Background/Question/Methods Plants with photosynthetic stems are common in both temperate and tropical regions. In Mediterranean drylands, advantages associated with stem photosynthesis include extra carbon gain, improved whole-plant water-use efficiency, and supply of carbon for the production of leaves, flowers and fruits. It is unknown whether these advantages extend to plants in more mesic ecosystems such as moist tropical forests, and whether there are differences between trees and lianas. Stem photosynthetic capacity of 48 species of trees and lianas was studied in two forests in the Republic of Panama during wet and dry seasons of 2021-2023: Parque Natural Metropolitano (PNM) with a mean annual precipitation of 1900 mm, and Parque Nacional Soberanía (PNS) with a mean annual precipitation of 2250 mm. We measured leaf and stem CO2 exchange, chlorophyll concentration, stomatal density, and area and biomass investment into leaves and stems.
Results/Conclusions All sampled species exhibited either stem net photosynthesis or stem recycling photosynthesis, both of which improve the carbon economy of plants. Leaf photosynthetic rate (leaf A) was generally greater in the slightly drier forest (PNM), where trees maintained greater leaf A than lianas throughout the year, with leaf A being greater during the wet than the dry season in both growth forms. At the slightly wetter forest (PNS), leaf A did not differ between the two growth forms and seasons. At PNS during the wet season, lianas had greater stem CO2 photosynthetic re-assimilation capacity than trees. At PNM, there were no differences in stem re-assimilation between growth forms or seasons. For 50 cm long terminal branches, the proportion of photosynthetic stem area to total photosynthetic area was greater in lianas than trees. Our results indicate that leaf A tends to be greater in trees than in lianas, and that lianas tend to have a greater stem re-assimilation rate than trees, but there is variation between forests and seasons. The contribution of stem photosynthesis to overall carbon gain seems particularly important in lianas as they allocate more area to green stems than trees.
