Summary
Tropical forests are important to the regulation of climate and the maintenance of biodiversity on Earth. However, these ecosystems are threatened by climate change, as temperatures rise and ...droughts' frequency and duration increase. Xylem anatomical traits are an essential component in understanding and predicting forest responses to changes in water availability.
We calculated the community‐weighted means and variances of xylem anatomical traits of hydraulic and structural importance (plot‐level trait values weighted by species abundance) to assess their linkages to local adaptation and community assembly in response to varying soil water conditions in an environmentally diverse Brazilian Atlantic Forest habitat.
Scaling approaches revealed community‐level tradeoffs in xylem traits not observed at the species level. Towards drier sites, xylem structural reinforcement and integration balanced against hydraulic efficiency and capacitance xylem traits, leading to changes in plant community diversity.
We show how general community assembly rules are reflected in persistent fiber–parenchyma and xylem hydraulic tradeoffs. Trait variation across a moisture gradient is larger between species than within species and is realized mainly through changes in species composition and abundance, suggesting habitat specialization. Modeling efforts to predict tropical forest diversity and drought sensitivity may benefit from adding hydraulic architecture traits into the analysis.
Understanding how climate affects xylem formation is critical for predicting the impact of future conditions on tree growth and functioning in the Mediterranean region, which is expected to face ...warmer and drier conditions. However, mechanisms of growth response to climate at different temporal scales are still largely unknown, being complicated by separation between spring and autumn xylogenesis (bimodal temporal pattern) in most species such as Mediterranean pines. We investigated wood anatomical characteristics and carbon stable isotope composition in Mediterranean Pinus pinea L. along tree-ring series at intra-ring resolution to assess xylem formation processes and responses to intra-annual climate variability. Xylem anatomy was strongly related to environmental conditions occurring a few months before and during the growing season, but was not affected by summer drought. In particular, the lumen diameter of the first earlywood tracheids was related to winter precipitation, whereas the size of tracheids produced later was influenced by mid-spring precipitation. Diameter of latewood tracheids was associated with precipitation in mid-autumn. In contrast, tree-ring carbon isotope composition was mostly related to climate of the previous seasons. Earlywood was likely formed using both recently and formerly assimilated carbon, while latewood relied mostly on carbon accumulated many months prior to its formation. Our integrated approach provided new evidence on the short-term and carry-over effects of climate on the bimodal temporal xylem formation in P. pinea. Investigations on different variables and time scales are necessary to disentangle the complex climate influence on tree growth processes under Mediterranean conditions.
Conifer tree rings are generally composed of large, thin-walled cells of light earlywood followed by narrow, thick-walled cells of dense latewood. Yet, how wood formation processes and the associated ...kinetics create this typical pattern remains poorly understood.
We monitored tree-ring formation weekly over 3 yr in 45 trees of three conifer species in France. Data were used to model cell development kinetics, and to attribute the relative importance of the duration and rate of cell enlargement and cell wall deposition on tree-ring structure.
Cell enlargement duration contributed to 75% of changes in cell diameter along the tree rings. Remarkably, the amount of wall material per cell was quite constant along the rings. Consequently, and in contrast with widespread belief, changes in cell wall thickness were not principally attributed to the duration and rate of wall deposition (33%), but rather to the changes in cell size (67%). Cell enlargement duration, as the main driver of cell size and wall thickness, contributed to 56% of wood density variation along the rings.
This mechanistic framework now forms the basis for unraveling how environmental stresses trigger deviations (e.g. false rings) from the normal tree-ring structure.
