Tree growth and wood density
In our study, RHRL treatment did not affect the tree dimensions (height or diameter) after 5 years of treatment. Nevertheless, we showed that the RHRL treatment decreased tree growth (newly formed rings) in the beech plots. A number of studies have also reported that increasing biomass exportations from forest stands resulted in a decrease in the growth rate of trees (see Achat et al., 2015). However, these studies reported growth rates of very young trees after a clear-cut, while our study focused on 30-year-old trees. This effect is most likely due to the decreased availability of nutrients in the soil. For the Darney site, Maillard et al. (2019) showed that the RHRL treatment significantly decreased the contents in soil nutrients and the cation exchange capacity. Other studies have reported that depletion or enrichment of soil nutrients conveys a direct relationship with tree ring width and wood density, in oak (e.g., Bergès et al., 2008; Kint et al., 2012; Ponton et al., 2019) and beech (e.g., Elhani et al., 2005) trees.
In our study, the RHRL treatment decreased ring width and wood density in oak trees but non-significantly. In general, tree growth of diffuse porous species such as beech is found to be more responsive to stress than ring porous species such as oak (Meyer et al., 2020). This may explain the differences observed in this study between the beech stands at the Darney site and the oak stands trees at the Champenoux site. This difference could also be explained by the differences in soil nutrient availability between the two sites. The availability of soil nutrients at the Darney site was lower than that at the Champenoux site. This may have led to more acute effects at the Darney site.
Wood density variability is found to be less affected by radial growth components in the diffuse porous species, compared to ring porous species (Bouriaud et al., 2004; Diaconu et al., 2016). Along with other components such as- cambial age and climatic variables, ecological factors (e.g., soil fertility nutrients etc.) are also found to explain variability in radial growth and wood density. However, in the related studies, factors such as climate, water and nutrients were not distinguished (Bergès et al., 2008), and site condition variability was largely not accounted for; for instance, in most cases, extreme site conditions were not sampled or were undersampled (e.g., Polge and Keller, 1973; Becker, 1979; Guilley et al., 2004). In our study, for the beech trees, the ring width in the treatment plots (i.e., RHRL) explains less than 1% of the wood density reduction in the newly formed rings; to the remaining variation (of the 2.56% observed) comes from the treatment itself (after the ring width effect is removed). There might be a modification of the cell properties in addition to the reduction in the ring width. However, additional anatomical studies are required to verify this hypothesis. This could not be conducted in our study because the subsamples were milled to analyse their nutrient content. It is also likely that a 5-years period of intensive harvest might not be long enough to observe the effect and distinguish the variability in wood density among species; however, preliminary effects were seen in beech at the Darney site, suggesting that this species may be used as a sentinel.
Two particular phenomena about the dynamics of nutrients as a consequence of removing harvest materials could be identified from our study: (i) there was a sharp decrease in nutrients for both the near bark and near pith regions in the treatment plots and (ii) the results suggest that certain nutrients were translocated from the near pith region to the near bark region in the beech trees to compensate for the lower availability of nutrients in the soil.
The loss of nutrients in the soil as a consequence of removing harvest materials from forests had been reported in many parts of the world. Tamminen et al. (2012), Brandtberg & Olsson (2012), Kaarakka et al. (2014) and Vangansbeke et al. (2015) found depletion of one or multiple soil base cations (K, Ca, and Mg) due to WTH in the Scandinavian trial studies. The same was also reported in North America (e.g., Ponder et al., 2012- from the North American long-term soil productivity (LTSP) network; Johnson et al., 2015; Johnson et al., 2016) and also from the tropical studies (see workshop proceedings from- Nambiar, 2008; Kumaraswamy et al., 2014). The concentration of nutrients in the stem heartwood can act as an indicator of the nutrient status in the soil (Lévy et al., 1996). We may assume that the loss of nutrients from trees in response to our treatment, was a consequence of losing nutrients from the soil, due to removing harvest residues. Maillard et al., 2019 (see subsection 2.2) conducted a parallel study on the same experimental site and found significant loss of soil nutrients and cation exchange capacity (CEC) in treatment plots at the Darney site. There were few/no cases in existing literature that studied the harvest removal effects on wood chemistry and nutrient translocation, but for both species, the wood nutrient values reported in our study were closer to those, where trees were growing in moderate to poor soil fertility (Table 4, 5).
