Significant tree damage implies severe winds and should be reported to the NWS.  You should report any time trees are snapped or uprooted, and when limbs 2" or larger are broken off.  When you give your report, try to estimate the diameter of trees that have been uprooted or snapped, or the size of limbs that are broken.  Also, state the extent to which the damage has occurred (one tree or limb versus numerous trees down).  Give the approximate time the damage occurred, if possible.