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A Comparison of Native Tree Seedling Growth on Fallen Hapu'u Ferns and the Adjacent Forest Floor in Volcano, Hawaii

Kolea intro

Large, lush hapu'u ferns

Large, lush hapu'u ferns dominate the mid-level canopy of the rain forest.


 

"Kolea, follow me!" Naupaka yelled. Faithfully I followed my older brother into the forest. We were on our way to "the island," as we called it, a densely vegetated hill bisected by a 10-foot-deep fault that had a cave on one side. Later I learned that this raised geological formation is called a tumulus. At the time (10 years ago), I was only interested in exploring the vastness of the dark, mysterious rain forest that surrounds our house. My brother and I made a habit of exploring the forest. Each time we would go a little deeper, a little farther away from our house. Much to our mother's constant worry, we explored the lava tubes, holes, and cracks that were the products of ancient lava flows. On our adventures we would build forts from the dried-up, crinkly-brown hapu'u fronds and pretend we lived there. We slid in the mud, balanced precariously on fallen trees, looked at plants, and watched birds. The forest became our home.

These fond childhood memories always included being respectful of all the native plants in the forest. We were taught from a very young age to respect the hapu'u and the other native plants of our forest. When we trimmed the hapu'u around our house, my brother and I had to be very careful of the new shoots (the keiki, or children of the hapu'u) coming up. If we accidentally cut one, my mom taught us to stand next to the hapu'u and silently apologize to the spirit of the hapu'u for the loss. We also learned about the horrible "kahili ginger," a fast-spreading broad-leafed exotic plant. Kahili ginger has the potential to destroy the forest by choking out the native vegetation and replacing it with dense thickets of impenetrable ginger. The eradication of this noxious weed provided much entertainment and pocket money for Naupaka and me. We would slaughter thickets of ginger with hard wooden sticks, then dig up the huge, potato-like roots. Doing this service for the forest provided us with a great sense of satisfaction, and Mom paid us for the dug-up tubers. Now my approach to protecting our forest is somewhat less brutal (although my mom still has me dig ginger roots). I'm trying to learn how each component of the rain forest serves in the functioning and sustainability of the whole.

a small kahili ginger plant

A fallen, moss-covered hapu'u log with ohi'a seedlings and a small kahili ginger plant in the lower left corner.


To understand the forest, my first goal was to identify and describe the wealth of native Hawaiian flora that surrounds our house. The forest that surrounds my house is virtually untouched by extremely destructive ungulates such as the feral pig. This has allowed for the survival of some of the more rare native shrubs and smaller trees of the forest. This leaves us with a wealth of native species on our property. I have already mentioned the hapu'u tree fern, Cibotium glaucum. This fern is the main component of the mid-level canopy and the ground environment. Hapu'u are nicknamed "tree ferns" for a reason. Some of the older ferns—and by older, I mean more than 500 years old—have trunks that tower 20 to 30 feet into the sky, and long fronds that reach even higher. These fronds are constantly regenerating, leaving a tangle of dead, dry-brown fronds draped around their bases. There are two types of hapu'u that thrive in my forest: the hapu'u Cibotium glaucum, and the less common and morphologically smaller species, hapu'u i'i, Cibotium shamissoi, and Cibotium menziesii. This species differs not only in size, but also by a more distinguishable characteristic—the fronds' tips are covered with many brittle, reddish-brown hairs. Also, the undersides of the hapu'u i'i fronds are white when young, unlike the pale green undersides of hapu'u. Both species of tree ferns are threatened with destruction from feral pigs, which break open the trunks to get at the soft, nutritious, gelatinous cores.

Peperomia, a small stature plant.

Peperomia, a small stature plant.


The next major component of my rain forest is the species that dominates the upper-level canopy, the ohi'a-lehua, Metrosideros polymorpha. This species provides the habitat for many of the forest's native bird species, such as the i'iwi, 'apapane, 'oma'o, i'o, and 'elepaio. Hawaii's native bird species are the most threatened of Hawaii's fauna, and many efforts are being made to aid in their survival. The ohi'a tree has small, delicate, pompom-like blossoms that range in color from the most common red to orange, yellow, and even white. In the forest around my house, I have seen some of the biggest and oldest ohi'a trees in Volcano. Huge trunks more than six feet in diameter jut out of the hapu'u fronds to tower more than 200 feet above us. Some of these trees have a hollow area under their bases, making the trees look like huge claws.

