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.

揭示树皮（bark）多功能性及其潜在性状的生态效应，已成为植物生态学领域一个相对新兴但至关重要的研究方向。尽管树皮最核心的两个层级——即内皮（inner bark）与外树皮（outer bark）——在结构与功能上的显著差异已被广泛认知，但在绝大多数树皮相关研究中，整体树皮仍被视作均质组织。

这一误区导致，在进一步评估树皮所支撑的关键生态系统功能（尤其是在驱动树皮分解变异层面）时，我们对不同树种内皮与外树皮性状的功能独立性、专属贡献以及潜在关联的认知仍极为匮乏。

为填补这一研究空白，我们针对荷兰温带森林中的6种裸子植物枯木，开展了为期四年的同质园实验（common garden experiment），以探究其分解过程。我们评估了不同树种在分解早期阶段，内皮与外树皮的初始功能性状、分解速率，以及性状的死后效应（afterlife effects）在驱动原位（in-situ）树皮分解过程中的差异与关联。

本研究得到四项核心结论：1）不同树种的内皮与外树皮性状存在显著差异，且彼此间并无协调关联；2）相应地，内皮与外树皮的分解过程并不同步，且在不同树种间亦无协调关联，此外内皮的分解速率普遍快于外树皮；3）驱动树皮分解性（decomposability）的强预测性状具有层级特异性：多个内皮性状调控内皮的分解速率，但外树皮的分解性难以通过外树皮性状进行有效预测；4）除受内皮性状调控外，内皮的分解过程还间接受到若干功能性状以及外树皮结构相关性状谱（trait spectrum）的调控。

综合而言，本研究首次定量关联了树皮内、外两层的功能性状、分解性及其死后效应。我们强调，在树皮生态学与枯木动态研究中，应当区分内皮与外树皮的功能性状及其生态效应，而非错误地将树皮视为均质组织。此类研究将有助于更精准地评估树皮在局地至全球尺度上，对森林碳周转与生物地球化学循环的功能导向贡献。