The cost of capillary integration for bryophyte canopy water and carbon dynamics

Due to their high shoot density and ability to store and transport water externally, ectohydric bryophytes may share water laterally among shoots. Given that rates of evaporation may be spatially variable across the canopy due to structural irregularities that cause surface roughness, lateral water transport via integrated capillary networks may affect shoot water status and influence carbon dynamics. The interaction between capillary integration and surface roughness was explored using simulation modeling. A shoot-level model of water and carbon dynamics and their interaction was developed to model acrocarpous or Sphagnum species. Shoots were distributed regularly in a 20 × 20 grid with the same mean height, but with different standard deviations to represent variation in surface roughness. Evaporation and light availability were modeled as a function of the shoot height relative to its neighbors. A drying cycle was simulated whereby canopies were fully recharged with water and allowed to dry during a 14-day period. Plants could either share water with their immediate neighbors or not. At low surface roughness, there was no difference in canopy average water or carbon balance; however at high surface roughness, canopies without capillary integration experienced lower overall rates of evaporation and greater net carbon uptake. This was due to emergent shoots desiccating, but reducing the rate of water loss from lower shoots, which remained photosynthetically active. Consequently, there is a physiological cost to capillary integration in rough bryophyte canopies, a condition that differs from the benefits normally attributed to physiological integration in vascular plant colonies. The degree of capillary integration may be an important functional trait of bryophyte canopies.