Short communicationRainfall interception and stem flow by eucalypt street trees – The impacts of canopy density and bark type
Introduction
Trees provide many ecosystem service benefits in the urban landscape, such as habitat provision, carbon sequestration, reduced particulate pollution, a ‘sense of place’, contact to nature, microclimate cooling and mediated hydrological processes (Chiesura, 2004, McPherson et al., 2005). The mental health, cultural and economic service benefits that trees provide within an urban landscape are now well recognised and can be experienced at an individual, an organisation, or an entire community/city level (Westphal, 2003, Nowak and Dwyer, 2007). Urban trees also play an important role in urban catchment hydrology through canopy interception of rainfall, which reduces beneath canopy throughfall and therefore catchment peak flows. When trees are widely distributed throughout the urban landscape and these trees have dense, broad canopies the amount of storm water that reaches sewerage and river systems can be reduced and the peak flows delayed (Xiao et al., 2000, Xiao et al., 2007, Wang et al., 2008). In addition, tree architecture and bark properties greatly influence the proportion of intercepted rainfall that becomes stemflow and is directed to the base of the stem and root bowl (Johnson and Lehmann, 2006). Stemflow can be extremely important for precipitation and nutrient re-distribution in arid environments (Li et al., 2008), in agricultural and forest ecosystems (Levia and Frost, 2003). Likewise, in an engineered impervious urban landscape, stemflow can be important for the hydrology and nutrient availability and cycling of individual street trees.
Canopy interception is the difference between gross rainfall and the amount of rain that passes through the canopy (Xiao et al., 2000, Barbier et al., 2009). The process of canopy interception is influenced by three main factors: (i) the type of rainfall event (magnitude, intensity and duration), (ii) the tree species and canopy structure, and (iii) the antecedent weather (Crockford and Richardson, 2000). Most studies of canopy interception have been made in natural or managed forest systems, where up to 50% interception has been measured (Schellekens et al., 1999). In urban systems, canopy cover is discontinuous, tree canopies are often isolated and there is high species and canopy/leaf trait variation which means that interception is difficult to measure and even more difficult to predict. Wind direction during rainfall events is important but total rainfall interception can be expected to be less than in natural forest, continuous canopy situations (Guevara-Escobar et al., 2007). Xiao et al. (2000) developed a complete under-canopy collector to determine interception rates of 15% and 27% for Pyrus calleryana and Quercus suber, respectively. This approach enabled the collection of high quality data but can only be applied in controlled research situations for tree canopies without interference from adjacent vegetation or built structures. A simpler method is required so that canopy interception (CI) and stemflow (SF) can be measured in real urban settings and for mature tree canopies.
Understanding how different tree canopy characteristics (dense/clumped/sparse), leaf traits (pendulous, compound, large blade, needle) or stem properties (smooth/fibrous/fissured bark) influence net rainfall input, distribution and runoff will greatly help in attempts to model urban catchment hydrology. Mechanistic models that are semi- or fully distributed (i-Tree HYDRO; (Yang et al., 2011), MUSIC; (Fletcher et al., 2001), SWMM; (Barco et al., 2008), TOPLATS; (Bormann, 2006)) require a process based understanding of tree impacts upon hydrology across the range of rainfall events (Elliott and Trowsdale, 2007, Cuo et al., 2008, Wang et al., 2008). Modelling and planning of urban systems should consider both the beneficial (stormwater reduction) and negative (reduced rainfall recharge) impact of tree canopy cover upon rainfall inputs and urban catchment hydrology. The benefits are optimised when canopies cover impervious surfaces and the negatives are greatest when canopies cover pervious green spaces. Modelling how tree canopies impact urban hydrology requires models with at least hourly and not daily time steps, because recognising short, discrete rainfall events and differences in rainfall intensity may be critical for accurately predicting rainfall redistribution and runoff (Xiao et al., 2000, Wang et al., 2008).
Under-canopy troughs and stem helix troughs with tipping buckets were used to test three simple hypotheses:
- 1.
an isolated tree with greater plant area index would lead to greater canopy interception of rainfall,
- 2.
percentage rainfall intercepted would decrease with increasing magnitude of rainfall event, and
- 3.
an isolated tree with smooth bark would have greater stemflow than a rough bark species, given similar branching architecture and form.
In the process, the study tested the applicability of these simple field methods. The canopy interception and stemflow of a mature, smooth-barked E. saligna and mature, rough-barked E. Nicholii tree with contrasting bark characteristics were made near continuously for five months in Melbourne, Victoria. The impact of tree canopy interception is discussed with regards to two scenarios of street stormwater runoff and green space soil water recharge.
Section snippets
Materials and methods
The study was at the Burnley campus of The University of Melbourne, Victoria, Australia (37.82° S, 145.01° E). The average annual rainfall is 687 mm (since 1972) and mean monthly temperature minimum 15 °C and maximum 30 °C. Inter-annual rainfall is highly variable in south-eastern Australia, but in Melbourne the variation amongst monthly mean rainfall (since 1972) is small, ranging between 46.3 mm (March) and 66.2 mm (October). The minimum rainfall recorded within one month may be as low as <2 mm and
Results
The gross rainfall estimated by the roof-top trough (0.72 m2) was consistently less than that measured by the traditional roof-top rainfall gauge (0.02 m2). A linear function of y = 0.8179x + 0.121 (R2 = 0.983) was used to correct all trough data (GR and TF). It was assumed that this small discrepancy was due to retention and evaporation from the larger surface area of the troughs.
There was a strong linear correlation between GR and TF beneath the canopy of E. Saligna (R2 = 0.973) such that CI only
Canopy interception according to canopy and rainfall characteristics
Tree canopies will always intercept more rainfall for small or low intensity, scattered showers than under large storm rainfall events (Barbier et al., 2009). The same is true for these two eucalypt tree species grown in isolation in an urban setting, however, they show distinct differences in their magnitude of CI and response to increasing rainfall magnitude. In support of the first hypothesis, the more dense canopy of E. nicholii was able to intercept the majority of rainfall events that are
Conclusions
Through continuous measures of canopy throughfall and stemflow this study has demonstrated the importance of canopy and bark characteristics to how trees can modify their own and the surrounding urban hydrological processes. The study demonstrates that trees with greater plant area index (canopy density) intercepted a greater amount of gross rainfall. For a typical rainfall year, this equated to a 45% reduction in rainfall reaching the ground surface, reduced from 680 to 378 mm. Canopy
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