Total Belowground Carbon Allocation Research Articles

Differential Response of Soil Respiration and Total Belowground Carbon Allocation to Simulated Nitrogen and Phosphorus Deposition in Moso Bamboo Forests

Elevated nitrogen (N) and phosphorus (P) depositions have greatly affected belowground carbon processes in forest ecosystems. However, open questions still remained on the effects of N and P depositions on belowground carbon processes, including soil respiration (RS), its source components—autotrophic respiration (RA) and heterotrophic respiration (RH), and total belowground carbon allocation (TBCA) in Moso bamboo forests—one of the most important forest types with wide distributions in subtropical China. To fill this knowledge gap, a two-year N, P, and NP experiment was conducted in Moso bamboo forests. Results showed that RS, RA, and RH had a strong seasonal variability and were exponentially correlated with soil temperature. N and P depositions did not change RS and RA. However, P deposition increased RH due to the stimulation of microbial activities, indicating a significant soil carbon loss under P deposition. N and P depositions did not affect TBCA. However, NP deposition significantly increased root carbon-use efficiency. Net ecosystem production (NEP) varied from 198 ± 104 to 529 ± 225 g C m−2 year−1, indicating that Moso bamboo is an important carbon sink. P deposition marginally decreased NEP, while N and NP depositions did not affect NEP, which indicates that N deposition alleviated the suppression of P deposition on NEP. These findings highlight the inconsistent responses of RA, RH, and NEP to N, P, and NP depositions, which should be differently considered to increase the accuracy of predicting belowground carbon dynamics.

Belowground productivity varies by assessment technique, vegetation type, and nutrient availability in tidal freshwater forested wetlands transitioning to marsh.

Wetlands along upper estuaries are characterized by dynamic transitions between forested and herbaceous communities (marsh) as salinity, hydroperiod, and nutrients change. The importance of belowground net primary productivity (BNPP) associated with fine and coarse root growth also changes but remains the dominant component of overall productivity in these important blue carbon wetlands. Appropriate BNPP assessment techniques to use in various tidal wetlands are not well-defined, and could make a difference in BNPP estimation. We hypothesized that different BNPP techniques applied among tidal wetlands differ in estimation of BNPP and possibly also correlate differently with porewater nutrient concentrations. We compare 6-month and 12-month root ingrowth, serial soil coring techniques utilizing two different calculations, and a mass balance approach (TBCA, Total Belowground Carbon Allocation) among four tidal wetland types along each of two river systems transitioning from freshwater forest to marsh. Median values of BNPP were 266 to 2946 g/m2/year among all techniques used, with lower BNPP estimation from root ingrowth cores and TBCA (266–416 g/m2/year), and higher BNPP estimation from serial coring of standing crop root biomass (using Smalley and Max-Min calculation methods) (2336–2946 g/m2/year). Root turnover (or longevity) to a soil depth of 30 cm was 2.2/year (1.3 years), 2.7/year (1.1 years), 4.5/year (0.9 years), and 1.2/year (2.6 years), respectively, for Upper Forest, Middle Forest, Lower Forest, and Marsh. Marsh had greater root biomass and BNPP, with slower root turnover (greater root longevity) versus forested wetlands. Soil porewater concentrations of NH3 and reactive phosphorus stimulated BNPP in the marsh when assessed with short-deployment BNPP techniques, indicating that pulses of mineralized nutrients may stimulate BNPP to facilitate marsh replacement of forested wetlands. Overall, ingrowth techniques appeared to represent forested wetland BNPP adequately, while serial coring may be necessary to represent herbaceous plant BNPP from rhizomes as marshes replace forested wetlands.

Global patterns of soil autotrophic respiration and its relation to climate, soil and vegetation characteristics

