Molecular Soil Biology 2024, Vol.15, No.4, 172-182 http://bioscipublisher.com/index.php/msb 177 studies on Mediterranean oak and pine species have shown significant population genetic differentiation in traits related to drought tolerance, suggesting that these species have evolved increased drought tolerance as an adaptation to xeric and warm areas of the Mediterranean climate (Ramírez-Valiente et al., 2021). 6.2 Tropical forest tree species In tropical forests, tree species exhibit diverse root-water dynamics to cope with seasonal droughts. For example, a study on tropical dry forest (TDF) trees identified three functional guilds based on their drought survival mechanisms: drought avoiding, drought resistant, and drought tolerant. These guilds showed significant differences in species richness, stem density, and biomass accumulation capacity across a soil moisture gradient, with drought avoiding species dominating drier sites and drought tolerant species thriving in moister areas (Chaturvedi et al., 2020). Another study highlighted the species-specific impact of drought on root metabolic profiles and carbon allocation pathways in tropical rainforest species, demonstrating different defense mechanisms such as enhanced root structural defense and biochemical defense (Honeker et al., 2022). Furthermore, a meta-analysis of Neotropical humid forests revealed that wood density is a good proxy for predicting leaf- and tree-scale responses to drought, with higher wood density correlating with better drought resistance (Janssen et al., 2020). 6.3 Arid and semi-arid tree species Tree species in arid and semi-arid regions have developed root adaptations to survive extreme drought conditions. For instance, conifers in these regions depend on established roots to access reliable water sources during prolonged droughts. A dynamic root-hydraulic modeling framework showed that trees with root access to bedrock groundwater could maintain non-lethal water potentials by shifting water uptake among existing roots rather than growing new roots during drought (Mackay et al., 2019). Additionally, a study on fine-root dynamics in temperate forest trees under repetitive seasonal droughts found that species-specific responses, such as reduced fine-root production and prolonged root lifespan, play a crucial role in maintaining belowground productivity and root-derived organic matter supply to the soil (Zwetsloot and Bauerle, 2021). These adaptations are essential for the survival and ecological functioning of tree species in arid and semi-arid ecosystems. 7 Impact of Root Structure on Water Use Efficiency 7.1 Water use efficiency mechanisms Tree root systems play a crucial role in optimizing water use under limited water availability. The architecture of the root system, including the number and length of main and lateral roots, as well as the density and length of root hairs, exhibits significant plasticity in response to drought conditions. This plasticity allows trees to enhance their water uptake efficiency by adjusting their root growth and development to access deeper soil moisture reserves (Maurel and Nacry, 2020; Ranjan et al., 2022). Additionally, the hydraulic characteristics of roots, such as their ability to adjust water transport capacity, are essential for maintaining water uptake during periods of water scarcity (Vadez, 2014). The coordination between root architecture and leaf-level water use efficiency (WUE) is also critical, as deeper root systems can access stable water sources, thereby supporting higher WUE and reducing drought vulnerability. 7.2 Role of root architecture in reducing water loss Trees employ various strategies to minimize water loss through their root systems. One key strategy is the development of deep root systems that can access groundwater or deep soil moisture, which is less prone to evaporation compared to surface water (Christina et al., 2017; Mackay et al., 2019). This deep rooting not only provides a reliable water source during drought but also reduces the need for frequent water uptake from the upper soil layers, where water loss through evaporation is higher. Additionally, trees can modulate their root hydraulic properties to control water uptake and loss. For instance, by adjusting root hydraulic conductivity, trees can regulate the rate of water transport to the aerial parts, thereby optimizing water use and minimizing unnecessary water loss (Bucci et al., 2009). The ability to shift water uptake among existing roots rather than growing new roots during drought also helps in conserving energy and reducing water loss (Mackay et al., 2019).
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