Abstract
Riparian plants in arid regions face a highly variable water environment controlled by hydrological processes. To understand whether riparian plants adapt to such environments through plastic responses, we compared the root traits, biomass allocation and growth of Populus euphratica Oliv. Seedlings grown in lysimeters filled with clay or clay/river sand sediments under inundation and varying water table conditions. We hypothesized that adaptive phenotypic plasticity is likely to develop or be advantageous in seedlings of this species to allow them to adapt desert floodplain environments. Growth was significantly reduced by inundation. However, rather than following relatively fixed trait and allocation patterns, the seedlings displayed adaptive mechanisms involving the development of adventitious roots to enhance plant stability and obtain oxygen, together with a lower proportion of root biomass. At the whole-plant level, at deeper water table depths, seedlings allocated more biomass to the roots, and total root length increased with decreasing water table depths, regardless of the sediment, consistent with optimal partitioning theory. The sediment type had a significant effect on seedling root traits. P. euphratica displayed very different root traits in different sediment types under the same hydrological conditions, showing a greater first-order root number in clay sediment under shallower water table conditions, whereas rooting depth was greater in clay/river sand sediment under deep water table conditions. In clay sediment, seedlings responded to lower water availability via greater root elongation, while the root surface area was increased through increasing the total root length in clay/river sand sediment, suggesting that seedlings facing deeper water tables are not always likely to increase their root surface area to obtain more water. Our results indicate that P. euphratica seedlings are able to adapt to a range of water table conditions through plastic responses in root traits and biomass allocation.
Highlights
Desert floodplains are characterized by extreme temporal heterogeneity in hydrological variability, which is widely considered a major ecological and evolutionary driver in these environments [1,2,3]
We sought to evaluate the adaptive phenotypic plasticity of the root traits of P. euphratica seedlings in relation to the highly variable water environments where they become established. Because this species occurs in a variety of floodplain soils [28, 35], we considered whether the response of root traits to hydrological conditions would be modified by the sediment type
For seedlings under varying water table conditions, the patterns between treatments varied only in seedlings in the clay sediment type, where the biomass associated with a 70 cm water table depth was significantly greater at 45 days of growth, but in the clay/river sand sediment, this pattern only occurred at the end of experiments (Fig. 1A, B; Table 3)
Summary
Desert floodplains are characterized by extreme temporal heterogeneity in hydrological variability, which is widely considered a major ecological and evolutionary driver in these environments [1,2,3]. Plants inhabiting such areas must accommodate a wide range of hydrological conditions. Seedling responses to hydrological fluctuations are important in determining the distributions of woody species in these environments, as juvenile plants are usually more susceptible to environmental changes [4,5,6]. How the root systems of riparian plants at the seedling stage develop in response to hydrological fluctuations has not been fully elucidated
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