Soil autotrophic respiration (RA) is one of the key components of carbon and nutrient cycling in terrestrial ecosystems. Despite conflicting findings on the dominant climatic, edaphic and vegetation controls of RA from localized studies, little is known about global RA patterns and their potential drivers. To fill the knowledge gap, we evaluated the direct and indirect effects of climatic, edaphic and vegetation variables on global RA using regression analysis and a structural equation model (SEM) based on the most updated Global Soil Respiration Dataset with 499 observations. Results showed that mean (±standard deviation) RA was 358 ± 283 g C m−2 yr−1 across global terrestrial ecosystems and differed significantly among forest ecosystems, croplands and grasslands (p < 0.01), highlighting the importance of vegetation on RA. Mean RA was the highest in evergreen broad-leaf forests (633 ± 397 g C m−2 yr−1) and the lowest in deciduous needle-leaf forests (226 ± 170 g C m−2 yr−1). RA was positively correlated with mean annual temperature and precipitation, gross primary production (GPP), ecosystem respiration (ER) and total belowground carbon allocation (TBCA), whereas it was negatively correlated with silt and clay contents, and a unimodal relation was observed between RA and sand content with a maximum at 68%. These findings implied that the amount of carbon respired from roots back into the atmosphere may increase in the context of long-term climate change, mediated by soil and vegetation. SEM analysis indicated that TBCA was the most important factor controlling RA, accounting for 50% of the variation in RA at the global scale. Globally, RA ratios to GPP and ER were 20% and 23%, respectively, which, combined with developed global GPP and ER products, provide a new concept to estimate the spatiotemporal patterns of RA at regional or global scales. However, to increase the reliability of global RA estimates using these two ratios, more simultaneous observations of RA, ER or GPP from different ecosystems are strongly recommended.

Does plant biomass partitioning reflect energetic investments in carbon and nutrient foraging?

Abstract Studies of plant resource‐use strategies along environmental gradients often assume that dry matter partitioning represents an individual's energy investment in foraging for above‐ versus below‐ground resources. However, ecosystem‐level studies of total below‐ground carbon allocation (TBCA) in forests do not support the equivalency of energy (carbon) and dry matter partitioning, in part because allocation of carbon to below‐ground pools and fluxes that are not accounted for by root biomass (e.g., mycorrhizal hyphae, rhizodeposition; root and soil respiration) can be substantial. Here, we apply this reasoning to individual plants in controlled environments and ask whether dry matter partitioning below‐ground (root mass fraction, RMF) accurately reflects TBCA in studies of optimal partitioning theory. We quantified the relationship between RMF and TBCA in individual plants, using 311 observations from 51 studies that simultaneously measured both allocation variables. Our analysis included tests of whether the RMF‐TBCA relationship depended on mutualist soil microbes, plant growth form, age and study methodology including isotopic pulse–chase duration. We found that RMF was a poor proxy for below‐ground energy investment. This disconnect of RMF and TBCA was driven in part by plants of low RMF (&lt;0.4) exhibiting significantly higher rates of root and soil respiration per unit root mass than plants of high RMF. Root colonization by mutualist microbes, including arbuscular mycorrhizal fungi and nitrogen‐fixing bacteria, increased TBCA by 5%–7%, and TBCA was lower in grasses than other species by 9%–16%. These patterns were evident for relationships assessed both within and between species. We conclude that optimal partitioning studies of plants along environmental gradients are likely to underestimate plant energy allocation below‐ground if the C costs of root and soil respiration are ignored, especially under conditions favouring low RMF. Because energy rather than biomass better reflects how assimilated C supports fitness, this omission of respired C suggests existing studies misrepresent the significance of below‐ground processes to plant function. A free Plain Language Summary can be found within the Supporting Information of this article.

Allocation of assimilated carbon in paddies depending on rice age, chase period and N fertilization: Experiment with 13CO2 labelling and literature synthesis

Quantification of total belowground carbon (C) input and allocation to various pools in rice–soil systems depending on plant age, chase period, and nitrogen (N) availability. Rice plants with and without N fertilization were 13CO2 pulse labelled at tillering, elongation, heading, and filling stages and were destructively sampled after 6 h of labelling and at the final harvest. The allocation of C by rice was also generalized based on literature pertaining to 94 studies with respect to plant age and chase period. The C allocation in roots and soil strongly decreased with plant age. The literature review showed that C allocation to roots increased within the first 10 days of labelling and remained stable thereafter. Nitrogen fertilization had no effect on C allocation immediately after assimilation, but increased C remained belowground at 1.7-times that at final harvest. The total belowground net C input by one rice crop was 630–1080 kg C ha−1, including rhizodeposition of 160–330 kg C ha−1. Multiple pulse labelling at various plant growth stages and taking multiple subsequent samples as well as nutrient availability should be considered for tracing C flows more accurately for precise C balance in rice paddy systems.

