E. Charles Morris

Germination response of seven east Australian Grevillea species (Proteaceae) to smoke, heat exposure and scarification

Seeds of plant species from fire-prone vegetation often show dormancy, and germinate in response to fire-related cues. Seeds of some east Australian Grevillea species were known from earlier studies to germinate in response to fires, but gave mixed results when heat shock was tested as a germination stimulus. In the current study, seeds of seven east Australian Grevillea species were tested for their germination response to smoke, to short heat exposure, and to scarification. Seeds were smoked in a chamber by using a mixture of native woodland vegetation as fuel. The smoking and heating treatments were administered to seeds in bulk: this meant that subsequent germination data for these two treatments were not independent. Cumulative germination in the laboratory was scored for up to 60 days. All three germination treatments, when administered singly, increased germination: smoke increased germination in all seven species, heat in four species, and scarification in a different four species. Pairwise combinations of the three treatments also increased germination in some species e.g. smoke and heat treatments combined led to the highest germination observed in four species, and to increased germination compared with either treatment singly, in another species. Some treatment combinations appeared to interact: heat and scarification combined led to decreased germination in three species. The role of heat exposure and scarification in increasing germination was not due to the breaking of an impermeable seed coat: water uptake of both scarified and unscarified seeds was rapid and equal (after 24 h) in all seven species. The results indicate that seeds of the Grevillea species investigated are responsive to one or more fire-related germination cues.

How does fertility of the substrate affect intraspecific competition? Evidence and synthesis from self-thinning

When dense populations of even-aged plant monocultures are subject to intense competition, mortality can occur in a process known as self-thinning, in which changes in biomass are accompanied by decreases in density. On a plot of log biomass versus log density, self-thinning populations show a linear relationship called the self-thinning line. Variations in the fertility level of the substrate are known to affect self-thinning in a number of ways. Populations from substrates with different fertility levels have been observed to self-thin along the same line, or along different lines. A review of several experiments using the one species grown at different fertility levels was undertaken to look for any mechanisms that might account for the different patterns observed. It was postulated that the critical difference between whether populations followed a common or different line was the way in which competition developed in the stands as biomass accumulated. For the common-line pattern, data on the canopy volume required to support a given biomass showed that biomass packing did not differ between fertility levels, supporting the model of a common competitive mechanism operating at all fertility levels. When different lines were observed, the development of competition differed as plants increased in size and biomass accumulated at each fertility level. Over the upper range of fertility levels, biomass packing values per plant increased as fertility declined and the position of self-thinning lines followed predictions from biomass packing data. At the low end of the fertility scale, biomass packing values still decreased with fertility level, but the position of self-thinning lines was not linked to the biomass packing of individual plants: root interactions were presumed to dominate competition and the trajectory of self-thinning lines.

Self-thinning lines differ with fertility level

The effect of variations in fertility level of the substrate on the self-thinning lines followed by populations of Ocimum basilicum L. was investigated experimentally by establishing populations over a range of densities at two fertility levels. Populations from each fertility level followed different self-thinning lines for shoot biomass. Self-thinning began at a lower biomass in populations grown at the higher fertility level; the subsequent slope of the thinning line was −0.5 for these stands on a log shoot biomass versus log density plot. The slope of the self-thinning line was flatter (−0.29) at the lower fertility level. Fitting the self-thinning line by the Structural Relationship rather than the Major Axis made little difference to line estimates. Biomass packing differed with fertility level, with plants from the higher fertility stands requiring more canopy volume for given shoot biomass than plants from lower fertility levels. Biologically, this would mean shoot competition intensified more rapidly at the higher fertility level as biomass accumulated in stands. The difference in slope between fertility levels was associated with changes above- and belowground. The radial extension of the canopy versus shoot mass relationships of individual plants differed with fertility level. Plants at the lower fertility level allocated more biomass to root growth, and had less leaf area per unit root length. The differences in slope of the self-thinning lines may have been because of differences in the radial extension of the canopy versus shoot mass relationships of individual plants at each fertility level, and/or to an increase in root competition at the lower fertility level.

Carnivory and nitrogen supply affect the growth of thebladderwort Utricularia uliginosa

