Leaf Life History Strategies Of Major Woody Species From Tiantong Subtropical Evergreen Forests

Leaf functional traits and their relationships have been widely used to characterize plant life history strategies and to explore the mechanisms underlying species adaptation and community coexistence.Typically,leaf life span and leaf mass per area(LMA)are negatively associated with leaf nutrient concentration and photosynthetic capacity(Amax),indicating a life history strategy spectrum ranging from high resource use efficiency and high turnover rate to low efficiency and long life span.However,these previous studies and models incorporate static parameters(e.g.measurements for mature leaves)only,and they overlook the ecological importance of the variation in the parameters values.I hypothesize that,leaf population dynamics and variation in leaf functional traits may also reflect how species adapt to environmental changes,and therefore they can be used to characterize plant species life history strategies.This study,taking the major woody species from Tiantong subtropical evergreen forests as an example,investigated the seasonal variation pattern of leaf functional traits and discussed its ecological significance.I investigated leafing process for 19 species from 3 functional species groups since 2011,and then I followed population dynamics of leaf cohorts(related to the static parameter leaf life span)and the variation in LMA,N concentration,and Amax(the longest was 6 years for Cinnamomum japonicum).I fitted the leafing and falling processes using logistic functions,the season variation in LMA,N concentration,and Amax using damping function.I also used ordinary regression model and Pearson correlation model to determine the relationship between parameters when needed.Measurements for mature leaves showed that LMA and Amax were higher in canopy tree species than shade-adapted shrub species,and sun-adapted shrub species were between them.Moreover,LMA and Amax were positively correlated across species,in contrast to the negative relationship derived from the pooled data collected from different stands.This discrepancy is likely due to the fact that all the study species from the same stand share similar climate condition.Investigations of leafing process showed that shrub species,especially shade-adapted shrub species,emerged earlier than canopy tree species,but they emerged and expanded their leaves more slowly than the canopy species;the leaf emergence rate positively correlated with leaf expansion rate.Such a leafing pattern might be related to light condition and leaf traits of the study species.Specifically,light condition of shrub species is relatively more favorable(due to sparse leaved canopy trees)albeit low temperature in early spring than canopy species,but light condition became worse due to leafing canopy species although temperature rises in late spring.This may favor the observed leafing process of shade-adapted shrubs.For the canopy species,the high emergence and expansion rates are possibly driven by high selective pressure of herbivory during leafing.Investigations of leaf falling process showed that canopy trees defoliated more in winters than shrub ones;shade-adapted shrubs defoliated mostly in spring and summer,and sun-adapted shrubs defoliated mostly in summer and autumn.Such an observed leaf falling pattern may be ascribed to the change in the factors limiting Amax.Specifically,leaf Amax canopy species was most limited by low temperature in winters,shade-adapted shrubs by low light in summer and later spring,and sun-adapted shrubs possibly by water deficit or high light in summer.Investigations of mature leaves showed that LMA,N concentration,and Amax all presented significant season variation particularly in the shade-adapted shrub species.LMA was the highest in summer and lowest in winter,but N concentration showed a contrasting pattern.Moreover,the seasonal variation in N content per area was not apparent,while the C/N ratio was apparent with a low peak in winter and high peak in summer.One possible explanation for the observed pattern is that more carbohydrate was removed from leaves than N-containing compounds.In addition,leaf Amax was higher in winter and low in the winter,and it generally declined with leaf age.This could be attributed to leaf aging(e.g.the decline of enzyme activity for photosyntheses).In summary,my study results indicate a large seasonal variation in leaf functional traits,and the variation differs between functional species groups and between species.In particular,these differences suggest different life history strategies,which may facilitate community species coexistence.Future research should investigate leaf life history and follow seasonal variation in leaf functional traits for more species from more biomes.