Don't let the nutrients fly away!

Pitcher plants are beautiful but disturbing. Their vase-shaped pitchers are so elegant to look at, yet violate our notion of plants as passive and peaceful, because they are death traps for unwary insects and other small animals that fall in and drown in the pitcher fluid. Pitcher plants (genus Nepenthes) are one of the few carnivorous plants, who also include the Venus Fly Trap and the Bladderwort, that invert the food chain by gaining some of their nutrients from animals.

Nepenthes bicalcarata (by David Sucianto, via Wikimedia Commons)

Why would a photosynthetic organism need to trap animals? After all, they still have green leaves and chlorophyll, like any other plant. But other nutrients, especially nitrogen, are important to plant growth as well. Pitcher plants can grow in nutrient poor soil because they can supplement their intake of nitrogen and other nutrients by trapping and digesting animals, which are especially nitrogen-rich.

Not all animals will die in pitchers, however. Some insects, including the larvae of several dipteran (fly) species, can live in the pitchers and feed on the organic matter found there. The pitcher species Nepenthes bicalcarata also plays host to an ant species, Camponotus schmitzi, that is found only with N. bicalcarata. The ants are somehow able to walk on the slippery inner surface of the pitcher, and predate upon the fly larvae and other organic material, and also feeds on nectar from the plant.

It would seem at first that this is a lousy deal for the plant. The flies and ants are stealing its food right from its mouth! A new research paper published in PLoS ONE shows how the ants and pitcher plants actually derive mutual benefit.

By looking at the nitrogen isotope ratios in the plant tissue, and using isotope labeling experiments, the researchers showed that nitrogen is being transferred from the ants to the nutrients. They also observed how the ants predate upon the fly larvae that live and mature inside the pitchers. Left to their own devices, these larvae would consume the pitcher's nutrient supply, and then literally fly away with the stolen nutrients when they metamorphose into adults. For this they are (harshly) called kleptoparasites, or "thief-parasites". By capturing and eating the flies while they are still larvae or pupae, the ants put a stop to this thievery. The plant itself then recovers these nutrients in the form of the ant colony's waste products.

The ants are hence not only improving the pitcher's prey-capture efficiency, by keeping the slippery pitcher walls clean, but also prevent the nutrients from escaping with the insects. A fascinating story of symbiosis, that reveals just how dynamic and interconnected all these nutritional and behavioral relationships are in Nature.

Plant collectors are endangered species

The days of the botanical adventurer, a relic of the western Age of Discovery, may be over:

The modern botanist tends to focus on one plant group and uses DNA sequences to decode evolutionary history and relationships. “We'll see fewer collections per individual because people are becoming so specialized. Just collecting a lot of specimens isn't something people have much respect for,” says Robbin Moran, who studies ferns at the New York Botanical Garden. The shifts in botany have had costs, he says. “The really big collectors have been tremendous generalists, and that's something that's being lost.”

This news feature in Nature discusses the work of some of the last botanical generalists - plant collectors who venture into the field to collect widely, rather than just particular groups that they specialize in. Why is their number dwindling? Among the reasons: fewer opportunities for botanical fieldwork, the increasing specialization of academic botany, and tighter controls on collection and export of plant specimens by countries which have the forests.

The lifestyle can be punishing. Among the explorers mentioned in the article are Alwyn Gentry of the Missouri Botanical Garden, who died in a plane crash in 1993, and Leonard Co of the Philippines, who was accidentally killed in a military operation while collecting in the field.

For science, the decline of general taxonomic knowledge is worrisome. Specialization is a natural consequence of the growth of knowledge, because as the amount of information grows, the proportion that one can master is correspondingly smaller. No one today would dare like Linnaeus to claim that he or she has constructed a complete System of Nature. But this doesn't mean that being able to name the plants and animals around one's home is irrelevant. As the article points out, one has to start early, when the mind is still impressionable and soaks up information readily. Humans are natural pattern-recognizers, and we have an innate urge to put names to things and classify them.

As a scientist I also value this sort of general knowledge. Science seeks to test hypotheses and answer questions, but these hypotheses and questions have to come from somewhere. In biology, the interesting questions tend to come from observing nature and pondering why something is the way it is: natural history in its most basic sense. Our world would be much poorer without it, and without the people who keep this knowledge alive.

