What is decolouring the bottom of my conifers?

Looks like drought to me. Conifers are surprisingly shallow rooted compared to other trees - we've had some very windy weather, which dries things out enormously, and not much rain (well, in the south anyway).

We been having a few of these kind of conifer questions lately on GOY. Different variations on dry looking conifers. There has been so little rain for such a long time, and then all of a sudden we hit the first hot (and indeed windy) temperatures of the early summer. In no time at all these discoloured brown looking patches appear. Best to give it a good deep drink with a hose. In the long term there is the need to aim to build up the humus content of the soil, so that it will have a moisture retaining capacity during dry periods. Manure, compost from a compost heap, whatever you use. Let the soil evolve ever better, then it will take care of whatever grows in it. Best wishes.