Xylem plasticity allows rapid hydraulic adjustment to annual climatic variability

Thanks to acclimation, trees overcome environmental changes and endure for centuries. The anatomy of water conducting cells is an important factor determining plant success. Forming cells are coupled with the environment and their properties are naturally archived in the wood. Its variability across tree rings can thus provide a retrospective of plant's hydraulic adjustments. In this work, we measured lumen and wall thickness of tracheids along tree-rings to explore how trees regulate their conducting system under variable plant-water conditions. Tracheids were measured along 51 dated rings of five mature Larix decidua and Picea abies trees from a low elevation site. Anatomical-based chronologies of annual growth performance, hydraulic conductance and safety, and construction costs were built. Similarities among chronologies and the relation to monthly climate data were analyzed. Most parameters displayed high annual plasticity which was partly coherent among trees and mostly associated with radial growth. In general, summer drought reduced growth and potential hydraulic conductivity of the forming ring, and increased hydraulic safety and construction costs. To evaluate the functional relevance of the annual acclimation, the conductivity of the forming ring relative to the entire sapwood needs to be assesse

Radial Split Resistance of Chestnut Earlywood and Its Relation to Ring Width

New equipment was developed to measure the maximal radial split resistance of individual annual rings in green European chestnut wood (Castanea sativa Mill.). This equipment was then used to compare the split resistance in chestnut trees with and without ring shake taken from three differently managed coppices from the southern part of the Swiss Alps. Results indicate that within these stands radial split resistance and annual ring width are positively correlated, and that the rates of ring-shake occurrence increase with narrow and weak growth rings. Forest management of chestnut coppices that leads to an increase in growth thickness might, therefore, be a way of reducing the risk of ring shake

Duration and extension of anatomical changes in wood structure after cambial injury

Cambial injury has been reported to alter wood structure in broad-leaved trees. However, the duration and extension of associated anatomical changes have rarely been analysed thoroughly. A total of 18 young European ash (Fraxinus excelsior L.) trees injured on the stem by a spring flood were sampled with the aim of comparing earlywood vessels and rays formed prior to and after the scarring event. Anatomical and hydraulic parameters were measured in five successive rings over one-quarter of the stem circumference. The results demonstrate that mechanical damage induces a decrease in vessel lumen size (up to 77%) and an increase in vessel number (up to 475%) and ray number (up to 115%). The presence of more earlywood vessels and rays was observed over at least three years after stem scarring. By contrast, abnormally narrow earlywood vessels mainly developed in the first ring formed after the event, increasing the thickness-to-span ratio of vessels by 94% and reducing both xylem relative conductivity and the index for xylem vulnerability to cavitation by 54% and 32%, respectively. These vessels accumulated in radial groups in a 30° sector immediately adjacent to the wound, raising the vessel grouping index by 28%. The wound-induced anatomical changes in wood structure express the functional need of trees to improve xylem hydraulic safety and mechanical strength at the expense of water transport. Xylem hydraulic efficiency was restored in one year, while xylem mechanical reinforcement and resistance to cavitation and decay lasted over several year

Expeditious building of ring-porous earlywood vessel chronologies without loosing signal information

Chronologies of earlywood vessel size of ring-porous trees contain valuable ecological information, but long preparation procedures limit their application in ecological studies. Recent and fast techniques for wood surface preparation combined with automated image analysis are reducing the work needed to build chronologies, but might also entail measurement inaccuracy. In this study, we aim to evaluate the effect of a possible efficiency-accuracy trade-off on ecological signal strength. To this end, we compare measurements of mean vessel area from two recent and fast procedures carried out on sanded wood surfaces with a reference procedure based on an accurate survey from thin sections. Measurements were performed on increment cores of 15 sessile oaks (Quercus petraea (Mattuschka) Liebl.) for the period 1956-2006. Dissimilarities in results with the reference procedure were quantified and evaluated. Our data show that the workload can be reduced by more than 20-fold when using the highly automated procedure. Signal weakening caused by measurement errors is negligible for vessels >6,000μm2 and can be easily compensated by increasing the sample size. Manual correction of misrecognized vessels hardly reduced this error further. The new procedures constitute a major step towards an efficient and accurate analysis of earlywood vessel chronologies of ring-porous tree specie

Responses of wood anatomy and carbon isotope composition of Quercus pubescens saplings subjected to two consecutive years of summer drought

• To withstand and to recover from severe summer drought is crucial for trees, as dry periods are predicted to occur more frequently over the coming decades. • In order to better understand growth-related tree responses to drought, wood formation, vessel characteristics and stable carbon isotope composition (δ13C) in tree rings of Quercus pubescens saplings imposed to two consecutive summer droughts were compared with regularly watered control trees. • In both years, photosynthetic activity was strongly inhibited during the drought periods of five to seven weeks but quickly restored after re-watering, reinitiating wood formation. Stress caused more than a 20% reduction in ring width, a 0.5‰ increase in latewood δ13C and changes in vessels characteristics in both the current year latewood and the next year earlywood. The latewood displayed up to 90% increased hydraulic conductivity than control trees, likely to compensate for a cavitation-induced reduction of water transport. • The earlywood after the first drought year was characterized by more but smaller vessels suggesting the attempt of restoring conductivity while minimizing the risk of hydraulic failure. However, after the second year, the reduction of hydraulic conductivity and the increased δ13C values indicate a structural adjustment towards a reduced growth induced by exhaustion of carbon reserve

Tree growth response along an elevational gradient: climate or genetics?