Do plants quiver in fear?

Many trees have leaves that quiver in the wind. This property lends its name to the quaking aspen (below left), whose scientific name Populus tremuloides literally means "trembling poplar". In tropical Asia, the banyan tree (below right) is a commonly-encountered species with quivering leaves. They tend to have fairly broad, thin leaves and narrow, elongated petioles.

Quaking aspen (Populus tremuloides, left) and banyan (Ficus religiosa, right)

But what exactly is quivering good for? Biophysicists have looked at the way that leaves deform in strong winds, and some have hypothesized that this helps them avoid mechanical damage. On the other hand, it may simply be an unintended consequence of adaptation for other reasons, such as having an appropriate shape and orientation for the efficient capture of sunlight.

A new paper recently available online has suggested that quivering may be a means of avoiding herbivores. The author cites other examples of plant "behavior", such as the Sensitive Plant Mimosa pudica, whose leaflets fold up when touched, startling herbivores while simultaneously exposing the thorns on its twigs. Could quivering serve a similar function?

The author suggests that shaking leaves not only make it harder for small herbivores like insects to land on leaves, they may also cause pests and parasite propagules to be literally shaken off. He also hypothesizes that cryptic herbivores, like leaf and stick insects, would stand out from a background of quivering leaves and be made more vulnerable to their own predators. 

At the moment this stands as only an interesting hypothesis. Most biologists would acknowledge that "theory is cheap, data is expensive". It's fun to think about, but it would be lovely if someone could do some experimental work on it. As a topic in sensory ecology, it reminds me of the hypothesis on why autumn leaves turn red, which was covered in this blog a few years ago. Yet I cannot help thinking that maybe this is reading too much adaptationist significance into what may turn out to be a "spandrel", a trait which is merely a by-product of other traits (a term coined by Steve Gould). 

In the meanwhile, we can look at the leaves that flutter in the breeze and wonder: are they nervously trying to shake off their enemies?

Source:

Kazuo Yamazaki (2011) Gone with the wind: Trembling leaves may deter herbivory. Biological Journal of the Linnean Society, published online before print 3 Oct 2011, doi:10.1111/j.1095-8312.2011.01776.x

How the bean got its twist

Legumes are among the most diverse and successful families of plants in the world. In the Neotropical rainforests, the dominant tree species are mostly legumes (unlike in Southeast Asia, where dipterocarps predominate).

Their defining feature are their eponymous fruits, which appear to have a myriad ways of breaking up to release the seeds within. Some drop to the ground and shatter, like the tubular pods of Cassia:

Broken Cassia fistula pod exposing seeds

... while others twist and contort as they dry out to present seeds to dispersers, like Acacia:

Dried up and twisted Acacia auriculiformis seed pods

Legumes are certainly not the only plants with structures that change their shapes as they desiccate. They're not the only organisms to do so, either (think of mushrooms with caps that curl upwards as they mature to release their spores). However, their ubiquity and convenient size makes them a good subject for study and experimentation.

A team of physicists and mathematicians from Israel have recently figured out the rules behind the seed pod's twist, using a legume Bauhinia variegata as the model (paper in Science behind paywall). The basic idea is that the pod wall is made of an anisotropic material, that is, its material properties are not uniform but depend on the direction that it's being manipulated in. In this case, the anisotropy results from the orientation of fibres in the wall. Expansion or shrinkage tend to happen transversely to the aligned fibres. It's also a composite material, being made from at least two layers of wall sandwiched together. These two layers, however, have their fibres aligned in different orientations. As a result, when they dry out, they want to shrink in different directions. This conflict results in a deformation of the wall that produces a helical pattern.

Plant collecting and ex-situ conservation

Earlier this month I blogged about how beautiful wild orchids are under threat from indiscriminate plant collecting to satisfy the itch of orchid-fanciers to own the rarest and most delicate species in their gardens.

However, plant collecting does not always have a sad ending. The Victorian plant collectors who harvested rhododendrons and orchids in obscene quantity from the Himalayan uplands also ventured to other countries. In Japan, they came after the country ended its isolation from the West in 1854, and collected many species which found their way into British gardens and parks. Now, these same species are endangered in their native habitat, but relatively common in cultivation.

