Originally written by Nathalie Shanstrom in 2014; reviewed and updated by DeepRoot in 2026.

As discussed in previous blogs, trees and soil systems provide stormwater benefits in many different ways:

• Cleansing: Trees clean stormwater through many different mechanisms, including filtration, adsorption, and plant uptake.
• Interception: Interception is the amount of rainfall temporarily held on tree leaves and stem surfaces. This rain then drips from leaf surfaces, flows down the stem to the ground, or evaporates.
• Infiltration: Infiltration is the movement of surface water into soil. Once there, water can be temporarily stored, move into underlying soil, discharge to an underdrain, evaporate back into the atmosphere, or be taken up by plants.
• Transpiration: Transpiration is a process in which plants absorb water through their roots and transfer it to their leaves, where it evaporates into the atmosphere through small pores called stomata. Transpiration continues to reduce stormwater volume stored in the soil long after a rainfall event ends.

Many of these mechanisms rely on adequate infiltration rates in order to function properly, so this article examines the effects of trees on soil infiltration in greater detail. Since surface clogging and declining hydraulic conductivity can negatively affect the other mechanisms through which trees provide stormwater benefits, maintaining adequate infiltration in Stormwater Control Measures (SCMs) is crucial.

Without adequate infiltration rates, less water enters the soil. As a result, less water can be cleansed by soil, soil microbes, and roots, and less water is available in the soil for transpiration.

Infiltration rate and saturated hydraulic conductivity are closely related, but they are not identical. Infiltration describes water entering soil at the surface, while saturated hydraulic conductivity describes how readily water moves through saturated soil. Studies use both measurements to help explain how trees influence water movement through soil.

Studies conducted in forest, pasture, urban, and experimental bioretention settings have frequently found higher infiltration rates or hydraulic conductivity in soils influenced by trees. The magnitude of the effect varies with soil type, compaction, moisture, tree species, root characteristics, sediment loading, and system design. For example:

• Several studies of infiltration rates before and after deforestation or forest fires found decreased infiltration rates after the trees were gone (e.g., Wondzell and King 2003, as cited in Herrera 2008).
• Gonzalez-Sosa et al. (2010) found higher saturated hydraulic conductivity in areas of broadleaved forests and small woods than in permanent pasture soil and cultivated lands.
• Skorobogatov et al. (2013) compared saturated hydraulic conductivity in woody vegetation plantings (such as shelterbelts, tree groupings in urban parks, and tree rows on golf courses) with surrounding areas that had similar soil and topography but no trees. They found that trees had a significantly greater impact on soil permeability than lawn without trees.
• Chandler and Chappell (2008) found that the median and mean saturated hydraulic conductivity three meters from individual oak trees were factors of 2.3 and 3.4, respectively, greater than those of the surrounding grassland without trees. Their literature review also cites 12 other studies in which the ratio of saturated hydraulic conductivity in the upper soil horizon beneath trees to that beneath adjacent pasture ranged from 2 to 140.

How Trees Increase Infiltration Rates

So how do trees influence soil infiltration rates?

Living and decaying roots create a network of well-connected channels in the soil called macropores. Flow through these macropores can be up to several hundred times faster than flow through the surrounding soil matrix (Aubertin 1971 and Buttle and House 1997, as cited in Chandler and Chappell 2008).

In addition, organic matter from leaf litter and tree roots improves soil structure, which can increase infiltration rates. Soil structure is improved as soil particles are cemented together by humus, organic glues created by fungi and bacteria decomposing organic matter, and polymers and sugars excreted from roots.

Areas with trees generally have greater infiltration rates than lawn or pasture without trees because trees create more stable macropores. They do this for several reasons. The roots of dicotyledonous plants, which include most trees, grow in thickness through secondary growth as well as in length through primary growth. The roots of monocotyledonous plants, which include grasses, usually do not exhibit secondary growth, so the channels created by their roots may be more prone to collapsing than the thicker channels created by woody roots.

Woody roots and the organic materials left behind as they grow and decay can also help stabilize these channels. These are broad botanical tendencies rather than guarantees, however. More recent research indicates that root diameter, depth, density, architecture, and total root biomass can all influence hydraulic performance.

Impact of Trees on Infiltration Rates in Bioretention

Several studies have documented that vegetation can help maintain adequate saturated hydraulic conductivity over time in bioretention areas (e.g., Lucas and Greenway 2011; Hatt et al. 2009).

Breen et al. (2004) specifically compared unsaturated infiltration rates in model soil profiles contained in above-ground columns with trees to otherwise comparable columns without trees. The planted profiles had higher infiltration rates. This demonstrates that even at a young age, trees can begin to positively affect hydraulic conductivity.

