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

CSIRO. Detecting tree mortality severity

Precipitation mm/year

Current monitoring systems

• Aerial sketch mapping and field surveys

CSIRO. Detecting tree mortality severity

May 2007

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

CSIRO. Detecting tree mortality severity

• A critical need for a forest health monitoring system

• MODIS satellite imagery

• Frequent coverage over large areas

• Spatial resolution: 250 - 1000m

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

CSIRO. Detecting tree mortality severity

• 15 cm spatial resolution

• Visual dead tree detection

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)

• ~ “greenness”

NIR redNDVI

NIR red

−=

+

CSIRO. Detecting tree mortality severity

• ~ “greenness”

• Leaf chlorophyll, leaf area, canopy cover and architecture

• Enhanced Vegetation index (EVI)

• 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)

6 7.5 1

NIR redEVI

NIR red blue

−=

+ × + × +

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

( , )1

x tVIRI

VI= −

CSIRO. Detecting tree mortality severity

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

( )

1median t

RIVI

= −

Time series analysis

CSIRO. Detecting tree mortality severity

Statistical analysis

• Tree Mortality

• VI2005 ~ amount of alive trees in 2005

• VI2007 ~ amount of alive trees in 2007

• Analysis

• 4 indices: NDVI, EVI, RI.NDVI, RI.EVI

2005 2007TreeMortality VI VI= −

CSIRO. Detecting tree mortality severity

• 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

Preliminary results

• 4 VI based models

model R2 (*)

NDVI 0.32

RI.NDVI 0.32

EVI 0.13

RI.EVI -0.07

CSIRO. Detecting tree mortality severity

(*) index corrected for over-estimation

RI.EVI -0.07

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

CSIRO. Detecting tree mortality severity

• 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

Sustainable Ecosystems

Dr. Jan Verbesselt, Dr. Darius Culvenor

Phone: 03/95452265

Email: Jan.Verbesselt@csiro.au

Contact Us

Phone: 1300 363 400 or +61 3 9545 2176

Email: enquiries@csiro.au Web: www.csiro.au

Thank you

Acknowledgements This work was undertaken within the CRC for Forestry

Program 1: Monitoring Measuring. Discussions with Andrew Robinson,

Christine Stone, and Angus Carnegie contributed to this study.