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Thursday, 12 May 2016

The resilient plants of the western Mediterranean

By Helen Roberts

A recent talk to the Friends by Dr Chris Thorogood on the flora of the western Mediterranean was tonic for those of us longing for warmer weather. For Chris, the western Mediterranean has always had great appeal having spent many summers teaching field courses to undergraduate students at the University of Bristol and the last five years conducting fieldwork for his new book, a field guide to the area.

"The flora of the Western Mediterranean is really special. The plants are able to grow in some fascinating but really harsh places," explains Chris. "Because of these severe conditions, plants have evolved numerous coping mechanisms in order to survive."

The region is extremely rich botanically, with over 10,000 different species, all of which are specially adapted to particularly taxing conditions. The area covers a huge geographical expanse incorporating the westerly Portuguese Algarve, to Italy in the east, the islands (Balearic Islands, Corsica and Sardinia) and North Africa from Morocco to Tunisia (see my post last week, which discusses these regions in more detail).

A bounty of habitats: scrubby landscapes

There is a diverse range of floral habitats in the region from the scrubby maquis to forests with wonderful understories of orchids. The bare and arid habitats are home to 'experts' in drought tolerance; and at the other end of the watery spectrum are the seasonal lakes where deadly predatory plants reside. Humans have shaped the flora as well through thousands of years of agriculture, which has produced a visually evocative landscape throughout the whole Mediterranean basin.

Cistus ladanifer, the common gum cistus.
Photo credit: Henry Bush [via Flickr, CC]
A habitat that we so often associate with the Mediterranean landscape is maquis, which is specific to the Mediterranean area. It is comprised of spiny sclerophyllous  (a fancy word for hard-leafed) tough vegetation, which is specially adapted to cope with severe drought. There are often small trees and shrubs dotted about, often with beautiful understories of bulbs and short-lived annuals. Many of the species are aromatic. Typical species include prickly juniper (Juniperus oxycedrus) and common gum cistus (Cistus ladanifer), both highly pungent plants that are used by the cosmetic industry for their oils.

"The smells that exude from maquis vegetation are wonderful," exclaims Chris, "and the scent from these aromatic plants just seems to hang on your clothes long afterwards."

Garrigue vegetation is similar to that of maquis. It differs slightly in that it is low growing in form, occurs closer to the coasts and grows on shallower soils. Due to its close proximity to the coast it is battered by winter storms and winds. The low stature of the garrigue evergreen scrub means that perennials and bulbs are highly visible. The flowering displays in spring are visually spectacular and include wild tulips, crocuses, thymes, mints, helichrysums and lavenders.


Into the woods

The native forests of the western Mediterranean form prominent landscapes occupying quite different terrain than the shrubland regions. The oak woodlands are dominated by the holm and cork oak, with a spectacular understory that offers a refuge for many animals, including the endangered Iberian lynx.

The pine forests of Pinus halepensis and Pinus pinaster occupy coasts and cliffs. Healthy habitats have a particularly distinctive flora and fauna with rarities such as the semi-parasitic Violet Limadore orchid found in the maritime pine forest of Landes in France.

Ceratonia siliqua, commonly known as the carob tree.
Photo credit: Jesus Cabrera [via Flickr, CC]  
Humans have also shaped the forest landscapes of the region to a certain degree. Traditional farming practices in the Mediterranean have created unique assemblages of plants. Olive, carob, fig and almond groves represent landscapes people often associate with the Mediterranean. No other landscape denotes the true essence of the Mediterranean like a grove of olives. The olive is engrained in the lives and culture of the people of the region. Carob groves are also stunning in their composition with the gnarly dark trunks contrasting brilliantly against the green understory. The carob, Ceratonia siliqua, is a member of the ‘peas’ (Fabaceae), the most speciose family in the Mediterranean.


Rare aquatic habitats

Most of the western Mediterranean habitats are dry and parched, but surprisingly there are some wet ecosystems too. These unusual habitats are rare and include some curious species like the carnivorous bladderworts, found in seasonal lakes, which catch insect prey using sticky hairs and trap doors.

"The aquatic habitats of the western Mediterranean are interesting because they are pretty rare," explained Chris. "Many are only seasonal but they support a wealth of interesting species from tiny forget-me-nots, tongue orchids and the carnivorous bladderworts."