Effects of understory removal on root production, turnover and total belowground carbon allocation in Moso bamboo forests

Abstract: Although the role of roots has been highlighted in carbon and nutrient cycles in forest ecosystems, root production, turnover and total belowground carbon allocation (TBCA) under different management regimes in Moso bamboo forests have not been determined to date. In this study, sequential soil cores were collected at two soil depths (0-20 cm and 20-40 cm) to assess the effects of understory removal on root production, turnover, and TBCA in Moso bamboo forests in subtropical China. A total of 1080 sequential soil cores were collected from April 2011 to March 2012. Understory removal significantly reduced fine root biomass and production for both soil layers (p stand II (457.7 g C m-2 a-1) > stand III (404.9 g C m-2 a-1), though the differences were not statistically significant (p > 0.05). However, TBCA calculated from the mass balance approach showed a reverse trend compared to the component cumulative approach. The TBCA of stand III was significantly higher than that of stand I and stand II (p < 0.05), demonstrating that the belowground process is complex and standardizing the method of estimation of TBCA is extremely important in global carbon cycle modeling.

Carbon fluxes in coniferous and deciduous forest soils

Our aims were to identify responsible factors for the site-to-site variability in soil CO2 efflux and to assess the sources of soil CO2 of different forest types on a regional scale. Soil CO2 effluxes were measured over 1–4 years in four coniferous and three deciduous forests of Bavaria, Germany, and related to climate, soil properties and forest productivity. Total belowground carbon allocation (TBCA) was assessed using soil CO2 effluxes and aboveground litterfall. Additionally, CO2 production of organic layers was examined over 10 months under constant conditions in an incubation experiment. Annual soil CO2 effluxes were not different among the forest sites, but predicted effluxes at a given temperature of 10°C revealed some significant differences and correlated with the phosphorous stock of the organic layers. The incubation study indicated 50% faster decomposition of organic layers from deciduous than from coniferous forests. TBCA related to soil CO2 efflux was smaller in the deciduous than in the coniferous forests. The ratio of TBCA to soil CO2 efflux was positively correlated with the C stock of organic layers. Our results suggest that marked differences in site characteristics have little impact on soil CO2 effluxes at the regional scale, but the contribution of soil CO2 sources varies among the forest types.

Long- and short-term precipitation effects on soil CO2 efflux and total belowground carbon allocation

Soil CO2 efflux (Esoil), the main pathway of C movement from the biosphere to the atmosphere, is critical to the terrestrial C cycle but how precipitation and soil moisture influence Esoil remains poorly understood. Here, we irrigated a longleaf pine wiregrass savanna for six years; this increased soil moisture by 41.2%. We tested how an altered precipitation regime affected total belowground carbon allocation (TBCA), root growth, soil carbon, and Esoil. We used two methods to quantify Esoil: daytime biweekly manual measurements and automated continuous measurements for one year. We hypothesized that the low-frequency manual method would miss both short- and long-term (i.e., subdaily to annual, respectively) effects of soil moisture on Esoil while the high-frequency data from the automated method would allow the effects of soil moisture to be discerned. Root growth was significantly higher in irrigated plots, particularly at 0–20cm depth. Irrigated annual Esoil was significantly greater than that of the control when estimated with the continuous measurements but not when estimated from biweekly measurements. The difference in annual Esoil estimates is likely due to (1) the delayed increase in Esoil following irrigation pulses of soil moisture (i.e., variation that the biweekly manual measurements missed) and (2) the diel timing of biweekly manual measurements (they were completed early to mid-day before peak efflux). With irrigation, estimates of TBCA increased almost two-fold with automated measurements but only 36% with intermittent measurements. Relative to controls, irrigated treatments stored almost 2MgCha−1year−1 more in soils and 0.26MgCha−1year−1 more in roots. High-frequency measurements of Esoil were essential to estimate total belowground carbon allocation. With irrigation, soil carbon pools were not at steady-state, so shifts in soil carbon storage must be considered in TBCA estimates.

