Stable isotopes in tree rings.

Forest age, which is affected by stand-replacing ecosystem disturbances (such as forest fires, harvesting, or insects), plays a distinguishing role in determining the distribution of carbon (C) pools and fluxes in different forested ecosystems. In this synthesis, net primary productivity (NPP), net ecosystem productivity (NEP), and five pools of C (living biomass, coarse woody debris, organic soil horizons, soil, and total ecosystem) are summarized by age class for tropical, temperate, and boreal forest biomes. Estimates of variability in NPP, NEP, and C pools are provided for each biome-age class combination and the sources of variability are discussed. Aggregated biome-level estimates of NPP and NEP were higher in intermediate-aged forests (e.g., 30–120 years), while older forests (e.g., >120 years) were generally less productive. The mean NEP in the youngest forests (0–10 years) was negative (source to the atmosphere) in both boreal and temperate biomes (−0.1 and –1.9 Mg C ha−1 yr−1, respectively). Forest age is a highly significant source of variability in NEP at the biome scale; for example, mean temperate forest NEP was −1.9, 4.5, 2.4, 1.9 and 1.7 Mg C ha−1 yr−1 across five age classes (0–10, 11–30, 31–70, 71–120, 121–200 years, respectively). In general, median NPP and NEP are strongly correlated (R2=0.83) across all biomes and age classes, with the exception of the youngest temperate forests. Using the information gained from calculating the summary statistics for NPP and NEP, we calculated heterotrophic soil respiration (Rh) for each age class in each biome. The mean Rh was high in the youngest temperate age class (9.7 Mg C ha−1 yr−1) and declined with age, implying that forest ecosystem respiration peaks when forests are young, not old. With notable exceptions, carbon pool sizes increased with age in all biomes, including soil C. Age trends in C cycling and storage are very apparent in all three biomes and it is clear that a better understanding of how forest age and disturbance history interact will greatly improve our fundamental knowledge of the terrestrial C cycle.

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Carbon cycling and storage in world forests: biome patterns related to forest age.

Models of vegetation function are widely used to predict the effects of climate change on carbon, water and nutrient cycles of terrestrial ecosystems, and their feedbacks to climate. Stomatal conductance, the process that governs plant water use and carbon uptake, is fundamental to such models. In this paper, we reconcile two long-standing theories of stomatal conductance. The empirical approach, which is most commonly used in vegetation models, is phenomenological, based on experimental observations of stomatal behaviour in response to environmental conditions. The optimal approach is based on the theoretical argument that stomata should act to minimize the amount of water used per unit carbon gained. We reconcile these two approaches by showing that the theory of optimal stomatal conductance can be used to derive a model of stomatal conductance that is closely analogous to the empirical models. Consequently, we obtain a unified stomatal model which has a similar form to existing empirical models, but which now provides a theoretical interpretation for model parameter values. The key model parameter, g1 ,i s predicted to increase with growth temperature and with the marginal water cost of carbon gain. The new model is fitted to a range of datasets ranging from tropical to boreal trees. The parameter g1 is shown to vary with growth temperature, as predicted, and also with plant functional type. The model is shown to correctly capture responses of stomatal conductance to changing atmospheric CO2, and thus can be used to test for stomatal acclimation to elevated CO2. The reconciliation of the optimal and empirical approaches to modelling stomatal conductance is important for global change biology because it provides a simple theoretical framework for analyzing, and simulating, the coupling between carbon and water cycles under environmental change.

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Reconciling the optimal and empirical approaches to modelling stomatal conductance

We revisit the relationship between plant water use efficiency and carbon isotope signatures (δ13C) of plant material. Based on the definitions of intrinsic, instantaneous and integrated water use efficiency, we discuss the implications for interpreting δ13C data from leaf to landscape levels, and across diurnal to decadal timescales. Previous studies have often applied a simplified, linear relationship between δ13C, ratios of intercellular to ambient CO2 mole fraction (C
 i/C
 a), and water use efficiency. In contrast, photosynthetic 13C discrimination (Δ) is sensitive to the ratio of the chloroplast to ambient CO2 mole fraction, C
 c/C
 a (rather than C
 i/C
 a) and, consequently, to mesophyll conductance. Because mesophyll conductance may differ between species and over time, it is not possible to determine C
 c/C
 a from the same gas exchange measurements as C
 i/C
 a. On the other hand, water use efficiency at the leaf level depends on evaporative demand, which does not directly affect Δ. Water use efficiency and Δ can thus vary independently, making it difficult to obtain trends in water use efficiency from δ13C data. As an alternative approach, we offer a model available at http://carbonisotopes.googlepages.com
 to explore how water use efficiency and 13C discrimination are related across leaf and canopy scales. The model provides a tool to investigate whether trends in Δ indicate changes in leaf functional traits and/or environmental conditions during leaf growth, and how they are associated with trends in plant water use efficiency. The model can be used, for example, to examine whether trends in δ13C signatures obtained from tree rings imply changes in tree water use efficiency in response to atmospheric CO2 increase. This is crucial for predicting how plants may respond to future climate change.

