• Energy Research

AbstractSubarctic ecosystems are exposed to elevated temperatures and increased cloudiness in a changing climate with potentially important effects on vegetation structure, composition, and ecosystem functioning. We investigated the individual and combined effects of warming and increased cloudiness on vegetation greenness and cover in mesocosms from two tundra and one palsa mire ecosystems kept under strict environmental control in climate chambers. We also investigated leaf anatomical and biochemical traits of four dominant vascular plant species (Empetrum hermaphroditum, Vaccinium myrtillus, Vaccinium vitis‐idaea, and Rubus chamaemorus). Vegetation greenness increased in response to warming in all sites and in response to increased cloudiness in the tundra sites but without associated increases in vegetation cover or biomass, except that E. hermaphroditum biomass increased under warming. The combined warming and increased cloudiness treatment had an additive effect on vegetation greenness in all sites. It also increased the cover of graminoids and forbs in one of the tundra sites. Warming increased leaf dry mass per area of V. myrtillus and R. chamaemorus, and glandular trichome density of V. myrtillus and decreased spongy intercellular space of E. hermaphroditum and V. vitis‐idaea. Increased cloudiness decreased leaf dry mass per area of V. myrtillus, palisade thickness of E. hermaphroditum, and stomata density of E. hermaphroditum and V. vitis‐idaea, and increased leaf area and epidermis thickness of V. myrtillus, leaf shape index and nitrogen of E. hermaphroditum, and palisade intercellular space of V. vitis‐idaea. The combined treatment caused thinner leaves and decreased leaf carbon for V. myrtillus, and increased leaf chlorophyll of E. hermaphroditum. We show that under future warmer increased cloudiness conditions in the Subarctic (as simulated in our experiment), vegetation composition and distribution will change, mostly dominated by graminoids and forbs. These changes will depend on the responses of leaf anatomical and biochemical traits and will likely impact carbon gain and primary productivity and abiotic and biotic stress tolerance.

The changing climate will expose boreal forests to rising temperatures, increasing soil nitrogen (N) levels and an increasing risk of herbivory. The single and interaction effects of warming (+2 °C increase), moderate N addition (30 kg ha-1 year-1) and bark herbivory by large pine weevil (Hylobius abietis L.) on growth and emissions of biogenic volatile organic compounds (BVOCs) from shoots of Scots pine (Pinus sylvestris L.) seedlings were studied in growth chambers over 175 days. In addition, warming and N addition effects on shoot net photosynthesis (Pn) were measured. Nitrogen addition increased both shoot and root dry weights, whereas warming, in combination with herbivory, reduced stem height growth. Warming together with N addition increased current-year shoot Pn, whereas N effects on previous-year shoot Pn were variable over time. Warming decreased non-oxygenated monoterpene (MT) emissions in June and increased them in July. Of individual MT compounds, α-pinene, δ-3-carene, γ-terpinene and terpinolene were among the most frequently responsive compounds in warming treatments in the May-July period. Sesquiterpene emissions were observed only from warming treatments in July. Moderate N addition increased oxygenated monoterpenes in May, and MTs in June and September. However, N addition effect on MTs in June was clearer without warming than with warming. Bark herbivory tended to increase MT emissions in combination with warming and N addition 3 weeks after the damage caused by weevils. Of individual compounds in other BVOC blends, herbivory increased the emissions of methyl-benzene, benzene and hexanal in July. Hence, though both warming and N addition have a potential to change BVOC emissions from Scots pines, the N effect may also be partly cancelled by warming. Furthermore, herbivory pressure in combination with climate warming and N addition may, at least periodically, increase BVOC release to the atmosphere from young Scots pine seedlings.

In the present experiment, the single and combined effects of elevated temperature and ozone (O(3)) on four silver birch genotypes (gt12, gt14, gt15 and gt25) were studied in an open-air field exposure design. Above- and below-ground biomass accumulation, stem growth and soil respiration were measured in 2008. In addition, a (13)C-labelling experiment was conducted with gt15 trees. After the second exposure season, elevated temperature increased silver birch above- and below-ground growth and soil respiration rates. However, some of these variables showed that the temperature effect was modified by tree genotype and prevailing O(3) level. For instance, in gt14 soil respiration was increased in elevated temperature alone (T) and in elevated O(3) and elevated temperature in combination (O(3) + T) treatments, but in other genotypes O(3) either partly (gt12) or totally nullified (gt25) temperature effects on soil respiration, or acted synergistically with temperature (gt15). Before leaf abscission, all genotypes had the largest leaf biomass in T and O(3) + T treatments, whereas at the end of the season temperature effects on leaf biomass depended on the prevailing O(3) level. Temperature increase thus delayed and O(3) accelerated leaf senescence, and in combination treatment O(3) reduced the temperature effect. Photosynthetic : non-photosynthetic tissue ratios (P : nP ratios) showed that elevated temperature increased foliage biomass relative to woody mass, particularly in gt14 and gt12, whereas O(3) and O(3) + T decreased it most clearly in gt25. O(3)-caused stem growth reductions were clearest in the fastest-growing gt14 and gt25, whereas mycorrhizal root growth and sporocarp production increased under O(3) in all genotypes. A labelling experiment showed that temperature increased tree total biomass and hence (13)C fixation in the foliage and roots and also label return was highest under elevated temperature. Ozone seemed to change tree (13)C allocation, as it decreased foliar (13)C excess amount, simultaneously increasing (13)C excess obtained from the soil. The present results suggest that warming has potential to increase silver birch growth and hence carbon (C) accumulation in tree biomass, but the final magnitude of this C sink strength is partly counteracted by temperature-induced increase in soil respiration rates and simultaneous O(3) stress. Silver birch populations' response to climate change will also largely depend on their genotype composition.

