Therefore, it is likely that we have slightly underestimated coarse root biomass, and similarly slightly overestimated shoot to root ratio. Soil characteristics, calculated coarse root biomass data at the plot level and shoot to root ratios were analysed using a two-way ANOVA in a fixed factorial design. ANOVA tables were constructed in accordance with Petersen , where degrees of freedom, sum of squares, mean squares and F values were computed.
Pairwise correlations were used to identify which of the following environmental factors were significantly correlated with shoot to root ratios: site elevation; soil pH, CEC and base saturation; soil organic matter, clay, silt and sand content, and soil supply rate of NO 3 , NH 4 , P, Ca, K and Mg.
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Thereafter, regressions were developed between key environmental variables and shoot to root ratios. Both of these sites are located at low elevation and shared similar soil characteristics in terms of clay content, pH, percent organic matter, CEC and base saturation. Compared to the two moderate fertility sites Melbourne and Ste-Catherine , much higher NO 3 and Ca supply rates were also observed in late summer , at the two high fertility sites Bedford and Brompton. At Bedford and Brompton, NO 3 supply rate, respectively, reached Allometric relationships between diameter at breast height DBH, in centimetres , as the predictor variable x , and coarse root biomass in kilogrammes per tree , as the response variable Y.
The general model combines data from the four sites, the moderate site fertility model combines data from the Melbourne and Ste-Catherine sites, while the high site fertility model combines data from the Bedford and Brompton sites. For each model, goodness of fit expressed by the Shapiro—Wilk statistic W is presented with its associated p value. General allometric relationship between diameter at breast height DBH; in centimetres and coarse root biomass in kilogrammes per tree in year-old hybrid poplar plantations.
Environment-specific allometric relationships between diameter at breast height DBH; in centimetres and coarse root biomass in kilogrammes per tree for two classes of soil fertility high and moderate in year-old hybrid poplar plantations. Linear relationships between calculated mean aboveground woody biomass and mean coarse root biomass at the plot level for two classes of soil fertility high and moderate in year-old hybrid poplar plantations. Mean aboveground biomass data are from Truax et al. Coarse root biomass and the shoot to root ratio were calculated using a general allometric relationship, but also using the environment-specific relationships plastic allometry developed for the two site fertility classes.
Data for aboveground woody biomass were taken from Truax et al. Shoot to root ratio in hybrid poplar plantations as a function of a soil NO 3 supply rate, b soil Ca supply rate, c soil clay content, d soil base saturation, e soil organic matter content and f site elevation. This study provides evidence that site fertility or quality influences the trajectory, or shape, of the allometric relationship between DBH and coarse root biomass of mature hybrid poplar trees grown on abandoned fields Fig.
In other words, coarse root biomass allocation in year-old poplars is plastic, being both tree size and environment dependent. Consequently, environment-specific equations describe more accurately the relationship between tree diameter growth and coarse root biomass growth in mature poplar plantations.
The different slopes of the relationships between estimated mean aboveground biomass and mean coarse root biomass per tree also support the evidence of a flexible allometry change in tree architecture that optimises biomass allocation along a site fertility gradient Fig. These results corroborate previous glasshouse experiments showing that a favourable N soil environment triggers changes in hybrid poplar tree architecture, which favours aboveground structures over the root system to increase C fixation capacity [ 37 , 36 ].
Evidence of plastic allocation patterns in response to resource availability have also been shown for seedlings of different tree species, including Betula pendula [ 67 ] , Fagus sylvatica and, to a much lower extent, Picea abies [ 39 ]. Similar observations have been reported for herbaceous plants [ 68 , 69 ] and for different tree species growing in forest environments [ 31 , 70 , 71 ]. In willow bioenergy crops, site fertility also causes changes in the relationship between stem diameter and aboveground biomass growth [ 28 , 29 ].
Being pioneer riparian species, the parental species of studied hybrid poplars Aigeiros and Tacamahaca sections often colonise nutrient poor mineral substrates such as sand or gravel bars along streams, but also sand dunes and beaches [ 72 ]. In these low fertility and well-drained soil habitats, newly established seedlings often have to grow under the additional stress of a low or rapidly declining water table [ 73 ]. The rapid development of root systems for resource acquisition, at the expense of shoot growth, is therefore critical to ensure seedling survival when resource availability is low or rapidly declining [ 74 ].
