Abstract
Designing where to plant riparian vegetation is a component of many
river projects. Several mechanistic models have been developed
considering biological, soil, hydrological, and hydraulic requirements
that influence riparian vegetation growth. However, many models are not
spatial explicit and there remains high uncertainty as to where
plantings will survive or die. This study sought to determine if a
machine learning (ML) algorithm could be trained to accurately
characterize the complex set of site attributes that promote survival,
and do so exclusively using metrics derived from airborne LiDAR. Results
could then be used to guide planting strategies. The selected testbed
river was 34 km of alluvial, regulated, gravel/cobble river where
planting projects are common and have high mortality. The lower Yuba
River, California, USA was mapped at sub-meter resolution in 2017. Our
approach has four steps. First, a set of 32,000 vegetation
presence/absence observations were randomly selected from LiDAR-derived
polygons of naturally occurring established vegetation. Second, the
river was split into 75 training, validation and test areas. Third, a
set of 17 LiDAR-derived topographic potential predictors were computed
at 0.91-m (3-ft) resolution. Finally, a Random Forest machine learning
model was trained to best predict vegetation presence. The model results
in a riparian vegetation presence probability map and has a “Area Under
the Curve” (AUC) of 0.77. As probability values are difficult to
interpret, a forage ratio electivity index analysis was performed with
statistical bootstrapping. Results show that points with probability
values > 0.8 had ~ 8.5 times more riparian
vegetation present than would be likely from random chance at the 95%
confidence level. Microtopographic ‘vector ruggedness’ was identified as
the main driver for vegetation presence, followed by Terrain Ruggedness
Index and Roughness. In conclusion, a ML model can identify where
riparian vegetation planting are most likely to succeed and guide
design. Our results also suggest that more attention should be paid to
creating rugged microtopography under plantings to help cuttings and
seedlings establish deposition critical for nutrition.