Explore the vast range of titles available.

This paper reviews the role of soil respiration in determining ecosystem carbon balance, and the conceptual basis for measuring and modeling soil respiration. We developed it to provide background and context for this special issue on soil respiration and to synthesize the presentations and discussions at the workshop. Soil respiration is the largest component of ecosystem respiration. Because autotrophic and heterotrophic activity belowground is controlled by substrate availability, soil respiration is strongly linked to plant metabolism, photosynthesis and litterfall. This link dominates both base rates and short-term fluctuations in soil respiration and suggests many roles for soil respiration as an indicator of ecosystem metabolism. However, the strong links between above and belowground processes complicate using soil respiration to understand changes in ecosystem carbon storage. Root and associated mycorrhizal respiration produce roughly half of soil respiration, with much of the remainder derived from decomposition of recently produced root and leaf litter. Changes in the carbon stored in the soil generally contribute little to soil respiration, but these changes, together with shifts in plant carbon allocation, determine ecosystem carbon storage belowground and its exchange with the atmosphere. Identifying the small signal from changes in large, slow carbon pools in flux dominated by decomposition of recent material and autotrophic and mycorrhizal respiration is a significant challenge. A mechanistic understanding of the belowground carbon cycle and of the response of different components to the environment will aid in identifying this signal. Our workshop identified information needs to help build that understanding: (1) the mechanisms that control the coupling of canopy and belowground processes; (2) the responses of root and heterotrophic respiration to environment; (3) plant carbon allocation patterns, particularly in different forest developmental stages, and in response to treatments (warming, CO₂, nitrogen additions); and (4) coupling measurements of soil respiration with aboveground processes and changes in soil carbon. Multi-factor experiments need to be sufficiently long to allow the systems to adjust to the treatments. New technologies will be necessary to reduce uncertainty in estimates of carbon allocation, soil carbon pool sizes, and different responses of roots and microbes to environmental conditions.

Many conifer forests experience stand-replacing wildfires, and these fires and subsequent recovery can change the amount of carbon released to the atmosphere because conifer forests contain large carbon stores. Stand-replacing fires switch ecosystems to being a net source of carbon as decomposition exceeds photosynthesis—a short-term effect (years to decades) that may be important over the next century if fire frequency increases. Over the long term (many centuries), net carbon storage through a fire cycle is zero if stands replace themselves. Therefore, equilibrium response of landscape carbon storage to changes in fire frequency will depend on how stand age distribution changes, on the carbon storage of different stand ages, and on postfire regeneration. In a case study of Yellowstone National Park, equilibrium values of landscape carbon storage were resistant to large changes in fire frequency because these forests regenerate quickly, the current fire interval is very long, the most rapid changes in carbon storage occur in the first century, and carbon storage is similar for stands of different ages. The conversion of forest to meadow or to sparser forest can have a large impact on landscape carbon storage, and this process is likely to be important for many conifer forests.

We examined a chronosequence of subalpine lodgepole pine stands to test the hypothesis that low net primary production in older forest stands is caused by higher maintenance respiration costs of woody tissue. We predicted that respiration of woody tissue (particularly stem sapwood) would be greater in older stands and that the higher maintenance costs would account for observed low wood production. For a unit of ground surface, the carbon flux involved in wood production and association constructed respiration was 210 g^.m^-^2^.yr^-^1 in a 4-yr-old stand, but declined to 46 g^.m^-^2^.yr^-^1 in a 245-yr-old stand. However, maintenance respiration of woody tissue in stems and branches consumed only 61 g^.m^-^2^.yr^-^1 in the 40-yr-old stand and 79 g^.m^-^2^.yr^-^1 in the 245-yr-old stand. The slight, nonsignificant increase in maintenance respiration of woody tissues could not explain the dramatic decline in aboveground wood production in the old-growth stand.

