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Friday, 23 January 2015

Some like it hot

By Helen Roberts

With the dismal wet wintry weather prevailing in the UK at this time of year, most people look forward to the return of warm long days, evenings outside and picnics on the beach. Things can look a little dreary and dull in people’s gardens at the moment, before the arrival of spring and its profusion of bulbs and other spring flowers. The Mediterranean collection at the University of Bristol Botanic Gardens helps to remind me of summer sun and balmy places.

Maquis: scrubland vegetation of the Mediterranean


Maquis shrubland in Conca de Dalt of Catalonia, northeastern Spain.
Photo by Gustau Erill i Pinyot.
Licensed under CC BY-SA 3.0 via Wikimedia Commons 
The Mediterranean Basin of Europe and North Africa display shows examples from different vegetation biomes; the collection includes evergreen forest, maquis or macchie and garique. Many of the highly aromatic and fragrant plants are associated with maquis vegetation. The maquis biome, which often includes leathery broadleaves, evergreen shrubs and small trees, usually occurs on the lower slopes of mountains bordering the Mediterranean Sea.

The climate of the maquis biome is characterised by long dry summers and short mild wet winters, with low annual rainfall. Ocean currents and fog influence the temperature and limit the growing season. The plants that grow here cope with the very hot summers by entering a slow growth period or dormant phase so they can resist long periods of drought.

Plant defences in this harsh environment are numerous. Many shrubs and low growing vegetation have thick tough leathery leaves, which reduce water loss but also deter hungry herbivores from grazing. Many plants contain volatile organic compounds (VOCs). These chemicals help facilitate interactions between plants and the environment - from attracting pollinators and seed dispensers to warding off pathogens, parasites and herbivores. When a plant is under attack from a herbivore, for example, it will release a chemical cocktail of VOCs, some of which have been found to attract natural predators of the herbivores. VOCs are also good at leaching into surrounding soil and so will deter the growth of other plants.

Plants containing VOCs are often very flammable, which is partly why areas of maquis are prone to wild fires.

Pyrophytes need a little fire in their lifecycle


Montpelier cistus (Cistus monspeliensis).
Photo by Jean-Pol GRANDMONT
via Wikimedia Commons
Many of the plants in the maquis biomes are pyrophytes (fire loving plants) and require wild fires for reproduction, recycling of nutrients and removal of dead vegetation. Pyrophytes can be classed as active or passive. Active pyrophytes actually encourage fires as they often require fire in order to reproduce. Passive pyrophytes resist the effects of fires.

An active pyrophyte growing in the University of Bristol Botanic Garden maquis biome is Cistus monspeliensis, commonly known as the Montpelier cistus or rock rose. When a fire starts, Cistus has evolved to burn. This combustible conclusion to the adult plant helps destroy competing plants near to it. However, unlike its competitors, the fire will mechanically rupture the seeds of Cistus and the smoke and heat will trigger germination of the next generation. The Cistus seedlings have the advantage of beginning life in a competitor-free environment.

A closed cone and foliage of Pinus pinea.
 Licensed under CC BY-SA 2.5 es via Wikimedia Commons 
Pinus pinea (the stone pine) is also in the Botanic Garden’s collection and it is a passive pyrophyte. It has a thick bark with high moisture content to help it resist fire. Its hard-coated fire resistant cones open when exposed to high temperatures. This is particularly useful in the harvest of the desirable seeds, known to many of us as pine nuts. As the tree grows, it self-prunes the lower branches, which helps prevent the fire travelling at ground level from jumping up into the canopy and potentially destroying the tree.

Some species spring back from fire events by sprouting new growth from the old. There are two such sprouter species in the Botanic Garden’s collection. The carob tree (Ceratonia siliqua) re-sprouts from epicormic buds that lie dormant beneath the bark. These buds are normally suppressed by hormones produced by active shoots, but after a fire event and the destruction of the foliage, the buds are activated. Erica arborea (tree heath) re-sprouts from root crowns so is known as a below ground sprouter. It produces seed in the first post-fire summer.

Fire adapted plants of the maquis biome are only a handful of those plants making up the Garden’s Mediterranean collection. The collection also includes plants (some of which are also pyrophytes) from the southwestern tip of South Africa and displays are also under development from the Western andSouthern Australian, Chilean and Californian Mediterranean climate regions.
The half hardy plants of the Mediterranean collection are planted out in each respective biome in the summer, whilst more tender plants are protected from our cooler Bristol temperatures in the warm temperate and cool zones of the glasshouses. The outdoor collection sits on a south facing rocky bank to maximise the amount of sun and aid drainage. The path meanders through the collection allowing visitors to appreciate the appearance and fragrances of the different plants. These are all good things to look forward to in the summer months, but meanwhile if you have the chance to warm yourself in front of a crackling fire, think of those volatile fire-loving plants! 

