Heavy browsing affects the hydraulic capacity of Ceanothus rigidus (Rhamnaceae)

Defoliation by herbivores can reduce carbon assimilation, change plant water relations, and even shift the biotic structure of plant communities. In this study, we took advantage of a long-term deer exclosure experiment to examine the consequences of persistent deer herbivory on plant water relations and the xylem structure–function relationships in Ceanothus rigidus, a maritime chaparral shrub in coastal California. Browsed plants had thicker stems with many intertwined short distal twigs, and significantly higher sapwood-to-leaf area ratios than their non-browsed counterparts. Leaf area-specific hydraulic conductivity was similar in both browsed and non-browsed plants, but xylem area-specific conductivity was significantly lower in the browsed plants. Vessel diameters were equivalent in both plant groups, but the number of vessels on a transverse area basis was nearly 40 % lower in the browsed plants, accounting for their lower transport efficiency. Mid-day in situ water potentials and losses of hydraulic conductivity due to embolism were similar in both groups of plants but stomatal conductance was higher in the browsed shrubs in the early part of the growing season. We discuss our findings in the context of whole-plant ecophysiology, and explore the consequences of herbivory on hormonal signals, wood anatomy, and xylem function.     

Drought and changes in leaf orientation for two California chaparral shrubs: Ceanothus megacarpus and Ceanothus crassifolius

Summary Predawn xylem pressure potentials were measured on two California chaparral shrubs, Ceanothus megacarpus and Ceanothus crassifolius, throughout the winter and spring growing season and into the summer drought. On the days xylem pressure potentials were measured, leaf orientation measurements were made on a population of marked leaves from the same shrubs. Predawn xylem pressure potentials decreased from -0.1 MPa in both species to -7.8 and -6.6 MPa in C. megacarpus and C. crassifolius, respectively, between May and August, 1981. Leaf inclinations became more vertical during this period with the greatest change observed in C. crassifolius. This change in leaf inclination was reversible, and, in the late winter and early spring, one year old leaves became more horizontal. Leaf azimuths were random and did not change seasonally. Simulations of solar radiation interception indicated that the increase in leaf inclination associated with summer drought reduced the absorption of solar radiation in August by 6% for C. megacarpus and 20% for C. crassifolius. Standard leaf energy budget calculations suggest that steep leaf inclinations would result in slightly lower leaf temperatures and transpiration rates under summer conditions.     

Exudate flavonoids of eight species of Ceanothus (Rhamnaceae)

Leaf glands of Ceanothus species excrete a lipophilic material that contains a variety of flavonoids. Most of these are aglycones, but some glycosides were also observed. Seven out of eight species exhibit flavonols, whereas flavones are excreted by only one species. Four species produce flavanones and dihydroflavonols; one excretes a remarkable quantity of flavonol glycosides. The exudate flavonoids thus form different patterns that might be characteristic for different Ceanothus species.     

Molecular phylogeny of the genus Ceanothus (Rhamnaceae) using rbc L and ndh F sequences

 Intrageneric phylogeny among ten representative Ceanothus species was investigated using DNA sequences of the chloroplast encoded ndhF and rbcL genes. Parsimony analysis of the ndhF sequences identified two main clades corresponding to two subgenera Ceanothus and Cerastes. The phylogenetic results suggest that three monophyletic clades within the subgenus Ceanothus can be delimited on the basis of (1) evergreen or (2) deciduous leaves and (3) thorn presence within the evergreen clade. The estimated divergence time based on rbcL sequences suggests that the two subgenera diverged 18–39 million years ago whereas species within each subgenus diverged more recently. Taken together, the results support the division of Ceanothus into two monophyletic subgenera and are consistent with the postulated recent divergence of many species within each subgenus. Received: 25 September 1996/Accepted: 8 November 1996     

Relationships among xylem transport, biomechanics and storage in stems and roots of nine Rhamnaceae species of the California chaparral

Here, hypotheses about stem and root xylem structure and function were assessed by analyzing xylem in nine chaparral Rhamnaceae species. Traits characterizing xylem transport efficiency and safety, mechanical strength and storage were analyzed using linear regression, principal components analysis and phylogenetic independent contrasts (PICs). Stems showed a strong, positive correlation between xylem mechanical strength (xylem density and modulus of rupture) and xylem transport safety (resistance to cavitation and estimated vessel implosion resistance), and this was supported by PICs. Like stems, greater root cavitation resistance was correlated with greater vessel implosion resistance; however, unlike stems, root cavitation resistance was not correlated with xylem density and modulus of rupture. Also different from stems, roots displayed a trade-off between xylem transport safety from cavitation and xylem transport efficiency. Both stems and roots showed a trade-off between xylem transport safety and xylem storage of water and nutrients, respectively. Stems and roots differ in xylem structural and functional relationships, associated with differences in their local environment (air vs soil) and their primary functions.     

