Influence of number and depth of
tap holes on maple sap flow
Robert R. Morrow
Bulletin 982, July 1963
Cornell University Agricultural Experiment Station
New York State College of Agriculture, Ithaca, New York

The reasons for great variation in sap flow in sugar maples are many: weather; heredity; exposure; microorganism activity; tree size and vigor; crown size and openness. In all probability, variation is also affected by other environmental factors, including edaphic and physiographic features of the locality. The tap hole, the number of holes per tree, and the depth of the holes influence variation.

In some earlier studies of numbers of tap holes, it was noted that on large trees several taps could be made without much decrease in flow per tap. Results of such tests are confounded with factors of tree and crown size and condition; the effect of number of taps on sap flow is not evaluated. This bulletin presents a method of testing this effect, together with results of tests from 4 sugar bushes and up to 7 years of replication.

Despite the fact that tap hole depth is less important than the number of tap holes, around the turn of the century several small experiments were made to determine the importance of depth of tap holes. These tests, usually made on fewer than a half dozen trees, indicated that there was often about a quarter more sap flow from taps 4 to 6 inches deep than from taps half as deep. Much of this increased flow came late in the season. Jones, Edson, and Morse (3) found that the sap sugar percentage decreased as the sap came from nearer the center of the tree. In 1957, Cool (1) reported results of more intensive tests in Michigan. Three groups of 16 trees were tapped to depths of 2, 4, and 6 inches, respectively. For 3 years the flow from 2-inch taps was about 70 percent of that from 4-inch taps, but flow from the latter also exceeded that from the 6-inch taps. When the trees were re-randomized with respect to treatment, the 2-inch taps slightly exceeded the inch taps; the 6-inch taps exceeded both the 4- and 2-inch taps. These somewhat contradictory results tended to confirm the results of the earlier tests. A study of the effect of depth of tap holes on sap flow over a 7-year period is presented.

Method of Determining Number of Tap Holes
The amount of sap flow, as influenced by the number of tap holes, was tested at various times from 1953 through 1962 in 4 sugar bushes located southwest of and within 20 miles of Ithaca, New York. Note in the following description of these bushes that they are typical of many commercially tapped bushes:

Bush A
Faces southeast, protected by high hills to the east and west, elevation about 1000 feet above sea level; good growing site; trees of medium age, 12-22" diameter breast height, medium to large crowns, medium growth; had been tapped up to about 10 years prior to test, also the 2 years just prior to test. Three groups of 5 trees each, tested with 1, 2, and 3 taps for 7 years.

Bush B
Level, exposed to west winds, about 900-foot elevation; very good site; trees fairly old, 12-30" d.b.h., medium to large crowns, poor to medium growth; previously tapped up to about 6 years prior to test. Three groups of 10 trees each, tested with 1, 2, and 3 taps for 6 years.

Bush C
Faces southeast, protected by high hills to the east and west, elevation about 1200 feet; medium to good site; trees of medium age, ~16" d.b.h., mostly small crowns, poor to medium growth; no previous tapping. Three groups of 10 trees each, tested with 1, 2, and 3 taps for 2 years. Nine other specially treated trees (see p. 5).

Bush D
Faces east, partially protected by low hill to the west, about 1900-foot elevation; medium site; old trees 14-36" d.b.h., large and open crowns, slow growth because of age; tapped 2 years just prior to test. Three groups of 9 trees each, tested with 2, 3, and 4 taps for 2 years. Because of the great variability in sap flow between trees, test trees were not selected on a random basis. They were selected instead, on the basis of uniformity in characteristics known to affect sap flow; and were divided into 3 groups for treatment so that the average value of these characteristics was nearly equal for each group. Treatments were then assigned to each group by chance. In the second year treatments were reassigned to the groups by chance, except that no group was allowed the same treatment as in the first year. Treatments in the third year, in Bushes A and B only, were arbitrarily assigned so that each group had received each of the treatments by the end of the third year. It is known that for any tree, sap flow is related to previous flow (5), to the size, vigor, and openness of tree and crown (4, 6), and to exposure. Therefore only healthy trees were selected; and trees near the edge of the bush were avoided. When available, measurements of previous sap flow were used to help group trees. Measurements of d.b.h., average crown diameter, average live crown ratio (length of live crown divided by total tree height), and average number of open sides were also used to group trees. The variations in these measurements, as well as the averages, are given in table 1 for each group of trees in each bush.

