Seasonal Microclimate and Carbon–Water Responses of Tree–Tea Agroforestry: Contrasting Instantaneous Photosynthesis and Harvestable Shoot Fresh Weight

Abstract

Tree–tea agroforestry can reduce radiation and thermal load, but lower irradiance may also constrain instantaneous photosynthesis. We examined whether these responses were accompanied by parallel changes in harvestable shoot fresh weight across three tea-harvest periods. In 2024, microclimate, leaf gas exchange, and 100-shoot fresh weight were measured in tea monoculture (CK), Osmanthus fragrans Lour.–tea (S1), Magnolia chapensis (Dandy) Sima–tea (S2), and mixed O. fragrans–M. chapensis–tea (S3), with three fixed plots as independent replicates per treatment. Measurements were conducted on four clear-sky days in each of spring, summer, and autumn (12 observation days in total). Relative to CK, S3 reduced the net photosynthetic rate (Pn) by 13.2%, 31.4%, and 48.5%, yet 100-shoot fresh weight was 29.7%, 105.6%, and 45.3% greater in spring, summer, and autumn, respectively. In spring, Tr declined by 52.8%, producing a 103.6% increase in instantaneous WUE. In summer, the largest reduction in leaf temperature (3.74 °C) coincided with the largest fresh-weight difference. In autumn, lower Pn and WUE were consistent with a greater apparent physiological limitation under the observed conditions, while fresh weight remained higher than in CK. These seasonal contrasts show that instantaneous photosynthesis alone did not predict harvestable shoot fresh weight under the observed field conditions.