The Tree Climber’s Guide. Jack Cooke

become mass sewers, and tributaries that once served as transport links now ferry human effluent and the floating fat of restaurant and home.

      Trapped and ignored, it is easy to suppose that, like the tree falling unheard in the forest, if a river flows unseen it has ceased to really flow at all. Yet these waterways are older than the city – older than England even. While their springs still flow, a thousand years interred is a fleeting moment in the life of a river. They surface in secret, running in concrete channels or narrow ditches, and a line of trees is the surest way to trace their covert passage. Where there’s water there will grow life, even if that same water is choked with plastic bags, shopping trolleys and sunken glass.

      There is a strong compulsion to climb trees over water. Drawn to long branches above rivers and canals, we are imbued with a misplaced confidence, something in the brain associating water with soft landings and summers past. Inner-city streams conceal submerged dangers and still pools stagnate, but these are superficial deterrents. A tree overhanging the current combines the two fundamentals of wood and water, an elemental landscape in the midst of the man-made. These mesmeric haunts tempt the climber like few other urban spaces.

      Perched over water, whether in the arms of a weeping willow or a straight-backed alder, networks of branch and leaf reflect upwards, enshrining the climber in a double image of the tree. The play of shadows and light hypnotises the most care-worn commuter as the water wages an endless battle to lick the city clean.

      All London’s streams flow into the wide blue artery of the Thames. Look at a satellite image of the city – snaking lines of trees hug the great river’s bends, clinging to the water’s edge as if trying to escape the metropolis altogether. Some of these are being toppled by riverside development, while others stand proud, like the uninterrupted march of London planes that edges the river from Blackfriars to Fulham. Climbing branches over the Thames we hang over the heart of the city and, if we listen closely, a rare natural sound can be heard – running water.

      Canals, brooks and creeks offer an alternative environment, tight channels shaded by trees whose roots thread the water like long white eels. The Thames forms an abrupt gulf between north and south, while these smaller, circumscribed rivers are fissures in suburbia, boundaries crossed by irregular bridges but numberless branches. Many species of tree crowd the long, empty stretches of their straight-sided banks.

      When storms lash the city the old waterways show their wrath, heavy rainfall swelling their channels and leaking into our streets, bubbling over manholes and seeping through brick and mortar. Sometimes I long for London’s waters to burst their artificial bonds, purging themselves of their sordid cargo and making islands of bank-side trees. J. G. Ballard imagines such a future in A Drowned World, where the city lies buried in silt beneath a deep lagoon and a primordial hierarchy has re-established itself, rampant plants colonising the stairwells of tower blocks. Floating over a street still visible sixty feet below the water’s surface, the narrator describes the sunken lines of London’s buildings, ‘like a reflection in a lake that had somehow lost its original’. Suspended on the long arm of a London plane or in the tresses of a willow, it would be easy to forget a city ever existed on these banks.

      Waterside trees are prime lookouts, places to watch canal boats, Thames Clippers, and the tide of objects lost to London’s current. Whether clutching a high branch over the river or perching above a creek, we enact a two-fold escape, climbing off the ground and then leaving the land altogether. Traversing branches over water allows us to cast off in our imagination; the current takes us with it on a journey, floating past long rows of riverside houses, shipyards and factories, beside green fields and long sandbars and then out into the open ocean. The climber is like lost timber, fallen from the deck of a container ship and set adrift.

Brothers in Arms, Bishop’s Park

      Platanus × acerifolia/London plane & Ilex aquifolium/Common holly

      An avenue of London planes runs along the riverside at Bishop’s Park. With their branches curling over the path, you walk under the arms of a cheering crowd. In season, great curtains of leaves cascade over the embankment wall, seeming to stretch out towards the river. Where these branches join the trunks, perfect saddles are formed for the climber.

      One of these planes shares its soil with a holly. Hollies are well adapted to thrive in shadow and this one has made a deep impression, stiff branches embedded in the side of its overlord. I use the holly as a mast to step up into the plane, taking a seat in the elbow where the two cross. Beneath me is the freckled wood of one; all around and above the leaves of the other.

      At this height the holly’s leaves are smooth, not spined, safe from browsing animals, although the only passing threat is an overweight Labrador. Shuffling along towards the river, I find that a holly branch has crossed the plane, rubbing up against it. The branch shifts in the wind, its underside like a flat tyre from the friction.

      Beyond the footpath I edge out over the wall and the long drop down to the river. The tide is out and the sand exposed, a beach littered with lumps of stone from the wall and a scattering of flotsam. What looks like an anchor lies half-buried in the mud. Other pieces of rusted metal could be forgotten treasure or scaffolding; near the waterline the clay pipes of Victorian London are a scattering of white shards, roll-ups from another era.

      A crow pecks on the foreshore at a flash of silver – foil or a bottletop – while a black-headed gull dive-bombs it from above. Leaves drift down the river and I make a promise to return in autumn, when the plane will shed its burden to make an armada on the water.

      Retreating to the landward side of the tree, I see the branches are covered in lichen and the wood has a curious pitted appearance, whole sections with fossil-like indentations where the bark has flaked away. I climb higher and lean my back against the trunk. On the opposite bank the London Rowing Club’s slipway is jostled with cars parked at steep angles to the water, only their hand brakes saving them from immersion. Out on the wind-ruffled river, four women pull hard against the waves in a yellow scull.

The Helping Hand, Regent’s Canal

      Populus alba/White poplar

      Standing in a narrow corridor of grass by the canal in Mile End Park are two poplars. Behind them the single chimney of a Victorian brick kiln rises above a wall of graffiti. The chimney is mirrored in the canal’s green water, and drifting clouds join it in the depths.

      The dried grass beneath the southernmost poplar is thick with crickets, a raucous mating song in the July heat wave. There exists a city all of its own in the shade of the tree, replete with ring roads and intersections among the roots. As I step into the shade, the building site beyond the grass fades to a dull rumble under the canopy’s thrall.

      I stand on a fairy ring of carved logs at its base, staring up at the cut-diamond patterns that decorate the bark. One great suckering root passes between my feet – I can almost feel the tree’s thirst.

      The upsweep of branches above me ends in great clusters of leaves, their contrasting sides of green and white giving a sense of motion, even without a breath of wind. I try to flat-foot up the poplar’s slope and retreat dispirited, having moments before fallen from the first branches of another close to Limehouse Basin. Drenched in sweat, I begin to question the merits of climbing in thirty-degree heat.

      Then an angel appears on the tow path, a man in a hi-vis jacket carrying a spade in one hand and a lunch bag in the other. He watches me repeatedly sliding down the trunk, then hops the railing and walks over. I turn, expecting some kind of mockery, but instead he drops the spade and asks, ‘Need a leg-up?’ This remains the sole occasion I’ve been helped into a tree by a total