Angiosperms originally evolved in moist and shady habitats, but later spread to other ecosystems as the climatic started to be drier and/or colder. Among angiosperms, herbaceous species largely avoid harsh climates by senescing above-ground tissues and persisting through adverse seasons below ground in the form of dormant bud-bearing organs.
Understanding the changes in below-ground bud bank density and composition along a climatic gradient is essential to explore the species distribution pattern and vegetation composition in response to climatic changes. However, investigation on bud banks along climatic are still scarce.
Dr. QIAN Jianqiang and his colleagues from the Institute of Applied Ecology of the Chinese Academy of Sciences investigated how total bud density and composition (different bud bank types) change with aridity (0.43-0.91), mean annual precipitation (MAP: 93-420 mm) and mean annual temperature (MAT:-1.51 to 6.93°C) at 21 sites along a 2500 km climatic gradient in the temperate steppe of northern China.
They aimed to test the hypotheses that belowground bud bank is expected to reduced in size in arid conditions, and costly, bud-bearing organs with long spacers would be replaced by more compact forms with buds are better protected than those found in moist conditions.
The relationship between belowground bud bank density and precipitation/aridity was fundamentally changed along the climatic gradient. Bud bank density increased first and then decreased along the aridity and MAP gradients, the turning points were aridity=0.67 and MAP=260 mm respectively, while it decreased consistently with increasing MAT.
The proportion of the bud bank that comprised tiller buds or dicotyledonous herb buds fluctuated along the aridity gradient. The proportion of rhizome buds was higher at relatively moist sites (aridity <0.75), while that of bulb buds was higher at drier sites (aridity>0.75).
Results in the present study indicate that belowground bud bank density decreases towards the dry, hot end of the climate gradient. Based on the distribution of bud types along the climatic gradient, bulb buds and tiller buds of tussock grasses seem to be more resistant to environmental stress than rhizome buds.
The dominance of annual species and smaller bud banks in arid region implies plant reproductive strategies and vegetation composition will be shifted of increased drought under future climatic change.