I fertilized the new trees I planted in my yard about a week ago, and afterwards my 7-month old puppy started digging around my trees. I guess something in the fertlizer attracts her. So I covered the base of my trees with pine straw and then a rubber mulch mat. She has started ripping apart my expensive mulch mats and chewing on them. She has never messed with the trees before, only since I fertilized them. I’m afraid to take the mulch mats off because I don’t want her to dig around my trees and kill them. Any tips? I tried spraying the mats with bitter spray and within minutes she had one in her mouth. She is an inside dog, but we let her out for a couple minutes to go to the bathroom and it’s like she goes right to them. Help?
is there anything I can spray or put on the mats to discourage her during the times that my kids let her out and I’m not there quick enough to catch her? My kids are too young to remember not to let her out. She was wonderful outside before the fertilizer so we let her out to play with them whenever.

I would recommend taking her out on a leash for the next few days, even in your back yard. Then when she starts to chew the mats say "no" firmly to her and take her back inside and ignore her for 5 minutes. Ignoring a puppy is the best way to "punish" it for doing a bad behaviour. A puppy would much rather have your attention than chew that mat, so if you have her on the lead you can control her not to do it, and be there to correct her if she does chew it. After you do this "punishment" a few times, she will get the hint.

Adding more of that spray also couldn’t hurt.