Inside the Dragon: non-invasive analysis of the trunks of the monocot Dracaena draco via electrical resistivity tomography.

Dracaena draco is an evergreen monocotyledonous species endemic to the Macaronesian region and the Moroccan Anti-Atlas, and is endangered due to severe habitat fragmentation. Additionally, many D. draco specimens growing in public and private gardens have recently suffered substantial canopy damage, with some trees losing their entire canopy. We used minimally invasive electrical resistivity (ER) tomography to assess the trunks of D. draco individuals suffering different levels of canopy damage. The extent of canopy damage was used to categorize trees into three groups: visibly undamaged (0%), moderately damaged (25-55%) and severely damaged (70-95%). ER tomograms enabled novel insights into the trunks of D. draco, by displaying homogeneous and concentric patterns corresponding to primary (higher ER) and secondary growth (lower ER). A progressive and significant increase of ER was found with increasing percentage of canopy damage. Visibly undamaged trees exhibited significantly lower ER (<250 Ωm) than severely damaged trees (>250 Ωm), while moderately damaged trees had ER falling between the two extremes. This study demonstrates that ER tomography can be a useful tool for analyzing structural and functional traits also in the trunks of monocotyledonous trees having secondary growth. Results showed an imbalanced ER distribution and higher values in the trunk of trees with a severely damaged canopy. This indicates possible wood dehydration and/or decreased electrolyte content and variation in tissue density.