| Low-light environment in forest provides an unfavorable condition for the survival and growth of regenerating tree seedlings. To survive in forest understorey, seedlings must depend on carbohydrate reserves when they experience negative carbon balance imposed by occasional light reduction and tissue loss to herbivores and diseases. A greenhouse experiment was conducted to investigate shade tolerance in seedlings of two deciduous broad-leaved tree species, Quercus acutissima and Platycarya strobilace, in a hilly region of southeast Hubei province. Since the two tree species are exposed to a very heterogeneous light environment, their seedlings are expected to express morphological and physiological plasticity in response to changes in the light environment, and to show different shade tolerance in low light.Here we studied how different tree seedlings maintain their low-light carbon balance and trade-off between survival and growth, in order to provide theoretical basis in support of afforestation effort and help with understanding forest succession. We compared the growth, photosynthetic characteristics and biomass allocation between seedling of the two tree species under four controlled light regimes (3%,6%, 15%, and 25% natural irradiance), in order to assess their low-light carbon balance and life history growth strategy when imposed to low irradiance. Results show strong differences between seedlings of the two tree species in growth, photosynthetic characteristics and biomass allocation under different light regimes. Q. acutissima seedlings had higher survival rate than P. strobilacea seedlings under all light regimes, especially in the low irradiance (3% and 6% natural irradiance) treatment where P. strobilacea seedlings suffered complete death in 80 days but Q. acutissima seedlings grew well with at least 35% of survival. Light had significant effects on the photosynthetic characteristics in seedlings of the two tree species. Q. acutissima seedlings had higher apparent quantum yield (AQY) and maximal net photosynthetic rate (Pmax), but lower light saturation point (LSP) and dark respiration (Rd) than P. strobilacea seedlings, indicating stronger low-light carbon assimilation and carbon capture capability. There was a negative relationship between survival rate and relative growth rate:Q. acutissima seedlings had a high survival rate but poor relative growth, whereas P. strobilacea seedlings had a high relative growth rate but poor survival. Despite small differences in net assimilation rate (NAR), the light-demanding P. strobilacea seedlings had greater relative growth rate (RGR) than the shade tolerant Q. acutissima seedlings under the four light regimes. Compared with the high root mass ratio (RMR) in Q. cutissima seedlings, P. strobilacea seedlings allocated more resources to leaves and less to roots, hence relatively greater leaf mass ratio (LMR), leaf area ratio (LAR) and specific leaf area (SLA).In conclusion, further to a trade-off between survival and growth, seedlings with different shade tolerance may adopt specific life history strategies in response to low irradiance. Light-demanding tree seedlings have higher growth potential but weaker self-protection, which can be disadvantageous to survival at low light because high growth potential results in greater rates of whole-plant respiration, tissue turnover, herbivory and mechanical damage and decreased storage. In the contrary, the shade tolerant tree seedlings accumulate more biomass, maintain positive carbon balance, and have low-light competitive advantage, and are characterized by traits that maximize survival and net growth.
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