Comparison of hybrid walnut trees growth in an agroforestry system and under forestry condition

doi:10.21203/rs.3.rs-2595200/v1

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Comparison of hybrid walnut trees growth in an agroforestry system and under forestry condition

https://doi.org/10.21203/rs.3.rs-2595200/v1

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The services provided by agroforestry trees are numerous, especially on the economic front. They help to improve the business model of the farm by diversifying activities and incomes for the farmer. However, agroforestry trees grow in very different conditions than in traditional forests; mainly because of the specific environmental conditions like their higher exposure to wind and light and the strong interactions with annual crops. Moreover, numerous human operations on branches (pruning) and the soil tillage, that can affect the root systems, modify the growing conditions too. Production level and quality such as anatomical, chemical and technological properties of wood coming from agroforestry systems have not yet been investigated. The present study aims to understand how the particular agroforestry growing conditions affect the trees’ development - especially primary and secondary growths. A comparative study on the growing kinetics in height and diameter, from 1995 to 2014 of hybrid walnut trees in an agroforestry plot and in a forest control plot, was performed at the Restinclières experimental Platform, in France. The results show that the quantities of biomass produced by the agroforestry walnut trees are higher than those produced by their forestry control trees. However, these tendency need to be qualified, because the two plots showed a great deal of variability, particularly in terms of water availability, which is one of the parameters that has been detailed in this work.

Such results could provide interesting data to the farmers to promote access of agroforestry wood to conventional and/or niche markets.

Agroforestry

Growth dynamics

Juglans regia × nigra

Water tables

Wood productivity

For centuries, agroforestry systems were an important part of European agriculture and an integral part of the landscape. These systems have gradually declined with the intensification of agriculture practices: trees and hedges gradually lost their place in agricultural models (Bazin and Schmutz 1994; Barr and Gillespie 2000; Pointereau 2002; Pointereau and Coulon 2006), in order to increase the areas of agricultural plots and to facilitate the mechanization. But since the 1970s, the new intensive agricultural models have shown economic, environmental, and social weaknesses (Nair 1993), leading scientists to define and implement alternative agricultural patterns that reintegrated environmental and social issues into economic development (Millenium Ecosystem Assessment. 2005).

New agricultural models have been developed in opposition to the intensive models. This is the case of agroecology, which integrates ecological issues and the preservation of natural resources into production systems (Stassart et al. 2012; Guillou et al. 2013; Schaller 2013). As a result, policies and strategies have been put in place to replant trees and hedges into the fields and to resurrect the interest of agroforestry systems in order to address environmental issues in the heart of agricultural production. Now, agroforestry systems are also becoming more and more numerous thanks to the evolution of agriculture towards practices combining both agroecology and increased area-based biomass production. In Europe, agroforestry areas, including silvopasture, silvoarable and homegarden practices, are now estimated to occupy 19.77 Mha (Mosquera-Losada et al. 2018). These areas are gradually increasing and Zomer et al. estimated a 1.6% surface increase between 2000 and 2010 (Zomer et al. 2014; FAO 2019). It is hence essential to consider trees as a production output in their own right (complementary to annual agricultural production) and no longer as simple crop auxiliaries. In this context, a better understanding of their growth and development dynamics is essential in order to predict wood quality and to optimize both the valorization and transformation of agroforestry timbers.

However, while tropical agroforestry systems have been widely studied to date, this is not the case for agroforestry systems in temperate regions, either in Europe or in North America. Usually, studies in these systems focus more on the impact of the trees on the crops and there is very little information on the impact of the crops on the trees (e.g. Cutter and Garrett 1993) and on the growth dynamics of the trees.

