Hello Jennifer:

    Good question.  Since wood are widely available in Alaska, many people will have similar question like you have.
     Wood chips play a role to sperate soil surface from air.  As such, it first can prevent water losses from soil, and 2nd, it keeps soil cooler in the summer and warmer in the winter.  The wood chip mulches sperate soil from the air and keep soil from the Sun, as such, it is cooler in the summer.  Also in the winter time, it will keep the cold air being transmitted to soil, therefore soil is warmer in the winter. It will also keep away of the weeds growing from the soil where you put the wood chip mulches because of no sunshine.  

  Size of the wood chips is also important in heat transmission from air to soil in the summer and also in keeping soil temperature warmer in the winter.  General speaking, large woodchips has more large air pockets and air is a poor heat transmitter, therefore they can keep soil cooler in the summer and warmer in the winter.  For small wood chips, the small pocket tend to hold water, and water is good heat transmitter.  So wet small wood chips will delay longer of soil from warming up in the spring time because of the wetness of the woodchips due to smaller air pocket.

   Please let me know if you still have questions!

Both (alder and pine) woods are fine for making chips.  However, there is a little difference between the two.  Alder wood chips will decompose fast than pine wood chips.  This is because alder is a nitrogen fixation tree.  Because of that, it serves as a pioneer species in the disturbed land to build up nitrogen nutrient in soil.  Also, it is a poor competitor.  Once it prepare the land rich in nitrogen, other species start to come.  It can't compete and dies, then decompose and release nutrients.  As such, in the the bushes, one usually can't see the remains of alder tree trucks.   Have a good day!