Considering inner and outer bark as distinctive tissues helps to disentangle the effects of bark traits on decomposition

Revealing the ecological consequences of bark multifunctionality and its underlying traits has become a relatively new but essential focus in plant ecology. Although the enormous differences between the most crucial bark layers, i.e., inner and outer bark, in structure and functions have been widely recognized, the overall bark has been regarded as a homogenous tissue in most bark-related studies. This has led to poor knowledge on the functional independence, specialized contributions and possible linkages of inner and outer bark traits across tree species when further evaluating the crucial ecosystem functions that bark provides, especially in driving variation in bark decomposition. To fill this research gap, we used a "common garden experiment" on deadwood of six gymnosperms in a temperate forest in the Netherlands over four years of decomposition. We evaluated the differences and associations between inner and outer bark in initial functional traits, decomposition rates and afterlife effects of traits in driving in-situ bark decomposition across tree species at the earlier decomposition stage. We report four main findings: 1) inner and outer bark traits varied significantly and were not coordinated across tree species; 2) correspondingly, the decomposition of inner and outer bark were asynchronous and not coordinated across species and inner bark generally decomposed faster than outer bark; 3) the strong predictive traits driving bark decomposability were bark layer-specific, with several inner bark traits controlling inner bark decomposition rates but outer bark decomposability being poorly predicted by outer bark traits; 4) besides being controlled by inner bark traits, inner bark decomposition was also indirectly regulated by several functional traits and the structure-related trait spectrum of outer bark. Synthesis. This is the first study that has linked functional traits, decomposability and afterlife effects of inner and outer bark within the bark quantitatively. We highlight the significance of separating functional traits and ecological consequences of inner and outer bark in research in bark ecology and deadwood dynamics, rather than erroneously considering bark as a homogeneous tissue. Such research will help to better evaluate the function-oriented contribution of bark to the turnover of forest carbon and biogeochemical cycles from local to global scale.