Do tall tree species have higher relative stiffness than shorter species?

Associations between shade tolerance and wood specific gravity for conifers in contrast to angiosperm trees: Foundations of the conifer fitness-enhancing shade tolerance hypothesis

For decades, researchers have held that wood specific gravity was an indicator or surrogate for both shade tolerance and successional status. However, recent research in dry tropical forests has shown very different associations regarding wood specific gravity. Past analyses of the tolerance and wood properties of tree species have focused on pooled coniferous and angiosperm species in temperate regions; fewer analyses have been conducted separately for conifers and angiosperm species. A database was compiled for the wood properties and/or tolerance scores of 542 temperate Northern Hemisphere conifer and angiosperm trees. Plant strategy was defined by shade tolerance (T shade), drought tolerance (T drought), and polytolerance (T poly = T shade + T drought) and fundamental wood properties were represented by basic specific gravity (SGbasic), relative stiffness (MOE/SGbasic), and relative strength (MOR/SGbasic). Simple linear regressions tested the significance (p < .05) of correlations between plant strategy and wood properties. Conifers, unlike angiosperm trees, showed a negative correlation between T shade and SGbasic and a positive correlation between T shade and both MOE/SGbasic and MOR/SGbasic. Only angiosperm trees had a significant correlation between T poly and both SGbasic and MOE/SGbasic, but both conifers and angiosperm trees had a significant correlation between T drought and both SGbasic and MOE/SGbasic. Shade tolerance, as a plant strategy, has functional implications for wood properties in temperate Northern Hemisphere conifers but not in associated angiosperms. The implied functional link between wood properties (SGbasic) and shade tolerance hypothetically extends to other fitness-enhancing traits impacted by SGbasic, such as growth rates and species maximum height.

Parenchyma fractions drive the storage capacity of nonstructural carbohydrates across a broad range of tree species

PREMISE

Nonstructural carbohydrates (NSCs) play a key role in tree performance and functioning and are stored in radial and axial parenchyma (RAP) cells. Whether this relationship is altered among species and climates or is linked to functional traits describing xylem structure (wood density) and tree stature is not known.

METHODS

In a systematic review, we collated data for NSC content and the proportion of RAP in stems for 68 tree species. To examine the relationships of NSCs and RAP with climatic factors and other functional traits, we also collected climatic data at each tree's location, as well as wood density and maximum height. A phylogenetic tree was constructed to examine the influence of species' evolutionary relationships on the associations among NSCs, RAP, and functional traits.

RESULTS

Across all 68 tree species, NSCs were positively correlated with RAP and mean annual temperature, but relationships were only weakly significant in temperate species and angiosperms. When separating RAP into radial parenchyma (RP) and axial parenchyma (AP), both NSCs and wood density were positively correlated with RP but not with AP. Wood in taller trees was less dense and had lower RAP than in shorter trees, but height was not related to NSCs.

CONCLUSIONS

In trees, NSCs are stored mostly in the RP fraction, which has a larger surface area in warmer climates. Additionally, NSCs were only weakly linked to wood density and tree height. Our analysis of evolutionary relationships demonstrated that RAP fractions and NSC content were always closely related across all 68 tree species, suggesting that RAP can act as a reliable proxy for potential NSC storage capacity in tree stems.

Resource availability and disturbance shape maximum tree height across the Amazon

Tall trees are key drivers of ecosystem processes in tropical forest, but the controls on the distribution of the very tallest trees remain poorly understood. The recent discovery of grove of giant trees over 80 meters tall in the Amazon forest requires a reevaluation of current thinking. We used high-resolution airborne laser surveys to measure canopy height across 282,750 ha of old-growth and second-growth forests randomly sampling the entire Brazilian Amazon. We investigated how resources and disturbances shape the maximum height distribution across the Brazilian Amazon through the relations between the occurrence of giant trees and environmental factors. Common drivers of height development are fundamentally different from those influencing the occurrence of giant trees. We found that changes in wind and light availability drive giant tree distribution as much as precipitation and temperature, together shaping the forest structure of the Brazilian Amazon. The location of giant trees should be carefully considered by policymakers when identifying important hot spots for the conservation of biodiversity in the Amazon.

Biomechanical properties of marsh vegetation in space and time: effects of salinity, inundation and seasonality

BACKGROUND AND AIMS

Over the last decade, the importance of plant biomechanical properties in shaping wave dissipation efficiency of marsh vegetation has gained growing attention. Here we provide the first analyses of how biomechanical stem properties vary with seasons and along environmental gradients in coastal and estuarine marshes, which is essential to enable accurate assessments of flood defence value of marsh vegetation.

METHODS

We quantified both spatial and seasonal variation in stem flexibility and breakability for a variety of common marsh vegetation (Spartina anglica, Scirpus maritimus, Phragmites australis, Elymus athericus, Suaeda maritima, Aster tripolium, Saliconia procumbens) distributed along both salinity and inundation gradients.

KEY RESULTS

Increasing salinity tends to induce a shift from species with tall shoots, high flexural stiffness (stem resistance to bending; N mm2) towards species with shorter and more flexible stems. The same trend was found with increasing inundation stress (i.e. decreasing elevation) from the higher part of the low marsh towards the pioneer zone. Stem breakability (the force required to break or fold a stem, N) followed the same pattern of stem stiffness due to the positive relationship between flexural strength (material resistance to flexure, N mm-2) and Young's bending modulus (material resistance to bending; N mm-2). Shifts in stem stiffness and breakability at the community level were found to relate positively to the variation in canopy height between species, highlighting the concurrence of changes in morphological and biomechanical traits under environmental changes. Compared to the differences between species, within-species variability between sampling locations and between seasons is generally minor.

CONCLUSIONS

Our findings imply that environmental changes may significantly modify wave attenuation capacity of coastal vegetation by inducing species shifts. This emphasizes the need to understand the response of community composition to climate change and human disturbances, when using nature-based flood protection by coastal vegetation as an adaptive response to global change.