The response of coarse root biomass to long-term CO2 enrichment and nitrogen application in a maturing Pinus taeda stand with a large broadleaved component.

Elevated atmospheric CO2 (eCO2 ) typically increases aboveground growth in both growth chamber and free-air carbon enrichment (FACE) studies. Here we report on the impacts of eCO2 and nitrogen amendment on coarse root biomass and net primary productivity (NPP) at the Duke FACE study, where half of the eight plots in a 30-year-old loblolly pine (Pinus taeda, L.) plantation, including competing naturally regenerated broadleaved species, were subjected to eCO2 (ambient, aCO2 plus 200 ppm) for 15-17 years, combined with annual nitrogen amendments (11.2 g N m-2 ) for 6 years. Allometric equations were developed following harvest to estimate coarse root (>2 mm diameter) biomass. Pine root biomass under eCO2 increased 32%, 1.80 kg m-2 above the 5.66 kg m-2 observed in aCO2 , largely accumulating in the top 30 cm of soil. In contrast, eCO2 increased broadleaved root biomass more than twofold (aCO2 : 0.81, eCO2 : 2.07 kg m-2 ), primarily accumulating in the 30-60 cm soil depth. Combined, pine and broadleaved root biomass increased 3.08 kg m-2 over aCO2 of 6.46 kg m-2 , a 48% increase. Elevated CO2 did not increase pine root:shoot ratio (average 0.24) but increased the ratio from 0.57 to 1.12 in broadleaved species. Averaged over the study (1997-2010), eCO2 increased pine, broadleaved and total coarse root NPP by 49%, 373% and 86% respectively. Nitrogen amendment had smaller effects on any component, singly or interacting with eCO2 . A sustained increase in root NPP under eCO2 over the study period indicates that soil nutrients were sufficient to maintain root growth response to eCO2 . These responses must be considered in computing coarse root carbon sequestration of the extensive southern pine and similar forests, and in modelling the responses of coarse root biomass of pine-broadleaved forests to CO2 concentration over a range of soil N availability.

Biomass increases attributed to both faster tree growth and altered allometric relationships under long-term carbon dioxide enrichment at a temperate forest.

Increases in atmospheric carbon dioxide (CO2 ) concentrations are expected to lead to increases in the rate of tree biomass accumulation, at least temporarily. On the one hand, trees may simply grow faster under higher CO2 concentrations, preserving the allometric relations that prevailed under lower CO2 concentrations. Alternatively, the allometric relations themselves may change. In this study, the effects of elevated CO2 (eCO2 ) on tree biomass and allometric relations were jointly assessed. Over 100 trees, grown at Duke Forest, NC, USA, were harvested from eight plots. Half of the plots had been subjected to CO2 enrichment from 1996 to 2010. Several subplots had also been subjected to nitrogen fertilization from 2005 to 2010. Allometric equations were developed to predict tree height, stem volume, and aboveground biomass components for loblolly pine (Pinus taeda L.), the dominant tree species, and broad-leaved species. Using the same diameter-based allometric equations for biomass, it was estimated that plots with eCO2 contained 21% more aboveground biomass, consistent with previous studies. However, eCO2 significantly affected allometry, and these changes had an additional effect on biomass. In particular, P. taeda trees at a given diameter were observed to be taller under eCO2 than under ambient CO2 due to changes in both the allometric scaling exponent and intercept. Accounting for allometric change increased the treatment effect of eCO2 on aboveground biomass from a 21% to a 27% increase. No allometric changes for the nondominant broad-leaved species were identified, nor were allometric changes associated with nitrogen fertilization. For P. taeda, it is concluded that eCO2 affects allometries, and that knowledge of allometry changes is necessary to accurately compute biomass under eCO2 . Further observations are needed to determine whether this assessment holds for other taxa.

Dynamics of soil CO2 efflux under varying atmospheric CO2 concentrations reveal dominance of slow processes.

