Rapid photosynthetic acclimation of Shorea johorensis seedlings after logging disturbance in Central Kalimantan

This study examined the photosynthetic acclimation of pre-existing Shorea j ohorensis (Dipterocarpaceae) seedlings to the change in conditions that occ urs at the time of logging in Central Kalimantan, Indonesia. The hypothesis was that the seedlings would be unable to acclimate beyond partially open conditions after canopy disturbance caused by logging, therefore limiting t he potential for regeneration in the most open areas. Bleaching and reducti ons in the predawn ratio of variable to maximum fluorescence (F-v/F-m) indi cated chronic photoinhibition and damage to the previously shade-adapted le aves of seedlings in an area logged 2 weeks earlier. The majority of seedli ngs in partially open and open environments of an area logged 3 months earl ier were already growing fast. Leaves that had developed in the new environ ment showed only small reductions in predawn F-v/F-m and large increases in the light saturated rate of photosynthesis (A(max)) per unit area when com pared to shaded seedlings. Leaves in the most open environments had higher but more variable nitrogen concentrations, A(max) per unit area and A(max) per unit mass when compared to seedlings in partially open environments. In creases in dark respiration were disproportionately large compared to incre ases in A(max) and may have been the result of increased investment in phot oprotective mechanisms. The response of stomatal conductance to the vapour pressure deficit and leaf temperature was examined, but it suggested only a 10% reduction in daily leaf level carbon gain in open environments. The ra tio of leaf area to fine root mass was highest in shade-suppressed and newl y exposed seedlings, suggesting a potential hydraulic limitation to transpi ration during acclimation. However, rainfall during this period was high an d leaf water potentials did not differ between disturbed and undisturbed en vironments. S. johorensis seedlings were capable of significant acclimation to conditions more extreme than partial canopy opening, Low seedling densi ty after logging during the wet season cannot be explained by a limited pot ential for photosynthetic acclimation.