Roger F. Walker

Soil pCO 2 , soil respiration, and root activity in CO 2 - fumigated and nitrogen-fertilized ponderosa pine

The purpose of this paper is to describe the effects of CO2 and N treatments on soil pCO2, calculated CO2 efflux, root biomass and soil carbon in open-top chambers planted with Pinus ponderosa seedlings. Based upon the literature, it was hypothesized that both elevated CO2 and N would cause increased root biomass which would in turn cause increases in both total soil CO2 efflux and microbial respiration. This hypothesis was only supported in part: both CO2 and N treatments caused significant increases in root biomass, soil pCO2, and calculated CO2 efflux, but there were no differences in soil microbial respiration measured in the laboratory. Both correlative and quantitative comparisons of CO2 efflux rates indicated that microbial respiration contributes little to total soil CO2 efflux in the field. Measurements of soil pCO2 and calculated CO2 efflux provided inexpensive, non-invasive, and relatively sensitive indices of belowground response to CO2 and N treatments.

PRESCRIBED FIRE EFFECTS ON FOREST FLOOR AND SOIL NUTRIENTS IN A SIERRA NEVADA FOREST

The objectives of this study were to quantify the effects of prescribed fire on forest floor C and nutrient content, soil chemical properties, and soil leaching in a Jeffrey pine (Pinus jeffreyi [Grev. and Balf.]) forest in the eastern Sierra Nevada Mountains of California. The study included a prescribed fire and three timber harvest treatments: whole-tree (WT) thinning, cut-to-length (CTL) thinning, and no harvest (CONT). Prescribed fire resulted in significant decreases in forest floor C (−8 to −23 mg ha−1, or 39% to 61% decrease), N (−114 to −252 kg ha−1, or −31% to 51% decrease), S (0 to −15 kg ha−1, or 0% to 48% decrease), and K (−3 to −45 kg ha−1, or 12% to 51% decrease) contents but no significant change in Ca or Mg contents. In each case, the decreases were greatest in the CTL treatment, where slash accumulation before burning was greatest. Burning caused statistically significant effects on soil total nitrogen, C:N ratio, pH, water-extractable ortho-P, and water-extractable SO42− in some cases, but these effects were generally small, inconsistent among harvest treatments and horizons, and in the case of ortho-P much less than the temporal variation in both burned and unburned plots. There were no statistically significant effects of burning on total C, Bray-extractable P, bicarbonate-extractable P, and exchangeable Ca2+, K+, or Mg2+. Burning had no significant effect on soil solution pH, ortho-P, SO42−, NO3−, or NH4+ as measured by ceramic cup lysimeters and no effect on the cumulative leaching of ortho-P, NO3−, or NH4+ as measured by resin lysimeters. Burning had no effect on needle weight or nutrient contents as measured by the vector analysis. We conclude that prescribed fire had minimal effects on soils or water quality at this site, and that the most ecologically significant effect was the loss of N from the forest floor.

Effects of CO2 and nitrogen fertilization on vegetation and soil nutrient content in juvenile ponderosa pine

This paper summarizes the data on nutrient uptake and soil responses in opentop chambers planted with ponderosa pine (Pinus ponderosa Laws.) treated with both N and CO2. Based upon the literature, we hypothesized that 1) elevated CO2 would cause increased growth and yield of biomass per unit uptake of N even if N is limiting, and 2) elevated CO2 would cause increased biomass yield per unit uptake of other nutrients only by growth dilution and only if they are non-limiting. Hypothesis 1 was supported only in part: there were greater yields of biomass per unit N uptake in the first two years of growth but not in the third year. Hypothesis 2 was supported in many cases: elevated CO2 caused growth dilution (decreased concentrations but not decreased uptake) of P, S, and Mg. Effects of elevated CO2 on K, Ca, and B concentrations were smaller and mostly non-significant. There was no evidence that N responded in a unique manner to elevated CO2, despite its unique role in rubisco. Simple growth dilution seemed to explain nutrient responses in almost all cases.There were significant declines in soil exchangeable K+, Ca2+, Mg2+ and extractable P over time which were attributed to disturbance effects associated with plowing. The only statistically significant treatment effects on soils were negative effects of elevated CO2 on mineralizeable N and extractable P, and positive effects of both N fertilization and CO2 on exchangeable Al3+. Soil exchangeable K+, Ca2+, and Mg2+ pools remained much higher than vegetation pools, but extractable P pools were lower than vegetation pools in the third year of growth. There were also large losses of both native soil N and fertilizer N over time. These soil N losses could account for the observed losses in exchangeable K+, Ca2+, Mg2+ if N was nitrified and leached as NO3−.

