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Timing of resin-tapping operations in maritime pine forests
in Northern Spain
Roberto Touza1, Margarita Lema2, Rafael Zas3*
1Extracción de Resina. Lugar de Maúnzo 25 Xeve 36150 Pontevedra. 2Department of Functional Biology, Faculty of Biology, Universidad de Santiago de
Compostela, Spain. 3Misión Biológica de Galicia. Consejo Superior de Investigaciones Cientícas. Apdo 28, Pontevedra, 36080, Spain.
Abstract
Aim of study: To optimize the timing of resin-tapping activities for maximizing the economic eciency of resin tapping in Atlantic
maritime pine forests.
Area of study: Northern Spain.
Materials and methods: We conducted three small experiments in a mature maritime pine forest aimed to test: i) the impact of groove
frequency on resin production, ii) the eect of previous grooves as a driver of temporal patterns of resin production along the seasons and
iii) the impact of previous tapping on resin production in the following campaign.
Main results: The resin produced decreased as groove frequency decreased, but the reduction was low. Considering that the number of
trees that a worker can tap increases with more spaced grooves, higher tapping eciency can be achieved with monthly grooves. Previous
tapping increased resin yield during the following campaign but resin production was not aected by the previous grooves during the current
tapping campaign.
Research highlights: Responses to wounding seem to require time to be eective and temporal patterns of resin production appear to be
driven by weather conditions alone.
Keywords: resin yield; Pinus pinaster; seasonality; induced responses; wounding.
Authors’ contributions: RT, ML, and RZ conceived the idea, designed the experiments and provided the founding. RT performed the
eld work and processed the datasheets. ML and RZ carried out the statistical analyses. All authors contributed to the writing through suc-
cessive revisions of the text.
Citation: Touza, R., Lema, M., Zas, R. (2021). Timing of resin-tapping operations in maritime pine forests in Northern Spain. Forest
Systems, Volume 30, Issue 3, eSC05. https://doi.org/10.5424/fs/2021303-18414.
Received: 02 May 2021. Accepted: 21 Sep 2021.
Copyright © 2021 INIA. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 Interna-
tional (CC-by 4.0) License.
Competing interests: The authors have declared that no competing interests exist.
Correspondence should be addressed to Rafael Zas: rzas@mbg.csic.es
Forest Systems
30 (3), eSC05, 6 pages (2021)
eISSN: 2171-9845
https://doi.org/10.5424/fs/2021303-18414
Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA)
OPEN ACCESS
SHORT COMMUNICATION
Funding agencies/institutions Project / Grant
Regional Government (“RESDRON”, Xunta de Galicia/FEADER) Feader2018/066B
Spanish Government (“ACREMA”, MAPA/AEI-Agri/FEADER, UE) O00000226e2000043659
Introduction
Resin tapping of pine forests does not only provide
valuable economic complements (Susaeta et al., 2014),
but also high-value and broad-scale ecosystems services
(e.g. re prevention and surveillance (Solino et al., 2018),
promotion of rural employment and retention of local po-
pulations (Justes & Solino, 2018), recreation and people
welfare (Heinze et al., 2021), substitution of petroleum
derivatives by renewable bioproducts in the industrial
sector (Rodrigues-Correa et al., 2012)).
Given the multiple direct and indirect benets, resin
tapping activities should be seen as a key tool to rekindle
many pine plantations of southern Europe with low tim-
ber protability and high risk of wildres. However, the
cost eciency of resin tapping is low and often below the
economic minimum for eld workers (Justes & Solino,
2018). In addition, the high volatility of resin price in the
markets, strongly linked to that of petroleum, adds great
uncertainty to the sustainability of resin tapping (Heinze
et al., 2021). This has resulted in drastic swings of the
surface tapped and the number of resin workers in Euro-
pean pine forests during the last decades (Ortuño Perez et
al., 2013).
Two main lines of research are nowadays aiming to
overcome these drawbacks through the quantication of
ecosystem services searching for compensatory measures
(Solino et al., 2018) or the optimization of management
2Roberto Touza, Margarita Lema and Rafael Zas
Forest Systems December 2021 • Volume 30 • Issue 3 • eSC05
operations for maximizing the eciency of working ope-
rations (e.g. groove mechanization (Rodríguez-García et
al., 2016), development of new chemical stimulants (Mi-
chavila Puente-Villegas et al., 2021), genetic improve-
ment (Vázquez-González et al., 2021).
