| Specific leaf area(SLA)is defined as the leaf area per unit weight and it is a key trait in plant growth and reflects plants'ability to acquire resources and adapt to different habitats.It also has important directive significance for vegetation regeneration,community and ecosystem succession.However,variations in SLA over resource gradients remain unclear.We systematically surveyed 4895 plant species in 56 natural terrestrial ecosystems(26 forest ecosystems and 30 grassland ecosystems)in China.It straddles the Tibetan Plateau subcold zone,Tibetan Plateau temperate zone,cold temperate zone,middle temperate zone,warm temperate zone,northern sub-tropical zone,central sub-tropical zone,southern sub-tropical zone,tropical zone.Vegetation types include desert,meadow,grassland,evergreen broad-leaved forest,deciduous broad-leaved forest,evergreen needle-leaved forest,deciduous needle-leaved forest,covering almost all climate types and vegetation types in the northern hemisphere.Furthermore,these ecosystems have large-scale hydrothermal gradients.Based on this,they were used to explore the variation and its control mechanism of SLA in China.Meanwhile,we measured SLA of 1357 species from the Tibetan,Loess,and Mongolian Plateaus,limited by temperature,soil nutrient,and precipitation,respectively.In each plateau,we set up an east-west transect of 10investigation sites,covering meadow,grassland,and desert,alongside controlled experiments,were conducted.We also consider both all species and the dominant species in the analysis to reveal the variations of SLA and explore the changes of SLA along the resource gradients under extreme environments,respectively.The results showed:(1)The average SLA was 22.02 m2kg-1,with a range from 0.9 to 110.24 m2kg-1in grassland.SLA was obviously different in different ecosystems,climatic zones,vegetation types,and functional groups and it was larger growing in forest than in grassland.With the improvement of resources,SLA in each climatic zone and vegetation type increased first and then decreased.For SLA of different functional groups,it showed herb>shrub>tree,broadleaf>coniferous,deciduous plants>evergreen plants and gymnosperms had the smallest SLA.SLA first increased and then decreased with the increasing mean annual temperature(MAT),mean annual precipitation(MAP),and de Martonne aridity index(DI)and it decreased with the increasing ultraviolet radiation(UV)and soil pH and increased with increasing soil nitrogen.However,SLA did not change with increasing climate and soil factors in arid region.SLA was controlled by radiation,soil nutrients,climate,and soil environment and there were direct and indirect effects between each factor.In the medium environment,SLA was the largest.We suggest SLA increased with increasing hydrothermal gradient(competition priority strategy),and then decreased with increasing hydrothermal gradient(survival priority strategy).(2)The average SLA was 14.12 m2kg-1,with a range from 0.9 to 58.90 m2kg-1in grassland.SLA was significantly larger in deserts than in meadows and grasslands,and was significantly different among the plant functional groups.Plants from the Tibetan Plateau had a smaller SLA than those from the Loess and Mongolian Plateaus.Particularly,SLA increased with the increasing resources of temperature,precipitation and soil nitrogen in the controlled experiments,while SLA was relatively conservative along the resource gradients in natural grasslands of all plateaus.From the results of transect investigations and controlled experiments,we suggest that plants adopt survival priority strategy with a stable SLA along the natural resource gradients as a long-term adaptation with periodical and severe limiting factors,however,plants adopt competitive priority strategy with an increased SLA along the resource gradients in controlled experiments as a short-term response to resource availability.Knowing these two divergent strategies will help us better understand plant adaptation and evolution under changing resource environments. |
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