Module responses in a tropical forest tree analyzed with a matrix model

Module dynamics were studied for the shade-tolerant canopy tree species Vouacapoua americana in a French Guiana rain forest. A module life cycle graph was constructed, including all the possible transitions between four module states: apically growing (G), apically dormant (D), apically arrested (A), and branching (J). Transitions (module level) were translated to the module population growth rate lambda (tree level) and related to the variance in lambda among 18 different trees. This variance was also related to light availability (1-60% of ambient PAR) and tree height (5-30 m). Three module life cycle pathways (or loops) were dominant in their contributions to lambda: persistent apical dormancy (D-->D), biannual apical growth (G-->D-->G), and biannual branching by dormant modules (J-->D-->J). This suggests that biannual or even slower module production rates predominate in the module life cycle. The positive covariance between biannual loops seems the result of synchronization in apical and axillary activity. Slow production rates and synchronization allow trees to accumulate carbon, flush massively, and escape from herbivore attacks, and at the same time allow the tree to replace its leaves. The variance in lambda (V(lambda)) among trees was low. Apical trade-offs, which occur as one apical fate excludes other apical fates by definition, lead to negative covariances between apical growth and apical dormancy, and thus reduced the net contributions of the apical transitions to the variance in lambda among trees. Branching (D-->J) was independent of such trade-offs, was highly variable, increased with light availability, and almost fully accounted for V(lambda). Module fates and V(lambda) were unaffected by tree height. The module mechanisms underlie the rather invariable module population growth rate lambda in the shade, as well as the increasing lambda during higher light episodes, enabling shade-tolerant canopy trees to grow up and survive in a heterogeneous forest light environment.