Experimental Test Of Intra- And Interspecific Trait Variation In Response To Seedling Competition

The response of plant functional traits to competition can adjust intraspecific and interspecific trait variation,ultimately affecting the state of species competition and even coexistence.For example,the individual variation hypothesis suggests that competition can induce increased intraspecific trait variation and increased interspecific trait overlap,and therefore competition-induced trait variation may be harmful to species coexistence.In contrast,the niche packing hypothesis suggests that species under competitive pressure will reduce intraspecific trait variation and decrease interspecific trait overlap,potentially promoting species coexistence.All these hypotheses agree that trait variation can respond to competition,but the lack of strong experimental evidence has led to ongoing debates on the specific ways to respond.Moreover,studies related to trait variation and competition have so far remained at the stage of theoretical studies of competition between two species in homogeneous environments.And it is unclear whether the above theories or hypotheses hold in multispecies competition in homogeneous or heterogeneous environments due to the abiotic environmental dependence of species competition and the complexity of multi-species competition systems.Based on this,in order to systematically explore the response of trait variation to two-species competition,multi-species competition under a homogeneous environment,and multi-species competition under a heterogeneous environment,this thesis conducted a 3-year potted competition experiment on seedlings of 10 common tree species in a subtropical broadleaf evergreen forest in Tiantong,Zhejiang Province.The experiment was set up with a monoculture planting as the control group,two-species and seven-species competition treatments under a homogeneous environment,and seven-species competition treatments under 3 horizontal gradients of 3 types of environmental factors（light,soil moisture,and phosphorus content）for a total of 3840 seedlings.The competitive intensity of the target species was estimated by biomass per seedling,while changes in competition-induced intra-and interspecific trait variation were quantified based on 10 key functional traits per seedling（chlorophyll content,leaf mass per area,leaf dry matter content,leaf toughness,stem specific density,stem moisture content,specific root length,specific root area,root tissue density,and rootshoot ratio）.This thesis aims to answer the following scientific questions: 1)How does intraspecific trait variation respond to two-species competition in a homogeneous environment? Do these responses differ between competitively inferior and superior species? 2)How does interspecific trait variation respond to two-species competition in a homogeneous environment? 3)Are the changes in trait differences induced by multi-species competition similar to those in two-species competition in a homogeneous environment? 4)Does multi-species competition in a heterogeneous environment induce similar changes in trait variation as described above?The main results of this thesis are as follows:（1）Two-species competition in the homogeneous environment significantly increased intraspecific trait variation,with an average increase of 1182.7% ± 14.6%compared to the competition-free control.At the same time,the increase in intraspecific trait variation was significantly and positively correlated with the competitive intensity of the species,i.e.,the greater the competitive suppression of the species,the greater the increase in intraspecific trait variation.This ultimately resulted in significantly higher intraspecific trait variation in inferior species than in superior ones.These results support the view that competition-induced trait variation is not negligible and provide the first experimental confirmation of the mean-variance tradeoff hypothesis: inferior species have greater intraspecific variation than superior species.Theoretical studies have shown that competition-induced intraspecific trait variation can promote species coexistence when this assumption holds.（2）Two-species competition in a homogeneous environment significantly altered interspecific trait variation with an average variation of 6.4% ± 2.3%,as shown by a significant decrease in interspecific trait overlap,and these changes occurred simultaneously with the increase in intraspecific trait variation.This suggests that a)the actual changes in trait variation under competition do not support the individual variation hypothesis and the niche packing hypothesis,because both hypotheses suggest that intraspecific and interspecific trait variation have the same trend of change under competition induction.However,our results show that competition can reduce interspecific trait overlap while increasing intraspecific trait variation;b)such changes in both intraspecific and interspecific trait variation may reduce species competition for common resources and thus reduce the intensity of interspecific competition.As a result,competition-induced changes in functional traits may promote species coexistence by weakening competition.（3）The increase in intraspecific trait variation induced by multi-species competition in a homogeneous environment was 14 times as large as that in two-species competition,with an average increase of 17609.69%±740.54% compared to the competition-free control.The other patterns of intraspecific trait variation were similar to the results of the two-species competition.Thus the mean-variance tradeoff assumption still holds in multi-species competition in homogeneous environments.However,the change in interspecific trait overlap was the opposite of the two-species competition result（16.04%±5.73% increase）.In this case,competition significantly increased interspecific trait overlap.This result illustrates the context-dependent properties of trait variation.The change of functional traits in multi-species competition requires simultaneous consideration of multiple competitors’ strategies.The degree of change of interspecific trait overlap may be more restricted at this time than in twospecies competition.Thus,species may not have sufficient space for variation to offset the pressure of increased interspecific trait overlap due to greater intraspecific variation under multi-species competition in a homogeneous environment.（4）Changes in intraspecific trait variation in multi-species competition in heterogeneous environments remain similar to the results of the two-species competition,with an average increase of 1221.71%±114.29% compared to the competition-free control.This further extends the applicability of the mean-variance trade-off hypothesis,and the pattern of inferior species with greater intraspecific variation may prevail.Meanwhile,the changes in interspecific trait overlap were similar to the results of two-species competition in a homogeneous environment（3.89%±0.24%reduction）.This provides side evidence that species may not have enough space for variation in multi-species competition in a homogeneous environment;it also implies that the heterogeneous abiotic environment introduced in a multi-species competition system may provide additional space for functional trait variation,allowing species to maintain low interspecific trait overlap while niche width（e.g.,intraspecific trait variation）increases.（5）Intraspecific variation in single functional traits in a competitive environment is similar to the overall（multidimensional）change pattern,but there are some differences.Mainly,the increase in intraspecific variation of fine root traits（specific root length,specific root area,and root tissue density）was significantly higher than that of other traits,indicating that plants compete more intensely for belowground resources and fine root traits are more sensitive to competitive responses.Meanwhile,the intraspecific variation in leaf toughness and stem specific density increased less or even tended to decrease（in a homogeneous two-species competition environment）,which may be due to their closer association with plant fitness,suggesting that leaf hardness and stem density of plants in a homogeneous two-species competition environment clustered toward optimal traits.The results based on single functional traits provide more detailed information for understanding the mechanisms of plasticity in plant responses to competition.In summary,this thesis systematically explored the response of intraspecific trait variation and interspecific trait overlap to competition from a two-species competitive system to a multispecies competitive system,and from a homogeneous to a heterogeneous environment.These results indicate that competition increases intraspecific trait variation,and that intraspecific trait variation increases more for inferior species at least for seedlings of common tree species in Tiantong.The competition also reduced or increased interspecific trait overlap,depending on the complexity of the competitive system and the heterogeneity of the abiotic environment.These results not only confirmed experimentally the validity of the mean-variance tradeoff assumption for the first time,but also corrected the misconceptions of previous studies regarding intra-and interspecific trait variation in competition.These new findings provide experimental evidence to resolve the debate between the individual variation hypothesis and niche packing hypothesis and also provide new ideas for constructing more reasonable and accurate models of species coexistence.