This adaptive strategy in response to riparian habitat features could explain the plasticity in root biomass allocation observed in this study. Differences in elevation between moderate and high fertility sites could have also contributed to the plastic allometry pattern observed. Lower elevation sites benefit from a warmer climate in the region [ 46 ], and therefore higher soil temperature conditions, which may reduce the need to invest C in the root system because higher root ion uptake rates are generally observed at higher soil temperature [ 63 ].
To a certain extent, the interpretation that higher coarse root biomass allocation on lower fertility sites occurs to allow a higher nutrient uptake capacity is debatable. It is widely assumed that nutrient uptake is restricted to the narrow region of young fine roots located just behind the root tip [ 75 ]. Yet, recent evidence suggests that the majority of N uptake occurs in older woody roots of both coniferous and broadleaved seedlings [ 76 ].
Nitrogen uptake rate in older woody roots was found to be lower than in young fine roots, but the much higher total surface area of these woody roots provides the higher total N uptake [ 76 ]. Small woody roots also had important P and K uptake rates in year-old slash pines growing under field conditions [ 77 ]. Consequently, we cannot completely discard the idea that larger woody roots of poplars can contribute to nutrient uptake, although rates of nutrient or water uptake by coarse roots might be very low. The large surface area of coarse roots could be a determinant factor in nutrient absorption, especially on low fertility sites.
Still, clear demonstration of the nutrient uptake capacity of larger woody roots of different tree species remains to be shown under field conditions. On the other hand, the plastic allocation to coarse biomass observed in this study Figs. As trees become more mature, internal remobilisation of stored C and N may become proportionally more important than uptake to fulfill nutritional requirements, because mature trees have larger storage pools and often slower uptake rates [ 79 ].
It is well-known that poplar coarse root biomass is an important storage site for non-structural carbohydrates, but also for amino acids in the form of bark storage proteins [ 43 , 44 ]. On lower fertility sites, it may be advantageous for poplars to allocate more belowground biomass to build a larger coarse root system for storage purposes.
Stored carbohydrates and amino acids in roots could then be used locally, and in timely fashion, to sustain a higher level of fine root growth, symbiotic associations with fungi [ 80 , 81 ] and root exudation [ 82 , 83 ]. These three processes have an increasing importance for nutrient capture as site fertility decreases [ 84 , 85 , 86 ]. This interpretation is consistent with the observations of Kobe et al. These authors showed that total root mass, as a proportion of whole-plant mass, was higher in low vs. The build-up of a larger pool of non-structural carbohydrates in roots could provide flexibility to temporal resource variations in soil environments [ 87 ], with such resource variations being typical of riparian habitats where parental species of the studied hybrid poplars naturally grow [ 74 , 72 ].
During periods when nutrients become scarce, non-structural carbohydrates stored in roots could be used to increase access to soil nutrients by providing an energy source for mycorrhizae and organic matter decomposers in the rhizosphere [ 87 ]. This energy source could then be used for the rapid production of fine roots and to support metabolic costs of nutrient uptake, as nutrients become more available [ 87 ].
These low soil richness indicators and the presence of grey birch and willow seedlings in the plot understory were likely linked due to the poorer drainage conditions that were observed during root sampling of this outlier tree J. Fortier, personal observation. Still, this outlier is in agreement with the idea that poplars allocate a higher proportion of biomass to coarse roots in low fertility environments. Consequently, much higher allocation to coarse root biomass than what is reported in this study may occur if hybrid poplars are grown on very low quality sites ex: in acidic soils of clear-cut forest sites of the boreal shield ecoregion of Canada.
In this study, we have only sampled coarse root biomass located at a limited distance from the tree base, where most root biomass may be expected to provide a stability or anchorage function. This stability function may be especially important when plantation soil had been cultivated for site preparation prior to tree establishment, as was the case in this study. However, if the stability function of coarse root biomass would have been of overriding importance in the studied mature hybrid poplar trees, biomass allocation to coarse roots would have only been dependant of aboveground biomass allocation or DBH, regardless of site quality.
