Improving the Sustainability of Pollarding in Multifunctional Agro-Forestry Plantations

Abstract

Pollarding is an ancient agroforestry practice that greatly contributes to the sustainability of farming but is slowly becoming extinct because traditional pollards are not viable from a financial and social viewpoint. In particular, the cutting of pollards is too slow, expensive and dangerous for modern farmers to apply. This study presents the first test of mechanized pollarding, performed with two different devices: a set of shears and a disc saw. Both devices were fitted to the boom tip of a tracked excavator and were tested on poplar rows in a typical alley-cropping system. The introduction of those simple devices restored productivity and safety to pollarding as a modern practice. Tree topping incurred a cost around 1 € tree⁻¹, or 250–350 € ha⁻¹. This cost would need to be balanced against the revenue obtained from the treetops sold as biomass and the increased yields of the alley crop, prolonged for several years. Mechanization also allows cutting the treetops several metres above ground level, so that the trunks of the pollarded trees may yield valuable timber when they are eventually harvested.

1. Introduction

Pollarding is a specific coppice system whereby resprouts originate from artful cuts performed two or more metres above ground level [1]. The main reason for pollarding is to train the new sprouts above the reach of livestock, which can be introduced to the same field right after harvest without any risk for browsing damage [2]. For centuries, pollarding has been one of the most popular strategies of European agroforestry [3,4] and represented a key element of subsistence farming [5].

Pollarding is a very ancient practice: its origins have been traced to the development of animal husbandry, approximately 4000 years ago in the mountains of northern Iran [6]. Pollarding was devised as a strategy for the effective combination of livestock farming and wood production [7], and its success can be judged by its rapid spread and prolonged use [8]. In many cases, pollards became a characterizing element of the cultural landscape, as witnessed by such famous masterpiece as “Landscape with Pollard Willows” painted by V. Van Gogh in 1882. Until the last decades, pollards remained a common sight in places as far apart as the Basque country, the Pyrenees, Sicily and Crete [9,10]. Then, the transition towards industrial agriculture caused a steady decline of pollarding, which is now on the brink of extinction [4].

However, even industrial agriculture may fall behind the increasingly complex requirements of economic development, which now places multiple competing demands on the agrarian landscape: food security, energy production and a whole range of ecosystem services [11]. As a result, the integration of trees and agriculture is gaining renewed interest as a viable agroecological solution to sustainable intensification [12]. The old systems must adapt to the new goals and methods of modern societies, but a major revival of agroforestry is within reach. Agroforestry could become a key element in increasing the sustainability of farming, because it makes full use of the many benefits offered by any given field. If the same area becomes capable of providing food, timber, livestock and amenity at one time, then resource use efficiency increases, and farming achieves a higher level of sustainability.

A key factor for the successful integration of trees and conventional agriculture is canopy management, given that the crowns of maturing trees will progressively cover the underlying agricultural crops and eventually suppress them. In that case, pollarding can be used to restore light availability [13], thus offering an essential contribution to the development of efficient agroforestry systems [14]. The revival of pollarding as a modern coppicing technique depends on the capacity of developing new work methods that can match the current high requirements for work safety and cost-effectiveness [15]. Traditional methods based on manual work are unable to match those requirements: cutting large branches while perched on a ladder is neither fast nor safe. This deprives pollarding of social sustainability even more than financial sustainability. The mechanization of pollarding is crucial to sustainability and may come in the simple shape of a conventional feller, that is: a tree cutting device mounted on the tip of a mechanical boom. The simplest such machine consists of a tracked excavator equipped with a shear or a saw head. The introduction of such a machine offers three main advantages. First, it makes work safe, because operators sit comfortably inside a safety cab, protected against the risk of falling stems and of falling themselves; second, it makes work much more productive, because the time to cut a branch is dramatically reduced, and the wood can be cut and stacked at once using the mechanical boom; third, it enables pollarding trees higher than two metres, because cutting height is not limited by the reach of a portable ladder. A mechanical boom can extend at least 6 m above ground level, so that topping the tree at a height of 5 m is relatively easy. By extending cut height one can add a further product to the mix: the tree trunk, from which one can obtain valuable timber when the pollards are renewed. Timber production is a strategic goal of the European Union, and the new pollarding systems would then contribute to it [16].

