Plant diversity in mediterranean-climate regions.

The high plant diversity of mediterranean-climate regions has attracted much attention over the past few years. This review discusses patterns and determinants of local, differential and regional plant diversity in all five regions. Local diversity shows great variation within and between regions and explanations for these patterns invoke a wide range of hypotheses. Patterns of regional diversity are the result of differential speciation and extinction rates during the Quaternary. These rates have been influenced more by the incidence of fire and the severity of climate change than by environmental heterogeneity. All regions have a high number of rare and locally endemic taxa that survive as small populations, many of which are threatened by habitat transformation.

Winter growth phenology and leaf orientation in Pachypodium namaquanum (Apocynaceae) in the succulent karoo of the Richtersveld, South Africa.

Pachypodium namaquanum (Nyley ex Harb.) Welw., an unusual arborescent stem succulent from the succulent karoo of the arid Richtersveld in north-western South Africa and adjacent Namibia, is characterized by a striking curvature of the terminal 20-60 cm of the trunk toward the north. This orientation displays the single terminal whorl of drought-deciduous leaves with their flat surface angled at a mean inclination of 55° from horizontal. Inclination of 50-60° was found in 65% of individuals sampled, and 85% were inclined between 45 and 65°. Northward azimuth was also quite regular, but varied slightly between populations. The fixed leaf orientation in P. namaquanum maximizes radiation absorption during the winter months when leaves are present. Leaves normally form in early fall (April) and abscise early in spring (October). Growing season conditions in the Richtersveld are relatively mild, with mean maximum temperature dropping only to 21.6°C in July, the coldest month of the year. Frosts are rare. By the fixed orientation of its leaf whorl, P. namaquanum is able to maintain nearly twice the midwinter radiation absorptance that it would have with horizontal orientation. Over an annual cycle the angled leaves receive more radiation than would horizontal leaves for each of the 6 months in which they are present on the plant. This increased winter irradiance is hypothesized to singificantly increase net primary production by concentrating growth activities in winter months and allowing the species to remain dormant during the hyperarid conditions of the hot summer months. Midwinter flowering from apical buds in P. namaquanum may also be aided by its stem orientation. The evolution of this characteristic pattern of winter growth phenology and nodding stem orientation may have come about because of low but relatively regular autumn precipitation and moderate winter temperatures. Slow and regular growth of P. namaquanum leads to long lifespans which may reach 300 years or more.

Carbon isotope composition in relation to leaf gas exchange and environmental conditions in Hawaiian Metrosideros polymorpha populations.

Carbon isotope composition, photosynthetic gas exchange, and nitrogen content were measured in leaves of three varieties of Metrosideros polymorpha growing in sites presenting a variety of precipitation, temperature and edaphic regimes. The eight populations studied could be divided into two groups on the basis of their mean foliar δ13C values, one group consisting of three populations with mean δ13C values ca.-26‰ and another group with δ13C values ca.-28‰. Less negative δ13C values appeared to be associated with reduced physiological availability of soil moisture resulting from hypoxic conditions at a poorly drained high elevation bog site and from low precipitation at a welldrained, low elevation leeward site. Gas exchange measurements indicated that foliar δ13C and intrinsic wateruse efficiency were positively correlated. Maximum photosynthetic rates were nearly constant while maximum stomatal conductance varied substantially in individuals with foliar δ13C ranging from-29 to-24‰. In contrast with the patterns of δ13C observed, leaf nitrogen content appeared to be genetically determined and independent of site characteristics. Photosynthetic nitrogenuse efficiency was nearly constant over the range of δ13C observed, suggesting that a compromise between intrinsic water- and N-use efficiency did not occur. In one population variations in foliar δ13C and gas exchange with leaf cohort age, caused the ratio of intercellular to atmospheric partial pressure of CO2 predicted from gas exchange and that calculated from δ13C to be in close agreement only in the two youngest cohorts of fully expanded leaves. The results indicated that with suitable precautions concerning measurement protocol, foliar δ13C and gas exchange measurements were reliable indicators of potential resource use efficiency by M. polymorpha along environmental gradients.

Photosynthesis by inflated pods of a desert shrub, Isomeris arborea.

