These are strips of alder bark, left over from making fence posts. When I first peel them off the alder logs, the insides of the bark are a light, cream color. As they dry in the sun, they turn bright red, which is why they are called Red Alder.

From the Slater Museum:

Alders are among the few higher plants that have the special ability to fix nitrogen, so they can take atmospheric N2 and convert it to ammonia (NH3), which then is available to be used in nucleotides and amino acids, basic building blocks of life. Thus these plants can grow on newly created soils that lack the nitrogenous compounds that act as natural sources of nitrogen for most plants.

Which explains why alder trees grow like weeds here. In just ten to twelve years they grow to be 40 foot tall trees a foot or more in diameter, so they are very useful trees.