We evaluated the effect on soil CO2 efflux (FCO2 ) of sudden changes in photosynthetic rates by altering CO2 concentration in plots subjected to +200 ppmv for 15 years. Five-day intervals of exposure to elevated CO2 (eCO2 ) ranging 1.0-1.8 times ambient did not affect FCO2 . FCO2 did not decrease until 4 months after termination of the long-term eCO2 treatment, longer than the 10 days observed for decrease of FCO2 after experimental blocking of C flow to belowground, but shorter than the ~13 months it took for increase of FCO2 following the initiation of eCO2 . The reduction of FCO2 upon termination of enrichment (~35%) cannot be explained by the reduction of leaf area (~15%) and associated carbohydrate production and allocation, suggesting a disproportionate contraction of the belowground ecosystem components; this was consistent with the reductions in base respiration and FCO2 -temperature sensitivity. These asymmetric responses pose a tractable challenge to process-based models attempting to isolate the effect of individual processes on FCO2 .

On the complementary relationship between marginal nitrogen and water-use efficiencies among Pinus taeda leaves grown under ambient and CO2-enriched environments.

BACKGROUND AND AIMS: Water and nitrogen (N) are two limiting resources for biomass production of terrestrial vegetation. Water losses in transpiration (E) can be decreased by reducing leaf stomatal conductance (g(s)) at the expense of lowering CO(2) uptake (A), resulting in increased water-use efficiency. However, with more N available, higher allocation of N to photosynthetic proteins improves A so that N-use efficiency is reduced when g(s) declines. Hence, a trade-off is expected between these two resource-use efficiencies. In this study it is hypothesized that when foliar concentration (N) varies on time scales much longer than g(s), an explicit complementary relationship between the marginal water- and N-use efficiency emerges. Furthermore, a shift in this relationship is anticipated with increasing atmospheric CO(2) concentration (c(a)). METHODS: Optimization theory is employed to quantify interactions between resource-use efficiencies under elevated c(a) and soil N amendments. The analyses are based on marginal water- and N-use efficiencies, λ = (∂A/∂g(s))/(∂E/∂g(s)) and η = ∂A/∂N, respectively. The relationship between the two efficiencies and related variation in intercellular CO(2) concentration (c(i)) were examined using A/c(i) curves and foliar N measured on Pinus taeda needles collected at various canopy locations at the Duke Forest Free Air CO(2) Enrichment experiment (North Carolina, USA). KEY RESULTS: Optimality theory allowed the definition of a novel, explicit relationship between two intrinsic leaf-scale properties where η is complementary to the square-root of λ. The data support the model predictions that elevated c(a) increased η and λ, and at given c(a) and needle age-class, the two quantities varied among needles in an approximately complementary manner. CONCLUSIONS: The derived analytical expressions can be employed in scaling-up carbon, water and N fluxes from leaf to ecosystem, but also to derive transpiration estimates from those of η, and assist in predicting how increasing c(a) influences ecosystem water use.

Relationships between stem CO(2) efflux, substrate supply, and growth in young loblolly pine trees.

*We examined the relationships between stem CO(2) efflux (E(s)), diameter growth, and nonstructural carbohydrate concentration in loblolly pine trees. Carbohydrate supply was altered via stem girdling during rapid stem growth in the spring and after growth had ceased in the autumn. We hypothesized that substrate type and availability control the seasonal variation and temperature sensitivity of E(s). *The E(s) increased and decreased above and below the girdle, respectively, within 24 h of treatment. Seasonal variation in E(s) response to girdling corresponded to changes in stem soluble sugar and starch concentration. Relative to nongirdled trees, E(s) increased 94% above the girdle and decreased 50% below in the autumn compared with a 60% and 20% response at similar positions in the spring. *The sensitivity of E(s) to temperature decreased below the girdle in the autumn and spring and increased above the girdle but only in the autumn. Temperature-corrected E(s) was linearly related to soluble sugar (R(2) = 0.57) and starch (R(2) = 0.62) concentration. *We conclude that carbohydrate supply, primarily recently fixed photosynthate, strongly influences E(s) in Pinus taeda stems. Carbohydrate availability effects on E(s) obviate the utility of applying short-term temperature response functions across seasons.

