Jan Verbesselt - Assessing Severity Of Tree Mortality In Pinus Radiata Plantations With Modis Time Series

Detecting tree mortality severity using MODIS satellite data

Jan Verbesselt 14 ARSPC Conference 30 September 2008

Tree mortality • Severe Ips bark beetle damage

CSIRO. Detecting tree mortality severity

Tree mortality • Caused by drought stress

Precipitation mm/year

CSIRO. Detecting tree mortality severity

Current monitoring systems • Aerial sketch mapping and field surveys

May 2007

CSIRO. Detecting tree mortality severity

Current monitoring systems • Aerial and field surveys • Few highly skilled individuals in the country • Expensive and labour demanding • Infrequent coverage (aircraft hire, cloud coverage) • -> only annually

• A critical need for a forest health monitoring system • MODIS satellite imagery • Frequent coverage over large areas • Spatial resolution: 250 - 1000m

CSIRO. Detecting tree mortality severity

Detecting tree mortality • Aim • Assess sensitivity of MODIS satellite data • Which VI’s are most sensitive to tree mortality? • When are VI’s most sensitive?

• Using aerial ADS 40 digital imagery • 15 cm spatial resolution • Visual dead tree detection

CSIRO. Detecting tree mortality severity

Data

CSIRO. Detecting tree mortality severity

MODIS satellite data • MOD13Q1 product • • • •

250 m spatial resolution 16 daily composited images 2000-2008 Vegetation indices • Normalized Difference Vegetation Index (NDVI)

NDVI =

• ~ “greenness” • Leaf chlorophyll, leaf area, canopy cover and architecture

NIR − red NIR + red

• Enhanced Vegetation index (EVI)

EVI =

NIR − red NIR + 6 × red + 7.5 × blue + 1

• Improved sensitivity in high biomass regions • Minimizing soil and atmosphere influence

* red (620-670 nm), near-Infrared (NIR; 841 - 876 nm), blue (459-479 nm)

CSIRO. Detecting tree mortality severity

Time series analysis

CSIRO. Detecting tree mortality severity

Time series analysis

CSIRO. Detecting tree mortality severity

Time series analysis

CSIRO. Detecting tree mortality severity

Time series analysis • Apply the Relative Index (RI) • To remove seasonality • Reduce general atmospheric noise • Increase sensitivity

RI =

VI ( x ,t ) VI median ( t )

−1

• VI median is median at time t ( t: 2000 -> 2007) for current selected pixels within an area of 10 by 10 km.

CSIRO. Detecting tree mortality severity

Time series analysis

CSIRO. Detecting tree mortality severity

Statistical analysis • Tree Mortality

TreeMortality = VI 2005 − VI 2007

• VI2005 ~ amount of alive trees in 2005 • VI2007 ~ amount of alive trees in 2007

• Analysis • 4 indices: NDVI, EVI, RI.NDVI, RI.EVI • 3 periods: Median VI of whole year, Feb – Mar (a), and Jun – Aug (b) • 2 manifestations: Difference approach, 2007 value

• Modelling • OLS model, step-wise selection of variables based on AIC value • correct for over-estimation by resampling of the OLS model

CSIRO. Detecting tree mortality severity

Preliminary results • 4 VI based models model

R2 (*)

NDVI

0.32

RI.NDVI

0.32

EVI

0.13

RI.EVI

-0.07

(*) index corrected for over-estimation

CSIRO. Detecting tree mortality severity

Discussion • Remote sensing challenges • Tree Mortality only up to maximally 30% • Drought stress caused overall decline in forest health in 2007 (~ defoliation) • Understorey influence (e.g. weeds)

CSIRO. Detecting tree mortality severity

Conclusions & further work • NDVI is more sensitive then EVI • Sensitivity changes within a year • NDVI : Feb-Mar • EVI: Jun-Aug ⇒Development of specific change detection metrics for each VI

• Further work • Verify EVI • Include SWIR based indices • Derive amount of alive trees in 2005 and 2007 from Quickbird data • Tree density estimation using local Maxima and image segmentation

CSIRO. Detecting tree mortality severity

Sustainable Ecosystems Dr. Jan Verbesselt, Dr. Darius Culvenor Phone: 03/95452265 Email: [email protected]

Thank you Acknowledgements This work was undertaken within the CRC for Forestry Program 1: Monitoring Measuring. Discussions with Andrew Robinson, Contact Us Christine Stone, and Angus Carnegie contributed to this study. Phone: 1300 363 400 or +61 3 9545 2176 Email: [email protected] Web: www.csiro.au