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California Sustainability Indicators Plant Growth Index

Final Project Report November 2, 2012 Project Team: Forrest Melton, Hirofumi Hashimoto, Carolyn Rosevelt, Pamela Krone-Davis Ecological Forecasting Lab CSU Monterey Bay & NASA Ames Research Center Moffett Field, CA

MODIS Rapid Response, Oct 30, 2012

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Plant Growth Index

•  Plant growth index: Proposed as indicator for trends in vegetation condition from long-term satellite data records

•  Derived from trends in peak annual NDVI from two satellite data records:

•  AVHRR GIMMS, monthly, 8km, 1982-2010

•  MODIS, 8-day, 250m, 2001-2011

•  Linear trends calculated with least squares regression

•  Screened for statistical significance using the Mann-Kendall trend test, at p > 0.90

Nemani et al., Remote Sensing of Env., 2009

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Plant Growth Index: Tracking Trends in Ecosystem Conditions

• Included in the 2008 Heinz Center State of the Nation’s Ecosystems Report

• Based on NDVI calculated from AVHRR GIMMS data (1982-2010) and MODIS (2000-present)

• Changes in the amount of energy captured by plants over large areas, as reported in this indicator, may signal significant changes in ecosystem functioning.

• Indicator can provide information about influence of climate, nitrogen deposition, disturbance, etc.

Significant trends in NDVI. Green values represent an increase in index values over the period for a particular 5-mile square pixel, and red values represent a relative decrease. White represents areas with no significant trend up or down.

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PGI for California

Statistically significant trends in the plant growth index from 1982-2010, calculated from the GIMMS AVHRR 8km data record.

Statistically significant trends in the plant growth index from 2001-2011, calculated from the MODIS 1km data record.

PGI from AVHRR (8 km) PGI from MODIS (250m)

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PGI by Ecosystem Type

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Separating Response to Anthropogenic and Natural Disturbance Events

SERGoM, Bierwagen et al., 2010

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PGI and Fire History

Fire history polygons from 1991 to 2011 overlaid on the trends in the plant growth index calculated from the MODIS NDVI data record from 2001 to 2011.

Fire history polygons overlaid on trends in the plant growth index east of Clear Lake, California. Notable features include the Fork Fire (large blue polygon), which occurred in 1996 and is associated with a large cluster of pixels exhibiting a positive trend in the plant growth index, and the Walker Fire (largest red polygon), which occurred in 2008.

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PGI and Fire History

Fire history polygons overlaid on trends in the plant growth index east of Willow Creek, California. Much of this area was burned during the period from 1991 to 2001.

Fire history polygons overlaid on trends in the plant growth index in Big Sur, California. Notable features include the Basin Complex and Indians fires (red polygons) that occurred in 2008, and the Kirk Fire (1999) and Wild Fire (1996) (blue polygons).

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PGI and Urbanization

Examples of spatial comparisons between trends in the plant growth index and increases in urban land cover from the 2001 and 2006 NLCD data. Figures (a) and (b), from Lathrop and Stockton, provide examples of cases where there is reasonable agreement between negative trends in the plant growth index and expansion of urban land cover.

(a) (b)

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PGI and Urbanization

Examples of spatial comparisons between trends in the plant growth index and increases in urban land cover from the 2001 and 2006 NLCD data. Figures (c) and (d), from Turlock and Stockton, provide examples of locations where there changes in urban land cover and trends in the plant growth index do not align spatially.

(c) (d)

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Conclusions

•  Long-term trends in the plant growth index in California present a mixed pattern of positive and negative trends, with a majority of the state exhibiting no significant trend.

–  AVHRR: much of the state shows no significant trends, and overall pixels showing positive trends outnumber pixels exhibiting negative trends. 1,377 pixels exhibited a statistically significant trend over the study area, representing 88,128 km2, or 21.3% of the state. Of these, 1,096 pixels (80%) exhibited a positive trend, representing 70,144 km2. A total of 281 pixels (20%) exhibited a negative trend, representing 17,984 km2.

–  MODIS: of the 414,000 km2 encompassed by California, 33,104 km2 (529,656 250m pixels) or 8% of the state exhibited statistically significant trends in the plant growth index from 2001 to 2011, with the total area of negative trends slightly outweighing positive trends

•  Comparisons between the plant growth index and fire history polygons show clear spatial relationships at a resolution of 250m, demonstrating that the plant growth index is sensitive to local changes associated with ecosystem disturbance.

•  Spatial agreement with changes in urban land cover was less well defined, and generally the spatial agreement was not strong, even using the MODIS 250m dataset.

–  Urban development, especially for residential housing, typically results in a short-term reduction in the plant growth index, as irrigated lawns and other landscaping can achieve maximum NDVI values that are comparable to agricultural lands, and grassland, shrubland, and savannah ecosystems. Other satellite derived indicators may be much more sensitive to changes in land cover type, however, and can be used to reliably map increases in urban land cover and ISA (Homer et al., 2004).

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Conclusions

•  Previously published results for the U.S. (Heinz Center, 2008) and North America (Nemani et al., 2009) demonstrate the ability of the plant growth index to capture long-term trends in vegetation condition over large areas.

•  In California, substantial year-to-year variability in weather conditions, coupled with the

natural role of fire in California’s ecosystems, may make it harder to detect emerging long-term trends in the plant growth index.

•  In the short-term, the plant growth index may have limited utility for communicating climate

vulnerabilities to the citizens of California. As warming continues to advance over the coming decades, however, the plant growth index may serve as a useful indicator for mapping regional trends in ecosystem condition as trends begin to emerge against the historic interannual variability in temperature and precipitation in California.

•  There are also numerous other satellite indicators that may provide highly useful spatial

summaries of changes in land cover, snow cover, groundwater, evapotranspiration, evaporative stress, biomass and carbon fluxes, and other key indicators of the condition of California’s natural resources.