Vegetation Analysis in Santa Monica Mountains

Vegetation Analysis in Santa Monica Mountains

The objective of this lab is to understand the vegetation habitat in Santa Monica Mountain by conducting terrain analysis in the ArcGIS environment. Esri’s spatial analysis tools will be used analyze the Digital Elevation Model (DEM) of Santa Monica Mountains to discover patterns in vegetation habitats across the mountains at various elevations. Certain plants require specific amounts sunlight and shade to grow at various elevations. Spatial analysis of DEM will allow us to reveal interesting patterns of vegetation growth in Santa Monica Mountain ranges.

The basic idea behind analysis is to compute the hillshade of Santa Monica DEM for various seasons such as Spring, Summer, Fall and Winter. Hillshade can be used   in combination with other raster analysis tools to calculate solar insolation and insolation statistics. This will help us to understand why certain types of species grow in certain locations. We will observe the influence of elevation, the direction of sun and degree of slope on the   various species of vegetation found in Santa Monica mountains.

            For analysis, I utilized two datasets sets; 30m elevation dataset (downloaded from USGS) and a California vegetation shapefile provided at class website at UCLA.  Both files were clipped to the extent of the study area. The resulting vegetation layer had four species namely, Annual Grass, Chamise-Redshank Chaparral, California sagebrush, and Mixed Chaparral. The study will focus on determining the impact of elevation and insolation on the growth pattern of these species.

Spatial Analyst tools were used to calculate slope and aspect of the DEM. To calculate the amount of solar insolation received by vegetations species during Spring Equinox, summer Solstice, fall equinox and winter solstice, we needed azimuth and altitude angle   to create accurate hillshades for all those desired dates. I made use of solar calculator available at “http:// susdesign.com/sunangle/ to calculate azimuth and altitude angle for all the fours seasons. I used a longitude of 119.084 West and latitude of 34.30 North as the location on DEM that will be used to calculate azimuth and altitude. Following values were found for various seasons:

Spring Equinox - March 20th, 2012:  altitude angle 55.93 while azimuth angle 178.40

Summer Solstice - June 20th, 2012: altitude angle:  79.13, azimuth angle: -177.65

Vernal Equinox - September 22nd, 2012 altitude angle:  55.52, azimuth angle: -175.04

Winter Solstice - December 20th, 2012 :altitude angle:  32.25 ,azimuth angle: -178.59

I used the values stated above to calculate the amount of solar insolation received by all species of  vegetation  found  in  the  Santa  Monica  mountain  region  in  a  given  season.To  calculate insolation I made use of Raster Calculator tool in ArcGIS tool box. I used the formula provided in our lecture slide which is (1000/255)*hillshade, with 1000 being the insolation constant of

1000 watts/sq.m for a flat slope on a clear day. Once we got four insolation raster maps for four different earth’s season, we used these map in conjunction with clipped vegetation file of Santa Monica mountains to calculate statistics on zones defined by vegetation. I used spatial analyst tool in ArcGIS to determine insolation statistics for each vegetation zone. In the zonal statistics dialog box, I used vegetation layer and Santa Monica insolation rasters   to calculate statistics where zones were defined by vegetation layer. This table summarizes the influence of elevation and insolation on vegetation occurrence in the Santa Monica Mountain Ranges. These summary tables were created for all four seasons. As listed above, the area we selected contained four vegetation types : Annual Grass, Chamise-Redshank Chaparral, California sagebrush, Mixed Chaparral - as well as areas defined as Urban/Agriculture.  The distribution of these vegetation at different elevations can be seen in the maps. In addition, I also present graphs for slope, aspect, elevation and solar regime showing mean insolation for all four seasons.

The chaparrals species are a low lying shrubs and are found all over the mountain. They appear to tolerate lack of water and ranges of warmer and cooler temperatures well. Consequently, they are found in areas with the lowest mean insolation throughout all seasons.  They also tolerate variation in elevation, slopes and aspect well, and as a result are the most widespread species in the Santa Monica Mountains. The annual grasses are found in areas with slope of around 20 degrees facing south and west, and these species also tolerate scarcity of water, though they are less tolerant than chaparrals, and therefore are less available on Santa Monica Mountains.