In the RHRL plots, the relative decrease of near pith concentrations of K was above threefold higher than that of near bark tissues in beech trees. For Mg and Ca, the relative decrease was also comparatively higher by 8% and 47% respectively. Besides, the difference in nutrient concentration (‘near bark’ minus ‘near pith’) was also significantly greater for K (p < 0.001) and for Mg (non-significant) in RHRL plots compared to the control plots. This trend indicates that these elements may have been internally translocated from the near pith tissues to the near bark tissues, probably to compensate for the nutrient loss, as described by Meerts (2002). The difference was not significant for Mg, with a 54% proportional increase computed from the mean values. However, the internal compensation might not be as efficient for Mg as it is for K, and Mg was still significantly lower in the near bark samples from the RHRL plot than it is in the samples from the control plots. However, our study supports the hypothesis stated by Penninckx et al. (2001) that the efficiency of the resorption of Mg is higher in sites with the lowest availability, as a possible mechanism to compensate the deficit from soil resources. Calcium, which is less mobile than Mg or K in trees (Mc Laughlin and Wimmer 1999, Lautner and Fromm, 2010), had a decreasing trend regarding treatment and also for the abovementioned difference value. Ca is found to be the least translocated element in trees, if not translocated at all (e.g., Colin-Belgrand et al., 1996; Fife et al., 2008) and often reported to be immobilized in great portion in the tree bark and stem wood tissue (Ferguson, 1979; Katainen and Valtonen, 1985). However, part of the Ca in aboveground biomass may be stored in an exchangeable/adsorbed and mobilizable form (van der Heijden et al 2015; van der Heijden et al 2017), Overall, in our study, the behaviour of Mg and Ca were similar in the RHRL plots in respect to remobilization from the near pith tissues. The Sulphur is found to be extensively lost from both the near bark area (55%) and near pith area (45%) in the beech trees and from the near bark area (41%) in the oak trees. The forest floor is recognised as the major pool of S available to trees (Cronan et al., 1978; Yanai, 1998), and the availability of S in a soluble form is highly correlated with fungi and bacterial interactions in forest soils (Strickland and Fitzgerald, 1978). Thus, the removal of soil litter as well as the significant reduction in topsoil enzymatic activities due to the removal of harvest residues (Maillard et al. 2019) can explain the extensive loss of S in the treatment plots.
In the oak trees, the concentrations of K, Ca, Mg, Na and S were significantly depleted in both the near bark and near pith tissues (except for Mg and S in the heartwood) in the treatment plots. While the concentrations of K, Ca, and Mg were found to be significantly higher in the near bark area than that in the near pith area, which can indicate potential nutrient translocation (Lévy et al., 1996; André and Ponette., 2003), but these trends were not significant of the treatment. In contrast to most existing literature (Peri et al., 2006; André et al., 2010; Peri et al., 2010), we found a positive effect of tree diameter on the nutrient concentration in the oak near bark zone, but this could be because our stands are much younger than those assessed in published studies. Colin-Belgrand et al. (1996) found the same effect on stemwood in relatively young chestnut trees.
After 3 years of carrying out treatments in the same experimental plots as those used in our study, Maillard et al.(2019), did not find any change in the soil pH of the treatment plots, but the CEC of the soil was significantly decreased. At present, after 5 years, we have found a significant increase in the ratio of Al/Mg (p = 0.001) in the treatment plots (Fig. 6), which potentially indicates increased acidity in the soil. (Lévy et al., 1999). The aberrant increase in the concentration of Fe in the treatment plots may also be related to high soil acidity (Meisch et al., 1986); however, this may be more likely due to instrument error related to the use of an increment borer to collect the stem wood samples (Augusto and Bert, 2005).