Acacia mangium growth in relation to light and nutrients

In a recent experiment, we grew Acacia mangium under five light intensities: 4, 7, 25, 50 and 100% side by side with four other rainforest species under conditions free of water stress. Acacia mangium outgrew all the other species as expected, not only under 100% light but also under 50% and 25%. It had the highest leaf turnover rate due to its short leaf life span, which is about 300 days at 4% light dropping to 100 days at 100% light. Its leaf life span drops at a rate of 40 days for each doubling of light intensity. Compared to the next fastest growing species, Shorea roxburghii, Acacia mangium grows 4 times as fast but takes up 15 times as much N, 10 times as much P and 16 times as much K.

Our paper has been published in the Journal of Tropical Forest Science 20 (3) 218-234 (2008). P.S. Tong and F.S.P. Ng: Effect of light intensity on growth, leaf production, leaf lifespan and leaf nutrient budgets of Acacia mangium, Cinnamomum iners, Dyera costulata, Eusideroxylon zwageri and Shorea roxburghii.

http://info.frim.gov.my/cfdocs/infocenter/booksonline/index.cfm?menu=jtfs

Leaf demography, phenology and light

In the humid tropics there is no obvious climatic stimulus to make a tree shed its leaves, but leaf-shedding occurs nevertheless. Some trees shed old leaves and produce new ones all through the year, good examples being the papaya, the oil palm and the coconut. The majority shed leaves all at one go and replace them simultaneously with new leaves. The switchover is usually so smooth that the trees never appear to be bare, so tropical humid forests appear evergreen. In most years there is little or no synchronization between trees, but once in a long while there is a baffling mass synchronization event.

In the 1930s, R.E. Holttum, then Director of the Botanic Gardens Singapore, had the idea that leaves have an inherent life span and are shed when their time is up. He monitored the leaf change cycles in many trees for 10 years or more and found that each species follows an approximate periodicity, with a variation of up to a couple of months. He could not explain the variation.

Recent studies have shown that leaves exposed to full sun have shorter life spans than leaves in shade. A just-published paper by Ms Tong Pei Sin (my student) and myself describes an experiment comparing 5 species (Acacia mangium, Shorea roxburghii, Dyera costulata, Eusideroxylon zwageri and Cinnamomum iners) under 5 light intensities (4, 7, 25, 50 and 100%). This study shows that leaf life span is shortened by a constant amount for every doubling of light intensity. For example, Acacia mangium leaves have a life span of about 300 days at 4% dropping to about 100 days at 100% losing about 40 days per doubling of light intensity. Shorea roxburghii leaves have a life span of about 550 days at 4% dropping to about 200 days at 100%, losing about 70 days for each doubling of light intensity.

Hence leaves do have a life span, but it is a service life span, not a calendar life span. We propose photosenescence of leaves as one of the drivers of phenology in the humid tropics.

Trees in nature, being subject to varying cloud cover, would vary in periodicity of leaf fall because no two periods would ever experience exactly the same combination of cloud cover conditions. A long period of cloudy weather would prolong the leaf life span of many trees and if this is followed by clear sunny days, many trees would trip over their tresholds and shed leaves simultaneously. This would be accompanied by production of new leaves and inflorescences in mass synchrony. The full paper is availble:

http://info.frim.gov.my/cfdocs/infocentre/Korporat/2003Publications/Links/JTFS%2020(3)/CONTENTS.html

or

http://info.frim.gov.my/cfdocs/infocenter/booksonline/index.cfm?menu=jtfs

Creating cannas for the ever-flowering tropical garden

The creation of new flowers is something most gardeners leave to professional plant breeders. Since there are very few professional plant breeders in the humid tropics, the rate of development of new varieties in the humid tropics is very slow. I would like to change this by getting more gardeners into plant breeding, starting with something really easy -- tropical cannas.

Cannas are good subjects because they come in many different floral colours including red, yellow, orange, pink, cream, near-white and mixed or mottled. The leaves are green, red, or striped. There are talls, mediums and dwarfs. Each flower lasts usually two days but a head of flowers may carry up to 20 flowers. An inflorescence usually bears 2-3 heads of flowers in succession and some bear up to 15heads. For every shoot in flower, another should be half-way and a third pushing up from the rhizome underground. Flowering can be prolonged for years.

Here are some other statistics.
Time from pollination to seed-ripening: about 20 days
Time for treated seeds to germinate: about 7 days
Time from germination to flowering: about 3 months
Time for doubling of plants by division of rhizomes: any time after 4th month

There are few plants that one can hybridize and evaluate within 6 months. This is a phenomenally rapid rate compared to tulips, curcumas and orchids.

