Soil compaction is the loss of “spaces” between soil particles. These spaces are where soil moisture and air necessary for trees’ survival reside. Soil compaction can be caused by many things, but today let’s just focus on cars.
I grew up in a rural environment where it was common for folks to park on their front lawns. It’s less common in southern California, but I still see it happening in less developed and/or economically depressed areas. The weight and pressure of cars driving and parking on soil compacts the soil, making it less friendly to roots and other soil organisms.
Wet soil is damaged more severely than dry soil when it is subjected to excessive weight such as that caused by parked cars. The reason for this is that wet soil particles have less adhesive ability. The water lubricates the particles and makes them less able to maintain their structure. So when wet soil is subjected to extreme pressure it collapses, and all the spaces vanish.
The photo below is a good illustration of a dirt parking lot that invites cars to park beneath a native oak tree. The photo was taken after some rainfall, so the ground is wet. This is an invitation to severe soil compaction beneath the oak tree and may well lead to its decline over time. It would be better for the tree if cars were not allowed to park within its dripline.