A glasshouse experiment was designed to determine whether the terrestrial bladderwort species Utricularia uliginosa Vahl. gained any growth advantage from carnivory at different levels of nitrogen enrichment of the substrate. Three organism treatments were used: a control, a protist-only treatment (Euglena spp.) and a protist + meiofauna treatment (Euglena spp. plus nematodes, copepods, ostracods, cladocerans and Acarina). The organism treatments were factorially combined with three nitrogen levels (N0, N1, N2). Populations of Euglena were successfully established in the substrate of pots in the protist, and meiofauna + protist treatments, and were found in the traps of plants growing in these treatments. Populations of the meiofauna from the inoculating cultures were successfully established in the meiofauna + protist treatment, and were trapped by plants in these treatments. A background contamination of the control and protist-only treatments by Acarina occurred over the course of the experiment; however, numbers of Acarina in these treatments were at least an order of magnitude lower than numbers of meiofauna in the meiofauna + protist treatment. Non-acarinid meiofauna were restricted solely to the meiofauna + protist treatment. Organism treatment interacted significantly with the nitrogen level of the substrate to affect growth of Utricularia. Plants trapping Euglena (+ Acarina) had significantly less dry weight than control plants at the N0 base level of nitrogen; this negative effect of trapping Euglena on plant growth disappeared at the two higher nitrogen levels. The dry weight of plants trapping the full range of meiofauna at the N0 level was comparable with control plants. Plants trapping the full range of meiofauna + Euglena at the N1 level had significantly more dry weight than plants trapping Euglena only, and the highest dry weight of any treatment; the benefit of trapping the full range of meiofauna + Euglena was non-significant at the N2 level. Dry weight of plants was not significantly affected by nitrogen in any organism treatment. These results suggest that carnivory was overcoming a nitrogen deficiency induced by Euglena at the N0 level, but was overcoming deficiencies of other nutrients at the N1 level.

Stem, branch and leaf biomass-density relationships in forest communities

Defining and quantifying biomass–density relationships in dense plant stands has been a long-standing issue in both theoretical and empirical studies. Most existing/traditional studies focus on whole plant individuals, without considering different plant components (e.g., stem, branch and leaf). However, the analysis of biomass–density relationships for different plant parts is linked to those for whole plants, and thus important for understanding plant strategies for utilizing resources and community dynamics. In our study, we investigated standing stem (MS), branch (MB) and leaf (ML) biomass–density relationships, across a range of forest communities in China. The results showed that there was no constant predicted value (e.g., −1/2 or −1/3 for MS; −1/2, −1/3 or 0 for MB and ML) that can describe all the relationships, and that the scaling exponents for stem, branch and leaf biomass varied across different forest types. In particular, standing leaf biomass (leaf biomass per unit area) was not constant in these forest communities. Furthermore, stem biomass–density lines were steeper than corresponding branch and leaf lines across most of these forest communities.

Seed-coat Dormancy in Grevillea linearifolia: Little Change in Permeability to an Apoplastic Tracer after Treatment with Smoke and Heat

BACKGROUND AND AIMS Seeds of Grevillea linearifolia germinate following fire, and have seed-coat dormancy broken by smoke and heat shock. Smoke breaks seed coat dormancy in Emmenanthe penduliflora by altering the permeability of the seed coat to an internal germination inhibitor, which subsequently escapes. This model was tested for in G. linearifolia by investigating the permeability of the seed coat to diffusion of high-molecular-weight compounds, and whether this changed after exposure to fire cues. METHODS Germination response of the seeds to heat shock, smoke or heat + smoke was tested. Penetration of Lucifer Yellow dye into intact seeds was examined after 24 and 48 h of exposure, and penetration of the dye from the inside of the seed coat outwards was examined after 24 h. Histochemical staining with Nile Red and Acridine Orange was used to locate cuticles, suberin and lignin. KEY RESULTS Twenty-three per cent of untreated seeds germinated; heat shock and smoke increased germination additively up to approx. 80 % for both cues combined. Lucifer Yellow did not penetrate fully through the seed coat of untreated seeds, whether diffusing inwards or outwards. Three barriers to diffusion were identified. Treatment with heat or smoke slightly increased penetration of the dye, but did not completely remove the barriers. Suberin was identified in secondary walls of exotestal and mesotestal cells, and was absent from primary cell walls. Movement of Lucifer Yellow occurred through the middle lamella and primary cell wall of suberized cells; movement of the dye was impeded where suberin was absent. CONCLUSIONS Fire cues did not significantly decrease barriers to diffusion of high-molecular-weight compounds in the seed coat of Grevillea, and must be breaking dormancy by another mechanism.

Population structure, seed loads and flowering phenology in three common (Melaleuca styphelioides, M. thymifolia, M. nodosa) and one rare (M. deanei) Melaleuca (Myrtaceae) species of the Sydney region

Melaleuca deanei F.Muell. is listed under state and national protection legislation occurring as small, disjunct populations restricted to the Central Coast botanical district of New South Wales. This paper reports on the population structure, reproductive output and phenology of large and small field populations of M. deanei, compared with three common congeneric species in the area, namely, Melaleuca nodosa (Sol. ex Gaertn.) Sm., Melaleuca thymifolia Sm. and Melaleuca styphelioides Sm. Both M. deanei and M. styphelioides had few seedlings per population at the sites studied. Results indicated that seedling recruitment is rare and would appear to require specific conditions. In addition, M. deanei had a low incidence of flowering within the small populations, significantly fewer fruiting plants per population and significantly lower numbers of viable seeds per square metre, most likely compounding its limited recruitment. Flowering, when it occurred in M. deanei, was from mid to late October through to late November–early December with increased flowering in response to fire and along road edges. Further study is needed to determine reproductive success at the individual-plant level, the breeding system and recruitment requirements of these species.

