光照对鄂东南2种落叶阔叶树种幼苗生长、光合特性和生物量分配的影响

通过设置4个光照梯度(25%、12%、6%和3%自然光)模拟鄂东南低山丘陵地区落叶阔叶林林下的光环境,研究了2种耐荫性不同的树种幼苗--麻栎(Quercus acutissima)和化香(Platycarya strobilacea)不同光强下的存活率、光合特性、生长和生物量分配,探讨了低光环境中耐荫性不同的树种幼苗维持自身碳平衡的机制和权衡"存活-生长"选择的生活史策略。结果表明:(1)低光下的2个树种幼苗的生长、光合特性和生物量分配具有显著性差异。(2)各个光照梯度下麻栎幼苗都生长良好,存活率保持在35%以上,而化香幼苗遭遇高的死亡率,80d后3%和6%自然光下的幼苗全部死亡;低光环境中麻栎幼苗比化香幼苗具有更大的表观光量子(AQY)和最大净光合效率(Pmax),更低的光补偿点(LCP)和暗呼吸效率(Rd),即耐荫性较强的麻栎幼苗比耐荫性较弱的化香幼苗具有更高的低光碳同化率和碳捕获能力。(3)2个树种幼苗的成活率与RGR呈负相关关系,各个光照梯度下耐荫性较弱的化香幼苗的相对生长率(RGR)显著高于耐荫性较强的麻栎幼苗,而两个树种幼苗的净同化率(NAR)无明显差异。相对于麻栎幼苗较高的根生物量比(RMR),化香幼苗将更多的生物量分配给叶部,因而具有较高的叶生物量比(LMR)、叶面积比(LAR)和比叶面积(SLA)。不同耐荫性的幼苗生长及生物量分配方式的差异是植物"存活-生长"权衡后的结果,耐荫性弱的化香幼苗具有较高的生长潜力和较弱的自我保护能力,而耐荫性强的麻栎幼苗具有更高的低光碳储量,能够维持更好的低光碳平衡,具有竞争优势。

The effect of low irradiance on growth, photosynthetic characteristics, and biomass allocation in two deciduous broad-leaved tree seedlings in southeast of Hubei Province

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.