Diversification of the North American shrub genus Ceanothus (Rhamnaceae): conflicting phylogenies from nuclear ribosomal DNA and chloroplast DNA

Ceanothus comprises ～55 morphologically and ecologically diverse species of woody perennials endemic to North America. Interpretations of the natural history of Ceanothus have served as a general model of evolution for woody perennials with simple entomophilous pollination systems, but these interpretations lacked explicit phylogenetic context. We used cladistic analysis of sequences of the chloroplast-encoded matK and the internal transcribed spacers (ITS) and 5.8S coding region of nuclear ribosomal DNA (nrDNA) to reconstruct the phylogeny of Ceanothus. The nuclear and organellar phylogenies exhibited very low levels of both topological and character congruence. Subgenera Ceanothus and Cerastes are monophyletic sister taxa in both phylogenies, but both data sets suffer from a lack of resolution below the level of subgenus. Lack of taxonomic congruence between the two data sets may be a result of introgression and/or lineage sorting. The ITS tree was accepted as the better estimate of a species phylogeny for Ceanothus, on the assumption that nuclear markers are less prone to introgression. Three of five polytypic species in the ITS data set were paraphyletic, and four of six polytypic species in the matK data set were paraphyletic. This study demonstrates the degree to which matched independent data sets can produce conflicting summaries of evolutionary history.     

Low genetic diversity among Frankia spp. strains nodulating sympatric populations of actinorhizal species of Rosaceae, Ceanothus (Rhamnaceae) and Datisca glomerata (Datiscaceae) west of the Sierra Nevada (California)

Frankia spp. strains typically induce N2-fixing root nodules on actinorhizal plants. The majority of host plant taxa associated with the uncultured Group 1 Frankia strains, i.e., Ceanothus of the Rhamnaceae, Datisca glomerata (Datiscaceae), and all actinorhizal members of the Rosaceae except Dryas, are found in California. A study was conducted to determine the distribution of Frankia strains among root nodules collected from both sympatric and solitary stands of hosts. Three DNA regions were examined, the 5' end of the 16S rRNA gene, the internal transcribed spacer region between the 16S and 23S rRNA genes, and a portion of the glutamine synthetase gene (glnA). The results suggest that a narrow range of Group 1 Frankia spp. strains dominate in root nodules collected over a large area of California west of the Sierra Nevada crest with no apparent host-specificity. Comparisons with Group 2 Frankia strain diversity from Alnus and Myrica within the study range suggest that the observed low diversity is peculiar to Group 1 Frankia strains only. Factors that may account for the observed lack of genetic variability and host specificity include strain dominance over a large geographical area, current environmental selection, and (or) a past evolutionary bottleneck.     

Distribution of plant life history types in California chaparral: the role of topographically‐determined drought severity

Abstract. Spatial patterns of shrub life history and Ceanothus distribution are examined in relation to topographically‐mediated differences in drought severity within 3 watersheds on the coastal and inland flank of the Santa Ynez Mountains, California. Spatially distributed fields of drought severity are simulated for the studied watersheds using high‐resolution digital terrain data and daily climate data in combination with a process‐based hydro‐ecological model (RHESSys). Field samples of species composition are spatially integrated with the distributed drought data for analysis of ecological relationships. Patterns of seedling recruitment type correspond to topographic variability in drought severity in ways that are consistent with concepts presented in the literature. Species that depend on fire for recruitment are increasingly represented with increasing drought severity, the converse also applies. Sites that experience moderate drought severity permit co‐dominance of species from both recruitment modes. Residual analysis suggests that some of the unexplained variability is related to substrate. Analyses also indicate that the distribution of 5 Ceanothus shrubs reflect differences in drought severity in ways that are consistent with their resistance to water stress‐induced xylem dysfunction. Species from the subgenus Cerastes sort in accordance with moisture availability and have unique spatial distributions. Results are evaluated and discussed with respect to studies on plant morphology, resource use and seedling establishment patterns.     

