Seasonal patterns of LAI and microclimate variables
The five vegetation types represented in this study showed distinct seasonal patterns of canopy leaf development and microclimate effects. These seasonal variations in urban biophysical properties were more strongly related to differences in percent tree cover and canopy density than to plant functional type.
We found that the five vegetation types represented in this study had distinctly different seasonal patterns of relative soil and surface temperature, yet did not differ in their seasonal patterns of soil water content. Over the course of the growing season, sites with greater tree cover and higher LAI had consistently cooler soil and surface temperatures than did open or low tree cover sites, regardless of plant functional type (Fig.
2b, c). Consistent with this finding, the only significant differences among vegetation types in mid-season relative soil and surface temperatures were between the low tree cover, low LAI sites and high tree cover, high LAI sites (Fig.
3b, c). These differences in relative soil temperatures were largest during the middle of the growing season and minimal at the beginning and end of the growing season, while differences in relative surface temperature were most noticeable during the first half of the growing season. It is more likely that the seasonal dynamics of soil water content are controlled by precipitation events and irrigation practices, which were not sampled systematically in this study. These ideas are consistent with previous studies that have discussed the importance of irrigation as a driver of spatial patterns of soil moisture in urban and suburban landscapes (Scharenbroch et al.
2005; Byrne et al.
2008).
As expected, plant functional types differed in their seasonal LAI dynamics, most markedly with deciduous-dominated sites showing larger variation in LAI across the growing season than evergreen sites (Fig.
2a). However, plant functional types did not explain the large differences in mid-season LAI among sites (Fig.
3a), suggesting that structural characteristics such as percent tree cover, stand density, or species composition are more important for explaining stand-level variability in urban LAI. Consistent with this, the only significant difference we found in mid-season LAI among the vegetation types was between the deciduous low and all other high tree cover sites (Fig.
3a).
Our stand-level LAI measurements obtained with an optical plant canopy analyzer were similar to those found in another urban study from Terre Haute, Indiana (Hardin and Jensen
2007). LAI values at our suburban study sites were also similar to, or slightly lower than, those observed in temperate and boreal forests with tree species similar to those in our study area (Chen et al.
2006; Lindroth et al.
2008). This result is consistent with studies showing that the aboveground biomass of open-grown urban trees can be 20% less than forest-grown trees of the same diameter (Nowak
1994). Stand-level LAI values are also likely reduced in urban and suburban areas because of landscaping practices that maintain park-like spacing between trees and prevent natural succession and canopy closure.
Microclimate variables showed strong functional responses to changes in LAI across the growing season (Fig.
4). Regardless of vegetation type, surface temperature decreased by ∼1°C for every unit (m
2 m
−2) increase in LAI (Fig.
4b). In other words, surface temperatures under dense tree canopies (
e.
g., sites with LAI = 6 m
2 m
−2 in Fig.
4b) were reduced up to 6°C, relative to sparse canopies with near-zero LAI. Soil temperature also declined with increasing LAI. When compared to sparse tree canopies with near-zero LAI, soil temperatures under dense tree canopies (
e.
g., LAI = 6 m
2 m
−2) were reduced up to 7°C. However, the slope and intercept of this relationship was significantly different for evergreen-dominated versus deciduous-dominated sites (Fig.
4a). For every unit increase in LAI, soil temperature decreased by ∼3.1°C at evergreen-dominated sites and by ∼1.2°C at deciduous-dominated sites. A possible explanation for the difference at the evergreen sites is that they had much less variation in LAI over the course of the growing season than did the deciduous and mixed sites. All sites with high tree cover (DH, EG, MX) showed a similar range of relative soil temperature (Fig.
