I found this. https://www.researchgate.net/figure/Illustration-testing-and-monitoring-series-of-movement-and-soil-moisture-3-Result-and_fig1_331347689

Not sure if this is fully functional or it's just idea. But it's based on arduino, which makes it cheap. Knowledge how to do it and program it is the only issue so better connect with some arduino community.

I have no background in this, but it seems people are mostly detecting wetness of soil and movement of soil. So some plain gps for movement and moisture sensors should do the job. Positioning those sensors might be crucial. Tech might help to save lives, but for long term I'd go natural way and try to find experts who knows about the soil and trees. Planting trees with deep roots could help in some cases.