This new system is now being tested on fast-growing poplar tree plantations, where the intercropping of poplar and cereals (rice or wheat) is common practice [17]. This new pollarded variant has been installed at several experimental farms, with plans to pollard the poplars every four years, then cut them at the base between the 12th and the 16th year, prior to replanting [18].

The goal of this study was to obtain science-based data on the productivity, the cost and the work quality of mechanized pollarding, performed at a height of 5 m with modern excavator-based fellers. The study explored the two main technology options available on the market, namely: shears and disc saw. The hypothesis underlying the technology comparison is that of no significant difference in the productivity, cost and work quality between shears and disc saw (null hypothesis).

2. Materials and Methods

A two-day trial was conducted on 16 and 17 December 2021. The trial was conducted at the Sasse Rami farm, located in Ceregnano near Rovigo, an area with a long tradition in poplar farming (Figure 1). The farm is owned by the Regional Authority for Veneto and managed by Veneto Agricoltura, the Regional Agency for farming research and extension services.

The plot marked for the experiment was placed near the farm centre (45°03′05.92″ N; 11°52′49.90″ E); it measured 0.22 ha and contained 121 poplar trees, set at a spacing of 3 m × 6 m. The plantation had been established in 2014 with 2-years old cuttings of the hybrid clone I-214, one the oldest, most successful and most widespread poplar clones. The poplars formed three parallel rows with a length of approximately 130 m each; the rows were divided into two equal segments, and three half rows were assigned to each of the two technologies, in an alternate pattern. On the first row, the shear took the half nearer to the access, while the disc took the other half; on the second row the order was reversed; on the third row the order was repeated as for the first row. This arrangement was considered as the best solution for an even spread of eventual site gradients, while mitigating the error possibly introduced by the overly complex operational environment that would have been created by a purely randomized design.

The two technologies on test were a 22-ton tracked excavator fitted with a double-blade shear head, and a 13-ton tracked excavator fitted with a disc-saw head (Figure 2). They represented the shear and the disc treatment, respectively. The shear head was connected to the excavator boom tip through an active joint, while the saw-head was a dangle type. Both machines were operated by professional operators, who had the necessary specific qualification and experience. Both had been operating their respective machines for >3 years, performing thousands of cuts.

Before starting the trial, all trees were attributed ID numbers, which were written on their trunks using high-visibility paint, for easy identification during cutting. A calliper was used to determine the diameter at breast height (DBH) of all trees with the accuracy of 0.5 cm. The diameter at 5 m was estimated by assuming a taper of 0.6% [19], given the difficulty of taking a reliable direct measure of diameter at that height.

During the trial, a researcher recorded the time taken to cut the top off each tree, and to lay it down on the ground [20]. Treating one tree was the observation unit and the time required to do that was divided into the following tasks: moving the machine from one workstation to the next; extending the boom and grabbing the top to be cut with the grapple arms; cutting the top with the shears or the disc; laying the cut top on the ground. The short descriptors for those tasks were, respectively, “move”, “grab”, “cut” and “dump”. Delay time was kept separate and excluded from the analysis, given that the short observation time could not offer a fair representation for its actual impact. A 25% delay factor was used, instead [21]. Work time was recorded using an iPad equipped with the dedicated software Laubrass UMT Plus 2.0 (https://laubrass.com/solutions/umt-plus/, accessed on 1 December 2024). All time records were associated with the respective tree IDs.

Machine costs were calculated with the method developed within the scope of COST Action FP0902 [22]. Costing assumptions were obtained directly from each individual machine owner, or from the manufacturers. Labour cost was assumed to be 20 € per scheduled machine hour (SMH), according to the approved regional price lists (

Table 1. Costing: Assumptions, estimates and total cost.