Mortality of resprouting chaparral shrubs after a fire and during a record drought: physiological mechanisms and demographic consequences

We examined postfire regeneration of chaparral shrubs during an intense drought. This study focused on the demography and physiology of shrub species that resprout from a basal lignotuber following fire. We found significant levels of resprout mortality when intense drought occurred in the year following fire during the period of shrub recovery. Three of the seven sampled resprouting species had the greatest or near greatest levels of mortality ever recorded when compared to previous studies. Most shrub mortality occurred during the drought after individuals had resprouted (i.e. individuals survived fire, resprouted and then subsequently died). Physiological measurements of species with high mortality suggested that resprout stems were highly embolized and xylem hydraulic conductivities were close to zero during the peak of the drought. In addition, lignotubers of two of the three species experiencing high mortality were depleted of starch. Population densities of most shrub species declined after the drought compared with their prefire levels, with the exception of one drought tolerant obligate seeding species. Resprouting shrub species may deplete their carbohydrate reserves during the resprouting process, making them particularly vulnerable to drought because of the need to transpire water to acquire the CO₂ that is used to supply energy to a large respiring root system. Drought appears to interact with fire by altering postfire shrub recovery and altering species abundances and composition of chaparral communities.     

Resilience of fishes and invertebrates to prolonged drought in two California streams

Long-term data are needed to assess spatial and temporal variability of communities and their resilience to natural and anthropogenic disturbances, particularly in climatic regions marked by high interannual variability (e.g. mediterranean-climate). A long-term study at four sites on two streams in mediterranean-climate California (annual sampling over 20  yr) allowed us to quantify the influence of a 5-yr prolonged drought on stream invertebrate and fish communities. Invertebrate community composition did not show recovery following drought. The primary environmental factors influencing community composition, identified through principle components and multiple correspondence analyses were precipitation and flow permanence. Invertebrate taxon richness and abundance exhibited few responses (some site specific) and recovered quickly. Native fish abundance was lowest during the drought period and highest during the wet years that occurred at the end of the study period. Importantly, the prolonged drought facilitated the establishment and success of the invasive green sunfish (Centrarchidae: Lepomis cyanellus ) that was then resilient to subsequent large flow events. There was high spatial synchrony in the temporal changes among all four sites, and three distinct periods were identified: early drought, late drought, and post-drought years. However, we still found differences among sites along the flow permanence gradient from temporary to perennial in the response to drought of both invertebrate and fish assemblages likely as a result of changes in substrate, vegetation, and other habitat characteristics. The observed lack of resilience and negative impacts to biodiversity due to prolonged drought associated with long-term habitat changes is important because hydroclimatic extremes are predicted to increase in frequency and magnitude with global climate change.     

Canopy recovery after drought dieback in holm-oak Mediterranean forests of Catalonia (NE Spain)

Climate change is likely to produce more frequent and longer droughts in the Mediterranean region, like that of 1994, which produced important changes in the Quercus ilex forests, with up to 76% of the trees showing complete canopy dieback. At the landscape level, a mosaic of responses to the drought was observed, linked to the distribution of lithological substrates. Damage to the dominant tree species (Q. ilex) and the most common understorey shrub (Erica arborea) was more noticeable on the compact substrates (breccia) than on the fissured ones (schist). This result was consistent with observations documenting deeper root penetration in schist than in breccia materials, allowing the plants growing on fissured substrates to use water from deeper soil levels. Smaller plants were more vulnerable to drought than larger plants in the trees, but not in the shrubs. Overall, Q. ilex was more affected than E. arborea. The resilience of the system was evaluated from the canopy recovery 1 year after the episode. Stump and crown resprouting was fairly extensive, but the damage pattern in relation to substrate, plant size, and species remained similar. The effect of recurrent drought episodes was studied on vegetation patches of Q. ilex located on mountain slopes and surrounded by bare rock. We observed that plants that resprouted weakly after a previous drought in 1985 were more likely to die or to produce poor regeneration in 1995 than plants that had resprouted vigorously. Vegetation patches located on the lower part of the slope were also less damaged than patches situated uphill. The study provides evidence of relevant changes in forest canopy as a consequence of extreme climate events. The distribution of this effect across the landscape is mediated by lithological substrate, causing patchy patterns. The results also support the hypothesis that recurrent droughts can produce a progressive loss of resilience, by depleting the ability of surviving plants to regenerate.     