Tuesday, 13 January 2015

A walk through the mendips

By Helen Roberts

A few weeks ago our family, had a great day out walking on the Mendip Hills. We set off in autumn sunshine, through pretty deciduous woodland, to an Iron Age hill fort called Dolebury Warren - an upland area of calcareous grassland. Having lived on the edge of the Mendips during my childhood, I am always keen to show my children where I used to explore as a youngster.

Part of Cheddar Gorge, Somerset, England, seen from a
light aircraft. Photo by Adrian Pingstone (1975).
The Mendips are a range of mainly carboniferous limestone hills comprised of at least four convex fold structures formed between 363 and 325 million years ago, during the end of the Carboniferous Period. Weathering of the limestone has resulted in features including gorges, dry valleys, screes and swallets (sink-holes) and incorporates the famous Cheddar Gorge and Burrington Combe, each with extensive cave systems. The area also has interesting landscape characteristics like limestone pavements and other karst structures.

The geology of the Mendips makes for interesting ecological communities and consists of large areas of open calcareous grassland with many rare flowering plants. For instance, Dolebury Warren owned by the National Trust and managed by Avon Wildlife Trust, sits within the Mendip Hills Area of Outstanding Natural Beauty and is a Site of Special Scientific Interest due to its important limestone flower communities. These flower communities attract up to 70% of all British butterfly species. Dolebury Warren has a gradation of communities from species rich calcareous grassland, through acid grassland to limestone heathland, with large areas of mixed scrub.

Cheddar Pink (Dianthus gratianopolitanus).
Photo by Paul Harvey, via Wikimedia Commons
The charity Plantlife has identified the Mendips as an Important Plant Area (IPA), which is an area of landscape that has very high botanical importance. Some plant species found on the Mendips are found nowhere else. Due to factors such as over grazing, poor land management, scrub encroachment and agricultural intensification, these plants are declining in numbers and some are threatened with extinction. The University of Bristol Botanic Garden has a local flora and rare native plant collection, which includes a sub-collection from the Mendip Hills, Limestone Cliffs and Coastal Islands. The collection was developed to help grow and interpret some of the rare and threatened plant species found in these habitats. The collection represents an important habitat and phytogeographic display and is helping meet the objectives of the ‘Global Strategy forPlant Conservation.

Rare plants of the Mendips 


One of the Mendip plants in the Botanic Garden’s collection is the Cheddar Pink (Dianthus gratianopolitanus). This is a very pretty scented pink flower that grows in a few places on the Mendips but mostly at the original site of Cheddar Gorge. This plant was originally discovered about 300 years ago and is considered the pride of Somerset and was voted the County Flower. It grows best in rock crevices, high on the limestone crags of the Gorge, and can be seen in June and July using binoculars to search patches of colour visible on the cliffs, just above the road.

Also growing at the Gardens is the interestingly named Starved Sedge (Carex depauperata). This is an exceptionally endangered plant that is only found in one local area - in the woods and on a hedge bank near the small town of Axbridge. This is one of only two sites in the whole of the UK. Fifteen years ago, Starved Sedge had declined to such an extent that there was only one plant in the whole of Britain. As appearances go, it’s not much to look at. It’s a tussocky plant with trailing leaves and gigantic seeds and can easily be mistaken for some common woodland grasses. A reintroduction programme has improved the status of this plant by using cuttings and seed collecting to re-establish it at other sites in the UK.

The University Botanic Garden are also helping to preserve another plant at high risk of extinction and classed as nationally rare, known as Somerset Hair-grass (Koeleria vallesiana). Again, this is a fairly innocuous grass restricted to the Mendip Hills with very slow spreading habit. The Bristol Botanic Garden’s specimens were collected from Brean Down, which is the most westerly part of the Mendip Hills, as well as the interesting tiny islands of Steep Holm and Flat Holm.

Brean Down is an outstanding example of calcareous grassland and supports endemic plant communities that provide for many important insect communities. Other important plant species growing at Brean Down and now growing at Bristol Botanic Gardens include the White Rockrose (Helianthemum appenninum). This is an attractive white flowered perennial sub shrub, which frequently grows on southern slopes. White Rockrose, Somerset Hair-grass and Dwarf Sedge (Carex humilis) are all particularly at risk due to scrub colonization. This highlights the importance of grazing to maintain grassland habitats. The National Trust introduced grazing by long horned White Park Cattle and British WhiteCattle (feral goats are already on Brean Down) to help keep the grass short and scrub species controlled. 

The rarity of the plants found on the Mendip Hills highlights how important collections, such as those held at the University Botanic Garden, are for ensuring the survival of plants teetering on the brink of extinction. Equivalent to a botanical savings account, these collections help ensure that if plant species are lost, they can be reintroduced back into the wild.