Perpetuating old ponderosa pine

We review current knowledge about the use of management treatments to reduce human-induced threats to old ponderosa pine (Pinus ponderosa) trees. We address the following questions: Are fire-induced damage and mortality greater in old than younger trees? Can management treatments ameliorate the detrimental effects of fire, competition-induced stress, and drought on old trees? Can management increase resistance of old trees to bark beetles? We offer the following recommendations for the use of thinning and burning treatments in old-growth ponderosa pine forests. Treatments should be focused on high-value stands where fire exclusion has increased fuels and competition and where detrimental effects of disturbance during harvesting can be minimized. Fuels should be reduced in the vicinity of old trees prior to prescribed burns to reduce fire intensity, as old trees are often more prone to dying after burning than younger trees. Raking the forest floor beneath old trees prior to burning may not only reduce damage from smoldering combustion under certain conditions but also increase fine-root mortality. Thinning of neighboring trees often increases water and carbon uptake of old trees within 1 year of treatment, and increases radial growth within several years to two decades after treatment. However, stimulation of growth of old trees by thinning can be negated by severe drought. Evidence from young trees suggests that management treatments that cause large increases in carbon allocation to radial xylem growth also increase carbon allocation to constitutive resin defenses against bark beetle attacks, but evidence for old trees is scarce. Prescribed, low-intensity burning may attract bark beetles and increase mortality of old trees from beetle attacks despite a stimulation of bole resin production.     

Prescribed fire effects on bark beetle activity and tree mortality in southwestern ponderosa pine forests

Prescribed fire is an important tool in the management of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) forests, yet effects on bark beetle (Coleoptera: Curculionidae, Scolytinae) activity and tree mortality are poorly understood in the southwestern U.S. We compared bark beetle attacks and tree mortality between paired prescribed-burned and unburned stands at each of four sites in Arizona and New Mexico for three growing seasons after burning (2004–2006). Prescribed burns increased bark beetle attacks on ponderosa pine over the first three post-fire years from 1.5 to 13% of all trees, increased successful, lethal attacks on ponderosa pine from 0.4 to 7.6%, increased mortality of ponderosa pine from all causes from 0.6 to 8.4%, and increased mortality of all tree species with diameter at breast height >13 cm from 0.6 to 9.6%. On a per year basis, prescribed burns increased ponderosa pine mortality from 0.2% per year in unburned stands to 2.8% per year in burned stands. Mortality of ponderosa pine 3 years after burning was best described by a logistic regression model with total crown damage (crown scorch + crown consumption) and bark beetle attack rating (no, partial, or mass attack by bark beetles) as independent variables. Attacks by Dendroctonus spp. did not differ significantly over bole heights, whereas attacks by Ips spp. were greater on the upper bole compared with the lower bole. Three previously published logistic regression models of tree mortality, developed from fires in 1995–1996 in northern Arizona, were moderately successful in predicting broad patterns of tree mortality in our data. The influence of bark beetle attack rating on tree mortality was stronger for our data than for data from the 1995–1996 fires. Our results highlight canopy damage from fire as a strong and consistent predictor of post-fire mortality of ponderosa pine, and bark beetle attacks and bole char rating as less consistent predictors because of temporal variability in their relationship to mortality. The small increase in tree mortality and bark beetle attacks caused by prescribed burning should be acceptable to many forest managers and the public given the resulting reduction in surface fuel and risk of severe wildfire.     

Soil and microbial respiration in a loblolly pine plantation in response to seven years of irrigation and fertilization

Because soil CO₂ efflux or soil respiration (R_S) is the major component of forest carbon fluxes, the effects of forest management on R_S and microbial biomass carbon (C), microbial respiration (R_H), microbial activity and fine root biomass were studied over two years in a loblolly pine (Pinus taeda L.) plantation located near Aiken, SC. Stands were six-years-old at the beginning of the study and were subjected to irrigation (no irrigation versus irrigation) and fertilization (no fertilization versus fertilization) treatments since planting. Soil respiration ranged from 2 to 6 μmol m⁻² s⁻¹ and was strongly and linearly related to soil temperature. Soil moisture and C inputs to the soil (coarse woody debris and litter mass) which may influence R_H were significantly but only weakly related to R_S. No interaction effects between irrigation and fertilization were observed for R_S and microbial variables. Irrigation increased R_S, fine root mass and microbial biomass C. In contrast, fertilization increased R_H, microbial biomass C and microbial activity but reduced fine root biomass and had no influence on R_S. Predicted annual soil C efflux ranged from 8.8 to 10.7 Mg C ha⁻¹ year⁻¹ and was lower than net primary productivity (NPP) in all stands except the non-fertilized treatment. The influence of forest management on R_S was small or insignificant relative to biomass accumulation suggesting that NPP controls the transition between a carbon source and sink in rapidly growing pine systems.     

