Arboriculture & Urban Forestry 36(5): September 2010 the plant community as a whole (Dawson 1993; Dawson 1996). In addition to “lifting” water, trees may redistribute water into deeper soil regions, possibly improving groundwater recharge (Burgess et al. 1998; Burgess et al. 2001). Tree roots may also have indirect effects on the hydrologic cycle through their role in nutrient and carbon cycling and improvements in soil structure. Nutrient Cycling Plant nutrient content of urban soils can range from highly de- ficient due to interrupted nutrient cycles and disturbed soils to overly abundant due to misapplication of fertilizers and other an- thropogenic sources. Nitrogen deposition from the atmosphere has increased considerably over the past 150 years, and the con- sequences of this change are still uncertain (Holland et al. 2005). Urban ecosystems have been identified as sources of nutrient pol- lution to receiving waters (Boyer et al. 2002), particularly N and P. Urban and suburban watersheds have much higher N losses than completely forested watersheds (Groffman et al. 2004). The input of reactive N compounds in urban areas is also much higher than surrounding, less populated areas, with sources ranging from automobile engines and excessive N fertilization to pet urine and feces (Zhu et al. 2004). Rates of denitrification in urban areas can be very high compared to other ecosystems and N distribution is influenced by stormwater capture systems (Zhu et al. 2004). The effect of such nutrient hotspots on urban tree root systems is poorly documented. However, tree roots can help regulate nutri- ent cycles by influencing the supply and availability of nutrients in the soil via root turnover, root exudates, and nutrient uptake. Trees can affect nutrient export by reducing stormwater runoff and soil erosion (see Hydrology); stormwater may carry nutri- ents as well as sediment laden with nutrients that may be tightly bound to soil (e.g., P). Trees can influence nutrient supply in the rhizosphere by biological N fixation, extracting nutrients – especially nitrate – from below the root zone of other plants, and reducing nutrient losses from processes such as leaching and ero- sion (Buresh and Tian 1997; Jama et al. 1998). Roots influence a complex set of nitrogen transformations that regulate produc- tion, flow, and loss of N in ecosystems (Fornara et al. 2009). In a Jamaican study, proximity to Casuarina cunninghamiana (river sheoak) trees increased N, NO3 , organic matter, P, Mg, K, Ca, pH, and CEC (Zimpfer et al. 1999). The researchers attributed this response to a complex symbiotic relationship with particu- lar mycorrhizal species. On a global scale, nutrient cycling by plants alters vertical distribution of nutrients within the soil pro- file, keeping nutrients available nearer the soil surface (Jobbágy and Jackson 2001). For example, sloughed root cells and muci- lage contain substantial amounts of soluble C and N (Jones et al. 2004), which is a source of energy for rhizosphere flora and fauna that in turn contribute to a consistent supply of N for plants. ment because of both high emissions and fuel use and minimal C sequestration (Kaye et al. 2006). In addition, daily average atmo- spheric CO2 Carbon Cycling, Soil Organic Material, and C Sequestration Urban regions are large contributors to atmospheric CO2 plant growth (Gregg et al. 2003), and trees fix this CO2 via photo- whereas global mean concentrations are 379 ppm (Pataki et al. 2007; Lorenz and Lal 2009). Higher CO2 concentrations in city centers can exceed 500 ppm, concentrations enhance 199 synthesis and sequester it into the soil through litter and root in- puts. Urban soils have the potential to store large amounts of root- supplied soil organic carbon (SOC) and therefore to contribute to mitigation of increased atmospheric CO2 concentrations (Lorenz and Lal 2009). The amount of SOC that can be stored is highly variable – the SOC pool at 0.3-m depth may range between 16 and 232 Mg/ha and between 15 and 285 Mg/ ha at 1-m depth (Lorenz and Lal 2009). SOC storage is also dependent on the local climate, land use, and parent material. For example, the cool, wet climate of northeastern United States favors higher accumulation of soil organic carbon than dry, rocky, arid climates (Pouyat et al. 2006). The role of urban tree root systems in carbon storage has re- ceived limited attention, and research rests primarily on results from other ecosystems and laboratory studies. However, the potential for carbon storage through root deposition is consid- erable. Besides the deliberate incorporation of organic matter, carbon enters soil from plant litter, the release of carbon-rich root exudates, and root death along with associated mycorrhi- zae (i.e., turnover) (Grayston et al. 1997; Young 1998; Farrar et al. 2003). It has been estimated 2%–4% of net fixed C in plants may be directly deposited into the soil via root exudates (for a review, see Jones et al. 2004). These carbon compounds can also be taken back up by the plant in a controlled fashion (Farrar et al. 2003). Trees direct a greater proportion of their fixed carbon below ground when compared to annual plants, with rates from 40%–73% of assimilated C being demonstrated in studies with trees (Grayston et al. 1997). Up to 47% of carbon allocated to fine roots and mycorrhizae is deposited into soils through root turnover (Fogel and Hunt 1983). Not only does SOC increase activity of microorganisms, but the presence of the microorgan- isms can initiate a feedback system that increases root exuda- tion (Meharg and Killham 1991). Carbon from plant roots there- fore exerts a large control on the soil microbial community and consequently on overall soil health (Brant and Myrold 2006). As previously discussed, urban soils are often very inhospita- ble to root growth. Stripping urban land of its vegetation and top- soil, coupled with elevated temperatures, also depletes soil organ- ic matter and consequently decreases soil microbial populations, particularly in newly disturbed soils (McDonnell et al. 1997; Scha- renbroch et al. 2005). Soil microorganisms are very important to tree growth because they are critical drivers of nutrient cycling, N fixation, nitrification, and the aggregation of clay particles (i.e., building of soil structure) (Lee and Pankhurst 1992). Urban sites in Colorado, U.S., that were fertilized and irrigated had greater microbial biomass than adjacent agricultural land that was not fertilized or irrigated (Kaye et al. 2005). Takahashi et al. (2008) compared soil C concentrations of different land uses [turf, trees “with management” (weed and litter removal), and trees “without management” in urban parks], and found that at 0–10 cm soil depth there were similar soil C concentrations, but at 10–30 cm, average C concentrations were lower for turf than they were for trees “with management.” Trees “without management” result- ed in far greater soil C concentrations than the other land uses. enrich- CONCLUSIONS AND FUTURE RESEARCH This review has focused on the ecophysiology of tree roots in the urban environment and how they interact with this human-dominated world. There are many unanswered ques- tions that relate to management of urban tree root systems, ©2010 International Society of Arboriculture
September 2010
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
