Functional type classification of southern Californian vegetation using spectral reflectance

ZUTTA, B.R, D.SIMS and J.AGAMON

California State University of Los Angeles, Los Angeles, CA 90032 USA



 
 
 
 

ABSTRACT

Southern Californian vegetation, exposed to strong seasonal weather cycles and dynamic disturbance regimes, undergo wide variations in photosynthetic fluxes and other ecosystem processes. Sorting vegetation by "functional types," based on their resource use patterns and seasonal response, provides one simplification to help understand changing ecosystem fluxes in these systems. For this approach to be broadly applicable, these functional types must be detectable through remote sensing. Our objective was to measure the seasonal change in spectral reflectance of Southern Californian vegetation and to use the seasonal and spectral variability to categorize species into optically distinct types. A field spectrometer was used to measure reflectance at 1-meter intervals along 100 meter transects in a variety of vegetation types, located in the Santa Monica Mountains of Southern California. Several spectral indices were used in principal components analysis to characterize the variability in spectral reflectance in time and space. Initial results of this study indicate limited power to separate functional groups (e.g. associated with contrasting phenology or photosynthetic rates) when spectra from a single date were used. The ability to separate optical types was enhanced when multiple sampling dates were included in the analysis. Further work is focusing on the physiological characteristics of the optically determined functional groups. Development of this approach may lead to better characterization of functional types and an understanding of the impacts of disturbance and global change on ecosystem processes.
Keywords: Remote sensing, functional types, ecosystem function
 
 
 
 

INTRODUCTION

Reflectance indices can be used to show contrasting spectral patterns of distinct vegetation types or reflect seasonal changes.

This study proposes a new way to interpret remotely sensed data to understand large scale vegetation patterns and processes. Further development of this approach can be applied for understanding the impacts of disturbance and global change.

Objective

1- Categorize southern Califonian vegetation into functional types through their optical response, detectable with remote sensing.

2 - Examine functional type seasonal changes.
 
 

METHODS

Optical sampling (remote sensing) from the ground employs a spectrometer, held above the canopy, and samples vegetation along a transect or within a quadrat. Eight 100 meter transects were established at each site to represent the dominant functional types of the area. These types included annuals, drought deciduous, evergreen, and winter deciduous vegetation. Each point was sampled repeatedly through the study period.
 
 

Site Locations

Rancho Sierra Vista (left square) and Stunt Ranch (right square) in the Santa Monica Mountians were chosen for their diversity of functional types (map courtesy of the National Parks Service).

 

Field Spectrometry

A field spectrometer (UniSpec, PP Systems, Haverhill, MA.), was used at the stand scale during the three seasons (winter, spring, summer).


Optical View of Vegetation

Figure 1. Different functional types have different "optical patterns" during any point in time (December 1999)
Spectral Reflectance and Indices


Figure 2. Reflectance indices can be used to show contrasting spectral patterns of distinct vegetation types.
 
 
 
 

Table 1. Examples of common physiological indices derived from "optical" sampling (see Figure 1).
 
 

From Two Dimensions to a Multi-Dimensional Graph

Figure 3. Using two indices, functional types appear along known relationships (I.e WBI vs. NDVI).

 

Figure 4. Different relationships among three indices distinguish groups in a 3D plot.

 

Figure 5. Multiple indices and variables differentiate groups in a 2D plot of a multi dimensional graph.

 
 
 

Dominant Southern Californian Vegetation


The Santa Monica Mountains posses dominant vegetation types found in Southern California: (from left) Winter Deciduous, Evergreen, Annual grasslands, and Drought Deciduous vegetation.

 
 

Species and Functional Type
 

Table 2. We focused on the preceeding species due to their dominance in the canopy, availabilty at both sites, and different seasonal responses. Note that many species found on transects dominated by evergreens (i.e. chaparral) may also be found with drought deciduous vegetation.

 

Seasonal Changes of Vegetation

Seasonal changes are most noticeable in the annual grassland transect from Rancho Sierra Vista. The vegetation types and patterns change dramatically with water availability (From left: Fall 1999, Winter 2000 and Spring 2000).


RESULTS/DISCUSSION

Categorizing functional types through their spectral reflectance

Spectral indices and principle component factors were used to discriminate optical patterns into separate groups (See Table 2 for functional type list) for each month.

We found that many functional types could be distinguished optically from one another at seasonal extremes. After high precipitation in February and high water stress in July, groups seperated more due to changes in canopy structure (see Figure 6 and 10).

Figure 6. The beginning of winter initiated new growth in annuals. Many evergreens and deciduous vegetation responded similar to each other.
 
 

Figure 7. The unusual lack of precipitation in January had stressed many plants types. Evergreen shrubs (i.e. Malosma laurina) showed little physical effect.
 
 

Figure 8. The return of precipitation initiated a large quantity of growth. All functional reacted with vegetative growth.
 
 

Figure 9. Vegetative growth continued and many species began their reproductive cycle. However, chaparral vegetation (I.e. Ceanothus species) began their decline.
Figure 10. Most functional types plateau or began their decline.

Figure 11. Increased water stress forced many functional types decline. Winter deciduous vegetation were still photosynthetically active.
 
 

Figure 12. All functional types were in different stages of decline due to severe water stress.

 
 

Summary of seasonal changes

Figure 13. Unique seasonal response of winter deciduous vegetation allowed for better functioanl type differentiation in March and July. This graph shows NDVI from December 1999 through July 2000.

Figure 14. Seasonal location of functional types were related to changes in their canopy structure.
 
 
 

Figure 15. Delta indices from months with opposite water availability, March and July (see Figures 6 and 10), clearly classified functional types, and separated drought deciduous into their respective species (M. fasciculatus above, S. melifera below).

 
 

CONCLUSIONS

1- Spectral classification of functional types worked best after the high precipitation of early spring and the sever water stress of mid-summer.

2- Seasonal changes in annuals and winter deciduous canopy structures allowed for relatively distinct spectral groups throughout the year. Drought deciduous and evergreen vegetation mixed regularly.

3 - Additional research is needed to determine if spectral classification of functional types change during episodes of exteme drought or percipitation.
 
 

ACKNOWLEDGEMENTS

We would like to thank Cea-Crest (Center for Environmental Analysis) for their continued support of this study, Dafna Kohn and the National Park service for supplying maps, and the students and professors of the California State University, Los Angeles for their continued support.