For Victorians hoping to deck their halls with traditional boughs of holly, Christmas in 1876 would be one of deep disappointment and leave James McNab, curator of the botanical gardens in Edinburgh, perplexed.

He had received correspondence from all over reporting the unusual scarcity of holly berries: at the Edinburgh gardens clusters of berries were said to be “few and scanty” and in some trees barely present at all.

In Liverpool, the lack of red berries became known as ‘the holly famine” and sparked a mini-industry of fakes made using wax and red paint.

In Dublin, holly trees were a mass of white and cream blooms, but no precious berries.

A late snow and harsh frost which swept parts of the country the previous April, McNab later told the Edinburgh Botanical Society, may well have been to blame.

The curator of what would become the Royal Botanic Garden Edinburgh (RBGE) had a particular interest in the weather’s impact on when plants sprout and flowers bloom.

Concerned over, of all things, the changing climate, he was overseeing innovative phenology research – the study of how climate affects plants, animals, birds and bugs.

In those days, however, the worry was that the thermometer was pointing downwards and the climate cooling.

Generations on and now, with temperatures rising, the RBGE’s modern phenology researchers are taking his work further in an effort to understand the implications of climate change on plants - aided by an army of devoted volunteers who measure their every tiny change.

Launched in 2002 and now marking its 20th anniversary, the modern RBGE phenology project monitors the annual life cycle of over 150 species at its four Gardens - Edinburgh at Inverleith, Benmore in Argyll, Dawyck in the Scottish Borders and Logan in Dumfries & Galloway.

Phenology research has shown that with each degree rise in average temperature, plants advance by between a week to ten days – raising a host of questions over what earlier flowering may mean across a range of areas, from food security to pollination.

“Phenology began in the 1850s when there were concerns about the climate getting colder rather than warmer,” says Dr Antje Ahrends, who leads the RBGE’s modern phenology research programme.

“At that time, it was at the end of a cooling period, they noted it was getting colder and were worried about how that would affect the plants.

“So, when they started phenology research, it was with the opposite concern to what we have today.”

In McKay’s era, the flowering dates of more than 60 species of mostly rhododendrons and roses were recorded.

Today’s research sees daily and weekly recordings of the initiation and duration of flowering and leaf development for over 150 species, with staff and volunteers noting the tiniest changes in growth, the timings when buds and blossoms appear, leaf changes and autumn leaf falls.

A major part of the RBGE study focuses on the phenology of almost 60 Rhododendron species.

Because they are closely related and highly sensitive to changes in temperature, data from the RBGE’s Rhododendron collections provides critical information about the ability of plants to adjust their life cycles in response to environmental cues.

Volunteers and researchers also collect data from fruit trees and cloned poplar trees.

Their findings feed into a Europe-wide network of gardens which use the same recording methods and grow plants in relatively similar surroundings ensuring maximum comparability across the large-scale study.

“Changes in the annual life cycle of plants can have profound consequences for ecological processes, forestry, agriculture, food security, and the global economy,” adds Dr Ahrends.

“The rhododendrons we are monitoring are closely related to each other, so the question is whether they respond similarly because the genes might cause the plant to behave in a certain way or are the responses variable.

“And it seems they are surprisingly variable.”

The research poses questions over whether plants which are highly sensitive to temperature change may have an advantage because they can easily extend their time in bloom, or whether blooming too early means they might miss precious time with pollinators.

Another question also hangs over whether early flowering also leaves plants susceptible to frost and other weather events.

The project relies on the efforts of dedicated volunteers who, regardless of weather, spend hours every week checking for tiny changes in each plant in the programme.

Christine Thompson, who has been involved in the project at the Edinburgh garden for 15 years, says while it sounds like a slow process, the plants can be surprisingly active.

“We are a little team, we look at around 150 plants belonging to about 50 species and each time we look we assess their flowering, their leafing and their fruiting.

“In springtime there’s lot of activity, the rhododendrons are busy putting out new leaves, then as the year goes by that activity slows down. But there is always something to record.

“Once you start looking at rhododendrons – or any plant – you realise that there’s quite a lot going on all around you.”

The project is now seeking to establish how a warming climate will adjust plant behaviour in the future.

She added: “We have observed that spring-flowering rhododendrons are fully sensitive to temperature changes. At the Inverleith Garden they change their flowering date by almost 10 days for every degree change in average temperature in the month before flowering.

“This ‘plastic’ behaviour may help them to cope with moderate climate-change effects.

“However, it may also make them more vulnerable to late frosts and other climatic extremes in their places of origin, such as high-mountain habitats in the Alps and Himalayas.”

With computer modelling suggesting temperatures in the Himalayas may rise by six degrees by 2300, rhododendrons could eventually be flowering 60 days earlier.

“That can throw relationships they have with other plants and with animals that pollinate them out of synch,” she adds.”

Having started in the mid-19th century, Dr Ahrends says it’s now hoped the RBGE phenology research project might expand to include research in Chile, where the temperate rainforest enables comparisons with the few remaining pockets of rainforest in Scotland.

While technology may be used to keep a constant eye on fragile Alpine species in the Cairngorms.

With limited opportunities to retreat to higher, cooler ground and the threat of being overwhelmed by migrating plants seeking more suitable habitats, they are at particular risk of being lost.

Dr Ahrends added: “I am very grateful to our dedicated team of volunteers who have undertaken weekly phenology recording over many years.

“Long-term data collected by the same observers is invaluable for understanding the impact of climate change on vegetation.”