| Abies fargesii var.faxoniana coniferous forests are identified and managed as a water conservation forest in the subalpine dark coniferous forest in western Sichuan,China.Forests in the region were extensively logged from the 1950 s to the 1990 s.The regeneration of A.fargesii var.faxoniana in this area is the key of the forest restoration.Temperature,light,soil nutrients and other abiotic factors in the habitat are the key factors affecting sapling growth.It is important that saplings adapt to the changes of these factors for the saplings successfully establishment in the field.Ectomycorrhizal fungi(EMF)play a significant role in mediating habitat adaptive responses of host plants,and EMF may influence the distribution range of their host plants.It is important that the association between the habitat adaptation of A.fargesii var.faxoniana saplings and the EMF for the forest regeneration and restoration in the subalpine dark coniferous forest in western Sichuan.Therefore,this study focus on the A.fargesii var.faxoniana saplings and its root-associated EMF to study the changes of the EMF communities and their relationship along the different environment factors gradient(altitudinal gradient with temperature change,different forests with soil nitrogen(N)change,under canopy and canopy gap with illumination change),as well as the suitable habitat(north-west aspect)and unsuitable habitat(south-west aspect);to clarify the role of EMF in mediating the environmental adaptation of A.fargesii var.faxoniana saplings.The main results are as follows:1.On the contrary of the decreased EMF species with the increasing elevation in bulk soil and rhizosphere,the endospheral EMF species were increased with the increase elevation,which may be the adaptation of A.fargesii var.faxoniana saplings to the stressful high altitude habitat.With the increase of altitude,the soil N limitation was increased.And the relative abundance of endospheral EMF OTUs increased with the increase elevation,which benefit their host plants with expanding nutrients sources,i.e.cortinarius spp.and inoxybe spp.(account to 50%).These results suggested that EMF communities may help A.fargesii var.faxoniana saplings to expand the N-source and benefit saplings adapting to the high stressful altitude habitat with low soil nitrogen.Thus,EMF may be beneficial for A.fargesii var.faxoniana saplings to adapt to high altitude habitat.2.Compared with the unrelated correlation between rhizospheral and endospheral observed EMF species with soil TN and N:P in forest plot with higher soil nitrogen,the rhizospheral and endospheral observed EMF species were significantly negatively correlated with soil TN and N:P in the forest plot with lower soil nitrogen.And there are more rhizospheral and endospheral OTUs,which was significantly negatively correlated with TN,in plot with lower soil N than plot with higher soil N.More than 53% of those OTUs were Cortinarius spp.and Inocybe spp.,which were beneficial for the host to expand nutrient sources.These may improve the uptake of N of sapling in lower N habitat.Compared with the plot with higher soil N,the plot with lower soil N had more rhizospheral and endospheral OTUs which were positively correlated with sapling basal diameter.In the plot with lower soil nitrogen,rhizospheral and endospheral OTUs and observed EMF species were significantly positively correlated with the sapling base diameter,while there was no similar positive correlation in the plot with higher soil N.These were consistent with the "stress gradient hypothesis",and might be beneficial for the saplings to adapt to low soil nitrogen habitat.Thus,A.fargesii var.faxoniana saplings may be beneficed from EMF in adapting to low soil nitrogen.3.EMF may enhance the gap-advantage of A.fargesii var.faxoniana regeneration.However,the positive correlation between the A.fargesii var.faxoniana sapling and EMF was closer under the canopy than canopy gap.The observed EMF species in the bulk soil,rhizosphere and endosphere at the canopy gap were significantly more than that under the canopy.The relative abundance of OTUs,which were significant difference between under the canopy and canopy gap,were higher in the canopy gap than which under the canopy.In particular,the Cortinarius spp.,which can mobilize organic N,was higher in the canopy gap than that under the canopy.This suggested that EMF may be beneficial to the higher density and better growth of A.fargesii var.faxoniana saplings in the canopy gap.The relative abundance of EMF and endospheral OTUs and the observed EMF species under the canopy were positively correlated with the sapling basal diameter,while there was no similar positive correlation in the forest canopy gap.These results were consistent with the stress gradient hypothesis,which suggested that EMF might be beneficial for the shadow canopy adaptation of A.fargesii var.faxoniana saplings.4.The relationship between EMF and A.fargesii var.faxoniana saplings in north-west aspect was positive,while the relationship in south-west aspect was negative.Tylospora,which is common mycorrhizal of spruce,was richer in the south-west aspect than north-west aspect.And the relative abundance of Tylospora was significantly negatively related to the sapling basal diameter(DBH)in the north-west aspect,and the relationship between EMF and Tylospora was negative in north-west aspect and was positive in south-west aspect.The relative abundance of EMF and the observed EMF species in the north-west aspect were higher than that in the south-west aspect.The observed EMF species was significantly positivily correlated with sapling DBH in the north-west aspect,wihle significantly negativily correlated with sapling DBH in the north-west aspect.100% OTUs,which relative abundance were significantly correlated with seedlding DBH,were significantly positively correlated with sapling DBH in the north-west aspect,while were siginificantly negatively correlated with sapling DBH in the south-west aspect.The relative abundance of Tylospora were significanlty negatively correlated with endospheral observed EMF species and the relatve abundance of73% endospheral OTUs(the relative abundnace of the OTU was significantly correlated with the relative abundance of Tylospora)in the north-west aspect;while were significanlty positively correlated with endospheral observed EMF species and the relatve abundance of93% edospheral OTUs.The antagonistic relationship between EMF and Tylospora in northwest aspect and the cooperative relationship between EMF and Tylospora in south-west aspect may be related to the growth and distribution of A.fargesii var.faxoniana in the suitable habitat(north-west aspect)and unsuitable habit(south-west aspect).A.fargesii var.faxoniana saplings may be beneficed from EMF in the suitable habitat(north-west aspect),while saplings may be maladapted to their local EMF in the unsuitable habitat(south-west aspect).In the suitable habitat,the changes of endospheral EMF may be beneficial for the habitat adaptation of A.fargesii var.faxoniana along the altitude gradient with the temperature change,different forests with soil nitrogen change,canopy gap and under canopy with the illumination change.The positive relationship between EMF and sapling basal diameter in suitable habitat was closer as the habitat was more stressful,which was consistent with the "stress gradient hypothesis".The changes of EMF may be beneficial to A.fargesii var.faxoniana saplings in the suitable habitat,but may be inhibited to saplings in the unsuitable habitat.This may affect the distribution range and ecological amplitude of A.fargesii var.faxoniana saplings.This study explored the relationship between EMF and the habitat adaptability of A.fargesii var.faxoniana saplings along the habitat gradient.The results provided reference for EMF technology to promote the regeneration and forest restoration of A.fargesii var.faxoniana forest in the subalpine dark coniferous forest in western Sichuan,China. |
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