Bartens et al. (2008) also found that tree roots affected soil hydraulic conductivity at a young age. The researchers concluded that woody roots can increase infiltration before very large roots or substantial root turnover have developed, suggesting that water was traveling through channels alongside existing living roots.

In one controlled portion of the Bartens study, green ash roots penetrated a geotextile and an underlying layer of compacted clay-loam soil. The planted treatment recorded an average infiltration rate 27 times that of the unplanted control. This was a specific experimental configuration rather than a universal multiplier, but it demonstrates the considerable influence roots can have on water movement through compacted soil.

Research published since this article was originally written has continued to support its central conclusion. A 2020 review found that long-term bioretention performance is shaped by interactions between growing media and living vegetation, particularly the influence of roots on pore structure, infiltration, storage, and treatment. A 2023 comprehensive review similarly found that research generally converged on a positive effect of vegetation on infiltration and percolation. It also found that plant selection and root characteristics can affect the degree to which initial infiltration rates are maintained.

Trees can therefore help support the long-term hydraulic function of a bioretention system, but they cannot compensate for every design or maintenance problem. Improperly specified or compacted soil, excessive sediment loading, inadequate system sizing, and poorly managed inlets can still restrict infiltration.

Conclusion

Tree-and-soil systems can function as highly effective green infrastructure. Since surface clogging and compaction negatively affect the mechanisms through which trees and soils provide stormwater benefits, maintaining adequate infiltration is crucial. Trees can help.

Many studies demonstrate that trees and their roots have a positive impact on soil structure, creating stable macropores that help maintain infiltration rates and keep these systems functioning over time. This does not eliminate the need for appropriate soil specification, system sizing, sediment management, and maintenance. Instead, it demonstrates why living roots should be understood as a functional part of long-term stormwater performance.

References

Aubertin, G.M. 1971. Nature and Extent of Macropores in Forest Soils and Their Influence on Subsurface Water Movement. USDA Forest Service Research Paper NE-192. Northeastern Forest Experiment Station, Upper Darby, Pennsylvania.

Bartens, J., S.D. Day, J.R. Harris, J.E. Dove, and T.M. Wynn. 2008. “Can Urban Tree Roots Improve Infiltration Through Compacted Subsoils for Stormwater Management?” Journal of Environmental Quality 37(6): 2048–2057.

Breen, P., L. Denman, P. May, and S. Leinster. 2004. “Street Trees as Stormwater Treatment Measures.” Proceedings of the Third International Conference on Water Sensitive Urban Design.

Buttle, J.M., and D.A. House. 1997. “Spatial Variability of Saturated Hydraulic Conductivity in Shallow Macroporous Soils in a Forested Basin.” Journal of Hydrology 203(1–4): 127–142.

Chandler, K.R., and N.A. Chappell. 2008. “Influence of Individual Oak (Quercus robur) Trees on Saturated Hydraulic Conductivity.” Forest Ecology and Management 256(5): 1222–1229.

Gonzalez-Sosa, E., I. Braud, J. Dehotin, L. Lassabatère, R. Angulo-Jaramillo, M. Lagouy, F. Branger, C. Jacqueminet, S. Kermadi, and K. Michel. 2010. “Impact of Land Use on the Hydraulic Properties of the Topsoil in a Small French Catchment.” Hydrological Processes 24(17): 2382–2399.

Hatt, B.E., T.D. Fletcher, and A. Deletic. 2009. “Hydrologic and Pollutant Removal Performance of Stormwater Biofiltration Systems at the Field Scale.” Journal of Hydrology 365(3–4): 310–321.

Herrera Environmental Consultants, Inc. 2008. The Effects of Trees on Stormwater Runoff. Prepared for Seattle Public Utilities.

Lucas, W.C., and M. Greenway. 2011. “Hydraulic Response and Nitrogen Retention in Bioretention Mesocosms with Regulated Outlets: Part I—Hydraulic Response.” Water Environment Research 83(8): 692–702.

Skorobogatov, A., W. Thorne, and B. Amell. 2013. “Biological Elements in Rain Garden Design.” Presentation at the International Low Impact Development Symposium, August 18–21, 2013, Saint Paul, Minnesota.

Skorobogatov, A., J. He, A. Chu, and B. van Duin. 2020. “The Impact of Media, Plants and Their Interactions on Bioretention Performance: A Review.” Science of the Total Environment 715: 136918.

Técher, D., and E. Berthier. 2023. “Supporting Evidences for Vegetation-Enhanced Stormwater Infiltration in Bioretention Systems: A Comprehensive Review.” Environmental Science and Pollution Research 30(8): 19705–19724.

Wondzell, S.M., and J.G. King. 2003. “Postfire Erosional Processes in the Pacific Northwest and Rocky Mountain Regions.” Forest Ecology and Management 178(1–2): 75–87.