How megadrought causes extensive mortality in a deep-rooted shrub species normally resistant to drought-induced dieback: The role of a biotic mortality agent.

Southern California experienced unprecedented megadrought between 2012 and 2018. During this time, Malosma laurina, a chaparral species normally resilient to single-year intense drought, developed extensive mortality exceeding 60% throughout low-elevation coastal populations of the Santa Monica Mountains. We assessed the physiological mechanisms by which the advent of megadrought predisposed M.

Positive root pressure is critical for whole-plant desiccation recovery in two species of terrestrial resurrection ferns.

Desiccation-tolerant (DT) organisms can lose nearly all their water without dying. Desiccation tolerance allows organisms to survive in a nearly completely dehydrated, dormant state. At the cellular level, sugars and proteins stabilize cellular components and protect them from oxidative damage.

Acclimation of mechanical and hydraulic functions in trees: impact of the thigmomorphogenetic process.

The secondary xylem (wood) of trees mediates several functions including water transport and storage, mechanical support and storage of photosynthates. The optimal structures for each of these functions will most likely differ. The complex structure and function of xylem could lead to trade-offs between conductive efficiency, resistance to embolism, and mechanical strength needed to count for mechanical loading due to gravity and wind.

Dynamic control of osmolality and ionic composition of the xylem sap in two mangrove species.

• Premise of the study: Xylem sap osmolality and salinity is a critical unresolved issue in plant function with impacts on transport efficiency, pressure gradients, and living cell turgor pressure, especially for halophytes such as mangrove trees.• Methods: We collected successive xylem vessel sap samples from stems and shoots of Avicennia germinans and Laguncularia racemosa using vacuum and pressure extraction and measured their osmolality. Following a series of extractions with the pressure chamber, we depressurized the shoot and pressurized again after various equilibration periods (minutes to hours) to test for dynamic control of osmolality.

Effects of phyllotaxy on biomechanical properties of stems of Cercis occidentalis (Fabaceae).

Premise Of The Study: Phyllotaxy, the arrangement of leaves on a stem, may impact the mechanical properties of woody stems several years after the leaves have been shed. We explored mechanical properties of a plant with alternate distichous phyllotaxy, with a row of leaves produced on each side of the stem, to determine whether the nodes behave as spring-like joints.

Methods: Flexural stiffness of 1 cm diameter woody stems was measured in four directions with an Instron mechanical testing system; the xylem of the stems was then cut into node (former leaf junction) and nonnode segments for measurement of xylem density.

A global analysis of xylem vessel length in woody plants.

Premise Of The Study: Vessels are the chief conduit for long-distance water transport in the majority of flowering plants. Vessel length is a key trait that determines plant hydraulic efficiency and safety, yet relatively little is known about this xylem feature. •

Methods: We used previously published studies to generate a new global data set of vessel length in woody plants.

Allocation tradeoffs among chaparral shrub seedlings with different life history types (Rhamnaceae).

Premise Of The Study: California chaparral shrub species have different life history types: Nonsprouters (NS) are killed by fire and persist through a fire-stimulated seed bank; facultative sprouters (FS) reestablish by a combination of vegetative sprouting and seeding; and obligate sprouters (OS) reestablish exclusively by sprouting. Nonsprouters and FS establish seedlings in open-canopy postfire environments, whereas OS establish seedlings between fires in the shady understory. We hypothesized that allocation differences among seedlings of postfire sprouters and nonsprouters and regeneration niche differences would lead to contrasting patterns in biomass accumulation (NS > FS > OS, in sun; OS > FS > NS, in shade).

Photosynthetic, hydraulic and biomechanical responses of Juglans californica shoots to wildfire.

Leaf gas exchange and stem xylem hydraulic and mechanical properties were studied for unburned adults and resprouting burned Juglans californica (southern California black walnut) trees 1 year after a fire to explore possible trade-offs between mechanical and hydraulic properties of plants. The CO(2) uptake rates and stomatal conductance were 2-3 times greater for resprouting trees than for unburned adults. Both predawn and midday water potentials were more negative for unburned adult trees, indicating that the stems were experiencing greater water stress than the stems of resprouting trees.

Water stress tolerance of shrubs in Mediterranean-type climate regions: Convergence of fynbos and succulent karoo communities with California shrub communities.

Mediterranean-type climate regions are highly biodiverse and predicted to be particularly sensitive to climate change. Shrubs of the mediterranean-type climate region of South Africa are highly threatened, and their response to water stress has been comparatively little studied. Resistance to water stress induced xylem cavitation (P(50)) and xylem specific hydraulic conductivity (K(s)) were measured in 15 shrub species from fynbos and succulent karoo communities of South Africa.

Comparative community physiology: nonconvergence in water relations among three semi-arid shrub communities.

Plant adaptations to the environment are limited, and therefore plants in similar environments may display similar functional and physiological traits, a pattern termed functional convergence. Evidence was examined for functional convergence among 28 evergreen woody shrubs from three plant communities of the semi-arid winter rainfall region of southern California. Both leaf and water relations traits were examined, including seasonal stomatal conductance (gs), specific leaf area (SLA), leaf specific conductivity (Kl), seasonal water potential (Psi w), stem cavitation resistance (Psi 50), and xylem density.

Cavitation resistance and seasonal hydraulics differ among three arid Californian plant communities.

Vulnerability to water stress-induced cavitation was measured on 27 woody shrub species from three arid plant communities including chaparral, coastal sage and Mojave Desert scrub. Dry season native embolism and pre-dawn water potential, and both wet and dry season xylem specific hydraulic conductivity (Ks) were measured. Cavitation resistance, estimated as water potential at 50% loss in conductivity (Psi50), was measured on all species during the wet season and on a subset of species during the dry season.

