From Carlquist's ecological wood anatomy to Carlquist's Law: why comparative anatomy is crucial for functional xylem biology

Water storage capacity is inversely associated with xylem embolism resistance in tropical karst tree species

Tropical karst habitats are characterized by limited and patchy soil, large rocky outcrops and porous substrates, resulting in high habitat heterogeneity and soil moisture fluctuations. Xylem hydraulic efficiency and safety can determine the drought adaptation and spatial distribution of woody plants growing in karst environments. In this study, we measured sapwood-specific hydraulic conductivity (Ks), vulnerability to embolism, wood density, saturated water content, and vessel and pit anatomical characteristics in the branch stems of 12 evergreen tree species in a tropical karst seasonal rainforest in southwestern China. We aimed to characterize the effects of structural characteristics on hydraulic efficiency and safety. Our results showed that there was no significant correlation between Ks and hydraulic safety across the tropical karst woody species. Ks was correlated with hydraulic vessel diameter (r = 0.80, P < 0.05) and vessel density (r = -0.60, P < 0.05), while the stem water potential at 50 and 88% loss of hydraulic conductivity (P50 and P88) were both significantly correlated with wood density (P < 0.05) and saturated water content (P = 0.052 and P < 0.05, respectively). High stem water storage capacity was associated with low cavitation resistance possibly because of its buffering the moisture fluctuations in karst environments. However, both Ks and P50/P88 were decoupled from the anatomical traits of pit and pit membranes. This may explain the lack of tradeoff between hydraulic safety and efficiency in tropical karst evergreen tree species. Our results suggest that diverse hydraulic trait combination may facilitate species coexistence in karst environments with high spatial heterogeneity.

Angiosperms follow a convex trade-off to optimize hydraulic safety and efficiency

Intervessel pits are considered to function as valves that avoid embolism spreading and optimize efficient transport of xylem sap across neighbouring vessels. Hydraulic transport between vessels would therefore follow a safety-efficiency trade-off, which is directly related to the total intervessel pit area (Ap ), inversely related to the pit membrane thickness (TPM ) and driven by a pressure difference. To test this hypothesis, we modelled the relative transport rate of gas (ka ) and water (Q) at the intervessel pit level for 23 angiosperm species and correlated these parameters with the water potential at which 50% of embolism occurs (Ψ50 ). We also measured ka for 10 species using pneumatic measurements. The pressure difference across adjacent vessels and estimated values of ka and Q were related to Ψ50 , following a convex safety-efficiency trade-off based on modelled and experimental data. Minor changes in TPM and Ap exponentially affected the pressure difference and flow, respectively. Our results provide clear evidence that a xylem safety-efficiency trade-off is not linear, but convex due to flow across intervessel pit membranes, which represent mesoporous media within microporous conduits. Moreover, the convex nature of long-distance xylem transport may contribute to an adjustable fluid balance of plants, depending on environmental conditions.

Plant morpho evo-devo

Evo-devo is often thought of as being the study of which genes underlie which phenotypes. However, evo-devo is much more than this, especially in plant science. In leaf scars along stems, cell changes across wood growth rings, or flowers along inflorescences, plants trace a record of their own development. Plant morpho evo-devo provides data that genes could never furnish on themes such as heterochrony, the evolution of temporal phenotypes, modularity, and phenotype-first evolution. As plant science surges into increasingly -omic realms, it is essential to keep plant morpho evo-devo in full view as an honored member of the evo-devo canon, ensuring that plant scientists can, wherever they are, generate fundamental insights at the appropriate level of biological organization.

Functional traits and water-transport strategies of woody species in an insular environment in a tropical forest

PREMISE

Plants survive in habitats with limited resource availability and contrasting environments by responding to variation in environmental factors through morphophysiological traits related to species performance in different ecosystems. However, how different plant strategies influence the megadiversity of tropical species has remained a knowledge gap.

METHODS

We analyzed variations in 27 morphophysiological traits of leaves and secondary xylem in Erythroxylum pulchrum and Tapirira guianensis, which have the highest absolute dominance in these physiognomies and occur together in areas of restinga and dense ombrophilous forest to infer water-transport strategies of Atlantic Forest woody plants.

RESULTS

The two species presented different sets of morphophysiological traits, strategies to avoid embolism and ensure water transport, in different phytophysiognomies. Tapirira guianensis showed possible adaptations influenced by phytophysiognomy, while E. pulchrum showed less variation in the set of characteristics between different phytophysiognomies.

CONCLUSIONS

Our results provide essential tools to understand how the environment can modulate morphofunctional traits and how each species adjusts differently to adapt to different phytophysiognomies. In this sense, the results for these species reveal new species-specific responses in the tropical forest. Such knowledge is a prerequisite to predict future development of the most vulnerable forests as climate changes.

