Initial Carbon, Nitrogen, and Phosphorus Fluxes Following Ponderosa Pine Restoration Treatments

Abstract

Southwestern ponderosa pine forests were dramatically altered by fire regime disruption that accompanied Euro‐American settlement in the 1800s. Major changes include increased tree density, diminished herbaceous cover, and a shift from a frequent low‐intensity fire regime to a stand‐replacing fire regime. Ecological restoration via thinning and prescribed burning is being widely applied to return forests to the pre‐settlement condition, but the effects of restoration on ecosystem function are unknown. We measured carbon (C), nitrogen (N), and phosphorus (P) fluxes during the first two years after the implementation of a replicated field experiment comparing thinning and composite (thinning, forest floor fuel reduction, and prescribed burning) restoration treatments to untreated controls in a ponderosa pine forest in northern Arizona, USA. Total net primary productivity (260 g C·m−2·yr−1) was similar among treatments because a 30–50% decrease in pine foliage and fine‐root production in restored ecosystems was balanced by greater wood, coarse root, and herbaceous production. Herbaceous plants accounted for <20% of total plant C, N, and P uptake in the controls but from 25% to 70% in restored plots. Total plant N uptake was ∼3 g N·m−2·yr−1 in all treatments, but net N mineralization was just one‐half and two‐thirds of this value in the control and composite restoration, respectively. Element flux rates in controls generally declined more in a drought year than rates in restoration treatments. In this ponderosa pine forest, ecological restoration that emulated pre‐settlement stand structure and fire characteristics had a small effect on plant C, N, and P fluxes at the whole ecosystem level because lower pine foliage and fine‐root fluxes in treated plots (compared to controls) were approximately balanced by higher fluxes in wood and herbaceous plants.

Short-Term Belowground Responses to Thinning and Burning Treatments in Southwestern Ponderosa Pine Forests of the USA

“…Our 1-and 2-year post-treatment samples reflect these same overall FFS Network patterns. Kaye et al [6] also maintained that fire alone produced greater short-term (2-year) net nitrification rates than thinning plus fire. Similar to the fire only treatment, fungal populations in the forest floor were reduced, yet neither bacteria in the forest floor or mineral soil populations were affected.…”

“…An unexpected outcome was that the fire only treatment had no effect on net N mineralization or nitrification, substrate utilization, or enzyme activity. Previously in southwestern ponderosa pine forests, Kaye and colleagues [6,21] found that fire alone increased short-term (2-year) net nitrification rates. Significant differences between SWP and this previous study include a reduction and change in composition of the forest floor prior to prescribed fire, and the relatively small plot sizes (0.25 ha).…”

“…A meta-analysis by Johnson and Curtis [80] concluded that changes in total mineral soil C and N contents in coniferous stands following thinning were the result of harvest method and treatment of residues. Total C and N contents in the mineral soil have initially been unaffected following restoration treatments that included thinning and prescribed fire in other ponderosa pine stands in northern Arizona [6,79], but long-term increases in total soil N have also been reported [31,74]. The limited effects of the FFS thinning treatments are due in part to modest stand density reductions from a single application.…”

“…We suspect that a more intense alteration to forest structure with repeated burning could change microbial community structure and biomass. These changes would have the potential to alter decomposition and nutrient mineralization processes they mediate [6]. Given the large spatial scale that fuel treatments are typically applied, continued monitoring of treatments with repeated burning is essential to determine the longer-term implications to the structure and function of these forests.…”

“…Frequent fires (every 2-20 year), herbaceous competition, and periodic drought maintained the pre-Euro-American forest structure with densities of 30-140 trees ha´1 [3,4]. Following Euro-American settlement, fire exclusion, livestock grazing, and removal of large pre-settlement trees increased stand densities dramatically [5][6][7]. Removal of the dominant trees, coupled with a large seedling recruitment of ponderosa pine in 1919 [8,9], increased stand densities to the current 727 trees ha´1 on average in Arizona [5], with some stands exceeding 2000 trees ha´1 [6,7].…”