Tough but not invincible

While the native species of the western Mediterranean might be seen as tough and indestructible in the harsh landscapes they occupy, they are extremely susceptible to invasive alien species (IAS). Chris explained that the two most invasive plants in the region include certain species of Eucalyptus and Acacia. People assume these species are native, probably because they fit into the landscape visually and they can tolerate harsh climatic conditions. However, both these trees can alter native ecosystems and have a negative impact on biodiversity. Unfortunately, many are still planted as ornamental shrubs despite measures drawn up to prohibit the cultivation of them.

Chris explains the danger of one such exotic: "Acacia cyclops is an invasive species that is likely to become the next big invasive in the western Mediterranean. This species forms a mass of vegetation in barren landscapes due to its ability to cope with extremely dry and saline conditions. Ultimately it outcompetes native species."

These invasive pests steal native plants’ water and change the biochemistry and microbiology of the soil. The native flora is sensitive because the western Mediterranean is exceptionally biodiverse in a relatively small area, with high levels of endemism, particularly on some of the islands. As well as the threat from IAS, there is also intense pressures on these fragile native habitats from humans due to urbanisation, afforestation, and coastal and agricultural development. Effective and timely conservation measures are vital to ensure the survival of these beautiful and botanically rich habitats before it is too late and they go into decline.

Helen Roberts is a trained landscape architect with a background in plant sciences. She is a probationary member of the Garden Media Guild and a regular contributor to the University of Bristol Botanic Garden blog.

Friday, 6 May 2016

The evolution of a predatory plant

By Nicola Temple

We keep a Venus flytrap (Dionaea muscipula) in our bathroom. My son begged me for it, which inevitably means I look after it. Having seen these carnivorous little delights in the glasshouses at the University of Bristol Botanic Garden, I have learned that humidity and moisture are key to its happiness - hence it's bathroom location and its constant immersion in a tray of water.

The leaves of  the Venus flytrap, open (foreground) and
wrapped around its prey (background, right).
Photo credit: Shelby Temple
While I mostly leave my son to do the part he loves best - feeding - I can't deny my own fascination with it. The leaves, converted to ambush traps through evolution, have to have enough stimulation by an unsuspecting insect to warrant the plant investing the energy to snap the trap shut.  Once the trap is shut, the plant estimates the size of the prey based on the amount of stimulation of the sensory 'hairs' triggered by the trapped (and no doubt panicked) insect. If there is a sufficient signal from the sensory hairs, the plant starts to produce enzymes and proteins that will help it digest and absorb the prey. It's the stuff of nightmares...for the insect.

So what evolutionary steps transformed a leaf designed to harvest light from the sun into a leaf designed to trap prey? New research published this week in the journal Genome Research has provided some insight into the origins of the Venus flytrap's trap.

It's a leaf with a hint of root and a dash of...tongue?

Professors Rainer Hendrich and Jörg Schultz led a team of scientists from Julius-Maximilians-Universität Wüuzburg (JMU) in Bavaria, Germany who looked at the genes being expressed by the traps. They found that the traps not only had active genes typical of leaves, but also those typically found in roots.

A close up view of the trap, which shows the sensory 'hairs'.
Photo credit: Shelby Temple
There are dome-shaped glands on the surface of the trap. The outer layer of each gland secretes the digestive enzymes, but the middle layer has foldings that increase the surface area - reminiscent of microvilli in the human intestine. It is thought that this is where nutrient absorption takes place. As this is a major function of roots, it is not surprising that some of the same genes are required.

Now...about about that tongue. I mentioned above that the plant releases digestive enzymes if it receives enough stimulation within the closed trap. But what if the insect dies very quickly after being trapped? The plant has a receptor in the trap that can detect chitin - the main constituent of an insect's exoskeleton.  So even if the insect is no longer moving, the plant can 'taste' the insect in the trap and begin digesting.


Switching from defence to offence

When non-carnivorous plants come into contact with chitin, it is usually not going to turn out well for the plant -  they are under attack by herbivorous insects. Henrich and Schultz looked at the defence mechanism triggered by insects feeding on the non-carnivorous plant thale cress (Arabidopsis thaliana). They found that the plant in defence mode activates the same genes in the same pattern as the Venus flytrap in attack mode.

"In the Venus flytrap these defensive processes have been reprogrammed during evolution. The plant now uses them to eat insects," explains Hedrich.

