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Mesic–dry tundras are widespread in the Arctic but detailed assessments of net primary production (NPP) and ecosystem carbon (C) stocks are lacking. We addressed this lack of knowledge by determining the seasonal dynamics of aboveground vascular NPP, annual NPP, and whole-ecosystem C stocks in five mesic–dry tundras in Northern Sweden with contrasting microtopography, altitude, and dominant species. Those measurements were paralleled by the stock assessments of nitrogen (N), the limiting nutrient. The vascular production was determined by harvest or in situ growing units, whereas the nonvascular production was obtained from average species growth rates, previously assessed at the sites. Results showed that aboveground vascular NPP (15–270 g m−2), annual NPP (214–282 g m−2 or 102–137 g C m−2) and vegetation biomass (330–2450 g m−2) varied greatly among communities. Vegetation dominated by Empetrum hermaphroditum is more productive than Cassiope tetragona vegetation. Although the large majority of the apical NPP occurred in early-mid season (85%), production of stems and evergreen leaves proceeded until about 2 weeks before senescence. Most of the vascular vegetation was belowground (80%), whereas most of the vegetation production occurred aboveground (85%). Ecosystem C and N stocks were 2100–8200 g C m−2 and 80–330 g N m−2, respectively, stored mainly in the soil turf and in the fine organic soil. Such stocks are comparable to the C and N stocks of moister tundra types, such as tussock tundra. [ABSTRACT FROM AUTHOR]

Bryophytes and lichens abound in many arctic ecosystems and can contribute substantially to the ecosystem net primary production (NPP). Because of their growth seasonality and their potential for growth out of the growing season peak, bryophyte and lichen contribution to NPP may be particularly significant when vascular plants are less active and ecosystems act as a source of carbon (C). To clarify these dynamics, nonvascular and vascular aboveground NPP was compared for a subarctic heath during two contrasting periods of the growing season, viz. early-mid summer and late summer-early autumn. Nonvascular NPP was determined by assessing shoot biomass increment of three moss species ( Hylocomium splendens, Pleurozium schreberi and Dicranum elongatum) and by scaling to ecosystem level using average standing crop. For D. elongatum, these estimates were compared with production estimates obtained from measurements of shoot length increase. Vascular NPP was determined by harvesting shrub and herb apical growth and considering production due to stem secondary growth of shrubs. Hylocomium splendens and Pleurozium schreberi showed highest biomass growth in late summer, whereas for D. elongatum this occurred in early summer. Maximum relative growth rates were ca. 0.003–0.007 g g−1 d−1. For D. elongatum, production estimates from length growth differed from estimations from biomass growth, likely because of an uncoupling between length growth and biomass shoot growth. Nonvascular NPP was 0.37 and 0.46 g dry weight m−2 d−1, in early and late summer, respectively, whereas in the same periods vascular NPP was 3.6 and 1.1 g dry weight m−2 d−1. The contribution of nonvascular NPP to total aboveground NPP was therefore minor in early summer but substantial in late summer, when 25% of the C accumulated by the vegetation was incorporated into nonvascular plant tissue. The expected global change-induced reduction of nonvascular plant biomass in subarctic heath is likely therefore to enhance C release during the late part of the growing season. [ABSTRACT FROM AUTHOR]

Abstract: Stem diameter variations are not solely driven by the water status of a tree, but also by the carbon status in the phloem. We mechanically changed the carbon status in young oak trees (Quercus robur L.) by girdling the stem in order to investigate its effect on stem diameter variations. The stem was girdled at two heights by removing the bark which contains the carbon conducting phloem. The upper stem zone (U) still received new assimilates from the leaves, while the lowest stem zone (L) received only stored carbon from the roots. The middle stem zone (M) was completely isolated from crown and roots. As downward carbon transport was interrupted after girdling, the stem expansion and the roots. We argue that expedited growth in U was driven by both irreversible radial growth and reversible swelling. In contrast to U, stem expansion and carbohydrate content decreased in the two lower stem zones (M and L). Time lags were observed between the moments of morning shrinkage when comparing the three stem zones (U, M and L). Since these time lags correlated with the soluble carbohydrate content, it seems that daily dynamics in stem diameter variations were influenced by changes in carbon status. Furthermore, a decrease in xylem sap flow rate was observed, which could be attributed to an indirect effect of girdling. This decrease seems to be provoked by a feedback inhibition of net photosynthesis rate via stomatal closure. Such feedback inhibitions are often described in literature as a consequence of the reduced sink strength after girdling. This study, hence, demonstrates that a disturbance in tree carbon status not only changes carbon-related processes (i.e. radial stem growth, photosynthesis, carbon storage), but also water-related processes (i.e. stem swelling and shrinkage, transpiration and sap flow). [Copyright &y& Elsevier]

Abstract: Reducing energy use in greenhouses contributes to the profitability of horticulture. Important energy savings can be realized through the use of temperature integration. However, such a greenhouse heating strategy is only acceptable for commercial purposes if there are no adverse effects on plant growth and quality. During this 3-month study, Hedera helix ‘Green Ripple’ and H. helix ‘Shamrock’ were subjected to a day/night temperature regime of 20/20°C (control) and two treatments with temperature integration over 24h and 4d, respectively, based on a DIF of 13.5°C, maintaining the average temperature at the same level of the control. Temperature treatments resulted in a promotion of stem elongation. After 3 months, shoot length rose up to maximum 37.3% when temperature integration was applied. However, temperature integration reduced total dry weight and particularly root dry weight was negatively affected. In addition, assessments of relative growth rate, shoot extension rate, specific leaf area, total leaf area and pigment concentrations were performed and it was concluded that dynamic temperature regimes with longer integration periods support commercial production of English ivy. [Copyright &y& Elsevier]