Long-term post-wildfire dynamics of coarse woody debris after salvage logging and implications for soil heating in dry forests of the eastern Cascades, Washington

Long-term effects of salvage logging on coarse woody debris were evaluated on four stand-replacing wildfires ages 1, 11, 17, and 35 years on the Okanogan-Wenatchee National Forest in the eastern Cascades of Washington. Total biomass averaged roughly 60 Mg ha⁻¹ across all sites, although the proportion of logs to snags increased over the chronosequence. Units that had been salvage logged had lower log biomass than unsalvaged units, except for the most recently burned site, where salvaged stands had higher log biomass. Mesic aspects had higher log biomass than dry aspects. Post-fire regeneration increased in density over time. In a complementary experiment, soils heating and surrogate-root mortality caused by burning of logs were measured to assess the potential site damage if fire was reintroduced in these forests. Experimentally burned logs produced lethal surface temperatures (60 °C) extending up to 10 cm laterally beyond the logs. Logs burned in late season produced higher surface temperatures than those burned in early season. Thermocouples buried at depth showed mean maximum temperatures exponentially declined with soil depth. Large logs, decayed logs, and those burned in late season caused higher soil temperatures than small logs, sound logs, and those burned in early season. Small diameter (1.25 cm), live Douglas-fir branch dowels, buried in soil and used as surrogates for small roots, indicated that cambial tissue was damaged to 10 cm depth and to 10 cm distance adjacent to burned logs. When lethal soil temperature zones were projected out to 10 cm from each log, lethal cover ranged up to 24.7% on unsalvaged portions of the oldest fire, almost twice the lethal cover on salvaged portions. Where prescribed fire is introduced to post-wildfire stands aged 20–30 years, effects of root heating from smoldering coarse woody debris will be minimized by burning in spring, at least on mesic sites. There may be some long-term advantages for managers if excessive coarse woody debris loads are reduced early in the post-wildfire period.     

Effect of collar insertion on soil respiration in a larch forest measured with a LI-6400 soil CO2 flux system

Little information is available on the effect of root cutting by the collar pre-insertion technique on soil respiration. In this study, we found that soil respiration rates decreased with increasing depth of collar insertion in both the with live roots intact and with live roots severed treatments, but the rate of decrease was substantially higher in the former. The cutting of roots, especially fine roots, may be responsible for this result.     

Thinning intensity effects on carbon and nitrogen stores and fluxes in a Norway spruce (Picea abies (L.) Karst.) stand after 33 years

The present paper deals with C and N storage in soil and vegetation, litter fall and CO₂ efflux from the soil 32–33 years after early thinning in a Norway spruce (Picea abies (L.) Karst.) stand in order to evaluate the effect of thinning regime on C sequestration. At 22 years old, the stand was reduced from 3190 to 2070, 1100 and 820 trees per hectare in four replicates. The N2070 treatment represents the recommended start density in practical forestry, while the other represent a moderate to large reduction in tree number at the present stand age. Aboveground biomass was estimated from single tree measurements of diameter and height based on allometric functions. Litter fall was collected during one and a half years and soil respiration was measured on five occasions during one summer. Ground vegetation was mapped and sampled for biomass, C and N determination. A significant decrease in aboveground tree (including stump-root system) C storage of 27% and 22% due to thinning was found in the N820 and N1100 treatments, respectively, compared to the N2070 treatment. Ground vegetation C storage was little affected by treatment, while litter fall C showed a non-significant decrease in the N820 and N1100 treatments compared to the N2070 treatment. Soil respiration was significantly lower in parts of the summer in the N2070 treatment compared to the N820 treatment. The reason for this is still unexplained since no differences in soil temperature, soil moisture or litter fall chemistry was found between the treatments. No significant treatment effects on humus and mineral soil C storage could be detected. With the present soil variability, the time period of 32 years is probably too short to detect soil C differences due to thinning. The N storage followed the same pattern as for C.     

The effect of forest management on trace gas exchange at the pedosphere–atmosphere interface in beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) forests stocking on calcareous soils

The effect of forest management (thinning) on in situ carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) trace gas exchange between soil and atmosphere was studied in three consecutive years at three beech forest sites, which differ in aspect [southwest (SW), northeast (NE), northwest (NW)]. At all sites adjacent thinning plots (“T”) and untreated control plots (“C”) were established. Measurements at the SW and NE sites covered the years 4–6 after thinning while at the NW site measurements covered the year before and the first 2 years after thinning. Mean N₂O fluxes were <3 μg N₂O–N m⁻² h⁻¹ at all plots except for the newly thinned NWT plot. CH₄ uptake was rather low, too. Very low CH₄ oxidation rates during dry periods are explained by physiological drought stress for CH₄ oxidizers. Heterotrophic litter decomposition constitutes the largest part of total soil respiration. On the whole, no significant positive or negative effects of the silvicultural treatment on the magnitude of CO₂-, CH₄- and N₂O-trace gas exchange could be observed at the SW site 4–6 years after thinning. Also at the NE site, no effects of thinning on CO₂ and N₂O fluxes could be demonstrated. However, at this site a significant moisture-induced lower CH₄ uptake could be shown. At the NW site forest management led to a dramatic increase in N₂O emissions in the first two summers after thinning and to distinct effects on CO₂ emissions and CH₄ uptake in the first year after the felling. The unambiguous effects of thinning at the NW site are mainly related to higher C input by dead residues leading to enhanced mineralization activity, to a shift in the competition for nutrients favoring microorganisms as compared to trees and to changes in the soil water availability at the thinned plot. Considering the data obtained from the NE and SW site we expect that with the development of an understorey vegetation at the NW site the observed effects on the magnitude of trace gas exchange due to thinning will continue to decline in the following years. Our results implicate that it is indispensable to take account of the effects of forest management in order to accurately calculate trace gas emission inventories for the investigated forest ecosystem in case thinning took place immediately before.