# Green infrastructure and urban forestry The physiology behind urban trees: why compacted soil and heat fail them, how rooting volume sets attainable canopy size, where tree cooling actually comes from, and why uncompacted soil does double duty as stormwater storage. A street tree is a plant whose entire resource base is a box of soil under pavement; rooting volume, compaction and aeration decide the canopy, and the canopy decides every service the tree is credited with. Source: https://en.bioecon.ru/docs/ecology-restoration/ecosystem-restoration/green-infrastructure-urban-forestry/ Updated: 2026-09-07 Urban vegetation is credited with shade, cooling, stormwater and air quality, and nearly all of those services fail for the same physical reason: a street tree's entire resource base is a box of soil under pavement, and the box is whatever the construction left behind. The hardware of green infrastructure exists to re-specify that box. ## Soil: the constraint that is built, not found Roots grow by deforming pores. Construction traffic compacts urban soil until the large pores collapse, mechanical resistance exceeds what a root tip can exert, and the soil stops exchanging gas: oxygen reaches roots and microbes by diffusion through air-filled pores, and when those pores are collapsed or waterlogged, the root zone suffocates. The visible symptoms — shallow, circling roots, small and sparse crowns, trees that die in one or two decades well before maturity — are consequences of the substrate, not of the species choice. Because root system and crown scale together, the rooting volume available to a tree sets the canopy it can ever carry, and the canopy is what delivers every service the tree is planted for. Rooting volume is therefore the limiting quantity of urban forestry, and structural soil cells have exactly one job: hold the pavement's traffic load on a frame while the volume between the frames stays uncompacted, aerated and infiltrable. Urban soils add chemical hostility on top — de-icing salt, alkaline debris, contamination — but the geometric constraint comes first. ## Heat: where the cooling actually comes from The urban heat island is an energy-balance phenomenon: paved surfaces absorb shortwave radiation, store it in thermal mass and release it through the evening, while almost no absorbed energy goes into evaporation. Vegetation counters with two mechanisms. Shade intercepts radiation before it reaches the hot surface. Transpiration converts sensible heat into latent heat carried away as vapour — the same evaporation that cools a wet cloth. The second mechanism has a catch: stomata close under water stress, so a drought-stressed tree cuts its transpiration precisely during the hottest hours. The cooling service is coupled to the soil constraint — an uncompacted, water-holding root volume is part of the cooling mechanism, not an amenity around it. ## Stormwater: one volume, two functions Uncompacted engineered soil does a second job: it is a detention reservoir. Rain infiltrates instead of running off, the pore space stores the event's volume, and evapotranspiration empties the store between rains so the capacity returns. Compacted ground sheds water nearly as completely as pavement, which is why the same structural cells that host roots are often specified to receive street runoff. The two functions compete for the same volume and reinforce each other: the drainage design wants storage, the tree wants exploitable soil, and a green street is two systems sharing one excavation — which is also why under-sized pits fail both at once. The soil physics here is the same physics that decides whether any restored plant cover functions, at landscape scale, in [ecosystem restoration](../ecosystem-restoration/); what differs in the city is that every parameter — volume, density, water supply — is an explicit purchase decision.