Now I have at least part of the answer.
My initial guess had been that this symbol was used for young persons who had not yet married and therefore had no offspring.
But there were too many broken trees.
My next guess was that the symbol could mean somebody who had died before the age of 30 - 35, and that he/she was married but did not have any children.
Out of the identified graves, at least 30 graves had a broken tree, so I had 30 possibilities to see how old these persons were when they died.
Age 29 years and younger: 11 persons.
ZIEGELHEIM, Jakob Salomon , 16 years old, died in 1920. Buried in Row 22, Grave 1.
ROSENBACH, Simche, 17 years old, died in 1915. Buried in Row 20, Grave 48.
ROKACH, Sara Bila, 20 years old, died in 1920. Buried in Row 8, Grave 7.
GOTLIEB, Eliasz , 22 years old, died in 1936. Buried in Row 43, Grave 10
FLIESER Majer, 22 years old, died in 1931. Buried in Row 25, Grave 30.
KATZ, Ita, 22 years old, died in 1921. Buried in Row 28, Grave 36.
LILIENFELD, Kalman, 24 years old, died in 1922. Buried in Row 25, Grave 31.
WEISS nee WEINTRAUB, Beila, 26 years old, died in 1931. Buried in Row 29, Grave 18
STRASSBERG, Tauba, 26 years old, died in 1930. Buried in Row 6, Grave 1.
STEINBRUCH, Josef, 26 years old, died in 1926. Buried in Row 28, Grave 2.
SALZBERG, Lazar Lieber, 29 years old, died in 1919. Buried in 26, Grave 12
SPINDEL , Josef , 13 years old, and Moses SPINDEL, 18 years old. Row
Josef and Moses SPINDEL
Age 30 - 39: 5
HERZBERG Salamon , 30 years old, died in 1924. Buried in Row 25, Grave 32
HELLSINGER, Hersz Leib, 32 years, died in 1930. Buried in Row 28, Grave 13.
Hersz Leib HELLSINGER
BODENSTEIN nee HERZBERG, Laje, 34 years old, died in 1922. Buried in Row 10, Grave 1
MAJER nee KORN, Reisel, 35 years old, died in 1925. Buried in Row 16, Grave 1.
HOFFMAN, Malka, 38 years old, died in 1915. Buried in Row 2, Grave 67.
Age 40 - 49: 10
SANDBANK nee RISS, Chaje, 40 years old, died in 1916. Buried in Row 2, Grave 24.
KATZ, Chana, 40 years old, died in 1919. Buried in Row 8, Grave 4
KIRSCHENBAUM nee KNOPF, Hinda, 40 years old, died in 1922.
Buried in Row 18, Grave 23.
KELCZ nee DANENHIRSCH, Chana, 40 years old, died in 1917. Buried in Row 20, Grave 51.
Chana KELCZ nee DANENHIRSCH
STELCER, Szymon, 41 years old, died in 1926. Buried in Row 5, Grave 78.
TEUCH nee KORN, Marjem Rifka, 41 years old, died in 1935. Buried in Row 20, Grave 1.
MAJER, Hersz , 42 years old, died in 1930. Buried in Row 4, Grave 12.
SONENSCHEIN Izrael, 46 years old, died in 1923. Buried in Row 27, Grave 10.
SAMBOL, Moses, 48 years old, died in 1925. Buried in Row 28, Grave 30.
SANDGARTEN, Moses, 49 years old, died in 1922. Buried in Row 20, Grave 25.
Interestingly enough, four were in their fifties.
REICHLER, Szewach, 52, died in 1920. Buried in Row 24, Grave 41.
KOENIG nee BIENENFELD, Cywia, 59, died in 1929. Buried in Row , Grave
WALDER nee ZIEGELHEIM, Gitla, 52 years old, died in 1927. Buried in Row 14, Grave 1.
LEINSIEDER Jakob, 56 years old, died in 1916. Buried in Row 11, Grave 5.
Also check Row 10, Grave 3.
Cywia KOENIG nee BIENENFELD
From my own thoughts:
Using the symbol of a broken tree, can refer to a young person who had not yet married and had no children.
But also for a married person leaving behind young children and a spouse, there is a strong feeling of something broken.
If I should try to understand why the symbol was used for those who died in their forties and fifties, perhaps it meant the close family members felt this had been an especially tragic loss? A person in the prime of his/her life who suddenly died? An accident?
After all, it was the family of the person buried who decided what would be on the gravestone, not any chart with strict rules for the use of the symbol. Perhaps one can say that the symbol expressed a feeling?
When I now understand that many young persons seem not to have gotten a gravestone at all, I also understand that the number of gravestones of those under the age of 30, is much lower than the actual number of persons buried in that age group. There would have been many more broken trees if those young persons had gotten an inscribed gravestone.
When and how did the broken tree come into use as a symbol in Judaism? Perhaps you can tell me?
A broken tree as a symbol of grieving in a non Jewish environment:
In the area I live in Norway, in order to reach some of the houses and farms you use small dirt roads going through the forest.
Around twenty years ago, my mother saw, on both sides of such a road, many young fir trees with the tops broken. It turned out that the old lady who lived on that farm had died, and her children and grandchildren had used this way of showing their grief.
It must be an extremely old custom, not related to religion. My mother also knew about this custom from her native Sweden.