The photosynthetic capacity and carbon metabolism of the fruits of Isomeris arborea (Capparidaceae), an evergreen shrub endemic to the desert and coastal habitats of Southern California and Baja California, are described. The inflated structure of the pods of I. arborea provides a model system for experimental studies of fruit photosynthesis in native plants since the gas concentration of the internal space can be manipulated and monitored separately from the external pod environment. CO2 released by seed respiration is partially contained in the inner gas space of the pods, resulting in an elevated CO2 environment inside the fruit (500 to 4000 µmol mol-1 depending on the stage of fruit development). A portion of this CO2 is assimilated by the inner layers of the pericarp, but a larger fraction leaks out. The photosynthetic layers of the pericarp use two different sources of CO2: the exocarp fixes exogenous CO2 while the endocarp fixes CO2 released by seed respiration into the pod cavity. Even though the total weight of the fruit increases during development, the combined rates of fixation of externally and internally supplied CO2 remained constant (10-11 µmol CO2 pod-1 h-1). After the pods attain maximum volume, the major change in gas exchange that takes place during fruit growth is a gradual increase in the amount of respiratory CO2 released by the seeds. This shifts the CO2 balance of the fruit from positive, in young fruits, to negative in mature fruits. Pericarp photosynthesis helped support not only the cost of fruit maintenance, but also the cost of fruit growth, particularly during the first stages of fruit development. During later fruiting stages insufficient carbon is fixed to fully supply either respiration or growth.

Compound interactions effects of plant antioxidants in combination with carbaryl on performance ofTrichoplusia ni (Cabbage Looper).

Plant chemicals naturally exist in complex mixtures, which can interact either additively, synergistically, or antagonistically. We investigated the potential interactions of three naturally occurring antioxidants- nordihydroguaiaretic acid (NDGA), safrole, and α-tocopherol-with the general insecticide carbarayl to affect the performance of cabbage looper larvae (Trichoplusia ni). The cabbage looper is known to produce a mixed-function oxidase enzyme system in response to the presence of carbaryl. We proposed that plant antioxidants would interfere with enzymatic oxidation, enhancing the susceptibility of this insect to carbaryl. Insects were fed artificial diets containing each antioxidant alone or in pairwise combinations with the insecticide carbaryl to test for their effects on the insect's nutritional measurement indices. The three antioxidants tested were not equally effective individually against insect survivorship and interacted differentially in combination with the insecticide. The nutritional indices were measured on insects fed diets containing the chemicals at nonlethal doses. Insects fed 0.001 % wet wt NDGA diets grew 1.62 times less, and had gross and net conversion efficiencies reduced 3.20 and 3.63 times, respectively, compared to the control larvae. Carbaryl (0.002% wet wt) in combination with NDGA acts as an antagonist to the effects mentioned above, while safrole (1 × 10(-4) wet wt) had an additive effect when combined with the insecticide, reducing 1.76 times larval relative growth rate and efficiency of conversion of ingested food in respect to the control. The larvae fed significantly more (1.2 times) on both insecticide and safrole diets than on the controls or their combined diets. Larvae fed α-tocopherol alone or in combination with carbaryl had similar growth and conversion efficiencies as controls. We conclude that the effects of different combinations of compounds cannot be predicted a priori and must be determined experimentally.

Water relations of stem succulent trees in north-central Baja California.

Water relations of several stem succulent trees were measured in north-central Baja California in comparisons to other growth forms in the same habitat. Our research concentrated on three stem succulent species (Idria collumnaris, Pachycormus discolor and Bursera microphylla) each with a different succulent stem morphology. The stem succulent trees had 1 to 4 kg H2O/m3 of trunk while the other trees and shrubs in the same habitat had 0.6 to 0.8 kg H2O/m3. The diurnal and seasonal variation in leaf water potential was small for the stem succulent species in comparison to deciduous and evergreen species as a consequence of the stem-water, buffering capacity. In addition, the leaf conductance of the stem succulent species was low (60 mmol m-2 s-1) and yet, the leaf conductance decreased through the day similar to adjacent evergreen and deciduous species. The leaves of the stem succulent trees lost turgor at low saturated water deficits (0.06 to 0.14), had comparatively high osmotic potentials, and high values of elastic modulus in comparison to adjacent evergreen and deciduous species. The stem acts as an important buffering mechanism allowing for the maintenance of leaf turgor in these stem succulent trees. The low transpiration rates of the stem succulent trees may be a mechanism to minimize leaf saturated water deficit and extend leaf longevity.

Stem photosynthesis in Psorothamnus spinosus (smoke tree) in the Sonoran desert of California.

