Aircraft wood starts with the logging process. The tree must be felled so that there are no induced cracks in the wood. There's a lot of issues with aircraft wood, and a lot more than ring counts and linear runout. The wood has to be treated right throughout the ENTIRE process. One way to know for sure is to stress test the wood by pulling on it. I'm sure that fir will do well, and the weight difference in negligible.
But, be sure the integrity of the wood is correct. Ron Freiberger EAA 28328, Tech Counselor 4125
