Hydraulic plasticity and limitations of alpine Rhododendron species

Abstract

In the European Alps, Rhododendron ferrugineum grows in silicate regions while Rhododendron hirsutum is restricted to limestone areas. At geologically mixed sites, also hybrids (Rhododendron × intermedium) can occur. We hypothesised that hydraulic properties would vary with the species' habitat requirements. Key hydraulic parameters (vulnerability to drought-induced embolism, stomata regulation) and related wood characteristics as well as diurnal courses of water potential (Ψ) and stomatal conductance were analysed on plants growing on a silicate, a limestone and a geologically mixed site. Highest embolism resistance[Ψ at 50% loss of conductivity (Ψ (50)), -3.24 ± 0.18 MPa] and the highest safety margin between the Ψ at stomata closure (Ψ (SC) at 10% of maximal leaf conductance) and Ψ (50) were observed in R. hirsutum at the limestone site (1.57 MPa). Like in R. ferrugineum, hydraulic parameters indicated less resistance at the geologically mixed site. Highest Ψ (50) (-1.95 ± 0.12 MPa), corresponding to wide conduits and a reduced conduit wall reinforcement, was found in R. × intermedium. Diurnal courses indicated a rapid stomata closure in response to low Ψ in R. hirsutum and R. × intermedium. The plasticity in drought adaptation of R. hirsutum corresponds to its ability to colonise dry limestone areas. In contrast, hydraulic limitations of R. × intermedium may explain restrictions to rather moist sites. This study provides insight into the role of xylem hydraulics and stomata regulation in shrub water relations, interspecific and site-specific differences in drought adaptation, as well as effects of hybridisation on plant hydraulics.

Dwarf shrub hydraulics: two Vaccinium species (Vaccinium myrtillus, Vaccinium vitis‐idaea) of the European Alps compared

“…Low k s cause steep Ψ gradients in plants, but due to short transport distances in dwarf shrubs, this effect may be of minor relevance. As d and d h (Table ) were similar to other alpine shrubs with higher k s ( J. communis , Rhododendron ferrugineum and Rhododendron hirsutum ; Beikircher and Mayr , Mayr et al ), low conductivities in Vaccinium may not only be the result of small conduits but of high pit resistance and low proportion of conducting conduits in the xylem (Tyree and Zimmermann ) or non‐functional conduits in the older growth rings.…”

“…In comparison, Ψ PLC50 of coniferous trees and shrubs of the timberline ecotone is between −3.39 ( Larix decidua ) and −3.98 MPa ( Picea abies ; Mayr et al , ). However, vulnerability thresholds of V. myrtillus and V. vitis‐idaea were similar to alpine Rhododendron shrubs (−1.95 to −3.24 MPa; Mayr et al ). Rhododendron ferrugineum and R. hirsutum reduce the risk of xylem embolism via stomata closure 0.89–1.57 MPa above Ψ PLC50 (Mayr et al ).…”

“…Zimmermann , Tyree and Ewers , Martinez‐Vilalta et al , McCulloh and Woodruff ), and comparably few studies dealt with the hydraulics of shrubs (e.g. Wheeler et al , Beikircher and Mayr , , Bucci et al , Mayr et al , Vilagrosa et al ). In the database of Choat et al (), xylem functional traits for 331 tree species and 140 shrubs are listed, and most shrubs represented were from arid environments.…”

Allometric equations for biomass assessment of subalpine dwarf shrubs

“…Only one abundant species is absent from our data set, which is R. hirsutum L., an abundant species co-existing with R. ferrugineum only in the central and the eastern Alps. The physiognomy of R. hirsutum and R. ferrugineum is very close, but differs in terms of chemical-related soil preferences (Larcher and Wagner 2004) and drought-related physiological plasticity (Mayr et al 2010). But, we assume that our equation could also be used to estimate the biomass of R. hirsutum, because of the great similitude between the two rhododendron species.…”

Phenotypic and developmental plasticity of xylem in hybrid poplar saplings subjected to experimental drought, nitrogen fertilization, and shading

“…While xylem anatomy can provide a good proxy of xylem function in some cases, solid knowledge of how specific growing conditions influence xylem anatomy and how patterns in xylem structure link with xylem function is an essential prerequisite for such approaches. Changes in K S and P50 in response to environmental conditions such as drought (Beikircher and Mayr, 2009; Fichot et al ., 2010), irradiance (Cochard et al ., 1999), salinity (Stiller, 2009), nutrient availability (Harvey and van den Driessche, 1997; Hacke et al ., 2010), and soil type (Hacke et al ., 2000; Mayr et al ., 2010) have been shown. However, a broader range of species and environmental conditions should be tested and links to genetic underpinnings should be elucidated (Lamy et al ., 2011; Wortemann et al ., 2011) in order to gain a better understanding of the acclimation potential of xylem.…”