Effects of fire mitigation on post-settlement ponderosa pine non-structural carbohydrate root reserves
This investigation involved post-colonial (∼85 years old), ponderosa pine (Pinus ponderosa var. scopulorum Engelm.) growing at ∼2,255 m in northern Arizona an area that historically experienced a wildfire event every 5-20 years. Such fires were typically limited to surface fuels and thus seldom effected stand replacement. However, since the settlement of European peoples in approximately the 1880's fire cycles have been dramatically altered by the systematic suppression of all wildland fire. This change greatly increased seedling survivorship, and has resulted in a forest with historically unprecedented stem densities. Under current conditions, with individual trees receiving insufficient nourishment, many ponderosa pine forests have demonstrated stagnated growth and increased risk of pathogenic attack or catastrophic wildfire.
In response to deteriorating forest conditions and the building threat of wildfire many forest managers are exploring methods of reestablishing natural cycles to restore historical stand dynamics. Typically this effort has included timber harvesting and/or the use of prescribed fire. In this application timber harvesting typically involves removing the younger cohort of post-settlement trees. While the objectives of prescribed fire are similar to timber harvesting, fire is by its nature less precise; treatment can not be limited to a target demographic. As a result, the application of prescribed fire can lead to different stand conditions than tree removal. It is also common (especially under extreme stem densities) to have a timbering operation precede a fire treatment. The impact of these three scenarios on residual ponderosa pine tree reserves is not well understood.
This project has taken part in cooperation with the USDA Forest Service and the national Fire and Fire Surrogate task group. The experiment was arranged with a 2 x 2 factorial design which resulted in four treatments. Plots were either timber harvested, burned with prescribed fire, cut than burned, or left untreated with treatment application occurring in 2002-03, three years prior to initial sampling. Trees were sampled with one of two methods. Initially a randomly selected coarse lateral root was mechanically exposed for three meters; roughly the average canopy drip line radius whereupon root tissue was extracted at intervals starting at the root collar. This disruptive method was eventually replaced by limiting sampling to the root collar. To slow metabolic activity root tissue samples were field chilled before being oven dried. Dried samples were ground through a 40# mesh and then triple-extracted with ethanol for carbohydrate quantification with high performance liquid chromatography (HPLC). Carbohydrate samples were processed at the University of Massachusetts Cranberry Experiment Station in Wareham MA. with an ion exchange column and a refractive index detector using water as a mobile phase. Starch quantifications were conducted on sub-samples by Cumberland Valley Analytical Services INC. (Maugansville, MD.) using an alpha-amylase digestion and color metric evaluation on an Astoria auto analyzer.
As a result of this project it was determined that the ponderosa pine root non-structural carbohydrate components are sucrose, glucose, fructose, xylose and starch. Reserve concentrations of soluble (EtOH) carbohydrates declined significantly between May and August on plots that were not treated with prescribed fire, burning eliminated this seasonal difference. Starch and total non-structural carbohydrates were also found to be lower in August (compared to May) suggesting that during the summer (May–August), ponderosa pine must draw upon reserves. It was also found that carbohydrate concentrations increased significantly with distance from the root collar. When sampled on a monthly basis it was determined that carbohydrates, both individually and collectively, were significantly affected by the month of sampling and that overall timber harvesting reduced total soluble (EtOH) sugar concentrations. It is believed that collectively carbohydrate concentrations were a reflection of the bi-modal precipitation pattern of northern Arizona; spring snow-melt in particular, appears to have significant increased stored reserves.
Key Words: Pinus ponderosa, prescribed fire, restoration, Southwest, thinning
0768: Environmental science
0817: Plant biology