The growing conditions of agroforestry trees are very different from those in conventional trees plantations. In temperate agroforestry systems, planting densities are much lower than those in forests are. In France, agroforestry plantation densities are usually between 30 and 200 trees/ha according to the associated production and the strategy of the farm (Liagre et al. 2012), whereas those forest densities usually range from 1000 to 1500 trees/ha (IGN 2017). At the individual level, these differences lead to modification of many parameters of the tree-growing environment, such as higher exposures to wind and light, while the competitiveness between trees is mainly replaced by trees/crops interactions. Some of these interactions have some positive effects; eg facilitation or complementarity between species for the resources. For example, competition for water availability leads to the formation of more developed and deeper tree roots (Mulia and Dupraz 2006), which improves the trees' resistance to drought (Dupraz and Liagre 2008; Dupraz et al. 2012). In addition to their interactions with annual crops, numerous human operations on their branches (pruning) may affect the development of agroforestry trees and therefore the formation of wood, both quantitatively and qualitatively (Plomion et al. 2001; Shanavas and Kumar 2007; Taghiyari and Efhamisisi 2012).

In order to anticipate the best use of wood in the agroforestry systems, it is necessary to study both trees’s productivity, through the quantity of wood they produce, and their quality as a material, the valuation that yields the most added value.

There are many definitions of wood quality and its meaning often differs, whether from the point of view of foresters, manufacturers or customers (Zhang 2003). It can be define as the suitability of wood for a particular end-use (Jozsa and Middleton 1994) and could be the result of physical and chemical characteristics of a tree, or a part of a tree, that enable it to meet the property requirements for different end products (Mitchell 1961). These characteristics, such as wood density, fiber length, proportion of sapwood and heartwood, proportion of clear wood, presence of reaction wood, or the amount of extractives, can be directly related to the species, but are also determined by the growing conditions of the environment. This implies a management of forest and agroforestry plots adapted to each site, whether in terms of species selection, planting density, fertilization, pruning, etc. (Jozsa and Middleton 1994).

The low planting densities of agroforestry systems are one of the major elements that may modulate tree growth and wood formation by inducing greater exposure to wind and reducing competition for light (Benomar et al. 2013). During their development, trees are regularly subjected to mechanical stimuli such as those induced by wind. In response to these mechanical disturbances, trees modulate their growing development by changing the distribution of their biomass (Niez et al. 2018). This acclimation phenomenon, called thigmomorphogenesis (Jaffe 1973), generally results in a decrease in aerial and reproductive growth in favor of root growth. In terms of aerial growth, there is generally a significant reduction in primary growth and an increase in secondary growth of their trunk (Niez et al. 2018). These changes are also coupled with a modification of the anatomy of their wood, then called flexure wood, and of its mechanical properties (Telewski 1989, 1995, 2006; Niklas 1996, 1998; Meng et al. 2006b, a; Coutand 2010; Bonnesoeur et al. 2016; Niez et al. 2018; Roignant et al. 2018). These responses to mechanical disturbances are still poorly understood, but they could strengthen the tree against the wind by improving its anchorage in the ground, reducing its drag and increasing the strength of its trunk (Gardiner et al. 2000; Nicoll et al. 2008; Niez et al. 2018).

In forest stands, it has been shown that a decrease in plantation density, is generally correlated with an increase in diameter growth (Bonnesoeur et al. 2016) and a decrease in the height growth of their trunk. As agroforestry trees are more exposed to wind due to lower planting densities than in forest, it can be assumed that they could be smaller and wider than those growing in forest plantations could.

Thanks to their late budburst (Dufour et al. 2013), which delays competition for light with neighboring arable crops, their heliophilicity and their fairly narrow crown, walnut trees are well suited for use in temperate agroforestry models with the production of high quality timber as one of the economic objectives. They are economically interesting because of their fruits and their valuable timber (Reisner et al. 2007; Mohni et al. 2009) and thus allow diversification of products and income within the agroforestry plots. Two main species of walnut trees are cultivated: the common walnut (Juglans regia) native to Central Asia and the black walnut (Juglans nigra) native to North America. For many decades, hybrids of these two species (Juglans x Regia x Nigra) have been created in order to improve the behavior of trees with a better disease resistance, a lower sensitivity to phototropism and finally better resistances to drought and spring frost. They are also more vigorous and have good crown dominance (Beritognolo 2001). The hybrid walnut species prefer well-watered climate, without drought, and a well-drained soil (Coello et al. 2008).