Preserving an endangered species outside its native environment is called ex-situ conservation. It's not ideal: they're largely removed from their natural interactions, the context is lost, and reintroducing them could be problematic, especially for animals with complex behaviors. Furthermore, only a few individuals can typically be preserved, so much of the genetic diversity is inevitably lost. However, with careful curation, like with the captive breeding programmes carried out in zoos, it is possible to minimize loss of genetic diversity in the existing captive gene pools and avoid inbreeding.

The article I linked to ends on a pessimistic note: all countries are having to deal with extinction of their native flora and fauna, to a greater or lesser extent. Ex-situ conservation can only help to a small extent, though anything is better than nothing at all.

In our anthropogenic age (what some people are calling the Anthropocene), with species being shuffled all over the globe and having their ways of life permanently changed, maybe it won't make sense to talk about native or non-native any more, except as a sort of historical documentation of what we have lost.

I'm not saying that tracts of the remaining "wild" nature are not worth preserving: the rain forests, taiga, and coral reefs are valuable for being as close to "native" as we can get. What I'm thinking of are the extreme cases of domesticated landscapes–much of old Europe, East Asia, and Mesopotamia–where any hope of reviving a pre-human species palette is impossible. Barring any great human cataclysm, the rest of the world will inevitably become more and more like these places over time.

The agave's big bang

One of the condominiums near my home has a bit of fancy horticulture at the entrance. Lined up along the road is a row of big agave plants, and lately four of them have been at various stages of flowering. If you haven't seen an agave plant (also called "century-plants") flowering, it's a sight to behold: the succulent leaves are thick and waxy, forming a rosette close to the ground, and from the middle emerges a spike-shaped stalked reaching taller than a person, from which the flowers will eventually unfurl.

Agave plants flowering in Portugal (Wikimedia)

The agave plants (see the Agavaceae webpage for more photographs) are also known as century plants because of a misconception that they only flower once a century. It is true however that it can take up to several years or even decades before an individual plant flowers. Once it does so, it apparently withers away and dies. For this reason, probably, in some of the plants that I saw, the flower stalks were cut away soon afterwards.

Why would any organism only flower once in its lifetime? Doesn't natural selection reward those who have more offspring? Isn't having only one crop of offspring akin to putting all your eggs in one basket?

Plant gives birds a place to stand

The Cape of Good Hope on the Southern tip of Africa is one of the world's great floristic zones: mega-centers of plant biodiversity where the weird and wonderful have made their home. One of these residents, a member of the Iris family called Rat's Tail, Babiana ringens, has an unusual structure that sticks out vertically, looking much like a rodent's nether appendage.

Botanists from the University of Stellenbosch in South Africa have lately suggested that this structure has evolved as a built-in perch for birds that pollinate the plant's flowers (BBC News, Univ. Stellenbosch). They observed that sunbirds were the only pollinators of these plants, and that the birds perched on the stalk while reaching down towards the flowers. Even more compellingly, in regions where the birds had access to other plant species for nectar, the perches were smaller, an instance of "relaxed selection", where a trait is less pronounced when natural selection is acting less strongly.

All this only serves to confirm my prejudice that animals are merely vehicles for plants to move around in.

Perils of beauty - Attack of the "Orchid snatchers"

Collectors of rare orchids are stripping them from the rainforest, driving the most beautiful and unique species to rapid extinction. Is beauty a curse? Perhaps so, if you're an orchid!

This is nothing new, of course. Even the Victorian Britons suffered the same collecting-mania, which they called "orchidelirium". The period saw a fashion for hothouses among the wealthy, and professional plant-collectors were despatched to far-flung parts of the world to procure new and interesting plants for horticulture. This is the subject of a book, The Plant Hunters, by Tyler Whittle.

Eminent botanists were not innocent parties either:

"Sir Joseph Hooker, traveling in India, remarks that Vanda caerulea is 'The rarest and most beautiful of Indian orchids'. Yet in a footnote he mentions that he collected 'seven men's loads' of this plant (few of which reached England alive); he goes on to suggest that collectors with better facilities for getting the plants home 'might easily clear from £2,000 to £3,000 in one season, by the sale of Khasia orchids' - and this is just what happened. ... Few orchids now remain in the Himalayan foothills."

This is quoted from Anthony Huxley's very readable book Green Inheritance: The WWF Book of Plants, originally published in 1984 but still a good source of information about the ways people make use of plants and relate to our green environment.