When I was much younger, I remember being able to stand up completely in one of these hollows. I didn't mind the damp darkness. I can remember spending hours in one particularly great hollow, reading a book with a flashlight and enjoying my secret solitude.The third major component of the native forest is the mid-level canopy tree, o'lapa, Cheirodendron tigynum. O'lapa, a member of the ginseng family, has broad, light-colored leaves that quiver in the wind. This characteristic is how it gets its Hawaiian name: o'lapa means to "dance in the wind." O'lapa has clusters of small, purplish-black berries, the most common native berry, which serve as a major component in the diet of the 'oma'o, a fruit-feeding bird of the forest. These berries were often the main ingredient in the "concoctions" and "magic potions" my brother and I mixed in large glass pickle jars. The o'lapa's leaves, when crushed, have the distinct smell of a green mango.

Deer tongue's fern.

Deer tongue's fern.


I identified the plants that composed the understory of my forest. However, I am waiting for the name of a species that a neighbor and botanist, Julie Williams, could not identify. She proposed that it was probably a subspecies of pu'ahanui, Broussaisia arguta, a relative of the ornamental hydrangea. This dwarf subspecies reaches a maximum height of eight inches and grows in several niches throughout my forest. The discovery of species such as this one prompted my newfound interest in protecting and restoring my own patch of native forest.  

An ohi'a seedling.

An ohi'a seedling.


While identifying the plant species of my forest, I noticed that on fallen hapu'u logs there was an abundance of ohi'a, o'lapa, and tree ohelo seedlings compared to the surrounding ground. I hypothesized that these fallen hapu'u logs serve as "nursery logs" for the development and growth of certain species. It is known that large ohi'a trees serve as nursery logs as well, but very little research has been done concerning the important role that fallen hapu'u play in the cyclic regeneration of the forest. The "hollows" that I played in when I was younger may at one time have been filled by huge hapu'u logs. As centuries passed, these logs rotted away and the trees grew over them, leaving a hollow play area. By evaluating the importance of hapu'u in the growth and regeneration of the forest, I will be better able to understand the unique functioning of our forest.

To test this hypothesis, I would need to compare the number of seedlings and saplings on hapu'u logs to the number of seedlings and saplings on an area of ground equal to the area occupied by the tree ferns. This would allow me to compare the ground and logs without having to worry about a variance in microclimate within the forest. I defined a seedling as any plant species under one foot tall, and saplings as plants between one and eight feet tall. To obtain my sample of logs, I plotted out a half-acre area on our one-and-a-half-acre property. These rectangular half-acres were 103 feet wide by 217 feet long. To plot this area out, I used several markers on our property, and recorded the latitude and longitude coordinates of the four corners using a Navman GPS unit attached to a Palm IIIXE that runs Cetus GPS. I used string to mark the boundaries of the half-acre. The string would let me know if I had ventured outside the half-acre while collecting information. If I stayed within the boundaries of the string, I would be fine. 

The next step was to decide which logs I would survey. To do this without bias, I randomly chose logs by walking in several lines parallel to the back end of the parcel. I walked in six evenly spaced lines across the property, sometimes deviating from these transects because of geological and biological formations (faults, holes, densely vegetated areas, fallen trees). In total, I recorded 25 fallen hapu'u logs. My measurements and observations included the percentage of cover (how much of the sunlight is blocked by the canopy), the length of the log in feet, the number of seedlings on the log, the number of saplings on the log, the number of seedlings on the adjacent ground, and the number of saplings on the adjacent ground. I also recorded what species the seedlings and saplings were. Along with this, GPS measurements were made of each log to determine the location and distribution of the logs in the half-acre.

Getting the GPS signal in the thick rain forest was a challenge. I had to move to the least-covered part of the area I was in. Getting completely turned around in the forest was also a constant possibility; I had to keep certain landmarks always in sight, which, in the thick vegetation, were often hard to distinguish. Walking in the rain forest, I encountered numerous faults and holes that were most concentrated near the tumulus, a raised portion of land with a fault going through the middle that formed its own island of land. Walking in the fault was an awe-inspiring experience. Cool, wet liverworts lined the sides of the fault, while large tree ferns laid their majestic trunks across the top. These ferns were the habitat for a number of primeval-looking plants that gave the whole area a Jurassic look.

