Impacts Of Herbaceous Plants On The Relationship Between Tree Functional Diversity And Aboveground Productivity In A Young Experimental Forest

Understanding the ecological processes underlying the relationship between biodiversity and ecosystem productivity has been a central pursuit of ecological research.Over the last three decades,numerous experiments involving the manipulation of plant diversity have been implemented in grassland and forest ecosystems to elucidate how plant species loss influences ecosystem functioning.It has been widely evidenced that functional diversity can be employed to identify mechanisms that underly the effects of biodiversity on productivity.However,such experiments typically prevent immigration by continuously removing non-target species,thereby questioning the generality of these findings to real-world ecosystems.This is particularly true in forest ecosystems where herbaceous plants are usually weeded in tree diversity experiments to avoid the competition between herbaceous plants and tree individuals.Consequently,little is known about how herbaceous plants influence the relationship between tree functional diversity and productivity,and therefore contribute to the total productivity of forest communities.To answer this question,this study was conducted based on a four-year tree functional diversity experiment including both weeded and non-weeded treatments.Eight species that differ characteristically in economic strategies,and are located in different positions of the'fast–slow'economics spectrum,were selected for this experiment.These species were divided into four economic segments from the conservative to acquisitive side(two species per segment),according to their positions in the leaf and wood economics spectrum.There are 17 kinds of tree compositions in this high-density(i.e.,0.6m apart)experiment,including eight monocultures,four single-segment(two-species)mixtures,four two-segment(four-species)mixtures,and one four-segment mixture including all eight species.The temporal dynamics of tree growth and herbaceous properties were investigated across four years to test the role of herbaceous plants on the relationship between tree functional diversity and forest productivity in the early-stage of the experiment.The major results are outlined below:1.Overall,the total community aboveground productivity under the non-weeded treatment was significantly higher than that under the weeded treatment during the first two years.As succession proceeded,tree species gradually contributed more to the community productivity than herbaceous plants,whereas the relative contribution of herbaceous plants remain large in some plots,suggesting that the previous traditional tree diversity experiment underestimated the total community aboveground productivity at the early stages.After four experimental years,tree above-ground productivity varied significantly across tree functional diversity gradients.Under the weeded treatment,Liquidambar formosana monoculture(15.11 Mg ha^-1 yr^-1),Neolitsea sericea-Machilus thunbergii-Sapium sebiferum-L.formosana mixture(10.87 Mg ha^-1yr^-1),and S.sebiferum-L.formosana mixture(10.79 Mg ha^-1 yr^-1)showed the highest tree aboveground productivity;N.sericea monoculture(1.25 Mg ha^-1 yr^-1),N.sericea-M.thunbergii mixture(2.27 Mg ha^-1 yr^-1),and M.thunbergii monoculture(2.96 Mg ha^-1 yr^-1)showed the lowest aboveground productivity.Under the non-weeded treatment,L.formosana monoculture(10.82 Mg ha^-1 yr^-1),S.sebiferum-L.formosana mixture(10.76 Mg ha^-1 yr^-1),and Schima superba-Cyclobalanopsis glauca mixture(10.04 Mg ha^-1 yr^-1)showed the highest aboveground productivity;N.sericea monoculture(0.93Mg ha^-1 yr^-1),N.sericea-M.thunbergii mixture(1.87 Mg ha^-1 yr^-1),and Koelreuteria bipinnata-Sapindus mukorossi mixture(1.99 Mg ha^-1 yr^-1)showed the lowest aboveground productivity.2.After the removal of herbaceous plants,there were non-significant relationships between tree species richness and aboveground productivity as well as accumulated aboveground biomass.Tree functional diversity(represented by functional dispersion)significantly positively correlated with aboveground productivity and the relationship strengthened through time,suggesting that niche complementarity effect among tree species increases through time.The positive relationship between tree community-weighted mean(CWM)of leaf economics traits(i.e.,specific leaf area,leaf nitrogen content)and tree aboveground productivity also strengthened through time,suggesting that selection effect also drives the temporal dynamics of tree aboveground productivity.There was a significant positive relationship between tree aboveground biomass and tree CWM of leaf economics traits,suggesting that the resource competition of acquisitive tree species determines the storage of forest aboveground biomass at the early stages.3.The positive relationships between tree functional diversity and tree aboveground productivity as well as accumulated aboveground biomass were robust under both weeded and non-weeded treatments,whereas herbaceous plants weakened the relationship between tree CWM of economics traits and aboveground productivity as well as accumulated aboveground biomass.Herbaceous plants weaken the relationship between tree functional diversity and complementarity effects based on accumulated aboveground biomass.4.There were weak negative effects of tree functional diversity on herbaceous aboveground productivity,but the positive effects on community total productivity increased through time.These results suggested that the strong competition between tree and herbaceous plants get strengthened through time,whereas tree functional diversity promotes the total aboveground productivity of forest community through time.The results of structural equation models showed that tree diversity increased the dominance of herbaceous species with functional traits typical of low-light conditions(lower CWM of maximum plant height,but larger CWM of specific leaf area),and promoted the shifts from herb-dominated to tree-dominated community through time.In summary,based on the exploration of the mechanisms of tree functional diversity and aboveground productivity relationships,this study quantifies the influence of herbaceous plants on the relationship between tree diversity and aboveground productivity,and revealed how tree and herbaceous species interactively drive total productivity of forest communities.This study clarifies the importance of herbaceous plants on the relationship between tree functional diversity and forest productivity during the early stage.This study also highlights the significance of the natural community processes between target and non-target species in deeply understanding the effects of tree diversity on the temporal dynamics of forest ecosystem functioning.