Experiments were conducted to determine two pieces of information essential to identify practices necessary to ensure tapping trees for birch sap collection is both sustainable and profitable—the selection of the time to initiate tapping birch trees to obtain maximum yields, and the volume of nonconductive wood (NCW) associated with taphole wounds in birch trees. The yields obtained from various timing treatments varied between sapflow seasons, but indicate that using test tapholes to choose the appropriate time to initiate tapping is likely to result in optimum yields from birch trees. The volume of NCW associated with taphole wounds in birch trees was highly variable and generally quite large, averaging 220 times the volume of the taphole drilled, and requiring relatively high radial growth rates to maintain NCW at sustainable levels over the long-term. However, more conservative tapping practices, including reduced taphole depth and increased dropline length, as well as thinning and other stand management practices, can be used to reduce the minimum growth rates required. Producers can use this information to ensure that they use tapping practices that will result in sustainable outcomes and obtain the maximum possible sap yields from their trees.
Tapping and collecting sap from birch trees (Betula, sp.) for the production of beverages and syrup is gaining increased levels of interest in the maple-producing region of North America [
First, there are limited data available on production practices required to ensure optimum yields are obtained from birch trees tapped for sap collection. In particular, a key information shortfall is with respect to the optimum timing of tapping birch trees each year―there is currently no scientific information on which to base decisions of when birch trees should be tapped to achieve the best sap yields. Currently, birch producers rely on a wide variety of informal techniques and indicators to determine when trees should be tapped, including tapping based on the rate of sap flow from test trees, or tapping when puddles first appear in the woods [
Also, each year, tapping a tree for sap collection permanently removes a small portion of wood where the taphole is drilled and the spout inserted [
Twenty-six healthy paper birch (Betula papyrifera) trees at the University of Vermont Proctor Maple Research Center (UVM-PMRC) in Underhill, Vermont (USA) were assigned to one of three timing of tapping treatments:
• Early: Trees were tapped before stem pressure development was observed or test taps were exuding sap.
• Test taps: Trees were tapped after test tapholes in nearby trees began exuding sap at a substantial rate, >1 drop per second.
• End of maple season: Trees were tapped immediately after the maple production season at UVM-PMRC ended.
Tree diameter ranged from 8.8 - 14.0'', and was stratified across treatments so that it did not differ significantly between treatments (p < 0.6200, Y ¯ Early = 11.2'', Y ¯ Testtaps = 11.9'', Y ¯ Endofmaple = 11.5''). At the appropriate time for each treatment, each tree was tapped with a standard 5/16'' maple spout and connected to a plastic chamber that enables the collection and quantification of sap from individual trees under vacuum. Vacuum was maintained throughout the duration of the experiment at standard maple industry levels (~25'' Hg). The volume and sugar content of sap produced by each tree was measured daily throughout the production season. Sap sugar content was measured with a Misco PA202X refractometer. (It should be noted that maple refractometers are calibrated for sucrose solutions; although birch sap is predominantly a solution of glucose and fructose, at concentrations below 10%, the difference between invert and sucrose refractometer scales is negligible [
All data were analyzed using JMP Pro software version 13.0 (SAS Institute, Cary, NC). Homogeneity of variance assumptions were verified using Levene’s tests and normality assumptions were verified using Shapiro-Wilk tests. One-way analysis of variance was used to determine if significant differences existed between the overall mean syrup equivalent yields of the three treatments in 2015 and 2016, and Students t-tests were used to conduct pairwise comparisons between individual treatments within each year. Wilcoxon Rank Sums tests were used for data that were not normally distributed.
Thirty-nine healthy paper birch trees with an average dbh of 9.6'' (range, 7.9 - 12.7'') growing in a stand in located in Underhill, VT (USA) were selected and tapped during the spring 2015 sapflow season following current standard maple tapping practices (5/16'' spout, 2'' tapping depth). Each tree was subsequently felled in late-autumn 2015, and a portion of the stem containing the taphole wound (approximately 4’ above and below the taphole) was cut and removed (
Beginning at the center of the taphole, each stem segment was subsequently cut with a circular saw into 2''-wide segments (
discoloration [
NCW volume data were then input into a mathematical model of the “tapping zone” of a tree, the portion of the stem available for tapping and sap collection [
The birch sapflow seasons of 2015 and 2016 were extremely different from one another (
In 2015, there was an overall significant difference in total yield between the timing treatments (p < 0.0138), with pairwise comparisons indicating greater yields were obtained from both the Early (p < 0.0062) and Test Taps (p < 0.0485) treatments compared to yields from trees tapped at the End of the Maple season (
In contrast, in 2016 there was only a marginally significant overall difference in the yields between the three timing treatments (p < 0.0829) (
†p-values are for one-way analysis of variance comparing overall mean syrup yields between the treatments, and individual Students t-tests between each timing treatment. ‡indicates comparison made with nonparametric Wilcoxon Rank Sums test. *denotes statistically significant differences (p ≤ 0.05). One tree each in the Test Taps and End of Maple treatments was lost to mortality after the 2015 season.
