A. I am on the side of "it doesn't make much difference". Yes, pine needles contain a lot of resin and they don't break down quickly. But having them on the surface of the soil, or mixed into the soil at about 5% of the soil volume, makes no difference at all!
Besides the resin, there is some slight lowering of pH (more acidic) which in our soils is actually helpful. Hopefully you amended the soil under pine trees with compost at the time of planting.
Digging a hole large enough for plants under pine trees can be difficult because of the roots of the pine. Don’t be afraid to cut pine roots when planting. Allow these cut roots to heal 24 to 48 hours before they are buried in the soil again.
Look closely at the amount of light these plants are receiving. Some plants grow nicely with 50% or more sunlight available to them. Others need much more than this for good growth. As an example, Bermudagrass can handle about 10% shade while tall fescue can handle nearly 50% shade.
Generally speaking, flowering plants need lots of light compared to non-flowering plants. Signs of a lack in light can be a general failure of the plants to thrive, leaf drop and a general thinning of the canopy, and leaf yellowing. You may have to remove some of the lower limbs to allow enough sunlight to the plants growing beneath the canopy.
This link about pine needle mulch is from Clemson University in South Carolina where most soils are acidic. Our soils are nearly 100 times less acidic (more alkaline) than those in many parts of South Carolina!
The pH scale is logarithmic, not linear. A pH of 5.5, a common pH value for acid soils, is 100 times more acidic than our soils with a pH of 7.5. Our soils commonly range from 7.8 – 8.5.Pine needles decompose more quickly if they are cut or shredded.