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ABSTRACT. Phenotypic plasticity in response to environmental heterogeneity is an important adaptive component of plant strategies. This study addresses plasticity of Aesculus glabra leaf traits in response to small-scale (within-canopy, [...]

Both engineered hydraulic systems and plant hydraulic systems are protected against failure by resistance, reparability, and redundancy. A basic rule of reliability engineering is that the level of independent redundancy should increase with increasing risk of fatal system failure. Here we show that hydraulic systems of plants function as predicted by this engineering rule. Hydraulic systems of shrubs sampled along two transcontinental aridity gradients changed with increasing aridity from highly integrated to independently redundant modular designs. Shrubs in humid environments tend to be hydraulically integrated, with single, round basal stems, whereas dryland shrubs typically have modular hydraulic systems and multiple, segmented basal stems. Modularity is achieved anatomically at the vessel-network scale or developmentally at the whole-plant scale through asymmetric secondary growth, which results in a semi-clonal or clonal shrub growth form that appears to be ubiquitous in global deserts. plant hydraulic systems | wood anatomy | hydraulic redundancy | xylem structure and function

We conducted a long-term rodent exclosure experiment in native grass- and shrub-dominated vegetation to evaluate the importance of top-down and bottom-up controls on plant community structure in a low-productivity aridland ecosystem. Using multiple regressions and analysis of covariance, we assessed how bottom-up precipitation pulses cascade through vegetation to affect rodent populations, how rodent populations affect plant community structure, and how rodents alter rates of plant community change over time. Our findings showed that bottom-up pulses cascade through the system, increasing the abundances of plants and rodents, and that rodents exerted no control on plant community structure and rate of change in grass-dominated vegetation, and only limited control in shrub-dominated vegetation. These results were discussed in the context of top-down effects on plant communities across broad gradients of primary productivity. We conclude that bottom-up regulation maintains this ecosystem in a state of low primary productivity that constrains the abundance of consumers such that they exert limited influence on plant community structure and dynamics. Key words: Chihuahuan desert; grassland; long-term study; precipitation; shrubland; small mammals.

The effect of differences in nitrogen (N) availability and source on growth and nitrogen metabolism at different atmospheric C[O.sub.2] concentrations in Prosopis glandulosa and Prosopis flexuosa (native to semiarid regions of North and South America, respectively) was examined. Total biomass, allocation, N uptake, and metabolites (e.g., free N[O.sub.3.sup.-], soluble proteins, organic acids) were measured in seedlings grown in controlled environment chambers for 48 d at ambient (350 ppm) and elevated (650 ppm) C[O.sub.2] and fertilized with high (8.0 mmol/L) or low (0.8 mmol/L) N ([N.sub.level]), supplied at either 1 : 1 or 3 : 1 N[O.sub.3.sup.-] : N[H.sub.4.sup.+] ratios ([N.sub.source]). Responses to elevated C[O.sub.2] depended on both [N.sub.level] and [N.sub.source], with the largest effects evident at high [N.sub.level] A high N[O.sub.3.sup.-] : N[H.sub.4.sup.+] ratio stimulated growth responses to elevated C[O.sub.2] in both species when N was limiting and increased the responses of P. flexuosa at high [N.sub.level] Significant differences in N uptake and metabolites were found between species. Seedlings of both species are highly responsive to N availability and will benefit from increases in C[O.sub.2], provided that a high proportion of N[O.sub.3.sup.-] to N[H.sub.4]-N is present in the soil solution. This enhancement, in combination with responses that increase N acquisition and increases in water use efficiency typically found at elevated C[O.sub.2], may indicate that these semiarid species will be better able to cope with both nutrient and water deficits as C[O.sub.2] levels rise. Key words: ammonium; C[O.sub.2]; controlled environments; nitrate; nitrogen metabolism; nitrogen uptake; Prosopis flexuosa; Prosopis glandulosa.