Early treatment in 2016 was initiated much earlier than in 2015, after several consecutive days with temperatures above 50˚F (
Taken together, these results indicate that inherent variability in sap yields between years might sometimes confound impacts of the selection of the time to tap birch trees. However, despite the variability observed, the data do indicate an optimum timing of tapping that producers can use to obtain optimum yields. In both years, the Test Taps treatment resulted in yields that were equal to or not significantly different from the highest-yielding treatments. Thus, tapping trees based on observations of when test tapholes begin running at a substantial rate (>1 drop per second) appears to be an effective strategy to ensure optimum yields are obtained. In addition, the results indicate that 1) waiting to tap birch trees until the maple season concludes could result in significantly lower yields in some years, 2) tapping slightly early and before test taps are observed to be exuding sap (when using vacuum) is not likely to significantly negatively impact yields, and 3) tapping very early in response to aberrantly early warm temperatures like those observed in 2016 may result in yield reductions compared to tapping at later times closer to the “standard” birch sapflow season.
The volume of NCW columns was highly variable between trees (
The data on the average volume of NCW generated by taphole wounds in birch trees were next used to estimate the practices required for tapping and sap
n | Mean | Minimum | Maximum | |
---|---|---|---|---|
Total NCW volume (cm3) | 39 | 557.2 ± 91.9 | 65.4 | 2275.3 |
NCW volume in proportion to taphole volume | 221.6 ± 36.6 | 26.0 | 905.1 |
collection from birch trees to be sustainable in the long-term. To do this, the NCW volume data were used with a model of the tapping zone of a tree [
The results of this study indicate that test tapholes can be a reliable indicator that producers can use to determine the appropriate time to tap birch trees to obtain
Tapping Practices | ||||||
---|---|---|---|---|---|---|
Tapping Depth (in.) | 1.5 | 1.5 | ||||
Spout size (in.) | 5/16 | 5/16 | ||||
Dropline length (in.) | 30 | 36 | ||||
DBH (in.) | BAI (cm2) | Ring width (cm) | BAI (cm2) | Ring width (cm) | ||
8 | 24.9 | 0.38 | 20.1 | 0.31 | ||
10 | 29.3 | 0.36 | 20.1 | 0.25 | ||
12 | 33.8 | 0.35 | 20.3 | 0.21 | ||
14 | 38.3 | 0.34 | 20.4 | 0.18 | ||
16 | 42.8 | 0.33 | 23.3 | 0.18 | ||
18 | 47.3 | 0.33 | 26.4 | 0.18 | ||
20 | 51.9 | 0.32 | 29.4 | 0.18 | ||
22 | 56.4 | 0.32 | 32.5 | 0.19 | ||
optimum yields. In addition, the results indicate that the volume of NCW associated with taphole wounds in birch trees is generally quite large, and that vigorous radial growth rates are required to ensure NCW does not accumulate excessively in the tapping zone. Producers should evaluate the growth rates of birch trees to be tapped for sap collection to ensure sustainability of tapping practices, and, if necessary, modify tapping practices to increase the likelihood of sustainability―increasing dropline length and reducing tapping depth can reduce the accumulation of NCW [
Funding support for this project was provided by the Northeastern States Research Cooperative (NSRC), a partnership of Northern Forest states (New Hampshire, Vermont, Maine, and New York), in coordination with the USDA Forest Service. We thank Robert White, Wade Bosely, Brian Stowe, and Teague Henkle for their assistance in completing this research.
van den Berg, A.K., Isselhardt, M.L. and Perkins, T.D. (2018) Identifying Sustainable Practices for Tapping and Sap Collection from Birch Trees: Optimum Timing of Tapping Initiation and the Volume of Nonconductive Wood Associated with Taphole Wounds. Agricultural Sciences, 9, 237-246. https://doi.org/10.4236/as.2018.93018