|Noteworthy Plants For February 1999|
Whirling Nut (Gyrocarpus)
Blowing In The Wind
Seeds & Fruits Dispersed By Wind
|Like an endless army of parachutists released from an airplane, seeds and fruits travel the wind currents and gentle breezes of the earth, possibly colonizing a distant mountain slope or fertile valley. Literally hundreds of species in many plant families have adopted this remarkable method of dispersal, including a variety of ubiquitous plants that we recognize as "weeds." The answer to why some weedy composites (such as the European dandelion) have worldwide distributions is truly "blowing in the wind" (to quote from the Peter, Paul and Mary song). Some of the ingenious adaptations for this method of wind dispersal include seeds that resemble parachutes, helicopters and gliders. In fact, one species (see opening photo) reportedly inspired the design of some early aircraft. An astronomer friend of Mr. Wolffia once observed a strange formation of flying objects through his telescope. He was focusing on a squadron of tiny parachute seeds high above his house. And the entire plant body of wolffia (the world's smallest flowering plant) may be transported by powerful cyclonic storms. In the southeastern United States there are records of wolffia plant bodies less than one millimeter long being carried by a tornado, and they have even been reported in the water of melted hailstones.|
An Introduction To The Botany of Seeds
Seeds provide the vital genetic link and dispersal agent between successive generations of plants. Angiosperm seeds are produced and packaged in botanical structures called fruits which develop from the "female" pistils of flowers. Immature seeds (called ovules) each contain a minute, single-celled egg enclosed within a 7-celled embryo sac. The haploid (1n) egg is fertilized by a haploid (1n) sperm resulting in a diploid (2n) zygote that divides by mitosis into a minute, multicellular embryo within the developing seed. A second sperm unites with 2 haploid polar nuclei inside a binucleate cell called the endosperm mother cell which divides into a mass of nutritive tissue inside the seed. In most seeds the embryo is embedded in this endosperm tissue which provides sustenance to the embryo during germination. In exalbuminous seeds (found in many plants such as the legumes), the endosperm tissue is already absorbed by the time you examine a mature seed within the pod, and the 2 white fleshy halves in the seed are really the cotyledons (components of the embryo). The 2 sperm involved in the double fertilization process originated within the pollen tube that penetrated the embryo sac. The pollen grain (and pollen tube) come from the "male" organs (called anthers) on the same plant or different parental plants in a remarkable process known as pollination. Pollination is also accomplished by the wind (or water), and it may also involve insects in some of nature's most fascinating relationships between a plant and an animal. This is especially true of the amazing fig trees and their symbiotic wasps.
One of the important functions of seeds and fruits is dispersal; a mechanism to establish the embryo-bearing seeds in a suitable place away from their parental plants. There are 3 main mechanisms for seed and fruit dispersal: (1) Hitchhiking on animals, (2) Drifting in ocean or fresh water, and (3) Floating in the wind. This article concerns one of the most remarkable of all seed dispersal methods, riding the wind and air currents of the world.
Helicopters: A. Box Elder (Acer negundo, Aceraceae); C. Big-Leaf Maple (Acer macrophyllum, Aceraceae); E: Evergreen Ash (Fraxinus uhdei, Oleaceae); F. Tipu Tree (Tipuana tipu, Fabaceae).
Flutterer/Spinners: B. Empress Tree (Paulownia tomentosa, Scrophulariaceae); D. Tree Of Heaven (Ailanthus altissima, Simaroubaceae); G. Jacaranda (Jacaranda mimosifolia, Bignoniaceae).
Note: The tree of heaven seed (D) actually spins along its longitudinal axis like a rolling pin.
|The remarkable winged seed of the tropical Asian climbing gourd Alsomitra macrocarpa. The entire seed has a wingspan of 5 inches (13 cm) and is capable of gliding through the air of the rain forest in wide circles. This seed reportedly inspired the design of early aircraft and gliders.|
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Gliders include seeds with 2 lateral wings that resemble the wings of an airplane. They become airborne when released from their fruit and sail through the air like a true glider. One of the best examples of this method is Alsomitra macrocarpa, a tropical vine in the Gourd Family (Cucurbitaceae) native to the Sunda Islands of the Malay Archipelago. Football-sized gourds hang from the vine high in the forest canopy, each packed with hundreds of winged seeds. The seeds have two papery, membranous wings, with combined wingspans of up to 5 inches (13 cm). They reportedly inspired the wing design of some early aircraft, gliders and kites. Although the seeds vary in shape, some of the most symmetrical ones superficially resemble the shape of the "flying wing" aircraft or a modern Stealth Bomber. According to Peter Loewer (Seeds: The Definitive Guide to Growing, History, and Lore, 1995), the aerodynamic seeds spiral downward in 20 foot (6 meter) circles, although a gust of wind would probably carry them much farther away.