Soil incorporation of logging residue affects fine-root and mycorrhizal root-tip dynamics of young loblolly pine clones.

Loblolly pine (Pinus taeda L.) plantations cover a large geographic area of the southeastern USA and supply a large proportion of the nation's wood products. Research on management strategies designed to maximize wood production while also optimizing nutrient use efficiency and soil C sequestration is needed. We used minirhizotrons to quantify the effects of incorporating logging residues into soil on fine-root standing crop, production and mortality, and mycorrhizal root tips in young loblolly pine clones of contrasting ideotypes. Clone 93 is known to allocate more C to stem growth, while clone 32 allocates less C to stems and more to leaves. The relative allocation by these clones to support fine-root turnover is unknown. Clone 32 exhibited 37% more fine-root mortality than clone 93, which was mainly the result of a greater standing crop of fine roots. Fine-root standing crop in plots amended with logging residue was initially higher than control plots, but 2.5 years after planting, standing crop in control plots had exceeded that in mulched plots. Production of mycorrhizal root tips, on the other hand, was initially higher in control than mulched plots, but during the last 9 months of the study, mycorrhizal tip production was greater in mulched than control plots, especially for clone 93. As expected, turnover rate of fine roots was greater in surface soil (0-25 cm) compared with deeper (25-50 cm) soil and for small roots (< 0.4 mm diameter) compared with larger fine roots (0.4-2.0 mm diameter). Rates of fine-root turnover were similar in both clones. Organic matter additions reduced survivorship of individual roots and increased turnover rates of fine-root populations. Results indicate that management decisions should be tailored to fit the growth and allocation patterns of available clones.

Acclimation of leaf hydraulic conductance and stomatal conductance of Pinus taeda (loblolly pine) to long-term growth in elevated CO(2) (free-air CO(2) enrichment) and N-fertilization.

We investigated how leaf hydraulic conductance (K(leaf)) of loblolly pine trees is influenced by soil nitrogen amendment (N) in stands subjected to ambient or elevated CO(2) concentrations (CO(2)(a) and CO(2)(e), respectively). We also examined how K(leaf) varies with changes in reference leaf water potential (Psi(leaf-ref)) and stomatal conductance (g(s-ref)) calculated at vapour pressure deficit, D of 1 kPa. We detected significant reductions in K(leaf) caused by N and CO(2)(e), but neither treatment affected pre-dawn or midday Psi(leaf). We also detected a significant CO(2)(e)-induced reduction in g(s-ref) and Psi(leaf-ref). Among treatments, the sensitivity of K(leaf) to Psi(leaf) was directly related to a reference K(leaf) (K(leaf-ref) computed at Psi(leaf-ref)). This liquid-phase response was reflected in a similar gas-phase response, with g(s) sensitivity to D proportional to g(s-ref). Because leaves represented a substantial component of the whole-tree conductance, reduction in K(leaf) under CO(2)(e) affected whole-tree water use by inducing a decline in g(s-ref). The consequences of the acclimation of leaves to the treatments were: (1) trees growing under CO(2)(e) controlled morning leaf water status less than CO(2)(a) trees resulting in a higher diurnal loss of K(leaf); (2) the effect of CO(2)(e) on g(s-ref) was manifested only during times of high soil moisture.

Pinus taeda clones and soil nutrient availability: effects of soil organic matter incorporation and fertilization on biomass partitioning and leaf physiology.