Lessons from Lysimeters: Soil N Release from Disturbance Compromises Controlled Environment Study

A controlled environment study of the effects of carbon dioxide (CO2) and nitrogen (N) on growth of ponderosa pine seedlings produced results contradictory to those obtained in the field with the same species, soil, and treatments. In the controlled envi- ronment study, there was a significant negative growth response to N fertilization, whereas in the field there was a significant positive response to N. The difference was due to high rates of native N mineralization after soil disturbance during potting. This was evident from soil solution N03- concentrations that peaked at -5000 Kmol/L in the unfertilized pots and 20000 Kmol/L in the fertilized pots. These concentrations are orders of magnitude greater than those typically observed in the field. The effects of soil disturbance on N mineralization and nitrification need to be carefully considered before initiating controlled environment studies. The results of this study show that excessive N mineralization caused by soil disturbance can seriously compromise the results of controlled environment studies

Fire Effects on Carbon and Nitrogen Budgets in Forests

Estimates of C and N loss by gasification during a wildfire in a Jeffrey pine (Pinus Jeffreyii [Grev. and Balf.]) forest in Little Valley, Nevada are compared to potential losses in more mesic forests in the Integrated Forest Study (IFS). In Little Valley, the fire consumed the forest floor, foliage, and an unknown amount of soil organic matter, but little standing large woody material. on an ecosystem level, the fire consumed approximately equal percentages of C and N (12 and 9%, respectively), but a considerably greater proportion of aboveground N (71%) than C (21%). Salvage logging was the major factor in loss, and C lost from the site will not be replenished until forest vegetation is established and succeeds the current shrub vegetation. N2 fixation by Ceanothus velutinus [Dougl.l in the post-fire shrub vegetation appears to have more than made up for N lost by gasification in the fire over the first 16 yr, and may result in long-term increases in C stocks once forest vegetation takes over the site. N loss from the fire equaled > 1,000 years of atmospheric N deposition and > 10,000 years of N leaching at current rates. Calculations of C and N losses from theoretical wildfires in the IFS sites show similar patterns to those in Little Valley. Calculated losses of N in most of the IFS sites would equal many centuries of leaching. Conceptual models of biogeochemical cycling in forests need to include episodic events such as fire.

Effects of elevated CO2 and nitrogen on nutrient uptake in ponderosa pine seedlings

This paper reports on the results of a controlled-environment study on the effects of CO2 (370, 525, and 700 μmol mol-1) and N [0, 200, and 400 μg N g soil-1 as (NH4)SO4] on ponderosa pine (Pinus ponderosa) seedlings. Based upon a review of the literature, we hypothesized that N limitations would not prevent a growth response to elevated CO2. The hypothesis was not supported under conditions of extreme N deficiency (no fertilizer added to a very poor soil), but was supported when N limitations were less severe but still suboptimal (lower rate of fertilization). The growth increases in N-fertilized seedlings occurred mainly between 36 and 58 weeks without any additional N uptake. Thus, it appeared that elevated CO2 allowed more efficient use of internal N reserves in the previously-fertilized seedlings, whereas internal N reserves in the unfertilized seedlings were insufficient to allow this response. Uptake rates of other nutrients were generally proportional to growth. Nitrogen treatment caused reductions in soil exchangeable K+, Ca2+, and Mg2+ (presumably because of nitrification and NO3- leaching) but increases in extractable P (presumably due to stimulation of phosphatase activity).