|An individual parachute of western salsify (Tragopogon dubius) showing an umbrella-like, plumose crown of hairs (pappus) above a slender one-seeded fruit (called an achene). These fragile units can become airborne with the slightest gust of wind, and can literally sail across valleys and over mountain slopes.|
|Western salsify or goatsbeard (Tragopogon dubius) showing dense, puff-like cluster of numerous parachute seeds (one-seeded achenes). Each achene has an umbrella-like crown of plumose hairs and may literally be carried into the atmosphere by strong ascending air currents.|
|A population explosion of western salsify(Tragopogon dubius) near Mono Lake, on the east side of the Sierra Nevada of Central California. This ubiquitous species is actually native to Europe and Asia.|
Parachutes include seeds or achenes (one-seeded fruits) with an elevated, umbrella-like crown of intricately-branched hairs at the top, often produced in globose heads or puff-like clusters. The slightest gust of wind catches the elaborate crown of plumose hairs, raising and propelling the seed into the air like a parachute. This is the classic mechanism of dispersal for the Eurasian dandelion (Taraxacum officinale) and includes numerous weedy and native members of the Sunflower Family (Asteraceae). A giant Eurasian version of the dandelion called salsify or goat's beard (Tragopogon dubius), is one of the most successful wind-travelers in North America. Its seeds have literally blown across mountain ranges, colonizing vast fields of open land in the western United States. Three weedy species of salsify (T. dubius, T. pratensis and T. porrifolius) have been introduced into the western United States, 2 with yellow dandelion-type flowers and one with purple flowers. The latter, purple-flowered species (T. porrifolius) has a large, edible tap root with a flavor resembling oysters, hence the name "oyster plant."
|Inflorescence and mature, seed-bearing head of the Eurasian dandelion (Taraxacum officinale). The slightest gust of wind catches the elaborate crown of plumose hairs, raising and propelling each seed-bearing achene into the air like a parachute. This successful weed thrives in a wide range of climates and has become naturalized throughout North America.|
In some parachutes, the crown of silky hairs arises directly from the top of the seed (not on an umbrella-like stalk). Again, the Sunflower Family (world's largest plant family with about 24,000 described species) contains many weedy representatives with this type of parachute seed. One of the most troublesome weeds of farm land in the western United States is wild or thistle artichoke (Cynara cardunculus). The large seed head of this weedy composite releases hundreds of parachute seeds which fly through the air and invade vast areas of grazing land with spiny, perennial bushes that literally take over. The large leaf stalks (resembling giant celery stalks) are edible and are sold under the name of "cardoon." Populations of wild artichoke often contain so much variation between spiny and non-spiny plants, that some experts believe that they belong to one variable species. In fact, some botanists believe that the cultivated artichoke (C. scolymus) may be a cultivated variety of the wild C. cardunculus. Incidentally, the delicious artichoke is really a cooked flower head in which the outer bracts (phyllaries) and central basal portion (receptacle) are dipped in butter and eaten.
|Brown puffs (Stebbinoseris heterocarpa), formerly Microseris heterocarpa, an interesting member of the sunflower family. A closely related species is called silver puffs (Uropappus lindleyi syn. Microseris lindleyi). In Stebbinoseris the pappus paleae are bifid at the apex. Unlike the weedy dandelions (Taraxacum) and salsify (Tragopogon), this is a native species in California.|
Another plant family which has evolved this parachute method of seed dispersal is the Milkweed Family (Asclepiadaceae). Hundreds of parachute seeds (each with a tuft of silky hairs) are produced within large, inflated pods called follicles. So abundant are the silky hairs, that they were actually collected and used as a substitute for kapok during World War II. Kapok comes from masses of silky hairs that line the seed capsules of the kapok tree (Ceiba pentandra), an enormous rain forest tree of Central and South America. Kapok is used primarily as a waterproof filler for mattresses, pillows, upholstery, softballs, and especially for life preservers. The floss silk tree (Chorisia speciosa), another member of the Bombax Family (Bombaceae) also produces large seed capsules lined with masses of silky hairs. This tree with its distinctive thorny trunk and showy pink flowers is commonly planted in southern California. The seeds of kapok and floss silk trees are embedded in these silky masses which aid in their dispersal by wind; however they probably belong in Section 5 below (Cottony Seeds & Fruits).