Treatment		Shear	Disc
Investment—Excavator	Euro	150,000	150,000
Investment—Head	Euro	30,000	45,000
Investment—Total	Euro	180,000	195,000
Resale	Euro	54,000	58,500
Service life	Years	10	10
Utilization	h year−1	1000	1000
Interest rate	%	3	3
Depreciation	Euro year−1	12,600	13,650
Interests	Euro year−1	3699	4007
Insurance	Euro year−1	2500	2500
Fuel and lube	Euro year−1	23,760	31,680
Repairs	Euro year−1	6300	6825
Total	Euro h−1	48.9	58.7
Crew	n.	1	1
Labour	Euro h−1	20.0	20.0
Overheads (20%)	Euro h−1	13.8	15.7
Total rate	Euro h−1	82.6	94.4

Notes: h = scheduled machine hour, including delay time; Cost in Euro (€) as on 1 December 2024.

).

Work quality was estimated based on the conditions of the cut surface and the vigour of resprouting. For that purpose, all trees were inspected one year after pollarding, when one researcher on a lifting platform visually inspected the surface of all cuts to determine the presence of splits and whether the cut end was live or dry. In the latter case, the length of the dried stem portion was also measured. Furthermore, the researcher counted the number of live shoots on the cut and the diameter of the three largest shoots taken at 30 cm from their insertion, with the accuracy of 1 mm [23].

Data analysis consisted in the extraction of simple descriptive statistics, aimed at providing solid indicators for centrality and variability. Then, the statistical significance of the differences between the two treatments was checked using non-parametric statistics, which are robust against any eventual violations of the statistical assumptions. The statistical significance of any differences in the distribution of different cutting quality classes between the two treatments was assessed using the χ² test. Finally, regression analysis was used to check the statistical significance of any trends. The chosen significance level was α < 0.05.

3. Results

The DBH of pollarded trees varied from 9 cm to 33 cm, with a mean value at 19.6 cm (median = 19 cm). However, the largest number of trees had a DBH between 17 and 22 cm (interquartile range). Despite all efforts to achieve an even distribution of tree size between treatments, the trees negotiated by the disc saw were significantly larger than those handled by the shears (

Table 2. Time consumption, productivity and cost: summary table.

		Shears (n = 53)			Disc (n = 68)
		Mean	SD	Median	Mean	SD	Median	p Value
DBH	cm	18.5	5.5	18.0	20.3	3.9	20.0	0.0215
Move	s	8.6	6.6	7.0	9.8	7.3	9.0	0.3852
Reach	s	12.6	7.7	11.0	12.7	6.0	10.0	0.5680
Cut	s	6.1	5.3	5.0	3.4	1.9	3.0	<0.0001
Dump	s	9.3	4.9	8.0	6.7	4.0	6.0	0.0006
Total	s	39.1	16.1	35.0	33.8	10.8	32.5	0.1426
Productivity	Trees PMH−1	105	35	103	117	39	111	0.1426
Productivity	Trees SMH−1	87	29	86	98	33	92	0.1426
Cost	€ Tree−1	1.08	0.44	0.96	1.07	0.34	1.02	0.6049

Notes: n = number of observations (i.e., work cycles); DBH = Diameter at breast height; PMH = productive machine hour, excluding delay time; SMH = scheduled machine hour, including delay time; p Value = estimated according to the Mann–Whitney non-parametric unpaired comparison test.

). Of course, the cut was performed almost 4 m above DBH, and therefore the diameter actually cut was much smaller: the mean diameter actually cut was 15.6 cm and 17.0 cm, for the shears and the disc saw, respectively.

The entire topping cycle took less than one minute per tree, including delays, regardless of machine type, which corresponded to a productivity slightly below 100 trees per scheduled machine hour (SMH). Compared with the shears, the disc-saw was significantly faster when cutting and when laying down the cut top, but not when moving along the row and reaching out to the treetop (

Table 3. Resprouting vigour as a function of cut quality (above) and treatment (below).