Stem photosynthetic responses to environmental parameters were investigated with Psorothamnus spinosus in the Sonoran Desert of California. Light saturation of stem photosynthesis was equal to maximum midday summer irradance (1600-2000 µmol·m-2·s-1). The optimum temperature for stem photosynthesis was 39°C, and lower stem temperatures (27-35°C) caused significant decreases (up to 50%) in stem photosynthesis. Positive stem photosynthesis was maintained up to 51°C. Stem photosynthesis was relatively insensitive to increasing vpd up to 5 kPa; However, stem conductance decreased by 25% at a vpd of 5 kPa. At vpd greater than 5 kPa stem photosynthesis decreased relatively more than that of stem conductance causing a decrease in water use efficiency and an increase an intercellular carbon dioxide concentration. Maximum stem photosynthetic rates were low (6.2-10.6 µmol·m-2·s-1) on a stem surface area, but, stem photosynthetic rates of young shoots were substantially higher (19.5-33.3 µmol· m-2·s-1) on a projected area basis.

Effects of manipulation of water and nitrogen regime on the water relations of the desert shrub Larrea tridentata.

Water and nitrogen regimes of Larrea tridentata shrubs growing in the field were manipulated during an annual cycle. Patterns of leaf water status, leaf water relations characteristics, and stomatal behavior were followed concurrently. Large variations in leaf water status in both irrigated and nonirrigated individuals were observed. Predawn and midday leaf water potentials of nonirrigated shrubs were lowest except when measurements had been preceded by significant rainfall. Despite the large seasonal variation in leaf water status, reasonably constant, high levels of turgor were maintained. Pressure-volume curve analysis suggested that changes in the bulk leaf osmotic potential at full turgor were small and that nearly all of the turgor adjustment was due to tissue elastic adjustment. The increase in tissue elasticity with increasing water deficit manifested itself as a decrease in the relative water content at zero turgor and as a decrease in the tissue bulk elastic modulus. Because of large hydration-induced displacement in the osmotic potential and relative water content at zero turgor, it was necessary to use shoots in their natural state of hydration for pressure-volume curve determinations. Large diurnal and seasonal differences in maximum stomatal conductance were observed, but could not easily be attributed to variations in leaf water potential or leaf water relations characteristics such as the turgor loss point. The single factor which seemed to account for most of the diurnal and seasonal differences in maximum stomatal conductance between individual shrubs was an index of soil/root/ shoot hydraulic resistance. Daily maximum stomatal conductance was found to decrease with increasing soil/root/ shoot hydraulic resistance. This pattern was most consistent if the hydraulic resistance calculation was based on an estimate of total canopy transpiration rather than the more commonly used transpiration per unit leaf area. The reasons for this are discussed. It is suggested that while stomatal aperture necessarily represents a major physical resistance controlling transpiration, plant hydraulic resistance may represent the functional resistance through its effects on stomatal aperture.

Ecological tannin assays : Evaluation of proanthocyanidins, protein binding assays and protein precipitating potential.

Estimations of condensed tannin content are generally based on calibration standard curves from Quebracho condensed tannins. We generated calibration standard curves from eight Sonoran Desert species for comparison with estimates of tannin concentrations derived from the Quebracho standard curve. Estimates of leaf tannin concentrations of each of the eight species using each species standard curve differed significantly with the estimates given by the Quebracho standard curve. Standard curves constructed from tannins from different individuals of three of the species varied significantly between, but not within, species. The efficiency of precipitation of protein bovine serum albumin (BSA) by each different tannin varied up to a factor of fifty for tannins of different species. Ordering species from highest to lowest based on tannin concentrations or binding efficiencies gave two different ranks. We argue that concentration or efficiency alone do not describe adequately tannin ecological activity. Instead, we suggest combining tannin concentrations and binding efficiencies to measure the protein precipitating potential of a leaf. Leaf protein precipitating potential is a more ecologically realistic parameter, we feel, for between-species comparisons than tannin content or binding efficiencies alone.

Stable isotopes in physiological ecology and food web research.

Mention the word 'isotopes' and most people will think of short-lived radioactive isotopes. Yet far more abundant than the radioactive forms and perhaps much more useful for ecological studies are the stable isotopes. Early interest in stable isotope analyses developed in the geological sciences, and their application in environmental biology has developed slowly until recently. Over the past few years, innovative applications of stable isotope analyses to studies of biological processes have been expanding rapidly, and there is every indication that stable isotope approaches will lead to major advances over the next decade in our understanding of physiological processes and fluxes through ecological systems. Studies of plant and animal physiological ecology and food webs will benefit greatly.