The BBC article cites the case of a new orchid species, Bulbophyllum kubahense, that was introduced into the black market even before its discoverer, the orchidologist Jaap Vermeulen, had even published the formal description in a botanical journal.

All orchids are in Appendix II of the CITES treaty (see list - scroll down), which means that permits are required for them to be exported. A number of rare species, including all the slipper-orchids (Paphiopedilium) are Appendix I species, where trade in wild-caught individuals is illegal except with special permits.

Monitoring the trade and policing the rainforest is a mammoth task, which perhaps can never be fully accomplished. The only hope may lie in cutting back demand. Most orchids on sale in garden shops are artificially propagated, with Singapore having played a major role in the development of orchid propagation techniques. Nonetheless, for the conscientious shopper, it is probably worth paying more attention to the provenance of your garden plants before buying them, just as people are paying attention to the provenance of the fish they eat and the coffee they drink.

No more bananas by mid-century?

Four varieties of banana (Wikimedia Commons)

They can see it coming. Most bananas sold today belong to the Cavendish variety, which is grown in extensive monoculture plantations around the world. The plants are virtually sterile, and are propagated by transplanting suckers or cuttings. The cultivar Gros Michel, which was the predecessor to the Cavendish, was wiped out in the early 20th century by a fungus called the Panama disease, Fusarium oxysporum f. sp. Cubense, which as the names suggest showed up in the Caribbean. A new strain of this fungal pathogen has emerged in recent decades to attack the Cavendish variety, which is now under threat. The new strain, called Tropical Race 4, has spread through the Asia-Pacific region, and has yet to hit Latin America, that other bastion of banana agriculture, but it's only a matter of time.

Possible solutions include genetically-modified strains of the Cavendish banana plant, or increasing the genetic variety of bananas in cultivation. Preserving genetic variation in crop foods is now an important concern, given the susceptibility of Green Revolution-style monoculture projects to pathogens. Seed banks around the world store varieties of important crops like the potato and rice as an insurance policy, as traditional cultivars are being abandoned for 'modern' high-yield plants. There is also a thriving trade in so-called 'heirloom seeds' among gardening enthusiasts and smaller-scale farms.

Will such measures be enough to save the banana? Time will tell, but the days of the big, starchy, and seedless Cavendish may be numbered. (Another fun new fact: India "grows and consumes more bananas than any other country in the world." Who knew?)

(via Gawker)

Spellchecker to Weed Out Botanical Typos [Updated]

[Update 17/6/11]
Following the comments by araygoza to this article (see below), I ran my list of misspelled plant names through the TNRS spellchecker and voila! it works!

That's really cool! The new URL is here.

****
Scientific nomenclature - the business of giving names to organisms - is a huge book-keeping exercise that is notoriously error-prone. Not only may one species be given multiple binomials, as scientists argue whether it should be classified one way or another, but misspellings and typos are easy to make.

Typos directly affect scientific research, which often relies on analyzing data from species databases. For example, a simple count of how many species are present in a certain place may be inflated because some name records have been misspelled. For botanists, there is now a possible solution available online. A team from several institutions has launched the Taxonomic Name Resolution Service, which uses technology similar to spell-checking software to detect erroneous names and suggest corrections.

I decided to give the system a try (click on "Try it now!" on the main page), using the names of four common plants with single-letter misspellings, listed below:

• Cocos nuciferra
• Samana saman 
• Pterocarpus indicum
• Ficus grossulariodes

The results showed that these names were not found in the database, but it couldn't suggest any possible matches, which was a disappointment. Just in case they weren't actually in the database (it's based on TROPICOS, which focuses on plants from the Americas), I ran the correctly-spelled names through the checker and got a 100% match on each one.

From this small trial, which admittedly is not very thorough, I would say that the fuzzy-logic system for suggesting correct names is not really effective right now. However, the batch-check function is useful if one has a large database of records to sort through for validity. They have posted their source code and also are developing an API (application programming interface) for the service, so developers (or botanists with some computing savvy) can readily plug in their own systems.

(Via Nature News.)

Where Do Petals Come From?