Hydraulic impairment due to xylem embolism and carbon starvation are the two proposed mechanisms explaining drought‐induced forest dieback and tree death. Here, we evaluate the relative role played ...by these two mechanisms in the long‐term by quantifying wood‐anatomical traits (tracheid size and area of parenchyma rays) and estimating the intrinsic water‐use efficiency (iWUE) from carbon isotopic discrimination. We selected silver fir and Scots pine stands in NE Spain with ongoing dieback processes and compared trees showing contrasting vigour (declining vs nondeclining trees). In both species earlywood tracheids in declining trees showed smaller lumen area with thicker cell wall, inducing a lower theoretical hydraulic conductivity. Parenchyma ray area was similar between the two vigour classes. Wet spring and summer conditions promoted the formation of larger lumen areas, particularly in the case of nondeclining trees. Declining silver firs presented a lower iWUE than conspecific nondeclining trees, but the reverse pattern was observed in Scots pine. The described patterns in wood anatomical traits and iWUE are coherent with a long‐lasting deterioration of the hydraulic system in declining trees prior to their dieback. Retrospective quantifications of lumen area permit to forecast dieback in declining trees 2–5 decades before growth decline started. Wood anatomical traits provide a robust tool to reconstruct the long‐term capacity of trees to withstand drought‐induced dieback.
Conifer trees possess a typical anatomical tree‐ring structure characterized by a transition from large and thin‐walled earlywood tracheids to narrow and thick‐walled latewood tracheids. However, ...little is known on how this characteristic structure is maintained across contrasting environmental conditions, due to its crucial role to ensure sap ascent and mechanical support. In this study, we monitored weekly wood cell formation for up to 7 years in two temperate conifer species (i.e., Picea abies (L.) Karst and Larix decidua Mill.) across an 8°C thermal gradient from 800 to 2,200 m a.s.l. in central Europe to investigate the impact of air temperature on rate and duration of wood cell formation. Results indicated that towards colder sites, forming tracheids compensate a decreased rate of differentiation (cell enlarging and wall thickening) by an extended duration, except for the last cells of the latewood in the wall‐thickening phase. This compensation allows conifer trees to mitigate the influence of air temperature on the final tree‐ring structure, with important implications for the functioning and resilience of the xylem to varying environmental conditions. The disappearing compensation in the thickening latewood cells might also explain the higher climatic sensitivity usually found in maximum latewood density.
The structure of conifer wood cells depends on the speed and duration of processes shaping their formation. In this study, we show for the first time that cells growing at colder sites increase their duration of their processes to compensate for a speed reduction. This compensation allows conifers to mitigate the effect of air temperature to maintain a more similar tree‐ring structure despite contrasting conditions.
• Vulnerability to cavitation and conductive efficiency depend on xylem anatomy. We tested a large range of structure-function hypotheses, some for the first time, within a single genus to minimize ...phylogenetic ‘noise' and maximize detection of functionally relevant variation. • This integrative study combined in-depth anatomical observations using light, scanning and transmission electron microscopy of seven Acer taxa, and compared these observations with empirical measures of xylem hydraulics. • Our results reveal a 2 MPa range in species' mean cavitation pressure (MCP). MCP was strongly correlated with intervessel pit structure (membrane thickness and porosity, chamber depth), weakly correlated with pit number per vessel, and not related to pit area per vessel. At the tissue level, there was a strong correlation between MCP and mechanical strength parameters, and some of the first evidence is provided for the functional significance of vessel grouping and thickenings on inner vessel walls. In addition, a strong trade-off was observed between xylem-specific conductivity and MCP. Vessel length and intervessel wall characteristics were implicated in this safety-efficiency trade-off. • Cavitation resistance and hydraulic conductivity in Acer appear to be controlled by a very complex interaction between tissue, vessel network and pit characteristics.
Because of global warming, high‐latitude ecosystems are expected to experience increases in temperature and drought events. Wood formation will have to adjust to these new climatic constraints to ...maintain tree mechanical stability and long‐distance water transport. The aim of this study is to understand the dynamic processes involved in wood formation under warming and drought. Xylogenesis, gas exchange, water relations and wood anatomy of black spruce Picea mariana (Mill.) B.S.P. saplings were monitored during a greenhouse experiment where temperature was increased during daytime or night‐time (+6 °C) combined with a drought period. The kinetics of tracheid development expressed as rate and duration of the xylogenesis sub‐processes were quantified using generalized additive models. Drought and warming had a strong influence on cell production, but little effect on wood anatomy. The increase in cell production rate under warmer temperatures, and especially during the night‐time warming at the end of the growing season, resulted in wider tree‐rings. However, the strong compensation between rates and durations of cell differentiation processes mitigates warming and drought effects on tree‐ring structure. Our results allowed quantification of how wood formation kinetics is regulated when water and heat stress increase, allowing trees to adapt to future environmental conditions.
