Error estimates for biometric measurements of NEP Mike Ryan and Rudy King USDA Forest Service Motivation: Understand how age, fertility, species change NEP; compare with Eddy Flux NEE

One Method of Estimating NEP from Biometric Measurements NEP = GPP – R H - R A = Annual change in ecosystem C storage =  [C DW (dead wood) + C W (live biomass)+ C S (soil) + C R (Roots) + C L (fine litter)]

Myths Variability is too large to do this accurately Compounding error will give very uncertain estimates of NEP However, most variability is local (scale of m 2 ) and not important for our questions (scale of ha or larger)

Two Cases Replicate Plots: Inference at a landscape or experiment scale Single site: Inference at a site scale (for example Ameriflux tower footprint)

Replicate Plots Sub-sample variance not included in replicate plot is negligible and does not have to be propagated (n/N ~ 0) : Poor sub-sampling will give larger variance among replicate plots, and good sub- sampling will give smaller variance among replicate plots, but you do not need to propagate any additional sub-sample variance.

Replicate Plots Cumulative Error: NEP calculated using plot-level estimates of all components contains cumulative (  variance + covariance) and sub-sample variance. Allometric equations? Working on it.

Results For Eucalypt plantation in Hawaii: CV for NEP ~40%. 95% CI = 20 – 30% of mean for n=6. Larger plots for biomass sampling would probably lower CV. 50% higher NEP in high fertility plots

Results CV for lodgepole pine chronosequence suggests similar variance for NEP: 5-25%Soil C 8-12%Forest Floor C 17-45%Dead Wood C 12-20%Biomass C

Single Site CI and SE will decrease with sqrt(sample size). Plots (with sub-sampling) can be used to minimize small-scale variability. Cumulative Error: NEP calculated using plot-level estimates of all components contains cumulative (  variance + covariance) and sub-sample variance.

What can improve precision? Replicate sample units Time (Precision of D increases linearly with number of years) Increased subsamples/replicate plot will decrease overall variance-- especially important with small n.

Needs for Ameriflux Commitment to measuring C stocks at all sites Commitment to measuring NEP based on changes in C stocks over some standard interval (5-10 years). Development of standard methodology for measuring C stocks and changes at sites. Research into efficient sampling methods.