Under-story contributions to stand level GPP using the process model SECRETS

We used the forest process model SECRETS to examine the influence of life-form and species structure on stand level gross primary production (GPP) in a mixed coniferous–deciduous temperate forest. In this study we focus on pedunculate oak (Quercus robur L.) and Scots pine (Pinus sylvestris L.), which occur as over-story species, and rhododendron (Rhododendron ponticum L.) and black cherry (Prunus serotina Ehrh.) that co-occur in the under-story. We contrast six patches using oak and pine alone, and in combination with rhododendron or cherry. We found good correspondence between simulated and measured estimates of sub-canopy photosynthetic photon flux density (PPFD). Maximum simulated GPP in the oak-only patch reached 15 g C m−2 day−1 at peak leaf area index (LAI). Annual GPP for this patch was 1751 g C m−2. Pine at this site exhibited positive photosynthesis throughout the year, with maximum rates of almost 11 g C m−2 day−1 at peak LAI – 35% lower than oak – and with minimum rates of roughly 1 g C m−2 day−1 observed during winter. Rhododendron or cherry, under oak, substantially increased early season GPP. Rhododendron and cherry contributed about 21% and 11% to the annual GPP for the patch, respectively. When co-occurring with pine, rhododendron, and cherry both contributed about 28% to the patch total GPP. Both under-story species exhibited phenological traits that enabled them to maximize early season C gain prior to peak LAI of the over-story; photosynthesis was always light limited under pine. Our simulations suggest that photosynthesis of rhododendron and cherry was predominantly driven by diffuse beam PPFD, although direct beam radiation was important for rhododendron when co-occurring with oak prior to, and following, oak leaf-out. We conclude that the paired over- and under-story species structure, life-form, and growth phenology determine the under-story species contribution to total stand photosynthesis in a mixed species-structure temperate forest.