Excerpt from Ong, B. L. (2002). Green Plot Ratio: An ecological measure for architecture and urban planning. Journal of Landscape and Urban Planning, 965, 1–15.
LAI (Leaf Area Index) is related to a range of ecological processes like photosynthesis, transpiration and metabolism. These plant processes in turn enable the prediction of net primary production, rates of energy exchange between plants and the atmosphere, rates of future growth and yield, and the monitoring of changes in canopy structure due to pollution and climate change. The ability to estimate LAI is therefore a valuable tool in modelling the ecological processes occurring within a forest and in predicting ecosystem responses. Actual plant productivity is dependent on chlorophyll concentration, stomatal density and other physiological and environmental factors.
If we were to consider a lawn (Fig 1), i.e. a patch of ground covered with grass—we may imagine that the ratio of grass to the ground is roughly 1:1. The LAI in this case is 1 and the corresponding green plot ratio is 1:1. A closer inspection will reveal that it is actually more variable depending on the plant species and other factors but for the sake of discussion, the figure is close enough.
If this patch of ground were to be covered instead by a clump of bushes, the total surface area of leaves ex- posed to the sun may be expected to be more. Leaves are translucent and leaves within a canopy are able to tap the transmitted light through the upper layer of leaves. Current research (e.g. Deshmukh, 1986; Jones, 1992; Scurlock et al., 2001) suggests that a reasonable figure for this is about 3:1. That is, the total area of leaves exposed to the sun is about three times that of the area of the ground itself. If the ground is covered instead with full grown trees, with a dense canopy and closely planted individual trees, the ratio would be yet higher—6:1 or even 10:1.
The green plot ratio has been defined as the average LAI of the greenery on the site. It can also be equivalently defined as the ratio of the total single-side leaf area of the planted landscape to the plot or site area. Its value is effectively the same as the average LAI but expressed as a ratio rather than a single figure to reflect the normal practice for calculating maximum allowable floor area in architectural and and planning practice. GPR is a more reasonable indication of the effectiveness of the greenery on a site than current metrics based on the extent of site coverage because it is directly related to the amount of photosynthetically active leaves on site.
The use of GPR carries some caveats. GPR alone cannot indicate species richness on site. Leitao and Ahern (2002), for example, have provided a more detailed discussion of applicable metrics in a broader ecological context. The determination of which set of metrics should finally be used lies outside the scope of the present paper. Issues to be answered include the definition of urban ecology, not as a broad field encompassing various research directions, but as a planning and design target with quantifiable criteria to be met and regulated. While additional ecological criteria are needed, they do not, however, replace the usefulness of GPR but complement it. It is argued here that GPR, and its correlate LAI, are metrics fundamental to the development of this larger set of ecological metrics.
One of the primary strategies in the greening of the built environment is the incorporation of trees and plants. There are several benefits to increasing the amount of greenery in cities—environmentally, aesthetically, and recreationally. Current research not only supports the argument that plants are an important ingredient in balancing the polluting effects of cities, but also found plants to be aesthetically and emotionally beneficial. In fact, it is not the beneficial significance of plants that is surprising but the ability of modern society to camouflage our dependence on plants.
References:
Deshmukh, I., 1986. Ecology And Tropical Biology. Blackwell Scientific Publications, Palo Alto, CA.
Jones, H.G., 1992. Plants and Microclimate, Second ed. Cambridge University Press, Cambridge, UK.
Leitao, A.B., Ahern, J., 2002. Applying landscape ecological concepts and metrics in sustainable landscape planning. Landsc. Urban Plann. 59, 65–93.
Scurlock, J.M.O., Asner, G.P., Gower, S.T., 2001. Global LeafArea Index Data from Field Measurements, 1932–2000. Available On-Line From the Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, University of Colorado, TN, U.S.A., http://www.daac.ornl.gov.
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