Abstract
The vertical root distribution and rooting depth are the main belowground plant functional traits used to indicate drought resistance in arid and semiarid regions. The effects of the slope aspect on the aboveground traits are visible but not the belowground deep root traits. We aimed to investigate the fine root traits of the locust tree (Robinia pseudoacacia L.) planted on southerly and northerly aspects, and the variations in the rooting depth in regions with different rainfall, as well as assessing how deep rooting, might affect the response to drought in a loess region. We selected three study sites with different rainfall amounts, with six sampling plots at each site (three each with southerly and northerly aspects). Soil core samples were collected down to the depth where no roots were present. The locust trees tended to develop deeper fine roots rather than greater heights. The tree height and diameter were greater for locust trees on northerly aspects, whereas trees on southerly aspects had significantly deeper rooting depths. Fine root traits (root length, root area, and root dry weight density) were higher in the southerly aspect for both Changwu and Ansai, but lower in Suide. The ratio of the root front depth tree height ranged from 1.04 to 3.17, which was higher on southerly than northerly aspects, and it increased as the rainfall decreased. Locust tree growth traits (belowground fine root and aboveground tree height) were positively correlated with the mean annual rainfall. The soil moisture content of the topsoil decreased as the rainfall decreased, but the pattern varied in the deep layer. Our results suggest that the variations in the belowground rooting depth under different slope aspects may be related to plant survival strategies. The vertical extension of the rooting depth and tree height may be key functional traits that determine plant growth in drought-prone regions.
Highlights
Determining plant functional traits might help us to understand the responses of plants to environmental variations (Reich et al, 2003)
Locust trees on southerly aspects had higher root biomasses, but the opposite was found in Suide, possibly because the lower root diameter and weaker growth led to smaller biomasses (Table 3) due to severe soil moisture depletion lower than wilting point
A previous study showed that the root biomass was four times higher in mesic populations (Kurze et al, 2017)
Summary
Determining plant functional traits might help us to understand the responses of plants to environmental variations (Reich et al, 2003). The contrasting effects of northerly and southerly slopes can produce different climates, where the greater solar radiation on southerly slopes can increase the temperature and evapotranspiration to yield more arid conditions in the Northern Hemisphere (Finkel et al, 2001; Gutiérrez-Jurado et al, 2013) These climatic differences can cause plants to develop different functional traits, especially in terms of the tree height, canopy size, leaf structure, and branch density (Niinemets, 2006; Rozendaal et al, 2006). Finkel et al (2001) and Nevo (2012) showed that the aboveground biomass was lower in a plantation on a southerly slope, whereas Kou et al (2013) and Fan et al (2012) obtained the opposite results These differences of slope aspect and soil moisture in environmental factors could affect the aboveground morphological traits of plants (Vitória et al, 2019) and they might reflect the growth strategies employed by plants to resist drought (Liu and Ma, 2015). Little information is available regarding the deep root traits of plants
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