Cell size and wall dimensions drive distinct variability of earlywood and latewood density in Northern Hemisphere conifers

Abstract

Interannual variability of wood density - an important plant functional trait and environmental proxy - in conifers is poorly understood. We therefore explored the anatomical basis of density. We hypothesized that earlywood density is determined by tracheid size and latewood density by wall dimensions, reflecting their different functional tasks. To determine general patterns of variability, density parameters from 27 species and 349 sites across the Northern Hemisphere were correlated to tree-ring width parameters and local climate. We performed the same analyses with density and width derived from anatomical data comprising two species and eight sites. The contributions of tracheid size and wall dimensions to density were disentangled with sensitivity analyses. Notably, correlations between density and width shifted from negative to positive moving from earlywood to latewood. Temperature responses of density varied intraseasonally in strength and sign. The sensitivity analyses revealed tracheid size as the main determinant of earlywood density, while wall dimensions become more influential for latewood density. Our novel approach of integrating detailed anatomical data with large-scale tree-ring data allowed us to contribute to an improved understanding of interannual variations of conifer growth and to illustrate how conifers balance investments in the competing xylem functions of hydraulics and mechanical support.

Scientific Merits and Analytical Challenges of Tree‐Ring Densitometry

“…In temperature‐limited environments, the density of the latewood and thus the MXD parameter are tightly correlated with growing season air temperature. While high temperatures result in both larger cells and more deposited cell wall material, the MXD exhibits a net increase (Björklund et al, ). The cell dimensions that drive earlywood density variation are also controlled by growing season temperature, but because variations in deposited cell wall material is secondary to earlywood cell enlargement variations, high temperatures mainly mean larger cells and thus lower density (Björklund et al, ).…”

“…While high temperatures result in both larger cells and more deposited cell wall material, the MXD exhibits a net increase (Björklund et al, ). The cell dimensions that drive earlywood density variation are also controlled by growing season temperature, but because variations in deposited cell wall material is secondary to earlywood cell enlargement variations, high temperatures mainly mean larger cells and thus lower density (Björklund et al, ). Likewise, in drought prone environments, dry years appear to have a negative effect on earlywood cell enlargement and to yield high density (Camarero et al, ; Camarero et al, ).…”

“…The strong positive association between year‐to‐year MXD and temperature variation often include the warmer half of the year, a substantially longer period than for ring width (e.g., Anchukaitis et al, ; Björklund et al, ; Briffa et al, , ; Frank & Esper, ; Wilson et al, ). The latitudinal and altitudinal temperature response space is greater for MXD than for ring width, where MXD displays a significant positive response in many areas where ring widths do not (Björklund et al, ). In addition to this, MXD measurements are less susceptible than ring width to some types of non‐climatic noise, such as large‐scale disturbances (Rydval et al, ).…”

The climatic drivers of normalized difference vegetation index and tree‐ring‐based estimates of forest productivity are spatially coherent but temporally decoupled in Northern Hemispheric forests

“…Our results extend what was previously shown by for semi-arid ecosystems and by Kaufmann et al (2004) for middle and high latitudes, namely that the relationship between NDVI and TRW varies over the growing season and is strongest in mid-summer. Compared with TRW, the MXD parameter from high-latitude and high-elevation sites contains a stronger temperature signal that integrates a longer warm season, extending to the peripheral ends of the growing season (Björklund et al, 2017). Compared with TRW, the MXD parameter from high-latitude and high-elevation sites contains a stronger temperature signal that integrates a longer warm season, extending to the peripheral ends of the growing season (Björklund et al, 2017).…”

“…However, in spite of the recent research advances in understanding the development cycle of woody plants (e.g., Björklund et al, 2017;Rathgeber, Cuny, & Fonti, 2016), a comprehensive picture of links amongst leaf phenology, cambial phenology, canopy growth and xylem formation has yet to emerge (Čufar, Luis, Saz, Črepinšek, & Kajfež-Bogataj, 2012;Sass-Klaassen, Sabajo, & Ouden, 2011). Leaves are the primary plant organs that interact with the environment, whereas wood is the primary long-term biological reservoir for C. These two growth components are intimately linked, enabling plants to optimize the uptake, assimilation and allocation of resources.…”