Cut Quality		Sound Stub			Split Stub
		Mean	SD	Median	Mean	SD	Median	p Value
Shoots	n	13.8	6.9	12.0	14.8	6.9	15.0	0.5675
Mean D	mm	32.9	7.4	33.3	33.9	8.8	34.0	0.9111
Max D	mm	39.3	9.2	39.0	39.6	9.9	38.0	0.9692
Dry	cm	54.4	31.6	42.5	63.1	42.5	50.0	0.8035
Treatment		Shears			Disc
		Mean	SD	Median	Mean	SD	Median	p Value
Shoots	n	14.2	6.4	12.5	13.9	7.5	12.0	0.6412
Mean D	mm	33.1	7.8	32.8	33.2	7.7	33.5	0.8074
Max D	mm	39.7	9.6	38.0	32.1	17.1	37.0	0.0945
Dry	cm	56.5	39.6	40.0	60.8	25.4	50.0	0.1858

Notes: SD = Standard Deviation; n = number of shoots; Mean D = Mean of the diameters of the three largest shoots on the pollard; Max D = Largest of the three diameters for the three largest shoots on the pollard; Dry = length of the dry butt, taken from the surface of the cut; p Value = estimated according to the Mann–Whitney non-parametric unpaired comparison test.

). Fast cutting is a well-known quality of disc-saws and therefore this finding is not surprising; it rather confirms the good quality and the general reliability of the stop watching sessions. In contrast, it is a bit harder to explain why the disc saw was also faster than the shears when laying down the cut tops. A most likely explanation is the smaller size of the excavator carrying the disc-saw. While both excavators were the same power and cost, the unit carrying the disc saw was a compact zero-tail swing model. It stands to reason that a compact machine may enjoy some advantage over a standard one, when working within a 6 m interrow, where it is constrained on both sides by standing trees. The alternative explanation is that of different operator skills and work techniques, which is also likely; yet, both operators had similar age, service experience and apparent competence, which may suggest they would perform quite similarly under the same operational conditions.

In any case, the variability introduced by the other work tasks was large and it offset any advantage gained by the disc-saw when cutting and laying down the treetops. Therefore, productivity was not significantly different between the two treatments, and neither was cost. In fact, the disc-saw was more expensive to operate, but the cost increase was not too large (15%) and was largely absorbed by its higher productivity (11%, although not significant). As a result, the topping cost was just a few cents higher than 1 € tree⁻¹, regardless of treatment.

Tree size had a very small effect on time consumption: regression analysis showed that the time to top a tree increased with tree DBH and was shorter when the disc saw was used, but this relationship was very weak (r² < 0.1), at least within the range of explored tree sizes. Both coefficients were highly significant (p = 0.0025 for DBH, p = 0.0069 for treatment), but the overall effect was modest: it took between 25 s and 35 s to top a tree with a DBH of 10 cm, and between 30 s and 40 s for a tree with DBH of 30 cm (Figure 3).

Cut quality was much lower for the shears, compared with the disc saw: over 40% of the cuts performed with the shears presented visible splitting, while no splitting was found on any of the cuts performed with the saw (Figure 4).

The occurrence of splitting did not seem to be associated with tree size, since the diameter of trees with splits on their cut surfaces did not differ significantly from the diameter of those without any splits (p = 0.51).

After one year of vegetation, pollards with a split top did not seem to have lost any resprouting vigour; there was no significant difference between sound and split pollards regarding the number of shoots and the size of the three dominant shoots (Table 3).

The drying of the stem below the cut surface was observed on 40% of the stems, under both treatments and for both cut qualities—sound or split. Drying was more frequent for split cuts than for sound ones (67% vs. 36%), although such difference was not significant for the χ² test.

4. Discussion

Pollarding can contribute to alley-cropping through sheltering and product integration, and therefore it has a definite role within agroforestry. Modern technology can restore financial and social sustainability to this ancient and valuable practice. The introduction of simple felling machines makes the operation fast and safe. Once mechanized, tree topping incurs a cost that can be estimated at approximately 1 € tree⁻¹, or 250–350 € ha⁻¹. In return, the farmer will obtain a certain amount of biomass and an increase in the yields of the alley crop, extended over several years. Mechanization also allows cutting the treetops several metres above ground level, so that the trunks of the pollarded trees may yield valuable timber once they are renewed. In turn, structural use of the harvested timber is a better CO₂ sink than any of the fast-cycling traditional uses (e.g., energy, consumable handles or ties, etc.) and determines an increase in the sustainability of pollarding as an agroforestry practice.