In the present study, we focus on two factors that can
strongly determine the eciency of resin tapping opera-
tions but have received comparatively less attention: the
tapping frequency and the variation of resin production
throughout the season. In Spain, resin tapping has been
traditionally concentrated in the Central Plateau on ma-
ture maritime pine forests growing on sandy and poor
soils under Mediterranean climates with dry and hot sum-
mer seasons. Conventional resin tapping is performed by
doing fortnightly grooves and applying a sulfuric-acid-ba-
sed stimulant paste (Rodríguez-García et al., 2016). To
our knowledge, this periodicity has little experimental
support despite directly determining the number of trees
that a worker can process within a campaign. With more
spaced grooves, a single worker could tap signicantly
more trees with the same eort, maximizing, thus, the
protability of the labor.
Another important point that deserves attention is when
and where a pine forest could be tapped. Nowadays, resin
tapping is being incorporated in timber-oriented maritime
pine forests growing under Atlantic climates with much
more humid and fresh summer seasons, and there are
some doubts about whether the tapping period should be
adjusted for this new climatic scenario (Zas et al., 2020a).
Previous investigations have shown pronounced seasonal
patterns of resin production in these Atlantic areas, with
a peak in summer (Zas et al., 2020a) associated to the
increase of mean temperature and water decit (Zas et al.,
2020b). However, seasonal patterns of resin production
are not only determined by weather variation, but also
they can be largely inuenced by the induced responses
of pines to previous and successive wounding (Hood &
Sala, 2015; Rodríguez-García et al., 2014). To avoid con-
founding these two sources of variation, dierent starting
points (within the season) of resin tapping should be com-
pared (Zas et al., 2020a).
Here, we conducted three small experiments aimed to
i) quantify the inuence of groove periodicity on resin
yield and ii) determine the inuence of previous grooves
(within the same campaign and across campaigns) on re-
sin yield along the tapping period. The experiments were
conducted on a mature maritime pine forest located close
to the Atlantic coast of northwestern Spain.
Material and Methods
The study was conducted in a naturally-regenerated
mature maritime pine (Pinus pinaster Ait.) forest loca-
ted in Campo Lameiro, Galicia (NW Spain, 42.5566°N,
8.54392°W). The stand has a tree density of 652.5 ± 43.4
trees ha-1 (mean ± s.e.) and a basal area at breast height
of 36.4 ± 2.2 m2 ha-1. Trees were around 25 years-old,
16.5 ± 0.29 m tall and 26.5 ± 0.8 cm in diameter at breast
height (dbh) at the time of experimental setup. Clima-
te in the area is humid and mild, with 13.4 °C of mean
annual temperature, around 1600 and 150 mm of annual
and summer (July-September) precipitation, respectively,
and low temperature oscillation between the mild winters
and the fresh summers (average climate obtained from
www.meteogalicia.gal for the period 2007-2020). Three
independent experiments were conducted between June
2019 and October 2020.
Experiment 1 was designed to test the eect of three
dierent tapping frequencies (weekly, biweekly and
monthly grooves) on resin yield. Twenty-four neighbo-
ring trees were selected within the stand and randomly
assigned to one of the three frequencies. Starting on
February 5, 2020 all the experimental trees were resin
tapped following the conventional method commonly
used in central Spain (Rodríguez-García et al., 2016) but
varying the frequency of the grooves. In all cases, 12-cm
wide and 3-cm high striped wounds (“grooves”) were pe-
riodically made on the southern side of the trunk moving
upwards. Mechanical wounds were enhanced by applying
a strip of stimulant paste in the upper-inside border of the
practiced wound. The paste used is that commonly used
for resin tapping in Spain and is based on sulfuric acid
(see details in Zas et al., 2020a). A total of 8, 16 and 30
grooves were practiced on monthly, biweekly and weekly
frequencies, respectively, with all treatments ending at the
same moment (October 10, 2020). In all cases, wounding
was started at 25 cm from the ground and ended at around
50, 75 and 115 cm for monthly, biweekly and weekly fre-
quencies, respectively. For all treatments, resin ow was
collected in pre-weighed 2-L pots every two weeks (i.e.
just before the biweekly grooves were applied) and the
amount of resin determined gravimetrically (0.1 g).
In Experiment 2 we tested whether previously tapped
trees on the preceding campaign produce more resin than
previously untapped trees. To this end, 16 trees conven-
tionally-tapped during spring-summer of 2018 and 16
nearby untapped trees were selected. All these trees were
tapped following conventional procedures applying bi-
weekly grooves (same details as before) during a whole
year (June 2019 – June 2020). The resin ow exuded after
each groove was determined every two weeks.
