Towards the Development of Historical Indicators of Spatial Carbon Dynamics in Drylands: A Blue Oak Woodlands of California Case Study

Abstract:

The role of drylands in global terrestrial carbon dynamics is understudied and thus is not well understood. Yet, drylands cover 41% of the terrestrial land surface and their net primary productivity (NPP) can influence the amount of atmospheric carbon dioxide by sequestering carbon through photosynthesis. The blue oak woodlands of California represent a large component of the region’s terrestrial vegetation for which 300-year tree ring width (TRW) chronologies have been developed. Consequently, the goal of this study is to reconstruct the historical spatial and temporal variation in NPP using the observed correlation of TRW with a remotely sensed proxy for NPP: the time-integrated normalized difference vegetation index (iNDVI). Thus, this study’s objectives are to 1) determine the best grain resolution of 8 km pixel resolution Advanced Very High Resolution Radiometer iNDVI to achieve the highest correlation with blue oak TRW, 2) use this correlation to develop iNDVI chronologies at each site, and 4) generate 300 years of NPP spatial maps by interpolating the iNDVI chronologies with soil and topographic factors using cokriging regression. We hypothesize that (1) neither the TWR or their residual chronologies will impart a large difference in correlation to iNDVI, and (2) that the best correlations of TRW with iNDVI will be at coarser grain resolutions. We found that linear regression of TRW to mean NDVI from 1982 to 2003 at selected chronology sites had significant adjusted R2-values of 0.1823 to 0.2572. We expect these relationships will strengthen in comparison to iNDVI and adjustment of grain resolution. Additionally, we will compare the contemporary record of the historical NPP from 2001 to 2003 to both historical Palmer Drought Severity Index and net ecosystem exchange (NEE) maps.