City Trees

be available to plants at adequate soil moisture levels because the soil pH was within the range between 6.0 and 7.3. It is relatively easy to main- tain adequate soil moisture levels in the sampled areas because plant available water was generally above 20% of gravimetric (dry) soil water content. Also, active carbon content, organic matter, and soil respiration rates generally received "fair" or "good" ratings, which are likely to indicate ade- quate nutrient cycling and sufficient pools of nutrients in the soils at the Mall. The less-than-ex- cellent ratings for active carbon (the fraction of organic matter used by soil microorganisms) and soil respiration are likely exacerbated by soil com- paction. All these soil properties are adequate yet have been underutilized by the trees due to the high levels of soil compaction across the Mall. Species Distribution Maintaining a diverse species composition is a good way to maintain a healthy, resilient tree pop- ulation. A general guideline in urban forestry sug- gests that an urban tree population should include no more than 10% of any one species, 20% of any one genus, or 30% of any one family. While it is debatable if this guideline is appropriate for a monumental landscape such as the National Mall, it underscores the importance of tree diver- sity to reduce the risk of catastrophic loss. All 550 trees inventoried on the National Mall are elm trees. This number excludes the five bald cypress trees (Taxodium distichum) found growing in front of the National Museum of Natural History, as this study is focused specifically on the elm planting. It is a monoculture landscape, and the vast majority of the trees are just one species, the American elm (Ulmus americana). While it is possible that a hand- ful of trees are Ulmus × hollandica or newer hybrid elms developed for their DED resistance, U. amer- icana dominates. Younger trees tend to consist of American elm cultivars planted because of their resis- tance to DED, especially 'Princeton' and 'Jefferson'. Another prominent cultivar on the mall is 'Augustine Ascending', which stands out visually from the other trees due to its markedly upright form (see photo). Ground Penetrating Radar Sampling of Elm Roots In November of 2017, 17 elm trees in twelve pan- els were selected for deeper more intense inves- tigation using Ground Penetrating Radar (GPR). These represented some of the oldest and larg- est trees at the east end of the Mall with the greatest observed soil disturbance as well as trees from the west end, which has the best turf and showed the least observed disturbance. GPR has been shown to successfully locate tree roots non-invasively and in three dimensions in forest soils. Although this technology has a long history in archaeology and engineering to locate antiquities and utilities, the practice of using it to map roots in urban soils, which can be compacted, layered and discontinuous, is comparatively new. Root detection is possible in principle because of the moisture content within the woody root that provides an excellent contrast with the surrounding soil. Although roots are detectable, it may only be possible to estimate a bulk property of the roots' structure, such as root frequency at different dis- tances from the trunk and different depths in the soil. The tall, narrow form of the cultivar 'Augustine Ascending' makes it easy to spot among the other elms. Photo by Nina Bassuk 30 CityTREES

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