United States 

Department of 

Agriculture Ray Tissues as an Indirect 

Forest Service Measure of Relative Sap-sugar 

Northeastern Forest 

Experiment Station Concentration in Sugar Maple 

Research Paper NE-626 

Peter W. Garrett 

Kenneth R. Dudzik 
i Abstract 

Attempts to correlate ray tissue as a percentage of total wood volume with sapsugar concentrations of sugar maple progenies were unsuccessful. These results 

raise doubts about our ability to use a relatively constant value such as ray-tissue 

volume in a selection program designed to increase the sap-sugar concentration 

of sugar maple seedlings. 

The Authors 

PETER W. GARRETT is Project Leader of the research unit on the ecology and 

I management of northern hardwoods located at the USDA Forest Service 

Northeastern Forest Experiment Station, Durham, New Hampshire. He received a 

M.S. degree in ecology and a Ph.D. in physiology from the University of Mighican 

in 1962 and 1969, respectively. He joined the Northeastern Forest Experiment 

Station in 1966. 

KENNETH R. DUDZIK is a forester with the research project on the physiological 

and biochemical mechanisms of tree response to injury and infection at the 

Northeastern Station at Durham, New Hampshire. He received a B.S. degree in 

forestry from the University of New Hampshire in 1981 and has specialized in 

research on anatomy. He joined the Northeastern Forest Experiment Station in 

1980. 

Manuscript received for publication 7 April 1989 

Northeastern Forest Experiment Station 

370 Reed Road, Broomall, PA 19008 

November 1989 
Introduction 

For hundreds of years in the United States and Canada, 

sugar maple, red maple, and several birch species have 

been wounded to provide sweet sap. Because sugar maple 

produces the largest quantities and the sweetest sap, 

investigators have been studying this species in some detail 

for more than a century. Yet, there remain important 

questions that need to be answered. Gabriel and Seegrist 

(1977) studied 21,000 trees in several states and found a 

considerable amount of variation in sapsugar content within 

and between stands. The average reading of all trees was 

2.5 percent. Stressed trees in pastures and along roads had 

slightly higher readings. There are reports in the literature 

stating that trees producing sap with a high sugar content 

also produce large volumes of sap (Marvin et al. 1967). 

Other researchers have maintained that such a correlation 

does not exist (Tressler and Zimmerman 1942). Some 

reports suggest that trees with one or both of these traits 

tend to maintain their relative ranking year after year 

despite fluctuations in absolute values due to environmental 

factors (Mclntyre 1932; Tressler and Zimmerman 1942; 

Taylor 1956). 

If it is true that individual trees are always sweeter, or 

always produce larger volumes of sap than other trees, 

there is little research to suggest why this might be so, and 

even less evidence that these are heritable traits that will be 

expressed in progeny of "select" trees (Wilkinson 1985). 

Nor have we produced data to show that vegetatively 

propagated material is similar to the ortet from which it was 

derived, or that it is more uniform than nonrelated ramets. 

In fact, the only data we have suggest that this probably is 

not true. Santamour and Cunningham (1964) looked at 

variation in sugar content on budded sugar maple of a 

single clone and compared that with sugar content of 

seedlings of unselected parents. Their data showed that the 

variation among ramets of this clone was as great as the 

variation among seedlings. Demeritt (1985) obtained similar 

results with grafted trees in the clone bank at Grand Isle, 

Vermont. The variation among clonal ramets was as great 

as that among clones. 

Recent analysis of long-term data from progeny trials of 

"sweet" trees has raised questions about the original 

selection methods. Sap-sugar concentrations in sugar 

maple vary from season to season, day to day, and even 

hour to hour, making it difficult to assign a "sweetness" 

value to individual trees. Finding a trait that is correlated 

with sweetness, and one that remains relatively constant 

over time, might be a better method of predicting the 

potential of a tree to produce sap of a given sugar content. 