Tap location, spacing, and timing are important in any test of numbers of tap holes. In Michigan and Ohio studies (1, 4, 8), yearly flow totals differed little with the direction the tap faced, although in some years north taps were somewhat poorer than others. Studies in southern New York gave similar results (unpublished data on file in the Department of Conservation, Cornell University); yearly flow totals differed little with tap direction, except that flow totals for south taps exceeded those for north taps by 20-25 per cent in 3 of 6 years. Usually, however, south taps run better early in the season and north taps better at the end of the season. Because of both the dead wood created as a result of tapping, and the possible flow interaction between taps, tap holes on multiple-tapped trees were made as far apart as possible. In this experiment, tapping direction was preplanned to provide nearly equidistant spacing of taps around the tree in both the first and in succeeding years, and to allow a relatively unbiased set of tap directions for each treatment. An exception is the case of single taps, where southerly directions were generally favored over northerly (a logical practice in some bushes where single, rather than multiple, taps are used). All tap holes were bored 2 inches deep, and to obtain fresh wood, were made at different elevations between 3 and 5 feet. The tap directions are in tables 2, 3, 4, and 5. The flow data in these tables generally agree with the previously cited published data with respect to effect of tap direction.

With proper weather conditions, sap flow will occur any time from fall through early spring. The best time for sap collection will obviously depend primarily on size and continuity of flows, sugar percentage, and sap quality. Each year the beginning and end of the season was determined by these factors. The date of flow of the first sap collected varied from February 17 to March 19. The date of flow of the last sap collected varied from March 29 to April 16. In early flow seasons, southerly taps were generally favored, while northerly taps were favored in late flow seasons, and these seasonal variations tended to balance each other over several seasons.

Trees grouped for uniformity, rather than selected randomly, save a large amount of work by greatly reducing the variability of groups of highly variable individuals. Such a grouping permits reduced sampling of the population to obtain results of a given accuracy. The assignment of all treatments to each group provides a final reduction in variability, as well as a check on the uniformity of the groups for testing purposes. In Bush A, for example, the 5 trees in Group 1 yielded an average yearly flow (for the first 3 years) of 109 quarts per tree. Groups 2 and 3 yielded an average of 99 and 100 quarts, respectively. The average yearly flow per tap (each yearly average based on a different number of taps) for Groups 1, 2, and 3, respectively, was 56, 59, and 57 (average per tree is less than twice the average per tap because yield per tap is reduced with additional taps). These flow results are so uniform that the exchange of any tree from one group to another would probably have increased the differences between the groups. Yet this uniformity of groups for the purpose of testing was obtained from a sample of only 5 trees which had single tap hole flows varying from 34 to 125 quarts one year, 49 to 150 quarts another year, and 39 to 105 quarts in a third year.

After 3 years' results established the uniformity2 of the groups for testing, it appeared unnecessary to continue the practice of yearly reassignment of treatments to groups. Instead, each group in Bush A and B was assigned a permanent treatment to test cumulative results of the treatments. Over a long period of time, multiple tap holes create more dead wood and possibly cause progressively decreased yield, relative to single taps. However, no trend in this direction is apparent after 4 years oŁ testing in Bush A and 3 years of testing in Bush B.

