Branching architecture limits the rate of somatic mutation accumulation in trees

Trees are long-lived plants that develop complex, highly branched shoot systems as they grow. Their extended lifespan allows somatic mutations to accumulate along these branching structures, ultimately becoming fixed in reproductive and vegetative tissues such as leaves, flowers, and fruits. Mature trees can easily sustain tens of thousands of terminal branches, each potentially carrying mutated gametes. To avoid mutational meltdown and inbreeding depression, long-lived plants appear to have evolved mechanisms that slow mutation accumulation per unit time. How this is achieved remains unclear. Here, we show that branching architecture can limit mutation accumulation to the same extent as reducing the mutation rate itself. Tree structures that maximize branch path sharing during development constrain mutational diversity in the crown. These architectural factors can drive differences in mutation burden by orders of magnitude, even when mutation rates and terminal branch numbers are identical. Building on these insights, we show that current estimates of somatic mutation rates in trees are actually upward biased by a factor that scales with the topological characteristics of the tree. These results raise the deeper question whether somatic mutation rate differences, recently detected among tree species, reflect variation in branching strategies rather than variation in the rates themselves. It is possible that specific branching architectures have evolved not only to optimize resource allocation and structural stability but also to limit mutational load.