The objective of the present study is to compare the growth dynamics of agroforestry hybrid walnut trees (Juglans regia × nigra cv. NG23) with walnut trees grown in more conventional plantation density. We hypothesized that due to the lower planting density in the agroforestry plot, the agroforestry walnut trees could have a lower height growth but a higher width growth than their forest controls.

2.1 Study site

The study was conducted at the Restinclières experimental Platform, located near Montpellier, in South of France (longitude 04°01’ E, latitude 43°43’N, elevation 54 m a.s.l.). This platform was created by INRAE in 1995, and now it is one of the biggest experimental agroforestry systems in France It covers 45 ha of plantations composed by agroforestry and forest plots, and by additional crops areas without trees consisting in agricultural sole crop control plots. In this way, it allows comparison of production and growth between the three systems.

The climate is subhumid Mediterranean with an annual average temperature of 15,4°C and a mean annual rainfall of 873 mm (years 1995 – 2013,). Soils are deep carbonated sandy loam Fluvisol (IUSS Working Group WRB 2007). However, the Restinclières domain is a large site, but with contrasting soil types and a highly variable distance to the water table within the plots. It is crossed by streams, in particular the Lez in the context of our study. It flow varies little, being the result of a resurgence connected to an underground network that is extremely large in comparison.

In 1995, four plots were dedicated to the study of agroforestry hybrid walnut (Juglans regia × nigra cv. NG23), on a total area of 12,5 ha. Our study focused on two walnut trees plots: an agroforestry plot (AF plot in Fig.1) and a forest control plot (FC plot on Fig.1). Both were roughly North-South oriented and were bordered by the Lez River on the West side. Four piezometers based systems (Fig.1, numbers in blue correspond to the name of the piezometer) were installed in the soil of the AF plot and three in the FC plot to measure the distance to groundwater in the plots . Since 2002, a reading of the depth of the water table has been recorded every month for each piezometer. The water table heights were determined from the depth of the water column in each piezometer. The data used were those obtained after levelling, the reference piezometer being 6_35 (plot FC). Unfortunately, levelling was not carried out on piezometer 5_7 located to the north of plot FC, so it was not considered in our study.

Initially the walnut planting density was identical in both plots: 200 trees ha^-1 in 1995, trees spacing: 13x4 m in AF plot and 7x7 m in FC plot. Nine years after planting, in 2004, a thinning process reduced the density of the AF plot to 100 trees ha^-1. The walnut trees became spaced 4, 8, 12 or 16 m on a same planting line and the cultivated inter-row was 13 m wide. From 1995, winter cereal crops were cultivated between each line with alternating durum wheat, rape and protein peas. In AF plot, the annual crop was fertilized with approximately 150 kg N ha⁻¹yr⁻¹, except when composed of leguminous, and the soil in the inter-row was usually ploughed to 20 cm depth every year before sowing. In the forest control plot, half of the trees were fertilized from 1999 to 2010 with ammonitrate (50 units of nitrogen/ha), and the other half not.

Moreover, the walnut trees in the FC plot were combined with alders (Alnus cordata), a nitrogen-fixing species, to reach a global plantation density of 400 trees ha^-1. Alder are often used as a complementary species in walnut plantations to promote the growth of the trunk and the good development of the crown of walnut trees (Becquey 2008). These associations also make it possible to limit the need for inputs within the plot: species such as alder, by fixing atmospheric nitrogen, improve the nutrient richness of the soil. In the case of walnut, several studies have shown an improvement in the primary and secondary growth of common walnut trees (Juglans regia) and in the shape of their trunks and crowns when associated with alders, compared with pure plantations (Salbitano and Bagnara 2004; Tani et al. 2006; Mohni et al. 2009).

FC plot consisted in eleven lines, numbered from 1 to 11. Line 1 was near the Lez (to the West) and line 11 was the opposite side, close to the road (to the East). AF plot was composed of eight lines numbered from 1 to 8. Line 1 was close the Lez side and line 8 was close to the roadside. As the plantation lines near the river Lez consisted of very few trees, we grouped lines 1 and 2 of the AF plot and lines 1, 2 and 3 of the FC plot for statistical analysis (Fig.1).