Sap Flow Yields with Different Numbers of Tap Holes
The data in tables 2, 3, 4, and 5 [not shown on this webpage] show that added numbers of tap holes usually increase the sap flow per tree, but considerably reduce the sap flow per tap. When the results of all years are averaged, and 2 taps accepted as the norm, the following flow relationships (percents) occurred:

The tables also show that the results were remarkably uniform from year to year and, when differences due to tapping direction are accounted for, uniform throughout the season. The biggest differences in sap flow occurred in Bush A in 1957 when the flow relationship between 1, 2, and 3 taps was 65, 100, and 145; indeed this was the only case in which the yield of trees with three taps was more than double that of trees with single taps. When the data from the somewhat similar Bushes A and B are combined, the mean yield of trees with single taps was 76+5 percent (at the 5 percent level of significance) of that of trees tapped twice; the mean yield of trees tapped 3 times was 122+9 percent of that of trees tapped twice.

In Bush B, the average yearly flow per tree was 120, 111, and 113 quarts for Groups 1, 2, and 3 respectively. The average yearly flow per tap hole was 67, 67, and 63 respectively. By exchanging one tree from Group 1 to Group 3 and vice versa on the basis of the 3-year flow records, these figures became 117, 111, and 117 quarts per tree and 65, 67, and 65 quarts per tap. This exchange was made for continued testing after the third year.

In Bush C, 9 additional trees were tapped excessively for 8 years, beginning in 1952. These trees had similar size and crown characteristics to the other test trees in the bush. Sap flow was measured over 4 years (table 6). The average flow from 20 similar nearby trees, tapped only once a year, is given for 3 years. These data confirm the earlier results from the other 30 test trees in Bush C and emphasize that additional taps yield little more sap for such small trees.

In sugar bushes with small trees, as in Bush C, it appears that little would be gained by making more than one tap per tree. Where larger trees are present, as in Bushes A and B, a moderate increase in sap flow occurs with addition of a second and third tap hole. The data from Bush D suggest that a fourth tap will yield little more sap, even on trees over 20 inches in diameter. It should be pointed out, however, that Bush D, tested for only 2 years, was made up of old trees with poor vigor; large fast-growing young trees may yield differently.

Depth of Tap Holes
In 5 sugar bushes the amount of sap flow from tap holes of different depths was measured at various times between 1953 and 1960 (table 7). The test trees were located in the same general area as those used to test the numbers of tap holes. Stand conditions were similar except for Bush 2, which consisted of roadside trees on a wind-exposed hill. In no case was there more than 2 years' previous tapping.

Two tapping depths that averaged 2 and 3 1/2 inches (not including bark) were tested. In Bushes 3, 4, and 5 only one tap hole was made each year. The experimental procedure was similar to that used in the study of numbers of tap holes. Trees were divided into 2 groups with nearly equal sap flow characteristics, treatments were alternated annually in each group, and tapping direction was selected to avoid bias. Two taps per tree per year, one for each treatment, were used in Bushes 1 and 2, and the direction of tap for each treatment was alternated between successive trees. It should be noted that for 5 years half of the trees were tapped earlier than the others. Although this was in connection with another experiment, it is interesting to see that there is apparently no interaction between time of tapping and depth of tap hole. Table 7 shows that tapping depth influenced sap flow less than did the number of tap holes. The deeper taps yielded significantly more sap in only 2 years (1955 and 1957 when there were more bushes tested) and in only one bush (Bush 2~. Nevertheless there was a tendency toward more flow from the deeper taps in all bushes and in all years. When average yearly totals of all bushes and all years are treated as units of observation, the yield from 31/2-inch tap holes exceeded that from the 2-inch holes by 12+5 per cent (at the 5 percent level of significance). Table 7 shows that this relation ship held throughout most of the season.

