In my own research, I do not sample live trees for fire history. Because fire scars can occur on any side of a tree, we generally need a full cross section (sometimes called a tree cookie) for the fire-history work. We focus on collecting these cross sections from snags (standing dead trees) and stumps. There is a tree-ring method of sampling live trees without them - a pencil thin core is extracted using a tool known as an increment borer. However, this method is not useful for fire history; it’s used in studies that examine things like tree age and climate-growth relationships. You can check out cool photographs of working with stumps and live trees on my research website:

https://sites.google.com/view/monicarother/photo-gallery