1. We investigated experimentally differences in abiotic stress tolerance and the effects of plant-plant interactions for two desert shrubs, Chrysothamnus nauseosus and Sarcobatus vermiculatus, along a soil salinity (NaCl) and boron (B) gradient at Mono Lake, California, USA. Based on differences in natural distribution, and the classical expectation of a trade-off between competitive ability and stress tolerance, we hypothesized that (i) Chrysothamnus would have greater competitive ability than Sarcobatus at the low salinity end of the gradient, and that (ii) Sarcobatus would be more stress tolerant than Chrysothamnus. 2. Juvenile target plants of Chrysothamnus and Sarcobatus were planted into four sites along the gradient. Biomass was determined by destructive harvests over two growing seasons. At each site, interspecific relative competitive ability was assessed as the effect of Sarcobatus neighbours on Chrysothamnus targets compared to the effect of Chrysothamnus neighbours on Sarcobatus targets. Stress tolerance was assessed as the ability of each species to survive and grow, in the absence of neighbours, at different sites along the gradient. 3. The two species did not differ in the relative strength of plant-plant interactions, providing no support for the expectation that Chrysothamnus had greater competitive ability than Sarcobatus. Furthermore, there was no evidence for competition or facilitation, either interspecific or intraspecific, at any site in either year of the study. However, fertilization treatments demonstrated nutrient limitations, soil water reached limiting levels and root systems of targets and neighbours overlapped substantially. It is therefore surprising that plant-plant interactions among juveniles apparently play little role in the growth and survival of shrubs in this saline desert habitat. 4. Sarcobatus was more stress tolerant than Chrysothamnus and the two species performed optimally at different sites along the gradient. Sarcobatus juveniles grew best at the two most saline sites and survived at all sites, whereas Chrysothamnus juveniles grew best at a low-salinity site and did not survive at the most saline site. The difference in site of optimal performance may be due to differences in nutrient limitations or to interactions between nutrient availability and sodium (Na) and B tolerance. Keywords: boron, Chrysothamnus nauseosus, salinity, Sarcobatus vermiculatus, sodium

Morphology and physiology of fruit and seed development were compared in Rhus aromatica and R. glabra (Anacardiaceae), both of which produce drupes with water-impermeable endocarps. Phenology of flowering/fruiting of the two species at the study site was separated by [approximately]2 mo. However, they were similar in the timetable and pattern of fruit and seed development; it took [approximately]2 mo and [approximately]1.5 mo for flowers of Rhus aromatica and R. glabra, respectively, to develop into mature drupes. The single sigmoidal growth curve for increase in fruit size and in dry mass of these two species differs from the double-sigmoidal one described for typical commercial drupes such as peach and plum. Order of attainment of maximum size was fruit and endocarp (same time), seed coat, and embryo. By the time fruits turned red, the embryo had reached full size and become germinable; moisture content of seed plus endocarp had decreased to [approximately]40%. The endocarp was the last fruit component to reach physiological maturity, which coincided with development of its impermeability and a seed plus endocarp moisture content of Key words: Anacardiaceae; embryo germinability; endocarp impermeability; fruit development; mass allocation to fruit components; Rhus aromatica; Rhus glabra; seed development.

Electrophoretic analysis of progeny arrays was used to determine whether Ardisia escallonioides (Myrsinaceae), a self-compatible tropical understory shrub in south Florida, exhibits a mixed-mating system. Ten seedlings from seven to 16 maternal plants from four populations were analyzed using five polymorphic enzyme systems. Multilocus population outcrossing rates ranged from 0.39 to > 1, with three populations showing high levels of selfing. Both single and multilocus inbreeding coefficients (F) indicated excess homozygosity in seedlings but not adults for three of the four populations. Population outcrossing rate was not positively correlated with increased numbers of flowering plants. The importance of annual variation in population outcrossing rates is discussed with regard to the high temporal variability in seedling recruitment in this species. Key words: Ardisia escallonioides; Florida; mating system; Myrsinaceae; outcrossing rate; reproductive ecology; tropical shrub.