Relationships between time since fire and honeyeater abundance in montane heathland

Abstract Montane heath communities in south-eastern Australia provide a valuable nectar resource for honeyeaters (Meliphagidae) but are subject to variable fire regimes that may influence the production of this resource and the abundance of honeyeaters. Little is known about changes in honeyeater communities through time after fire. We examined seasonal relationships between characteristics of the honeyeater community and time since fire in Blue Mountains Heath using regression analysis. Positive trends were expected, with possible eventual declines owing to senescence of the vegetation community. Twelve sites ranging from 2 to 39 years since fire were sampled in late winter-spring (cool season) and in late summer (warm season). In the warm season, abundance of all honeyeaters and abundance of New Holland Honeyeaters showed saturation relationships with time since fire. A steep rate of increase in abundance reached a plateau 10 years after fire for all honeyeaters and New Holland Honeyeaters. In the cool season, New Holland Honeyeater abundance was positively influenced by time since fire. This study showed that honeyeaters used heath across a broad range of post-fire intervals >10 years and there was no evidence of a decline as the heath matured (to 39 years post-fire). Our research has contributed to the understanding of the links between fire ecology and avian habitat use, which in turn provides a scientific basis for ecologically sustainable fire management of natural areas.

Comparison of biomass removal, nutrient manipulation and native seed addition to restore the ground layer of a degraded grassy woodland

This study reports on a trial of methods to overcome barriers to restoration of degraded Cumberland Plain woodland. Soil scalping was compared with fire or slashing to remove existing canopy. Fire and slashing were combined with soil carbon addition at two levels, to reduce soil nitrate. Native seed was added to overcome a lack of native propagules. Treatments, applied to 2 × 2 m plots, consisted of a control; scalped; fire and slash treatments without carbon addition; fire and slash treatments at the low and the high carbon addition levels; and a further fire and a slash treatment at the low carbon level with no native seed added, to give 10 treatments in total. Scalping eliminated the existing canopy and reduced weed seed and bud banks allowing native species (and some colonising exotics) to establish by 33 months. Rapid re-growth in the fire treatment resulted in plant canopy abundance returning to control levels by 12 months, and native species richness on burnt plots remained similar to the control. Canopy abundance in the slash treatment remained lower than in the controls for 20 months, and native species richness increased by then. Carbon addition reduced canopy re-growth in both fire and slash treatments: in the fire-low carbon plots with added native seed, native species richness was double that of the controls by 20 months. This increase did not occur on slash-low carbon plots. The highest level of carbon addition had negative effects on plant growth and survival, resulting in the lowest native species richness. The non-scalping treatments had little effect on exotic species richness in the absence of carbon or small negative effects if combined with carbon.

Plant-level fecundity and andromonoecy in three common (Melaleuca styphelioides, M. thymifolia, M. nodosa) and one rare (M. deanei) Melaleuca (Myrtaceae) species of the Sydney region

This paper reports plant-level measures of reproductive performance from large and small field populations of the vulnerable species Melaleuca deanei F.Meull. compared with three common species within the genus: Melaleuca nodosa (Sol. ex Gaertn.) Sm., Melaleuca thymifolia Sm. and Melaleuca styphelioides Sm. Measures reported include average seed numbers per capsule and per plant; average fruit loads per plant; the proportion of buds that become flowers and thence fruits (fruit set); the number of ovules per flower and seed : ovule ratios (seed set); and floral morphologies. Results indicate that when M. deanei flowers, bud to flower ratios and flower to fruit ratios are not reduced compared with the congeners or between large and small populations. Seed loads per plant held by fruiting plants were comparable between the species and between large and small populations of M. deanei. Seed to ovule ratios were in the order of 5–9% in M. nodosa; 5–7% in M. styphelioides; 7–10% in M. deanei; and 12–15% in M. thymifolia. Larval herbivory of ovules was recorded in M. nodosa and pre-dispersal seed predation noted in M. thymifolia. Andromonoecy was recorded in two of the species at rates of 0.9–3.1% (M. deanei) and 2.9–7% (M. thymifolia). Results suggest that seed production within smaller populations of M. deanei is poor because of a low frequency of flowering and a low proportion of flowering plants per population, rather than plant-level pollination, fruit- or seed-set barriers. Further study is needed to determine the triggers to flowering, the breeding systems, the extent of clonality and the germination and establishment requirements in these species.