|Skywatching Notes||Plant Notes||Invertebrate Notes||Vertebrate Notes|
Terrestrial Nonvascular Plants in Lincoln County
The "Other Nature Notes" column for nonbird field observations began in May 1994; previously, there were occasional articles about animals other than birds. Based on observations shared by observers, Range Bayer compiled this column through 2000, and Kathy Merrifield began writing the column in 2001. If you have any observations to share, email them to her.
Unfortunately, Range has lost his computer files for the Sandpiper prior to September 1994, so earlier notes would have to be scanned and converted to text. Below, Range has re-organized and done some editing of "Other Nature Notes" columns from September 1994 and some columns only about dragonflies. Notes are organized by plant or animal group.
Note that these are incidental observations and were not meant to be comprehensive. Nevertheless, there is so little information available about many of these plants or animals in Lincoln County that these notes provide information that is not otherwise available. Some notes are from Neskowin in Tillamook County or coastal Lane County because the observers were Yaquina Birders & Naturalists members.
Common names may be capitalized or not, although it is preferred to have only one style.
To save bandwidth, this material has been subdivided into four files:
|Skywatching Notes||Plant Notes||Invertebrate Notes||Vertebrate Notes|
Some Lincoln Co. site locations: Bayshore Beach=ocean beach along north Alsea Bay Spit, Bayview Pasture=field east of junction of Beaver Creek Road and North Alsea Bay Road, Beaver Creek=creek flowing through Ona Beach State Park, Coquille Point=SE Corner of Sally's Bend, Driftwood Beach State Park=park about halfway between Seal Rocks and Waldport, Eckman Lake=lake just east of Waldport along Hwy 34, HMSC=OSU Hatfield Marine Science Center, Idaho Flats=large embayment just east of HMSC, Lost Creek State Park=park about 4.75 mi south of Yaquina Bay Bridge, Mike Miller Park=county park just easy of HWY 101 in South Beach, Ona Beach State Park=park about 6.6 mi south of Yaquina Bay Bridge, Sallys Bend=large embayment east of the LNG tank at Yaquina Bay, Sandpiper Village=residential area west of HWY 101 and north of Waldport, Seawoods=residential area just north of Patterson State Park, Thiel Creek=creek about 3.5 mi south of Yaquina Bay bridge, Tidewater=about 8 mi east of Waldport, Wandemere=about 0.5 mi north of Ona Beach along Hwy 101.
Tangled piles of Nereocystis (bull kelp) began appearing on beaches and in bay waters in October 2001 (Kathy Merrifield). These organisms are usually annual but occasionally last for more than one season. These tangles of kelp on beaches, if they are not too ripe, are wonderful sources to search through for other deep intertidal organisms (Kathy Merrifield). Many other kinds of algae grow on Nereocystis stipes ("stems") and blades ("leaves"). Kelp holdfasts, the branched attachment structures, in turn provide attachment or lodging sites for countless cnidarians (jellyfish; coral-like animals), molluscs, worms of all kinds, arthropods, and sea stars and their relatives. Be sure to bring your tweezers and a jar of alcohol when playing on the beach.
It was just heavy surf on 5/29/2002, but it
produced up the kind of wrack that typically follows storms (Kathy
Merrifield). The sand between Seal Rock and Ona Beach held a fairyland
of Neat Stuff. Single hunks of big brown algae, some complete with
holdfasts, and one Gordian tangle of Macrocystis (giant kelp) accompanied
Fucus ("rockweed"), Postelsia ("sea palm"), and Alaria encrusted with
hydrozoans, bryozoans, and barnacles. One Postelsia holdfast completely
engulfed those of two small Alaria, suggesting a dramatic growth rate.
The 5/29/2002 beach display also featured pelagic goose barnacles anchored to free-floating substrates including Fucus, Postelsia, Halosaccion (a yellowish-grown sac-shaped red alga), driftwood, tufts of filamentous green algae, and itself. Among the flotsam were red crabs, comb jellies ("sea gooseberries"), and a delightful assemblage of jellyfish. All in all, it was a deeply rich and satisfying display, delivered by the faraway forces that make the waves (Kathy Merrifield).
