One of our specialties at The Kestrel Design Group is designing urban tree systems (especially designing urban tree systems to maximize stormwater benefits). One question we are regularly asked on urban tree projects is whether or not soil organic matter will become too low or depleted when trees are surrounded by pavement and leaves are no longer able to decompose in the soil in which the tree is growing.
Ideally, of course, much of a tree’s leaf litter would become incorporated back into the soil where it is growing (whether the tree is surrounded by pavement or not), but the proportion of soil organic matter that comes from tree leaf litter is much lower than most people intuitively think. In fact, the biggest contributors to tree soil organic matter (SOM) are fine tree roots and even more importantly, the mycorrhizae associated with tree roots. A mycorrhiza is an association between a fungus and the roots of a plant. Most tree roots grow in association with mycorrhizal fungi. The tree supplies carbohydrates and other growth requirements to the fungus, and the fungus gives the tree roots greater access to water and nutrients (particularly phosphorus) in the soil by greatly increasing the total absorptive area of the root system.
Why is soil organic matter important? It helps grow healthy trees by benefitting them in the following ways:
- helps build soil structure
- provides nutrient reservoir
- increases soil water holding capacity
Many urban trees in downtown streetscapes, parking lots, and plazas are growing in a small tree opening (often 5′ x 5′, and sometimes as small as 3′ x 3′) surrounded by pavement. How does being covered with pavement affect soil organic matter? The answer is that it depends mostly on the soil.
Godbold et al (2006), for example, studied the relative importance of leaf litter decomposition, fine root and fungal turnover for carbon (C) incorporation into soil organic matter for three growing seasons in a poplar forest in Italy, and found that 62 percent of the carbon that entered the SOM pool came from mycorrhizae. Their paper states that their research methods quantified the maximum carbon (C) input from leaf litter and overestimated C input from leaf litter. In other words, the relative contribution of C to the SOM pool from leaf litter is likely to be even less than this study quantified, and the relative contribution of fine roots and mycorrhizae is likely to be even greater. They also found that the fungal hyphae of the mycorrhizae had a very fast turnover rate of only nine days. This is much faster than that of leaves, which turnover once a year, and fine roots, which turnover about three times a year (Lukac et al 2003 in Godbold et al 2006).
Fogel and Hunt (1982) also found that return of organic matter to the soil by fine roots and mycorrhizae was much higher than that from leaf litter. In their two year study of a young Douglas fir forest in the Oregon Coast range, return of organic matter to the soil by fine roots and mycorrhizae ranged from 84 percent to 78 percent.
Knowing the relative influences of fine roots and the associated mycorrhizae vs. leaf litter on the formation of soil organic matter, how would we expect pavement to influence the release of organic matter from trees to soil?
Perhaps the most obvious impact of the pavement , and the one most people think of first, is that the soil under a deciduous tree canopy no longer has access to the tree’s leaf litter, so leaf litter can no longer contribute to soil organic matter.
Much more significant than the impact of the pavement on leaf litter carbon cycling, however, is the impact of the pavement on fine tree roots and mycorrhizae associated with the tree. If the pavement above the tree rooting zone compacts the soil to root limiting bulk densities or greater, root growth will be very limited under the pavement. One would therefore expect very limited renewal of soil organic matter under pavement if the soil under the pavement is overly compacted. Root limiting bulk density depends on soil type (see Figures 1 and table 1), but ranges from 1.39 g/cm3 for clay soils to 1.69 g/cm3 for sands and loamy sands. This generally corresponds to a Proctor density of 80% to 85%. In order to install pavement, soils are typically compacted to 95% proctor density, which is much higher than the limit for root growth. So, where pavement compacts underlying soils to root limiting bulk densities, so that the growth of roots and their associated mycorrhizae is very limited, we would not expect much organic matter to be produced.
If, however, the soil under the pavement can be protected from excessive compaction, and a healthy rooting environment can be provided under the pavement, we would expect that soil organic matter could be renewed by fine tree roots and mycorrhizae associated with tree roots. Protecting soil under pavement from excessive compaction is therefore expected to be crucial to maintain healthy levels of soil organic matter under pavement, much more important than whether or not leaf litter is returned to the soil. And, as mentioned above, maintaining adequate soil organic matter is crucial to grow healthy trees because it helps build soil structure, provides a nutrient reservoir, and increases soil water holding capacity.
Nathalie Shanstrom is a sustainable landscape architect with The Kestrel Design Group.
Top image Flickr Credit: Iain Turner