Ray parenchyma cells are known to be the primary storage 

sites for the sugars in trees (Gregory 1977, 1978; Gregory 

and Hawley 1983). If the quantity of this tissue as a 

proportion of total tissue in the stem is correlated with sapsugar concentration, one could select on the basis of the 

more constant tissue rather than fluctuating sugar 

concentration. Selection has been restricted to several 

weeks in the spring when sap is flowing, but the use of ray 

tissue would allow that activity to take place at any time. 

Gregory (1981) worked out simple techniques for estimating 

ray volume based on maximum ray width, maximum ray 

height, and number of multiseriate rayslmm* of tangential 

area. Gregory (1982) then suggested that individual trees 

may inherently produce more ray tissue, and that if this is 

so, the volume of ray tissue could be manipulated 

genetically. Morselli et al. (1978) and Gregory reported that 

ray area is strongly correlated with sap sugar. Wallner and 

Gregory (1980) reported that this positive relationship 

between sapsugar concentration and the amount of ray 

tissue per unit of xylem does exist, but they also found 

considerable overlap in the amount of ray tissue between 

individuals in the high and low sap-sugar groups of trees. 

They concluded that factors other than storage space are 

involved in regulating (increasing) sap-sugar concentrations. 

Fast-growing trees would use more reserve and currently 

produced carbohydrates, which would mean that less starch 

is available for overwinter storage despite greater storage 

space. Also, the secretion of sucrose into vessel sap is 

controlled by enzymes (Jones 1932; Jones and Bradley 

1933; Sauter et al. 1973) rather than passive flow as 

suggested by Munch (1930). And enzymatic activity may 

vary from tree to tree because of genetic control of those 

activities. 

The possibility of regulating sap-sugar concentration has 

long been a goal of researchers working with sugar maple. 

Earlier work had been based largely on stand manipulation. 

The work of Gregory, Morselli, and others with ray tissue, 

and their suggestions that real differences in sap sugar may 

be associated with volume of ray parenchyma tissue, 

prompted our study. If regulation of sap-sugar 

concentration is possible, it is essential that this be a 

heriteble trait that can be passed from a select tree to its 

progeny. In the 1960's, the USDA Forest Service 

established a series of sugar maple progeny trials in several 

states. These plantings contained material that could be 

used to study inheritance patterns useful in tree 

improvement programs to increase the quantity andlor 

quality of sap. 
Methods and Results 

In the spring of 1968, the Sugar Maple Project of the 

Northeastern Forest Experiment Station,. then located at 

Burlington, Vermont, established rangewide 

provenancelprogeny trials in several states. In 1982 that 

program was terminated and all of the tree improvement 

activities were transferred to the Genetics Project at the 

Northeastern Forest Experiment Station's laboratory at 

Durham, New Hampshire. When the trees in these progeny 

trials were 17 years old, the planting on the Fernow 

Experimental Forest near Parsons, West Virginia, was 

selected for additional studies on inheritance of response to 

wounding. One tree in each two-tree plot was wounded and 

7 months later the wounded trees were cut. Portions of the 

main stem were brought to the Durham Laboratory for 

additional sectioning and analysis. The trees averaged 3.16 

inches in diameter at 4.5 feet above ground. Stem sections 

from each cut tree were stored under refrigeration (340F) 

until used in the current study. 

Because this was an exploratory study, it was decided to 

limit our initial investigations to a few progenies and 

individuals that our records indicated had diverse sugar 

readings over the previous 6- to 10-year period, and later 

Figure 1 .-Microphotograph shows multi- and uniseriate 

rays on tangential surface of sugar maple (field of view 

expand it to other progenies if the data warranted it. For this 

study we chose two seed sources with relatively high sap 

sugar (31-Cass County, MN; 34-Mille Lacs County, MN) and 

five seed sources with lower readings (17-Berkshire County, 

MA; 23-Franklin County, ME; 28-Iron County, MI; 

29-Quebec, Canada; and 36-Chittenden County, VT). 