Environment and genetics combine to influence tree growth and should therefore be jointly considered when evaluating forest responses in a warming climate. Here, we combine dendroclimatology and population genetic approaches with the aim of attributing climatic influences on growth of European larch (Larix decidua) and Norway spruce (Picea abies). Increment cores and genomic DNA samples were collected from populations along a ~900-m elevational transect where the air temperature gradient encompasses a ~4°C temperature difference. We found that low genetic differentiation among populations indicates gene flow is high, suggesting that migration rate is high enough to counteract the selective pressures of local environmental variation. We observed lower growth rates towards higher elevations and a transition from negative to positive correlations with growing season temperature upward along the elevational transect. With increasing elevation there was also a clear increase in the explained variance of growth due to summer temperatures. Comparisons between climate sensitivity patterns observed along this elevational transect with those from Larix and Picea sites distributed across the Alps reveal good agreement, and suggest that tree-ring width (TRW) variations are more climate-driven than genetics-driven at regional and larger scales. We conclude that elevational transects are an extremely valuable platform for understanding climatic-driven changes over time and can be especially powerful when working within an assessed genetic framewor

A method to quantify and account for the hygroscopic effect in stem diameter variations

Dendrometers recording stem diameter variations (SDV) at high-resolution are useful to assess trees' water relation since water reserves are stored in the elastic tissue of the bark. These tissues typically shrink during the day as they release water when evaporative demand is high and swell during the night as they are replenished when evaporative demand is low, generating the typical SDV profile known as the diel SDV cycle. However, similar SDV cycles have been observed on dead trees due to the hygroscopic shrinking and swelling of the dead bark tissues. In order to remove this hygroscopic effect of the bark, dendrometers are applied as close as possible to the living bark tissues by removing the outer dead layer, however with questionable success. In this study, we used SDV time series from 40 point dendrometers applied on dead-bark-removed mature trees to assess and quantify the remaining hygroscopic effect on individual trees. To do so, we checked SDV behavior in the cold season and explored the relation between the diel SDV cycle and changes in relative humidity (RH). Our results showed that (a) the hygroscopic effect in SDV can be well-detected based on the amplitude of the diel SDV cycle (diel SDVampl) and the correlation between SDV and RH during both the cold and the warm season; (b) the level of the hygroscopic effect varies strongly among individuals; (c) diel SDVampl is proportional to both changes in RH and transpiration so that the hygroscopic effect on the diel SDV cycle can be quantified using a linear model where (diel SDVampl) is a function of RH changes and transpiration. These results allow the use of the model to correct the amplitude of the diel SDV cycles and suggest that this method can be applied to other ecological relevant water-related SDV variables such as tree water deficit

Temperature modulates intra-plant growth of Salix polaris from a high Arctic site (Svalbard)

Arctic ecosystems are important carbon sinks. Increasing temperatures in these regions might stimulate soil carbon release. Evidence suggests that deciduous shrubs might counteract these carbon losses because they positively respond to increasing temperature, but their role in ecosystem carbon budgets remains uncertain. Many studies dealing with large-scale tundra greening and carbon sequestration in relation to increasing temperature have usually based their estimations on the aboveground components, but very little is known about belowground growth. In this context, annual rings can provide a retrospective insight into intra-plant temperature responses and seasonal growth allocation. This study presents a 70-year-long and annually resolved intra-plant analysis of ring width and missing ring distribution from a comprehensive serial sectioning, including 142 cross-sections and the measurements of 471 radii from ten Salix polaris Wahlenb. dwarf shrubs growing in the high Arctic on Svalbard. Results indicate a high intra-plant and inter-annual growth variation, characterized by a high proportion of partially (13.6%) and completely (11.2%) missing rings. The annual growth and the frequency of completely missing rings were evenly distributed inside the plant and mainly controlled by summer temperatures. Radial growth in the belowground parts appeared to be proportionally higher during long and warm summers and lower in cold early growing seasons than in the aboveground parts. The results reveal a diverging allocation between aboveground and belowground growth depending on the climatic conditions. Favorable years promoted root allocation since root radial growth occurs after aboveground growth. The observed belowground responses suggest that shrub carbon allocation might be higher than estimated only from the aboveground compartment

On the need to consider wood formation processes in global vegetation models and a suggested approach

Dynamic global vegetation models are key tools for interpreting and forecasting the responses of terrestrial ecosystems to climatic variation and other drivers. They estimate plant growth as the outcome of the supply of carbon through photosynthesis. However, growth is itself under direct control, and not simply controlled by the amount of available carbon. Therefore predictions by current photosynthesis driven models of large increases in future vegetation biomass due to increasing concentrations of atmospheric CO2 may be significant over-estimations. We describe how current understanding of wood formation can be used to reformulate global vegetation models, with potentially major implications for their behaviour

A tree-centered approach to assess impacts of extreme climatic events on forests

Ajut Marie Curie IF fellowship (No 659191); COST Action FP1106 STReES