One tall ponderosa tree with a square silhouette dominates over the other large but conical shaped trees. Photo by Robert Dryja
Amateur Naturalist: The Growth of PonderosaBy ROBERT DRYJA
Los Alamos
A 60-foot wide area was created behind the Aquatic Center when it was constructed in 1988. It therefore would be reasonable to expect to see similar sized ponderosa pine trees that are nearly 30 years old. Assumedly the trees would have started to grow throughout this level area after the construction was completed.
Instead 81 small size ponderosa trees are growing. “Small” means they are from two to eight feet tall. In contrast there are only five medium size ponderosa trees in the same area. “Medium” means they are eight to 20 feet tall. How come there are so few medium sized trees growing after 30 years? What has happened in the more recent past so that small trees now are flourishing?
The puzzle deepens when looking at the area immediately beyond the 60 foot width of the construction area. There are 24 large ponderosa trees growing here, varying from 20 to 40 feet in height. However there is just one dominate ponderosa tree among the 24 large ponderosa trees. This tree is over 40 feet tall and has more of a square silhouette. This contrasts with the typically conical silhouettes of the other large trees.
A ponderosa tree has more of a conical shape while it is growing upward. A fully mature tree stops its upward growth. It becomes flattened at its top because the branches lower down continue to grow horizontally. The trunk of the dominant tree is nearly two feet in diameter, distinctly bigger than those of the nearby large trees. It has several massive side branches, unlike the other large ponderosa trees. Why is there only one fully mature tree?
A change then occurs when looking at the forest extending down the slope of Acid Canyon away from the Aquatic Center. Much more shrubbery is growing among the ponderosa trees near the Aquatic Center, unlike in the forest further away. The overall impression of the more distant forest is that the trees typically are from 10 to 20 feet apart with little shrubbery present. Indeed it is easy to hike through the forest except for an occasional arroyo or steep ledge along the way.
Straight and Curved Trunks
Conifer trees such as ponderosa, spruce or fir grow straight and tall. Their growth is directed at gaining height rather than width. A conifer tree may be 30 feet tall while only one foot wide at the base of it trunk. Height means a tree is in a better position to receive the most sunlight compared to other shorter trees and shrubs attempting to grow below them. This helps to explain why there is little growth below the more mature trees that are growing down in Acid canyon.
However a tall tree needs a trunk that can cope with the physics of height. A tree trunk is a pole and the top of a tall tree trunk can create much leverage. A long pole with weight at one side can create considerable stress for the other end. Conifer trees therefore grow straight vertical trunks to avoid this stress. Their branches also grow symmetrically around the trunk so their weight is distributed evenly. The symmetry of a conifer tree is a coping mechanism for the force gravity that is constantly pulling them down.
A conifer may loose branches on one side of its trunk. The remaining branches now pull the trunk toward their side with their weight. The tree potentially will fall to that side unless it somehow compensates. The trunk starts to grow in a long curve so that it remains as straight as possible for at least some of its length.
The trunk also grows more on one side from year to year to provide additional strength against the pull from the other side. The resulting tree rings reflect this lopsided growth. The rings on one side are much wider, reflecting the additional growth that has developed on that side of the trunk to strengthen the tree.
Short and Shared
A tree can minimize this kind of stress by remaining short and having a relatively thick trunk. Gambel oak trees follow this strategy. A Gambel oak tree is rarely over twenty feet tall. Gambol oak trees also may grow in clusters with their branches intertwining. They now stabilize one another against leaning too far to one side. The tree trunks in the cluster act as single trunk for vertical support. A cluster of Gambel oak trees may be 15 feet tall but have the equivalent of a one foot thick trunk among the several trunks supporting them.
A Gambel oak tree may gain stability by being short but faces a problem in terms of competing for sunlight. A tall conifer receives the most sunlight even if facing stability problems. A Gambel oak is faced with making due if it is in the shade of a conifer. The broad flat leaves of gambol oak help it catch what little sunlight reaches it.
A Gambel oak tree can avoid competing with conifer trees by growing in places that are unsuitable for conifers. These are steep slopes where a conifer is more likely to fall even when only slightly out of vertical. Lower Acid Canyon with its steep slopes demonstrates this nicely. Conifer trees have toppled regularly here.
The fallen trunks are typically those of relatively young trees based on their size. This area contains an extensive grove of Gambel oak trees. The lower canyon receives less sunlight compared with the top of the canyon. However Gambel oak trees now do not have to compete with the shade of conifers that would reduce sunlight even more.
Exploring down the Rabbit Hole —Learning more like Alice
Learn about ponderosa pine ecology at: https://www.fs.fed.us/psw/publications/documents/psw_gtr198/psw_gtr198_a.pdf
Ponderosa trees growing down the Acid Canyon slope are more spread out with little shrubbery growing around them. Photo by Robert Dryja
A conifer has lost its branches on one side and also is growing on a slope. It has grown into a curve to compensate for possibly leaning too far and falling. Photo by Robert Dryja
The trunk grew nearly three times as wide on one side of its core to provide additional strength to offset the extra weight of branches on the other side. Photo by Robert Dryja
Short Gambel oak trees are growing in a cluster of supporting branches and trunks. The leaves are brown in the autumn. Photo by Robert Dryja
Young conifer trees have fallen when growing on the steep slopes of lower Acid Canyon. The brown leaves of Gambel oak trees can be seen growing in place of the fallen conifer trees. The fallen conifer tree trunks lay above the snow and across the narrow, steep slopes of the lower canyon. Photo by Robert Dryja