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Thus, our results contrast with those of Resh et al. This result could be reflecting the high requirements for biomechanical stability in large Eucalyptus trees, or an indication of the non-plastic response of Eucalyptus coarse root biomass to nutritional environment. Our results also challenge the notion that allometry of biomass allocation in plants is rarely affected by environmental features, with plant form or biomass allocation being generally the same at a given size independently of soil nutrient availability in the local environment [ 89 , 90 , 62 ].
Many studies have reported relationships between shoot to root ratio and environmental factors, but these relationships were often the sole consequence of allometric growth or apparent plasticity [ 91 , 45 ]. As shown in Fig. However, when the shoot to root ratio was calculated using the environment-specific allometric relationships Fig. In a meta-analysis on the root biomass of upland forests, Cairn et al. In this study, shoot to root ratios of mature poplar plantations were strongly and positively correlated to several key indicators of soil fertility Ca supply rate, clay content and base saturation; Fig.
This provides additional evidence that N availability in soils is a key factor driving biomass partitioning towards aboveground structures to increase or maintain C fixation capacity in hybrid poplars. Strong negative correlations between soil organic matter or elevation and shoot to root ratio were also observed Fig. Biomass estimates calculated with the environment-specific relationships also suggest little variation of coarse root biomass in year-old poplar plantations. With the exception of clone DxN that produced very low aboveground and coarse root biomass at the Melbourne site, coarse root biomass estimates had a similar range on moderate fertility sites Greenhouse experiments also suggest that similar root biomass was observed between hybrid poplars growing under low and high N availability, with higher shoot biomass being observed when N was more available [ 37 , 36 ].
Our results are consistent with the analysis of Vicca et al. Finally, the results presented in this study have important implications for bioenergy crop models and C stock estimations because both rely on allometric equations developed for the different tree biomass compartments [ 16 , 20 , 21 ]. Being the largest biomass C pool after stems and branches, poplar coarse root biomass needs to be more accurately quantified [ 96 ].
Until now, only general allometric relationships had been developed for poplar coarse root biomass [ 24 , 26 , 22 , 23 , 19 , 27 , 25 ], with no consideration for the potential effect of plantation environment on allometric trajectory. In this study, using the general allometric relationship developed across the four sites Fig.
Increasing the accuracy of coarse root biomass estimates is also important for a better understanding of C cycling in the soils. Large roots have particularly slow decay rates, and they can contribute to the belowground biomass C pool over a century after harvest [ 97 ], while having a short-term positive effect on soil organic C stocks once incorporated into the soil [ 98 ]. Lastly, more accurate coarse root biomass estimates are also be needed, as coarse root biomass may eventually represent a valuable bioenergy feedstock in some countries [ 19 ].
A special thanks to Harry Isbrucker for providing us with a large amount of space for sample storage and preparation. Finally, we wish to thank the two anonymous reviewers for their positive comments and suggestions, which have contributed to improve this manuscript. Skip to main content Skip to sections. Advertisement Hide. Download PDF. Open Access. First Online: 12 May Introduction In many parts of the world, poplar Populus spp. The Bedford and Brompton sites have a high site fertility and high aboveground woody biomass yields The geology of the study area is complex, but the bedrock is almost completely covered with glacio-fluvial and glacial till deposits [ 46 ].
The PRS-probes consist of an ion exchange membrane encapsulated in a thin plastic probe, which is inserted into the ground with little disturbance of soil structure. Nutrient availability predicted with this method is generally significantly correlated with conventional soil extraction methods over a wide range of soil types [ 53 ], and which has also been corroborated in poplar riparian agroforestry systems of the study region [ 54 , 55 ]. In August , four pairs of probes an anion and a cation probe in each pair were buried in the A horizon of each plot for a day period.
Composite samples were made in each plot by combining the four pairs of probes. A large rectangular pit was excavated using a small mechanical excavator. The size of the excavated pits was 1. This approach was chosen because large monoliths are required for representative coarse root sampling [ 56 ]. The position of the quadrant sampled was randomly selected.
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