Effect of canopy composition on soil CO2 emission in a mixed sprucebeech forest at Solling, Central Germany

It was hypothesized that soil respiration can be affected by canopy composition. Hence, admixture of trees as a common forest management practice may cause significant change in the carbon cycling. This study was conducted in a mixed spruce-beech stand at Solling forest in central Germany to investigate the effect of canopy composition on soil respiration. The canopy cover was classified in four major canopy classes (pure beech, pure spruce, mixed and gap), and the area under each canopy class was identified as a sub-plot. Soil respiration in each sub-plot (n=4) was measured monthly from Jun 2005 to July 2006. Results show significant difference in annual soil respiration between the beech (359 g·m−2·a−1 C) and gap (211 g·m−2·a−1 C) sub-plots. The estimation of the total below-ground carbon allocation (TBCA) based on a model given by Raich and Nadelhoffer revealed considerably higher root CO2 production in the beech sub-plot (231 g·m−2·a−1 C) compare to the gap sub-plot (51 g·m−2·a−1 C). The contribution of the root respiration to the total soil respiration was higher in the soil under the beech canopy (59%) compared with the soil in the gap (29%). The findings suggested that the condition under the beech canopy may cause more desirable micro-site for autotrophic respiration and consequently higher CO2 release into the atmosphere.

Influence of stand density on soil CO2 efflux for a Pinus densiflora forest in Korea

We investigated the influence of stand density [938 tree ha(-1) for high stand density (HD), 600 tree ha(-1) for medium stand density (MD), and 375 tree ha(-1) for low stand density (LD)] on soil CO(2) efflux (R (S)) in a 70-year-old natural Pinus densiflora S. et Z. forest in central Korea. Concurrent with R (S) measurements, we measured litterfall, total belowground carbon allocation (TBCA), leaf area index (LAI), soil temperature (ST), soil water content (SWC), and soil nitrogen (N) concentration over a 2-year period. The R (S) (t C ha(-1) year(-1)) and leaf litterfall (t C ha(-1) year(-1)) values varied with stand density: 6.21 and 2.03 for HD, 7.45 and 2.37 for MD, and 6.96 and 2.23 for LD, respectively. In addition, R (S) was correlated with ST (R (2) = 0.77-0.80, P < 0.001) and SWC (R (2) = 0.31-0.35, P < 0.001). It appeared that stand density influenced R (S) via changes in leaf litterfall, LAI and SWC. Leaf litterfall (R (2) = 0.71), TBCA (R (2) = 0.64-0.87), and total soil N contents in 2007 (R (2) = 0.94) explained a significant amount of the variance in R (S) (P < 0.01). The current study showed that stand density is one of the key factors influencing R (S) due to the changing biophysical and environmental factors in P. densiflora.

Carbon and nitrogen balances for six shrublands across Europe

Shrublands constitute significant and important parts of European landscapes providing a large number of important ecosystem services. Biogeochemical cycles in these ecosystems have gained little attention relative to forests and grassland systems, but data on such cycles are required for developing and testing ecosystem models. As climate change progresses, the potential feedback from terrestrial ecosystems to the atmosphere through changes in carbon stocks, carbon sequestration, and general knowledge on biogeochemical cycles becomes increasingly important. Here we present carbon and nitrogen balances of six shrublands along a climatic gradient across the European continent. The aim of the study was to provide a basis for assessing the range and variability in carbon storage in European shrublands. Across the sites the net carbon storage in the systems ranged from 1,163 g C m−2 to 18,546 g C m−2, and the systems ranged from being net sinks (126 g C m−2 a−1) to being net sources (−536 g C m−2 a−1) of carbon with the largest storage and sink of carbon at wet and cold climatic conditions. The soil carbon store dominates the carbon budget at all sites and in particular at the site with a cold and wet climate where soil C constitutes 95% of the total carbon in the ecosystem. Respiration of carbon from the soil organic matter pool dominated the carbon loss at all sites while carbon loss from aboveground litter decomposition appeared less important. Total belowground carbon allocation was more than 5 times aboveground litterfall carbon which is significantly greater than the factor of 2 reported in a global analysis of forest data. Nitrogen storage was also dominated by the soil pools generally showing small losses except when atmospheric N input was high. The study shows that in the future a climate‐driven land cover change between grasslands and shrublands in Europe will likely lead to increased ecosystem C where shrublands are promoted and less where grasses are promoted. However, it also emphasizes that if feedbacks on the global carbon cycle are to be predicted it is critically important to quantify and understand belowground carbon allocation and processes as well as soil carbon pools, particularly on wet organic soils, rather than plant functional change as the soil stores dominate the overall budget and fluxes of carbon.

