Planting trees is a practical land-use strategy for managing stormwater. By changing how water moves across and into soil, trees can reduce runoff volume and peak flow while improving local water balance.

Core idea: trees increase infiltration

Trees promote infiltration by intercepting rainfall, slowing overland flow, and improving soil structure so more water soaks into the ground instead of running off.

Conceptual diagram of a root-zone water-balance model showing how precipitation is partitioned into evapotranspiration, changes in soil-water storage, and downward water movement that contributes to net infiltration (groundwater recharge). The figure helps connect tree-driven processes (root-zone uptake and evapotranspiration) to infiltration and subsurface flow pathways. Source

A site’s infiltration capacity depends on soil texture and structure, slope, vegetation cover, and compaction; trees influence several of these factors at once.

How trees slow runoff

• Canopy interception: Leaves and branches catch rainfall, reducing the energy of raindrops that would otherwise dislodge soil particles and seal soil pores.

• Stemflow and throughfall: Water is routed down trunks and drips from leaves more gradually, spreading delivery over time and lowering runoff rates.

• Surface roughness: Tree trunks, understory plants, and fallen debris create friction that slows moving water, increasing the time available for infiltration.

Soil processes: why infiltration improves under trees

Roots and soil structure

Tree roots help create and maintain macropores (large channels in soil) that act as pathways for water movement:

• Root growth pushes through soil, opening channels.

• Root decay leaves voids that persist and conduct water downward.

• Root exudates and associated microbes help bind soil particles into stable aggregates, improving porosity and reducing crusting.

Organic matter and the forest floor

Trees add organic material that improves soil conditions for infiltration:

• Leaf litter cushions the soil from raindrop impact and reduces surface sealing.

• Decomposing litter forms humus, which increases water-holding capacity and supports soil aggregation.

• Greater biological activity (earthworms, fungi) further mixes organic matter into soil and creates additional channels.

Reduced erosion supports infiltration

By slowing runoff and protecting soil surfaces, trees reduce erosion that can strip topsoil and leave behind compacted or less-permeable layers. Less sediment is transported downslope, helping maintain soil permeability where water enters the ground.

Where planting trees is most effective

Urban and suburban settings

In developed areas, soils are often compacted and water is rapidly delivered to drains. Strategic urban forestry increases infiltration by:

Street-side “tree filter” (a compact bioretention practice) designed to intercept runoff from pavement and temporarily pond water above a soil media before it infiltrates or drains. This illustrates how urban trees can be integrated with stormwater infrastructure to reduce runoff volume and peak flow while increasing infiltration opportunities. Source

• Breaking up compacted zones over time as roots grow and soils biologically recover.

• Capturing runoff from nearby hard surfaces when trees are planted in or near drainage pathways (e.g., along streets, parking-lot edges, or in small planted basins).

Along waterways and low-lying areas

Tree planting near streams can increase infiltration in adjacent soils by slowing surface flow before it reaches channels. This can reduce flashy flows that contribute to bank erosion, while encouraging shallow groundwater recharge.

Design and management considerations

Effective tree-based infiltration depends on matching species and planting practices to site conditions.

• Species selection: Choose trees tolerant of local moisture conditions; deep and spreading root systems often improve soil porosity more than shallow-rooted species.

• Soil preparation: Avoid compaction during construction; loosen soils and add organic amendments where appropriate to support root growth and pore formation.

• Placement: Plant where runoff naturally concentrates, but avoid creating standing-water hazards that could damage sensitive species.

• Maintenance: Mulch layers help protect soil and retain moisture; long-term canopy and root development increases benefits over time.

• Limitations: Infiltration gains are smaller where soils are heavily compacted, very clay-rich, or where high water tables limit downward water movement; tree survival and growth rates also control how quickly benefits appear.