The absence of any relationship between splitting and diameter suggests that the cause of splitting may not be the compression exerted by the shear mechanism, which is proportional to cut diameter and should result in an increase in splitting events with tree diameter—which was not the case here [24]. Therefore, a more likely cause of splitting might be the tension applied to the cut stem by the shear head. This machine is designed to perform the cut only after the top has been grabbed by the grapple arms, and therefore some pressure is applied to the stem before cutting. This is especially true for the model used in this trial, which was connected to the excavator boom through a stiff active joint that left no margins for free-floating adjustments. In contrast, disc-saw heads are designed in a way that the grapple arms firmly seize the cut stems only when the cut is completed, for the very purpose of avoiding that tension is applied to the stem during cutting and prevent splitting.

What are the consequences of splitting, then? One may imagine two main effects: weaker resprouting and wood decay [25]. In fact, the measurements taken one year after cutting did not bring any evidence of the former, and could not shed any light on the latter, which would take some time to show, if it did. A possible indicator of wood decay could be the drying of the stem below the cut surface, but the frequency of drying was not significantly associated with split damage or treatment, and one might need to look somewhere else for its probable cause—possibly bud position relative to cut. Furthermore, until one cuts the trunks and opens them, one cannot know if trunks with dry ends are more likely to present wood stain or decay, compared with the other trunks that do not show any drying, but just live scar tissue. This is definitely one of the main subjects of the new research planned in Ceregnano.

The introduction of relatively heavy machines (up to 20 t) also raises the question of soil disturbance and its consequences on water infiltration rates, soil biodiversity and tree root health—among others. We did not conduct any measurements to accurately determine soil disturbance, but visual inspection did not find visible signs of soil impacts. As a matter of fact, both machines were tracked and therefore, they spread their weight on a large footprint, thus exerting low ground pressure. Furthermore, they only performed a single pass, not repeated passes on the same trail. Finally, the agroforestry system proposed in the pilot plantation is based on the association of a tree crop with an annual farm crop, and therefore the soil would be regularly tilled at each sowing. This would remedy any potential compaction, especially if it is moderate, as it could be expected in this case.

Another question concerns the mass of treetops, which may be turned into a biomass product and sold to recoup part of the pollarding cost. However, that was not determined in this study and should be addressed in future research. In any case, that amount would be independent from the cutting method being applied and would not affect the comparison between the two technologies on test.

The main limitation of this study is the small scale of the test, which was performed on one stand only, and with one single operator per machine. This was the result of resource limitations, since the budget available was limited and the fields suitable for the test were very few. As a consequence, this study can only offer a preliminary contribution and its conclusions cannot be generalized without much caution, despite all our efforts to select a representative stand and representative professional operators. This is why the data collection technique was deliberately simplified: it did not make sense to analyze the fine details, if those details would be obscured when trying to make a general prediction. This also explains the cursory statistical analysis of data: elaborate analytical techniques would have generated a deceiving sense of confidence in a prediction that was approximate by its own nature. On the other hand, our study offers robust preliminary figures, supported by a good estimate of data variability.

Clearly, the present study is far from perfect, and yet it is the only one available so far on the technical and financial performance of pollarding—mechanized or otherwise. However preliminary and approximate, the knowledge obtained from this study represents the only solid foundation available to date for analyzing and hopefully improving the financial sustainability of a valuable tradition that can accrue strong environmental benefits through the multiple ecosystem services provided by pollarded trees [26]. The technology proposed is relatively simple and affordable; excavators can be found everywhere in the world and can be purchased second-hand at budget prices, if investment capacity represents a main hurdle. They can also be used in a variety of tasks, for better depreciation. The cost of a felling head is quite variable, but the simplest versions are also inexpensive. Furthermore, a felling head is simple enough that it can be maintained at relatively little cost and can be acquired second-hand (and eventually refurbished) with a moderate expense. For that reason, the technology we propose is widely applicable to many regions, even where economic conditions are not the most favourable. Large scale implementation of the proposed system is both feasible and desirable, since the system has a very large work capacity and any increase in the scale of the operation will result in a reduction in logistic and administration cost. Furthermore, implementation on a large scale would favour the efficient recovery of the cut treetops, which may not be economically viable on small operation due to the limited amount of material potentially recovered; any expansion of operational scale will further motivate biomass recovery, with meaningful benefits in terms on financial and environmental sustainability.