Impact of chaparral wildfire-induced sedimentation on oviposition of stream-breeding California newts (Taricha torosa)

We examined the effects of chaparral wildfire on stream-breeding California newts (Taricha torosa) in a 750-m stretch of a perennial Santa Monica Mountain stream (Los Angeles County). Detailed field surveys of 1992 and 1993 established the composition (run, riffle, pool) of this habitat and determined the oviposition sites of newts. We also quantified California newt egg mass density and estimated the density of newt adults. A chaparral wildfire burned the entire study site on 2 November 1993. Using the same methods, we collected field survey data in 1994 and 1996. Erosion following the 1993 wildfire produced major changes in stream morphology and composition. Pools and runs represented approximately 40–50% of pre-fire stream area. In the spring following the fire, the stream consisted of less than 20% run and pool. Pools that did remain were often smaller and shallower. The average density of adult California newts did not differ among years. The total number of newt egg masses observed in the spring after the fire was approximately one-third of egg mass counts from pre-fire surveys. Most California newt egg masses were laid in pools and runs; California newts prefer deeper slow-moving water. We conclude that fire-induced landslides and siltation have eliminated pools and runs, thus reducing the amount of habitat suitable for oviposition. Habitat alterations caused by fire likely account for the observed reduction of egg masses at the stream. Received: 11 June 1996 / Accepted: 18 December 1996     

Hydraulic failure and tree dieback are associated with high wood density in a temperate forest under extreme drought

Catastrophic hydraulic failure will likely be an important mechanism contributing to large‐scale tree dieback caused by increased frequency and intensity of droughts under global climate change. To compare the susceptibility of 22 temperate deciduous tree and shrub species to hydraulic failure during a record drought in the southeastern USA, we quantified leaf desiccation, native embolism, wood density, stomatal conductance and predawn and midday leaf water potential at four sites with varying drought intensities. At the two driest sites, there was widespread leaf wilting and desiccation, and most species exhibited predawn leaf water potentials of ≤3 MPa and >60% loss of xylem conductivity in branches. Although species with high wood density were more resistant to cavitation, they had higher levels of native embolism and greater canopy dieback than species with low wood density. This unexpected result can be explained by the failure of species with dense wood to avert a decline in water potential to dangerous levels during the drought. Leaf water potential was negatively correlated with wood density, and the relationship was strongest under conditions of severe water deficit. Species with low wood density avoided catastrophic embolism by relying on an avoidance strategy that involves partial drought deciduousness, higher sensitivity of stomata to leaf water potential and perhaps greater rooting depth. These species therefore maintained water potential at levels that ensured a greater margin of safety against embolism. These differences among species may mediate rapid shifts in species composition of temperate forests if droughts intensify due to climate change.     

A comparative study of oak (Quercus, Fagaceae) seedling physiology during summer drought in southern California

Natural recruitment of oaks appears to be declining throughout the northern hemisphere. Summer drought poses a potentially important barrier to oak recruitment in southern California. To evaluate this barrier, we grew evergreen Quercus agrifolia and deciduous Q. lobata from seeds near parental trees. We measured water relations, chlorophyll fluorescence, and gas exchange during these seedlings' fourth and fifth summers and compared them to neighboring adults. Most seedlings had substantially lower values for predawn xylem pressure potential (Ψ(pd)), minimum photosystem II (PSII) quantum efficiency (Φ(PSIIMIN)), maximum quantum efficiency for PSII under dark-adapted leaf conditions (Fv/Fm), and maximum photosynthetic assimilation (Amax), and higher values for maximum nonphotochemical quenching (NPQmax) than did conspecific adults. The high, unvarying Ψ(pd) values of the adults suggest they use perennially available groundwater. Quercus lobata seedlings commonly had lower values for Ψ(pd) than did Q. agrifolia, and values for Ψ(pd) and Φ(PSIIMIN) were significantly related to size in Q. lobata but not in Q. agrifolia. These data suggest important interspecific differences in root architecture. Lower values for Φ(PSIIMIN), Fv/Fm, and higher NPQmax in Q. agrifolia indicate that Q. agrifolia seedlings were usually under more stress than Q. lobata, which typically had higher Amax rates than did Q. agrifolia seedlings. Diurnal photosynthesis curves were quite flat for Q. agrifolia, but they peaked in the morning for Q. lobata. Established seedlings appeared to be under more stress than adults, but this stress did not appear severe enough to cause death. Access to perennially available groundwater may be crucial for the seedling to sapling transition.     

