## 4 Discussion
A positive temperature of \(\sim\)1-3 \({}^{\circ}\)C in the litter layer, where most larch fine roots expand, commenced the growth activity [65]. In the northern taiga, this usually happened during the last week/10 days of May, sometimes at the beginning of June, when air temperatures reached +10 \({}^{\circ}\)C and initiated thawing in trees openings. Simultaneously, the buds of the larch trees began to burst, revealing new larch needles, when photosynthetic active radiation, PAR, reached 600-700 umol m\({}^{-2}\)s\({}^{-1}\)[15]. Thus, in early June, the synchronized influence of air and soil temperatures, as well as of PAR, triggered the processes of CO\({}_{2}\) exchange. However, a very weak net CO\({}_{2}\) uptake of \(-\)1 umol m\({}^{-2}\)s\({}^{-1}\) was observed in early summer. [PERSON] et al. [66] showed that in the Gmelin larch forest on permafrost in Yakuttia, the maximum assimilation rate was reduced by 12% by low PAR and was reduced by 75% by low air humidity and water supply, including water uptake from thawing permafrost that is temperature limited. The weather conditions in our Gmelin larch site were characterized by the dry air and low soil moisture that explain the low assimilation activity of the forest. At the beginning of June, the process of the cambial division starts, the intensity of which resulted in the wood cell size and, finally, in wood stem productivity [67]. [PERSON] and [PERSON] [68] showed that the temperature threshold of 5 \({}^{\circ}\)C in pine non-permafrost forests is important for root physiological activities. The combination of temperatures below 5 \({}^{\circ}\)C with high photosynthetically active radiation, which often occurs in interior Siberia, may cause inhibited conifer photosynthesis in early summer [69]. Water use efficiency was 5.8 in the larch forest and 11 mg CO\({}_{2}\) g\({}^{-1}\)H\({}_{2}\)O in the pine forest. Thus, the water cost per unit C-assimilation was 1.5-fold greater in the larch ecosystem: 170 g vs. 260 g. This may be likely explained that a portion of water does not play a part in C-assimilation. Cold water drains along the frost table in the Gmelin larch ecosystem rather than it is consumed by roots unless water warms up during the summer. WUE of our Gmelin larch forest in central Siberia and that of a Gmelin larch forest in eastern Siberia [53] were similar. [PERSON] et al. [53] also found that the model and independent leaf-level measurements suggest that the marginal water cost of plant C-gain in the _L. gmelinii_ forest in East Siberia is very much similar to values from deciduous or desert species rather than other boreal forests. _L. sibirica_ forests growing in the cold semiarid environments in Mongolia at the southern limit of the Siberian taiga also exhibit a high water use efficiency [63]. WUE of both our ecosystems appeared to be consistent with those reported in the literature for a wide range of trees: 2.4-19.8 mg g\({}^{-1}\)[70]. Compared to some published results on WUE in temperate and boreal forests, our estimates were in this range: 8.5 mg CO\({}_{2}\) g\({}^{-1}\)H\({}_{2}\)O in a temperate mixed broadleaved-conifer (Mongolian oak and Korean pine) forest in NE China [70]; 11.3 mg g\({}^{-1}\) in a boreal mixed conifer (Norway spruce and pine) forest in Sweden [71]; 10-21 mg g\({}^{-1}\) in a boreal willow forest in Sweden [72]. Based on our findings, in \"hot spots\" where Gmelin larch forests on permafrost were predicted to be replaced by the Scots pine forest [73] that follow the permafrost retreat in a warming climate, C-sink would increase uptaking CO\({}_{2}\) from the atmosphere. Thus, positive potential feedback to the climate system that slows down climate warming may take place. However, as predicted with general circulation models, profound warming is expected at high latitudes. Thus Arctic-boreal ecosystems become especially vulnerable to warming, which may transform the Arctic-boreal ecosystems from the carbon sink to source [27; 28]. [PERSON] et al.