Results and Discussion            Top

Tapping frequency

Periodicity of grooves significantly affected 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 differences were only significant between monthly grooves and the other two tapping intervals (weekly and biweekly) (Fig. 1).

Despite the significantly lower resin production of monthly grooves, relative resin production efficiency was around 40% higher for monthly grooves than for the conventional 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 field workers, the profitability of resin tapping activities in Spain is strongly compromised by the effectiveness 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 fire damage, etc) (Génova et al., 2013; van der Maaten et al., 2017), or increased number of years that a tree can be tapped. The final profitability of lower groove frequencies (e.g. 4 weeks) must be, however, carefully estimated, as increasing the number of working trees implies some extra field 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 exploring the resin production efficiency of different frequency tapping interventions. Judging from the literature (e.g. Heinze et al., 2021; Yi et al., 2021), groove frequency largely varies (ca. from every 2 days to every 3 weeks) depending on the species, the climate, the stimulant paste used and, probably, the manpower costs. Results presented here contrast with previous experimental findings in Spanish maritime pine forests, showing that resin flow decreased exponentially with time since wounding, with most of the resin flow occurring during the first week post wounding (Zas et al., 2020a). However, those results were obtained through micro-tapping procedures and we cannot discard that resin flow in conventional tapping is maintained through longer periods because of reduced crystallization and higher fluidity of the resin produced on larger wounds.

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 efficiency (%) in terms of final yield per worker for each periodicity in relation to the conventional biweekly operations is also shown (circles, right axis). Relative efficiency 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

Figure 2.  a) Accumulated resin production from June 2019 to June 2020 in previously tapped and untapped trees. Differences between both groups were marginally significant (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 significant differences (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 significant (F22,660 =1.6, p = 0.036).

Across- and within-years induction

Trees subjected to resin-tapping in the previous year produced more resin than trees newly tapped, although the differences were only marginally significant (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 temporal patterns indicated that the effect of previous tapping significantly interacted with time, with significant differences in immediate resin flow after each groove between previously tapped and untapped trees occurring only during the first half of the assessed period, and tending to dilute thereafter (Fig. 2b). This result is consistent with previous findings (de Oliveira Junkes et al., 2019; Neis et al., 2018), and is likely due to the production of traumatic resin ducts as a response to wounding (Rodríguez-García et al., 2014).

Despite the observed effects of previous tapping on resin production in the following campaign, both accumulated resin production at the end of the tapping campaign (F3,27 = 0.47, p = 0.707, Fig. 3b), and temporal patterns of immediate resin flow after each groove (F21,196 = 0.15, p = 0.999, Fig. 3a) were not significantly influenced by the moment when trees began to be tapped within the current campaign. Pine responses to biotic or mechanical damage include both increased resin production and flow from existing resin ducts (Moreira et al., 2012), and the differentiation of new traumatic resin ducts (Vázquez-González 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 experimental period (April 2020, day 15) when resin flow was significantly lower in those trees with no previous wounds in relation to trees subjected to previous periodical wounds (Fig. 3a). However, these differences 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 significantly 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 first wounds, with the boost of resin production quickly exhausting the resin accumulated in the resin duct web. On the contrary, responses to wounds involving differentiation of traumatic resin ducts would require more time to be produced (Hood and Sala, 2015; Moreira et al., 2015), and their effects will be seen only in the following tapping campaign (Rodríguez-García et al., 2016). Within a single campaign, temporal patterns of resin flow 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.

Figure 3.  a) Immediate resin production (mean ± standard error) after each biweekly groove as a function of the starting point of the resin tapping operations (June 2019, February 2020, April 2020 and June 2020). Only the common assessment period (June 2020 – October 2020) is shown. Significant differences (p < 0.05) between treatments were only observed for the starting (day 15) and ending dates (day 124). For each of these dates, different letters denote significant differences between treatments. b) Accumulated resin production from June 2020 to October 2020 as a function of the starting point of the resin tapping operations. No significant differences (F3,27 = 0.47, p > 0.05) were observed.

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 interventions may result in increased production efficiency. These results should be, however, confirmed with further research, 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 influenced by previous recent wounds, and are mainly dependent on temperature and water deficit (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 tapping period. Third, pine responses to resin tapping take time to be effective and may influence resin production in successive tapping campaigns. Previous quantifications of resin yield potential based on newly-tapped trees (e.g. Zas et al., 2020a) may be thus underestimated.