Influences of microclimatic conditions and water relations on seasonal leaf dimorphism of Prosopis glandulosa var. torreyana in the Sonoran Desert, California.

Seasonal measurements of microclimatic conditions were compared to seasonal indices of leaf structural components and plant water relations in Prosopis glandulosa var. torryana. P. glandulosa had two short periods of leaf production which resulted in two distinct even aged cohorts of leaves. The two leaf cohorts (summer, winter) were concurrent in the summer and fall, contrasting to previous studies on other species in which one leaf form replaces a previous leaf type. The structural characteristics of these two cohorts differed significantly in two replicate year cycles. The leaves of the spring cohort were larger in weight and area but similar to the summer cohort in specific leaf weight and leaflet number. The second growth period leaves constituted only a small proportion of the total plant leaf area. The dimorphism between the two cohorts was best associated with plant water relations and not energy load. Second growth period leaves maintained turgor to greater water deficits but lost turgor at higher leaf water potentials. Seasonal osmotic adjustment occurred for first growth period leaves but not second growth period leaves. The small leaves produced during the hot climate were most likely the result of low turgor potential during development rather than an adaptation to tolerate stressful environments.

Morphological changes along an altitude gradient and their consequences for an andean giant rosette plant.

Selected morphological features were measured in five populations of the giant rosette plant Espeletia schultzii occurring along an elevation gradient from 2600 to 4200 m in the Venezuelan Andes. Pith volume per amount of leaf area increases with elevation resulting in significantly larger water storage capacity at higher elevations. Thickness of leaf pubescence and, therefore, leaf boundary layer resistance, also increases with elevation resulting in both potentially higher leaf temperatures relative to air temperature and higher leaf to air vapor pressure gradients. The net effect on transpiration rate would depend on ratios of stomatal to boundary layer resistance and leaf energy balance. At higher elevations the central rosette leaves are more vertically oriented and the leaf bases show a pronounced curvature as the intersection with the main axis is approached. This gives these rosettes a distinctly paraboloid appearance and probably enhances capture and retention of incident long and shortwave radiation by the apical bud and expanding leaves. Features which result in enhanced water storage capacity and higher plant temperatures relative to air temperature without greatly increasing water loss are adaptive in high altitude paramo habitats where water availability and growth are limited by year round low temperatures (mean 2-3° C).

Post-fire uptake of nutrients by diverse ephemeral herbs in chamise chaparral.

Diverse populations of ephemeral herbs form the dominant element of community biomass in the first year of growth following a fall burn in chamise chaparral. Ephemeral herbs constituted 337 kg ha-1 of above-ground biomass after the first season of post-fire growth. This was 64% of the total, with the majority of the remaining biomass being in resprouts of Adenostoma fasciculatum. Ephemeral herb biomass following fire in other stands was as high as 1117 kg ha-1. Nutrient contents of ephemeral herbs were 6.68 kg N ha-1, 0.71 kg P ha-1, 10.05 kg K ha-1, 4.75 kg Ca ha-1 and 0.91 kg Mg ha-1. These were 55, 54, 81, 71 and 70% respectively of the above-ground totals. In the second year following fire, the total herb biomass was 40% higher, but the nutrient pool in above-ground biomass of these herbs was only 30-60% of what it had been the first year. Resprouts of A. fasciculatum and short-lived wood shrubs constituted more than 90% of above-ground biomass at this stage of postfire succession.

Population structure of Adenostoma fasciculatum in mature stands of chamise chaparral in the southern Sierra Nevada, California.

In the low elevation chaparral areas of Sequoia National Park, California, pure stands of chamise (Adenostoma fasciculatum) are periodically rejuvenated by fire. Mature stands showed considerable variability in density and total biomass even though a positive correlation exists between the two. Mature stands showed a preponderance of individuals in the smaller size classes (inverse-J shape distribution). Dead shrubs found in mature stands also tended to be in the smaller size classes. This relatively high mortality of small individuals is important to post-fire stand development. In addition, resprout and seedling biomass one year after fire both showed inverse-J shaped size-class structures. A positive correlation existed between the preburn basal area of a shrub and its first year resprout biomass. Shrub biomass and distance to nearest neighbor were poorly correlated. A significant correlation existed between stand density and a stand's variance-to-mean ratio, indicating a trend toward more regular spacing as density increases. Pre-burn and fire-induced mortality tended to move the stand towards a more clumped distribution. Seedlings replaced dead individuals after a fire and thus restored regular spacing.