In 2014, the number of walnut trees was respectively 140 and 208 in the AF and the FC plots. All the trees were 4 m high pruned.

2.2 Diameter and height measurements of walnut trees

INRAE team monitored the growth of walnut trees by annual measurements of the diameter at breast height (DBH) since 1998 and an annual measurement of the total height since 1995. Height measurements (H) were carried out each year between November and April, using a laser dendrometer or a pole. The data was analysed until 2014 because from 2015 onwards the height of the trees was too high to allow for a sufficiently accurate measurement. In addition, since 2017, measurements of walnut trees are no longer done regularly. For each tree, the height to DBH (H/DBH) ratio (m.cm^-1) has been calculated until 2014.

2.3 Statistical analysis

Statistical analysis were performed with the free R studio software, version 4.0.4. They were considered significant at p-value ≤ 0.05. We used the non-parametric Wilcoxon test to compare the height and diameter growths and the ratio H/DBH of walnut trees depending on the plot and the type of fertilization. The comparisons of growths within the same plot (plantation line) and of the water table levels between the piezometers on the two plots were performed using a non-parametric Kruskall-Wallis test.

3.1 Trees dimensions

The growths in diameter and height of the agroforestry walnut trees (AF) were both greater than those of forest controls trees (FC). These differences are significant immediately from the first years of planting (Fig.2a and Fig.2b) (p-value < 0.001). In 2014, the average height of AF walnut trees was 13.20 m compared to 11.20 m for FC walnut trees (p-value < 0.001). The average diameter of AF walnut trees was also significantly greater than that of FC walnut trees, with average DBH values of 27.48 cm and 18.12 cm, respectively in 2014 (p-value < 0.001).

The monitoring of growth in the control forest plot indicated no significant difference between the mean height of the fertilized and unfertilized FC walnut trees since 1999 (Fig.3a). Similarly, the effect of fertilization on the diameter of FC walnut trees was not significant (p-value = 0.14) (Fig.3b). This growth dynamic remained identical after the cessation of fertilization (in 2010) on the FC plot. In 2014, the mean height and diameter of fertilized FC walnut trees were 11,54 m and 17,70 cm, respectively, compared to 12,00 m and 15,30 cm for unfertilized FC walnut trees, respectively (p-value = 0.39).

There is a progressive decrease in the H/DBH ratio from 1998 to 2014, the ratio being significantly higher in the FC plot than in the AF plot (p-value<0.05 for all years) (Fig.4).

3.2 Effects of the distance to the Lez river and the height of the water table

A significant decrease in the height of the walnut trees was observed when moving away from the western edge of the plots, along the Lez river (Fig.5a). On the AF plot, the average height of walnut trees decreased from 15.90 m on lines 1-2 near the Lez river, to 12.00 m on line 8 the farthest ones. In the FC plot, the average height of the walnut trees decreased much more, from 15.4 m to 7.40 m from lines 1-2-3 to line 11 (Fig.5a).

Regarding the average diameters of the walnut trees, we again observed a progressive and significant decrease in the average diameter when moving away from the Lez river. The average diameters decreased from 23.90 cm (lines 1.2.3) to 13.20 cm (line 11) in the FC plot (Fig.5b). On the other hand, in the AF plot, the decrease of the diameter seemed less pronounced: only lines 7-8 and 1 were significantly different, with average diameters of 32.10 cm and 24.50 cm respectively (Fig.5b). The average diameter of the walnut trees in the other lines was however not significantly different from each other.