Recovery of seed germination from NaCl salinity of desert shrubs (Haloxylon recurvum and Suaeda fruticosa, and the herbs Zygophyllum simplex and Triglochin maritima was studied under various thermoperiods. The percentage of ungerminated seeds that recovered when they were transferred to distilled water varied significantly with variation in species and thermoperiods. Zygophyllum simplex had little recovery from all NaCl concentrations in all thermoperiods. Haloxylon recurvum, S. fruticosa, and T. maritima showed substantial recovery. Percentage recovery was highest in S. fruticosa, followed by T. maritima, and H. recurvum. Thermoperiodic effects varied with the species investigated. There was little thermoperiodic effect on the percentage recovery of S. fruticosa, except in the higher salinity treatment at higher thermoperiods. Variation in thermoperiod appears to play an important role in recovery of germination of halophytes from salt stress when seeds are transferred to distilled water. Key words: halophyte; Haloxylon recurvum; recovery of seed germination; Suaeda fruticosa; thermoperiod; Triglochin maritima; Zygophyllum simplex

We studied various aspects of the fruiting biology of Cornus sanguinea (Cornaceae), a fleshy-fruit-producing deciduous shrub, in four populations in northwest Spain. One population was studied over a 5-yr period (1989-1994), and the remaining populations in 1994 only. Fruit-set level varied among years (range 11-18%) and among populations (range 8-22%), but was in all cases low. Within plants, fruit-set level did not vary significantly among inflorescences, indicating that inflorescence fruit set is independent of inflorescence position and inflorescence phenology. To investigate the function of surplus flowers, we carried out flower removal experiments. Inflorescence fruit-set level was unaffected by removal of up to [approximately equal to]75% of flowers. These results suggest that Cornus sanguinea regulates its fruit-set level via plasticity in the number of fruits aborted: if flower mortality has been high, fewer fruits will be aborted. Within the inflorescence, surplus flowers thus act as insurance against flower loss. Key words: Cornus sanguinea; fruit set; reproductive resources; reserve ovary hypothesis.

Pollination biology of two Gnetum species was studied in a lowland mixed dipterocarp forest in Sarawak, Malaysia. A dioecious shrub species, G. gnemon var. tenerum, flowered in the evening and its strobili emitted a stinking odor. Pollination droplets were secreted from ovules on female strobili and from sterile ovules on male strobili in the evening. These strobili were visited by nectar-seeking moths of Pyralidae and Geometridae. Sticky pollen was attached on proboscides and antennae of these moths. A dioecious woody climber, G. cuspidatum, had cauline strobili on woody stems near the forest floor. Its male strobili emitted a funguslike odor in the evening and secreted nectar on collars that subtended 'flower' rings. The strobili were visited by small flies of Lauxaniidae (Diptera), onto the antennae and legs of which pollen was attached. Evaporation of the exposed droplet/nectar of these Gnetum flowers seemed to be minimized by nocturnal flowering near the tropical rain forest floor. Another floral-structural constraint of gymnosperms, i.e., lack of showy petals, was compensated for by floral fragrancy. The prevalence of entomophily in the Gnetales (i.e., Gnetum, Ephedra, and Welwitschia) may suggest that unspecialized insect pollination originated before the divergence between the Gnetales and angiosperms.

Cephalanthus occidentalis L. is protandrous and presents pollen secondarily on the stigma surface. Because self-pollen is present on the stigma, the degree of selfing vs. outcrossing in this species will depend on 1) the phenology of pollen presentation and stigma receptivity; 2) whether the species is self-incompatible; and 3) the rates of self vs. crossed pollen tube growth. This study describes floral morphology and phenology, self-incompatibility, and pollen tube growth rates in self- and cross-pollinations of C. occidentalis. Light and scanning electron microscopy were used to study stigma morphology after flower opening, while controlled pollinations tested for incompatibility. Stigmas were unreceptive initially but became receptive by the second day after flower opening. Ninety-two percent of cross-pollinated flowers set fruit, compared to 12% fruit set in self-pollinations. Pollen tubes from selfed and out-crossed pollen initially had similar growth rates. Out-crossed pollen tubes began to grow rapidly ca. 5 hr after pollination of a receptive stigma, whereas selfed pollen tubes ceased growth or grew slowly after this time. Pollen tubes from out-crossed pollen grew the length of the style within 24 hr after pollination, while selfed pollen tubes were inhibited at the stigma-style junction. Our results indicate that C. occidentalis has self-incompatibility, in addition to protandry and secondary pollen presentation. Protandry allows removal of self-pollen from the unreceptive stigma, while self-incompatibility prevents fertilization by unremoved self-pollen.

