point does the
existence of bryophytes stop hindering seedlings and start paving the way for them? I learned later that
the existence of bryophytes actually increases tree seedling growth when their layers do not exceed 5cm
(Guo, 2011). The layers of bryophytes on the nurse logs at both sites did not exceed 5cm. My search for
answers continued. It was not until I noticed, a tiny Tsuga heterophylla seedling growing out of a nurse
log at site 2 that I had proof that tree seedlings could grow alongside common moss. Site 1 had given me
no hope. Why and how was that seedling able to grow there?
Finally, after comparing all of the notes from both sites (see appendix C) a promising pattern
emerged. The decay class in site 2 was higher overall. It had fewer organisms, but more plant diversity.
This led me to further research specific decay classifications and bark fragmentation. I learned that decay
class is highly influential in nurse log succession. It varies based on tree type because of chemical
components among a host of other variables (Guo, 2011). The system of decay classification is also
highly variable around the world. One study states, “Although there is no unifying system or index to use
in different ecosystems, the one thing that most systems share in common is that they have no objective
time frame linked to their decay classes”(Campbell and Laroque, 2006). This investigation used the class
system described in the book, Ancient Forests of the Pacific Northwest (page 50).
In the study, Distribution of Licorice Fern (Polypodium glycyrrhiza) in Conifer Forests of Seward
Park, Seattle, Washington, the word preference was used to describe the prolific amounts of licorice fern
growing atop Bigleaf maple. The word “prefer” is ubiquitous in scientific journals describing the
relationships between plants and their suitable substrates. In Ancient Forests of the Pacific Northwest,
written and edited by Elliot Norse, he writes,
Some organisms have preference behaviors: They actively choose one habitat over another. Others are
more abundant in some habitats than others not because they “prefer” them but because they simply are
more successful at surviving or reproducing there. A red crossbill can choose whether to alight in a
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particular stand, but a redcedar cannot prefer one habitat over another. Wherever a seed lands, it either lives
or it doesn’t (page 72).
I had read this section before, but have a deeper understanding now. What originally fascinated me about
this subject was the use of the word “preference” to describe why certain plants are found in similar
environments. I believe it is misleading to lay people, novice naturalists and children. It gives the
impression that plants, in some esoteric way, choose where they grow. This observational investigation
did not yield the definitive patterns, I had hoped for to assist in understanding plant “preferences”. It did
however introduce me to a breadth of information about forest ecology, and the many factors that play a
role in determining how plant communities (the forest landscape) form.
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References
Campbell, L., and Laroque, C., 2006. Decay progression and classification in two old-growth
forests in Atlantic Canada. Mount Allison University, Sackville, New Brunswick, Canada
E4L 1A7.
Frank, Evan. Date unknown. Distribution of Licorice fern (Polypodium glycyrrhiza) in conifer
forests of Seward Park, Seattle, Washington.
Guo, X. 2011. Natural Regeneration in Course Woody Debris. Graduation Essay. FRST 497
Huffman, D., Tappeiner, J., and Zasada, J., 1993. Regeneration of salal (Gaultheria shallon) in
the central Coast Range forests of Oregon. Can. J. Bot. 72: 39 – 51.
Jitjay, D., 2007. Nurse logs as substrates for vegetative growth in old-growth forests of the
Pacific Northwest: comparing the relative abundance of bryophytes on nurse logs to soils.
Seattle University, Seattle, WA 98122, USA.
Kennedy, P., and Quinn, T. 2000Understory plant establishment on old-growth stumps and the
forest floor in western Washington. University of Berkerly, CA 94720, USA
Luoma, Jon R.
MacKinnon, A., and Pojar, J. Plants of the Pacific Northwest. 1994. Lone Pine Publishing Vancouver,
BC, CAN V64 3N3
Packee, E.C., Date unknown. Tsuga Heterophylla (Raf.) Sarg.
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