胡杨当年生小枝茎构型对展叶效率的影响

Effect of current-year twig stem configuration on the leaf display efficiency of Populus euphratica

枝叶异速生长关系对理解荒漠植物生物量分配和生活史策略具有重要意义。该研究以小枝茎长度、茎纤细率和茎体积表征茎构型,以叶密度(单位茎长度的叶数量)、叶面积比(单位小枝茎干质量的总叶面积)和叶茎质量比(单位小枝茎干质量的总叶干质量)表征展叶效率,采用标准化主轴回归(SMA)方法研究胡杨(Populus euphratica)当年生小枝茎构型对展叶效率的影响及二者沿地下水埋深(GWD)梯度的权衡策略。结果显示,胡杨当年生小枝茎直径、展叶效率和比叶面积、叶大小随GWD增加而降低,小枝茎长度、茎纤细率及其上的叶数量则增大。小枝茎构型性状均与展叶效率呈显著负相关关系,即随小枝茎长度、茎纤细率和茎体积的增加,展叶效率逐渐降低,这可能是枝叶大小、水分传导与机械支撑间的权衡结果。小枝茎构型与展叶效率的异速生长指数(斜率)随GWD增加而增大,是由于单位小枝茎投资获得的叶面积或者叶质量降低所致,反映出随GWD增加胡杨采取了高消耗低收益的保守型策略。胡杨应对环境压力时,倾向于在长枝上着生数量较多的小叶,短枝上着生数量较少的大叶,体现出胡杨小枝的资源利用策略与枝叶大小的权衡机制。综上所述,GWD显著影响胡杨小枝茎构型-展叶效率的权衡关系,低展叶效率是胡杨应对日益旱化荒漠环境的适应策略。

Aims Plant functional traits reflect the resource trade-offs and allocation strategy between different organs and functions in the process of plant adapting to environmental changes.Branches and leaves are the key components of tree canopy,and understanding the relationship of leaf-stem allometry is important for revealing adaptive strategies of desert plants with changing environmental constraints.In this study,our objective is to explore the scaling relationships between leaf display efficiency and current-year twig stem configuration,and trade-off strategy shift along groundwater gradients in extremely arid region.Methods Leaf number,area,mass and stem length,diameter,volume,mass of current-year twigs were measured for Populus euphratica with 30 trees within three different groundwater depths in Tarim basin,Xinjiang,China.The stem length,stem slender ratios and stem volume of current-year twigs were used as the proxy of stem configuration traits.Density of leaf number(leaf number per stem length),leaf area ratio(total leaf area per stem mass)and leaf/stem mass ratio(total leaf mass per stem mass)were used as the proxies of leaf display efficiency.The standardized major axis(SMA)regression was used to examine the scaling relationship between stem configuration traits and leaf display efficiency within current-year twigs across groundwater gradients.Important findings Stem diameter,leaf display efficiency,specific leaf area,individual leaf mass and area,all decreased with the increase of groundwater depths(GWD).In contrast,stem length,stem slender ratio and leaf number per twig increased with GWD.There was significant difference in stem and leaf functional traits across groundwater gradients.The density of leaf number,leaf area ratio and leaf/stem mass ratio as the proxies of leaf display efficiency were all significantly negatively correlated with stem length,stem slender ratio and stem volume of the current-year twigs.These results suggest that leaf display efficiency decreases with stem configuration variation of current-year twigs,which may reflect the trade-off of twig-leaf size,water conduction and mechanical support.The allometric exponents(slope)of stem configuration and leaf display efficiency decreased with the increase in GWD,likely because the leaf area or mass per unit stem investment decreased with GWD.It may reflect that the desert woody plants tend to adopt a conservative adaptive strategy of high consumption and low benefit with habitat deterioration.When Populus euphratica responds to environmental stress,it tends to have a large number of small leaves on long twigs,or tends to have relatively few large leaves on short twigs,which reflects the strategy of resource utilization and trade-off mechanism of twig-leaf size.Our results demonstrate that GWD is a key factor for regulating the leaf-stem allometric relationship,and low leaf display efficiency is an adaptive strategy for P.euphratica to cope with the worsening arid desert environment.