Long-lived Perennial Plant Research Articles

WHY DON'T CARNIVOROUS PITCHER PLANTS COMPETE WITH NON-CARNIVOROUS PLANTS FOR NUTRIENTS?

Most field studies suggest that competition for nutrients is important in nutrient-limited systems. Nevertheless, the mechanisms by which species avoid competition for nutrients in nutrient-poor but species-rich communities remain poorly understood. I used a model system, the carnivorous pitcher plant Sarracenia alata, to measure competition for nutrients. In a field competition experiment, I increased the potential for nutrient-niche overlap between established juvenile pitcher plants and their non-carnivorous neighbors by denying a unique source of nutrients (prey) to pitcher plants. Contrary to predictions based on nutrient-niche complementarity, prey exclusion did not reduce the performance of pitcher plants competing with non-carnivorous neighbors. Hence, differences in nutrient-source specialization did not explain the coexistence of Sarracenia alata and its non-carnivorous neighbors in the field. Instead, prey exclusion reduced the positive growth response to increased light following the removal of neighbors. By reducing allocation to pitchers in the presence of neighbors, pitcher plants appeared to reduce their demand for nutrients when shaded. I hypothesize that carnivorous plants avoid competition with non-carnivorous plants by using changes in the light environment to cue adjustments in their demand for nutrients. I suggest that such plasticity is adaptive in any long-lived perennial plant that periodically experiences simultaneous pulses of light and nutrients during its lifetime (e.g., as might occur in fire-prone habitats). Corresponding Editor: E. S. Menges

Impact of flower harvesting on the salt marsh plant Limonium carolinianum

Because of the potentially detrimental effects of seed production on adult survivorship and growth, moderate flower harvesting may have little negative impact on population growth of long-lived perennial plants such as Limonium carolinianum (Walter) Britton. We examined this by collecting data on survivorship, growth, and fecundity of an unharvested population over a period of 5 years and conducted a controlled experiment to examine the effect of harvesting on adult survivorship and growth over a 3-year period. Data were summarized in the form of a stage-structured matrix population model with a stochastic element that incorporated year-to-year variation in transition probabilities. Contrary to our original hypothesis, we found that preventing seed set through removal of flowers did not increase adult survivorship or growth. By determining the harvest level that reduced population growth rate to 1.0, we estimated the maximum sustainable harvest level to be 16%, a value that is approximately half that of reported harvest levels on accessible marshes in the study area. In spite of this, the reported harvest levels are unlikely to drive local populations to extinction in the foreseeable future. Providing the adult population size is >100 and harvest levels are <90%, time to local extinction will exceed 100 years. This is a function of the very high survivorship of adults in this species and the fact that harvesting has no negative impact on adult survivorship or growth. However, because of the long preadult phase in this species (8–9 years) and the fact that fecundity of young adults is low, recovery from overharvesting is extremely slow. Adult population size can be reduced to 25% of its original value in 7 years at high harvest levels, but it will take 34 years on average to recover once harvesting is terminated.Key words: cost of reproduction, stage–structures matrix population model, sustainable harvest, conservation of biodiversity.

Special symposium: In vitro plant recalcitrance recalcitrance of woody and herbaceous perennial plants: Dealing with genetic predeterminism

As a general, long-lived perennial plants probably present the most challenging obstecles to the researcher, breeder or propagator utilizing microculture as a tool. These challenges appear during all stages of the microculture process, but are probably most resplendent during the stabilization phase. What may be particularly frustrating is that much of this ‘recalcitrance’ is genetically driven and is thus difficult to control by environmental and nutritional manipulations in microculture. Perennials have complex seasonal cycles and life cycles, which complicate control of their growth in microculture. As shoot cultures have provided useful tools for overcoming these limitations, the inability to establish stabilized shoot cultures is a major form of recalcitrance. Plants having seasonal growth dynamics dominated by strong episodic or determinant shoot growth are some of the most recalcitrant species because stabilized shoot cultures cannot be readily generated. In some cases, episodic growth may be tied closely to phase state and can thus be controlled by manipulating phase; nevertheless, adequate controls have not been identified for many problematic plants. Another trait contributing to recalcitrance of perennials is the relatively slow growth rate in microculture. Slow growth complicates such procedures as selection of transformed tissues. The high phenolic content of many perennial tissues can interfere with the efficacy of transgenic traits such as β-glucuronidase. Developmentally determined growth characteristics such as plagiotropism may persist through all stages of microculture and complicate the recovery of commercially useful micropropagules. Although some technical approaches can occasionally circumvent immediate microculture limitations, general solutions await the development of a deeper understanding of physiological bases of such genetically predetermined phenomena.

Germination Ecology of Some Common Forest Herbs in Yellowstone National Park, Wyoming, U.S.A.

