Not the 3-30-300, but the 10-20-30 rule to avoid tree monocultures

ARTICLE

MARKET & CURRENT AFFAIRS

Thijs Dolders (Boomkwekerij Ebben), Monday 27 May 2024

145 sec

Santamour's rule helps create an ecologically valuable urban tree stock

Tree monocultures remain common in urban settings. This poses significant challenges regarding planting strategies, especially in the context of climate change, heavily paved environments, and the rapid spread of diseases due to globalization. In this article, we explore the conditions for establishing a valuable ecological tree stock, emphasizing Dr. Santamour's 10-20-30 rule as guidance for urban planners and green managers.

Tree diversity in Delft (photo: Ebben Tree Nursery)

Monocultures and ecological risks

Planting large numbers of a single tree species in one area poses significant risks. The chance of cross-contamination increases when trees of the same species stand close together, potentially leading to large-scale losses if disease strikes. Oaks, known for their high biodiversity, have traditionally been extensively planted. They provide shelter and food for numerous insects, mites, moths, algae, fungi, and mosses. Although beneficial for biodiversity, extensive oak plantings have created new challenges, notably oak processionary caterpillars and oak bark beetles. Entire avenues planted with oaks heighten the risk of insect infestation. This vulnerability applies to other uniform tree stocks as well, such as ash dieback, bleeding canker of horse chestnuts, and sooty bark disease in maples. Therefore, diversity within the urban tree stock is crucial for spreading risk.

Santamour's 10-20-30 rule as a guideline for diversity

A diverse tree population consists of various species, sizes, and ages, offering aesthetic benefits and increased resistance to pests and diseases. The 10-20-30 rule, formulated by forester Dr. F. Santamour (1990), provides a practical framework to maintain such diversity. The rule specifies the maximum percentages allowed: no more than 30% from one family, 20% from one genus, and 10% from one species, thereby reducing risk. For example, applying this to the common oak (Quercus robur), no more than 10% should be of the same species (Quercus robur), no more than 20% from the same genus (Quercus), and no more than 30% from the same family (Fagaceae). Therefore, municipalities must accurately inventory their tree stocks by species, genus, and family. This rule applies not only across entire municipalities or neighborhoods but also to smaller scales such as avenues and groups of trees.

Challenging environments

However, certain (urban) environments are challenging, limiting the variety of tree species that can survive. The more challenging the conditions, the fewer species will thrive. In such situations, there should be no compromise regarding strong, well-adapted species, even if the 10-20-30 rule cannot fully be followed. Conversely, studies in the Netherlands suggest a stricter 5-10-20 diversity guideline for new public spaces: 5% of one species, 10% of one genus, and 20% of one family. Good growing conditions can undoubtedly increase diversity and resilience. Even within species, genetic diversity can be enhanced by planting seedlings rather than clones or genetically identical cultivars. Yet, proven resilient clones and cultivars (elms, maples, lindens) may be the best choice in challenging environments.

Ecologically valuable tree stock

Municipalities play a critical role in establishing ecologically valuable tree stocks. A customized tree selection list is essential, taking into account local conditions such as soil type, groundwater levels, wind exposure, and salt tolerance, alongside diversity needs. This sequence is crucial: a tree must first thrive in its location before contributing to a diverse and healthy tree population. A diverse tree population also provides alternative food sources for natural predators, thus reducing pest pressure. Additionally, losing a particular tree species in a diverse setting has less impact on the overall landscape, preserving the ecosystem services provided by urban trees.

Diverse tree stocks for a sustainable city

Diverse urban tree populations are not just a trend but a necessity, especially as climate change and new threats from diseases and pests challenge our green spaces. Green managers and urban designers play a crucial role in creating ecologically balanced urban landscapes and should rely on common sense. Santamour's 10-20-30 rule provides a valuable compass for achieving this diversity and maintaining tree health. In some contexts, even the ambitious 5-10-20 guideline could be the starting point. It is time to invest in sustainable green management and strive for environments where greenery is not only attractive but also resilient, biodiverse, and healthy.

The author, Thijs Dolders, is a landscape architect at Ebben Tree Nursery in Cuijk.

Thijs Dolders

Sources:

Trees for Urban Planting: Diversity, Uniformity, and Common Sense - Frank S. Santamour, Jr., U.S. National Arboretum Agricultural Research Service, U.S. Department of Agriculture, Washington D.C., 2002

This article was previously published on May 27, 2024, on the Boomzorg website.

Boomkwekerij Ebben B.V.

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