Miocene Cupressinoxylon from Gökçeada (Imbros), Turkey with Protophytobia cambium mining and the study of ecological signals of wood anatomy

Premise

The recognition of the Miocene Climate Optimum (MCO) in terrestrial palaeoenvironments of the Eastern Mediterranean is restricted to Lesbos and Lemnos Islands, Greece. This area is significant for its wood microfossils. A recently-discovered fossil wood assemblage from Gökçeada (Imbros) Island, Turkey, including tree species similar to the Greek findings, is thought to have an early Miocene age. Here, we revise the age of the latter plant fossiliferous locality, re-evaluate the area for the study of MCO for the terrestrial palaeoecosystems of the Eastern Mediterranean and the nomenclature errors referring to the occurrence of fossil wood. We present the plant-insect-environment interactions using detailed anatomical descriptions, of an extinct conifer and its extinct cambium miner feeding traces observed in its secondary xylem.

Methods

Three thin sections were prepared with standard palaeoxylotomical techniques from a small section of the silicified wood; the sections were observed under a light microscope. The anatomy of the conifer and its damage patterns were compared with those of extant and fossil Cupressaceae and Agromyzidae, respectively.

Pivotal results

The common anatomical features of the studied wood specimen and Hesperocyparis macrocarpa (Hartw.) Bartel and a shared characteristic (the number of the cross-field pits - a feature we consider of diagnostic value) with Xanthocyparis vietnamensis Farjon & T.H. Nguyên led to its assignment to the Hesperocyparis-Xanthocyparis-Callitropsis clade. The detailed study of the wound scars and anatomical abnormalities, the anatomical-environmental associations, and structural-functional reactions follow the identification of the wood's anatomy sensu Carlquist providing decisive results.

Conclusions

Based on the distinctive characteristics presented, we identify our macrofossil as Cupressinoxylon matromnense Grambast, a stem or an extinct lineage of the Hesperocyparis-Xanthocyparis vietnamensis-Callitropsis nootkatensis clade with feeding traces of the fossil cambium miner of the genus Protophytobia Süss (Diptera: Agromyzidae), and anatomical damage and reaction tissue on adventitious shoots. The use of Protopinaceae and Pinoxylon F. H. Knowlton from the eastern Mediterranean are re-evaluated and corrections are provided. The age of the studied plant fossiliferous locality in Gökçeada is revised as middle Miocene, allowing the proposal of an eastern Mediterranean MCO hotspot, including Lesbos, Lemnos, and Gökçeada (Imbros) Islands.

Functional xylem characteristics associated with drought-induced embolism in angiosperms

Hydraulic failure resulting from drought-induced embolism in the xylem of plants is a key determinant of reduced productivity and mortality. Methods to assess this vulnerability are difficult to achieve at scale, leading to alternative metrics and correlations with more easily measured traits. These efforts have led to the longstanding and pervasive assumed mechanistic link between vessel diameter and vulnerability in angiosperms. However, there are at least two problems with this assumption that requires critical re-evaluation: (1) our current understanding of drought-induced embolism does not provide a mechanistic explanation why increased vessel width should lead to greater vulnerability, and (2) the most recent advancements in nanoscale embolism processes suggest that vessel diameter is not a direct driver. Here, we review data from physiological and comparative wood anatomy studies, highlighting the potential anatomical and physicochemical drivers of embolism formation and spread. We then put forward key knowledge gaps, emphasising what is known, unknown and speculation. A meaningful evaluation of the diameter-vulnerability link will require a better mechanistic understanding of the biophysical processes at the nanoscale level that determine embolism formation and spread, which will in turn lead to more accurate predictions of how water transport in plants is affected by drought.

Anatomical adaptions of pits in two types of ray parenchyma cells in Populus tomentosa during the xylem differentiation

Pits in ray parenchyma cells are important to understand the functional anatomy of the ray parenchyma network in the xylem but have been less studied. Herein, pits in two types of ray parenchyma cells, contact cells and isolation cells, across different developmental stages were qualitatively studied using 48-year-old Populus tomentosa trees. The timing of differentiation and death was determined by histochemical staining and polarized light microscopy. The dimension, shape and density of pits as well as cell wall thickness were measured using SEM and optical microscopy images of semi-thin radial sections and macerated ray parenchyma cells, and analyzed by multi-factor analyses of variance. Results showed that secondary wall thickening and lignification of contact cells begun near the cambium, contrarily those of isolation cells have started until the transition zone. But even in the sapwood, contact cell walls were still much thinner than isolation cell walls. Moreover, district anatomical adaptions of pits during the xylem differentiation were present between horizontal walls and tangential walls, between contact cells and isolation cells. Ray pits were simple to slightly bordered, whereas sieve-like pits were only shown on tangential walls of isolation cells. Pit density of horizontal walls was similar between contact cells and isolation cells, nevertheless greater pits were present on tangential walls, especially for isolation cells. In addition, pits of ray parenchyma cells in the heartwood were smaller and more bordered than those in the sapwood, particularly on the horizontal walls. Moreover, isolation cells had pits with the smaller dimensions, greater pits on the tangential walls, more bordered pits on horizontal walls, as well as longer and narrower cell morphology with much thicker cell walls than contact cells. To a certain extent, all these anatomical adaptations were developed to ensure distinct functions of the two types of ray parenchyma cells in the xylem and finally to support tree growth in demand.

