Living on stone
There is a brick chimney stack near my home in Hemel Hempstead in southern England that has a tree growing out of it, and I have seen similar trees on rock faces and cathedral spires. How do these plants survive? Where do their roots go? Young trees in my garden struggle in well-tended, well-fed soil. How can a 1-metre tree survive in what is essentially a brick wall?
鈥� The absence of trees in most chimneys shows that a very favourable site is required. Damaged brickwork can provide a crevice for a seed to lodge in and send down a root, usually during a particularly wet year 鈥� such as this one. The extra wetness permits the roots to go deep enough into the brickwork to survive the next year, so the initial damage needs to be substantial.
A factory chimney has thick, solid brickwork, which protects the interior of the bricks from drying. Rain can enter through cracks at the top, and wind can drive it into the chimney from the side. The swelling root widens and deepens the cracks, making the site even more favourable. This process is helped by water freezing and expanding in the cracks in winter, and rain dissolving some of the lime in the mortar.
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Numerous herbaceous species, such as willow herbs, produce wind-dispersed seed, which can easily reach a stack top and will germinate and grow briefly before dying in the drier weather. Their decaying roots enhance the water-holding capacity of the brickwork before the tree seed arrives.
The 1-metre tree mentioned is only superficially similar to a 1-metre garden specimen. The chances are that it is much older than a garden tree of the same size, and manages very little growth except in wet years. Also it will be more branched and have smaller leaves darkened by red pigments, which all plants produce under stress. In extremis it can reduce its rate of water loss by shedding leaves.
Of course it may well be an exotic garden species, not a wild one. Here in north Nottinghamshire we have a low rainfall similar to Hertfordshire. Buddleia grows everywhere in these conditions, with plants up to 30 centimetres on flat wall tops. I have seen a plant up to 2 metres tall where a broken gutter pours copious water over brickwork. The roots penetrated and split the wall, which was built with soft lime mortar.
Ian Hartland
Worksop, Nottinghamshire, UK
鈥� Rocky walls can hold enough water for plants, and there is always enough air to provide carbon dioxide. Often bird droppings, dust and minerals dissolved from the rock supply enough of the other required nutrients. Indeed some epiphytes such as Tillandsia, or 鈥渁ir plants鈥�, get practically all their mineral nutrition from dust.
Garden trees are not generally epiphytes, and are not well-suited to clinging to life among rocks or on bark. But many a fig, especially strangler fig species, starts life as a bonsai growing from a seed in bird droppings on a wall, cliff face or tree trunk. There they cling to life, sometimes for centuries, until time and chance destroy them or lead a rootlet to good soil.
Jon Richfield
Somerset West, South Africa
鈥� I live in an area of Snowdonia where a once-thriving quarrying industry has left vast mountains of slate chippings. Now several organisations are using various methods to re-foliate this barren landscape. One method is to bring in lorry-loads of topsoil and replant the hillsides with saplings in an attempt to landscape the surroundings. There are numerous problems involved: the slate slopes drain exceptionally well and do not readily retain sufficient water for healthy plant growth; the frequent rain washes away the soil; and the slopes are not stable enough for larger plants to maintain their purchase.
A less futile and less labour-intensive method involves throwing lots of seeds for suitable shrubs and trees onto the slate piles, and knocking in perching posts at regular intervals. Birds eat the scattered seed, then rest on the posts, leaving behind their droppings. These contain some active seeds and also act as a fertiliser. The birds may even import seeds from the surrounding countryside.
When the shrubs begin to grow, they provide even more perches for the birds, and focus the fertiliser where it is most needed. These plants promote the growth of smaller plants such as mosses and grasses by providing shade and dropped leaves. The smaller plants help to retain water and begin to build soil as they break down.
鈥淏irds perching on brick structures provide seeds, and their droppings provide the fertiliser鈥�
I would suggest that this process is responsible for the survival of trees in seemingly soil-free stone and brick structures. Birds nesting or perching in these safe places provide seeds, and their droppings provide the fertiliser. The nutrients also encourage mosses, which retain moisture for the tree and eventually provide a kind of soil as they die and break down. The restriction to the tree roots would act to 鈥渂onsai鈥� these trees, so they do not outgrow their nooks and crannies.
Jeremy Watkins
Blaenau Ffestiniog, Gwynedd, UK
This week鈥檚 questions
Digital delay
I recently invested in a set-top box to receive my TV signals in digital format. While setting it up I noticed my analogue TV in the next room was showing the same channel, but the action was 2 or 3 seconds ahead. Why is a digital signal delayed? And what are the implications of attempting to set one鈥檚 watch by the time signal?
Thomas Holden
Cardiff, UK
Water works
I was fascinated by your recent feature describing the finite limits of sporting achievement (14 August, p 31). However, it seems some sports have reached their limits before others. Watching the recent Olympics I was struck by how frequently swimming world records are broken and how infrequently running world records fall. These sports have been around for as long as we can remember and neither has any obvious technological input. Why the discrepancy?
Joan Louis
Helsinki, Finland
Wave goodbye
Over the best part of two bottles of wine my wife and I were arguing whether the stones we were throwing into the Mediterranean harbour of Ciutadella in Minorca were creating waves that would eventually hit the shores of North America. She argued they wouldn鈥檛, having to pass through the Strait of Gibraltar and cross the Atlantic, while being assaulted by other waves, friction from the coast and seabed, and storms. I said that until they encounter a shore, waves are virtually infinite. Who is right?
Dave Johnstone
Hounslow, Middlesex, UK