Yeah but how do you measure it between two very different trees. What are we measuring and from where. Electric current?

"Microsatellite DNA markers were used to identify tree and fungal genotypes isolated from mycorrhizas and for comparison with reference tree boles above-ground." https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.12387#main1