Lengthening single-stem rotation improves biomass yield and water use efficiency in black poplar genotype multi-stem rotation coppice plantations
Introduction
A globally increasing demand for energy and the need to mitigate the effects of climate change emphasise the importance of the development of sustainable and renewable bioenergy resources at both the cultivation and industrial levels [1,2]. The European Union (EU) has set a target to decrease greenhouse gas emissions (GHG) by 40% compared to the levels of 1990, and increase renewable energy resources by 27% by 2030 [3]. Therefore, tree crop plantations are expected to become one of the most sustainable environmental solutions for the biomass and energy supplies of many countries, as well as matching energy and climate policy targets of the EU [4]. Nonetheless, the potential large-scale deployment of woody crops could be hindered by an multiple concerns related to the loss of biodiversity and the detriment of water resources [5,6]. Indeed, negative impacts on ecosystem services are commonly attributed to the large-scale extent of dedicated woody crops [7,8].
Trees managed as short rotation coppices (SRCs) represent a cultivation system dedicated to lignocellulosic biomass production [9]. Further, SRCs are locally available and utilisable across various bio-based conversion processes [[10], [11], [12]]. The development and improvement of provenance selection, breeding, cultivation, and management techniques are crucial steps to greatly reduce environmental impacts associated with defence against pathogens, irrigation, harvesting, pre-treatment, and increasing the efficiency of woody biomass bioconversion [13,14]. Although stem density affects poplar growth performance [15], under the limited water supply of Mediterranean environments, understanding water use efficiency [16] and the optimal length of rotation of different poplar genotypes cultivated in SRC is crucial.
Hybrid poplars in temperate regions in Europe and around the Mediterranean are commonly cultivated in SRC [10,11,17], while wild native European black poplar genotypes are often only planted sporadically [18]. European poplar breeding and clone selection programs were finalised to identify and understand the genes involved in poplar clone performance [19,20], as well as on resistance to viruses, fungi, and insect-related disease [19,21]. Together with these two primary target selection criteria, other traits involved in biomass allocation patterns and drought tolerance need to be considered when developing poplar clone breeding programs to make SRCs economically and environmentally sustainable.
Yield and biomass production are directly linked to soil water availability, supplemented by irrigation in intensive poplar SRC plantations under soil water deficit conditions [22]. A worldwide increase in water shortage has led to the development of irrigation management techniques that minimise water consumption [23] and the selection and identification of drought-tolerant poplar genotypes [24]. However, there is no consensus on the impact of poplar SRCs on water use. Some studies have attributed poplar genotypes to high water use [19], although more recent studies have reported that under comparable sites and climatic conditions, the evapotranspiration of poplar SRCs did not exceed the reference evapotranspiration values of well-watered grassland or reference crop evapotranspiration [[25], [26], [27]].
In poplars, high biomass allocation to coarse root systems are associated with plant to instable river substrates, access to resources over large volumes of soil, and the storage ability of photosynthates (as non-structural carbohydrates in coarse roots), which all support aboveground biomass when coppiced [28,29]. Moreover, it has been observed that some Mediterranean resprouter tree species (i.e., Quercus spp.), with high below-ground biomass allocation, mitigate the effects of soil water shortage [30]. The ontogenetic stage of the mono-stem, in which first coppicing occurs, is crucial for the legacy effect between the root system, aboveground biomass production, and the water use efficiency of the resulting multi-stemmed coppice phase. However, to our knowledge, such a linkage has never been investigated in poplar genotypes.
The identification of native wild poplar genotypes with favourable levels of lignocellulosic biomass production could represent a useful strategy for reducing the biodiversity footprint and increasing the environmental compatibility (sustainability) of SRC plantations. Further, when bioenergy cultivation is based on threatened wild native species, the objectives of conservation and production may be achieved simultaneously.
Despite the wide use of poplar SRCs, little is known about the effects of single-stem rotation length on biomass production and the water use efficiency of the following multi-stem rotation coppice, which is crucial knowledge in drought-prone Mediterranean climates. Therefore, we aimed to assess variations in biomass production and water use efficiency in the first multi-stem cycle of a comparative field study involving ten fast-growing poplar genotypes. We hypothesised that (1) a longer single-stem rotation duration results in a higher biomass yield in the subsequent multi-stem rotation coppice and (2) the origin of the genotypes affects the relationship between above-ground carbon gain and water use.
Section snippets
Study site, climate, and plant materials
The study site was located in the Sele river valley (Latitude 40° 33′ 33.21″ N; Longitude 14° 50′ 15.60″ E, 19 m a.s.l., Eboli, Salerno, Italy) in a flat area previously used for agricultural crops in the experimental farm ‘Improsta’ (Fig. 1). Groundwater level is approximately at a depth of 5 m during summer [31]. Poplar plantations were established in 2007 using mechanically planted unrooted dormant cutting of length 15–20 cm. Six hybrid poplar genotypes (I-214, Grimminge, Hoogvorst,
Growing seasonal length and leaf conductance
The length of the growing season varied considerably among the hybrid and native black poplar genotypes (Table 1). Black poplars exhibited an average growing season of 242 (±10) days, which began in approximately mid-March and ended at the beginning of November. In the hybrid genotypes, the length of the growing season was 197 (±13) days, which began in mid-May, and ended in early November. Leaf unfolding in the black poplars occurred an average of 37 (±4) days before that of the hybrid
Single-stem lengthening effects on multi-stem aboveground biomass production
Our hypothesis that the lengthening the duration of the single-stem rotation improves aboveground dry biomass production in the first multi-stem rotation coppice was confirmed only for the native black poplar genotypes. Lengthening the single-stem rotation did not improve aboveground dry biomass and water use efficiency in the hybrid genotypes. Such divergent responses in highly contrasting genotype groups could be linked to the different selection trajectories experienced by native and hybrid
Conclusion
Our study demonstrates that longer single stem rotation can improve biomass production and water use efficiency in black poplar multi-stem coppice rotation. Furthermore, the cultivation of native black poplar genotypes was found to be suitable for SRC, showing both consistent biomass productivity and potential positive impacts on the terrestrial water cycle and on biodiversity conservation efforts in P. nigra threatened populations. Therefore, the cultivation of autochthonous plant material,
Author contributions
This manuscript is part of LS PhD thesis. Conceptualization, LS and AS; methodology, LS; software and data analysis, LS, AS and SR; writing—original draft preparation, LS; writing—review and editing, LS, EA, SR, GM, NF and AS; supervision, AS. All authors have read and agreed to the published version of the manuscript.
Funding
This work was supported by: CRAA- Regione Campania - SMA Campania; UE-Regione Campania PSR 2007–2013 misura 124 (Tra.Tec.F.U.L.En. and Sinergia projects); Regione Campania-Sesirca. Funds have been granted to AS. LS was partially supported by the MIUR project PON01_01966/2 and within the PhD programme at the “School of Agricultural and Food Science” (tutor: AS) in the Department of Agricultural Sciences, University of Naples Federico II.
Acknowledgements
All authors are grateful to Giampiero Guida for field leaf-gas exchange measurements. We would like to thank Editage (www.editage.com) for English language editing. We thank two anonymous reviewers for their helpful comments which improved the manuscript.
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