Belowground biomass is currently underrepresented in ecosystem research in bioenergy crops under climate change scenarios such as drought (Freitas et al., 2021). The results of this study add new novelty data with some of the published results from poplar plantations in Europe, in particular, in the vertical distribution of fine root biomass and production under different soil moisture scenarios. Some studies have reported that drought stress can lead to a decrease in root production and in poplar trees (Meyer et al., 2021; Regier et al., 2009). As young roots generally are physiologically more active, an increase in root turnover can increase the water use efficiency, but with an additional carbon cost (Eissenstat, Wells, Yanai, & Whitbeck, 2000). We confirmed that fine root biomass was lower in the drought treatment compared to the control treatment, but that the biomass production of roots was higher in relative terms in the drought treatment. Drought stress on poplar plantations can vary depending on the genotype of the trees and the timing and severity of the drought stress.
In this study we estimated an average fine root productivity of 195 g DM m− 2 y− 1, a high estimate as compared with fine root production of 53 g DM m− 2 y− 1 in a SRC plantation in Belgium. However, our estimates are closer to a stablished poplar plantation in the USA, which estimated an average production of 1.18 g DM m− 2 d− 1 during a ~ 195 days study period (Sprunger, Oates, Jackson, & Robertson, 2017). The difference between sites might be attributed to differences in the age of the plantations. When young and mature plantations were compared in the same study, fine root productivity was lower in the younger plantation (Block et al., 2006). Regarding the root methodology used in our study, the in-growth method provides reliable estimates for production with much less labor time (Berhongaray et al 2017). Moreover, as an improvement to the methodology to quantify the biomass and picking duration periods proposed earlier (Berhongaray et al. 2013) we performed in this study different curves for four categories of roots (SM1) and found that dead roots and absorptive roots (0–1 mm) needed more time to be found, especially the dead roots. This has important implications when the objective of the study is the determination of root mortality.
Inversely to root production, fine roots die in a process known as root turnover. At global scale, the root turnover is positively correlated with the length of the growing season (Gill & Jackson, 2000). In hybrid poplars, root production and turnover start soon after the budbreak and finish with the leaf fall (Dickmann, Isebrands, Blake, Kosola, & Kort, 2001). In our study, the fine root turnover ranged from 1.21 to 1.73 y− 1 for absorptive and transport fine roots respectively, which is by 40% lower as compared to 2.85 and 2.75 y− 1 reported for a poplar plantation in Belgium (Berhongaray et al., 2013). The lower turnover rate of our current study might be explained by the 12% (25 days) shorter leaf area duration as compared to the milder Belgian site (Fischer et al., 2018), i.e. continental vs. oceanic climate. The different turnover rate for absorptive and transport fine roots also confirm previous results were root turnover tend to decrease with increasing root order and diameter (McCormack et al., 2015). Previously, an increase in root turnover in poplar trees has been observed under drought stress (Meyer et al., 2021; Regier et al., 2009). However, there was not effect of the throughfall reduction to the root turnover at our site, similarly to results at global scale where there was no relationship between precipitation and root turnover (Gill & Jackson, 2000).
Some studies have found that the majority of roots are located in the top 30 cm of soil (Berhongaray, Verlinden, Broeckx, & Ceulemans, 2015; Di et al., 2018), similar to the results of this study. In this regard, our findings indicate that the month of May plays a crucial role in determining the vertical distribution of fine roots. Previous research demonstrates the significance of May in stem growth. During May, poplars reach their highest growth rate and contribute at least with 25% of the total stem growth, with even higher percentages observed in dry years (Trnka et al., 2016). These findings hold implications for exploring new hypotheses and conducting ecophysiological experiments on the crucial role May plays in the root biomass distribution of poplar plants, determined by soil moisture.
In our study, we found that roots biomass and root mortality rate were evenly distributed in the soil profile. This is an evidence of close-fitting relationship between root biomass, root production and fixed root mortality rate by soil depth as has been reported elsewhere (Gill & Jackson, 2000). Maximizing the root depth of poplars would also imply greater carbon inputs into the subsoil, where it is more efficient to sequester carbon (Rumpel & Kögel-Knabner, 2011). New hypotheses could state that reduced soil water availability in the topsoil can support carbon sequestration in the deeper layers.
The study has further confirmed earlier findings (Orság et al. 2018) i.e that throughfall exclusion experiments in SRC are not only excellent source of data but at the same time loaded with considerable experimental uncertainty. Even very significant throughfall exclusion has not been translated into equally intensive soil moisture gradient between control and drought variants across the individual subplots. Our hypothesis that the reduction of throughfall precipitation would result in changes in the vertical distribution of fine roots, altering their annual production and turnover rate was not fully confirmed. Not the drought treatment by the throughfall exclusion but the variation in soil moisture content – caused by the combination of treatment and location– which caused changes in root distribution. Nevertheless, these findings are robust and consistent with some of the published results from poplar plantations in Europe and suggest that drought stress can have a negative impact on root production and turnover rate in poplar trees.
By understanding the response of fine root production and turnover to drought, one can better predict the impact of drought on carbon dynamics in SRC poplar systems in the Czech Republic and Europe and provide important information for sustainable management of these ecosystems. New hypotheses and experiments on the management of SRC crops can be raised from these results. For example, growing grass in the spring under poplar plantations could encourage the poplars to root deeper, while increasing carbon input from the grass. It could also be hypothesized that in irrigated sites causing water restriction early in the growing season could deepen roots and improve water use efficiency. These experiments allow further research into drought scenarios.