Thin sections of the last growth ring (tangential surface) 

were made of each stem section. Two sections were made 

from each block and microphotographs were made of three 

random fields on each of the two sections (Fig. 1). Using the 

same magnification for each photograph provided a uniform 

field size for further analysis. Tracings were then made of 

each photograph filling in every uniseriate and multiseriate 

ray (Fig. 2). The tracings were sent to Decagon Devices 

Inc., Pullman, WA, which used its Delta-T Area Meter to 

provide ray tissue as a percentage of total field. Each 

tracing passed through the meter 3 times with an accuracy 

of slightly better than 98 percent. 

In addition to the sections from the West Virginia planting 

that were taken from a position approximately 4.5 feet 

above ground, we looked at other portions of the stem of a 

sugar maple to see if there might be variation related to 

stem position. A single tree in southern Maine was felled 

Figure 2.-Tracing of Figure 1 showing only ray tissue. 

1.05 x 0.79 mm). 
Table 7.-Percenfage of ray tissue in sample sugar maple tree 

Height North East South West Average 

(feet) 

Stump 12.70 1 1.97 13.77 14.20 13.16 

3.5 14.37 14.43 14.60 14.30 14.43 

4.5 13.17 13.97 12.67 11.83 12.91 

5.5 10.50 11.13 t1.47 12.57 1 1.42 

14.0 11 57 12.00 10.87 10.57 1 1.25 

23.0 10.37 1 1.63 1 1 .03 11 .70 11.18 

Average 12.11 12.52 12.40 12.52 12.39 

and sections taken from the stump area and 3.5, 4.5, 5.5, 

14.0, and 23.0 feet above ground. At each of these points 

sections were made at each of the four cardinal directions 

for a total of 24 sampling points (6 heights x 4 directions). 

Again, two sections and three fields of each sections were 

photographed for a total of 144 measurements from this 

tree. There were no differences in amount of ray tissue due 

to direction (Table I), nor were there differences in ray 

tissue as a percentage of total wood tissue due to height 

except for a slight increase just above the stump. The 

amount of ray tissue in this sugar maple tree was 

essentially the same at all points in the main stem, which 

suggests that the sampling point in our main study should 

provide a valid measure of ray-tissue volume. 

Using one-way analysis of variance, we then looked at 

variation in percentage of ray tissue between sources and 

found highly significant differences (Table 2). Next, we used 

the same test to determine variation between families within 

sources, but we found little variation (Table 3). 

We then plotted ray tissue (percent) over sap-sugar 

concentration (percent) for every tree in the study 

regardless of source or family. We found no apparent 

correlation (Fig. 3), which suggests that trees with more ray 

tissue can have either high or low sugar concentrations, 

and that the same can be true of trees with small amounts 

of ray tissue. 

We also looked at possible correlations between ray tissue 

and sap sugar for individual trees within sources. Some 

were slightly positive while others were slightly negative, but 

all were weak (Fig. 4). 
I Table 2.-Comparison among sources using I 

Approximate Student Newman-Keuls test; one-way 

analysis of variance 

i 

Source Number of Percent 

No. samples ray tissue 

34 8 15.81 A 

28 19 15.63 A 

23 14 15.11 A B 

17 21 15.07 A B 

31 8 14.56 ABC 

29 15 13.95 B C 

36 26 13.43 C 

Variation d f SS MS F 

Total 110 367.03 3.34 

Source 6 81.35 13.56 4.94* ' 

Error 1 04 285.68 2.75 

Note: Ray tissue of sources sharing a common letter are not 

significantly different (P I 0.05). 

* * = < 0.01 level of significance. 