"From leaves, of course!" - would have been the answer given by Goethe, the German national poet and occasional scientist. He was one of the early proponents of the idea of homology, where two traits (which may be organs, limbs, tissues...) in two different species are said to be homologous if they are derived from the same organ in their common ancestor. Hence Goethe believed that "all plant is leaf", and that every part of the plant can be homologized with the leaves of the original, primeval Ur-plant.

We've come a long way since then. Floral development in particular has fascinated generations of botanists. Flowers comprise four parts, or 'whorls'–the sepals, petals, stamens, and carpel–listed from the outermost inwards. The sepals and petals, collectively called the perianth, are not always differentiated, and when they cannot they are called tepals. Non-flowering seed plants, or gymnosperms, such as pines, Ginkgo, and cycads, however, lack homologues to the perianth. Stamens and carpels, on the other hand, can be homologized to the male and female sexual organs of the gymnosperm cones (strobili). So where does the perianth come from?

Feeling the Burn

I love food, especially spicy food. The sting of chillies adds a good kick to any meal, and makes even a bowl of plain rice palatable. Why should it be so, though? After all, the pain we feel from eating chillies is very real - capsaicin, the active ingredient in chillies, directly stimulates pain receptors in our taste buds. It doesn't make sense that we should enjoy doing something that hurts us, and indeed other animals shy away from them. James Gorman at the New York Times writes about why we like to punish ourselves with chillies, and why these plants are so spicy to begin with.

Fossilized leaf-mimic lacewings

Lacewings, or Neuroptera, are a order of insects with very distinctively patterned wings, as their names (both in English and Latin) suggest. Among insects in general, many species, especially the well-known leaf katydids, are mimics of flowering-plant leaves. Such leaf mimicry presumably is a form of camouflage that functions in either defence or in aid of predation.

Paleontologists working in China have discovered a rare set of lacewing fossils (open access article) from Middle Jurassic rocks at a site in Inner Mongolia which are leaf mimics - a useful adaptation given their large size. However, their age means that they pre-date the Cretaceous radiation of flowering plants. The relationship between flowering plants (angiosperms) and insects has been characterized as a synergistic adaptive radiation - both groups flourished in diversity on similar timescales, an observation explained by pointing to the frequent close relationships and associations between insects and angiosperms. Therefore, leaf mimicry has long been thought to be a post-angiosperm phenomenon, and virtually all known leaf-mimics pretend to be angiosperm leaves. These lacewings, however, appear to mimic cycads or bennettiales, both of which are non-flowering seed plants (gymnosperms) that dominated the pre-angiosperm plant world. Both fossil and extant cycads have distinctive pinnate leaves, which the lacewings resemble. These are very different in form from the typical angiosperm leaf.

On the whole this is a very neat story - close insect-plant associations or coevolution appeared long before the rise of the angiosperms. The change in plant communities from gymnosperm-dominated to angiosperm-dominated, however, would have diminished the effectiveness of these cycad mimics, and might explain their disappearance. Perhaps some later fossil discoveries, likely coming from China, the new hot-bed for paleontology, might shed more light on the affair....

On Rice, Mosquito Ferns, and Cyanobacteria...

This blog has been veering towards the theme of food in recent posts. Perhaps the most important staple food in Asia is rice, traditionally grown in irrigated paddy fields. A piece of natural history trivia that has stuck with me from my childhood fascination with ferns is how paddy farmers encourage the growth of the mosquito fern Azolla, because it has nitrogen-fixing cyanobacteria that live in symbiosis with it, thereby contributing to the nitrogen content of the soil.

Norman Borlaug

Norman Borlaug, "The Father of the Green Revolution" passed away at the age of 95 on Sept 12 2009. He was instrumental in developing and introducing semi-dwarf, high-yield, disease resistant wheat varieties. NY times wrote of him as "...the plant scientist who did more than anyone else in the 20th Century to teach the world to feed itself..."

He won the Nobel Peace Prize for agricultural innovation and the development of high-yield crops in 1970. The Green Revolution has been touted to have averted a world-wide famine in the late 20th century.

Excerpts of his speech during the Nobel Prize award ceremony showed how committed he was in using science for the betterment of mankind:

"Accordingly, I shall not dwell upon the personal honor, for I have not done so even within myself. Instead, I want to devote my remarks to commendation of the Nobel Committee which had the perspicacity and wisdom to recognize the actual and potential contributions of agricultural production to prosperity and peace among the nations and peoples of the world.

