Two conifers which grow well as an understory are western hemlock and western red cedar. I wouldn't plant them throughout the wood as they would spoil a nice hardwood environment. They would be ideal along edges to keep the wind down and to act as flushing points. Planted sympathetically, they should benefit a native woodland environment within. You would want to cut them down at 10 years or so before they get too big and replant in stages. I find bare leaf litter areas under mature hardwoods are favoured by pheasants for scratching in, especially if you scatter grain now and then.
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