Terrestrial Nonvascular Plants in Lincoln County
Terrestrial Vascular Plants in Lincoln County
Several Amanita muscaria (fly agarics), the brilliant red-capped mushroom adorned with white floccules, were in their prime along the E shore of Eckman Lake on 11/19/2002 (Kathy Merrifield). Mature mushrooms measured up to 7 inches across and 8 inches high. Fly agaric is poisonous and hallucinogenic. Although fatalities from its ingestion are rare, the misery of the symptoms probably far outweigh any pleasure derived from the hallucinations.
Most Amanita species, including fly agaric, are mycorrhizal - that is, they form a mutually beneficial relationship with a plant. Mycorrhizal partnerships are so common that they are considered the normal condition for the majority of plants. In this symbiosis, the fungal partner derives carbohydrates from the plant, while the fungus/root complex absorbs far more water and nutrients than the plant could by itself.
There are at least three arrangements of mycorrhizae. In the arrangement characteristic of forest trees and shrubs, called ectomycorrhizae, the roots are swollen and more forked than roots without fungi. The fungal filaments produce a tightly interwoven mantle around the outside of small, absorptive roots. These fungal filaments grow both inward and outward. Growing inward, after penetrating the roots, they grow around the cortex cells - those inside of the epidermis but outside of the conducting tissue. Fungal filaments then replace the attachment material (middle lamellae) between root cortex cells. Growing outward, the fungal filaments branch widely and prolifically, absorbing water and nutrients from a tremendous soil volume.
The fly agarics next to Eckman Lake grew under spruce and beside evergreen huckleberry. Perhaps one or both of these plant species, and perhaps even more, were benefiting from these strikingly beautiful, very helpful fungi.
Also see Invertebrate Communities in Mosses by Kathy Merrifield and Lynn Royce.
Some mosses can be intertidal; on 12/24/1995, Kathy Merrifield saw high waves inundate her Schistidium maritimum study site near Yachats.
* * * * * * * * * * *KATHY MERRIFIELD'S 12 May 2002 VISIT TO SOUTH BEACH STATE PARK SAND DUNES FOR MOSSES AND LIVERWORTS
Bryophytes (mosses and liverworts) can occur on many sand dune substrates. The mosses Eurhynchium praelongum (a feather moss), Dicranum sp. (a broom moss), and Polytrichum sp. (a hair-cap moss) can form large to small turfs, tufts, or mats. Warnstorfia sp. (a side-curl moss) is more common in wetter areas.
Few bryophytes grow on tree trunks or branches among the sand dunes. Shore pine and Sitka spruce, the most common sand dune trees, have few bark-dwelling mosses even in upland sites. Among the dunes, these trees form compact, shrubby plants, providing even fewer bark expanses for mosses to grow. However, mosses do grow on dead branches of sand dune shrubs. Branches of Lonicera involucrata (twinberry, a shrubby honeysuckle) extending above the main part of the shrub tend to die, maybe from exposure to salt. Mosses grow on these dead branches. A more prolific source of dead branches is Scotch broom. When dead, it is an ideal surface for wood- and bark-dwelling mosses. Because it's a non- native, though, I'd rather search more deeply for mosses on the sparse dead twinberry branches than to have the banquet of mosses provided by dead Scotch broom, a harmfully invasive species.
Organisms, including bryophytes, that grow on other plants are called epiphytes. (In marine systems, little algae growing on big algae are termed epiphytes, too.) Epiphytes take nothing from the plants they grow on and do not harm them in any way. Epiphytic mosses on these dead branches among the dunes include Ulota phyllantha, Orthotrichum lyallii, and O. consimile. (Sorry - there are not good common names for these.) At least one species of Frullania, helmet liverwort, grows near and among the epiphytic mosses.
Many of these epiphytes absorb water so rapidly that when dry specimens are moistened, they look like a time-lapse Walt Disney video of a flower opening, in which days are condensed into seconds. These bryophytes, however, absorb water and fully expand within real seconds. You can have a fun and dramatic hand lens adventure by moistening dried epiphytic mosses and watching them remoisten. I sometimes do this before going to work as an inspirational start to the day. (No kidding!) As I often feel compelled to say in my own defense, life is more entertaining for the easily entertained.
On 20 January 1996, Dawson and Bobby Mohler hiked along the Coast Trail from Gwynn Creek to the Cape Perpetua Interpretative Center and found some scattered blossoms on Evergreen Huckleberry, Salal, and English Daisy; but they didn't find any plants flowering at Mike Miller Park.
