PEEC Amateur NaturalistBy ROBERT DRYJA
The Passage of Time and the Transition of Plants
We previously had discussed the kinds of plants seen in the Burnt Mesa area. Ponderosa trees are dominant at the higher elevations toward the Valle Grande.
In contrast, grasses dominate at lower elevations toward the entrance to Bandelier National Monument.
Elevation is a geologic variable that can be considered as setting the long term pattern for the variety of plants growing in area. A geologic time scale is over million of years as mountain ranges slowly rise and erode away.
The amount of rain and snow is influenced by elevation. More moisture occurs toward the higher elevations when considered on an average over hundreds to thousands of years as a part of a climatic time scale. Moisture can be considered as influencing the pattern for plant growth spread over several decades when considered on an ecological time scale.
Grasses, forbs and trees were burnt during the La Mesa fire in 1977. Photo by Terry Foxx
Finally there also is variation from year-to-year or decade-to-decade in the amount of moisture available on a seasonal time scale.
The size of the elk population has a short term influence on how abundantly which plants may grow in an area. The elk population has grown from an estimated 100 in 1978 just before the La Mesa forest fire to 1,500 in 1992. Grasses and forbs are among the first plants to grow in an area following a forest fire. A lot of grasses support a lot of elk.
The pace of change for grasses is fast when considered from year-to-year. The color of the landscape of Burnt Mesa was predominately brown to black following the La Mesa forest fire in June 1977. However the ground became predominately green with the rapid growth of grasses two months later in August. Grasses and forbs covered the ground a year later, growing among burnt but still standing ponderosa tree trunks.
Another eight years would pass before the majority of the dead trunks had fallen.
Grasses and forbs covered the same landscape one year later in 1978. Photo by Terry Foxx
Another seven years passed before the majority of tree trunks had fallen by 1985. They lay among species of ground cover plants that were different from those growing shortly after the fire. Photo by Terry FoxxAlthough first to grow back, different species of grasses and forbs may come to dominate over several years. For example false tarragon was not initially present in surveyed sections of Burnt Mesa. It then became the most commonly seen forb during the next thirty years. In contrast a species of geranium remained present but relatively uncommon throughout these years. Finally, Bahia dissecta is a plant that stands out with its yellow flowers but has disappeared in many areas.
One lone tree (right side of photograph) was growing 21 years after the forest fire in what otherwise was an open field. Photo by Terry FoxxPonderosa were the most commonly seen tree prior to the La Mesa forest fire. The extensiveness and intensity of the forest fire varied from area to area and this determined the survival potential of the trees. Trees growing closely together in clusters were killed during the fire. A partially burnt tree may have taken several years to die.
Twenty to 30 years passed for most of the trees to be clearly replaced by fields of grasses and forbs.
The dominant species of grasses and forbs had changed by 2011, 34 years after the forest fire. However young ponderosa trees now are beginning to grow in the area. Five trees can be seen in the distance. Photo by Terry FoxxHowever young ponderosa trees now are becoming visible after thirty years and are scattered across grassy fields. Trees may become the dominate species in another hundred years.
Instead of occasional trees scattered among grasses, occasional grasses may be scattered among trees in a maturing forest. Future climate, forest fires, and elk population will influence the pace of this change back to forest.