The Dogbane Family (Apocynaceae) also includes members with seed pods (follicles) and parachute seeds similar to those of milkweeds. One of the best examples is Nerium oleander, a drought-resistant, Mediterranean shrub planted throughout southern California. The foliage contains a powerful cardiac glycoside that can permanently relax the heart muscle.
|Parachute seeds escaping from the follicle of Nerium oleander. The crown of silky hairs arises directly from the top of the seed (not on an umbrella-like stalk. Unlike the seed-bearing achenes of the sunflower family (Asteraceae), these are true seeds.|
3. Helicopters (Whirlybirds)
|The South American tipu tree (Tipuana tipu) has one of the most unusual legumes in the world. Unlike the fruits of most members of the legume family (Fabaceae), the 3rd largest plant family, the fruits of this tree have a distinctive wing that causes the legume to spin as it falls from the rain forest canopy.|
Helicopters (also called Whirlybirds) include seeds or one-seeded fruits (samaras) with a rigid or membranous wing at one end. The wing typically has a slight pitch (like a propeller or fan blade), causing the seed to spin as it falls. Depending on the wind velocity and distance above the ground, helicopter seeds can be carried considerable distances away from the parent plant. The spinning action is similar to auto-rotation in helicopters, when a helicopter "slowly" descends after a power loss.
Numerous species of flowering trees and shrubs in many diverse and unrelated plant families have evolved this ingenious method of seed dispersal, good examples of convergent evolution. Representative examples of helicopter seeds and one-seeded fruits (called samaras) include the Maple Family (Aceraceae): Maples and box elder (Acer); Olive Family (Oleaceae): Ash (Fraxinus); Legume Family (Fabaceae): Tipu tree (Tipuana tipu); and the Protea Family (Proteaceae): Banksia and Hakea.
|An interesting one-seeded winged fruit that spins as it falls through the air. It is called "whirling nut" and belongs to the genus Gyrocarpus in the family Gyrocarpaceae. Note: This genus of tropical trees, shrubs and lianas is often placed in the hernandia family (Hernandiaceae). The unusual fruit shown above was collected and photographed at Ho'omaluhia Botanical Garden on the windward side of Oahu in the Hawaiian Islands. Identification provided by Ricarda Riina, Botany Department, University of Wisconsin.|
|Spinning fruits (seeds) from Thailand: A. Diptocarpus alatus (Diptocarpaceae). B. Diptocarpus obtusifolius. C. Gluta (Melanorrhoea) usitata (Anacardiaceae). The latter species is also called the Burmese lacquer tree and is well-known as a source of lacquer. The resin is chemically similar to the Japanese lacquer tree. Like the Japanese lacquer tree and poison oak, the resin canals also contain urushiol, a mixture of toxic phenolic compounds that cause a cell-mediated immune response in some people. Identifications courtesy of Dr. Tomiki Sando, Thailand.|
Although they are classified as gymnosperms with naked seeds arising from woody cones rather than flowers, the Pine Family (Pinaceae) contains many genera with winged seeds, including Pinus (Pine), Abies (fir), Picea (spruce), Tsuga (hemlock), and many additional genera. When shed from cones high on upper branches, they fly over slopes and across deep canyons. The natural reforestation of conifers following fire is proof of the flying ability of seeds from nearby forested slopes.
Maples have a double or twin samara composed of 2 winged one-seeded fruits (double samara) joined together at their bases. When they break apart, each winged fruit flies like a typical helicopter seed. Although the Legume Family (Fabaceae) is the third largest plant family with over 18,000 described species, the vast majority of legumes do not have winged seeds or fruits. The South American tipu tree (Tipuana tipu) is a notable exception, with beautiful yellow blossoms that give rise to pendant, samara-like legumes, each with a large wing on the lower end. The dried, winged legumes spin so neatly in the air that they could be marketed as a child's toy.
The remarkable Protea Family (Proteaceae) of Australia contains some truly amazing genera with winged seeds, including Banksia and Hakea. Although they are flowering plants, banksias produce a dense flower cluster (inflorescence) that gives rise to a cone-like structure containing many woody carpels. Each carpel bears 2 winged seeds and the entire cone-like structure superficially resembles a pine cone. In fact, some banksias release their seeds following fire and even resprout from subterranean lignotubers like chaparral shrubs.
