Response of plant growth to surface water balance during a summer dry period in Central Eurasian steppe

Introduction

Grasslands are located in the arid and semi-arid climate area and are one of the most widespread terrestrial ecosystems in the world. These regions are also sensitive to human- and/or climate-induced land degradation (desertification) mostly due to less water resources. It is likely that water deficits will increase in most of arid and semi-arid areas. In addition, interannual variability in above-ground net primary productivity (ANPP) in grasslands is greater and more strongly correlated with precipitation than any other ecosystems. Improved knowledge of large-scale climatic and environmental factors regulating local water balance and plant growth is required to deal with issues of sustainability of the grassland ecosystems that will respond to the possible future climate change.

In order to evaluate the impact of hydro-climatic conditions on the local surface energy balance and plant growth on seasonal and annual time scales, inter-seasonal measurements of the surface energy balance components and plant biomass were conducted since 2002 at natural grassland in north part of Kazakhstan. We focused on the responses of evapotranspiration and above- and below-ground biomasses to soil moisture content, during the development of a summer dry period.

Method

Meteorological observation and biomass measurements were carried out from 1 May to 1 November, 2002 at the natural grassland in Shortandy (51.3 N, 71.2 E, 427 m) in the northern part of Republic of Kazakhstan. The surface conditions of the site are mostly flat. This area is entirely cultivated for wheat, sugar beets and a variety of other crops because of the highly productive soil.

Surface heat budget was observed using the Bowen ratio-energy balance (BREB) method, installing a Bowen ratio system. Other supporting climatic variables, such as wind speed and direction, precipitation, incoming and reflected short-wave radiation, photosynthetically active radiation (PAR), and volumetric soil moisture content were also measured.

In order to estimate above- and below-ground biomass in the grassland, 4 sites of 1x1 m quadrate were set in 2 week interval from early May to mid-October, 2002. In the first site, plant coverage, height, above-ground alive and dead biomasses separately measured in each species. In addition, below-ground biomass down to 20 cm depth was also measured. In other three sites, measurements of coverage, height, above-ground alive and dead biomasses were identified between dominant grass (Stipa Capillata) and other species.

Results and discussion

Seasonal variations in energy balance, precipitation and soil moisture showed characteristic transition of water balance during the growing season. Soil moisture exhibits a remarkable difference before (wet) and after (dry) mid-July in conjunction with the frequency and intensity of precipitation. A gradual decrease of soil moisture began after mid-July and finally reached to the wilting point in late August.

A decrease in soil moisture content influences the evaporative fraction lE/Rn. The evaporative fraction ranged from 0.4 to 0.8 when there was abundant soil moisture during the wet season. In contrast, when soil moisture reduced during the early stage of the summer dry period, the evaporative fraction (around 0.4) remained nearly constant. These conditions indicate that evaporative water was effectively supplied by soil moisture. Soil moisture had already stored and kept since the snowmelt season and mitigated the reduction of evaporation in accordance with progressing summer dry season at this site. Subsequently, soil moisture decreased to less than 20% during peak stage of the dry period, corresponding with abrupt decrease in the evaporative fraction to 0.1.

Continuous difference in water balance occurred between seasonally accumulated evapotranspiration and precipitation. During May, evapotranspiration was active and equivalent to equilibrium evaporation, whereas the precipitation could not satisfy the evapotranspiration. The result showed that deficit in water balance between evapotranspiration and precipitation reached 40 mm during May and finally 93 mm on late August. According to seasonal course of soil moisture profile, soil moisture within the near-surface layers was substantially reduced before dry season in conjunction with plant growth. Then, soil moisture in deeper layers compensated the deficit during summer dry period.

Above-ground alive biomass consistently increased until the beginning of the dry period. Dead fraction of the above-ground biomass inversely decreased. During the summer dry period, alive biomass exhibited a gradual decrease, and dead biomass turned to increase. After mid-September, increasing trend of dead biomass indicated a transition to the senescence. The peak of vegetation height showed a large delay comparing with greenness and biomass variations.

During the wet season, active plant growth produced high transpiration. Subsequently, evaporative fraction (lE/Rn) was somewhat stable (0.4), until soil moisture reduced to the wilting point during the early stage of the dry period. The mature plant was likely to tap deeper sources of soil moisture during the early stage of the drought. After the reduction of soil moisture, the transpiration was substantially weakened. Thus, the abrupt reduction in transpiration during the peak stage of the dry period implied changes in physiological response of both above- and below-ground structure to the very dry soil.

The below-ground biomass temporally decreased during the summer dry period, while it increased again as the dry conditions progressed. It appeared that the response to the soil moisture variation was quite different between the above- and below-ground biomasses. The growth ratio of above-ground alive biomass is positively correlated with soil moisture content. In contrast, the below ground biomass did not show a significant correlation with soil moisture, whereas it grew even under the summer dry conditions. In brief, it implies that during dry period, the assimilation of plants could not sustain above-ground alive biomass, but a major portion of the assimilation was allocated to the below-ground biomass.