That's what I have been told by agriculture and forestry consultants, silviculturist's, and others more educated than me.
Do a google on mulch and nitrogen. Then find some new advisers!
Wood Mulch Pros and Cons:
Initial application can rob some nitrogen from the soil; you may need to supplement nitrogen for your plants. Since wood itself contains very little nitrogen it helps itself to N already present in the soil to break down its carbohydrates. When the decomposing process is complete the nitrogen will then be returned to the soil. See organic soil amendment chart for sources of nitrogen. A surface mulch of wood will only take nitrogen from the top of the soil surface whereas any wood mulch worked into the soil will readily consume the nitrogen.
Avoid aspen, black walnut, and black locust in wood mulches.
You do not want mulch that comes from diseased plants. This should be thrown out.
Wood mulches can sometimes harbor insects and rodents i.e.: watch out for borers (very destructive bugs) and mice. Here again it is best to keep mulch pulled back from the plant base to make it unattractive for rodent nesting and burrowing.
Hard to clean leaves and garden debris out of wood mulch.
Retains moisture very well.
Keeps the soil cool during hot weather and warms it in early spring.
Wood mulch is very appealing. Looks great when used around evergreens and perennial plantings.
Cedar makes for a superior mulch as it has insect repellant properties.
Some good choices: cypress, cedar, redwood, willow and pine.