Spinacia oleracea (Chenopodiaceae) is a potential model system for studies of mechanisms of sex expression and environmental influences on gender in dioecious species. Development of the male and female flowers and inflorescences of spinach were studied to determine when the two sex types can be distinguished. We found that female inflorescence apices are significantly larger than those of the male. Flower primordia are similar in size prior to perianth initiation, but the male primordia develop at a faster rate. Another distinguishing feature at this early stage is the larger bract subtending the female primordium. The two flower types become readily distinguishable when the perianth initiates. Male flowers produce four sepals and four stamens in a spiral pattern in close succession. Female flowers produce two alternate perianth parts that enlarge somewhat before the gynoecium becomes visible. There are no traces of gynoecia in male flowers or of stamens in female flowers. We propose that plant sex type is determined before inflorescence development, prior to or at evocation.

Generalization occurs when an animal responds to novel stimuli in much the same way it has responded to similar, familiar stimuli. Our objective was to determine whether goats generalized foraging skills from older growth blackbrush (Coleogyne ramosissima) to shrub live oak (Quercus turbinella). Blackbrush (OGBB) has small leaves that are difficult to ingest without the twigs, whereas shrub live oak (SLO) has larger leaves that can be ingested easily without twigs. In Yr 1, 6-mo-old goats and in Yr 2 18-mo-old goats were given experience browsing with OGBB for 0, 10, 20, or 30 d. They were next tested for 5 min/d for 2 d on OGBB, and then tested in the same way on SLO. The number of bites and bite rate increased significantly as goats of both ages gained experience browsing OGBB. Likewise, goats with more experience browsing OGBB took more bites during a 5-min period when foraging on SLO. Thus, goats generalized foraging skills from OGBB to SLO. Plant life form also affected the response of goats. Total intake, intake rate, and bite size were significantly higher for SLO than for OGBB. There was an interaction between level of experience and plant life form. The interaction occurred because goats with no experience browsing OGBB took more bites per 5 min of SLO than of OGBB, whereas goats with increasing levels of experience browsing OBGG generally took more bites of OGBB than of SLO. Controls took more bites per 5 min of SLO than of OGBB, because SLO leaves could be ingested (harvested and chewed) more easily than OGBB twigs.

Measurements of total leaf area, herbivory, number of herbivores, and growth on male, female and vegetative individuals of the dioecious shrub Lindera benzoin were made in different light environments. Herbivore abundance did not differ across habitat types and sexual morphs. Herbivory was greater in shade habitats than in sun habitats, and vegetative plants suffered greater herbivory than did flowering males or females which did not differ in the amount of leaf material eaten. Branch growth rates were higher in the sun than in the shade, and females grew less than male or vegetative plants. Because allocation to defense, growth and leaf characteristics are likely to be confounded, rates of herbivory may not always be predicted based on growth rate or gender alone.

We studied leaf area production in two annual plant species, Abutilon theophrasti and Amaranthus retroflexus, under three day/night temperature regimes (18 [degrees]/14 [ degrees], 28 [degrees]/22 [degrees], and 38 [degrees]/31 [degrees] C) and two concentrations of carbon dioxide (400 and 700 [mu]L/L). The production of whole-plant leaf area during the first 30 d of growth was analyzed in terms of the leaf initiation rate, leaf expansion, individual leaf area, and, in Amaranthus, production of branch leaves. Temperature and [CO.sub.2] influenced leaf area production through effects on the rate of development, determined by the production of nodes on the main stem (the plastochron index), and through shifts in the relationship between whole-plant leaf area and the number of main stem nodes. In Abutilon, leaf initiation rate was highest at 38 [degrees], but area of individual leaves was greatest at 28 [degrees]. Total leaf area was greatly reduced at 18 [degrees] due to slow leaf initiation rates. Elevated [CO.sub.2] concentration increased leaf initiation rate at 28 [degrees], resulting in an increase in whole-plant leaf area. In Amaranthus, leaf initiation rate increased with temperature, and was increased by elevated [CO.sub.2] at 28 [degrees]. Individual leaf area was greatest at 28 [degrees], and was increased by elevated [CO.sub.2] at 28 [degrees] but decreased at 38 [degrees]. Branch leaf area displayed a similar response to [CO.sub.2], but was greater at 38 [degrees]. Overall, whole-plant leaf area was slightly increased at 38 degrees] relative to 28 [degrees], and elevated [CO.sub.2] levels resulted in increased leaf area at 28 [degrees] but decreased leaf area at 38 [degrees]. The effects on leaf area closely parallel rates of biomass accumulation in the same experiment, suggesting that responses of developmental processes to elevated [CO.sub.2] and interacting factors may play an important role in mediating effects on plant growth.