Do xylem fibers affect vessel cavitation resistance?

Possible mechanical and hydraulic costs to increased cavitation resistance were examined among six co-occurring species of chaparral shrubs in southern California. We measured cavitation resistance (xylem pressure at 50% loss of hydraulic conductivity), seasonal low pressure potential (P(min)), xylem conductive efficiency (specific conductivity), mechanical strength of stems (modulus of elasticity and modulus of rupture), and xylem density. At the cellular level, we measured vessel and fiber wall thickness and lumen diameter, transverse fiber wall and total lumen area, and estimated vessel implosion resistance using (t/b)(h)(2), where t is the thickness of adjoining vessel walls and b is the vessel lumen diameter.

Mechanical perturbation affects conductivity, mechanical properties and aboveground biomass of hybrid poplars.

Xylem development in trees is affected by dynamic mechanical stresses imposed on stems by wind. To assess clonal differences in response to mechanical perturbation (MP), we subjected seven greenhouse-grown F1 hybrids of Populus trichocarpa Torr. and A.

Hydraulic conductivity and embolism in the mangrove tree Laguncularia racemosa.

We measured xylem pressure potentials, soil osmotic potentials, hydraulic conductivity and percent loss of conductivity (PLC) due to embolism, and made microscopic observations of perfused dye in the white mangrove tree, Laguncularia racemosa (L.) Gaertn. f.

Fluorescence shell: a novel view of sclereid morphology with the Confocal Laser Scanning Microscope.

Confocal Laser Scanning Microscopy (CLSM) was used to observe sclereids from stems of Avicennia germinans and from fruits of two species of pear (Pyrus calleryana "Bradford" and P. communis "Red Bartlett"). The images obtained from thick (25 to 100 microm) free-hand sections were, in certain respects, far superior to those obtained by other, more invasive and time-consuming microscopic techniques upon which previous reports of sclereid morphology were based.

Freeze/thaw stress in Ceanothus of southern California chaparral.

Freeze/thaw stress was examined in chaparral shrubs of the genus Ceanothus to determine the interactive effects of freezing and drought and to consider which is the more vulnerable component, the living leaves (symplast) or the non-living water transport system (apoplast). We hypothesized that where Ceanothus species co-occurred, the more inland species C. crassifolius would be more tolerant of low temperatures than the coastal species C.

Hydraulic, biomechanical, and anatomical interactions of xylem from five species of Acer (Aceraceae).

Possible trade-offs between hydraulic conductivity and mechanical properties of woody stems from five species were assessed. Acer negundo is a ruderal tree, A. saccharinum, and A.

Shoot dieback during prolonged drought in Ceanothus (Rhamnaceae) chaparral of California: a possible case of hydraulic failure.

Progressive diebacks of outer canopy branchlets of Ceanothus crassifolius were repeatedly observed after rainless periods up to 9 mo in duration in the Santa Monica Mountains of southern California. Mean xylem pressures of branchlets near the end of drought were as low as -11.2 MPa (N = 22) with a mean of about 60 dead branchlets per shrub.

Tradeoffs between hydraulic efficiency and mechanical strength in the stems of four co-occurring species of chaparral shrubs.

Possible tradeoffs between efficiency of water transport and mechanical strength were examined in stems of two congeneric pairs of co-occurring chaparral shrubs. First, since previously published results indicated that Adenostoma sparsifolium (Rosaceae) had greater specific conductivity (k or hydraulic conductivity per xylem transverse area) than A. fasciculatum, it was hypothesized that A.

A survey of root pressures in vines of a tropical lowland forest.

Pre-dawn xylem pressures were measured with bubble manometers attached near the stem bases of 32 species of vines on Barro Colorado Island, Panama, to determine if pressures were sufficient to allow for possible refilling of embolized vessels. Of 29 dicotyledonous species 26 exhibited only negative xylem pressures, even pre-dawn during the wet season. In contrast, three members of the Dilleniaceae exhibited positive pre-dawn xylem pressures, with a maximum of 64 kPa in Doliocarpusmajor.

Root-stem junctions of a desert monocotyledon and a dicotyledon: hydraulic consequences under wet conditions and during drought.

Junctions between shoot-borne main roots and stems, which are a crucial part of the water-flow pathway in plants, were investigated for the leaf succulent Agave deserti Engelm. and the sympatric stem succulent Ferocactus acanthodes (Lem.) Britton & Rose under wet conditions and during 21 d of drying in soil.

Why vines have narrow stems: Histological trends in Bauhinia (Fabaceae).

Xylem (wood) tissue in plants functions both for mechanical support and water transport. Since vines are mechanical parasites, they allocate less biomass for xylem tissue than do free-standing trees or shrubs. With-in the genus Bauhinia, stems of vine species were found to have not only less xylem per distal leaf area, but also less phloem and cortical tissue when compared to tree and shrub species.

A survey of vessel dimensions in stems of tropical lianas and other growth forms.

Vessel dimensions (total diameter and length) were determined in tropical and subtropical plants of different growth forms with an emphasis upon lianas (woody vines). The paint infusion and compressed air methods were used on 38 species from 26 genera and 16 families in the most extensive survey of vessel length made to date. Within most stems there was a skewed frequency distribution of vessel lengths and diameter, with many short and narrow vessels and few long and wide ones.

Longevity of needle fascicles of Pinus longaeva (Bristlecone pine) and other North American pines.

Variation in leaf longevity of gymnosperms has received surprisingly little attention despite its likely adaptive significance. Pinus longaeva, a pine of arid, subalpine environments in the western United States, has the record among conifers for needle longevity, with a maximum dwarf shoot (needle fascicle) retention time of up to about 45 years. Most low elevation pines have dwarf shoot retention times (DSRs) of two to four years.