Interactive Effects of Time, CO2, N, and Diversity on Total Belowground Carbon Allocation and Ecosystem Carbon Storage in a Grassland Community

Predicting if ecosystems will mitigate or exacerbate rising CO2 requires understanding how elevated CO2 will interact with coincident changes in diversity and nitrogen (N) availability to affect ecosystem carbon (C) storage. Yet achieving such understanding has been hampered by the difficulty of quantifying belowground C pools and fluxes. Thus, we used mass balance calculations to quantify the effects of diversity, CO2, and N on both the total amount of C allocated belowground by plants (total belowground C allocation, TBCA) and ecosystem C storage in a periodically burned, 8-year Minnesota grassland biodiversity, CO2, and N experiment (BioCON). Annual TBCA increased in response to elevated CO2, enriched N, and increasing diversity. TBCA was positively related to standing root biomass. After removing the influence of root biomass, the effect of elevated CO2 remained positive, suggesting additional drivers of TBCA apart from those that maintain high root biomass. Removing root biomass effects resulted in the effects of N and diversity becoming neutral or negative (depending on year), suggesting that the positive effects of diversity and N on TBCA were related to treatment-driven differences in root biomass. Greater litter production in high diversity, elevated CO2, and enhanced N treatments increased annual ecosystem C loss in fire years and C gain in non-fire years, resulting in overall neutral C storage rates. Our results suggest that frequently burned grasslands are unlikely to exhibit enhanced C sequestration with increasing atmospheric CO2 levels or N deposition.

Soil Respiration and Belowground Carbon Allocation in Mangrove Forests

Mangrove forests cover large areas of tropical and subtropical coastlines. They provide a wide range of ecosystem services that includes carbon storage in above- and below ground biomass and in soils. Carbon dioxide (CO2) emissions from soil, or soil respiration is important in the global carbon budget and is sensitive to increasing global temperature. To understand the magnitude of mangrove soil respiration and the influence of forest structure and temperature on the variation in mangrove soil respiration I assessed soil respiration at eleven mangrove sites, ranging from latitude 27°N to 37°S. Mangrove soil respiration was similar to those observed for terrestrial forest soils. Soil respiration was correlated with leaf area index (LAI) and aboveground net primary production (litterfall), which should aid scaling up to regional and global estimates of soil respiration. Using a carbon balance model, total belowground carbon allocation (TBCA) per unit litterfall was similar in tall mangrove forests as observed in terrestrial forests, but in scrub mangrove forests TBCA per unit litter fall was greater than in terrestrial forests, suggesting mangroves allocate a large proportion of their fixed carbon below ground under unfavorable environmental conditions. The response of soil respiration to soil temperature was not a linear function of temperature. At temperatures below 26°C Q10 of mangrove soil respiration was 2.6, similar to that reported for terrestrial forest soils. However in scrub forests soil respiration declined with increasing soil temperature, largely because of reduced canopy cover and enhanced activity of photosynthetic benthic microbial communities.

Production and carbon allocation in a clonal Eucalyptus plantation with water and nutrient manipulations

We examined resource limitations on growth and carbon allocation in a fast-growing, clonal plantation of Eucalyptus grandis × urophylla in Brazil by characterizing responses to annual rainfall, and response to irrigation and fertililization for 2 years. Productivity measures included gross primary production (GPP), total belowground carbon allocation (TBCA), bole growth, and net ecosystem production (NEP). Replicate plots within a single plantation were established at the midpoint of the rotation (end of year 3), with treatments of no additional fertilization or irrigation, heavy fertilization (to remove any nutrient limitation), irrigation (to remove any water limitation), and irrigation plus fertilization. Rainfall was unusually high in the first year (1769 mm) of the experiment, and control plots had high rates of GPP (6.64 kg C m −2 year −1), TBCA (2.14 kg C m −2 year −1), and bole growth (1.81 kg C m −2 year −1). Irrigation increased each of these rates by 15–17%. The second year of the experiment had average rainfall (1210 mm), and lower rainfall decreased production in control plots by 46% (GPP), 52% (TBCA), and 40% (bole growth). Fertilization treatments had neglible effects. The response to irrigation was much greater in the drier year, with irrigated plots exceeding the production in control plots by 83% (GPP), 239% (TBCA), and 24% (bole growth). Even though the rate of irrigation ensured no water limitation to tree growth, the high rainfall year showed higher production in irrigated plots for both GPP (38% greater than in drier year) and bole growth (23% greater). Varying humidity and supplies of water led to a range in NEP of 0.8–2.7 kg C m −2 year −1. This difference between control and irrigated treatments, combined with differences between drier and wetter years, indicated a strong response of these Eucalyptus trees to both water supply and atmospheric humidity during the dry season. The efficiency of converting light energy into fixed carbon ranged from a low of 0.027 mol C to a high of 0.060 mol C per mol of absorbed photosynthetically active radiation (APAR), and the efficiency of bolewood production ranged from 0.78 to 1.98 g wood per MJ of APAR. Irrigation increased the efficiency of wood production per unit of water used from 2.55 kg wood m −3 in the rainfed plot to 3.51 kg m −3 in irrigated plots. Detailed information on the response of C budgets to environmental conditions and resource supplies will be necessary for accurate predictions of plantation yields across years and landscapes.