Reproductive and growth parameters of Atriplex canescens were examined in a common garden study. Cloned ramets of male, female, and hermaphrodite individuals from two natural populations were planted in irrigated and nonirrigated portions of the garden. Sexual differences in growth and flowering schedules suggest tradeoffs in resource allocation between growth and reproduction. Males flowered with the greatest frequency and intensity while producing the smallest growth parameters. Females had the largest measures of vegetative growth, but flowered with the least frequency and intensity. Hermaphrodites appear to have a biology distinct from males and females, being more like females in growth and more like males in reproduction.

Research was conducted to find out whether noctuid larvae foraging on the shrub Daphne laureola exhibited within-plant selection is caused by plant architecture. It was found that leaf whorls were at lower branching orders and shorter supporting stems when visited by larvae than whorls without record of noctuid larvae. Thus, it was speculated that plant architectural traits are used by the larvae as cues in selecting food with reduced cost of travel.

I investigated the effects of display size and flowering phenology on fruit set in Aucuba japonica, an understory dioecious shrub pollinated by opportunistic insects. Natural variations in display size, flowering phenology, and fruit set were monitored in 1997. A hand-pollination experiment was also conducted to check whether pollen limitation was a factor in fruit set in the field. Increases in floral display size did not affect fruit set; the proportion of flowers that set fruit was almost constant irrespective of the total number of flowers per inflorescence, the total number of inflorescences per plant, and the total number of flowers per plant. The hand-pollination experiment showed that fruit set was not pollen limited despite the low mating probability that resulted from the combination of dioecism and the species' dependence on opportunistic pollinators. This was due, in part, to the fact that female flowers did not have a predetermined period of receptivity, but instead remained receptive until they received pollen. In contrast, flowering phenology did affect fruit set. Fruit set was most abundant when male and female flowering was most abundant. This suggests there was some degree of pollen limitation during the part of the flowering season when male flowers were scarce. Key words: Aucuba japonica; Cornaceae; dioecy; flowering phenology; fruit set, resource limitation.

We examined components of male and female reproductive success in protogynous and protandrous sexual morphs of the heterodichogamous and largely monoecious chenopod shrub Grayia brandegei. Percentage femaleness of flowering stalks ranged from 0 to 37.6% female ([bar]X = 15.5%) for protandrous plants and from 14 to 100% female ([bar]X = 55.8%) for protogynous plants. Functional gender estimates based on ovule production at two locations ranged from 23.0 to 31.8% female for the protandrous morph, and from 65.3 to 77.0% female for the protogynous morph. Realized gender estimates based on total seed production ranged in value from 3.6 to 16.8% female for the protandrous morph and from 76.5 to 96.4% for the protogynous morph, depending on location and year. Differences in reproductive success of the two morphs were largely due to a reduction in the female function of protandrous plants. Protogynous plants produced more female flowers per stalk and had a higher percentage of seed-filled fruits than did protandrous plants. Differences between sexual morphs were more pronounced in dry areas or years in which overall seed production was minimal. Differential seed production between morphs likely reflects temporal patchiness in environmental conditions, particularly in water availability. The significance of these findings in support of heterodichogamy as an evolutionary pathway to dioecy is discussed. Key words: Chenopodiaceae; dichogamy; dioecy; floral sex ratio; Grayia brandegei; reproduction; spineless hopsage; temporal patchiness; Zuckia.

Murray et al. analyzed the contents of Witheringia solanacea to determine the type of laxative chemicals that are contained in the Costa Rican cloud forest shrub and their effects on frugivorous birds and seed dispersal. Additional analysis of Witheringia sp. fruit juice indicated the absence of laxatives although the fruit juice exhibited a significant effect on seed retention. Furthermore, the weak seed coatings of Witheringia sp. also indicates its sensitivity to the digestive processes of frugivorous birds.