Carbon isotopes and water use efficiency: sense and sensitivity

We investigated the response of conifer trees in northern Eurasia to climate change and increasing CO2 over the last century by measuring the carbon isotope ratio in tree rings. Samples from Larix, Pinus and Picea trees growing at 26 high-latitude sites (59‐711N) from Norway to Eastern Siberia were analysed. When comparing the periods 1861‐1890 and 1961‐1990, the isotope discrimination and the ratio of the intercellular to ambient CO2 concentration (ci/ca) remained constant for trees growing in mild oceanic climate and under extremely cold and dry continental conditions. This shows a strong coordination of gas-exchange processes, consisting in a biochemical acclimation and a reduction of the stomatal conductance. The correlation for ci/ca between the two investigated periods was particularly strong for Larix (r 2 50.90) and Pinus (r 2 50.94), but less pronounced for Picea (r 2 50.47). Constant ci/ca under increasing CO2 in the atmosphere resulted in improved intrinsic water-use efficiency (Wi), the amount of water loss at the leaf level per unit carbon gain. We found that 125 out of 126 trees showed increasing Wi from 1861 to 1890 to 1961 to 1990, with an average improvement of

Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years

[1] Twenty tree ring 13C / 12C ratio chronologies from Pinus sylvestris (Scots pine) trees were determined from five locations sampled along the Yenisei River, spaced over a total distance of ∼1000 km between the cities of Turuhansk (66°N) and Krasnoyarsk (56°N). The transect covered the major part of the natural distribution of Scots pine in the region with median growing season temperatures and precipitation varying from 12.2°C and 218 mm to 14.0°C and 278 mm for Turuhansk and Krasnoyarsk, respectively. A key focus of the study was to investigate the effects of variations in temperature, precipitation, and atmospheric CO2 concentration on long- and short-term variation in photosynthetic 13C discrimination during photosynthesis and the marginal cost of tree water use, as reflected in the differences in the historical records of the 13C / 12C ratio in wood cellulose compared to that of the atmosphere (Δ13Cc). In 17 of the 20 samples, trees Δ13Cc has declined during the last 150 years, particularly so during the second half of the twentieth century. Using a model of stomatal behaviour combined with a process-based photosynthesis model, we deduce that this trend indicates a long-term decrease in canopy stomatal conductance, probably in response to increasing atmospheric CO2 concentrations. This response being observed for most trees along the transect is suggestive of widespread decreases in Δ13Cc and increased water use efficiency for Scots pine in central Siberia over the last century. Overlying short-term variations in Δ13Cc were also accounted for by the model and were related to variations in growing season soil water deficit and atmospheric humidity.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000GB001374

Response of central Siberian Scots pine to soil water deficit and long-term trends in atmospheric CO2 concentration

We present soil organic carbon (SOC) inventories and carbon isotope compositions from over 900 samples collected in areas of minimally disturbed mature vegetation on freely drained soils (excluding peatlands) on a 1000 km transect along the Yennisey River, central Siberia. Carbon inventories over 0–30 cm depth range widely from 1.71 to 7.05 kg m −2 . While an effect of changing climate or vegetation along the transect cannot be ruled out, the observed differences in SOC inventories are largely the result of variations in mineral soil texture, with inventories in fine-textured soils being approximately double those in coarse-textured soils. The δ 13 C values of SOC in the 0–5 cm interval ranged from −26.3 to −28.0‰ , with δ 13 C values for the 5–30 cm interval being 0.9 ± 0.8‰ (1σ) enriched in 13 C relative to the 0–5 cm samples. The average δ 13 C value for the 0–5 cm interval for all samples was −27.1 ± 0.6‰ (1σ) and for the full 0–30 cm interval the average was −26.5 ± 0.5‰ (1σ) . In general, δ 13 C values were higher in coarse-textured soils and lower in fine-textured soils. The results of detailed sampling of soils in Pinus sylvestris forest growing on sand near the Zotino flux tower suggest an SOC inventory in these soils of 2.22 ± 0.35 kg m −2 over 30 cm and an average δ 13 C value of −26.3 ±0.2‰ over the 0–5 cm depth interval and −25.9 ± 0.3‰ over 0–30 cm. Recent burning had no effect on SOC inventories, but clearing has led to an average 25% decrease on SOC inventories from 0–30 cm over 12 yr. Neither burning nor clearing had a discernible effect on the δ 13 C value of SOC. DOI: 10.1034/j.1600-0889.2002.01334.x

Y. N. Kalaschnikov

Papers

Response of central Siberian Scots pine to soil water deficit and long-term trends in atmospheric CO2 concentration

Soil carbon inventories and carbon-13 on a latitude transect in Siberia