The impact of interannual climate variability on the mean global vegetation distribution

Ecosystem distribution and structure is determined primarily by the mean climate(e.g.,annual mean precipitation and temperature),and their quantitative relationships have been addressed by numerous observational and modeling studies.However,the influence of interannual climate variability on terrestrial ecosystem remains poorly understood.In this paper,the impact of interannual climate variability on the global potential vegetation distribution is investigated using the revised Community Land Model-Dynamic Global Vegetation Model(CLM-DGVM). The control simulations use the observational atmospheric forcing data with seasonal and interannual variability,while the sensitivity tests use the mean annual cycle of the forcing data in which the interannual variability is not considered but the diurnal and seasonal variations are still considered.It is found that the spatial distributions of plant functional types(PFTs) differ in the control and sensitivity testes in three classical climatic regions: i.e.,the Tropical monsoon climate(Am),Semi-arid climate(Bs),and Subarctic climate(Dfc). The coverage of the dominant woody vegetation decreases,especially where the climate interannual variability is large.The global simulations show that the interannual climate variability decreases the mean global vegetation coverage by 5.9% by significantly decreasing woody vegetation(i.e.,tree and shrub) coverage but slightly increasing grass.The expansion of grass is mainly due to the reduction of tree and shrub.In response to the interannual climate variability,the absolute values of global coverage changes decrease for the PFTs of evergreen tree,grass,shrub,and deciduous tree,while the relative changes in the ratio of global average absolute difference to the mean coverage in the control simulation decrease for the PFTs of shrub,grass,deciduous tree,and evergreen tree. Among the various climatic factors considered(i.e.,temperature,precipitation,wind,specific humidity,solar radiation and atmospheric pressure),the interannual precipitation variability has the most significant impact on the mean vegetation distribution.For instance,under the influence of annual precipitation variation,the gross vegetation coverage decreases for annual precipitation 1200 mm,and such reduction increases with the precipitation variability.The responses of tree are similar,with the threshold changing to 1500 mm.While the coverage of evergreen trees decreases,which is independent of the annual mean precipitation,the coverage of deciduous trees increases for annual precipitation 1500 mm.Expansion of grass coverage is the largest when the annual mean precipitation is less than 1500 mm and its interannual variability is moderate.The impact of interannual temperature variability is more complicated.It has little effect on the coverage of deciduous trees but has a negative effect on the coverage of evergreen trees especially in cold regions.Grass coverage increases mainly for annual mean temperature -10℃,while shrub coverage increases for annual mean temperature 0℃. The influence of interannual temperature variability on gross vegetation coverage is negative when the annual mean temperature is higher than 0℃,and the areas with decreased coverage are primarily those with higher interannual temperature variability.These results imply that the changes in both the mean climate and climate variability should be taken into account when climate models or biogeography models are used to predict future terrestrial ecosystem distributions.