Thank you for your questions. First, pine needles decompose very slowly, so covering them up with compose will create a mat of needles under the compost, and the benefits of nutrients from the compost will be reduced. And you’ll find still-decomposing needles years from now. The following Extension article recommends no more than 10% of compost be needles: http://mgeldorado.ucanr.edu/files/270887.pdf Needles dry out quickly, so I’d recommend you rake out most of them before applying compost. Second, the fire danger will not be mitigated by adding composted material which will eventually dry out, too. I don’t know how close the trees are to your house, but here are some tips for wildfire danger control: https://disastersafety.org/wildfire/reduce-wildfire-damage-homes/ Finally, as to watering mature trees. I suggest you invest in a water moisture meter, which you can use in any garden, so you can see how dry the soil is. Although these trees’ roots are often deep, we are seeing many established trees suffering water stress in our summers. It is more effective to water less often, but thoroughly, rather than often and shallowly. But the probe can tell you how far down the water has penetrated. Twice a week may be enough. Hope this is helpful. Good luck!