Neal J. Enright

Canopy seed storage in woody plants

The retention of seeds in the plant canopy for one to 30 years or more is termed serotiny. It is well represented floristically and physiognomically in fire-prone, nutrient-poor and seasonally-dry sclerophyll vegetation in Australia, and to a lesser extent, South Africa followed by North America. While the seed-storing structures vary greatly, all will release their propagules following exposure to the heat of a fire (pyriscence). This phenomenon can be contrasted with seed release at maturity (non-storage) and soil storage of seeds. Although the evolutionary requirements for serotiny are clear, its adaptive advantages over other seed storage syndromes are largely the subject of conjecture in the absence of comparative experiments. Nine hypotheses were assessed here. Canopy storage maximises the quantity of seeds available for the next post-fire generation (unlike non-storage). Synchronized post-fire release satiates post-dispersal granivores (unlike non-storage and soil storage) and ensures arrival on a seed bed conducive to seedling recruitment (unlike non-storage). Canopy stored seeds are better insulated from the heat of a fire than non-stored, and probably soil-stored, seeds. Fluctuating annual seed crops, the opportunity for post-fire wind-dispersal, the possible advantages of dense stands of adults, short lifespan of the dispersed seeds and their optimal location in the soil for germination have only a limited role in explaining the advantages of serotiny. It is concluded that canopy seed storage is favoured in regions where seed production is restricted and inter-fire establishment and maturation are unlikely. In addition, these regions have a reliable seasonal rainfall and are subjected to intense fires at intervals occurring within the reproductive lifespan of the species.AbstraktDas Speichern von Samen für ein bis zu 30 Jahren im Blattwerk der Pflanzen bezeichnet man als ‘Serotiny.’ Es ist in zu Bränden neigenden, nährstoffarmen und periodisch trockenen Hartlaub-Vegetationen in Australien und in geringerem Ausmaß in Nordamerika und Südafrika häufig vertreten. Obwohl die Samenspeiche-rungsstrukturen stark variieren, werden alle ihre Brutkörper frei, nachdem sie der Hitze von Feuer ausgesetzt waren (pyrhiscene). Dieses Phänomen steht im Gegensatz zur Samenfreigabe bei Reife (Nicht-Lagerung) und Bodenlagerung. Obwohl die Entwicklungsvoraussetzungen für ‘Serotiny’ bekannt sind, ist die Überlegenheit gegenüber anderen Samenspeicherungserscheinungsbildern aufgrund der Anpassungsfä-higkeit, größtenteils Gegenstand von Vermutungen, da es vergleichende Experimente nicht gibt. Neun Hypothesen wurden hier bewertet. Blattwerkspeicherung maximiert die Menge des zur Verfügung stehenden Samens für die nächste Generation nach einem Feuer (im Gegensatz zur Nicht-Lagerung). Gleichzeitige Abgabe nach einem Feuer übersättigt die Körnerfresser (im Gegensatz zur Nicht-Lagerung und Bodenlagerung) und sichert so ein Auftreften auf dem Saatbeet, dieses ist für die Sämlingverstärkung von Nutzen. Samen welche im Blattwerk gelagert sind, sind besser gegen die Hitze des Feuers geschützt als nichtgespeicherte Samen und wahrscheinlich auch als bodengelagerte Samen. Schwankende jährliche Samenausbeute, die größere Möglichkeit für Ausbreitung durch den Wind, die möglichen Vorteile durch dichteres Zusammenstehen von älteren Pflanzen, kurze Lebensspanne von verstreuten Samen und die für die Keimung optimale Lage im Boden spielen nur eine begrenzte Rolle in der Erklärung der Vorteile der ‘Serotiny’. Es wird daher geschlossen, daß Blattwerksamenspeicherung in Regionen unwahrscheinlicher Zwischenfeuer-Etablierung und Reifung bevorzugt wird. Weiterhin haben diese Regionen einen verläßlichen saisonalen Regenfall und sind Gegenstand ausgedehnter Brände, die in Intervallen innerhalb der Fortpflanzungslebensspanne der Spezies auftreten.

A comparison of methods for the statistical analysis of spatial point patterns in plant ecology

We describe a range of methods for the description and analysis of spatial point patterns in plant ecology. The conceptual basis of the methods is presented, and specific tests are compared, with the goal of providing guidelines concerning their appropriate selection and use. