Experimental Test On The Relationship Between Tree Seedling Interaction And Interspecific Functional Trait Difference And Phylogenetic Distance In Tiantong Region

Interspecific interaction is one of the main ecological processes that affect the species diversity of local communities.However,due to the fact that the interaction between species cannot be directly measured in natural communities,indirect estimation via other measureable metrics is still one of the hot topic in community ecology.Interspecific trait differences?TDs?and phylogenetic distance?PD?are considered to be the most promising approaches.Because when TDs represent interspecific niche differences?such as different resource preferences?,two species with smaller TDs may have more similar niches,resulting in stronger negative interspecific interactions.If these related functional traits are evolved under conservative evolution,PD can approximate the TDs and thus predict interspecific interactions.However,TDs may also be related to the average fitness differences?e.g.,different tolerance of minimum resource concentration?.Then the smaller the TDs are,the smaller of the average fitness differences between species are,and the weaker the interspecific interactions are,and vice versa.Therefore,To what extent and which form of the niche difference related TDs,the average fitness difference related TDs and PD are connected to the interspecific interactions are still unknown,and have become one of the research focuses in this field.To fill these gaps,a two year common garden experiment involving pairwise interactions of 13 tree species?see Table 2-1?in subtropical evergreen broad-leaved forests in Tiantong,Zhejiang was carried out.The whole experiment contains 6,552 monoculture or bicuture of seedlings in 12 blocks.The strength of pairwise interspecific interaction was estimated by comparing the seedling biomass of the same species between monoculture and biculture treatments using relative interaction intensity?RII?index.The absolute trait difference(TD_ab)and hierarchical trait difference(TD_hr),approximately representing the niche difference and average fitness different respectively,were estimated for each species pair by six commonly used functional traits?Table 2-2?.PD of the experiment species was calculated based APG III.Our main conclusions are as following.?1?For negative interspecific interactions,the TD_hr of specific leaf mass has the largest positive effect.The TDshr of wood density,specific leaf area,leaf thickness,leaf dry matter content,and the maximum height of the adult tree were negatively correlated with RII.For the positive interaction between species,the TD_hr of leaf dry matter content was significantly and positively correlated with RII,and the TD_ab of wood density was significantly and negatively correlated with RII.Meanwhile,there are more TDs correlated with negative than positive interspecific interactions?7:2?,indicating that our measured TDs have closer connection with the negative interspecific interactions between seedlings in this study.?2?For negative interspecific interactions,the joint effect of TD_hr was much greater than that of TD_ab(10¹³:1).Similar pattern emerged for positive interspecific interactions?10:1?.These suggest that the average fitness difference among species is much more impact than the niche difference on the strength of the interactions between species.?3?At the community level,PD of experimental species had a significant linear relationship with the intensity of negative interspecific interactions,but the adjusted R square was extremely low?R²=0.005?.This result does not support the competition relatedness hypothesis,which assumes that the weaker negative interactions will appear between species with larger PD.Similar pattern exists between PD and positive interspecies interactions?R²=0.034?.At the species level,there are large interspecific differences in the relationship PD and RII,but explanation power are very low again.This result suggests that the same PD value may have different impact on the RII between different species pairs.In summary,a)TD_hr may have a stronger effect on the interactions between species than TD_ab,and deserves much attention in the future studies.b)Positive interactions commonly exists among seedlings of different species,but are weakly related to the commonly used aboveground functional traits in this study.Because positive interactions are often associated with soil nutrient resources and microbes etc.,additional traits reflecting the underground parts of seedlings are worth to collected for better understanding of the positive interactions.c)The PD has a neglectable linear relationship with pairwise interspecific interactions among seedlings.It is still unknown whether they have better relationship at nonlinear scale.This is worth to try in future studies.