Experiment 3 was established to test whether previous
wounds inuenced the resin produced along the resin
campaign or, alternatively, immediate resin ow is only
dependent on tree phenology and weather conditions.
Thirty two previously un-tapped trees were selected and
randomly assigned to each of four treatments correspon-
ding to four dierent dates of initiation of resin-tapping
(10 June 2019, 5 February 2020, 2 April 2020 and 2 June
Forest Systems December 2021 • Volume 30 • Issue 3 • eSC05
3
Timing of resin-tapping
2020). In all cases, trees were tapped since each initial
date, applying biweekly grooves and the same procedure
described before. Resin ow produced after each groove
was collected and weighted every two weeks during the
common tapping period for all treatments (2 June 2020 to
10 October 2020; 8 grooves in total).
In all experiments, immediate resin ow exuded every
ca. 14 days was analyzed with a repeated measures mixed
model in which the treatments (groove frequency (three
levels: every 1, 2 and 4 weeks), previous tapping (two
levels: tapped, untapped) and date of initiation (four le-
vels: Jun19, Feb20, Apr20, Jun20) for experiments 1, 2
and 3, respectively) acted as xed across-subject factors
and the time and the time × treatment interaction as repea-
ted measures within each subject (the individual trees). In
addition, total resin yield across the tapping campaign (or
the common period in experiment 3) was analyzed with
a one-way ANOVA. Dependent variables were log-trans-
formed (log(x+1)) if needed to achieve normality assump-
tions. Heterogeneous residual variances across treatments
were allowed if they signicantly improved the likelihood
of the models.
Results and Discussion
Tapping frequency
Periodicity of grooves signicantly aected total resin
production after a whole tapping campaign (F2,21 = 3.98,
p = 0.034). The more frequent the grooves the more resin
was produced, although dierences were only signicant
between monthly grooves and the other two tapping inter-
vals (weekly and biweekly) (Fig. 1).
Despite the signicantly lower resin production of
monthly grooves, relative resin production eciency was
around 40% higher for monthly grooves than for the con-
ventional biweekly grooves (Fig. 1). In other words, the
reduction in resin production with more spaced grooves
was largely compensated by the fact that a greater number
of trees could be processed if the interventions are less
frequent. This result is of remarkable applied relevance
as, given the current prices of resin in the market and the
economic needs of eld workers, the protability of resin
tapping activities in Spain is strongly compromised by the
eectiveness of the daily labors (Justes and Solino, 2018).
In addition, more spaced grooves imply lower number of
traumatic wounds along a resin campaign on a given tree,
and thus reduced impact on the trees (e.g. growth, wood
quality, protection from re damage, etc) (Génova et al.,
2013; van der Maaten et al., 2017), or increased number
of years that a tree can be tapped. The nal protabili-
ty of lower groove frequencies (e.g. 4 weeks) must be,
however, carefully estimated, as increasing the number of
working trees implies some extra eld works (e.g. initial
preparation of the trees by smoothing the bark), increased
movements of the workers within the stands, and the need
of more materials (pots, sheets).
We are not aware of previous published studies explo-
ring the resin production eciency of dierent frequen-
cy tapping interventions. Judging from the literature (e.g.
Heinze et al., 2021; Yi et al., 2021), groove frequency lar-
gely varies (ca. from every 2 days to every 3 weeks) depen-
ding on the species, the climate, the stimulant paste used
and, probably, the manpower costs. Results presented here
contrast with previous experimental ndings in Spanish
maritime pine forests, showing that resin ow decreased
exponentially with time since wounding, with most of the
resin ow occurring during the rst week post wounding
(Zas et al., 2020a). However, those results were obtained
through micro-tapping procedures and we cannot dis-
card that resin ow in conventional tapping is maintained
through longer periods because of reduced crystallization
and higher uidity of the resin produced on larger wounds.
Across- and within-years induction
Trees subjected to resin-tapping in the previous
year produced more resin than trees newly tapped,
although the dierences were only marginally signicant
(F1,30 = 3.1, p = 0.087, Fig. 2a). A previous campaign of
conventional resin tapping increased 41% the resin yield
in the following campaign (Fig. 2a). A closer look to the
Figure 1. Resin production (mean ±standard error, gray bars)
after a whole tapping campaign (February to October 2020) as
a function of the periodicity of the grooves (weekly, biweekly
and monthly). The relative eciency (%) in terms of nal yield
per worker for each periodicity in relation to the conventional
biweekly operations is also shown (circles, right axis). Relative
eciency was estimated assuming that the number of trees that
a worker can process with monthly and weekly grooves is 2 and
0.5 times greater than that processed with biweekly grooves,
respectively.