Although the two plots (AF and FC) were very close to each other, an analysis of the water table depth showed a great disparity between these two areas. The monitoring of the water table in both AF and FC soils indicated that the annual mean values of the water table depth was higher for FC (depth water table=151 cm) than for AF (depth water table=323 cm) (Fig.6). Between 2002 and 2014, fluctuations in the water table depth of both plots showed similar dynamics, with increases and decreases in the water table depth at the same times of the year, although the water table depth was always significantly higher in the FC plot (p-value < 0.01). These variations are seasonal, with a general decrease in spring and summer followed by an increase in autumn and winter. However, there was a very strong decrease in the water table depth in the AF plot after summer 2006 until winter 2007, which was not the case in the FC plot (Fig.6). Furthermore, there was no difference in water table depth within the FC plot, with both piezometers values being similar whatever the season was (p-value = 0.051). In the AF plot, on the other hand, there were significant differences with a higher water table depth in the North extremity (p-value < 0.001).

The results obtained for diameter growth are in agreement with literature results. They indicate that tree diameter growth increased with decreasing planting density (Jiang et al. 2007; Lasserre et al. 2009). Other studies of tree growth in agroforestry also reported greater secondary growth in agroforestry context than in forest controls (Cabanettes et al. 1998).

Concerning height growth, our results are not in agreement with other studies that reported that agroforestry trees are often smaller than in traditional forest areas (Cabanettes et al. 1998). However, some other studies reported that agroforestry trees could be taller than their forest controls (Balandier and Dupraz 1998; Thomas et al. 2020). In 1995, in the region of Puy de Dôme (France), INRAE designed an experimental setup to study the growth of hybrid walnut trees (NG38xRA) in case of standard walnut planting and in case of accompanying vegetation (with olive trees). The authors observed that growths in height and diameter were both impacted by the planting density (Balandier et al. 2005, 2008): Accompanied hybrid walnut trees were taller and had a smaller diameter than those planted without accompanying trees and therefore at a lower density. These results are consistent with the “thigmomorphogenetic” literature as the trees most exposed to the wind were smaller and wider than those accompanied; and therefore more protected from the wind (Bonnesoeur et al. 2016; Niez et al. 2018). In agroforestry conditions, trees being usually planted with low density, this leads to two major changes in these environments: a greater exposure to the wind and an increase in the light available to the trees. Therefore, their stems experience higher and more frequent mechanical deformations. In this configuration, the thigmomorphogenetic process is much higher for agroforestry trees compared to those of forestry trees and could explain a large part of their higher diameter.

Each year the H/DBH ratio was significantly higher in the FC plot. There is a gradual decrease in this ratio from 1998 to 2014 in both plots. These results are consistent with a 1998 study by Telewski and Pruyn who found a decrease in the H/d ratio (where d was the diameter of the stem) in response to increased bending (Telewski and Pruyn 1998). In addition to these thigmomorphogenetic processes, the reduction of competition for light induced by the opening of agroforestry environments is involved. In forest plantations, this reduction results in a reduction of tree height and an increase of tree circumference (Aussenac 2000); which is consistent with the results obtained by Cabanettes et al. (1998). In our system, even if AF trees were taller than FC one, the H/DBH ratio was lower for the AF walnut trees than for the FC (Fig. 4) ones; suggesting a strong thigmomorphogenetic response to wind-induced mechanical stress.

Other parameters could also explain the greater height of the AF walnut trees in the Restinclières system. First, the annual fertilization of the crops on the AF plot could be involved in the greater primary growth of the AF walnut trees, which would have benefited from the surplus of inputs. However, the fertilization of part of the control walnut trees on the forest plot showed that despite this fertilization, the fertilized FC walnut trees were still smaller than the AF walnut trees.