I gathered all my information during weather ranging from a clear blue sunny sky in the morning, to total cloud cover with a cold, drizzling rain falling in the afternoon. I found there was a striking difference between the number of seedlings and saplings growing on the fallen hapu'u logs compared to the surrounding ground. The average number of seedlings and saplings growing on fallen hapu'u logs was eight, while the average number of seedlings and saplings growing on the ground was one. The amount of cover seemed to play a role in the number of seedlings and saplings found on a log. The logs that had the least cover had the highest number of seedling and saplings (logs 8, 9, and 23). All had proportion of cover less than 0.5. Furthermore, the log with the least amount of cover, log 23, with 0.1 cover, had the highest number of saplings. Further study needs to be done to determine the exact role that light plays in the development of the understory forest.

I also recorded the species of the seedlings and saplings. Three species grew only on hapu'u logs. These were found to be ohi'a, o'lapa, and the tree ohelo kau la'au. The seedlings and saplings that were found on the ground were of many different shrub and small tree species, such as alani, manono, pu'ahanui, kawau, ha'iwale, oha, ie'ie, and pa'i'niu. These species showed no preferences in their choice of substrate; most of these shrubs have runners that climb along the forest floor, both on the ground and on hapu'u. For the climbing species such as ie'ie and pa'i'niu, the hapu'u trunks served as a perfect habitat. The data I collected show that what I had observed in my forest is actually true. Hapu'u ferns play an integral role in the growth of the forest and in its functioning as a whole.

The data I gathered provide insight into how the hapu'u function as "nursery logs" in the forest. The data collected shows not only a striking difference between the number of seedling and saplings growing on logs as compared to open ground, but also a divergence in the types of species present. Ohi'a, o'lapa, and ohelo kau la'au grow only on hapu'u logs. This may be because these species are much smaller in stature and size than the majority of shrub seedlings. Growing on the hapu'u log, these seedlings would be sheltered from the impact of fern fronds and other debris falling from the canopy layer. Also, the hapu'u logs are the perfect germinating medium for these species, which I observed to have extremely small seeds. The seeds of the ohi'a are produced in small packets and are the size of dust particles. These seeds are distributed by wind throughout the forest. When one of these seeds lands on the damp, moss-covered hapu'u, it germinates readily. However, when a seed lands on the ground, the area isn't constantly damp and the seed doesn't survive.

 

Data Gathered From Plot

log# latitude, longitude # of satellites cover living/dead log length (ft.) seedlings on log saplings on log seedlings on ground saplings on ground
log 1 19°26.8181',155°14.2059' 2 0.85 living 8 3 0 0 0
log 2 19°26.8257',155°14.1831' 4 0.7 living 11 3 0 0 0
log 3 19°26.8312',155°14.1803' 5 0.9 living 9 0 2 0 5
log 4 19°26.8354',155°14.1860' 4 0.75 dead 10 4 0 0 0
log 5 19°26.8342',155°14.1892' 4 0.7 living 10 4 0 0 1
log 6 19°26.8371',155°14.1819' 3 0.65 dead 5 4 1 0 0
log 7 19°26.8416',155°14.1856' 3 0.4 living 12 0 2 0 0
log 8 19°26.8400',155°14.1963' 3 0.5 living 11 26 1 0 0
log 9 19°26.8241',155°14.2120' 4 0.4 living 15 30 1 0 0
log 10 19°26.8233',155°14.2040' 6 0.7 living 6 14 0 0 0
log 11 19°26.8341',155°14.2018' 6 0.8 living 8 11 0 0 3
log 12 19°26.8342',155°14.1839' 4 0.5 living 8 0 0 8 0
log 13 19°26.8149',155°14.2075' 4 0.75 dead 10 9 0 2 0
log 14 19°26.8283',155°14.1914' 2 0.4 dead 12 12 0 0 0
log 15 19°26.8213',155°14.1950' 1 0.8 living 15 8 1 0 0
log 16 19°26.8256',155°14.1945' 4 0.9 dead 6 0 1 0 0
log 17 19°26.8302',155°14.2017' 5 0.75 dead 11 4 1 0 0
log 18 19°26.8242',155°14.1854' 3 0.85 living 10 3 0 0 3
log 19 19°26.8369',155°14.1894' 4 0.65 living 15 1 0 3 0
log 20 19°26.8232',155°14.2072' 3 0.6 living 11 10 0 0 0
log 21 19°26.8253',155°14.1956' 2 0.7 living 12 0 1 4 0
log 22 19°26.8194',155°14.1993' 2 0.7 living 15 2 9 0 0
log 23 19°26.8200',155°14.1894' 4 0.85 living 11 18 0 0 0
log 24 19°26.8189',155°14.1900' 4 0.9 dead 6 2 0 0 0
log 25 19°26.8192',155°14.1920' 4 0.672 living 7 4 0 0 0
Averages 10.16 6.88 0.8 0.68 0.48
 