|The native range of hopseed bush (Dodonea viscosa), a member of the Soapberry Family (Sapindaceae), extends from Arizona to South America. It is also commonly cultivated in southern California. The papery, winged fruits flutter and spin in the air, and may be carried short distances by the wind.|
|The jacaranda tree (Jacaranda mimosifolia) of northwestern Argentina. Like many other members of the Bignonia Family (Bignoniaceae), the papery, winged seeds flutter and spin as they are carried by the wind.|
Although their mode of dispersal is similar to single-winged helicopter seeds, the flutterer/spinners include seeds with a papery wing around the entire seed or at each end. When released from their seed capsules they flutter or spin through the air. Whether they spin or merely flutter depends on the size, shape and pitch of the wings, and the wind velocity. This method of wind dispersal is found in numerous species of flowering plants in many different plant families. Some examples of flutterer/spinner seeds include the Quassia Family (Simaroubaceae): Tree of heaven (Ailanthus altissima); Figwort Family (Scrophulariaceae): Empress tree (Paulownia tomentosa); Bignonia Family (Bignoniaceae): Jacaranda (Jacaranda mimosifolia), catalpa (Catalpa speciosa), desert willow (Chilopsis linearis), yellow bells (Tecoma stans), bower vine (Pandorea jasminoides), violet trumpet vine (Clytostoma callistegioides), and the fabulous trumpet trees (Tabebuia serratifolia and T. ipe); Elm Family (Ulmaceae): American and Chinese elms (Ulmus americana and U. parvifolia); Soapberry Family (Sapindaceae): Hop seed (Dodonea viscosa); and the Goosefoot Family (Chenopodiaceae): Four-wing saltbush (Atriplex canescens).
Any discussion of flutterer/spinners would not be complete without mentioning the quipo tree (Cavanillesia platanifolia), a massive rain forest tree in the bombax family (Bombacaeae) native to Panama. The enormous winged fruits of the quipo tree flutter through the air, carpeting the ground beneath the huge canopy of this striking tropical tree.
|The quipo tree (Cavanillesia platanifolia), a remarkable rain forest tree in the bombax family (Bombacaceae) with huge winged fruits. This massive tree is native to Panama.|
Some of the most beautiful flowering trees of the New World tropics belong to the Bignonia Family (Bignoniaceae). They typically produce long, slender (cigar-shaped) seed capsules containing masses of flat seeds with papery wings at each end. [The beautiful jacaranda of Argentina has flattened, circular seed capsules.] The lovely yellow bells (Tecoma stans) is native to Mexico and the Caribbean region, and is the official flower of the U.S. Virgin Islands. Some of the South American trumpet trees, including the pink-flowered Tabebuia avellanedae (listed as T. ipe in some references) and the yellow-flowered Tabebuia serratifolia, are also called ironwoods or axe-breakers (quebrachos) because of their dense, hard wood. The latter species is called "pau d'arco" and its wood actually sinks in water, with a specific gravity of 1.20. In South America, trumpet trees drop their leaves during the dry season and produce a profusion of pink or yellow blossoms. The crowns of these huge timber trees resemble gigantic floral bouquets in the midst of the forest. As with so many tropical species, some of the trumpet trees inhabit rain forest areas that are seriously threatened by slash and burn agriculture, large plantations of exportable products, and the general annihilation of the South American rain forests.
Other South American species of Tabebuia are also referred to as pau d'arco, including the pink-flowered T. impetiginosa and T. avellanedae. According to The New York Botanical Garden Encyclopedia of Horticulture Volume 10, 1982, T. avellanedae is a synonym for T. impetiginosa, and T. ipe " is so closely similar to T. impetiginosa that it can scarcely be more than a variety of that species." These attractive pink-flowered species are commonly used as landscape trees in temperate regions.
|The powdered inner bark of these pink-flowered species of pau d'arco is sold as a popular herbal remedy that reportedly stimulates the immune system. According to a book by Kenneth Jones (Pau d'Arco: Immune Power From the Rain Forest, Healing Arts Press, 1995), this valuable herb has been proven successful in the treatment of certain cancers, allergies associated with the Candida yeast syndrome, and in disorders involving a weakened immune system.|
Probably the best way to appreciate the relative hardness of different woods is the concept of "specific gravity," a numerical scale based on 1.0 for pure water. Without getting too mathematical, the specific gravity of a substance can easily be calculated by dividing its density (in grams per cubic centimeter) by the density of pure water (one gram per cubic centimeter). The brilliant Greek mathematician and inventor Archimedes discovered over 2,100 years ago that a body in water is buoyed up by a force equal to weight of the water displaced. Archimedes reportedly came upon this discovery in his bathtub, and ran out into the street without his clothing shouting "Eureka, I have found it." Since one gram of pure water occupies a volume of one cubic centimeter, anything having a specific gravity greater than 1.0 will sink in pure water. The principles of buoyancy and specific gravity are utilized in many ways, from scuba diving and chemistry to the hardness of dry, seasoned wood. Some of the heaviest hardwood trees and shrubs of the United States have specific gravities between 0.80 and 0.95; including shagbark hickory (Carya ovata), persimmon (Diospyros virginiana) and ironwood (Ostrya virginiana) of the eastern states, and canyon live oak (Quercus chrysolepis), Engelmann oak (Q. engelmannii), hollyleaf cherry (Prunus ilicifolia) and Santa Cruz Island ironwood (Lyonothamnus floribundus ssp. asplenifolius) of southern California. Although some of these trees are called ironwoods, their dense, dry wood will still float in water. Since the pure cell wall material (lignin and cellulose)) of wood has a density of about 1.5 grams per cubic centimeter, even the world's heaviest hardwoods generally have specific gravities less than 1.5 due to tiny pores (lumens) within the cell walls. True ironwoods include trees and shrubs with dry, seasoned woods that actually sink in water, with specific gravities greater than 1.0. They include lignum vitae (Guaicum officinale, 1.37); quebracho (Schinopsis balansae, 1.28); pau d'arco (Tabebuia serratifolia, 1.20); knob-thorn (Acacia pallens, 1.19); desert ironwood (Olneya tesota, 1.15); and ebony (Diospyros ebenum, 1.12). To appreciate the weight of these hardwoods, compare them with tropical American balsa (Ochroma pyramidale), one of the softest and lightest woods with a specific gravity of only 0.17.
