Photosynthetic and structural characteristics of canopy and shrub trees in a cool-temperate deciduous broadleaved forest: Implication to the ecosystem carbon gain

To reveal the seasonal change of leaf ecophysiological and canopy characteristics and to evaluate the functional role of canopy and shrub tree species in forest CO2 uptake, we measured forest canopy leaf area index (LAI) using a hemispherical canopy photography technique, leaf CO2 gas exchange and shoot architecture for canopy (Betula ermanii and Quercus crispula) and shrub (Hydrangea paniculata and Viburnum furcatum) tree species in a deciduous broadleaved forest in a cool-temperate region in central Japan. Canopy LAI and photosynthetic capacity of canopy tree leaves increased rapidly with leaf expansion. LAI reached its maximum in early summer but photosynthetic capacity reached its maximum in late summer. Development of photosynthetic capacity was dependent on the changes of leaf mass per area and leaf chlorophyll content (evaluated by SPAD). The seasonal maximum photosynthetic capacity of the leaves at the forest canopy top (B. ermanii and sun leaves of Q. crispula) was about more than double of the leaves in the shrub layer (H. paniculata, shade leaves of Q. crispula and V. furcatum). Light interception and photosynthetic carbon gain at a shoot level were simulated under three air temperature conditions by a three-dimensional canopy photosynthesis model (Y-plant) involving the combined leaf photosynthesis and stomatal conductance responses and shoot architecture. Results showed that (1) calculations without considering the heterogeneous light distribution in a foliage made by geometrical feature of plants would overestimate the photosynthetic carbon gain by +40% even at the canopy surface, and (2) the steep leaf angle in B. ermanii avoided midday depression of photosynthesis while the rather horizontal leaves in Q. crispula received excess light and heat load which led larger midday depression of photosynthesis. In addition to the large capacity of photosynthetic productivity of the canopy top foliage, our model also suggests the functional role of shrub species in forest ecosystem carbon gain, due to their high photosynthetic utilization efficiency of low light incidence available in the forest understory.