Effects of cloud,atmospheric water vapor,and dust on photosynthetically active radiation and total solar radiation in a Mongolian grassland

Photosynthetically active radiation （PAR） is an important input parameter for estimating plant produc- tivity due to its key role in the growth and development of plants. However, a worldwide routine network for sys- tematic PAR measurements is not yet established, and PAR is often calculated as a constant fraction of total solar radiation （SR）. Although the ratio of PAR to SR （PAR/SR） has been reported from many places, few studies have been performed for dry regions. The present study was therefore carried out in an arid region of Mongolia to obtain PAP-JSR and examine its dependency on sky clearness （the clearness index）, water vapor in the atmosphere and aeolian dust. Continuous measurements of PAR and SR were taken every one second using quantum and pyranometer sensors, respectively, and the readings were averaged and recorded at intervals of 30 minutes for a period of 12 months. The lowest monthly mean daily PAR/SR occurred in April （0.420）, while the highest ratio was observed in July （0.459）. Mean daily PAR/SR during plant growing season （May-August） was estimated to be 0.442, which could be useful for modeling plant productivity in the study area. The annual mean daily PAR/SR （0.435） was lower than the values reported in many previous studies. This difference could be explained with the regional variation in climate： i.e. drier climatic condition in the study area. PAR/SR was negatively correlated with the clearness index （r= -0.36, P〈0.001）, but positively with atmospheric water vapor pressure （r=0.47, P〈0.001）. The average PAR/SR was significantly lower （P=0.02） on the dusty days compared to the non-dust days. Water vapor in the atmosphere was shown to be the strongest factor in the variation of PAR/SR. This is the first study examining PAR/SR under a semi-arid condition in Mongolia.

Effects of grazing and precipitation variability on vegetation dynamics in a Mongolian dry steppe

Aims Grazing and water availability are the primary drivers of vegetation dynamics in grazing-dominated regions of Mongolia with a semi-arid climate and frequent droughts.Nomadic animal husbandry still plays a large part in the economy of Mongolia,but more variable precipitation regime and increase in livestock number have severely affected grassland ecosystems through overgrazing,leading to pasture degradation.This study aimed to examine the effects of grazing exclusion,interannual variation of plant-available precipitation(PAP)and their interaction on the aboveground biomass(AGB)of each dominant species,the AGB of annual species and the total AGB in a Mongolian dry steppe,using long-term field data.Methods To detect the effect of grazing on vegetation dynamics,vegetation surveys were conducted in a non-grazed exclosure zone and a fully grazed area outside the exclosure.We assessed the effects of grazing,PAP and their interaction on AGB parameters using a generalized linear model.A detrended correspondence analysis(DCA)was used to visualize the effects of grazing and PAP on the AGB of each species.Important Findings Grazing,PAP and their interaction had significant effects on AGB.The effect of grazing on AGB was larger with higher precipitation and higher amounts of AGB(i.e.forage)while AGB was strongly limited in drought years,which resulted in a smaller grazing effect.The current year PAP had the highest impact(r=0.88,P<0.01)on AGB.The dominance of annual species was characterized by the amount of PAP in the current and preceding years:annuals dominated in wet years that followed consecutive dry years.The DCA Axis 1 reflected the variation of AGB with interannual variation of PAP while the DCA Axis 2 differentiated the grazing effect.The DCA scatter diagram based on species score illustrated that Artemisia adamsii(an unpalatable herb)was clearly linked to grazing disturbance whereas palatable perennials such as Agropyron cristatum,Stipa krylovii and Cleistogenes squarrosa were related to grazing abandonment and wetter conditions.In brief,number of livestock,hence the grazing impacts on vegetation dynamic in this region could have driven by forage availability,which is mainly controlled by current-year PAP.