Establishment failure in biological invasions: a case history of Littorina littorea in California, USA

Background

The early stages of biological invasions are rarely observed, but can provide significant insight into the invasion process as well as the influence vectors have on invasion success or failure.

Methodology/Principal Findings

We characterized three newly discovered populations of an introduced gastropod, Littorina littorea (Linné, 1758), in California, USA, comparing them to potential source populations in native Europe and the North American East Coast, where the snail is also introduced. Demographic surveys were used to assess spatial distribution and sizes of the snail in San Francisco and Anaheim Bays, California. Mitochondrial DNA was sequenced and compared among these nascent populations, and various populations from the North American East Coast and Europe, to characterize the California populations and ascertain their likely source. Demographic and genetic data were considered together to deduce likely vectors for the California populations. We found that the three large California L. littorea populations contained only adult snails and had unexpectedly high genetic diversity rather than showing an extreme bottleneck as typically expected in recent introductions. Haplotype diversity in Californian populations was significantly reduced compared to European populations, but not compared to East Coast populations. Genetic analyses clearly suggested the East Coast as the source region for the California introductions.

Conclusions and Significance

The California L. littorea populations were at an early, non-established phase of invasion with no evidence of recruitment. The live seafood trade is the most likely invasion vector for these populations, as it preferentially transports large numbers of adult L. littorea, matching the demographic structure of the introduced California L. littorea populations. Our results highlight continued operation of live seafood trade vectors and the influence of vectors on the demographic and genetic structure of the resulting populations, especially early stages of the invasion process.     

Water uptake and structural plasticity along roots of a desert succulent during prolonged drought

Desert succulents resume substantial water uptake within 1–2 d of the cessation of drought, but the changes in root structure and hydraulic conductivity underlying such recovery are largely unknown. In the monocotyledonous leaf succulent Agave deserti Engelm. substantial root mortality occurred only for lateral roots near the soil surface; nearly all main roots were alive at 180 d of drought. New main roots were initiated and grew up to 320 mm at soil water potentials lower than – 5·0 MPa, utilizing water from the shoot. The hydraulic conductivity of distal root regions decreased 62% by 45 d of drought and 70% thereafter. After 7 d of rewetting, root hydraulic conductivity was restored following 45 d of drought but not after 90 and 180 d. The production of new lateral roots and the renewed apical elongation of main roots occurred 7–11 d after rewetting following 180 d of drought. Hydraulic conductivity was higher in the distal region than at midroot and often increased again near the root base, where many endodermal cells lacked suberin lamellae. Suberization and xylem maturation were influenced by the availability of moisture, suggesting that developmental plasticity along a root allows A. deserti to capitalize on intermittent or heterogeneous supplies of water.     

Effect of drought and prolonged refrigeration on senescence in cut carnation (Dianthus caryophyllus)

Severe water stress (40 and 50 h without water at 23°C) and long periods of refrigeration (4 and 5 weeks at 0°C) caused the peak of ethylene production by cut carnation flowers to appear soon after the return to normal conditions. Water stress caused a decrease in ψosm, but this increased back to the initial value on return to normal conditions. Accelerated wilting and massive ion leakage, probably a result of the Joss of membrane integrity, was associated with this premature burst of ethylene. Large amounts of acetaldehyde and ethanol accumulated during prolonged refrigeration (3, 4 or 5 weeks at 0°C). This accumulation of toxic metabolites may explain why the refrigeration of cut flowers for long periods causes a rapid wilting on return to normal conditions.     

Plant reproduction in the high-Andean Puna: Kentrothamnus weddellianus (Rhamnaceae: Colletieae)

The global picture of plant reproduction at high altitudes is still diffuse due to conflicting reports (e.g., about which are the prevalent breeding systems) and incomplete geographical and taxonomic coverage of high-altitude ecosystems. This paper reports on the reproductive biology of Kentrothamnus wedellianus, a shrub inhabiting the Puna semidesert in Argentina and Bolivia at ca. 3,600 m a.s.l. A set of four traits, including a high pollen/low nectar floral reward strategy, homogamy, a dry stigma, and partial self-compatibility, appear to be central for K. weddellianus to accomplish sexual reproduction in the high-altitude Puna. The existence of an entirely different set of characteristics in the related species Ochetophila nana suggests that adaptation to reproduction at high altitudes can be achieved through different pathways. Hence, the final results may strongly diverge from the patterns found in lowland species of each lineage, or not. More case studies still seem to be necessary before specific patterns can be discerned within the panorama of high-altitude plant reproduction.