Variation in below‐ground carbon fluxes along a Populus hybridization gradient

Here, soil CO(2) efflux, minirhizotron fine root production (FRP), and estimated total below-ground carbon allocation (TBCA) were examined along an elevation and hybridization gradient between two cottonwood species. FRP was 72% greater under high-elevation Populus angustifolia, but soil CO(2) efflux and TBCA were 62% and 94% greater, respectively, under low-elevation stands dominated by Populus fremontii, with a hybrid stand showing intermediate values. Differences between the responses of FRP, soil CO(2) efflux and TBCA may potentially be explained in terms of genetic controls; while plant species and hybridization explained variance in carbon flux, we found only weak correlations of FRP and TBCA with soil moisture, and no correlations with soil temperature or nitrogen availability. Soil CO(2) efflux and TBCA were uncorrelated with FRP, suggesting that, although below-ground carbon fluxes may change along environmental and genetic gradients, major components of below-ground carbon flux may be decoupled.

EFFECTS OF TREE DENSITY AND STAND AGE ON CARBON ALLOCATION PATTERNS IN POSTFIRE LODGEPOLE PINE

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 (&lt;1000 trees/ha), moderate (7000–40 000 trees/ha), and high tree densities (&gt;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·m−2·yr−1, and TBCA was 68, 237, and 306 g C·m−2·yr−1 for low‐, moderate‐, and high‐density young stands, respectively), and with stand age (ANPP was 218 g C·m−2·yr−1 and TBCA was 382 g C·m−2·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.

AN EXPERIMENTAL TEST OF THE CAUSES OF FOREST GROWTH DECLINE WITH STAND AGE

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 6½ 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·m−2·yr−1. Aboveground wood production declined from 1.4 kg C·m−2·yr−1 at age 2 to 0.60 kg C·m−2·yr−1 at age 6. Hypothesis 1 failed: GPP declined from 5.0 kg C·m−2·yr−1 at age 2 to 3.2 kg C·m−2·yr−1 at age 6. Aboveground woody respiration declined from 0.66 kg C·m−2·yr−1 at age 2 to 0.22 kg C·m−2·yr−1 at age 6 and TBCA declined from 1.9 kg C·m−2·yr−1 at age 2 to 1.4 kg C·m−2·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.

Primary production and carbon allocation in relation to nutrient supply in a tropical experimental forest

AbstractNutrient supply commonly limits aboveground plant productivity in forests, but the effects of an altered nutrient supply on gross primary production (GPP) and patterns of carbon (C) allocation remain poorly characterized. Increased nutrient supply may lead to a higher aboveground net primary production (ANPP), but a lower total belowground carbon allocation (TBCA), with little change in either aboveground plant respiration (APR) or GPP. Alternatively, increases in nutrient supply may increase GPP, with the quantity of GPP allocated aboveground increasing more steeply than the quantity of GPP allocated belowground. To examine the effects of an elevated nutrient supply on the C allocation patterns in forests, we determined whole‐ecosystem C budgets in unfertilized plots of Eucalyptus saligna and in adjacent plots receiving regular additions of 65 kg N ha−1, 31 kg P ha−1, 46 kg K ha−1, and macro‐ and micronutrients. We measured the absolute flux of C allocated to the components of GPP (ANPP, TBCA and APR), as well as the fraction of GPP allocated to these components.Fertilization dramatically increased GPP. Averaged over 3 years, GPP in the fertilized plots was 34% higher than that in the unfertilized controls (3.95 vs. 2.95 kg C m−2 yr−1). Fertilization‐related increases in GPP were allocated entirely aboveground – ANPP was 85% higher and APR was 57% higher in the fertilized than in the control plots, while TBCA did not differ significantly between treatments. Carbon use efficiency (NPP/GPP) was slightly higher in the fertilized (0.53) compared with the control plots (0.51). Overall, fertilization increased ANPP and APR, and these increases were related to a greater GPP and an increase in the fraction of GPP allocated aboveground.