On 7 April 1996, Dawson and Bobby Mohler did their annual Easter walk for wildflowers along the Giant Spruce Trail at Cape Perpetua on 4/7. They write: "We did not see a lot of abundance in either species in bloom or in abundance of flowers. We see flowers on salal, Salmonberry, Evergreen Huckleberry, and Red Elderberry shrubs. We see some Coltsfoot, English Daisy, abundant Yellow Violet, Candyflower, Bittercress, Smith's Fairybell, Skunk Cabbage, several Trillium, some Corydalis, rare Oxalis. There were only unopened buds on the Twisted Stalk, Bristle Flower, and on the Lily of the Valley."
On 14 April 1996, the Mohlers repeated their hike at the Giant Spruce Trail and write: "The flowers were still not blooming in great abundance. There were more Fairybell, Corydalis, and one patch of Oxalis with abundant flowers. One Twisted Stalk plant was in full bloom, the rest yet buds. The Bristle Flower buds had elongated but were not yet open, and the Lily of the Valley were not much advanced. New additions were a well-flowering Fringe Cup and a Stinking Currant that was just beginning to bloom."
On 12 January 1997, Dawson and Bobby Mohler saw a Candyflower with blossoms near Devils Churn at Cape Perpetua. A Salal had a blossom and a few English Daisies were blooming at Gywnn Creek Trail at Cape Perpetua on 1/24.
On 3/30/1997, Dawson and Bobby Mohler hiked along the Giant Spruce Trail at Cape Perpetua. They found few plants in bloom including Red Currant, Salmonberry, English Daisy, Yellow Violet, Bittercress, Candyflower, Skunk Cabbage, fairy bell, Corydalis, Oxalis, Coltsfoot, Bleeding Heart, Dandelion, a plum, Miner's Lettuce, and a mustard.
At Gwynn Creek Trail near the Lincoln/Lane Co. border on 4/26/1997, the Mohlers saw blooms on lots of Trillium, English Daisy, a lupine, Western Buttercup, Oxalis, Yellow Violet, Candyflower, Salal, Salmonberry, and Smith's Fairy Bell. On 5/9, flowers were in much greater abundance and additional species they saw included Monkey Flower, a giant pea, False Lily of the Valley, Bleeding Heart, blackberry, Bristle Flower, Miterwort, and Brewer's Bittercress--the Trilliums were turning to late phase red. On 5/11, the Mohlers also found Manroot, Cow Parsnip, a veronica sp., Thimbleberry, Milkmaids, and a blue violet.
On 5/31/1997 along Gwynn Creek trail near the Lincoln/Lane Co. border, Paul Reed saw white lumps in the ground that were sprouting Indian Pipes.
At Cape Perpetua, Dawson and Bobby Mohler hiked Gwynn Creek Trail on 6/9/1997, and Dawson noted: "Though the hike is mostly in deep forest shade, there are some wildflowers. The abundance is not impressive, but there is some variety." On 6/14, they hiked the Big Spruce Trail, Dawson writes: "This is a good wildflower walk in May, now in mid-June, there was very, very little to see. The one interesting flower which was not abundant in May is called Bristle Flower."
On 4 April 1999, Dawson and Bobby Mohler did their annual Easter wildflower walk at Cape Perpetua. Dawson reported that the numbers of flowers and varieties were not as spectacular as the first time they did the walk. But their flowering list included Salmonberry, Red Currant, Evergreen Huckleberry, Salal, Coltsfoot, Yellow Violet, Skunk Cabbage, Trillium, English Daisy, Oxalis, and Bittercress. Other plants, such as False Lily of the Valley and fairy bell had buds but were far from blooming.
On 1/20/1996 and 1/14/1997, Dawson and Bobby Mohler didn't find any plants flowering at Mike Miller Park, though they found some at Cape Perpetua.
At Mike Miller Park in South Beach on 4/20/1997, Dawson and Bobby Mohler found in bloom: Salal, Evergreen and Red Huckleberry, broom, Rhododendron, Trillium, Skunk Cabbage, and Salmonberry. The same day on the trail between the South Beach State Park and South Jetty, they also found flowering English Daisy, Shepherd's Purse, Wild Strawberry, Miner's Lettuce, Twinberry, Sea Thrift, and a purple pea; also a Sitka Spruce appeared ready to shed pollen.
On 12 May 2002, Kathy Merrifield visited South Beach State Park. The deflation plains were nearly dry. Among the few plants flowering so far were Carex pansa (sand dune sedge) in low, damp areas, and in drier areas, Anthoxanthum odorata (Sweet Vernal Grass) and Coast Strawberries (Fragaria chiloensis), which were producing runners as well.
