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How and when does climate influence carbon sink activity? Multi-temporal analysis of wood formation in conifers

Periodic Reporting for period 1 - INTREE (How and when does climate influence carbon sink activity? Multi-temporal analysis of wood formation in conifers)

Okres sprawozdawczy: 2018-08-01 do 2020-07-31

Wood is the main terrestrial biotic reservoir for long-term carbon sequestration. The amount of carbon stocked in the tree stem each year is highly variable, in relation to short- and long-term climate variability, variations in the stand dynamics (inter-tree interactions), and biotic and abiotic stress factors that influence tree physiology and growth processes. Better understanding of the mechanisms behind such variability would be critical for predicting future carbon sequestration by forests under global change.
Different approaches can be used to study environmental influence on tree radial growth (carbon sequestration). The tree-ring width analysis provides long-term information on environmental influence on tree growth at annual resolution. Direct xylogenesis observations inform on the timing of different phases of xylem (wood) cell formation, at the weekly scale. A third approach, the quantitative wood anatomy in tree rings, allows to retrospectively analyze of xylem anatomical traits along tree-ring series.
The INTREE project developed intra-ring quantitative wood anatomy and applied it to different tree species, constrained by different environmental drivers. In connection with outcomes from the other two approaches (xylogenesis observations and tree-ring analyses), this provided new information on how environmental variability (temperature, precipitation, drought, defoliator insects) affects xylem formation processes at different time scales and in different environments (project Objective 1). Research conducted in the project demonstrated that the amount of carbon in conifer tree rings is sensitive to variability in the anatomy of xylem cells (Objective 2). In particular, defoliation caused by insects can alter xylem cell formation and anatomy, causing strong reduction in the amount of carbon stored yearly in the tree stem (Objective 3). This calls for further quantitative investigation on how environmental-driven variations in xylem anatomy can influence the tree-ring biomass, which is the fundamental element for assessing forest biomass variations and climate change impacts on terrestrial carbon cycle.
Work activities included the analysis of wood anatomy along tree-ring series of black spruce in Quebec (Canada), Norway spruce and European larch in Loetchental (Switzerland), and other species not included in the initial plan such as Norway spruce and silver fir in the Black forest (Germany), and black spruce in Alberta (Canada). All activities were performed under the supervision of WSL researchers and technicians. These included the use of laboratory tools such as microtomes, embedding machine, fume hood, and the new laser scanner system for the automatic acquisition of micro-section images. Images were processed with the most recent version of ROXAS software, developed by WSL researchers. Retrieved data were elaborated with specific R packages for statistical and xylem anatomy analyses.
Results were communicated thought scientific manuscripts (3 published, one as last author, one as corresponding, one as first author, and one as first author in preparation), posters (3 as first author) and oral presentations to conferences (4) and workshops (1), seminars to researchers (1), PhD and master students (5). Besides, four informative papers were prepared in collaboration with other researchers and journalists. Finally, a workshop on quantitative wood anatomy with speakers from five different countries and more than 40 participants was organized at the Beneficiary Institute venue. Supervision activities included collaboration with PhD students and persons in charge of image analysis. Training activities comprised attendance to three courses, and participation to >10 seminars, 4 workshops and 6 conferences.
The project activities and results represent an important step forward for quantitative wood anatomy, an approach of increasing interest in the scientific community. The activity success is testified by publications in high-rank journals, high participation of students to seminars, of researchers to the organized workshop and conference contributions, and collaborations with research groups coming from different research areas (archaeology: Weizmann Institute of Science in Israel; ecohydrology: Waterloo University in Canada).
A few scientific results published during the project course were highly innovative, representing relevant advance in the current understanding of environmental influence on carbon accumulation in tree biomass. In particular, we discovered that early-summer drought negatively affects xylem hydraulic traits of boreal black spruce, in contrast to previous knowledge based on tree-ring width and xylogenesis analyses, that indicated positive influence of summer temperature on xylem formation processes. In another study, we evidenced the long-term impact of insect defoliation on the xylem structure of European larch. In parallel to xylogenesis observations during the last outbreak, our study evidenced that cell number is more and longer affected than cell size. Besides, anatomical analyses showed that biomass estimates from ring-width analyses underestimate the structural carbon loss during outbreaks. Such results go beyond the existing state of the art, coming mostly from ring-width analysis, xylogenesis monitoring, and application of wood anatomy at the ring level. Our findings evidence that intra-ring quantitative anatomy provides new and complementary information on xylem formation then classical approaches, and improves understanding of the mechanisms behind tree carbon sequestration processes.