An O'lapa sapling

An O'lapa sapling emerging from a moss covered hapu'u log.


The large number of seedlings compared to saplings is, from what I observed, the result of less-than-ideal conditions for the seedlings. Only when there is the right amount of light in the forest can the seedlings thrive and grow into adults. This is the forest's way of limiting itself, for if each seedling were to mature, the forest would be nothing but a dense, homogenous grove of ohi'a. However, the abundance of the various shrub species is not limited by the amount of light available, as most of these species were found to live in dark, shaded areas. I found that the majority of the shrub species had large, broad leaves in order to collect the maximum amount of light. This is an adaptation that allows them to survive on the shrouded forest floor. The factor that limits the abundance of shrub species is the availability of growing space. If there is a limit to the amount of open ground these species can inhabit, there are obviously going to be limits on the populations of these species. 

Tree ohelo

The new leaves of the tree ohelo are a beautiful orange-pink color.


Light and space are not the only limiters for these species; several new limiters have been introduced by humans in the past 100 years, such as kahili ginger and the Himalayan blackberry. Throughout the forest, I found thickets of ginger that covered both hapu'u logs and open ground, shading out any forest-floor growth with its large, shiny green leaves. The Himalayan blackberry—or "bramble," as it is commonly called—was also a prevalent nuisance, thriving with no preference on both hapu'u and the ground.

What I observed and learned about my forest is priceless. I was astounded by the uniqueness and diversity of the plant life and how it interacts to form, as a whole, the rain forest that has provided my family with much enjoyment and serenity. The hapu'u, one of the most threatened and important species in the rain forest, plays an integral part in the functioning and regeneration of the entire rain forest. Ohi'a, o'lapa, tree ohelo, and perhaps undiscovered plant species utilize the hapu'u as their means of survival. These trees in turn support even more plant and animal life. The too-weet of the 'apapane that I enjoy today when walking in my forest would not be so prevalent if the ohi'a trees or the hapu'u ferns were destroyed by noxious weeds, human development, or feral ungulates.

A proposed subspecies of Pu'ahanui

A proposed subspecies of Pu'ahanui, characterized by its small stature and tan fleshy fruits.


My children will not be able to enjoy the forest if we don't do something now to protect it. They won't be able to build forts, watch birds, explore ancient lava tubes, or play under the hollows of trees. My study of the role that the hapu'u and other species play in the sustainability of the forest is a step that cannot be skipped. We must strive to understand the forest so that it can continue to be a place where both children and adults can play, learn, explore, and have adventures.

 

References

Elbert, Samuel H. and Mary Kawena Pukui. Hawaiian Dictionary. Honolulu: University of Hawaii Press, 1986.

Hawaii Audubon Society. Hawaii's Birds. Honolulu: Hawaii Audubon Society, 1983.

Merlin, Mark D. Hawaiian Forest Plants. Honolulu: The Oriental Publishing Company, 1976.

S.H. Sohmer and R. Gustafson. Plants and Flowers of Hawaii. Honolulu: University of Hawaii Press, 1987.

Stone, Charles P. and Linda W. Pratt. Hawaii's Plants and Animals. Honolulu: Hawaii Natural History Association, 1994.

Williams, Julie. Personal interview. December 16, 2002.

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