In 1995, Terry Morse saw Siberian Candyflower and Trillium flowering on a cutbank at the middle reservoir of Newport Reservoir on 3/26, and Skunk Cabbage were then blooming in moist locations.
On 5/4/1996, Chuck Philo visited Grass Mountain (which is about 19 miles east of Waldport in Benton County). He found that the about 3 inch tall purple flowers of Snow Queen (Grouse Flower) were all over. Chuck notes that books say that when this flower is blooming that grouse hoot, and Blue Grouse were doing so when he visited!
In Newport, Range Bayer noted lots of pollen that was probably from Lodgepole Pine blowing around and covering windows and cars in April and early May 1997. When washed down and dried, it appeared as if there had been a "yellow rain."
The mild winter thus far is illustrated by flowering rhododendron, honeysuckle, English daisies, and an in-ground fuchsia in Shirley Schwartz's Neskowin neighborhood in Tillamook County on 12/21/1997, and a blooming crocus on 1/10/1998 at Dawson and Bobby Mohler's home south of Waldport.
Dave Pitkin found an old stump about 8 ft in diameter along Bayshore Beach about a quarter mile north of the north Alsea Bay Spit on Feb. 4, 1998. He counted 930 rings! It appears to have been dead for several years.
By 2/22/2001, red-flowering currant was slowly moving towards leaf expansion, and willow catkins were beginning to shed bud scales and show silver fuzz at Rebecca Cheek's Wandemere home.
On March 11, 2001, Skunk Cabbage along the coast and nearby was in full mid-flower, but plants up the Yachats River and Keller Creek were less mature (Kathy Merrifield). Wood Violets in the hills above Keller Creek produced their first flowers concurrently with the first leaves of the year. The first Salmonberry leaves there were expanding, and the first petals were just visible; Thimbleberry leaves were just starting to show green visible as a drive-by. Flower stalks of Sweet Colt's-foot along the south bank of the Yachats River were about a foot high, and a few are nearing full expansion. Red-flowering currant is in full flower at Eckman Lake; red alder leaf buds have just started to open, and staminate catkins are fully expanded and may be releasing their pollen (Kathy Merrifield).
In April 2001, Kathy Merrifield wrote that introduced plants have the potential to become big problems in natural systems, in Oregon and everywhere else in the world. If they are good competitors, non-natives may out compete nicely-balanced native species for water, nutrients, and light and even exclude natives. Glaring examples are the blanketing of fields and buildings by kudzu in the south, the choking of waterways by water hyacinth throughout the subtropics and tropics (allegedly the most serious weed problem in the world), and irreversible takeover of rangeland by several species of knapweed throughout the arid west. In northwestern Oregon, a search for diversity under English ivy or Himalaya blackberry is instructively sobering; very few other plant species survive under and among these invaders, and the few that do are often invaders themselves.
Wayne Hoffman, coordinator of the MidCoast Watersheds Council, in cooperation with Lincoln County Public Works, organized an April 7, 2001 weed-pulling party of Master Gardeners, including Rebecca Cheek, to remove French broom from the LNG road area. French broom is a non-native invasive yellow-flowered shrubby member of the pea family. Although it resembles Scotch broom, it is assigned to a different genus. French broom is a little less robust than Scotch, a little lighter green, and usually a little leafier. Its flowers are a little lighter yellow (and reportedly toxic), and in each flower cluster, the middle flower opens last. The French broom invasion along Yaquina Bay is at an early enough stage that there is still a good chance of eradicating it. (For Scotch broom all over the west, it's way too late.) Wayne said that Lincoln County Public Works received a grant from the state weed board to control French broom. Wayne, along with Rebecca and the rest of the weed crew, have performed a highly valuable service for us all.
Speaking of non-native yellow-flowered shrubby members of the pea family, there's a nice stand of gorse, another invader, in full flower on the eroding bank at the Inn at Otter Crest in April 2001. The Oregon Department of Agriculture (ODA) weed people told me (Kathy Merrifield) when I phoned that they are aware of it, that it is the only thing stabilizing that bank, and that it does not appear to be spreading at a serious rate - but they are not very happy about it. I forgot to ask them during that phone call if they were aware of the clematis along the north bank of the Yachats River that seems to be growing much more vigorously than our native species and covering everything in its path. They probably know about it, though. The ODA weed people keep themselves amazingly well informed about non-native plants in Oregon, and their vigilance is invaluable.