5. Cottony Seeds & Fruits
|Fuzzy brown cattail spikes (Typha latifolia) contain dense masses of tiny seeds, each with a tuft of silky hairs. Each spike contains about a million seeds. They are shed by the millions in a cloud of white fluff.|
Cottony seeds and fruits include seeds and minute seed capsules with a tuft (coma) of cottony hairs at one end, or seeds embedded in a cottony mass. Some of the examples in this group are very similar in function to parachute seeds, but probably are not carried as far by the wind. Many plant families have this type of wind dispersal, including the Willow Family (Salicaceae): Willows (Salix) and Cottonwoods (Populus); Cattail Family (Typhaceae): Cattails (Typha); Evening Primrose Family (Onagraceae): Willow-Herb (Epilobium) and California fuchsia (Zauschneria); Bombax Family (Bombaceae): Kapok tree (Ceiba pentandra) and floss silk tree (Chorisia speciosa); and the Sycamore Family (Platanaceae): Sycamore (Platanus).
In the California sycamore (Platanus racemosa), a common riparian (streamside) tree throughout the state, the one-seeded fruits (achenes or nutlets) are produced in dense, globose heads. The spherical heads hang from branches like little balls. Individual achenes have a tuft of hairs at the base which probably helps in their wind dispersal. Seeds of the South American kapok tree (Ceiba pentandra) and floss silk tree (Chorisia speciosa) are embedded in dense masses of silky hairs inside large woody capsules. This undoubtedly helps to disperse the seeds when seed-bearing masses of hair are carried by the wind. In tropical regions of the New World, the kapok grows into an enormous rain forest tree with a massive buttressed trunk. Kapok hairs are coated with a highly water-resistant, waxy cutin layer. The empty lumen (cavity) inside each hair is larger the cotton hairs; hence, the hairs are lighter. Unlike cotton hairs, kapok is difficult to spin and is not made into textiles. It is used primarily as a waterproof filler for mattresses, pillows, upholstery, softballs, and especially for life preservers. A kapok-filled life jacket can support 30 times its own weight in water.
One fuzzy brown cattail spike may contain a million tiny seeds. Each seed has a tuft of silky white hairs and is small enough to pass through the "eye" of an ordinary sewing needle. They are shed in clouds of white fluff and float through the air like miniature parachutes. A cattail marsh covering one acre may produce a trillion seeds, more than 200 times the number of people in the world. The fluffy seeds have been used for waterproof insulation and the buoyant filling of life jackets. In addition, each plant produces billions of wind-borne pollen grains; in fact, so much pollen that it was used as flour by North American Indians and made into bread. Cottonwoods and willows also produce masses of seeds, each with a tuft of soft, white hairs. Since they are dioecious, with pollen-bearing male and seed-bearing female trees in the population, only female trees produce the actual cotton. During late spring and summer in the western United States, the cottony fluff from cottonwoods resembles newly fallen snow. Because the wind-blown fluff can be quite messy in cultivated parks and gardens, male trees are generally planted. The discriminatory label of "cottonless cottonwood" refers to a male tree.
