Trees. Those huge, carbon sequestering plants which we are already cutting down faster than we can grow them? The large, tall stands prized for high strength in glulam and clt layups which are processed in enormous, energy-intensive plants with toxic adhesives? The percentage of energy used for steel and concrete is what it is, in part, due to the vast advantage in square feet constructed using these products. And energy use? Steel production at the mill can be virtually 100% recycled and 100% electric. How much post-use wood is actually being recycled into building products? Close to none.
Mining for iron and cement is certainly not carbon free, but neither is cutting down forests which are actively sequestering carbon. Harvesting beetle damaged trees is absolutely worthwhile, but there aren’t enough to supply even a small fraction of world building demand.