Abstract It is not clear how climate change in combination with increasing soil nitrogen availability and herbivory affects boreal forests, the largest terrestrial biome in the world. In this study, Scots pine ( Pinus sylvestris ) seedlings were exposed to moderate warming (ca. 1 °C), 1.5 × ambient ozone (O 3 ) concentration, fertilizer addition (120 kg N ha −1 yr −1 ) and shoot herbivory by pine sawfly ( Acantholyda posticalis ) alone and in combination. We measured fine root morphology, mycorrhizal colonization level, root fungal biomass (ergosterol), rhizosphere emission of biogenic volatile organic compounds (BVOCs), and microbial biomass (PLFAs) in the rhizosphere soil as well as seedling above- and below-ground growth. Warming and fertilization effects on fine root proportions or root fungal biomass occurred in combination with other factors, combination effects being usually negative on the studied variables, or then warming and fertilization in combination cancelled some other factors’ effects. O 3 effects on needle growth, root fungal biomass and BVOCs were more often seen after the third exposure year, and sometimes only in combination with other studied factors. Of abiotic factors, fertilizer addition had reducing effect on rhizosphere BVOCs. Though increased nitrogen availability and warming increased both shoot and root dry masses, growth allocation to above- and below-ground parts was not equally increased in the combined exposures. Thus, we conclude that climate change factors together with increased nitrogen availability and herbivory are likely to affect the below-ground compartments negatively, more often than shoots, and ultimately change in growth allocation pattern which may affect overall seedling growth and survival in later years.

ABSTRACTUnder elevated atmospheric CO2 concentrations, soil carbon (C) inputs are typically enhanced, suggesting larger soil C sequestration potential. However, soil C losses also increase and progressive nitrogen (N) limitation to plant growth may reduce the CO2 effect on soil C inputs with time. We compiled a data set from 131 manipulation experiments, and used meta‐analysis to test the hypotheses that: (1) elevated atmospheric CO2 stimulates soil C inputs more than C losses, resulting in increasing soil C stocks; and (2) that these responses are modulated by N. Our results confirm that elevated CO2 induces a C allocation shift towards below‐ground biomass compartments. However, the increased soil C inputs were offset by increased heterotrophic respiration (Rh), such that soil C content was not affected by elevated CO2. Soil N concentration strongly interacted with CO2 fumigation: the effect of elevated CO2 on fine root biomass and –production and on microbial activity increased with increasing soil N concentration, while the effect on soil C content decreased with increasing soil N concentration. These results suggest that both plant growth and microbial activity responses to elevated CO2 are modulated by N availability, and that it is essential to account for soil N concentration in C cycling analyses.

Climate change in the boreal forests include, e.g., warming, increased tropospheric ozone concentration, higher nitrogen (N) deposition and increased risk of insect outbreaks. Climate change influences emissions of biogenic volatile organic compounds (BVOCs) affecting plant defense, communication and atmospheric feedbacks. We studied the effects of elevated temperature (ca. 1 °C), elevated ozone (ca. 1.5 × ambient), two soil N availability levels (prevailing and 120 kg N ha−1 a−1) and herbivory on BVOC emission rates, net photosynthesis and resin canals (BVOCs storage), of Scots pine (Pinus sylvestris) seedlings in an open-field exposure in central Finland. Shoot BVOCs were collected in July 2012 within a few days after feeding by larvae of pine-sawfly Acantholyda posticalis, a month later in August, and in May 2013. Elevated temperature caused twofold to fourfold increases in total emissions of non-oxygenated monoterpenes (MTs), oxygenated MTs and sesquiterpenes (SQTs) and several reactive compounds, and higher N enhanced some of these changes. Ozone and higher N together increased emissions of several MTs and total SQTs. Higher number of resin canals and higher net photosynthesis might have contributed to BVOC increases. Herbivory had the strongest effect on SQT emissions (threefold increase) shortly after feeding. In the following spring, herbivory reduced emission rates of some MTs, but also synergistically increased MTs emissions with temperature but suppressed the increase caused by ozone. Results suggest that warming and ozone, particularly in areas with increased soil N availability, can increase BVOC emissions from young boreal forests in the near future, and herbivory may modify these responses.