6. Tumbleweed (Russian Thistle)
The common tumbleweed or Russian thistle is a rounded, bushy annual introduced into the western United States from the plains of southeastern Russia and western Siberia in the late 1800s. The name "thistle" comes from the stiff, sharp-pointed, awl-shaped leaves. Although it is depicted in songs of the old west, this species is a naturalized weed in North America. It is listed in most older references as Salsola kali or S. pestifer; however, the Jepson Flora of California (1993) lists it as S. tragus. Russian thistle belongs to the goosefoot family (Chenopodiaceae), along with many weedy species and some valuable vegetables, including beets (Beta vulgaris), goosefoot (Chenopodium album) and spinach (Spinacia oleracea).
|A large tumbleweed (Salsola tragus) in San Diego County, California. Tumbleweeds are pushed along by the wind, scattering thousands of seeds as they roll across open fields and valleys. A tumbleweed of this size is difficult to hold on to during a strong wind storm.|
Tumbleweed is a prolific seeder and rapid seed germination and seedling establishment occurs after only a brief and limited rainy season. A single plant may produce 20,000 to 50,000 seeds within numerous small fruits, each surrounded by a circular, papery border. Mature plants readily break off at the ground level and are pushed along by strong gusts of wind. As they roll along hillsides and valleys, the seeds are scartered across the landscape. Tumbleweeds often pile up in wind rows along fences and buildings. This is a troublesome weed in agricultural areas because it literally covers the farm land with bushy, prickly shrubs. One interesting use for this plant in arid regions of the American southwest is for a "snowman" at Christmas time. Three proportionally sized tumbleweeds are used to make the head, thorax and main body of a "snowman." Another suggested use is to compress tumbleweeds into logs and use them for firewood.
|A tumbleweed "snowman" in San Diego County made from three dried plants of Salsola tragus.|
|Squirrel-Tail Grass (Elymus elymoides), formerly named Sitanion hystrix is an attractive grass native to the mountains and plains of the western United States. Seed-bearing sections (spikelets) of the flower spike (containing one-seeded fruits called grains and very long awns) are carried short distances by the wind. Although not as efficient fliers, the long awns function like the parachute bristles (pappus) of composites.|
This miscellaneous category of wind-blown seeds and fruits includes plants that really don't fit the above 5 categories. The Grass Family (Poaceae) includes a number of species with plumose flower stalks that fragment into seed-bearing spikelets that blow into the wind. Some of these species have become troublesome weeds in southern California, including the South African fountain grass (Pennisetum setaceum). Although this tufted perennial makes an attractive, drought-resistant landscaping plant along walkways and roads, it is becoming a widespread weed in disturbed areas of San Diego County. Another species, called squirrel-tail grass (Elymus elymoides), resembles a weedy introduced grass, but it is actually a native perennial of dry, rocky mountains and open land in the western United States. To appreciate its airborne seeds, you really must see this grass during a strong gust of wind on the eastern slopes of the Sierra Nevada during late summer.
Mountain mahogany (Cercocarpus minutiflorus), a native shrub in the chaparral of southern California, produces a rather unique wind-blown fruit. The one-seeded fruit (achene) has a persistent, feathery style that glistens in the sunlight. Although they usually don't travel very far, the achenes are blown into the air by strong gusts of wind during the dry, fire season of late summer and fall. This species is not related to the West Indian mahogany (Swietenia mahagoni) or the Honduran mahogany (S. macrophylla), members of the true Mahogany Family (Meliaceae). Mountain mahogany actually belongs to the Rose Family (Rosaceae) and produces very hard wood that sinks in water when dry. In fact, the wood of a montane species (C. ledifolius), has a specific gravity of 1.12, as heavy and dense as ebony (Diospyros ebenum).
All text material & images on these pages copyright © W.P. Armstrong
Optical remote sensing investigations in tropical regions generally focus on the generation of land cover maps used to estimate tropical deforestation trends and habitat fragmentation at a regional and local scale , which in turn are used to estimate impacts on biological diversity in protected areas [2–4]. Other remote sensing monitoring efforts in tropical regions have focused on modeling ecosystem structure and composition [5,6], forest stand age [7–9], estimation of Leaf Area Index , Fraction of the Photochemical Active Radiation (FPAR) , and the identification and separability of tree species using hyperspectral imagery [11–13].