Obviously, I am personally honored beyond all dreams by my election. But the obligations imposed by the honor are far greater than the honor itself, both as concerns me personally and also the army of hunger fighters in which I voluntarily enlisted a quarter of a century ago for a lifetime term. I am acutely conscious of the fact that I am but one member of that vast army and so I want to share not only the present honor but also the future obligations with all my companions in arms, for the Green Revolution has not yet been won.

It is true that the tide of the battle against hunger has changed for the better during the past three years. But tides have a way of flowing and then ebbing again. We may be at high tide now, but ebb tide could soon set in if we become complacent and relax our efforts. For we are dealing with two opposing forces, the scientific power of food production and the biologic power of human reproduction.

Man has made amazing progress recently in his potential mastery of these two contending powers. Science, invention, and technology have given him materials and methods for increasing his food supplies substantially and sometimes spectacularly, as I hope to prove tomorrow in my first address as a newly decorated and dedicated Nobel Laureate. Man also has acquired the means to reduce the rate of human reproduction effectively and humanely. He is using his powers for increasing the rate and amount of food production. But he is not yet using adequately his potential for decreasing the rate of human reproduction. The result is that the rate of population increase exceeds the rate of increase in food production in some areas.

There can be no permanent progress in the battle against hunger until the agencies that fight for increased food production and those that fight for population control unite in a common effort. Fighting alone, they may win temporary skirmishes, but united they can win a decisive and lasting victory to provide food and other amenities of a progressive civilization for the benefit of all mankind.

Then, indeed, Alfred Nobel's efforts to promote Brotherhood between nations and their peoples will become a reality.

Let our wills say that it shall be so."

Further reading:

• "Agriculture pioneer Borlaug dies." BBC News, 13 Sep 2009.
• "Norman Borlaug, Plant Scientist Who Fought Famine, Dies at 95," by Justin Gillis. The New York Times, 13 Sep 2009.
• Wikipedia article: Norman Borlaug
• Norman Borlaug's Nobel Prize Speech: The Nobel Prize in Peace 1979, Norman Borlaug, nobelprize.org.
• "Norman Borlaugh and the high-yield dwarf spring wheat," by N. Sivasothi. Otterman speaks, 22 May 2005.

The Story of Maize

The story of how maize (or corn) was domesticated by human beings is a fascinating mix of biology (both classical and modern) and archaeology. It's a tale of careful observation and empirical reasoning that began not in the past century but over 9000 years ago:

"These people were living in small groups and shifting their settlements seasonally. Yet they were able to transform a grass with many inconvenient, unwanted features into a high-yielding, easily harvested food crop."

Cool fact from the article: George Beadle, of Beadle and Tatum fame, was the first modern biologist to suggest that teosinte was the wild ancestor of domesticated maize.

Fungus attacking Afghan opium

Opium, like any other crop, is prone to pests and diseases, and a fungal disease is making a big dent in this year's production. One wonders how well studied the fungal diseases affecting drug crops like opium and cannabis are. Farmers, of course, blame NATO and its anti-drug operations, saying that it looks like a white powder sprayed from the air, while officials deny any responsibility for the outbreak.

Flower-petal coccoons

Bees with a sense of style! The solitary bee Osmia (Ozbekosmia) avosetta from Turkey and Iran uses colorful petals from a legume plant to make its nests; each cell has two layers of petals sandwiching a thin layer of mud. Other Osmiine bees also use petals in their nest construction. A particularly beautiful example of animal architecture.

Original paper from the American Museum Novitates (AMNH), and NPR report with pictures.

(Via Sar)

Urban ecology: Revisiting weeds?

Urban landscapes seem very different from natural, 'pristine' landscapes, but many similar principles apply to understanding both of them. Those of us who are city-dwellers from birth, like most Singaporeans, have a very skewed vision of what urban vegetation 'should' be like, conditioned by heavily manicured and landscapes parks and plantings. We tend to look down on weeds and wildflowers as plant pests and ugly eyesores to be managed rather than celebrated. But weeds, or to use a more neutral term, 'spontaneous vegetation', have an important function in the urban environment, as Peter del Tredici argues.

