We hypothesized that photosynthesis and growth of tropical vegetation at its most northern distribution in Asia (Xishuangbanna, SW China) is adversely affected by seasonal drought and chilling temperatures. To test this hypothesis, we measured photosynthetic and growth characteristics of Zizyphus attopensis Pierre seedlings grown in three contrasting forest microhabitats: the understory, a small gap and a large gap. Photosynthetic capacity (light-saturated photosynthetic rate (A(max)), maximum rate of carboxylation and electron transport rate) and partitioning of leaf nitrogen (N) into carboxylation and electron transport differed significantly among seasons and microhabitats. Specific leaf area (SLA) did not change seasonally, but differed significantly among microhabitats and showed a negative linear relationship with daily integrated photon flux (PPF(i)). In contrast, leaf N concentration per unit area (N(a)) changed seasonally but did not differ among microhabitats. Measurements of maximum PSII photochemical efficiency (F(v)/F(m)) indicated that chronic photoinhibition did not occur in seedlings in any of the microhabitats during the study. Photosynthetic capacity was greatest in the wet season and lowest in the cool season. During the cool and dry seasons, the reduction in A(max) was greater in seedlings grown in the large gap than in in the understory and the small gap. Close logarithmic relationships were detected between PPF(i), leaf N(a) and photosynthetic capacity. Stem mass ratio decreased, and root mass ratio increased, in the dry season. We conclude that seasonal acclimation in growth and photosynthesis of the seedlings was associated with changes in biochemical features (particularly N(a) and partitioning of total leaf N between the different photosynthetic pools) and biomass allocation, rather than with changes in leaf morphological features (such as SLA). Local irradiance is the main factor driving seasonal variations in growth and photosynthesis in the study area, where the presence of heavy fog during the cool and dry seasons limits irradiance, but supplies water to the soil surface layers.
We selected five typical tree species, including one early-successional species (ES) Pinus massoniana Lamb., two mid-successional species (MS) Schima superba Gardn. et Champ. and Castanopsis fissa (Champ. ex Benth.) Rehd. et Wils. and two late-successional species (LS) Cryptocarya concinna Hance. and Acmena acuminatissima (BI.) Merr et Perry., which represent the plants at three successional periods in Dinghushan subtropical forest succession of southern China. Potted seedlings of the five species were grown under 12% of full sunlight for 36 months. The ES and MS showed the slowest and fastest responses to lightflecks, respectively, which correlated with the rate of stomatal opening. In contrast to P. massoniana and C. concinna, the other three species exhibited a high induction loss. Early-successional species showed the lowest specific leaf area and chlorophyll content, the highest photosynthetic capacity (A(max)) and respiratory carbon losses (R(d)). Compared with ES and MS, LS showed lower A(max) and R(d). The five tree species showed a similar chlorophyll a/b ratio after long-term low-light adaptations. On the other hand, LS had a relatively higher de-epoxidation state to protect themselves from excess light during lightflecks. Our results indicated that (i) slower responses to lightflecks could partially explain why ES species could not achieve seedling regeneration in low-light conditions; (ii) fast responses to lightflecks could partially explain why MS species could achieve seedling regeneration in low-light conditions; and (iii) smaller respiratory carbon losses might confer on the LS species a competitive advantage in low-light conditions.
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