The absence of trees in most chimneys shows that a very favorable site is required for growth.
Damaged brickwork can provide a crevice for a seed to lodge in and send down a root, usually during a particularly wet year.
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 favorable. This process is helped when water freezes and expands in the cracks in winter, and rain dissolves some of the lime in the mortar.
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 3-foot tree mentioned is only superficially similar to a 3-foot 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. Buddleia grows everywhere in these rainy conditions, with plants up to 1 foot on flat wall tops. We have seen a plant up to 6 feet tall where a broken gutter pours copious water over brickwork. The roots penetrated and split the wall, which was built with soft lime mortar.
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 “air plants,” 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 the figs cling to life, sometimes for centuries, until time and chance destroy them or lead a rootlet to good soil.
Some of us live in an area where a once-thriving quarrying industry has left vast mountains of slate chippings.
Now several organizations are using various methods to re-foliate this barren landscape. One method is to bring in truckloads 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 labor-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 fertilizer. 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 fertilizer 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.
We 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 fertilizer. 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 “bonsai” these trees, so they do not outgrow their nooks and crannies.