In both cases, mechanical stimulation (whether a chewing insect or a trapped one) generates an electrical impulse that activates the release of the hormone jasmonate. In Arabidopsis this hormone begins a cascade of events that starts the production of various chemicals that deter the insect or make the leaves hard to digest. In the Venus flytrap, however, jasmonate starts the digestion of the insect and uptake of the nutrients.

So, the ancestor of the Venus flytrap had all the machinery in place for detecting insects and triggering a chemical response to their presence, but evolution managed to shift it from a defensive strategy to a very effective offence.


Source: 

"Venus flytrap carnivorous life style builds on herbivore defense strategies", Felix Bemm, Dirk Becker, Christina Larisch, Ines Kreuzer, Maria Escalante-Perez, Waltraud X. Schulze, Markus Ankenbrand, Anna-Lena Keller Van der Weyer, Elzbieta Krol, Khaled A. Al-Rasheid, Axel Mithöfer, Andreas P. Weber, Jörg Schultz, Rainer Hedrich. Genome Research, DOI: 10.1101/gr.202200.115

Tuesday, 3 May 2016

Floral visits to the western Mediterranean

By Helen Roberts

A floral excursion to the western Mediterranean at this time of year appeals to many of us. The anticipation of warm weather, beautiful landscapes and a dizzyingly diverse range of exquisite wild flowers and I want to pack my bags in a flash. I certainly felt that way when I saw some of the images of the region’s wild flowers in a recent Friends' talk given by botanist Dr Chris Thorogood.

However, if you cannot escape overseas, then the Mediterranean collection at the University of Bristol Botanic Garden will give you a taste of some of the Mediterranean Basin species although you will have to wait till later in the year to see some of the flowers in bloom. 

If you do have a trip in mind though, here are a few of Chris Thorogood’s favourite spots to see the wild flowers of the western Mediterranean:
Cape St Vincent, Portugal. Photo courtesy of Peter Broster via
Flickr [CC license]

The Algarve, Portugal:

This area has a diverse flora due to varied geology and weather with numerous endemic species and beautiful wild flower meadows. Cape St Vincent, the most south-westerly point in the area and a vast nature reserve, has a spectacular display of flowers in the spring and early summer (January through to the end of May). There are many unique species of thyme and endemic rarities like the tiny diamond flower (Ionopsidium acaule).

Almeria, Spain:

This province is located in the southeast of the Iberian Peninsula and has a wealth of species adapted to cope with extremely dry conditions. Many plants are salt tolerant including sea lavenders like Limoniuminsigne and the rare low growing lily, Androcymbium europaeum whose flowers emerge on sand dunes in mid winter. An area called Cabo de Gata, an impressive tract of volcanic cliffs, is host to numerous unusual species. Many of these are freakishly odd looking from the succulent Caralluma europaea with its purple and yellow striped flowers to the phallic form of the parasitic Cynomorium coccineum.

Cap de Formentor, Mallorca:

This peninsula in the northeast of the island has many unique sea lavenders and orchids. Endemics are closely dotted only metres apart. Much of the landscape is fairly inaccessible due to its rocky and precipitous nature so one needs to be fairly adventurous to spot some species. Notable endemic species include Arum pictum, an arum that smells of rotten meat to attract its fly pollinators and a species of St John’s Wort unique to Mallorca, Hypericum balearicum.

Maremma, Southern Tuscany, Italy:

The Maremma region is rich in wild flowers and contains 25% of all Italian flora. It has a unique geology and extremely varied landscapes including the protected coast, swathes of pine forest and abandoned agricultural plains. The giant fennel, Ferula communis, is one such distinctive plant with its towering inflorescences that can take many years to develop.

Gargano National Park, Puglia, Italy:

The yellow bee orchid (Ophrys lutea) is one of the orchid
species found in Gargano National Park.
Photo credit: Alastair Rae [via Flickr, CC license]
This park has a unique flora and are highly specialised for growing in certain conditions many being endemic. The park covers a vast area and as a result the landscapes are varied from rich beech forests, steep cliffs, karstic plateaus and scrubby maquis. There are many orchid species here (over 65) including some unique bee orchids.


Helen Roberts is a trained landscape architect with a background in plant sciences. She is a probationary member of the Garden Media Guild and a regular contributor to the University of Bristol Botanic Garden blog.