C.Y. Jim points out that a lack of root support was the main cause of fallen trees; soil management techniques should be employed in replacing them

The unprecedented ferocity of Super Typhoon Mangkhut left a trail of devastation in Hong Kong. Our gray infrastructure, designed and built to high engineering and safety standards, weathered the storm pretty well and emerged largely unscathed. Our green infrastructure, represented mainly by urban trees, on the contrary, took the brunt of nature's wrath. Thus far, the officially reported tree loss cases have exceeded 46,000, which is probably the highest mortality in our history. Such a magnitude of tree demise could be regarded as calamitous and shocking.

The massive tree decimation has denuded the landscape and adversely affected the environmental quality of many roads and neighborhoods. While we lament the sudden loss of so many of our dear natural doyens, it is an opportune time to contemplate the future of our urban landscape. Learning from the ravages would allow us to improve our urban forestry techniques and skills. As climate change is expected to bring stronger typhoons, let us prepare and prime our next generation of greenery to meet the challenges.

Examination of hundreds of tree failure cases indicates two main modes of wind destruction. Firstly, the sheer force of the wind, including the short, sharp and unusually powerful gusts, could surpass the mechanical strength of tree trunks or main branches to break them outright. At the most exposed sites, even the strongest trees could succumb to extreme wind energy impacts which could exceed 200 kilometers per hour. Trees with compromised structure and wood decay at critical positions would be particularly vulnerable to snapping.

Secondly, the sail effect of trees could allow the enormous pressure exerted by wind to push them down. Many large trees that collapsed had their trunks and scaffold remaining rather intact. In other words, the wind did not break them. Instead, they were uprooted and toppled en masse. The exposed soil materials containing roots display several telltale traits. In comparison with the tree bulk, the rootable soil volume is inordinately small and shallow, often merely one to several square meters in area and less than a meter deep. The surrounding soil is heavily compacted to meet road-engineering requirements, rendering it unsuitable for root growth. The exposed soil volume tends to have a neat geometric shape, mainly square and sometimes circular, conforming to the configuration of the confined tree pit or planter. The roots are rather thin and they grow mainly downward. Few large and sturdy lateral roots which are critical for tree anchorage could be observed. Overall, the exposed rooting pattern indicates trapping of most roots in a tiny usable soil volume and a serious lack of roots that can grip the soil securely to stabilize the tree.

The evidence indicates convincingly that the design and management of soil present ample room for improvement. The bane of urban forestry in congested city areas is the serious lack of soil volume for roots to acquire stability. Over 360 million years of evolution, nature has endowed trees with strong structural roots that spread laterally to provide assured anchorage. They are tantamount to the foundation of our buildings, bridges or other artificial structures. In cramped urban planting sites, this innate stabilization ability unfortunately has been acutely stifled. As recent advances in urban tree and urban soil sciences can offer practical solutions, we do not have to follow the rut. Innovative methods could be adopted to expand the soil volume while maintaining usable pavement surfaces for pedestrian passage. Thinking out of the box and adopting the findings of applied research could resolve the vexing soil limitations.

Soil volume can be extended below the paving using pioneering approaches while maintaining the walkable surface area at roadsides. Wherever practicable, we can reduce the amount of hard paving and increase unsealed and uncompacted soil areas. This is especially suitable in open spaces situated away from roadsides where extensive hard paving is not necessary. Where pedestrian traffic is not heavy, some paved areas can be converted to lawns with trees. Tree pits could be enlarged from the standard 1 square meter to 2 or 3 square meters, with the ground surface covered by iron grille or reinforced concrete slabs supported by piers so that the soil underneath does not need to be compacted. Similarly, planters could be widened or enlarged. If the pavement is narrow and unsuitable for widening of the tree pit, it can be elongated along the road alignment. If possible, a tree strip (tree lawn) could be installed instead of a series of isolated tree pits. In addition to supporting trees, the increase in permeable soil areas can reap the bonus of surface and air cooling to combat the urban heat island effect.

Alternatively, the roadside strip could be underlain by a soil corridor to be filled with uncompacted and high-quality soil, and the pavement be made of reinforced concrete slabs supported by piers or proprietary metal or strong plastic frames. If the soil corridor is not feasible due to proliferation of underground utilities, a large-diameter storm-water drain pipe could be installed between two tree pits. The pipe will be filled with high-quality soil to be shared by the roots of the neighboring trees. Irrigation, aeration and drainage portals could be installed to enhance the conditions for root development. A tree pit or tree strip could be connected by soil corridor or soil pipe to adjacent green areas with open soil, so that roots can extend to a considerably expanded soil catchment area.

Tree management can move boldly from the conventional focus on the above-ground part to incorporate soil management. To a farmer, caring for the soil is de rigueur of good cropping practice. Similarly, urban soil should receive appropriate attention to nurture attractive, robust, stable and safe trees that are cost-effective and less burdensome to maintain. The soil component in the urban forestry regime has been neglected for too long in our city. It is high time that tree managers joined hands with engineers to revamp the modus operandi of tree planting, and to develop a synergistic union of hardscape and treescape that is tailor-made to match our congested urban environment. To fulfill our smart-cum-sustainable green city goal, let us make the best use of the current refurbishing opportunities to revolutionize our urban greening mindset and technology. We can inject and maintain considerable green features in our urban jungle to improve its livability with a combination of determination and adventurous spirit.

(HK Edition 10/08/2018 page8)