"I would not inflict this work on anyone else," says Sally Grace Bunker, a Hong Kong-based British botanical artist. "It's taking me six or seven hours a day, every day. And it's work that requires the utmost patience. Luckily, I have a lot of patience and I love detailed work."

Bunker is mid-project, recording the indigenous and significant trees of Hong Kong. She is collaborating with Richard Saunders, professor of plant systematics and phylogenetics at the University of Hong Kong's School of Biological Sciences, and Pang Chun-chiu, a pollination biologist and post-doctoral fellow in Saunders' lab. Together, they hope to produce the first Hong Kong heritage book to cross the boundaries of art and science.

"Trees are the most stable plants there are around," says Bunker. "They give us everything. Without them, we cannot live. Hong Kong is covered in amazing trees."

Today, Hong Kong is home to a verdant landscape but historical records dating back to the 18th century depict the territory as devoid of trees. In 1742, French landscape and botanical painter Jean-Louis Prevost observed that the territory's islands were "sterile and covered in rocks".

Nearly a century later, in 1841, the British colonised Hong Kong. That year, naturalist Richard Brinsley Hinds described it as "wild, dreary, bleak and apparently barren".

The British initiated a huge forestation programme, designed to provide fuel, charcoal and construction materials. By 1938, 70 per cent of Hong Kong Island was covered in tree plantations. But the second world war brought mass destruction to this forest cover. Fuel routes to Hong Kong were destroyed leaving beleaguered residents with no choice but to chop down the trees.

After the war, a second wave of forestation occurred. Many of the trees planted were a Chinese red pine, the Pinus massoniana. In the late 1970s, however, these trees were devastated by pine wilt disease.

The vacillation from no trees to plenty and back again has defined Hong Kong's landscape over the past few centuries. Happily, now, we are in a period of abundance.

There are 390 types of tree that are indigenous to Hong Kong. Many more exotic species have been introduced as ornamentals. Saunders has picked just 100 trees to include in the book.

"There were different criteria for picking the trees," he says. "Some are really conspicuous, like the Bombax tree. It's not a native species but it's distinctive and lines many roadsides, so it has to go in. Others are included because they have an interesting story. We are also trying to get species that occupy different habitats - upland forests, lowland forests, the mangroves - these all need including."

Each tree will have a double spread, one page dedicated to the description, history and scientific importance of the tree, the other featuring the botanical illustration. Saunders is in charge of the science, Bunker the art.

"Botanical illustration has a history going back hundreds of years," says Saunders. "It's a great art form and there are many structures that are much easier to illustrate than to capture on a photo. On the one hand it's a work of art, but it's much more than that because it also highlights the scientific information.

"The science is important. I don't want people to just pick up the book, look at the pretty pictures and then put it down. I want them to read the text and engage with the science. So every tree has a narrative. Every tree is interesting for a different reason. I try to bring that out in the text along with scientific hypotheses and also cross-reference it all so people can dig further if they want.

"[Take] the bauhinia, the symbol of Hong Kong. It has a great story.

"The tree is a hybrid between two species that are not native to Hong Kong. They were planted here in the 19th century because they have pretty flowers. Someone found a hybrid, Bauhinia blakeana, growing on Mount Davis in the late 19th century. The tree was removed and planted in the gardens at Bethanie [Sanatorium, in Pok Fu Lam] and a cutting was donated to the [Zoological and] Botanical Gardens. That tree became the only surviving specimen [but it] was blown down in 1906 by a huge typhoon. Botanists took cuttings and planted them. It's now a tree of horticultural importance and global significance and it all came from that one tree. It's sterile so it cannot produce seeds - it has to be artificially propagated - so there's a whole industry surrounding it. It therefore doesn't really qualify as a species but we are including it."

As an evolutionary biologist, Saunders is also fascinated by the way plants have evolved.

"Trees have an amazing diversity of flower and fruit structure, which has evolved in response to selective pressures. Macaranga tanarius, known in English as elephant's ear, has big, distinctive leaves … It is of scientific interest because of the way it has changed its pollination system. The trees in this genus have very small flowers that are usually pollinated by tiny insects called thrips. However, this species is pollinated by 'flower bugs', a predator of the thrips. They come to the tree to eat the thrips and the tree has adapted to be pollinated by them instead. This is associated with a change in the morphology of the flower."