In the context of species separability using leaf and airborne spectral datasets for tropical regions, significant advances have been achieved [11–15]. Cochrane  using an approach designed by Price , illustrated the possibility of remotely identifying species using mahogany (as a reference species) and several other hardwood species from the Brazilian Amazon, but the approach was limited to an evaluation of the spectra’s amplitude and shape. Later, Clark et al.  demonstrated, using leaf spectra, that significant differences can be observed in several spectral bands across the visible and short wave infrared range for species on a tropical dry forest of Costa Rica. Castro-Esau et al.  further explored the problem of intra- and inter-species variability using a comprehensive leaf data set of tropical dry and rainforest trees from Mesoamerica. The main contribution of Castro-Esau et al.  was a novel implementation of machine learning algorithms, to better understand the potential separability among tropical tree species. Castro-Esau et al.  concluded that some level of separability exists among different species at the leaf level, and that the level of intra-species variability is sometimes as wide as the differences among distinct species. Zhang et al.  explored the same problem at the canopy level using imaging spectral data rather than single leaf measurements for a site at La Selva, Costa Rica. Zhang et al.  concluded, using the energy levels derived from wavelet coefficients, that wavelet transforms presented a robust tool for the identification of tree species using hyperspectral data, but warned that it may be impractical to expect the identification of species using only hyperspectral signals, given the high level of spectral similarity that exists at the intra- and inter-species level, confirming the finding by . Similar studies at the leaf and plant level have been conducted by Tung et al. , Kamaruzaman and Ibrahim , Chaichoke et al. , Kelly and Carter  and Lucas and Carter . Of importance to this work is Chaichoke et al.  whose methods diverge from established classification approaches , and move toward advanced classification techniques for plant species discrimination. Finally, Rivard et al.  expanded these findings to include the short wave infrared spectrum, and concluded, in agreement with Clark et al. , that significant inter-species differences exist in the shortwave infrared region of the light spectrum, and that further work is necessary to explore those linkages toward species identification in tropical regions. Furthermore, the success of single-species identification in tropical environments is also affected by the presence of liana loads and other parasitic elements living in tree crowns because of their tendency to mask the true tree spectral reflectance [22–25].
In addition to the studies mentioned above, other studies have shown the validity of using hyperspectral approaches to characterize patterns of regional ecosystem structure and function [26,27], as well as biomass estimates and biological diversity [28–30]. Although many of these studies have shown some degree of success, many of these approaches tend to be site specific, presenting problems when applied to varying ecosystems .
Crown-level analyses in tropical environments have benefited from the use of high resolution remote sensing (pixel <5 m), but few high-resolution studies in the tropics have been conducted, in part due to data costs. Pioneer studies have demonstrated the value of high-resolution satellite imagery for monitoring crown diameter and tree mortality in tropical environments [32–34]. The work of Asner et al.  and Palace et al.  has been especially promising regarding crown delineation/identification in tropical regions, which, in turn, can be linked to tree architecture studies. Tree crown separation is also of value in guiding the analyses of hyperspectral datasets.
One aspect that has not been explored in tropical environments is the linkage between high-resolution remote sensing, tree phenology (defined here as tree flowering events) towards species identification and mapping. Such studies have the potential of providing important information on the extent of populations of threatened or endangered tree species. In addition, in the context of long-term ecological monitoring programs, these studies may provide important insights into the response of tropical ecosystems to climate change and habitat fragmentation via the quantification of the extent of specific tree populations using reproduction signals as a proxy.
In this context, this paper explores how a combination of spectral analysis techniques applied to two Quickbird satellite images can be used to map the spatial distribution of reproductively mature Tabebuia guayacan trees at Barro Colorado Island (BCI) in Panama based on flowering events. The T. guayacan tree is a hardwood tree, used extensively since colonial times in Central America for construction projects. T. guayacan has one of the most extensive flowering responses to precipitation after the dry season in the tropics , making it an excellent candidate to evaluate the effectiveness of high resolution remote sensing techniques for mapping an explicit phenological expression (e.g., flowering episode). Such information can be used in developing conservation and sustainable management plans for this and related species.
2.1. Description of the Image Data and the Study Site
Two high-resolution Quickbird satellite images (2.4 m pixels) acquired in 29April 2002 (5% cloud cover) and 21 March 2004 (0% cloud cover) over Barro Colorado Island (BCI) in Panama (Figure 1) were used for this study. Acquisition dates were selected to capture extensive flowering events of T. guayacan (Bignoniaceae).
T. guayacan flowering events are triggered at the end of the dry season (February–April), following short, intense precipitation episodes. T. guayacan flowering phases are “big bang” events characterized mostly by a “single brief highly conspicuous burst of mass flowering.” A T. guayacan canopy grouping typically presents up to 10,000 flowers in one single flowering event. Flowers can range from one to four inches in diameter, and grow in dense clusters. These flowers have a life expectancy of just two days . Further studies  indicate that adjacent T. guayacan trees display near-perfect inflorescence synchronization. Flowering events may occur one or two times in a year (Figure 2).
The aforementioned phenological traits of this species make it remarkably well-suited to Quickbird image evaluation for population estimates. Two T. guayacan traits stand out: (1) brief inflorescence periods due to short-lived flowering bodies, and (2) synchronization of inflorescence due to precipitation triggering. The first trait centers on the short lifespan of an individual T. guayacan flowers, which can survive for no longer than two days, supporting the assertion that the two selected images in this study captured the entire regional phenological expression. In this context, for example, the 29 April 2002 Quickbird image captured all trees that flowered either April 28th or the 29th In other words, if all flowers were present on April 28th, all flowers will be dead and undetectable by April 30th.
