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Validation

Source: vignettes/Validation.Rmd
Validation.Rmd
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How do we know if Real Twig returns accurate volume estimates? We rigorously tested our method against multiple high quality reference data sets that were both laser scanner and destructively sampled. The laser scanning was done in leaf-off conditions with a Riegl VZ-400. We used different versions of TreeQSM with the same input parameters per tree and different data sets to test our method. A detailed discussion of the results and implications can be found in Morales & MacFarlane (2024): https://doi.org/10.1093/forestry/cpae046

The graph and table below contain destructive sampling with total mass and basic density (wood + bark) across four data sets, three of which are publicly available and are linked below. The mass estimates and statistics use TreeQSM v2.4.1 with its built in tapering compared to Real Twig on the same QSMs.

Method Mean Relative Error (%) RMSE (kg) Relative RMSE (%) CCC
Real Twig (TreeQSM v2.4.1) -1.190 138.783 10.528 0.999
TreeQSM v2.4.1 76.754 641.746 48.683 0.982

TreeQSM v2.4.1

The following figures look at the Harvard Forest data set using different versions of TreeQSM. The destructive sampling data contains total branch and main stem dry mass, and also basic density for both the main stem and the branches. This allows us to test for compensating errors, and look at the true differences between TreeQSM versions. We used the same input parameters per tree and TreeQSM version to test our model.

TreeQSM v2.4.1
Metric Total Woody AGB Main Stem Biomass Branch Biomass
Mean Relative Error (%) 126.428 4.581 949.317
RMSE (kg) 325.973 22.217 317.471
Relative RMSE (%) 77.672 6.773 346.454
CCC 0.831 0.998 0.381
Real Twig (TreeQSM v2.4.1)
Metric Total Woody AGB Main Stem Biomass Branch Biomass
Mean Relative Error (%) 0.019 0.493 -0.691
RMSE (kg) 27.187 23.653 7.736
Relative RMSE (%) 6.478 7.210 8.442
CCC 0.998 0.997 0.997

TreeQSM v2.3.0

Below are the mass estimates and statistics using TreeQSM v2.3.0 with its built in tapering and Real Twig applied to the same QSMs.

TreeQSM v2.3.0
Metric Total Woody AGB Main Stem Biomass Branch Biomass
Mean Relative Error (%) 50.529 1.204 363.772
RMSE (kg) 156.127 23.138 146.843
Relative RMSE (%) 37.202 7.053 160.249
CCC 0.947 0.997 0.654
Real Twig (TreeQSM v2.3.0)
Metric Total Woody AGB Main Stem Biomass Branch Biomass
Mean Relative Error (%) 2.198 4.700 -9.095
RMSE (kg) 24.101 22.747 15.081
Relative RMSE (%) 5.743 6.934 16.457
CCC 0.998 0.997 0.989

SimpleForest

Real Twig was not tested with SimpleForest during its development. While Real Twig does improve volume estimates for SimpleForest versus its built in allometric corrections, there are still improvements to be made, as SimpleForest QSM cylinders are generally much more overestimated than TreeQSM cylinders, making the identification of “good” cylinders difficult.

Below are the mass estimates and statistics using SimpleForest v5.3.2 with its built in vessel volume correction, and Real Twig applied to the same QSMs.

SimpleForest
Metric Total Woody AGB Main Stem Biomass Branch Biomass
Mean Relative Error (%) 51.031 27.775 174.363
RMSE (kg) 177.462 48.287 141.470
Relative RMSE (%) 42.285 14.720 154.385
CCC 0.934 0.990 0.669
Real Twig (SimpleForest)
Metric Total Woody AGB Main Stem Biomass Branch Biomass
Mean Relative Error (%) 15.850 11.909 41.238
RMSE (kg) 27.605 24.525 18.651
Relative RMSE (%) 6.578 7.476 20.353
CCC 0.998 0.997 0.985