The trees were planted about 8years ago.  They were about 4 feet tall when they were planted We watered them for the first few years until they were well established. They are in fairly sandy soil and not in an area that floods. They have done really well up until this spring and have just died very suddenly. I’ve attached some pictures that will hopefully help. there doesn’t seem to be any damage to the trunk or any sap leaking from the tree.

From the closeup, this appears to be some kind of fir tree. The original photo came through at low resolution so I cannot zoom in closer to examine it. Is there a pattern that you noticed to the browning? For example, it appears that the second three has browned from the bottom up. Did you notice anything unusual with these trees prior to this spring?

If you do not see any signs of insects, cankers, etc., then it is likely that these two trees died from environmental stress or a problem with the roots. Although established, they may not have gotten sufficient water from rain during any prolonged dry periods. Sandy soils drain very quickly, and the larger trees appear to be very close to the trees that died. Larger trees can easily outcompete newly established trees for any available water in the soil. Fir trees may not brown immediately when stressed from lack of water. The fine roots that take up water and nutrients in the upper soil will dry up during drought, making it more difficult for the tree to take up water once drought conditions have ended. A period of high temperatures or bright sun with drying winds can quickly pull remaining water out of the needles, if the tree cannot pull up sufficient water to cool the needles and replenish the moisture that is drawn out through evaporation. The tree may "limp along" for some time before appearing to die suddenly, but it has been stressed and weakened for a long time. 

The other thing to discuss is planting. Were these bareroot, balled and burlapped, container grown, or transplanted from elsewhere as natural seedlings? Did you plant these trees yourself? If so, did you spread out the roots and remove any girdling roots? Did you dig the hole deep enough to allow the roots to all assume a natural shape? Sometimes people just press the roots into the hole, and a taproot if present may grow sideways, preventing adequate uptake of water. Did you amend the soil with anything at planting?

It can certainly be discouraging to see your beautiful trees dying. If you can answer some of the questions I posed, maybe we can figure out what may have gone wrong to prevent the issue in the future.