Wood Under the Microscope

Volume: 76Start Page: 18End Page: 2

Xylem anatomical traits and their relation to measures of hydraulic safety in a neotropical rainforest

International audienceIntroduction: The anatomical composition of wood underpins hydraulic transport and storage functions of trees. Together, anatomical traits of the vessels, fibers, and parenchyma influence the plants resistance to embolism formation (P50) and availability of internal water storage (sapwood capacitance), both of which can impact hydraulic vulnerability under drought stress. However, the degree to which interspecific variation in anatomical traits correspond with measures of hydraulic vulnerability has not been extensively studied in tropical angiosperms.Objectives: We compared a suite of xylem anatomical measurements to measures of embolism resistance and stem sapwood capacitance for branches from five individuals of twelve species representing a range of life history strategies in a tropical rainforest in Puerto Rico.We asked: (1) how do stem anatomical traits correlate with embolism resistance and sapwood capacitance? (2) how do structure-function relationships vary across species with different life-history traits? Methods: We used a sledge microtome and high-resolution slide scanner with digital analysis software to quantify a suite of anatomical traits (vessel dimensions, potential hydraulic conductivity, vessel grouping indices) of branch-wood cross sections, and compared these with hydraulic traits (P50 and stem capacitance) previously collected on the same individuals.Results: We found correlations at the site level between hydraulic traits (P50 and sapwood capacitance), and anatomical traits (including the size and grouping of vessels). Vessel diameter and vessel density were negatively and positively correlated, respectively, with more embolism resistant P50 values. While vessel diameter and the vessel grouping index were positively correlated with sapwood capacitance and vessel density was negatively correlated.Conclusions: At the site level, vessel characteristics were related to hydraulic traits indicating a link in how branches manage hydraulic stress. These results suggest that the anatomical design of a tree's xylem has implications on its ability to manage different kinds of drought stress; therefore, a better understanding of anatomical traits and their inter- and intraspecific variability will help to improve our understanding of trees' hydraulic vulnerability

Why trees grow at night

The timing of diel stem growth of mature forest trees is still largely unknown, as empirical data with high temporal resolution have not been available so far. Consequently, the effects of day-night conditions on tree growth remained uncertain. Here we present the first comprehensive field study of hourly-resolved radial stem growth of seven temperate tree species, based on 57 million underlying data points over a period of up to 8 yr. We show that trees grow mainly at night, with a peak after midnight, when the vapour pressure deficit (VPD) is among the lowest. A high VPD strictly limits radial stem growth and allows little growth during daylight hours, except in the early morning. Surprisingly, trees also grow in moderately dry soil when the VPD is low. Species-specific differences in diel growth dynamics show that species able to grow earlier during the night are associated with the highest number of hours with growth per year and the largest annual growth increment. We conclude that species with the ability to overcome daily water deficits faster have greater growth potential. Furthermore, we conclude that growth is more sensitive than carbon uptake to dry air, as growth stops before stomata are known to close

Why trees grow at night

- The timing of diel stem growth of mature forest trees is still largely unknown, as empirical data with high temporal resolution have not been available so far. Consequently, the effects of day–night conditions on tree growth remained uncertain. - Here we present the first comprehensive field study of hourly-resolved radial stem growth of seven temperate tree species, based on 57 million underlying data points over a period of up to 8 yr. - We show that trees grow mainly at night, with a peak after midnight, when the vapour pressure deficit (VPD) is among the lowest. A high VPD strictly limits radial stem growth and allows little growth during daylight hours, except in the early morning. Surprisingly, trees also grow in moderately dry soil when the VPD is low. Species-specific differences in diel growth dynamics show that species able to grow earlier during the night are associated with the highest number of hours with growth per year and the largest annual growth increment. - We conclude that species with the ability to overcome daily water deficits faster have greater growth potential. Furthermore, we conclude that growth is more sensitive than carbon uptake to dry air, as growth stops before stomata are known to close.ISSN:0028-646XISSN:1469-813

Variation in plant ecophysiological traits along a tropical aridity gradient explained with optimality theory (977)

International audienceThe ‘pure’ effect of aridity on photosynthetic and water-transport strategies is not easy to discern because of large-scale correlations between precipitation and temperature. We analyse traits collected along an aridity gradient in Ghana, West Africa, that shows very little temperature variation, in an attempt to disentangle thermal and hydraulic influences on plant traits. Theoretical predictions of the variation of key plant traits along the gradient are tested with field measurements. Most photosynthetic traits show trends consistent with theoretical predictions, including higher photosynthetic rates in the drier sites, and an association of higher photosynthetic rates with greater respiration rates and greater water transport. Hydraulic and leaf-economic traits show less consistency with previous theories, however. In particular the relationship between the sapwood-to-leaf-area ratio (AS/AL) and potential specific hydraulic conductance (Kp) is found to differ from that shown in a global dataset. Nonetheless, the link between photosynthesis and water transport holds: species with both higher AS/AL and Kp (implying greater water transport) (predominantly deciduous species found in drier sites) tend to have both higher photosynthetic capacity, and lower leaf-internal CO2, than others. These results indicate that aridity is a primary driver of the spatial pattern of photosynthetic traits, while plants show a greater diversity of water-transport strategies to support higher photosynthetic rate in arid environments

Vessel diameter is related to amount and spatial arrangement of axial parenchyma in woody angiosperms

Parenchyma represents a critically important living tissue in the sapwood of the secondary xylem of woody angiosperms. Considering various interactions between parenchyma and water transporting vessels, we hypothesize a structure–function relationship between both cell types. Through a generalized additive mixed model approach based on 2,332 woody angiosperm species derived from the literature, we explored the relationship between the proportion and spatial distribution of ray and axial parenchyma and vessel size, while controlling for maximum plant height and a range of climatic factors. When factoring in maximum plant height, we found that with increasing mean annual temperatures, mean vessel diameter showed a positive correlation with axial parenchyma proportion and arrangement, but not for ray parenchyma. Species with a high axial parenchyma tissue fraction tend to have wide vessels, with most of the parenchyma packed around vessels, whereas species with small diameter vessels show a reduced amount of axial parenchyma that is not directly connected to vessels. This finding provides evidence for independent functions of axial parenchyma and ray parenchyma in large vesselled species and further supports a strong role for axial parenchyma in long-distance xylem water transport

TRY plant trait database - enhanced coverage and open access

10.1111/gcb.14904GLOBAL CHANGE BIOLOGY261119-18