Table 3.-Comparison of variation between families 

within sources 

Variation d f SS MS F 

Total 

Family 

Error 

Total 

Family 

Error 

Total 

Family 

Error 

Total 

Family 

Error 

Total 

Family 

Error 

Total 

Family 

Error 

Source 17 (Berkshire County, MA) 

20 36.32 1.87 

7 7.93 1.13 .50 ns 

13 29.39 2.26 

Source 23 (Franklin County, ME) 

13 22.1 1 1 .70 

6 17.92 2.99 4.98 

7 4.20 .60 

Source 28 (Iron County, MI) 

18 54.55 3.03 

6 28.48 4.75 2.18 ns 

12 26.07 2.17 

Source 29 (Quebec, Canada) 

14 56.71 4.05 

6 41.17 6.86 3.53 ns 

8 15.54 1.94 

Source 31 (Cass County, MN) 

7 11.30 1.61 

4 7.95 1.99 1.78 ns 

3 3.35 1.12 

Source 34 (Mille Lacs County, MN) 

7 39.05 5.58 

6 37.25 6.21 3.44 ns 

1 1.80 1.80 

Source 36 (Chittenden County, VT) 

Total 25 64.64 2.59 

Family 7 11.55 1.65 56 ns 

Error 18 53.09 2.95 
PERCENT 

Figure 3.-Percentage of ray tissue over sap-sugar concentration for all trees. 

Conclusions 

The easiest conclusion to draw from these results is that 

there is no apparent correlation between the amount of 

sugar storage tissue (ray cells) and sap-sugar concentration 

(sweetness) in sugar maple trees. If this is true, then it 

would not be possible to use core samples of trees to 

predict and select "sweet trees." If our ray-tissue 

measurements reflect conditions across the growth rings 

and if this is constant with size of the tree, 17.9-percent ray 

cells (Meyer 1922) may be high. Our data indicate that this 

figure may be more on the order of 14.5 percent. 

Our failure to find correlations between sap-sugar readings 

for the parent trees and progeny from the parent tree in the 

bush or in the seed orchard, or to find any patterns between 

ray tissue and sap sugar, might mean there is no 

correlation. Or these negative results might be due to our 

inability to obtain reliable readings of sap sugar. If the 

standard method of taking one or several readings over 

several years produces inaccurate values, then anything 

correlated with the true values would appear to be 

uncorrelated. 

While our data failed to establish a connection between ray 
PERCENT 

Figure 4.-Source 17 (Berkshire County, MA)-correlation of -0.282 for ray tissue and 

sap sugar for all trees from this source. 

tissue and sap-sugar readings in sugar maple, we do know That ray cells are the primary storage sites for sugars in 

more about ray tissue in this species. We now know that the maple and other species is well documented. How the 

amount of rayiissue is similaron all sides of a tree and 

does not vary by distance along the main stem except for a 

small but nonsignificant increase just above ground level. 

We also know that there is significant variation between 

sources or populations of sugar maple, though we did not 

include enough sources in this study to determine if there 

are geographic patterns to this variation. Finally, our data 

indicate that there is little or no variation between families 

within any particular source. 

abundance of such tissue might influence the concentration 

of xylem sugar during the spring sap-flow period could not 

be answered by this study. Additional work is needed on the 

problem of evaluating relative sap-sugar concentration 

among trees. 

This research was funded by the North American Maple 

Syrup Council and the USDA Forest Service, Northeastern 

Forest Experiment Station. 
Literature Cited 

Demeritt, M. E., Jr. 1985. Sap sugar content of grafted 

sugar maple trees. Res. Note NE-328. Broomall. PA: 

U.S. Department of Agriculture, Forest Service, 

Northeastern Forest Experiment Station. 4 p. 

Gabriel, W. J.; Seegrist, D. W. 1977. Phenotypic variation 

in sap sugar among sugar maple stands in Northeast 

United States. In: Proceedings, 24th Northeastern forest 

tree improvement conference; 1976 July 26-29: College 

Park, MD. [Place of publication unknown]: 41-49. 

Gregory, H. A. 1978. Living elements of the conducting 

secondary xylem of sugar maple (Acer saccharum 

Marsh.). International Association of Wood Anatomists 

Bulletin. 4:65-69. 

Gregory, H. A. 1981. A rapid method of estimating the 

relative amount of vascular ray tissue. International 

Association of Wood Anatomists Bulletin. 2(2-3): 77-78. 

Gregory, R. A. 1982. Release of sap sugar and control of 

sap pressure. In: Sugar maple research: sap production, 

processing, and marketing of maple syrup. Gen. Tech. Rep. 

