Download Plants and Nutrients - Fort Hays State University

Plants and
Nutrients
Department of Biological Sciences, Fort Hays State University
Instructor: Mark Eberle
Course Homepage
Most plants of the western forests use inorganic nutrients available in the soil, air, and
water to build organic molecules through photosynthesis. However, some plant species
exhibit interesting strategies to improve their success in the competition for limited resources.
Nitrogen (as N2 gas) is the most abundant element in the air (about 78% of the
atmosphere), and nitrogen is an essential component of proteins and other organic molecules.
Some bacteria in the soil or water can convert N2 into ammonia (NH3). This process is
referred to as nitrogen fixation. The ammonia can be converted into ammonium ion (NH4+),
which is picked up from the soil by plants, especially trees and grasses, and by the symbiotic
fungi (mycorrhizal fungi) associated with the roots of plants. Other bacteria can convert
ammonia into nitrite (NO2‒) and then nitrate (NO3‒), which is used by most flowering plants.
This process is referred to as nitrification. In addition to nitrogen fixation and nitrification,
nitrogen becomes available to plants through decomposition of organic materials. Fungi and
bacteria are the dominant decomposers, breaking down complex organic molecules, including
proteins, into smaller molecules, such as amino acids and ammonia.
To assist them in obtaining nitrogen, legumes, such as the many species of lupine
(Lupinus; photograph next page), and Red Alder (Alnus rubra; a tree) have globular nodules
on their roots occupied by symbiotic bacteria that can fix nitrogen, some of which is used by
the plant. The plant provides high-energy molecules and other necessities to the bacteria.
Lupine, Mount Rainier National Park, Washington
Photograph by Curtis Wolf, July 2005
Bog soil presents a challenge to most plants because the soil is often saturated with water,
but it also is low in nitrogen (nitrate is less soluble in acidic soils, such as those in bogs).
Rather than assistance from symbiotic bacteria in their roots, Cobra Lilies (Darlingtonia
californica; photograph below) obtain their nitrogen from insects attracted to their modified,
tubular leaves that produce aromatic nectar. The insect moves inside the tube through an
opening on the underside of the “cobra head.” Long, downward-pointing hairs help to prevent
the escape of the insect, and an enzymatic fluid inside the tube helps to decompose it, which
provides nitrogen needed by the Cobra Lily.
Darlingtonia califronica (Cobra Lilies), Darlingtonia State Wayside, Oregon
Photograph by Eric Hoch, July 2002
Some flowering plants lack chlorophyll (achlorophyllous) and the metabolic (chemical)
pathways for photosynthesis. Thus, they rely on other photosynthetic plants directly or
indirectly for their organic molecules. The roots of Pinedrops (Pterospora andromedea) and
Indianpipe (Monotropa uniflora; photographs below) are associated with mycorrhizal fungi
connected with the roots of a conifer, forming what is called a common mycelial (or
mycorrhizal) network (CMN). Through this connection, the achlorophyllous plants indirectly
obtain their nutrients from the photosynthetic plant by way of the mycorrhizal fungus. In this
CMN, the conifer provides organic molecules derived from photosynthesis to the fungus, and
the fungus provides nutrients to the conifer. Whether the achlorophyllous plant provides
anything to the fungus from which it derives its nutrients is unknown, although the plant
might produce substances that stimulate the growth of mycorrhizal fungi (summary by
Trudell, 2002).
Pinedrops (left) and Indianpipe (right), Olympic National Park, Washington
Photographs by Mark Eberle, July 2004
Another feature of plant competition for nutrients in temperate rain forests is the nurse
log. When these trees fall, they open the canopy, allowing more sunlight to reach the forest
floor, and they serve as a home and nutrient source for a wide variety of organisms. Initially,
wood-boring arthropods (e.g., beetles, ants, termites, mites) tunnel into the wood to feed or
nest. Into these tunnels come the fungi and bacteria that will help decompose the wood.
Leaves and other forest litter, as well as nonvascular plants, accumulate on top of the log and
provide a place for seeds to germinate. Western Hemlock (Tsuga heterophylla), Sitka Spruce
(Picea sitchensis), and shrubs are among the plants that germinate on the log (next page, left
photograph). As the log continues to decompose, the tree roots, with the help of their
mycorrhizal fungi, continue to penetrate deeper into the moist microhabitat of the decaying
log toward the soil. Water produced by cellular respiration of the decomposers helps keep the
interior of the decaying log moist during the regular summer “drought,” and this improves the
chances of success for the young trees, with their active mycorrhizae, compared to those
growing on the soil, whose mycorrhizal fungi can become dormant during summer (summary
by Schultz, 1990:268‒279). As the process continues, some of the young trees do not survive,
but those that do eventually form a colonnade along the line of the largely decomposed nurse
log (photograph below, right).
Nurse Log with Fungi and Young Trees (left)
Colonnade of Trees along Line of Decomposed Nurse Log (right)
Hoh Rain Forest, Olympic National Park, Washington
Photographs by Mark Eberle, July 2004 and August 1998
Literature Cited
Schultz, S. T. 1990. The Northwest Coast: A Natural History. Timber Press, Portland, Oregon.
Trudell, S. 2002. Mycorrhizas (5): fall mushrooms, ghostly fungus-robbers, and a definition revisited.
Mushroom: the Journal of Wild Mushrooming, Issue 77, Fall. Accessed 21 June 2006 at
http://www.mykoweb.com/articles/Mycorrhizas_5.html.
Return to Course Homepage | Return to “Lecture” Notes