Global warming in high‐latitude ecosystems will affect wood formation. This work focuses on the dynamic processes involved in wood formation under experimental drought and warming treatments. Our study demonstrates that drought and warming have a strong impact on cell production, but a weak influence on xylem anatomy. Using innovative analysis of kinetics of tracheid development, we quantified how the compensation mechanisms between rates and durations of the xylogenesis sub‐processes strongly mitigate negative effects of multistress on wood structure.
Remains of gymnosperms are the most abundant plant macro-fossils in the outcrops of a fossil lagerstätte preserved in lacustrine limestones in the likely lower Aptian Crato Formation of the Araripe ...Basin in NE Brazil. However, the systematics of most fossil-taxa are mainly based on foliage and reproductive structures, whereas wood anatomy has rarely been used to study fossil plant diversity, despite the abundance of wood fragments. Here we describe a new fossil gymnosperm taxon based on anatomical characters of a well-preserved specimen from this lithostratigraphic unit. The new fossil-genus and fossil-species Cratoxylon placidoi gen. nov. et sp. nov. are established using characters of the secondary xylem and the bark, features that are also crucial for their affiliation into conifers. This finding reinforces the already known remarkable preservation potential of organisms in this konservat lagerstätte, since bark tissues are rarely preserved in the fossil record of Mesozoic gymnosperms. The results reveal the great potential of fossil woods for studying diversity of the Crato Formation flora and thus contribute to expand our knowledge on fossil vegetation in equatorial Gondwana during the early diversification of angiosperms.
•New fossil genus and species of conifer are established from Cretaceous of Brazil.•This species has bark preserved, which is rarely reported in Mesozoic gymnosperms.•This record expands the knowledge of Crato Fossil Lagerstätte paleoxyloflora.
Forecasted warmer and drier conditions will probably lead to reduced growth rates and decreased carbon fixation in long-term woody pools in drought-prone areas. We therefore need a better ...understanding of how climate stressors such as drought constrain wood formation and drive changes in wood anatomy. Drying trends could lead to reduced growth if they are more intense in spring, when radial growth rates of conifers in continental Mediterranean climates peak. Since tree species from the aforementioned areas have to endure dry summers and also cold winters, we chose two coexisting species: Aleppo pine (Pinus halepensisMill., Pinaceae) and Spanish juniper (Juniperus thuriferaL., Cupressaceae) (10 randomly selected trees per species), to analyze how growth (tree-ring width) and wood-anatomical traits (lumen transversal area, cell-wall thickness, presence of intra-annual density fluctuations-IADFs-in the latewood) responded to climatic variables (minimum and maximum temperatures, precipitation, soil moisture deficit) calculated for different time intervals. Tree-ring width and mean lumen area showed similar year-to-year variability, which indicates that they encoded similar climatic signals. Wet and cool late-winter to early-spring conditions increased lumen area expansion, particularly in pine. In juniper, cell-wall thickness increased when early summer conditions became drier and the frequency of latewood IADFs increased in parallel with late-summer to early-autumn wet conditions. Thus, latewood IADFs of the juniper capture increased water availability during the late growing season, which is reflected in larger tracheid lumens. Soil water availability was one of the main drivers of wood formation and radial growth for the two species. These analyses allow long-term (several decades) growth and wood-anatomical responses to climate to be inferred at intra-annual scales, which agree with the growing patterns already described by xylogenesis approaches for the same species. A plastic bimodal growth behavior, driven by dry summer conditions, is coherent with the presented wood-anatomical data. The different wood-anatomical responses to drought stress are observed as IADFs with contrasting characteristics and responses to climate. These different responses suggest distinct capacities to access soil water between the two conifer species.
PDF
