Response of central Siberian Scots pine to soil water deficit and long‐term trends in atmospheric CO₂concentration

Abstract

[1] Twenty tree ring 13 C/ 12 C 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 CO 2 concentration on long-and shortterm variation in photosynthetic 13 C discrimination during photosynthesis and the marginal cost of tree water use, as reflected in the differences in the historical records of the 13 C / 12 C ratio in wood cellulose compared to that of the atmosphere (D 13 C c ). In 17 of the 20 samples, trees D 13 C c 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 CO 2 concentrations. This response being observed for most trees along the transect is suggestive of widespread decreases in D 13 C c and increased water use efficiency for Scots pine in central Siberia over the last century. Overlying short-term variations in D 13 C c were also accounted for by the model and were related to variations in growing season soil water deficit and atmospheric humidity.

Stomatal Function across Temporal and Spatial Scales: Deep-Time Trends, Land-Atmosphere Coupling and Global Models

“…An equation for g w can be obtained from this relationship with A expressed in terms of the Farquhar et al (1980b) photosynthesis model, but the form of the equation varies with the Rubisco and RuBP regeneration-limited rates of A (Arneth et al, 2002;Buckley et al, 2002;Katul et al, 2010;Medlyn et al, 2011;Vico et al, 2013;Buckley and Schymanski, 2014;Buckley et al, 2017). Although the utility of water-use efficiency optimization has been demonstrated in leaf gas-exchange studies, it has not been implemented directly in land surface models.…”

“…The specificity of the MED model to the optimization of RuBP regeneration-limited photosynthesis is an important characteristic to consider when comparing the output of the MED model with a full water-use efficiency optimization model, as discussed below. The significance of the MED model is that it is derived by combining the standard leaf diffusion equations with an equation for optimum leaf internal CO 2 concentration, c i , in terms of the optimization criterion l (Arneth et al, 2002), thereby linking g 1 to l (as well as G*, which is temperature dependent). This connection is intuitive because g 1 , l, and, therefore, g 1M are all indexes of plant water-use efficiency (or more precisely its reciprocal, E/A, as traditionally defined).…”

Water use strategies and ecosystem-atmosphere exchange of CO<sub>2</sub> in two highly seasonal environments

“…The above expression has been successfully applied in a big-leaf approach to analyse the Scots pine canopy photosynthesis and conductance . It also provided the basis to interpret a tree-ring stable carbon isotope record in stems of Scots pine trees growing few hundred metres from the pine forest flux tower (Arneth et al, 2002b). Extending this analysis from the tree canopy to the entire ecosystem would be problematic in a multilayer-canopy with distinct understorey vegetation, due to the contribution of below canopy carbon and water fluxes to the measured total.…”

“…However, the significant difference in λ between years was evident right from the onset of the active period. In Scots pine trees growing nearby the Siberian eddy flux tower, the variation in stem cellulose carbon isotope ratios could be explained using a coupled assimilation-conductance model that accounted for a reduction of stomatal conductance, and λ, as soils dried (Arneth et al, 2002b). Since it is the combination of water and nutrient availability that influences stomatal and canopy conductance, photosynthetic capacity and hence water use strategies, the history in environmental conditions (e.g., soil water table level, soil temperatures) may be an important factor to explain variation in λ on interannual time steps.…”

A test of an optimal stomatal conductance scheme within the CABLE land surface model

“…An alternative approach, originally proposed by Cowan and Farquhar (1977) and Cowan (1982), is to model stomatal conductance using an optimisation framework (Hari et al, 1986;Lloyd, 1991;Arneth et al, 2002;Katul et al, 2009;Schymanski et al, 2009;Medlyn et al, 2011). This approach hypothesises that optimal stomatal behaviour occurs when the carbon gain (photosynthesis, A) is maximised, whilst minimising water loss (transpiration, E) over some period of time (t 2 -t 1 ).…”