Ground cover

Where we spread the phacelia brash from last year has now been covered by the 3 vigorously spreading butternut squash plants (with self seeded phacelia growing through as well).  The phacelia is consistently covered in bumble bees and from time to time a humming bird hawk moth spends time on the flowers with a quite distinctive way of moving between flowers.  All this should have kept the sun off the earth and may be preserving soil carbon and mineral nutrients from the phacelia.  Is the extensive disappearance of surface grass where it had been mown prior to the drought an illustration of how organic matter/carbon is lost from the soil without ground shading?
Mulched this area last autumn with dead leaves (rather than all to that year's leaf mould container).  It will be interesting to see what is left of them when we clear the squash plants this autumn.