Seasonal and annual variations in the photosynthetic productivity and carbon balance of a central Siberian pine forest

Abstract

We present a first analysis of data (June 1998 to December 2000) from the long-term eddy covariance site established in a Pinus sylvestris stand near Zotino in central Siberia as part of the EUROSIBERIAN CARBONFLUX project. As well as examining seasonal patterns in net ecosystem exchange (N E), daily, seasonal and annual estimates of the canopy photosynthesis (or gross primary productivity, G P) were obtained using N E and ecosystem respiration measurements. Although the forest was a small (but significant) source of CO 2 throughout the snow season (typically mid-October to early May) there was a rapid commencement of photosynthetic capacity shortly following the commencement of above-zero air temperatures in spring: in 1999 the forest went from a quiescent state to significant photosynthetic activity in only a few days. Nevertheless, canopy photosynthetic capacity was observed to continue to increase slowly throughout the summer months for both 1999 and 2000, reaching a maximum capacity in early August. During September there was a marked decline in canopy photosynthesis which was only partially attributable to less favourable environmental conditions. This suggests a reduction in canopy photosynthetic capacity in autumn, perhaps associated with the cold hardening process. For individual time periods the canopy photosynthetic rate was mostly dependent upon incoming photon irradiance. However, reductions in both canopy conductance and overall photosynthetic rate in response to high canopy-to-air vapour differences were clearly evident on hot dry days. The relationship between canopy conductance and photosynthesis was examined using Cowan's notion of optimality in which stomata serve to maximise the marginal evaporative cost of plant carbon gain. The associated Lagrangian multiplier (λ) was surprisingly constant throughout the growing season. Somewhat remarkably, however, its value was markedly different between years, being 416 mol mol −1 in 1999 but 815 mol mol −1 in 2000. Overall the forest was a substantial sink for CO 2 in both 1999 and 2000: around 13 mol C m −2 a −1. Data from this experiment, when combined with estimates of net primary productivity from biomass sampling suggest that about 20% of this sink was associated with increasing plant biomass and about 80% with an increase in the litter and soil organic carbon pools. This high implied rate of carbon accumulation in the litter soil organic matter pool seems unsustainable in the long term and is hard to explain on the basis of current knowledge.

Net ecosystem exchange of carbon dioxide and water of far eastern Siberian Larch (<I>Larix cajanderii</I>) on permafrost

“…The overall rates of NEE compare well with those given by Röser et al (2002) and Lloyd et al (2002) for Central Siberian pine forests, but are somewhat higher. The most noticeable phenomenon in the present case is the very sharp increase in negative NEE (uptake) after initial needle growth.…”

“…Particularly in July 2001 there is marked response in surface conductance but less so in evaporation. Such a response has also been observed by Lloyd et al (2002). We suggest that the response in transpiration to changes in g s may be masked by the interplay between on the one hand a reduction in conductance and on the other an increase in atmospheric demand.…”

“…In 2001 we had no observations of soil moisture. Lloyd et al (2002) found the somewhat surprising result that the water use efficiency of central Siberian pine, expressed as their parameter γ , describing the change of evaporation over a change of photosynthesis, was different for their two years of analysis. In Fig.…”

Mechanisms of water supply and vegetation demand govern the seasonality and magnitude of evapotranspiration in Amazonia and Cerrado

“…Higher (lower) iWUE implies vegetation is photosynthesizing at a lower (higher) internal to ambient CO 2 ratio (Lloyd et al, 2002;Beer et al, 2009), and its variation across sites in Amazonia and Cerrado may reflect site differences in soil fertility, vegetation composition, or both. It is an important diagnostic for modeled E in addition to g s because it governs how the light response of photosynthesis is translated into evaporative losses.…”

“…Canopy stomatal conductance and intrinsic water use efficiency (iWUE) are two key mechanisms controlling vegetation demand for water, respectively, in relation to atmospheric vapor pressure deficit (D) and ecosystem photosynthesis (GPP) arising from the 'photosynthesis-transpiration' compromise (Lloyd et al, 2002;Beer et al, 2009). The degree to which stomata regulate transpiration (E t ) independent of environmental conditions in the Amazon has been the topic of debate (Avissar and Werth, 2004;Costa et al, 2004).…”