The second trait that makes T. guayacan suitable for populations estimate by Quickbird image evaluation is synchronization. Inflorescence occurs in response to a large precipitation event, which synchronizes flowering for all adjacent individuals. Gentry , in documenting phenological observations of T. guayacan in Panama, indicated that this specific species presents “an amazing coordination of flowering between all the individuals of a population.” In other words, all trees observed on the Quickbird images are flowering at the same time, and they flower for two days only.
Both Quickbird images were georectified to UTM Zone 17 North. The April 2002 image was georeferenced using the Barro Colorado Island geo-spatial database and used as the master imagery for an image-to-image rectification of the March 2004 image. In both cases, a second-degree polynomial-resampling algorithm was used, given the relatively flat relief of the region. Root mean-square errors associated with the geo-rectification were estimated to be on the order of 1.5 m.
The imagery was atmospherically corrected by first converting the Quickbird data into radiance values using two different sets of absolute radiometric calibration factors (K values) provided by Digital Globe®. K values for 2002 and 2004 were different since they changed on 6 June 2003. Once that the images were converted into radiance values, an atmospheric correction using FLAASH with a tropical atmospheric model and sensor filter model was implemented using ENVI®.
To validate predictions from imagery, we used field data collected from a 50-ha long-term monitoring plot managed by the Smithsonian Tropical Research Institute (STRI, see insert in Figure 1). In this plot, STRI has identified all individuals of tree species. These data over the 50-ha plot reveal a population of 22 T. guayacan trees with a Diameter at Breast Height (DBH) equal to or higher than 0.20 m. Pursuing such an inventory for the entire island was not economically feasible.
2.2. Processing Algorithms
Image processing aimed at demarcating the centroid of flowering T. guayacan trees was conducted using two techniques: spectral angle mapping or SAM , and linear spectral unmixing, or LSU [40,41]. SAM computes the similarity of an unknown spectrum (of a pixel) in comparison to a reference spectrum, and has the advantage of allowing targeting of specific objects under variable illumination in the image . LSU determines the abundance of pure endmember (class) materials within a pixel, based on the assumptions the pixels are pure and that the reflectance of each pixel is a linear combination of the reflectance components of each endmember material within the pixel. The performance of these algorithms is dependent on several factors, primarily including the data used and the spectral endmember selected . Both algorithms (SAM and LSU) can be used to convert hyperspectral/multispectral imagery into biophysical information or thematic maps. Both SAM and LSU were used in this study to detect flowering T. guayacan trees, and the results were jointly analyzed to improve the mapping accuracy for mixed pixels. The two algorithms are similar in that both techniques require input of the target spectral signature (referred to herein as endmember), which in this case was obtained by averaging the brightest pixels of flowering T. guayacan trees as defined by One-Class Support Vector Machines (OCSVM).
2.2.1. One-Class Support Vector Machines (OCSVM)
The detection of the yellow flowering T. guayacan represents a one-class classification problem, a special case of the binary (two class) classification problem where data from only one class, the target class, are available and sampled well. The other class, the outlier class, is sampled sparsely or not at all, and the boundary between the two classes must be estimated from data of the available objects. Thus, the task is to define a boundary around the target class, such that it encircles as many target examples as possible and minimizes the chance of accepting outliers.
Thus, OCSVM, a recently developed one-class classifier and also a special type of Support Vector Machine (SVM), was adopted in the present study for the specific purpose of detecting pure pixels of flowering T. guayacan and excluding mixed pixels and pixels of background targets. The SVM is a statistical learning method , which can effectively handle high-dimensional data with a limited number of training samples. The SVM has shown considerable potential for the classification of remotely sensed data [44,45]. In the two-class formulation, the basic idea is to map feature vectors to a high dimensional space and compute a hyper-plane that not only separates the training vectors from different classes, but also maximizes this separation by making the margin as large as possible.
Scholkopf et al.  developed OCSVM to deal with the one-class classification problem. The OCSVM algorithm first maps input data into a high dimensional feature space via a kernel function, and then iteratively finds the maximal margin hyper-plane that best separates the training data from the origin. The OCSVM may be viewed as a regular two-class SVM where all the training data reside in the first class, and the origin is taken as the only member of the second class. Thus, the hyper-plane (or linear decision boundary) corresponds to the classification function:where w is the normal vector and b is a bias term. The OCSVM solves an optimization problem to find the function f with maximal geometric margin. This classification function can be used to assign a label to a test example x. If f (x) < 0, x is labeled as an anomaly, otherwise it is labeled normal.
Using kernel functions, solving the OCSVM optimization problem is equivalent to solving the following dual quadratic programming problem:subject to the conditions and where αi is a Lagrange multiplier, which can be thought of as a weight for example x, and ν