Belowground carbon allocation in forests estimated from litterfall and IRGA-based soil respiration measurements

Allocation of C to belowground plant structures is one of the most important, yet least well quantified fluxes of C in terrestrial ecosystems. In a literature review of mature forests worldwide, Raich and Nadelhoffer (1989) suggested that total belowground carbon allocation (TBCA) could be estimated from the difference between annual rates of soil respiration and aboveground litterfall. Here we analyze new measurements of soil respiration and litterfall, including data from the Ameriflux network. Our results generally agree with Raich and Nadelhoffer’s previous work. A regression analysis of data from mature forests produced the following relationship: annual soil respiration=287+2.80×annual litterfall. This regression slope indicates that, on average, soil respiration is roughly three times aboveground litterfall-C, which further implies that TBCA is roughly twice annual aboveground litterfall-C. These inferences are based on the uncertain assumption of soil C stocks being at steady state. Nevertheless, changes in soil C would have to be very large to modify the conclusion that TBCA is generally much larger than litterfall. Among only mature temperate hardwood forests, however, the correlation between litterfall and soil respiration was poor, and the correlation among years for a single site was also poor. Therefore, the regression cannot be relied upon to provide accurate estimates of soil respiration or TBCA for individual sites. Moreover, interannual variation in TBCA, short-term changes in C stocks, or different temporal scales controlling leaf litter production and soil respiration may cause important deviations from the global average. The regression slope for data from young forests is steeper, possibly indicating proportionally greater TBCA, but the steady-state assumption is more problematic for young forests. This method for estimating TBCA may be most appropriate where interannual variation is averaged over several years of observations and where a near-steady-state assumption of soil, litter, and root C stocks is least problematic.

Total Belowground Carbon Allocation in a Fast-growing Eucalyptus Plantation Estimated Using a Carbon Balance Approach

Trees allocate a large portion of gross primary production belowground for the production and maintenance of roots and mycorrhizae. The difficulty of directly measuring total belowground carbon allocation (TBCA) has limited our understanding of belowground carbon (C) cycling and the factors that control this important flux. We measured TBCA over 4 years using a conservation of mass, C balance approach in replicate stands of fast growing Eucalyptus saligna Smith with different nutrition management and tree density treatments. We measured TBCA as surface carbon dioxide (CO2) efflux (“soil” respiration) minus C inputs from aboveground litter plus the change in C stored in roots, litter, and soil. We evaluated this C balance approach to measuring TBCA by examining (a) the variance in TBCA across replicate plots; (b) cumulative error associated with summing components to arrive at our estimates of TBCA; (c) potential sources of error in the techniques and assumptions; (d) the magnitude of changes in C stored in soil, litter, and roots compared to TBCA; and (e) the sensitivity of our measures of TBCA to differences in nutrient availability, tree density, and forest age. The C balance method gave precise estimates of TBCA and reflected differences in belowground allocation expected with manipulations of fertility and tree density. Across treatments, TBCA averaged 1.88 kg C m 2 y 1 and was 18% higher in plots planted with 10 4 trees/ha compared to plots planted with 1111 trees/ha. TBCA was 12% lower (but not significantly so) in fertilized plots. For all treatments, TBCA declined linearly with stand age. The coefficient of variation (CV) for TBCA for replicate plots averaged 17%. Averaged across treatments and years, annual changes in C stored in soil, the litter layer, and coarse roots (0.01, 0.06, and 0.21 kg Cm 2 y 1 , respectively) were small compared with surface CO2 efflux (2.03 kg C m 2 y 1 ), aboveground litterfall (0.42 kg C m 2 y 1 ), and our estimated TBCA (1.88 kg C m 2 y 1 ). Based on