The combined effects of intensive management and planting of improved seedlings have led to large increases in productivity on intensively managed pine forests in the southeastern United States. To best match clones to particular site conditions, an understanding of how specific clones respond to changes in nutrition in terms of biomass partitioning, leaf physiology and biochemistry will be necessary. This study measured the response of biomass partitioning, light-saturated net photosynthesis (A(Sat)) and photosynthetic capacity to a range in soil fertility and fertilization between two contrasting Pinus taeda L. clone ideotypes: a 'narrow crown' clone (NC) that allocates more resources to stem growth and a 'broad crown' clone (BC) that allocates more resources to leaf area (LA). Under field conditions, we found consistent clone by environment (i.e., varying nutrient regimes) interactions in biomass as well as leaf physiology. Nutrient limitations induced by logging residue incorporation resulted in a 25% loss in stem growth in BC, while NC showed no response. We postulated that the decrease in BC was due to the differences in canopy architecture leading to a reduced canopy CO(2) assimilation, as well as to increased belowground maintenance costs associated with fine-root production. In contrast, N and P additions resulted in a 21% greater increase in stem volume in NC relative to BC. Fertilization increased A(Sat) temporarily in both clones, but A(Sat) eventually decreased below control levels by the end of the study. Although we found a clone by fertilization interaction in leaf physiology, the greatest genotype by environment interaction was found in the LA that appeared to have a greater influence than A(Sat) on growth. This research demonstrates the potential importance of selecting appropriate clonal material and silvicultural prescription when implementing site-specific silviculture to maximize productivity in intensively managed southern pine forests.

Short-term effects of fertilization on photosynthesis and leaf morphology of field-grown loblolly pine following long-term exposure to elevated CO(2) concentration.

We examined effects of a first nitrogen (N) fertilizer application on upper-canopy needle morphology and gas exchange in approximately 20-m-tall loblolly pine (Pinus taeda L.) exposed to elevated carbon dioxide concentration ([CO(2)]) for 9 years. Duke Forest free-air CO(2) enrichment (FACE) plots were split and half of each ring fertilized with 112 kg ha(-1) elemental N applied in two applications in March and April 2005. Measurements of needle length (L), mass per unit area (LMA), N concentration (N(l)) on a mass and an area basis, light-saturated net photosynthesis per unit leaf area (A(a)) and per unit mass (A(m)), and leaf conductance (g(L)) began after the second fertilizer application in existing 1-year-old foliage (F(O)) and later in developing current-year first-flush (F(C1)) and current-year second-flush (F(C2)) foliage. Elevated [CO(2)] increased A(a) by 43 and 52% in F(O) and F(C1) foliage, respectively, but generally had no significant effect on any other parameter. Fertilization had little or no significant effect on L, LMA, A or g(L) in F(O) foliage; although N(l) was significantly higher in fertilized trees by midsummer. In contrast, fertilization resulted in large increases in L, N(l), and A in F(C1) and F(C2) foliage, increasing A(a) by about 20%. These results suggest that, although both needle age classes accumulate N following fertilization, they use it differently-current-year foliage incorporates N into photosynthetic machinery, whereas 1-year-old foliage serves as an N store. There were no significant interaction effects of elevated [CO(2)] and fertilization on A. Elevated [CO(2)] increased the intercept of the A:N(l) relationship but did not significantly affect the slope of the relationship in either foliage age class.

Selfing results in inbreeding depression of growth but not of gas exchange of surviving adult black spruce trees.

In most tree species, inbreeding greatly reduces seed production, seed viability, survival and growth. In a previous large-scale quantitative analysis of a black spruce (Picea mariana (Mill.) B.S.P.) diallel experiment, selfing had large deleterious effects on growth but no impact on stable carbon isotope discrimination (an indirect measure of the ratio of net photosynthesis (A) to stomatal conductance (gwv)). It was hypothesized that selfing has no effect on carbon (C) fixation at the leaf level but impairs subsequent utilization of C. Alternatively, A and gwv may be impacted by selfing to the same extent. However, no gas exchange data were collected to test these hypotheses. We have now obtained photosynthetic gas exchange data from three selfed families and three outcrossed families (all the result of controlled pollination) from the same diallel experiment. Photosynthetic responses to intercellular CO2 concentration (A-Ci curves) were generated on four replicates per family, one block per day, over a 4-day period in July. There were no differences between selfed and outcrossed families in maximum carboxylation rate, maximum electron transport, A or gwv (both estimated at 370 ppm CO2), or the ratio A/gwv. Because selfed trees had higher mortality than outcrossed trees during the experiment, we cannot exclude the possibility that previously living selfed progeny had low A. Nevertheless, the data indicate that inbreeding can result in trees that have low productivity despite high A, supporting our hypothesis that gas exchange is similar between selfed and outcrossed progeny trees. We conclude that utilization of fixed C is modified in the surviving selfed progeny.