Second, the presence of alders in the FC plot may also have slowed down the growth of walnut trees, despite the benefits of a nitrogen-fixing species. Indeed, Thomas et al. (2020) in their study on the growth of poplars in agroforestry environments, found that in the forest mixture, poplars associated with alders were smaller than in their monoculture. Finally, the higher water table depth in the FC plot could cause a soil waterlogging during the winter period, which could partly explain the weaker growth of FC walnut trees, hybrid walnut trees being known to be sensitive to this factor. FC plot may then have suffered from waterlogging, generally leading to high root mortality (Dupraz and Liagre 2008). The death of part of the walnut trees root system at the beginning of their growth could therefore largely explain the lower heights of forest walnut trees at Restinclières (Talbot 2011). In addition, the competition between crop and tree root systems promotes the development of deep tree roots (Mulia and Dupraz 2006; Cardinael et al. 2015). This anchoring in the deep soil horizons would allow the trees to take up water and redistribute it to the superficial horizons especially when the latter are dry (Jose et al. 2004), such as in summer periods (Pierret et al. 2016). In our AF plot, the possible deeper rooting of agroforestry hybrid walnut trees could promote tree growth by improving their access to water and nutrients during summer drought. This hypothesis of water availability could also partly explain the higher stem diameter of these trees. Since, the water table depth in the FC plot probably limit root development at depth during winter period, trees have a limited access to water during summer period.

Another natural explanation could have been the distance to the Lez river since it seems to had a significant effect on the primary growth of trees in both plots. The trees planted close to the river were significantly taller than those planted at the opposite side were. Although, the observation of the different piezometers did not show a significant difference linked to the Lez distance. The difference of growing kinetic from West side to East side must be related to soil or depth nature. Soils characterizations of AF and FC plots were carried out by Cardinael et al. (2017). They highlighted that the Soil Organic Carbon stock was significantly greater at a depth of 30 to 50 cm in the FC plot than in the AF one (Cardinael et al. 2017). Overall, this past study showed the potential of agroforestry systems to store C in both soil and biomass in temperate regions. Such a soil characteristics difference between AF and FC plots could affect the tree growing kinetic.

After 19 years of growth, the agroforestry walnut trees are both taller and wider than the forest trees. These larger diameters in the agroforestry environment are consistent with the usual negative correlation between planting densities and the tree diameter increases. This effect is well known in sylviculture contexts and results from the combination of a deeper root development, better access to light and nutriments, and higher mechanical deformations due to the wind.

Assuming the thigmomorphogenesis hypothesis and the reduction of light competition, the height of the agroforestry walnut trees was expected to be lower than for the forest trees, which is not the case here since the agroforestry trees were taller. This effect of thigmomorphogenesis, which tends to reduce the height of the trees, seems to be compensated by the effect of the plantation characteristics, which provides more light and nutrient resources, which would explain why the agroforestry trees in our study are larger and taller than their forest controls. This probably suggests that the thigmomorphogenetic impact on height is low compared to the other environmental factors like the water table height or soil nature.

Thus, although our results indicate that agroforestry walnut trees produced more biomass at their stems level (especially because of their wider stems) than the forest trees do our study highlights the difficulties encountered when comparing these two systems in order to evaluate the impact of agroforestry practices on tree development. It highlights the complexity of creating strictly identical agroforestry and forestry experimental systems in terms of soil type, access to water and wind exposure.

Funding

This research was funded by the “Fondation de France” as part of its 2019 call for projects "Research on agroforestry".

Acknowledgments

We would like to thank INRAE (UMR ABSys) for sharing and providing access to their plots in Restinclières. The authors also gratefully acknowledge the “Fondation de France” association for its financial support attributed to Lucie Heim for her PhD thesis carried out at the “Arts et Métiers Institute of Technology”.

Availability of data and material

The datasets generated and/or analyzed during the current study are available on the INRAE data portal with the following references: https://doi.org/10.15454/KSB73Ga and https://doi.org/10.15454/KSB73G.

Ethics approval and consent to participate

Not applicable.

Consent for publication

All authors gave their informed consent to this publication and its content.

Competing interests

The authors declare that they have no conflict of interest.

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Comparison of hybrid walnut trees growth in an agroforestry system and under forestry condition

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Abstract

Figures

1. INTRODUCTION

2. MATERIALS & METHODS

2.1 Study site

2.2 Diameter and height measurements of walnut trees

2.3 Statistical analysis

3. RESULTS

3.1 Trees dimensions

3.2 Effects of the distance to the Lez river and the height of the water table

4. DISCUSSION

5. CONCLUSION

Declarations

References

Additional Declarations

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