The decline in aboveground wood production after canopy closure in even-aged forest stands is a common pattern in forests, but clear evidence for the mechanism causing the decline is lacking. The problem is fundamental to forest biology, commercial forestry (the decline sets the rotation age), and to carbon storage in forests. We tested three hypotheses about mechanisms causing the decline in wood growth by quantifying the complete carbon budget of developing stands for over six years (a full rotation) in replicated plantations of Eucalyptus saligna near Pepeekeo, Hawaii. Our first hypothesis was that gross primary production (GPP) does not decline with stand age, and that the decline in wood growth results from a shift in partitioning from wood production to respiration (as tree biomass accumulates), total belowground carbon allocation (as a result of declining soil nutrient supply), or some combination of these or other sinks. An alternative hypothesis was that GPP declines with stand age and that the decline in aboveground wood production is proportional to the decline in GPP. A decline in GPP could be driven by reduced canopy leaf area and photosynthetic capacity resulting from increasing nutrient limitation, increased abrasion between tree canopies, lower turgor pressure to drive foliar expansion, or hydraulic limitation of water flux as tree height increases. A final hypothesis was a combination of the first two: GPP declines, but the decline in wood production is disproportionately larger because partitioning shifts as well. We measured the entire annual carbon budget (aboveground production and respiration, total belowground carbon allocation [TBCA], and GPP) from 0.5 years after seedling planting through 6 1/2 years (when trees were ~25 m tall). The replicated plots included two densities of trees (1111 trees/ha and 10 000 trees/ha) to vary the ratio of canopy leaf mass to wood mass in the individual trees, and three fertilization regimes (minimal, intensive, and minimal followed by intensive after three years) to assess the role of nutrition in shaping the decline in GPP and aboveground wood production. The forest closed its canopy in 1-2 years, with peak aboveground wood production, coinciding with canopy closure, of$1.2-1.8 kg C\\cdot m^{-2}\\cdot yr^{-1}$. Aboveground wood production declined from$1.4 kg C\\cdot m^{-2}\\cdot yr^{-1}$at age 2 to$0.60 kg kg C\\cdot m^{-2}\\cdot yr^{-1}$at age 6. Hypothesis 1 failed: GPP declined from$5.0 kg C\\cdot m^{-2}\\cdot yr^{-1}$at age 2 to$3.2 kg kg C\\cdot m^{-2}\\cdot yr^{-1}$at age 6. Aboveground woody respiration declined from$0.66 kg C\\cdot m^{-2}\\cdot yr^{-1}$at age 2 to$0.22 kg C\\cdot m^{-2}\\cdot yr^{-1}$at age 6 and TBCA declined from$1.9 kg C\\cdot m^{-2}\\cdot yr^{-1}$at age 2 to$1.4 kg C\\cdot m^{-2}\\cdot yr^{-1}$at age 6. Our data supported hypothesis 3: the decline in aboveground wood production (42% of peak) was proportionally greater than the decline in canopy photosynthesis (64% of peak). The fraction of GPP partitioned to belowground allocation and foliar respiration increased with stand age and contributed to the decline in aboveground wood production. The decline in GPP was not caused by nutrient limitation, a decline in leaf area or in photosynthetic capacity, or (from a related study on the same site) by hydraulic limitation. Nutrition did interact with the decline in GPP and aboveground wood production, because treatments with high nutrient availability declined more slowly than did our control treatment, which was fertilized only during stand establishment.

The partitioning among carbon (C) pools of the extra C captured under elevated atmospheric CO₂ concentration ([CO₂]) determines the enhancement in C sequestration, yet no clear partitioning rules exist. Here, we used first principles and published data from four free-air CO₂ enrichment (FACE) experiments on forest tree species to conceptualize the total allocation of C to below ground (TBCA) under current [CO₂] and to predict the likely effect of elevated [CO₂]. We show that at a FACE site where leaf area index (L) of Pinus taeda L. was altered through nitrogen fertilization, ice-storm damage, and droughts, changes in L, reflecting the aboveground sink for net primary productivity, were accompanied by opposite changes in TBCA. A similar pattern emerged when data were combined from the four FACE experiments, using leaf area duration (L(D)) to account for differences in growing-season length. Moreover, elevated [CO₂]-induced enhancement of TBCA in the combined data decreased from approximately equal to 50% (700 g C m⁻² y⁻¹) at the lowest L(D) to approximately equal to 30% (200 g C m⁻² y⁻¹) at the highest L(D). The consistency of the trend in TBCA with L and its response to [CO₂] across the sites provides a norm for predictions of ecosystem C cycling, and is particularly useful for models that use L to estimate components of the terrestrial C balance.

Genetic changes in meristem tissue, nutrient limitation, respiration and hydraulic limitation are offered as explanations for the stoppage in tree growth with age. New approaches and resources are emerging that will enable ecologists and plant physiologists to assess the physiology of old trees.

Validating the components of the carbon (C) budget in forest ecosystems is essential for developing allocation rules that allow accurate predictions of C pools and fluxes. In addition, a better understanding of the effects of natural disturbances on C cycling is critical, particularly in light of alterations to disturbance regimes that may occur with global climate change. However, quantitative data about how postfire differences in ecosystem structure affect C allocation patterns are lacking. For this study, we examined how above- and belowground C pools, fluxes, and allocation patterns varied with fire-initiated differences in tree density and stand age in lodgepole pine stands in Yellowstone National Park of four forest types: low (<1000 trees/ha), moderate (7000-40 000 trees/ha), and high tree densities (>50 000 trees/ha) in 13-year-old stands, and in ~110-year-old mature stands. C pools in live biomass and detritus were estimated with allometric equations and direct sampling. Aboveground net primary productivity (ANPP) was estimated as aboveground biomass increment plus fine litterfall, and total belowground carbon allocation (TBCA) was estimated using a C balance approach. Our results indicate that the magnitude of C pools and fluxes varies greatly with fire-initiated differences in tree density and stand age. Coarse woody debris and mineral soil carbon accounted for the majority of total ecosystem C in young stands (91-99%), in contrast to mature stands where the largest amount of C was found in live biomass (64%). ANPP and TBCA increased with tree density (mean ANPP was 59, 122, and$156 g C\\cdot m^{-2}\\cdot yr^{-1}$, and TBCA was 68, 237, and$306 g C\\cdot m^{-2}\\cdot yr^{-1}$for low-, moderate-, and high-density young stands, respectively), and with stand age (ANPP was$218 g C\\cdot m^{-2}\\cdot yr^{-1}$and TBCA was$382 g C\\cdot m^{-2}\\cdot yr^{-1}$for 110-year-old stands). ANPP and TBCA were positively correlated, and both variables were well correlated with leaf area index. Notably, the ratio of TBCA to (TBCA + ANPP) remained remarkably constant (0.63-0.66) across extreme gradients of tree density and stand age, differing only slightly for the low-density young stands (0.54). These results suggest that C allocation patterns in a postfire lodgepole pine ecosystem are independent of tree density and stand age.

