The Effects of Fire Exclusion on Growth in Mature Ponderosa Pine in Northern Arizona

Dendrochronological techniques were used to assess the effect of fire exclusion on the radial growth of two age classes (approximately 150 to 300 years old) of mature ponderosa pine. Decline in average radial growth in both classes is coincidental with the establishment of a large ponderosa pine seedling crop in 1919 that has since become an extensive stand of stagnant, overcrowded saplings.F and t tests of tree ring indices comparing the time period before and after 1920 show that growth has significantly declined since 1920 in both age classes. F and t tests comparing the two age classes suggest that growth was similar before 1920, but the older age class shows a significantly stronger growth decline than the younger age class. Spearman Rank Correlation tests indicate that in both groups there was no trend or a tend toward increasing tree ring indices before 1920 in both age classes, but that after 1920 there was a strong, significant trend toward decreasing tree ring indices in both groups, and that the trend is stronger in the older age class. These results suggest that the older trees are experiencing a more pronounced growth suppression effect than the younger trees.October and July Palmer Drought Severity Indices from 1931 to 1976 were tested for trend toward drought using the Spearman Rank Correlation. There was no trend toward drought during these months, which have the most significant climatic relationship to ponderosa pine growth in northern Arizona. Therefore the growth decline at Chimney Spring may not be attributed to climate.No environmental factor has changed at Chimney Spring, other than fire exclusion and subsequent seedling establishment. Competition for soil moisture and nutrients, reduced nutrient cycling and soil moisture losses from litter interception may all be factors contributing to the growth decline in the mature ponderosa pines at Chimney Spring.Key words: age classes, competition, dendrochronology, “dog-hair” thickets, fire exclusion, forest, northern Arizona, Pinus ponderosa, radial growth10-year index:

• Declining radial growth in mature ponderosa pine
• Difference in growth rates between age classes
• Dendrochronological techniques to analyze growth rates
• Fire exclusion effects on radial growth rates
• Palmer Drought Severity Index and radial growth
• Reduced soil moisture from competition by young trees and from litter interception