NE-72. Broomall, PA: U.S. Department of Agriculture, Forest 

Service, Northeastern Forest Experiment Station: 1-7. 

Gregory, R. A.; Hawley, G. J. 1983. Sap extraction and 

measurement of soluble sap sugar in sugar maple. 

Canadian Journal of Forest Research. 13: 400-404. 

Jones, C. H.; Bradlee, J. L. 1933. The carbohydrate 

contents of the maple tree. Bull. 358. Burlington, VT: 

Vermont Agricultural Experiment Station. 147 p. 

Jones, E. 1932. Review of facts and observations by 

naturalists, botanists, historians and travellers in the 

properties and productions of the sugar maple tree. 

London: E. Colyer Publishing. 71 p. 

Marvin, J. W.; Morselli. M.; Laing, F. M. 1967. A correlation 

between sugar concentration and volume yields in sugar 

maple-an 18-year study. Forest Science. 13(4): 346-351. 

Mchtyre, A. C. 1932. The maple products industry of 

Pennsylvania. Bull. 280. State College, PA: 

Pennsylvania Agriculture Experiment Station. 

Meyer, J. E. 1922. Ray volumes of the commercial woods 

of the United States and their significance. Journal of 

Forestry. 20: 337-351. 

Morselli, M.; Marvin, J. W.; Laing, F. M. 1978. Imageanalyzing computer in plant science: more and larger 

vascular rays in sugar maple of high sap and sugar 

yield. Canadian Journal of Botany. 56: 983-986. 

Munch, E. 1930. Die Stoffbewegungen in der Pflanze. 

Jena, Germany: Gustav Fischer Verlag. 

Santamour, F. S., Jr.; Cunningham, F. E. 1964. Variation in 

sugar content in a budded sugar maple clone. Res. 

Note NE-17. Upper Darby, PA: U.S. Department of 

Agriculture, Forest Service, Northeastern Forest 

Experiment Station. 3 p. 

Sauter, J. J.; Iten, W.; Zimmerman, M. H. 1973. Studies on 

the release of sugar into the vessels of sugar maple 

(Acer saccharum). Canadian Journal or Botany. 51(1): 

1-8. 

Taylor, F. H. 1956. Variation in sugar content of maple 

sap. Bull. 587. Burlington, VT: Vermont Agriculture 

Experiment Station. 39 p. 

Tressler, C. J.; Zimmerman, W. 1. 1942. Three years' 

operations of an experimental sugar bush. Bull. 699. 

Ithaca, NY: New York Agriculture Experiment Station. 

Wallner, W. E.; Gregory, R. A. 1980. Relationship of sap 

sugar concentrations sugar maple to ray tissue and 

parenchyma flecks caused by Phytobia setosa. 

Canadian Journal of Forest Research. 10: 312-315. 

Wilkinson, R. C. 1985. Year-to-year variation in sap-sugar 

concentration of sugar maple progenies and its 

potential effects on genetic selection for high sap 

sugar. In: Proceedings, 29th northeastern forest tree 

improvement conference; 1984 July 18-20; Morgantown, 

WV. [Place of publication unknown]: Northeastern Forest 

Tree Improvement Conference: 120-1 33. 
Garrett, Peter W.; Dudzik, Kenneth R. 1989. Ray tissue as an 

indirect measure of relative sap-sugar concentration in sugar 

maple. Res. Pap. NE-626. Broomall, PA: U.S. Department of Agriculture, 

Forest Service, Northeastern Forest Experiment Station. 7 p. 

Attempts to correlate ray tissue as a percentage of total wood volume with 

sap-sugar concentrations of sugar maple progenies were unsuccessful. 

These results raise doubts about our ability to use a relatively constant 

value such as ray-tissue volume in a selection program designed to 

increase the sap-sugar concentration of sugar maple seedlings. 

I ODC: 161.34 

Keywords: Sugar maple, sap sugar, ray tissue, tree improvement 

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