| The loess hilly region is a typical ecological fragile region and a key area of ecological restoration in China.In recent years,with the implementation of ecological restoration measures,the vegetation coverage has been significantly improved in this region,and the ecological restoration was remarkable.Protecting natural secondary forests and artificial afforestation are important measures for the regional restoration.The natural secondary oak forests(Quercus wutaishanica),black locust plantations(Robinia pseudoacacia)and Chinese pine plantations(Pinus tabuliformis)are typical forest communities in this region.This region is located at the edge of typical forest distribution areas.The main determinants of forest productivity and their possible action pathways are not clear yet,and the relevant research information is limited.These three typical forest communities were selected in this study.Several sample plots were set up,and the soil nutrient and volumetric water content and stand structure characteristics measured.The effects of soil characteristics and stand structure properties on ecosystem production were explored.In addition,the trade-off relationships between the ecosystem service functions in black locust and Chinese pine plantations were quantitatively analyzed through the statistical parameters of root mean square deviation.Based on the evaluation of the ecological service functions in the two plantations along the stand density gradients,it is helpful to propose the suggestions for multi-function management of artificial plantations.The major results and conclusions are as follows:(1)The natural secondary oak forest in the loess hilly region was in a stable growth and biomass accumulation stage,and its aboveground primary production was5.80–10.23 t hm-2a-1.Litterfall production was higher than woody production.The effect of soil nutrient contents on the aboveground primary production and leaf production were near 60%and 70%,respectively.More than 70%variance of woody production was explained by stand structure,while litterfall production was largely dependent on stand vertical structure with effect size of 64%.The effect of soil nutrient content on stand structure indices was detected only on the vertical structure.The results suggested that soil nutrients could affect forest production both directly and indirectly in coordination with stand vertical structure,while the direct effect of stand vertical structure on forest production was relatively limited in natural secondary oak forest communities.(2)The aboveground primary production of black locust plantation in the loess hilly area was 7.30–9.74 t hm-2a-1,and that of Chinese pine plantation was11.31–12.64 t hm-2a-1.In both plantations,woody production was higher than litterfall production.The aboveground primary production of Chinese pine plantation decreased with the increasing stand density,which was maximized in middle density class in the black locust plantations.The change trend of woody production was consistent with aboveground primary production.Among the soil factors,nitrogen content was the major factor on the forest production in black locust plantations.While in Chinese pine plantations,there were no significant relationships between aboveground primary production and soil factors.There were significant correlations between soil volumetric water content and woody production,litterfall production,and leaf production.Compared to soil factors,stand structure indices were important factors affecting the forest production in the plantations.In black locust plantations,the diameter-related indices(stand basal area and the average DBH)were dominant factors;while tree height-related indices(the average stand height and maximum height)were main factors in Chinese pine plantation.(3)Fine root production was highest(4.23–6.69 t hm-2a-1)in oak forest,followed by 3.05–5.27 t hm-2a-1in the Chinese pine plantations,and the minimum was 2.26–4.64 t hm-2a-1in the black locust plantations.The fine root biomass characteristics were consistent with production in these three forest communities.Fine root turnover rate was maximized(0.73 a-1)in black locust plantations,followed by0.59 a-1in Chinese pine plantations,and was minimized(0.41 a-1)in oak forests.Fine root biomass and production increased with stand density increasing and decreased with soil depths.High fine root biomass was achieved through increases in the fine root production and turnover rate in the high-density black locust plantations and through an increase in fine root production in the pine plantations.In the high-density Chinese pine stand,there was a high fine root turnover which,coupled with high fine root production,contributed to a high fine root biomass.Overall,fine root production and turnover rate were closely related to soil volumetric water content in both plantations,while fine root biomass,especially the component of necromass,was related to soil nutrient status,which refers to phosphorous content in black locust plantations and nitrogen content in Chinese pine plantations.There was a close linkage between leaf area index and fine root dynamics in the black locust plantations but not in the pine plantations.(4)Litterfall production was the main component of the aboveground primary production in oak forests,while woody production was the main contributor of aboveground primary production in the black locust and Chinese pine plantations.In terms of the underground production,coarse root production was accounted for a small proportion,and almost two-thirds were provided by fine root in these forest communities.The proportion of woody production increased with the stand density increasing in black locust plantations,while the proportion of litterfall production increased in Chinese pine plantation.In the two plantations,the proportion of fine root production increased with the increasing stand density.The results of the correlation analysis between forest net primary production and its subcomponents showed that woody production might be a better indicator for predicting plantation forest production,especially in black locust plantations.(5)In black locust plantation,the relative benefits of various ecosystem service functions were highest in middle density class,and the relative benefits of multiple ecosystem service functions were highest in low density class in Chinese pine plantation.The relative benefits of ecosystem service function pairs with a higher trade-off were shifted in favor of soil water storage in black locust plantations,while the relative benefits of ecosystem service function pairs with higher trade-offs were shifted in favor of aboveground primary production,soil organic carbon and total nitrogen storages in Chinese pine plantations.The effects of tree species and stand density on the trade-off relationship between ecosystem service pairs were mainly detected in the ecosystem service function pairs related to stand large tree abundance.The interaction between tree species and stand density had significant effects on the ecosystem service function pairs with relative higher trade-offs.In this study,we selected regional typical forest types,natural oak forests,black locust and Chinese pine plantations,based on setting field plots and sampling surveys,the productivity characteristics and distribution of three typical forest communities in the loess hilly region were clarified.And the dominant factors were revealed in natural forests and plantations,which would be helpful to understand the forest ecosystem stability and sustainability.At the same time,the relative benefits of ecosystem service functions in the two plantations along the density gradient were systematically analyzed,and explored the trade-offs between ecosystem service function pairs.This study would have important guiding significance for the multi-functional management of plantations in loess hilly areas. |
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