Estimates of N2-fixation from variation in the natural abundance of 15N in Sonoran desert ecosystems.

The 15N abundance of tissues of five Prosopis specimens at our primary study site (a Prosopis woodland at Harper's Well in the Sonoran desert of Southern California) was determined over two growing seasons 1980 and 1981. The 15N abundance of soil and of tissues of presumed non-N2-fixing (control) plants was also measured. Prosopis tissues were significantly lower in 15N than either soil N or corresponding tissues of presumed non-N2-fixing plants which derive their N entirely from soil. Soil N was also significantly higher in 15N than atmospheric N2. We conclude that it is feasible to use variations in the natural abundance of 15N as an index of N2-fixation in this kind of ecosystem, and that N2-fixation is of considerable importance to Prosopis growing at this site.We also determined the 15N abundance of leaf tissue of presumed N2-fixing and control plants growing at the same site at six additional sites (five in the Sonoran desert of southern California and one in Baja California, Mexico near the town of Catavina). Four of these additional sites were dominated by Prosopis and two were mixed communities. There were statistically significant differences between the 15N abundances of the pooled legume population and control plants at all sites, although not every legume specimen exhibited this difference. From 15N abundance data we estimated the fractional contribution of biologically fixed N to the N economy of desert legumes. We concluded that N2-fixation is very important to Prosopis at six of seven sites in the Sonoran Desert. At the site where Prosopis did not appear to be fixing N2, N2-fixation was important only for legumes of the sub-family Papilionoideae, Lupinus, Dalea, Astragalus and Lotus.

Atmospheric water uptake by an atacama desert shrub.

Nolana mollis, a succulent-leaved shrub of the extreme coastal desert of Chile, has the capacity to condense water on its leaves out of unsaturated atmospheres, Metabolic energy would have to be expended to move this water either from the leaf surface directly to the mesophyll or, when dripped to the soil, from there into the roots. Because of the unusual aridity of its habitat and of the utilization of water-use-efficient metabolism by Nolana, at least during certain periods, such an energy expenditure could be effective.

Patterns of drought response in leaf-succulent shrubs of the coastal Atacama Desert in Northern Chile.

Despite the extrame aridity of the coastal Atacama Desert in northern Chile, sparse communities of leaf succulent shrubs and small cacti are regularly present. While most shrub species have small succulent leaves and accumulate high concentrations of salts in their tissues, the variable rooting patterns and mixed dominance of CAM and C3 species indicates a significant divergence in adaptive strategies. All dominant shrubs are readily surviving extended drought, but some species are much better able than others to maintain active growth and flowering. Regular flowering may not be a prerequisite for shrub population maintenance since large piles of viable seeds are present under the canopies of many species.

Further observations on the water relations ofProsopis tamarugo of the northern Atacama desert.

Prosopis tamarugo, a tree native to the Atacama desert of Chile apparently has unique water relations. It is proposed that in its native habitat, where there is essentially no precipitation, establishment occurs during the rare flooding periods, with water coming as runoff from the Andes. These plants subsequently exist as phreatophytes tapping the relatively shallow ground water. Although phreatophytic, the plants appears to come under increasing drought stress as the growing season progresses. Because of the very low water potentials of the salty surface soils, water evidently moves from the plant into the soil under certain conditions. This water may be reabsorbed subsequently and used by the plant as the water table capillary fringe is depleted toward the end of the leafy period.

Spatial relationships and competition in a Chilean desert cactus.

In the Atacama Desert of northern Chile, cacti of the genus Copiapoa occur in extensive, relatively dense, monospecific stands. The spatial distribution patterns within several stands of Copiapoa cinerea v. columna-alba were analyzed for evidence of competitive interactions among individuals. There was no indication that competition was affecting stand density. Mean annual precipitation is only 25 mm/yr, but estimates of maximum possible water use within a stand were much lower. It was shown that the time between rainstorms is long compared to the water storage capacity of the plant stems, especially in the case of small individuals. We conclude that population densities in Copiapoa are limited by the difficulty of establishment, not by competition.