Del Tredici is a scientist at the Arnold Arboretum and lecturer at the Harvard Graduate School of Design, and has just written a book titled Wild Urban Plants of the Northeast: A Field Guide, where he explains his philosophy of urban ecology in detail, along with descriptions of common species in the American Northeast. I attended a talk he gave in conjunction with a booksigning event, and was struck by how many of the things he observed about urban vegetation also applied to what we find in Singapore:

• Plants that thrive in urban open lots tend to be native to harsh environments. In the US, the 'Tree of Heaven' (Ailanthus altissima) is a cliff-dwelling species from East Asia and common in cities like Boston; in Singapore, Adinandra dumosa, which is common on wasteland, is actually native to limestone hills on the Peninsula.
  
• Bird-dispersed species with small seeds spread quickly and aggressively. Most of the successful species in urban landscapes are early successional species in 'nature'.
  
• The growth of spontaneous vegetation is inversely proportional to how wealthy the neighborhood is. In shrinking cities with declining economies, like Detroit, abandoned lots are being left to be grown over by wild grass and shrubs, and some companies even run pheasant hunts in the city because those birds are starting to colonize these spaces.
  
• Wayside trees planted for ornament and shade can escape and invade native habitats, displacing native species. Norway Maple is a big problem in Massachusetts, where it's illegal to plant this species, just like Albizzia might be in Singapore.
  

Ultimately what he argues for is a change of perspective - we can see these as ugly weeds, or we can incorporate them into our appreciation of the urban landscape, using and manipulating them to suit our needs. Even in their unmanaged form, urban wild plants provide many important services, not least among them shade and temperature regulation, soil consolidation, and even phytoremediation. With the economic downturn, less money is available for urban landscaping and planting, but if we can make use of these erstwhile invasive species, by weeding out the uglier and less cooperative species, making paths and borders, we could produce wild urban woodlands that enhance the urban experience, instead of detracting from it.

Looking at the plant list in his book, it is also striking how many of the species listed are actually common to both places. At the booksigning, he wrote in my copy that I "won't find any of these in Singapore," but at the genus or family level, there are quite a number of familiar names: Composites, Legumes, and Crucifers for sure, but also Euphorbiaceae, Scrophulariaceae, and Vitaceae. Familiar genera abounded especially among the grasses: Eragrostis, Digitaria, Poa. I'm sure someone has had similar thoughts before, but it would be instructive to look at the comparative community phylogenetics of weeds around the world - are certain groups of plants particularly predisposed to be successful in the urban landscape? If so we should start getting familiar with them, because these are the species that will be most familiar to most of the human population in time to come, if not already.

Why Autumn leaves are red (part II)

One of the hypotheses to explain the investment of anthocyanins in plants during autumn was very well detailed here previously (see the 14 Nov 2008 post). A recent TRENDS review by Archetti himself, with Doring and other experts in diverse fields lists the abiotic (photoprotection, osmotic regulation and warming) and other biotic factors (coevolution, fruit flag, direct defence, camouflage, anticamouflage and tritrophic mutualism). Here's a short note on the photoprotection hypothesis, as reviewed by Archetti et al., to complement the anti-herbivory theory.

As protection against photo-oxidative stress in autumn, the red anthocyanins may do one, two or all of three things:

(1) Directly shield leaf tissues from the sun's rays. There is some support for such a function in aging, young and evergreen leaves, as red leaves are less light stressed than non-red leaves in high light conditions.

(2) Indirectly protecting tissues from reactive oxygen species (ROS; i.e., antioxidant effect), hydrogen peroxide being the most likely as it is the only one known to penetrate both chloroplast (ROS produced) and vacuole (ROS stored).

(3) Enhancing nutrient (nitrogen) absorption through maintenance of light absorption function by anthocyanins.

The photoprotection hypothesis is significant due to the increased risk of damage in autumn as a result of: (i) lower photosynthetic capacity in the cold; (ii) increased light from thinning canopy, and (iii) decreased self-shading by chlorophyll due to its breakdown.

Some other untested hypotheses include:
-anthocyanins help decrease leaf osmotic potential, i.e., retain water to prevent drought stress when leaves start to be lost
-anthocyanins convert light to heat, protecting against cold temperatures
-anthocyanins make leaves unpalatable to herbivores
-anthocyanins inhibit fungal growth
-colours attract birds for seed dispersal
-colours expose camouflaged herbivorous insects
-colours attract aphids + ants (those that are symbiotic to each other) to defend the plant