Soil CO₂ flux can contribute as much as 60-80% of total ecosystem respiration in forests. Although considerable research has focused on quantifying this flux during the growing season, comparatively little effort has focused on non-growing season fluxes. We measured soil CO₂ efflux through snow in 50 and ∼300 year old subalpine forest stands near Fraser CO. Our objectives were to quantify seasonal patterns in wintertime soil CO₂ flux; determine if differences in soil CO₂ flux between the two forest ages during the growing season persist during winter; and to quantify the sample size necessary to discern treatment differences. Soil CO₂ flux during the 2002-2003 and 2003-2004 snow season averaged 0.31 and 0.35 μmols$\\text{m}^{-2}\\text{s}^{-1}$for the young and old forests respectively; similar to the relative difference observed during summer. There was a significant seasonal pattern of soil CO₂ flux during the winter with fluxes averaging 0.22 μmols$\\text{m}^{-2}\\text{s}^{-1}$in December and January and increasing to an average of 0.61 μmols$\\text{m}^{-2}\\text{s}^{-1}$in May. Within-plot variability for measurements used in calculating flux was low. The coefficients of variation (CV) for CO₂ concentration, snowpack density, and snow depth were 17, 8 and 14%, respectively, yielding a CV for flux measurements within-plot of 29%. A within plot CV of 29% requires 8 sub-samples per plot to estimate the mean flux with a standard error of ± 10% of the mean. Variability in CO₂ flux estimates among plots (size = 400 m⁲) was similar to that within plot and was also low (CV = ∼28%). With a CV of 28% among plots, ten plots per treatment would have a 50% probability of detecting a 25% difference in treatment means for α = 0.05.

We measured soil changes through a full rotation of a Eucalyptus saligna (Sm.) plantation. We hypothesized that accretion of C from Eucalyptus trees (C3-derived carbon, C3-C) would be balanced by an equal loss of older soil C derived from sugarcane (Saccharum officinarum L.) agriculture (C4-derived C, C4-C). We also hypothesized that large additions of N-containing fertilizer would increase C accretion by increasing the rate of C addition and decreasing the rate of C loss. The low spatial variability of the soil and the intensive sampling design provided precise tests of these hypotheses. Soil C averaged 13.8 kg m-2 for the O horizon plus the 0- to 45-cm depth mineral soil, with no change through the rotation [95% confidence interval (CI) ±0.057 kg m-2 yr-1], supporting the first hypothesis. Significant gains of C3-C (0.136 kg m-2 yr-1) balanced the losses of C4-C (0.144 kg m-2 yr-1). The second hypothesis was tested in the field using three levels of repeated, complete fertilization (including N at rates of 300, 700, and 1600 kg N ha-1), and in laboratory incubations with N addition. Addition of N had no effect on the accumulation of soil N and C3-C, nor on the rate of loss of older C4-C, refuting the second hypothesis. This first-rotation forest plantation was not able to increase soil C, even with heavy fertilization. These results contrast markedly from the soil changes under the influence of N-fixing trees, indicating that the effect of N fixation on soil C derives from factors other than N supply.

Maintenance respiration for boles of four temperate conifers (ponderosa pine, western hemlock, red pine, and slash pine) were estimated from CO2 efflux measurements in autumn, when construction respiration is low or negligible. Maintenance respiration of stems was linearly related to sapwood volume for all species; at 10 deg C, respiration per unit sapwood volume ranged from 4.8 to 8.3 micromol CO2 m** (3) per second. For all sites combined, respiration increased exponentially with temperature (Q(1O) = 1.7, r(2) = 0.78). It is estimated that maintenance respiration of aboveground woody tissues of these conifers consumes 52-162 g C m** (2) per year, or 5-13% of net daytime carbon assimilation annually. The fraction of annual net daytime carbon fixation used for stem maintenance respiration increased linearly with the average annual temperature of the site