Elevated levels of ground-level ozone threaten terrestrial ecosystems and biodiversity.

Silver birch (Betula pendula) is a pioneer boreal tree that can be induced to flower within 1 year. Its rapid life cycle, small (440-Mb) genome, and advanced germplasm resources make birch an attractive model for forest biotechnology. We assembled and chromosomally anchored the nuclear genome of an inbred B. pendula individual. Gene duplicates from the paleohexaploid event were enriched for transcriptional regulation, whereas tandem duplicates were overrepresented by environmental responses. Population resequencing of 80 individuals showed effective population size crashes at major points of climatic upheaval. Selective sweeps were enriched among polyploid duplicates encoding key developmental and physiological triggering functions, suggesting that local adaptation has tuned the timing of and cross-talk between fundamental plant processes. Variation around the tightly-linked light response genes PHYC and FRS10 correlated with latitude and longitude and temperature, and with precipitation for PHYC. Similar associations characterized the growth-promoting cytokinin response regulator ARR1, and the wood development genes KAK and MED5A.

Summary: The impacts of elevated atmospheric CO₂ and/or O₃ have been examined over 4 years using an open‐air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O₃‐sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera, and the determinate, late successional, O₃‐tolerant species Sugar Maple, Acer saccharum). The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O₃‐tolerant as well as O₃‐sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34–36 nL L⁻¹ during the summer daylight hours), O₃ offsets or moderates the responses induced by elevated CO₂. After 3 years of exposure to 560 µmol mol⁻¹ CO₂, the above‐ground volume of Aspen stands was 40% above those grown at ambient CO₂, and there was no indication of a diminishing growth trend. In contrast, O₃ at 1·5 × ambient completely offset the growth enhancement by CO₂, both for O₃‐sensitive and O₃‐tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO₂, and global models of forest productivity and climate change are presented.