Responses of soil-grown Scots pine seedlings to experimental warming, moderate nitrogen addition and bark herbivory in a three-year field experiment

Nitrogen availability mediates soil carbon cycling response to climate warming: A meta-analysis

Global climate warming may induce a positive feedback through increasing soil carbon (C) release to the atmosphere. Although warming can affect both C input to and output from soil, direct and convincing evidence illustrating that warming induces a net change in soil C is still lacking. We synthesized the results from field warming experiments at 165 sites across the globe and found that climate warming had no significant effect on soil C stock. On average, warming significantly increased root biomass and soil respiration, but warming effects on root biomass and soil respiration strongly depended on soil nitrogen (N) availability. Under high N availability (soil C:N ratio < 15), warming had no significant effect on root biomass, but promoted the coupling between effect sizes of root biomass and soil C stock. Under relative N limitation (soil C:N ratio > 15), warming significantly enhanced root biomass. However, the enhancement of root biomass did not induce a corresponding C accumulation in soil, possibly because warming promoted microbial CO₂ release that offset the increased root C input. Also, reactive N input alleviated warming-induced C loss from soil, but elevated atmospheric CO₂ or precipitation increase/reduction did not. Together, our findings indicate that the relative availability of soil C to N (i.e., soil C:N ratio) critically mediates warming effects on soil C dynamics, suggesting that its incorporation into C-climate models may improve the prediction of soil C cycling under future global warming scenarios.

Nitrogen and water availability control plant carbon storage with warming

Plants may slow global warming through enhanced growth, because increased levels of photosynthesis stimulate the land carbon (C) sink. However, how climate warming affects plant C storage globally and key drivers determining the response of plant C storage to climate warming remains unclear, causing uncertainty in climate projections. We performed a comprehensive meta-analysis, compiling 393 observations from 99 warming studies to examine the global patterns of plant C storage responses to climate warming and explore the key drivers. Warming significantly increased total biomass (+8.4 %), aboveground biomass (+12.6 %) and belowground biomass (+10.1 %). The effect of experimental warming on plant biomass was best explained by the availability of soil nitrogen (N) and water. Across the entire dataset, warming-induced changes in total, aboveground and belowground biomass all positively correlated with soil C:N ratio, an indicator of soil N availability. In addition, warming stimulated plant biomass more strongly in humid than in dry ecosystems, and warming tended to decrease root:shoot ratios at high soil C:N ratios. Together, these results suggest dual controls of warming effects on plant C storage; warming increases plant growth in ecosystems where N is limiting plant growth, but it reduces plant growth where water availability is limiting plant growth.

Volatile-mediated between-plant communication in Scots pine and the effects of elevated ozone

Conifers are dominant tree species in boreal forests, but are susceptible to attack by bark beetles. Upon bark beetle attack, conifers release substantial quantities of volatile organic compounds known as herbivore-induced plant volatiles (HIPVs). Earlier studies of broadleaved plants have shown that HIPVs provide information to neighbouring plants, which may enhance their defences. However, the defence responses of HIPV-receiver plants have not been described for conifers. Here we advance knowledge of plant-plant communication in conifers by documenting a suite of receiver-plant responses to bark-feeding-induced volatiles. Scots pine seedlings exposed to HIPVs were more resistant to subsequent weevil feeding and received less damage. Receiver plants had both induced and primed volatile emissions and their resin ducts had an increased epithelial cell (EC) mean area and an increased number of cells located in the second EC layer. Importantly, HIPV exposure increased stomatal conductance and net photosynthesis rate of receiver plants. Receiver-plant responses were also examined under elevated ozone conditions and found to be significantly altered. However, the final defence outcome was not affected. These findings demonstrate that HIPVs modulate conifer metabolism through responses spanning photosynthesis and chemical defence. The responses are adjusted under ozone stress, but the defence benefits remain intact.

Responses in growth and emissions of biogenic volatile organic compounds in Scots pine, Norway spruce and silver birch seedlings to different warming treatments in a controlled field experiment

We investigated the responses in growth and emissions of biogenic volatile organic compounds (BVOCs) in Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies [L.] Karst.) and silver birch (Betula pendula Roth) seedlings to a simulated climate warming of +2 °C (T2) and +4 °C (T4), compared to the ambient conditions, during two growing seasons (2019-2020) in a controlled field experiment in central Finland. In all seedlings, height was measured weekly. Diameter was measured continuously for one seedling from each tree species per plot. For shoot and root biomass measurements, half of the seedlings were harvested at end of the first growing season and the rest at the end of the second growing season. Foliage BVOC emission rates were measured at the end of the second growing season. Biomass, height, and diameter growth of silver birch did benefit the most from warming in both growing seasons. In the Scots pine and Norway spruce seedlings, height and diameter growth increased with increasing temperature in the second growing season, more so in Scots pine. Overall, the shoot and root biomass of conifer seedlings increased with increasing temperature. In the conifer seedlings, warming increased biomass and diameter growth more than height growth, due to their predetermined height growth pattern. The warming increased BVOC emissions more clearly in silver birch, whilst the BVOC emissions were in conifers less sensitive to temperature variation. Based on our findings, silver birch seedlings could be expected to benefit the most from warmer growing conditions and Norway spruce the least.