| Arceuthobium sichuanense, a hemiparasitic plant, is the most destructive pathogen of natural and man-made spruce forest in China. A vertical transect investigation on spruce stands of the different habitats was conducted in Sanjiangyuan Nature Reserve (Maixiu farm, Xianmi farm and Beishan farm), Relationships with occurrence characteristics of Arceuthobium sichuanense and site factors and stand characteristics were analyzed. And the morphological characters of needles, diurnal changes of photosynthesis and transpiration in healthy and infected P. wilsonii trees were measured. At last, through systematically organizing and converting the information from the previous records, using theories of macro-phytopathology, the temporal-spatial patterns of dwarf mistletoe in China was visualized and analyzed in my study. The main results were as follows:Picea wilsonii trees and Pinus tabuliformis trees naturally infected by A. sichuanense were firstly reported.A. sichuanense have had different occurrence degree in P. crassifolia, P. purpurea and P. wilsonii stands of different farm in Qinghai Province. A. sichuanense infection on spruce forest in slight disease stands were scattered, and the distribution of patches in severe disease stands was relatively centralized. patched. Spruce forest of different elevations was susceptible in survey region. Disease index of A. sichuanense was significantly negatively correlated with latitude and longitude, and the increase with, elevation, disease index of A. sichuanense increased. The disease index of A. sichuanense at low-elevation (n=27) within2,200-2600m was significantly lower than high-elevation (n=34) within3,000-3300m and middle-elevation (n=51) within2,600-3,000m (P<0.05), high-elevation and middle-elevation had no differences (P>0.05). A. sichuanense in semi-shady of the spruce forest was more abundant than shady (P<0.01). The disease index of A. sichuanense on upper slopes (n=39) was more abundant than middle slopes (n=39) or lower slopes (n=34)(P<0.05), but differences between middle slopes and lower slopes were not significantly different (P>0.05). Differences in disease index of A. sichuanense were not statistically significantly different between gentle (<10°, n=35) and moderat slopes (10°-30°, n=64)(P>0.05), but were significantly different between steep slopes (>30°, n=13) and either gentle or moderate slopes (P<0.05).Disease index of A. sichuanense was significantly negatively correlated with canopy density (P<0.001), and positively correlated with the crown or the base area (P<0.05), and no related with diameter at breast height, tree height or stand density (P>0.05). The disease index of A. sichuanense in woodland (canopy density<30%, n=11) and in middle woodland (canopy density30%-70%, n=71) had no differences (P=0.57), and midwood (canopy density>70%, n=30) was significantly lower than woodland and middle woodland (P<0.001). The disease index of A. sichuanense in a small crown (<2m, n=20) was significantlylower than in a middle crown (2-3m, n=60) and in a big crown (>3m, n=32)(P<0.05). Middle crown and large crown had no significant differences (.P>0.05). Disease index of A. sichuanense in a base area (<20m2ha-1,n=25) was significantly lower than other base areas (20-30m2ha-1, n=43;30-40m2ha-1,n=26;>40m2ha-1, n=19)(P<0.05), but other base areas had no differences (P>0.05). In addition, the disease index in different stands of spruce population was significantly higher than mixed spruce (host+broadleaf, host+conifer)(P<0.05).The length and width of needles from the infected P. wilsonii were significantly smaller than those from control trees (P<0.001), but the specific leaf area (SLA) of diseased P. wilsonii was significantly bigger than that of control trees (P<0.001). Further more, infection by A. sichuanense significantly reduced net photosynthesis rate (Pn), transpiration rate (Tr) and stomatal conductance (Gs) of P. wilsonii trees (P>0.05). However, substomatal CO2concentrations (Ci) were not influenced by A. sichuanense infection (P=0.32).Relationship analysis between environmental factors and photosynthesis-transpiration of P. wilsonii trees based on redundancy analysis (RDA) revealed that the importance of the tested environmental factors were changed by A. sichuanense infection, from ambient CO2concentration (CO2)>air temperature (Tair)>leaf temperature (T1)>vapor pressure deficit (Vpd)>air relative humidity (RH)>photosynthetic active radiation (PAR) for control trees to T1>RH>Tair>PAR> Vpd>CO2for diseased trees. T1became the key factor affecting Pn and Tr of diseased P. wilsonii trees. The Pn and Tr of diseased P. wilsonii trees also became more sensitive to RH and PAR. Moreover, water use efficiency (WUE) of diseased trees was significantly lower than that of control trees (P<0.05). Therefore, infection by A. sichuanense would weaken the adaptive capacity of P. wilsonii to environment and accelerate the death of the trees.A. sichuanense and A. chinense constituted most of the reported cases (45%and30%, respectively). However, for the host species, the spruce forest was the most commonly infected, with over50%of the cases. A. sichuanense is distributed in natural spruce forests of Tibet, Sichuan, Qinghai, and Gansu in China. Comparison of latitude and longitude of A. sichuanense and host range was relatively narrow, and A. sichuanense distribution areas mainly concentrated in high elevations. |
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