The vessel wall thickness-vessel diameter relationship across woody angiosperms

PREMISE

Comparative anatomy is necessary to identify the extremes of combinations of functionally relevant structural traits, to ensure that physiological data cover xylem anatomical diversity adequately, and thus achieve a global understanding of xylem structure-function relations. A key trait relationship is that between xylem vessel diameter and wall thickness of both the single vessel and the double vessel+adjacent imperforate tracheary element (ITE).

METHODS

We compiled a comparative data set with 1093 samples, 858 species, 350 genera, 86 families, and 33 orders. We used broken linear regression and an algorithm to explore changes in parameter values from linear regressions using subsets of the data set to identify a threshold, at 90-µm vessel diameter, in the wall thickness-diameter relationship.

RESULTS

Below 90 µm diameter for vessels, virtually any wall thickness could be associated with virtually any diameter. Below this threshold, selection is free to favor a very wide array of combinations, such as very thick walls and narrow vessels in ITE-free herbs, or very thin-walled, wide vessels in evergreen dryland pioneers. Above 90 µm, there was a moderate positive relationship.

CONCLUSIONS

Our analysis shows that the space of vessel wall thickness-diameter combinations is very wide, with selection apparently eliminating individuals with vessel walls "too thin" for their diameter. Most importantly, our survey revealed poorly studied plant hydraulic syndromes (functionally significant trait combinations). These data suggest that the full span of trait combinations, and thus the minimal set of hydraulic syndromes requiring study to span woody plant functional diversity adequately, remains to be documented.

Wood density relates negatively to maximum plant height across major angiosperm and gymnosperm orders

PREMISE

Wood density is a crucial plant functional trait related to plant life history strategies. Its ecological importance in small-stature growth forms (e.g., shrubs) has not been extensively examined. Given that hydraulic conduit dimensions vary positively with plant height and that there is a negative relationship between conduits' diameter and wood density, I hypothesized an also negative relationship between wood density and plant height. Knowing that bark and pith proportions are significant in small-diameter stems, I additionally disentangled the contribution of wood, bark, and pith to stem density.

METHODS

I determined density in small-diameter stems across 153 species spanning all major angiosperm and gymnosperm orders by considering a diversity of growth forms (trees, treelets, shrubs, vines, and hemiparasites). Stem cross sections were dissected to consider the densities of wood with bark and pith; wood with pith and without bark; wood with bark and no pith; and wood without bark and pith. Secondary growth was also measured.

RESULTS

Trees showed similar wood densities as non-self-supporting vines, and both showed significantly less dense wood than treelets, shrubs, and hemiparasites. General comparisons showed that wood was significantly denser than all other tissues, and these differences did not depend on growth form. Wood density was significantly and negatively related to growth rate and pith area proportions but not to bark thickness proportion.

CONCLUSIONS

An implicit negative relationship between maximum plant height and stem density emerges as a property of plants likely linked to hydraulic conductive size.

A poplar option: the 'within-individual approach' for elucidating xylem structure-function relationships

This article comments on:

Baer AB, Fickle JC, Medina J, Robles C, Pratt RB, Jacobsen AL. 2021. Xylem biomechanics, water storage, and density within roots and shoots of an angiosperm tree species. Journal of Experimental Botany 72, 7984–7997.

The role of ontogeny in wood diversity and evolution

Evolutionary developmental biology (evo-devo) explores the link between developmental patterning and phenotypic change through evolutionary time. In this review, we highlight the scientific advancements in understanding xylem evolution afforded by the evo-devo approach, opportunities for further engagement, and future research directions for the field. We review evidence that (1) heterochrony-the change in rate and timing of developmental events, (2) homeosis-the ontogenetic replacement of features, (3) heterometry-the change in quantity of a feature, (4) exaptation-the co-opting and repurposing of an ancestral feature, (5) the interplay between developmental and capacity constraints, and (6) novelty-the emergence of a novel feature, have all contributed to generating the diversity of woods. We present opportunities for future research engagement, which combine wood ontogeny within the context of robust phylogenetic hypotheses, and molecular biology.

Why is Tree Drought Mortality so Hard to Predict?

Widespread tree mortality following droughts has emerged as an environmentally and economically devastating 'ecological surprise'. It is well established that tree physiology is important in understanding drought-driven mortality; however, the accuracy of predictions based on physiology alone has been limited. We propose that complicating factors at two levels stymie predictions of drought-driven mortality: (i) organismal-level physiological and site factors that obscure understanding of drought exposure and vulnerability and (ii) community-level ecological interactions, particularly with biotic agents whose effects on tree mortality may reverse expectations based on stress physiology. We conclude with a path forward that emphasizes the need for an integrative approach to stress physiology and biotic agent dynamics when assessing forest risk to drought-driven morality in a changing climate.