According to the local news-minus (newsminer.com): "There has been an outbreak in Fairbanks of the amber-marked birch leaf miner (Profenusa thomsoni), an insect that came to North America in the early 1900s and arrived in Fairbanks by about 2002. Leaf miners overwinter in the ground as pupae and emerge as adults in the spring. The adults lay eggs at the tips of young birch leaves. The eggs hatch into larvae (small caterpillars) that eat the insides of the leaves, leaving yellow areas scattered with worm nuggets. After a few weeks, the larvae fall to the ground and pupate. The leaves that were mined then turn brown."
Their source: http://www.fs.usda.gov/detail/r10/forestgrasslandhealth/?cid=fsbdev2_038906
What's interesting is that in the past few years, the aspen trees had suffered from a similar infestation and turned a pale shade of yellow well before the birch. This year the aspen apparently were unaffected and exhibited the more vibrant colors.