On 3/25/2001, Kathy Merrifield observed skunk cabbage in full bloom along the Yachats River up to 7 miles away from coast, coast prickly- fruited gooseberry in full bloom at Mike Miller County Park, and sweet coltsfoot a few miles away from the coast was in full bloom but a little behind that at immediate coast. The earliest common horsetails had begun sporulating along the immediate coast.
On 4/1/2001 along Salmon Creek Road off Highway 20 in east Lincoln County, Kathy Merrifield also saw that small false Solomon's seal and false lily of the valley leaves were just emerging, bleeding hearts and toothwort were in early flower, trillium and red-flowering currant were in full flower, and salmonberry was flowering profusely, although leaves were still just emerging. Along Otter Crest Loop, fringe-cups were in very early flower, but sweet coltsfoot flowering stalks were beginning to age and die. Cow parsnip was in early leaf at many coastal localities.
Also on 4/1/2001, the earliest bigleaf maples were in very early flower from the west side of Cline Hill Summit of Highway 20 to the coast (Kathy Merrifield). None were flowering from the east slope of Cline Hill until the east slope leading down to Blodgett (Benton County); these were at about the same flowering stage as those on the coastal slope. Bigleaf maples in Philomath and Corvallis, however, were in full flower. Assuming that increasing temperature is one factor influencing flowering time, this provides clues to relative temperatures between the coast and the Willamette Valley and suggests that the air temperatures of urban environments are warmer than those of surrounding systems.
On 5/6/2001, purple beach pea was flowering at Bayshore Beach, and western buttercup, coast strawberry, English daisy, and sea thrift were flowering along the low bluffs just south of the Yachats River mouth (Kathy Merrifield). The Yachats population of yellow- eyed grass, a small iris native to coastal deflation plains which grows along small freshwater streams under the bluffs, flowered during the week of 5/21/2001. The assemblage of plants on these low bluffs is intriguing because it comprises coastal bluff upland species (sea thrift, seaside plantain, coastal strawberry, seacoast angelica, California fescue, sagina), interdune deflation plain species (Pacific silverweed, yellow-eyed grass), inland upland species (false lily-of-the-valley), and freshwater wetland plants (springbank clover, yellow monkeyflower, a small spikerush).
On 1/1/2002, "pussy willows" are emerging on a Hooker willow rooted from a shrub along Yaquina Bay (Kathy Merrifield). On 2/3/2002, pussy willows (probably Hooker willow) and Scotch broom were flowering along Yaquina Bay Road (Joel Geier).
On 6/4/2002, Sally Lockyear admired clustered rose and honeysuckle in bloom while eating wild strawberries along the 804 trail in Yachats.
In mid-August 2002, the mountain ash berries near Shirley Schwartz' Neskowin (Tillamook County) home were ripe on the top half of the tree, while the bottom half was in bloom. This is very unusual. Although the days there are sunny and warm, the nights are cold: the temperatures sometimes dropped to the upper 30s in mid-August, but they are usually in the 50s. SS lives in a valley behind Cascade Head. The setting sun disappears behind the head sooner than it might elsewhere, accounting for the early cooling down.
On 2/23/2003, yellow streamside violets (=Johnny jump-up), milkmaids (= bittercress, spring beauty), and sweet coltsfoot were flowering "everywhere" along Sam Creek between Logsden and Chitwood; by 3/4, western trillium and red-flowering currant flowered there (Chuck Philo). The same day along Nute Slough, the first Pacific silverweed and cow parsnip leaves emerged (Kathy Merrifield).
It happens every fall: not only do leaves change color, but people ask me why. Most of the time, I say something about chlorophyll breaking down and anthocyanins being synthesized, mumble a few Latin names to cover my ignorance, and creep back to my lab before anyone can ask any more questions.
I don't know why people ask ME this stuff. I'm not a plant physiologist, and the OSU Department of Botany and Plant Pathology, where I'm just a lil ole research assistant, is full of all kinds of botanists who could provide far better answers. However, a new graduate student who asked me this question last fall is such a thorough person and good scholar that I felt he deserved an accurate reply. I looked in actual references and amalgamated their comments for him. After condensation and removal of most of the smart-aleck comments, I thought it appeared fit to pass along to YB&N. Here goes....
