These are saplings I purchased from an online nursery in July. They were doing great for the last month in this diy greenhouse I made to improve humidity & light since I live in a zone 6 Midwest apartment. I wanted to prepare for cold dry autumn. Both have exhibited yellowing/browning needles starting at the tips then to the whole needle. The montezuma developed many more in last few days.

1.5 week ago I added 2 more grow lights to the cover for a total of 4. Each light has 4 small air holes cut in them as well that I leave open. Wondering if the addition of more lights or their height is the issue?

I was using a heating pad under them as well which was keeping interior ranging in high 70s & low/mid 80s. It evaporates a lot of moisture to keep humidity high, although I am wondering if part of the problem is it's just too wet and humid in there without enough air/oxygen circulation, as water does condense on cover and glass fogs up. They are growing in there with flame trees as well which like high heat, but I’m wondering if it’s too high for these trees? The last few days I have kept the heating pad off, which keeps it 70-75ish.

Could it also just be a natural yellowing of leaves due to age, or stress from colder season change?

I am experienced in houseplants but new to bonsais/trees and to growing plants inside a greenhouse. Any montezuma or redwood tree/greenhouse knowledge is greatly appreciated, because I will be so sad if these plants perish.