"I am learning a lot about the science," says Bunker. "It's pushed me to my limits to identify some of the trees. I read about it, look at the photo and then go out to try to find it. I am becoming more botanical in a scientific way and to translate that into painting is wonderful."

Bunker lives above Mui Wo, on Lantau Island. Botanical drawing is her second career. Prior to 2009, she was the principal of a small school on Lantau. Retirement gave her the time to pursue her passion, so she spent three years doing a distance course with the Society of Botanical Artists in London, for which she gained a distinction. She is a year away from full membership of the society.

Bunker's studio overlooks the sea. Once a week, she takes to the local hiking paths, mangroves and mountains on field trips.

"I have a hand-drawn map," she explains. "I go out and look for the trees and then mark them because I need to return several times to get the whole cycle of the tree. I get the bud, then three weeks later go back to get the flower, then I have to wait until the fruits come out. Each tree involves at least three, if not more, visits. Last year, I went on holiday and missed the Aleurites fordii flowering. When we came back, there had been a big storm and the flowers were all gone. So, I had to wait a year. This year, we are not going on holiday until I have drawn those flowers!"

Bunker either draws the tree then and there or takes a sample and works from it at home. Everything is drawn to scale in immense detail. For example, she collects large numbers of the flowers, dissects them and counts each and every stamen to get an accurate representation.

"People say to me, 'Why don't you just take a photograph?' But, if you look at a photograph, you simply cannot see the details. A botanical drawing gives the essence of a plant. It gives the details of its growth, its flowers, its fruit, its structure, everything about it that makes that plant. A photograph gives a shot of that flower at one given time. A botanical painting allows you the time to get the colours just right. I've seen purples and turquoises in husks that a photo will either miss or overexaggerate."

Each painting is in pencil and watercolour. Bunker is meticulous about the colours. All her watercolours are carefully labelled and her drawings are colour-coded. Unusually for a botanical artist, Bunker draws the trunk complete with its bark.

"I do a tracing of the different parts of the tree, then I stick the tracing paper to my window so the sun shines through from behind. Then, I put high-quality cartridge paper on top and lightly trace onto the paper. It means the paper is pristine - there's no rubbing out of lines, no mistakes, so you get a lovely clean composition.

"This is a beautiful tree," she says, indicating a drawing of Sterculia lanceolata. Next to a sketch of tiny, delicate pink flowers is a large, red-husked fruit with black, olive-shaped seeds. "The contrast between those delicate pink flowers and that brilliant fruit is an example of nature's great design. The fruit is smooth like velvet so I drew it with little tiny dots to make it look softer."

The work is hard but exciting: "There's one tree which grows on the top of Lantau Peak, so I will have to go up there to find it. It can be very frustrating at times, but when you find it, you get a real high."

Bunker has been stung and bitten. She is also not the only one interested in the trees. "I have got to trees only to find that all the fruit has disappeared, then realised the cows like to eat it. At the moment, the water buffalo are eating the Bombax fruit. I am in a race against them. The worst was when the camphor tree started to fruit in Mui Wo. Every time I got there, the cleaning ladies had swept up all the fruit. The ficus was the same because every morning one particular lady was cleaning them up. I was rushing down to get there before her."

And there are trees Bunker is less than keen on. "I hate the prickly ash [ Zanthoxylum avicennae]. It deserves its name. If you grab it falling down a slope, you lacerate your hands."

Portraits of Trees of Hong Kong is about a third of the way through, with the estimated publication of the book still three years off.

"I would hope, in the long run, the paintings will be bought as a collection," says Bunker. "I won't split them up because they are important to Hong Kong and should stay together. It's going to be a very large collection - we are looking at 100 trees, which means about 400 botanical paintings.

"I've got a lot to do, actually," she says, with a smile. "It's going to take me quite a while."

Six hours. Every day. For the next three years. Now, that requires patience.