| Rhododendron delavayi belongs to the Rhododendron family(Ericaceae)and is the dominant species of the Rhododendron community in the Baili Rhododendron Nature Reserve,playing an important role in the ecological function and ornamental value of the Baili Rhododendron forest.In recent years,the phenomenon of the gradual death of Rhododendron arborescens in the reserve for unknown reasons is most serious in the relatively low elevation areas of the scenic spot and where human interference is frequent,endangering the protection of rhododendron.The spatial distribution and growth of rhododendron and other plants are deeply influenced by topography and soil factors.The gradient changes of topographic factors reflect the differences of meteorological conditions and changes of physical and chemical properties of soil,and the thickness and p H of soil also profoundly affect the changes of moisture and nutrient conditions of soil.Therefore,it is necessary to investigate the spatial distribution characteristics and influencing factors of Rhododendron spp.in Baili Rhododendron Nature Reserve.40 sample plots of Rhododendron spp.in Baili Rhododendron Nature Reserve were investigated for ground diameter,plant height,crown width and density,and indoor analysis of soil physical properties and p H.We investigated the effects of soil physical and chemical properties and topography on the growth and community characteristics of different age classes(diameter classes)of Rhododendron.The main findings of the experiment were.1.In the three age classes of Rhododendron arboreum communities,the companion species mainly include:Myrica rubra,Rhododendron simsii,Rhododendron irroratum,Quercus,Eurya loquaiana,Betula luminifera,Rhododendron agastum,Litsea cubeba,decipiens Hemsl,Rhododendron arboreum,Quercus fabri,Camellia sinensis,Populus adenopoda,Corylus yunnanensis.Among them,Myrica rubra,Rhododendron simsii,Rhododendron irroratum,Quercus,Eurya loquaiana are found in all three age classes.The Populus adenopoda and Corylus yunnanensis are only found in the senior rhododendron community.The Betula luminifera,Rhododendron agastum,Litsea cubeba and Rhododendron arboreum were more common in the middle-aged rhododendron communities.The Betula luminifera,Rhododendron agastum,Litsea cubeba and decipiens Hemsl were found in the lower-aged rhododendron communities.The entire survey site showed that the number of associated species in the higher-aged Rhododendron delavayi stands was the lowest,only 57.5%of the middle-aged and lower-aged stands,showing a relatively homogeneous community structure.This indicates that as the growth of Rhododendron delavayi,plant canopy and stand depression gradually increase,the light in the forest understory decreases,and the invasion of pioneer species such as Betula luminifera and Litsea cubeba will be limited to some extent.In addition,the appropriate clearing of companion species in nature reserves to improve the ornamental effect of rhododendron is also a reason for the reduction of companion species.2.The effect of elevation on the spatial distribution of Rhododendron spp.in the lower and middle age classes is mainly reflected in the four indexes of ground diameter,plant height,crown width and density,but there is no significant effect on the spatial distribution of Rhododendron spp.in the higher age classes.The difference of each growth index with the slope direction(shady slope-semi-shady slope-sunny slope-semi-sunny slope)was not significant.The effect of slope on the overall growth of Rhododendron arboreum was small,and the height of trees growing on steep slopes was significantly higher than that on gentle slopes,and the effect of slope on Rhododendron arboreum at different ages was significant only in terms of density and basal area at middle ages.3.The overall effects of three topographic factors,namely elevation,slope direction and slope,on the soil physicochemical properties of Rhododendron arboreum were small,and only the effect of slope on soil porosity was significant.The effect of topographic factors on the physicochemical properties of different age classes of Rhododendron arboreum soils showed that the effect of elevation on the thickness of the soil at the lower age classes was the most significant,while the effect on the physicochemical properties of soils at other age classes was not significant.The effect of slope direction on the physicochemical properties of soil at all ages was not significant.The effect of slope on field(minimum)water holding capacity,maximum water holding capacity,total porosity and soil p H was significant for the middle age of Rhododendron arboreum.4.Regressing the density,basal area and importance value of Rhododendron arboreum on topographic and soil factors in segments,the results showed that the effects of elevation on both basal area and importance value of Rhododendron arboreum were not significant,and the linear pattern decreased with increasing elevation when the density of Rhododendron arboreum was>8000 plants/hm~2.The effect of slope direction on the density and basal area of Rhododendron arborescens was not significant,and when the importance value of Rhododendron arborescens was<0.6,it showed a trend of gradual decrease with the gradual increase of slope direction after the basic level at 30-150°.The effect of slope on the density and importance value of Rhododendron arborescens was not significant,but showed a trend of slowly increasing with slope when the basal area of Rhododendron arborescens was between 60-90 m~2/hm~2.The effect of soil thickness on the density of Rhododendron arboreum was not significant.When the basal area of Rhododendron arboreum was<60 m~2/hm~2,the basal area remained constant and then increased gradually with soil thickness;when the importance value of Rhododendron arboreum was>0.8,it decreased linearly with increasing soil thickness.The overall effect of soil p H on the density and importance value of Rhododendron arboreum was not significant,but gradually decreased with increasing soil p H when the basal area of Rhododendron arboreum was>90 m~2/hm~2and stabilized when the soil p H exceeded 4.4.There was no significant difference in the effect of soil density on the density and importance value of Rhododendron arborescens.When the basal area was<30m~2/hm~2,the basal area of Rhododendron arborescens decreased with increasing soil density and then increased slowly.When the basal area was>90m~2/hm~2,the basal area of Rhododendron arboreum increased gradually and linearly with the increase of soil density. |