4Roberto Touza, Margarita Lema and Rafael Zas
Forest Systems December 2021 • Volume 30 • Issue 3 • eSC05
temporal patterns indicated that the eect of previous
tapping signicantly interacted with time, with signi-
cant dierences in immediate resin ow after each groove
between previously tapped and untapped trees occurring
only during the rst half of the assessed period, and ten-
ding to dilute thereafter (Fig. 2b). This result is consis-
tent with previous ndings (de Oliveira Junkes et al.,
2019; Neis et al., 2018), and is likely due to the produc-
tion of traumatic resin ducts as a response to wounding
(Rodríguez-García et al., 2014).
Despite the observed eects of previous tapping on re-
sin production in the following campaign, both accumu-
lated resin production at the end of the tapping campaign
(F3,27 = 0.47, p = 0.707, Fig. 3b), and temporal patterns of
immediate resin ow after each groove (F21,196 = 0.15, p
= 0.999, Fig. 3a) were not signicantly inuenced by the
moment when trees began to be tapped within the current
campaign. Pine responses to biotic or mechanical dama-
ge include both increased resin production and ow from
existing resin ducts (Moreira et al., 2012), and the die-
rentiation of new traumatic resin ducts (Vázquez-Gonzá-
lez et al., 2020). Based on these responses, we expected
that resin production would be dependent on the number
of previous wounds practiced on the tree, but this pattern
was only observed at the very beginning of the experimen-
tal period (April 2020, day 15) when resin ow was signi-
cantly lower in those trees with no previous wounds in
relation to trees subjected to previous periodical wounds
(Fig. 3a). However, these dierences quickly disappeared,
and even switched at the end of the experimental period,
with trees tapped for the shortest period (7 grooves since
April 2020) producing signicantly more resin than those
previously tapped for the longest period (31 grooves since
June 2019) (Fig. 3a). This pattern is consistent with a rapid
but timid induction of resin production in preexisting resin
ducts after the rst wounds, with the boost of resin produc-
tion quickly exhausting the resin accumulated in the resin
duct web. On the contrary, responses to wounds involving
dierentiation of traumatic resin ducts would require more
time to be produced (Hood and Sala, 2015; Moreira et al.,
2015), and their eects will be seen only in the following
tapping campaign (Rodríguez-García et al., 2016). Within a
single campaign, temporal patterns of resin ow seem to be
more dependent on other factors (e.g. weather conditions,
Rodríguez-García et al., 2015; Zas et al., 2020b) than to
previous recent wounds. An increase in resin production in
successive resin campaigns should be, however, expected.
Practical recommendations
Three practical recommendations can be derived from
the results presented here. First, the tapping frequency
should be revised as reducing the frequency of interven-
tions may result in increased production eciency. The-
se results should be, however, conrmed with further re-
search, as the power of the experiment presented here is
limited (one single site, one single campaign). Second, as
temporal patterns of resin production are weakly inuen-
ced by previous recent wounds, and are mainly depen-
dent on temperature and water decit (Rodríguez-García
et al., 2015; Zas et al., 2020b), the resin tapping period
should be adjusted according only to the local climatic
conditions avoiding cold and wet periods during the
a) b)
Figure 2. a) Accumulated resin production from June 2019 to June 2020 in previously tapped and untapped
trees. Dierences between both groups were marginally signicant (F1,30 = 3.1, p = 0.087). b) Immediate
resin production (mean ± standard error) after each biweekly groove in previously-tapped and newly-tapped
trees during the assessed period (June 2019-June 2020). Note that signicant dierences (denoted with an
asterisk) between both groups occurred only between the 3rd and the 10th groove and disappeared thereafter.
The previous-tapping × time interaction was signicant (F22,660 =1.6, p = 0.036).
Forest Systems December 2021 • Volume 30 • Issue 3 • eSC05
5
Timing of resin-tapping
tapping period. Third, pine responses to resin tapping take
time to be eective and may inuence resin production in
successive tapping campaigns. Previous quantications of
resin yield potential based on newly-tapped trees (e.g. Zas
et al., 2020a) may be thus underestimated.
Acknowledgments
The authors thank the Forestry Communities of Cha-
cente and Paredes, the landowners of the experimental
stands for all the facilities. Assistance in eld assessments
by Jacobo Roselló is also acknowledged.
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