Interactions between xylem traits linked to hydraulics during xylem development optimize growth performance in conifer seedlings

Climate change has threatened forests globally, challenging tree species ability to track the rapidly changing environment (e..g., drought and temperature rise). Conifer species face strong environmental filters due to climatic seasonality. Investigating how conifers change their hydraulic architecture during xylem development across the season may shed light on possible mechanisms underlying hydraulic adaptation in conifers. Laser microscopy was used to assess the three-dimensional hydraulic architecture of balsam fir (Abies balsamea (Linnaeus) Miller), jack pine (Pinus banksiana Lambert), white spruce (Picea glauca (Moench) Voss), and black spruce (Picea mariana (Miller) Britton, Sterns & Poggenburgh) seedlings. We measured hydraulic-related xylem traits from early to latewood, during four years of plant growth. The xylem development of jack pine seedlings contrasts with the other species for keeping torus overlap (a hydraulic safety-associated xylem trait), relatively constant across the season (from early to latewood) and the years. The tracheids and torus expansion are positively associated with plant growth. Pit aperture-torus covariance is central to the seasonal dynamics of jack pine xylem development, which jointly with a rapid tracheid and pit expansion seems to boost its growth performance. Linking xylem structural changes during xylem development with hydraulics is a major issue for future research to assess conifers vulnerability to climate change.

Hydraulic architecture of seedlings and adults of Rhizophora mangle L. in fringe and scrub mangrove

Background: Mangrove plant species have distinctive anatomical and physiological responses to cope with a wide range of salinities and inundations. These strategies pertain a safe and efficient water use and transport, essential for survival. Questions: How are the anatomical and physiological attributes of the hydraulic architecture of seedlings and adults of Rhizophora mangle? what are the changes in hydraulic architecture of seedlings and adults of R. mangle in contrasting microenvironments? Studied species: Rhizophora mangle L. (Rhizophoraceae). Study site and dates: Scrub and fringe mangroves in Ria Celestún Biosphere Reserve, during the rainy season of 2013 (July to October). Methods: Hydraulic conductivity and leaf water potential, as well as xylem vessel density, length, transversal and radial diameter, and area were measured for seedlings and adults from both sites. The prevailing environmental conditions (soil water potential, salinity, photon flux density, air temperature and relative humidity) were also characterized. Results: A safer hydraulic conduction system, with narrow and more grouped vessels, was observed in seedlings than in adults of R. mangle in both sites. Adult individuals from the scrub mangrove, in the hyper saline microenvironment, had a safer hydraulic conduction system than adults in the fringe mangrove. Conclusions: The seedling stage of R. mangle showed a safer hydraulic system than adults in both types of mangroves. However, over time this hydraulic conduction system could become more efficient or remain safe depending on the microenvironment in which individuals are growing.

Towards the consideration of soil-root resistances in root water uptake models with macroscopic representations of hydraulic root architecture

<p>Plant water uptake from soil is an important component of terrestrial water cycle with strong links to the carbon cycle and the land surface energy budget. To simulate the relation between soil water content, root distribution, and root water uptake, models should represent the hydraulics of the soil-root system and describe the flow from the soil towards root segments and within the 3D root system architecture according to hydraulic principles. We have recently demonstrated how macroscopic relations that describe the lumped water uptake by all root segments in a certain soil volume, e.g. in a thin horizontal soil layer in which soil water potentials are uniform, can be derived from the hydraulic properties of the 3D root architecture. The flow equations within the root system can be scaled up exactly and the total root water uptake from a soil volume depends on only two macroscopic characteristics of the root system: the root system conductance, K<sub>rs</sub>, and the uptake distribution from the soil when soil water potentials in the soil are uniform, <strong>SUF</strong>. When a simple root hydraulic architecture was assumed, these two characteristics were sufficient to describe root water uptake from profiles with a non-uniform water distribution. This simplification gave accurate results when root characteristics were calculated directly from the root hydraulic architecture. In a next step, we investigate how the resistance to flow in the soil surrounding the root can be considered in a macroscopic root water uptake model. We specifically investigate whether the macroscopic representation of the flow in the root architecture, which predicts an effective xylem water potential at a certain soil depth, can be coupled with a model that describes the transfer from the soil to the root using a simplified representation of the root distribution in a certain soil layer, i.e. assuming a uniform root distribution.</p>

New plant hydraulic architecture reproduces impacts of droughts in the Amazon rainforest

<p>The Amazon rainforest has been hit by extreme drought events in recent decades. Thereby, plant hydraulics are essential to better understand the impacts of droughts on single plants and whole forest ecosystems. Plant hydraulic mechanisms such as stomatal closure and leaf water potential are very complex, still posing challenges for current vegetation model development and parameterization. Here, we present the new hydraulic architecture of the Dynamic Global Vegetation Model LPJ-GUESS, accounting for leaf stomatal responses to plant water status and subsequent drought-induced mortality. We show that when applying the model to the Amazon rainforest we can reproduce the observed increasing trend in carbon losses and the decreasing trend in net carbon sink from plot observations over the past two decades. Our model simulations suggest that the increasing historical trend in carbon losses from mortality can be explained by hydraulic failure and associated mortality.</p><p>The high biodiversity of the Amazon tropical rainforest poses further challenges for process-based models. Here we present an approach to include the diversity of plant responses to drought by simulating 37 individual Plant Functional Types (PFTs) differing in their leaf water potential regulation- and resistance to soil water stress, and provide a simple solution how to cover a wide range of species and species-specific parameters. Future modelling studies should also take species interaction and competition of different hydraulic strategies into account.</p>