2b) due to seasonal changes in solar radiation. Because those changes in relative soil temperature are seasonally correlated with changes in LAI across the growing season, the small LAI range at evergreen sites resulted in a steeper decline in relative soil temperature with every unit increase in LAI. Alternatively, if our study area had included evergreen sites representing a larger range of LAI, we may have observed a similar soil temperature response to increasing LAI as in other vegetation types. Our interpretation is that while the difference in response of the evergreen sites was statistically significant, it may not have been ecologically significant, especially in light of the fact that we found no vegetation type differences in the relationship between relative surface temperature and LAI (Fig.
4b).
The cooling effects of urban vegetation are well documented and are currently used as environmental design tools to reduce urban heat islands and home energy use (McPherson
1994). Previous field studies have found soil and surface temperatures of residential lawns to be several degrees cooler during the summer than other common ground cover types, such as bark, mulch (Byrne et al.
2008), and native grasses (Bonan
2000). This is largely because of the increased evaporative cooling by transpiring, and often well-irrigated, turfgrasses. Our results support these previous studies and they extend our understanding of how these cooling effects vary among urban vegetation types and over time with canopy leaf development. We found that tree canopies have a greater cooling effect on soils and the surface compared to open turfgrass lawns, which is likely due to the canopy intercepting solar radiation and shading the surface. Trees also have deeper roots and greater leaf area than turfgrasses, leading to greater evapotranspiration. Evapotranspiration from trees, however, occurs at the top of the canopy and does not necessarily mix throughout the vertical volume of air to significantly modify the local temperature surrounding an individual stand of trees (Oke
1989).
The differences in soil temperature we observed among our vegetation types could also have important implications for carbon cycling in urban areas. Because soil respiration rates increase exponentially with soil temperature between 0 and 40°C (Lloyd and Taylor
1994), our results suggest that CO
2 efflux from urban soils may be modulated by seasonal changes in canopy density as well as plant functional type (Fig.
4a). In natural systems, it has been shown that an increase in canopy density mediates soil respiration rates by reducing net radiation at the surface, causing lower soil temperatures (Smith and Johnson
2004; Tanaka and Hashimoto
2006). If we apply Smith and Johnson’s (
2004) temperature-response equations from a woodland-grassland study to our suburban ecosystem with a mean summer soil temperature of 25°C, the average soil respiration rate under a dense urban tree canopy (
e.
g. LAI = 6 m
2 m
−2) would be 56% lower than in an open turfgrass lawn (1.85 versus 4.17 μmoles CO
2 m
−2 s
−1, respectively). Our results suggest that the significant cooling effects of urban tree canopies on soil temperature should be accounted for in urban carbon budgets (Pataki et al.
2006; Churkina
2008). The greatest potential for reduced CO
2 emissions from lower soil respiration rates would be in sites where urban tree canopies occur over grass or bare soil ground covers, rather than impervious surfaces.
Site differences in mid-season microclimate and ground cover
We evaluated two stand-level metrics, LAI and percent tree cover, for their ability to explain spatial differences in mid-season microclimate, as well as mid-season ground cover composition, under the urban forest canopy. Overall, we found that mid-season LAI was a better predictor of both mid-season microclimate and ground cover variables than percent tree cover.
While we found mid-season LAI to be a better predictor of mid-season soil temperature (Fig.
6a, b), we also found that percent tree cover was a better predictor of mid-season surface temperature (Fig.
6c, d). Because surface temperature is most affected by the direct beam solar radiation penetrating a canopy at a given moment in time, it depends more on the extent and distribution of the canopy (indicated by percent tree cover) than the density of leaves per unit ground area (indicated by stand-level LAI). As a result, over the range of tree cover from 0 to 100%, the mean mid-season surface temperature was reduced by 6°C on average. In contrast, soil temperature is a more integrated measure of a site’s energy balance and is more strongly influenced by the total leaf area per unit ground area. A greater mid-season LAI by five units (m
2 m
−2) consequently reduced the average mid-season soil temperature across the sites by an average of 4°C.
Mid-season LAI was also a better predictor of site differences in percent ground cover than was percent tree cover (Fig.
7). The percent cover of turfgrass, in particular, showed the strongest correlation with mid-season LAI (Fig.