Elevated atmospheric CO2 and soil N fertility effects on growth, mycorrhizal colonization, and xylem water potential of juvenile ponderosa pine in a field soil

Interactive effects of atmospheric CO2 enrichment and soil N fertility on above- and below-ground development and water relations of juvenile ponderosa pine (Pinus ponderosa Dougl. ex Laws.) were examined. Open-top field chambers permitted creation of atmospheres with 700 µL L-1, 525 µL L-1, or ambient CO2 concentrations. Seedlings were reared from seed in field soil with a total N concentration of approximately 900 µg g-1 or in soil amended with sufficient (NH4)2SO4 to increase total N by 100 µg g-1 or 200 µg g-1. The 525 µL L-1 CO2 treatment within the intermediate N treatment was excluded from the study. Following each of three consecutive growing seasons, whole seedlings of each combination of CO2 and N treatment were harvested to permit assessment of shoot and root growth and ectomycorrhizal colonization. In the second and third growing seasons, drought cycles were imposed by withholding irrigation during which predawn and midday xylem water potential and soil water potential were measured. The first harvest revealed that shoot weight and coarse and fine root weights were increased by growth in elevated CO2. Shoot and root volume and weights were increased by CO2 enrichment at the second harvest, but growth stimulation by the 525 µL L-1 CO2 concentration exceeded that in 700 µL L-1 CO2 during the first two growing seasons. At the third harvest, above- and below-ground growth increases were largely confined to the 700 µL L-1 CO2 treatment, an effect accentuated by high soil N but evident in all N treatments. Ectomycorrhizal formation was reduced by elevated CO2 after one growing season, but thereafter was not significantly affected by CO2 and was unaffected by soil N throughout the study. Results of the xylem water potential measurements were variable, as water potentials in seedlings grown in elevated CO2 were intermittently higher on some measurement days but lower on others than that of seedlings grown in the ambient atmosphere. These results suggest that elevated CO2 exerts stimulatory effects on shoot and root growth of juvenile ponderosa pine under field conditions which are somewhat dependent on N availability, but that temporal variation may periodically result in a greater response to a moderate rise in atmospheric CO2 than to a doubling of the current ambient concentration.

Growth and nutritional responses of containerized sugar and Jeffrey pine seedlings to controlled release fertilization and induced mycorrhization

Abstract Two controlled release nutrient formulations, Sierra 17-6-10+Minors and High N 24-4-8, and ectomycorrhizal inoculation with pelletized basidiospores of Pisolithus tinctorius (Pers.) Coker & Couch were evaluated for their effects on containerized sugar pine (Pinus lambertiana Dougl.) and Jeffrey pine (Pinus jeffreyi Grev. & Balf.) seedlings. Low, medium, and high application rates of each amendment formulation were tested. Peters 20-10-20 water soluble nutrients, also applied with three rates, plus unfertilized and uninoculated seedlings of each species provided for comparison of treatment effects on growth and nutrition. Shoot growth in both sugar and Jeffrey pine, as indicated by dimensions, volume, and mass, increased with amendment application rate, and the response to controlled release nutrients exceeded that to the water soluble formulation. Jeffrey pine root growth, in terms of mass and total length, paralleled that of the shoots, but in sugar pine, the three fertilizers produced comparable root growth that did not increase with application rate. For both species, however, the Sierra formulation produced the greatest specific root lengths among fertilized seedlings. Inoculated sugar pine seedlings generally exhibited smaller dimensions and volume and less shoot and root mass than uninoculated sugar pine. Jeffrey pine with induced mycorrhization were also generally smaller in shoot dimensions and volume but had greater shoot and root mass than uninoculated seedlings. Total root length and specific root length were increased substantially in both species by mycorrhizal inoculation. The Sierra formulation applied at the high rate resulted in the most P. tinctorius mycorrhizae in Jeffrey pine, but in sugar pine, mycorrhizal counts and the percentages of root length colonized declined as fertilization rates rose although nutrient additions increased both relative to that found in unfertilized seedlings. Foliar analysis for a broad array of macro- and micronutrients revealed a coupled increase in nutrient concentrations, notably of N but also P and S among others, and amendment application rates in sugar and Jeffrey pine, which at least in part probably explains the growth responses to fertilization noted above. The High N amendment, which excelled in elevating N uptake, also had a propensity to depress P concentrations in both species, however. Inoculated sugar pine had higher foliar N and K but lower Ca and Mn overall than uninoculated seedlings, while inoculated Jeffrey pine had higher K and Zn but lower N, P, Ca, Mg, and S than their uninoculated counterparts. In the latter species, however, the depressed macronutrient uptake in inoculated seedlings was frequently offset by fertilization with specific formulations and/or application rates. Overall, these results indicate that the high rate of the Sierra amendment produced the most favorable array of attributes in both sugar and Jeffrey pine, but that P. tinctorius is likely a more promising mycobiont for inoculation of the latter species than the former.