Seed viability and dormancy mechanisms were investigated in seven herbaceous plant species and one low shrub species common in the first few years of postfire succession in subalpine forests of Yellowstone National Park, Wyoming, U.S.A. Seeds of Epilobium angustifolium (fireweed) had rapid and high percent germination in both the fall and spring after seeds were collected. Seeds of Lupinus argenteus (lupine) germinated well after several months of warm or cold storage, and the rate of germination but not the final percent germination was increased by scarification. Seeds of Arnica cordifolia (heartleaf arnica) were nearly all nonviable; only one seed germinated out of 650 tested, and no seedlings were observed in the field. Small sample sizes limited the scope of testing of the other species, but both Cirsium arvense (Canada thistle) and Epilobium ciliatum (willow-herb) showed moderate to high germination in the spring after several months of storage, Hieracium albiflorum (hawkweed) germinated better in the fall than in the following spring, Vaccinium scoparium (dwarf huckleberry) seeds germinated in the fall 1 mo after collection, and seeds of Antennaria racemosa (pussytoes) failed to germinate when tested in the fall 1 mo after collection. These results, combined with field studies, indicate that large-scale fires provide opportunities for recruitment of new genetic individuals, increased genetic diversity, and colonization of new patches on the forest floor in several species, including Epilobium angustifolium, Lupinus argenteus, Hieracium albiflorum, and Cirsium arvense. These effects are likely to persist for many decades in these long-lived perennial plant species.

How Do Trees and Other Long-Lived Plants Survive in Polluted Environments?

Most established vegetation within the vicinity or influence of industrial and urban areas is subjected to the regular or episodic impact of pollution. Complete decimation of the vegetation as a consequence of this is a relatively infrequent occurrence, although mining and industrial wastes containing metals or other toxins may remain virtually devoid of vegetation. The acute phytotoxic effects of many environmental contaminants are well known, but studying the possible chronic effects of lower concentrations of atmospheric contaminants on growth and vigour of plants is probably of more relevance, particularly to longlived plants. This has been a major focus of research in European and North American forests (Pitelka, 1988; Schulze, 1989) but much evidence is speculative due to the lack of long-term data (Blank et al., 1989). In many herbaceous plants the selection pressure of pollution has led to the natural evolutionary development of tolerant plant genotypes in response to a wide range of airand soil-borne pollutants and to herbicides (Bradshaw & McNeilly, 1981; MacNair, 1981; Murphy, 1983; Taylor, 1990). However, unlike the situation for many annual or colonizing plants, the long generation time of trees and other long-lived perennial and clonal plants prevents a rapid selection of tolerance. In this situation, as with other stresses which affect plants in a temporary or fluctuating manner, facultative adaptations (phenotypic plasticity) may be more appropriate (Bradshaw & Hardwick, 1989). This paper reviews the differing responses of plants to pollution, comparing the impact on individual plants, populations and more permanent plant communities, and considers how current knowledge explains the longevity of trees and clonal plants in polluted environments.

Atriplex species as irrigated forage crops

Field trials were conducted in the San Joaquin Valley, California, to evaluate accessions of Atriplex during the establishment year for overall plant performance as a forage crop when irrigated with saline drainage water. Productivity, forage quality and agronomic characteristics of both native and exotic species were promising. Of the exotic perennial species tested, total clipping harvest yields and overall nutritive values were highest for Atriplex barclayana (Benth.) Dietr. Productivity and nutritive value of harvested annual and short-lived perennial plants were comparable to clippings of the long-lived perennial plants. If the perennial species are used as a multi-clipped forage crop, irrigation and harvest management practices to provide optimum yields and quality forage would need to be developed. Average levels of ash, crude protein, acid detergent fiber, neutral detergent fiber, lignin and selenium of harvested clippings were 297, 166, 240, 423, 92 g kg −1 and 1 mg kg −1, respectively. Among the tested accessions, lignin levels were more variable than the other nutritive components.

Inheritance of isozyme variation and heterozygosity in Pinus ponderosa.

Techniques are presented to detect 23 isozyme loci in the long-lived perennial plant, ponderosa pine. Meiotically derived megagametophyte from seeds is used to examine directly the segregation of allelic variants. Approximately seven seeds were initially examined for 12 enzymes from each of 47 trees from ten stands throughout the northern Rocky Mountain region. Additional seeds were also examined from selected families to confirm the inheritance of observed electrophoretic variants at 13 polymorphic loci and to estimate linkage relationship. Significant norandom segregation was consistently detected for three pairs of loci: ADH-1:AAT-2, ADH-1:PGI-1, and LAP-2:6PG-1. Preliminary estimates of population parameters reveal a relatively high average heterozygosity (H = 0.123). This is partitioned into a high amont of genetic variation within local stands, with only approximately 12% of the total heterozygosity resulting from genic difference between stands.

The Influence of Breeding Habit on the Outcome of Natural Hybridization in Plants

Where the time available for reproduction by a natural hybrid is critical, as it is in annual herbs, biennials, and short-lived perennials, the natural hybridization will tend to run toward polyploidy under conditions of selffertilization and toward introgression under conditions of outcrossing. We accordingly find in some annual and biennial groups, where hybridization has occurred on an extensive scale, that polyploidy is closely associated with autogamy and introgression with outcrossing. In long-lived perennial plants these tendencies may be obscured and reversed by other factors. The association of allopolyploidy with an outcrossing breeding system, which was noted by Gustafsson (1947), is found chiefly in plants with a perennial habit and/or vegetative means of propagation.