Design in a Time of Uncertainty
By Rives Taylor, Principal, Global Resilience Research Lead
Part Two in a Three-Part Series on HARC’s program of work and research utilizing LiDAR in the Houston-Galveston region.
The City of Houston is a complex system comprised of built and natural features which, when considered as a whole, form an urban ecosystem. The spatial distribution of green space including various types of vegetation such as grasses, shrubs, and trees largely influence the plant and animal species present – which is one form of biodiversity. Green spaces are often measured in two dimensions that only account for coverage in terms of surface area. However, vertical characteristics of vegetation, the third dimension, can contain the majority of space for ecological communities to occupy. Three-dimensional features of vegetation remain unquantified, undescribed, and unmapped in Houston and most global urban centers due to difficulties of characterizing landscape scale vegetation with traditional methodologies1. The organizational structure of vegetation and woody plant material within the three-dimensional space of an urban ecosystem can be assessed using Vertical Structural Complexity (VSC) as a metric. Measuring vertical green space requires a technology with the capability to capture widescale height data – enter Light Detecting and Ranging (LiDAR).
Light Detecting and Ranging (LiDAR)
LiDAR is a remote sensing technology that uses light in the form of a pulsed laser to measure distances of various surfaces on the Earth. These light pulses—combined with other data recorded by the airborne system— generate a precise, three-dimensional representation of objects and surface characteristics of the Earth. The advent of LiDAR technology has made the fine scale mapping of detailed three-dimensional land surface characteristics, over large-scale geographies, a feasible endeavor2,3. LIDAR data is typically classified into categories that represent buildings, vegetation, and the ground surface, among other features. It is these classifications, in particular points of vegetation, that are useful because they contain information related to where vegetation occurs across a height gradient.
Biodiversity - the variation within and between life forms and the ecosystems they inhabit - is influenced by environmental conditions and geography. Biodiversity is important because it sets the foundation for ecosystem services and provides stability to ecosystems. The degree to which environmental conditions such as air temperature vary within a city is high because changes in land cover occur over short distances. In general, species richness is higher where more green space is present4. Moreover, the organizational structure of vegetation and woody plant material have been found to have a positive effect on biodiversity5. In the urban environment, where space at the ground level is often limited, increased VSC of vegetation can provide more habitat for species to occupy, enriching biodiversity. The VSC and other vegetative structural characteristics such as density, height, and spatial distribution can serve as an indicator for the potential of green space to support biodiversity and ecosystem services3, 5.
Application of LiDAR
Dr. Ryan Bare, a HARC Postdoctoral Research Scientist, will be undertaking a project to perform a comprehensive urban vegetative assessment by quantifying and mapping spatial differences of overall vegetative VSC as well as the coverage, height, and density of vegetation. These features represent structural characteristics of canopy, subcanopy, and ground vegetation in the Houston area. The vegetative data developed by this project can be applied to pursue future work to describe the amount of variation amongst layers of vegetation and, ultimately, how these differences influence ecological, hydrological, and social systems.
The urban ecosystem is complex and can vary over small scales. This variation, and the organization of green space on three-dimensional planes influences biodiversity. Understanding relationships between VSC of vegetation, vegetative structural characteristics, and the urban ecosystem is imperative because insights can lead to improved management, not only optimizing our environment for biodiversity, but also nature-based infrastructure (stormwater management), climate mitigation, public health, and social equity as well. HARC is striving "to build a sustainable future that helps people thrive and nature flourish" by providing this information on a broad scale to improve management of, policy concerning, and research opportunities related to the urban ecosystem.
1Caynes, R.J.C., Mitchell, M.G.E., Wu, D.S., Johansen, K. and Rhodes, J.R. (2016) Using high-resolution LiDAR data to quantify the three-dimensional structure of vegetation in urban green space. Urban Ecosystem 19, 1749-1765.
2Angelo, J.J. (2010) Characterizing the Vertical Structure and Structural Diversity of Florida Oak Scrub Vegetation Using Discrete-return Lidar. Electronic Theses and Dissertations 1553.
3Mitchell, M.G.E., Wu, D., Johansen, K., Maron, M., McAlpine, C. and Rhodes, J.R. (2016) Landscape structure influences urban vegetation vertical structure. Journal of Applied Ecology 53, 1477-1488.
4Norton, Briony A., Karl L. Evans, and Philip H. Warren. 2016. "Urban Biodiversity and Landscape Ecology: Patterns, Processes and Planning." Current Landscape Ecology 1:178-192.
5Beninde, J., Veith, M. and Hochkirch, A. (2015) Biodiversity in cities needs space: a meta-analysis of factors determining intra-urban biodiversity variation. Ecology Letters 18, 581-592.