Enriched atmospheric CO2 and soil P effects on growth and ectomycorrhizal colonization of juvenile ponderosa pine

Abstract Interactive effects of atmospheric CO 2 enrichment and soil P fertility on above- and below-ground development of juvenile ponderosa pine (Pinus ponderosa Dougl. ex Laws.) were examined. Seedlings were reared from seed in atmospheres with 700 μl 1 −1, 525 μl 1 −1, or ambient CO 2 concentrations, and in a potting mix with 68, 43, or 18 μg g −1 soil P, and all were inoculated with the mycobiont Pisolithus tinctorius (Pers.) Coker and Couch shortly after emergence. At 4-month intervals over the 1-year duration of the study, three whole seedlings of each combination of CO 2 and P treatments were harvested to permit detailed assessment of shoot and root growth and ectomycorrhizal colonization. After 4 months, shoot volume, root dry weight, and total root length of seedlings grown in 700 μl 1 −1 CO 2 were greater than those of seedlings grown in the other atmospheres regardless of P treatment, and shoot/root ratios decreased as the CO 2 concentration increased within each P treatment as well. After 8 months, the smallest shoot volumes and root weights and lengths within each P treatment were those of seedlings grown in ambient CO 2. Root weight and total length increased as the CO 2 concentration increased in high soil P, but the greatest root weights and lengths within the medium and low P treatments were those of seedlings reared in the 525 μl l −1 CO 2 atmosphere. Nevertheless, shoot/root ratios decreased with increasing CO 2 in both high and medium soil P at the second harvest, and the highest shoot/root ratio in low P was that of seedlings grown in ambient CO 2. After 1 year, the largest shoot and root volumes within the high and medium P treatments were those of seedlings grown in intermediate CO 2, while the reverse was true in low P. The effects of CO 2 concentration on dry weights, total root length, and shoot/root ratio at the final harvest were nonsignificant. As proved true with seedling growth, CO 2 effects on ectomycorrhizal colonization varied temporally, as mycorrhizal development was not affected by the atmospheric treatments after 4 months, while seedlings grown in ambient CO 2 exhibited the highest percent infections within each P treatment at the second harvest but those grown in 700 μl l −1 CO 2 had the highest percentages after 1 year. These results suggest that elevated CO 2 exerts stimulatory effects on shoot and root development of juvenile ponderosa pine which may be dependent on P availability to some degree, but these effects are somewhat transient and vary in magnitude over time.

Thinning and Prescribed Fire Effects on Forest Floor Fuels in the East Side Sierra Nevada Pine Type

Abstract Forest thinning accomplished with cut-to-length and whole-tree harvesting systems combined with prescribed underburning were assessed for their impacts on downed and dead fuel loading of all timelag categories in eastern Sierra Nevada Jeffrey pine (Pinus jeffreyi Grev. & Balf.). Cut-to-length harvesting resulted in an approximate doubling of total fuel loads with the greatest increases occurring in the 100-hr and 1000-hr categories, while changes associated with whole tree harvesting were marginal to the extent that overall posttreatment loading differed little between whole-tree and unfhinned treatments. Following the thinning operations, 1 + 10-hr and total fuel accumulations in the cut-to-lengfh treatment and 1000-hr fuels in the whole-tree treatment were positively correlated with harvested basal area and harvested foliage, branch, bole, and total tree biomass in simple regression models. Subsequent consumption during underburning eliminated 1 + 10-hr and 100-hr fuel additions from cut-to-lengfh harvesting along with a portion of the natural loading in these categories but was much less effective in reducing the 1000-hr fuels generated by this harvesting. Consumption of 1 + 10-hr, 100-hr, and total fuels in all thinning treatments was positively correlated with the amounts present within each category before underburning. Results reported here provide insight into fuel load modifications resulting from field practices that are being increasingly integrated into comprehensive management efforts to improve forest health and fire resilience in the western United States.