Seagrass response to burial and breakage of expanding horizontal rhizomes: implications for clone spread

In seagrasses, elongation of horizontal rhizomes, and hence clone expansion, is supported by physiological integration. However, horizontal rhizomes are subjected to breakage by physical disturbances and to burial, often simultaneously. Little is known about the combined impact of such factors on the growth of apical rhizome regions. We factorially manipulated connections on runners of Cymodocea nodosa and sediment height for 1 mo at 2 sites within a meadow to test the hypothesis that breakage of interconnections close (10 cm) to the rhizome apex causing premature loss of physiological integration would negatively affect the performance of runners and reduce their ability to tolerate burial (60% of shoot height). Shoot survival, length of vertical internodes and horizontal rhizome network, biomass production, partitioning, and costs were unaffected by the investigated factors. Rhizome breakage interacted with burial for some characteristics. Breakage reduced branch length and rhizome diameter only in unburied runners, while burial reduced branch length only in intact runners. Moreover, burial stimulated leaf sheath elongation and greater branching, irrespective of the status of connections. These findings indicate that breakage only delays lateral spread and increases the risk of fragmentation by subsequent disturbances, and, contrary to our hypothesis, has no substantial influence on the capacity of runners to withstand transitory and moderate burial. The findings also reveal that runners may escape from burial not only vertically, but also horizontally, changing growth pattern and direction from predominantly unidirectional to multidirectional. This additional strategy could allow buried runners to accelerate space occupation even if disconnected from clones.