Branch growth and gas exchange in 13-year-old loblolly pine (Pinus taeda) trees in response to elevated carbon dioxide concentration and fertilization.

We used whole-tree, open-top chambers to expose 13-year-old loblolly pine (Pinus taeda L.) trees, growing in soil with high or low nutrient availability, to either ambient or elevated (ambient + 200 micromol mol-1) carbon dioxide concentration ([CO2]) for 28 months. Branch growth and morphology, foliar chemistry and gas exchange characteristics were measured periodically in the upper, middle and lower crown during the 2 years of exposure. Fertilization and elevated [CO2] increased branch leaf area by 38 and 13%, respectively, and the combined effects were additive. Fertilization and elevated [CO2] differentially altered needle lengths, number of fascicles and flush length such that flush density (leaf area/flush length) increased with improved nutrition but decreased in response to elevated [CO2]. These results suggest that changes in nitrogen availability and atmospheric [CO2] may alter canopy structure, resulting in greater foliage retention and deeper crowns in loblolly pine forests. Fertilization increased foliar nitrogen concentration (N(M)), but had no consistent effect on foliar leaf mass (W(A)) or light-saturated net photosynthesis (A(sat)). However, the correlation between A(sat) and leaf nitrogen per unit area (N(A) = W(A)N(M)) ranged from strong to weak depending on the time of year, possibly reflecting seasonal shifts in the form and pools of leaf nitrogen. Elevated [CO2] had no effect on W(A), N(M) or N(A), but increased A(sat) on average by 82%. Elevated [CO2] also increased photosynthetic quantum efficiency and lowered the light compensation point, but had no effect on the photosynthetic response to intercellular [CO2], hence there was no acclimation to elevated [CO2]. Daily photosynthetic photon flux density at the upper, middle and lower canopy position was 60, 54 and 33%, respectively, of full sun incident to the top of the canopy. Despite the relatively high light penetration, W(A), N(A), A(sat) and R(d) decreased with crown depth. Although growth enhancement in response to elevated [CO2] was dependent on fertilization, [CO2] by fertilization interactions and treatment by canopy position interactions generally had little effect on the physiological parameters measured.

Relationship between stem CO2 efflux, stem sap velocity and xylem CO2 concentration in young loblolly pine trees.

We measured diel patterns of stem surface CO2 efflux (Es, micromol m(-2) s(-1)), sap velocity (vs, mm s(-1)) and xylem CO2 concentration ([CO2]) (Xs, %) in 8-year-old loblolly pine trees during the spring to determine how vs and Xs influence Es. All trees showed a strong diel hysteresis between Es and stem temperature, where at a given temperature, Es was lower during the day than at night. Diel variations in temperature-independent Es were correlated with vs (R2= 0.54), such that at maximum vs, Es was reduced between 18 and 40%. However, this correlation may not represent a cause-and-effect relationship. In a subset of trees, vs was artificially reduced by progressively removing the tree canopy. Reducing vs to near zero had no effect on Es and did not change the diel hysteretic response to temperature. Diel Xs tended to decrease with vs and increase with Es, however, in defoliated trees, large increases in Xs, when vs approximately 0, had no effect on Es. We conclude that at this time of the year, Es is driven primarily by respiration of cambium and phloem tissues and that sap flow and xylem transport of CO2 had no direct influence on Es.