SENESCENCE. Seasonal color change in leaves, along with leaf abscission, is an aging, or senescence, process. Senescence is a progression of irreversible change leading eventually to death. (Imagine all the comments that I, with my whitening hair, am tempted to insert here.) The phenomenon of leaf color change and abscission presents a paradox: this progression towards death is a mechanism for saving the tree's life during winter.
LEAF LOSS AS AN ENVIRONMENTAL ADAPTATION. The loss of leaves in the fall is an adaptation that keeps deciduous trees from desiccating during winter, when water cannot be drawn from the frozen ground. (This brings up two additional questions: 1) the ground doesn't usually freeze in northwestern Oregon, so why are leaves falling off in the first place? and 2) how can evergreen trees such as madrone (Arbutus menziesii) and most conifers live through the winter? The answers to these questions are beyond the scope of this little write-up.)
COLOR CHANGE AND ITS RELATIONSHIP TO ABSCISSION. Seasonal color change in leaves is intimately related to abscission. Abscission is the active separation of a leaf, fruit, or nonfertilized flower from a branch leaving branch tissue intact. Senescence of cells in the tip of the leaf, resulting in the mobilization of nutrients towards the base, is required for abscission in some species. Magnesium, nitrogen, amino acids, and sugars move to storage tissues in the stem in the fall (and then back to developing leaves the following spring). After the autumn loss of these substances, leaves stop making chlorophyll. (Magnesium is the central atom in chlorophyll, kind of like the iron atom in hemoglobin.) The remaining chlorophyll disintegrates, and green color is lost.
SUBSTANCES RESPONSIBLE FOR FALL COLORS. Three categories of substances are responsible for fall colors following chlorophyll disintegration. The first category of these newly visible substances are the ANTHOCYANINS, which are produced in the fall at the same time leaves stop producing chlorophyll. These crimson to purple water-soluble pigments occur in vacuoles (simple membrane-bound bodies in cells). Not all plant species have the genetic capability to produce anthocyanins. In those that do, environmental factors favoring anthocyanin production include low nitrogen levels, low temperatures, bright light, and drought, all of which contribute to a high sugar concentration in leaves. Anthocyanins are synthesized from some of these sugars. They produce the reds in the brighter fall leaves in our area, such as vine maple (Acer circinatum) and the dogwoods (Cornus species).
The second category of newly visible substances are the CAROTENOIDS. These yellow and orange pigments, soluble in alcohols and fats but not in water, are held in plastids (tiny organelles in cells containing multiple membrane layers housing pigments). Carotenoids - carotene and xanthophyll, for example - are present throughout the season but far less abundantly than chlorophyll, so most leaves appear green until senescence. Many leaves lacking anthocyanins, such as those of Acer macrophyllum (bigleaf maple), Fraxinus latifolia (Oregon ash), and Amelanchier alnifolia (serviceberry), turn yellow or orange due to carotenoids. Carotenoids also contribute to the color of leaves that do contain anthocyanins.
The third category of newly visible substances are the TANNINS, which like anthocyanins occur in vacuoles. Tannins are a heterogeneous group of phenol derivatives that are widely distributed throughout the plant body. They provide many of the browns in leaves and bark. The leaves of oaks such as Quercus garryana (Oregon white oak) are good examples of leaves that turn brown due to tannins. (In addition, the Kalapuya, Willamette Valley natives, leached Oregon white oak acorns to remove the tannins before eating them, and oak bark works well for tanning because of its high tannin content. No wonder there are so many tanning salons in the Willamette Valley.)
ABSCISSION. The abscission zone is the two-layered area at the base of the petiole (little stem-like structure at the leaf base) along which a senile leaf breaks naturally. Small, thin-walled cells form a separation layer where the leaf and the stem will separate, and cork cells form a scar on the stem side of the zone, preventing invasion of the plant by pathogens. The leaf falls off when enzymes degrade the walls of the separation layer. Formation of the abscission zone is controlled by a change in the balance of plant hormones (ethylene, auxin, and cytokinin). Two environmental changes - decreased daylength and cooling temperatures - stimulate leaf senescence. Senescence in different tree species is controlled to a relatively greater or lesser extent by each of these factors.
So there you have it. Leaves change color, and then they fall off. As I think through the role of leaf litter in plant and soil systems, however, it seems like the excitement is only starting when the leaves hit the ground. Hmm... I can feel another explanation coming on.
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