7a,
R
2 = 0.56). Although turfgrass species are adapted to a variety of light environments, in general turfgrass is less shade tolerant than many broad-leaved weed species (Fry and Huang
2004), which showed no trend with increasing LAI or percent tree cover (Fig.
7e, f). In high light environments, turfgrass can out-compete weed species, but turfgrass is less competitive in low light conditions (Fry and Huang
2004). Additionally, human management prevents competition from broad-leaved weeds through the use of herbicides, which are often applied to open canopy, high light lawns. The cover of turfgrass in urban and suburban areas has been much less frequently quantified (Milesi et al.
2005) than has tree cover, which is relatively easily assessed using forest inventories, aerial or satellite imagery (Nowak et al.
2008). Our results suggest that it would be possible to produce a first-order estimate of the density of turfgrass ground cover by using the more readily available data on urban tree canopies, although the predictive equations would likely need to be developed regionally to account for differences in climate and horticultural practices. Ultimately, this information could be used to account for understory vegetation cover in models of urban land–atmosphere exchanges of energy, water, and carbon (Rivalland et al.
2005).
Using tree cover to predict site differences in mid-season LAI
In this study, we evaluated the relative performance of two different measures of percent tree cover for predicting site differences in mid-season LAI. Measuring LAI in urban ecosystems is considerably more difficult than in natural forest systems because of numerous methodological constraints, including optical interference from buildings, a limited number of urban-specific allometric equations for trees, and spatially heterogeneous and isolated tree canopies (Peper and McPherson
2003). Although we found stand-level LAI was a better predictor of mid-season microclimates and ground cover than percent tree cover (Figs.
6 and
7), percent tree cover is an easily measured and more commonly used metric for evaluating the extent and distribution of urban forests, as well as the ecosystem services they provide (Nowak et al.
2008; Wang et al.
2008). We found that field-based estimates of percent tree cover were better than satellite-derived estimates at predicting the site-to-site variations in LAI in our suburban study area (Fig.
5). The satellite-derived land-cover map produced a narrower range of tree cover values compared to the field-based inventory using U.S. Forest Service urban forest inventory protocols (USDA Forest Service
2005). Although the field-based measures are more subjective, they take into account gaps within individual tree canopies that are too small to resolve in even high-resolution satellite imagery such as QuickBird (2.4 m). There was still considerable variation around the best-fit linear regression model (Fig.
5b) and, in general, LAI at evergreen sites was under-predicted by 30%, while LAI at deciduous sites was over-predicted by 40%. The model fit was largely driven by the strong positive relationship between stand-level LAI and field-measured tree cover at levels of <50% tree cover. At sites having >50% tree cover, there was considerable scatter in the relationship and field-measured percent tree cover poorly predicted mid-season LAI. This saturating effect of LAI with field-measured percent tree cover was likely due to the fact that both stand density and species composition contribute to a site’s LAI. At sites with low percent tree cover, stand density had the greater influence on stand-level LAI. In contrast, at sites with high percent tree cover, differences in the canopy structure of different plant functional types and species was more important in determining stand-level LAI.
Developing relationships between percent tree cover and mid-season LAI would be a useful step toward scaling up urban biophysical properties and providing the information required to implement complex urban land-surface models. Our results suggest that satellite imagery with a pixel size on the order of 2.4 m is unable to resolve the detailed tree canopy information needed to scale up urban forest biophysical properties. However, metropolitan-scale comparisons of different methods for estimating tree cover have found that high-resolution aerial photography (60 cm ground resolving distance) produces similar estimates to field-based urban forest inventories (Nowak et al.
1996; Walton et al.
2008). This suggests that next-generation, high resolution satellite imaging systems, such as GeoEye-1 (1.65 m multispectral and 0.41 m panchromatic resolution at nadir), could be used to produce maps of urban forest canopy characteristics that would be adequate to model LAI, soil temperature, surface temperature, and ground cover over relatively large urban areas.