Discussion
It has long been known that there is upward translocation in trees. Much less is known about lateral translocation in woody tissue. Increased sap flow per tap hole with fewer taps is empirical evidence that lateral translocation may be extensive, at least under certain conditions. Lateral translocation during sap flow may be caused by accompanying internal tree pressure (sometimes more than 15 pounds per square inch) which seeks release at the tap hole. This evidence agrees with that of Greenidge (2) who found that blockage of upward translocation by sawing halfway through the tree from different directions at successive heights had little restraining effect on moisture movement in sugar maple and several other species. On the other hand Morrow (7), in tests on several hardwoods, including sugar maples, found that application of 2, 4-D in spaced cuts around the tree deadened only the wood above and below the cuts. This suggests that in this case lateral translocation was negligible. However, neither positive internal tree pressure nor blockage of upward translocation was a factor; and there was only a very small amount of chemical. Apparently then, lateral translocation may be negligible in sugar maple under ordinary conditions, but considerable lateral translocation may take place under conditions of stress such as may occur with positive tree pressure or blockage of the upward translocation system.

Knowledge that lateral translocation may occur during sap flow has relevance in the design of experiments. Where multiple taps in the same tree are used for treatments and controls, a treatment that makes flow easier may be favored by any lateral translocation. Where 3 taps are located in one quadrant (to avoid direction bias), the middle tap may be at a disadvantage. These possible biases can be avoided of course by proper experimental design, including sufficient replication.

A number of factors may cause deep tap holes to exceed shallow taps in sap flow. The increased drainage area may be important when internal pressure is high. More sap is obtained from deep taps probably because temperatures are moderated in the inner part of the hole; both freezing and warm temperatures are delayed and the flow period may be lengthened. It has been suggested that deep taps are less quickly affected by microbial growth, which may slow or stop sap flow late in the season. This did not appear to be an important factor in this study since the deep taps were more productive throughout most of the season.

Practical Application
The number and depth of tap holes used by any producer may vary with his resources—especially with the number, size, and accessibility of his trees—and no general recommendation can be made. One objective of many producers, a sugar bush continuously producing over a long period of time, can, however, be approached mathematically. This is accomplished by calculating the number of taps that can be made over the tapping area of a tree and by calculating the time necessary to grow new wood for future taps. The latter calculation must consider both the growth rate and the tapping depth. The following formula is applicable:

                                 P • avg. diam. • 3 tapping levels
No. taps avg. =
                            avg.width between taps• rings per inch • tap depth

Because of the dead wood made by each tap hole and its possible effect on lateral translocation and sap flow, new taps should be located some distance from old taps. The minimum distance that would still result in full sap flow has not been determined. If an average lateral distance of 5 inches is maintained between taps (similar to practice in many sugar bushes) and tapping depth is 2 inches, the above formula reduces to, approximately:

                        avg. diam.
No. taps =    _____________
                       rings per inch

It is obvious from this formula that the number of taps is inversely proportional to the rings per inch and thus directly related to fast growth. Continuous production, additional tap holes, and deeper taps are therefore all related to growth and the proper management of the sugar bush.

References

1. Cool, B. M. 1957. An investigation of the effect of some production techniques and weather factors on maple sap flow and sugar yields in a central Michigan woodlot. Thesis for degree of Ph.D., Mich. State Univ. (Microfilmed.)
2. Greenidge, K. N. H. 1957. Rates and patterns of moisture movement in trees. In The physiology of forest trees, by K. V. Thimann. Ronald Press.
3. Jones, C. H., A. W. Edson, and W. J. Morse. 1903. The maple sap flow. Vt. Agr. Exp. Sta. Bul. 103.
4. Moore, H. R., W. R. Anderson, and R. H. Baker. 1951.Ohio maple syrup . . . some factors influencing production.Ohio Agr. Exp. Sta. Res. Bul. 718.
5. Morrow, R. R. 1952. Consistency in sweetness and flow of maple sap. J. Forestry 50: 130-131.
6. 1955. Influence of tree crowns on maple sap production. Cornell U. Agr. Exp. Sta. Bul. 916.
7. 1959. Chemi-thinning hardwoods in the dormant season. NE Weed Control Conf. Proc. 13: 356-359.
8. Robbins, P. W. 1948. Position of tapping and other factors affecting the flow of maple sap. Thesis for degree of M.S., Mich. State Coll. (Unpublished.)