JB ... I encourage you to use 4"-5" "bricks" of straw (if it's been wet, they'll peel off easily) as mulch on one of your pea-gravel mulch trees as an experiment. I'd bet money you will have no problems. Why do you suppose voles/mice will tunnel under wood mulch and not pea gravel? My guess is wood mulch gets wet and sticks together allowing the tunnel to be completed; however, the weight of pea gravel (and no sticking together) means the fabric/gravel collapses on the animal and doesn't allow tunnels/nests. Why place straw on top of pea gravel... several reasons including enhanced weed control, conserving moisture and keeping area out to drip line cooler in hot summer. the BIG reason is enhanced insulation which means the tree roots may keep growing well into late fall or winter (ground doesn't freeze). For trees, what's on top (structure including canaopy) is a funjction of what's down below (root structure); your tree will grow much bigger/faster over several years - perhaps fruiting sooner - with the extra straw mulch. You can do it! Gamble with one tree ... I'll bet you will be surprised and delighted. Good luck