As far as I know, all cannas, if fertile, will hybridize with each other. However, many garden forms are sterile. The fertile ones advertise themselves by producing fruits spontaneously and spradically. To start your breeding programme, make a collection of fertile varieties first.

To produce new hybrids, apply pollen from a fertile plant to the stigma of another fertile plant. This is best done in the afternoon, using freshly shed pollen.

Open a fully developed bud (tomorrow's flower). Strip off the sepals and petals until you are left with the two innermost members. One will be a stamen, somewhat like a small petal in appearance, which bears an anther on one side. The other will be a flattened pistil, at the tip of which is the stigma. The anther will already have split and deposited its pollen on to the side of the pistil. The pistil, with the pollen on its side, can then be used to dab the freshly shed pollen on to the stigmas of already opened flowers.

After about 20 days the seed will ripen. The seeds harden on drying and will keep for a year or more. To germinate, make a small cut in the hard seed coat with a wire cutter and plant about 1 cm deep in soil.

Springtime all the time--the ever-flowering garden

I gave a talk on this topic in Kuching a couple of days ago, and this is a brief summary.

The everflowering garden can only be created where it is warm and moist all the time, as in the humid tropics. The British had a go at it when they ruled what is now Malaysia and Singapore, but most of the flowering plants they used were annuals imported as seeds from Britain. In front of their bungalows, the British created lawns fringed by borders of flowering plants. The lawn tradition survives but the flowering border could not be sustained. By default, gardens in the humid tropics are evergreen--monotonously so! However, in the past 50 years, more and more tropical perennial ever-flowering plants have come into existence. I counted over 100species in my book Tropical Horticulture and Gardening. The ever-flowering garden is now well within reach and should be one of the aims of tropical gardening.

50 years ago, it was a problem to keep bougainvilleas in flower. Now there are ever-flowering bougainvilleas in a wide range of colours, thanks of plant breeding and selection. Other plants that have become ever-flowering are the drunken sailor Quisqualis indica and Kock's bauhinia Bauhinia kockiana. New forms of Hibiscus, all ever-flowering, have been bred in Hawaii and Australia. Ever-flowering heliconias have become common. Ever-flowering Canna were bred by the late Professor Holttum in Singapore but most of these have been lost; we have to start all over again.

I am particularly keen on cannas because they are easy to breed and select. I distributed hybrid seeds in Kuching and encouraged my audience to form an informal club for future breeding and dissemination of seeds. The breeding and selection of cannas will be my next blog.

A Natural History Museum in Malaysia

Natural history museums are organizations set up for research on animals, plants, and often of minerals, for the preservation of scientific reference specimens, and for public education through authoritative science-based exhibitions. Malaysia, though one of the top countries in biological diversity, does not yet have a natural history museum. Today a further step was taken to establish one, to be called the Natural History Museum Malaysia (NHM Malaysia).

Nine months ago, the Ministry for Natural Resources and Environment, with financial support from UNDP, appointed a team of consultants to prepare a framework proposal for such a museum. As leader of the team, I presented our report to the Steering Committee for the Museum today. Our report has been accepted, and a small project management team has been set up. The team will be based at the Forest Research Institute Malaysia and will be headed by Dr L.G. Saw.

This will be good news for the scientific community in Malaysia which has been lobbying for a natural history museum for over 20 years.

Bumper durian crop expected July 2008

There are usually two durian crops a year in Peninsular Malayia, in about December and about July. However, last December the crop was very poor. Last night I came across a few durians for sale. I stopped to talk to the stall keeper. He told me his fruits are the last of a miserable crop that he had obtained from Johore. However, he said there will be a bumper harvest in July, because durians all over Peninsular Malaysia are now heavy with little fruits.

Durians take 3-4 months from flowering to fruiting, so the flowering must have occured in end March to early April 2008. A big flowering event in durians usually coincides with a 'gregarious flowering' event in the forests affecting all kinds of trees. Hence I expect a bumper crop of all kinds of forest fruits, and a feast for the animals of the forest in July to August. It has been many years since the last such event.

A bumper crop is not good news for durian farmers, because it means the price will be too low for them to make a profit, or even to pay for the fertilizers and for the labour to pick up the fruits.

Cooked cabbage as snail bait in a water lily pond

The water lily pond in which I grow Victoria lilies (Victoria amazonica)was suddenly invaded by water snails two weeks ago. Although the undersides of the lily leaves are thorny, the thorns give no protection against snails. A friend who was with me recommended using boiled cabbage leaves to trap the snails. We did this, placing lightly cooked leaves on the water every evening. It works. Every morning, there are dozens of snails on each leaf, which our workers remove and destroy. I do not know if we can completely eliminate the snails by repeated trapping, but the Victoria lilies have recovered.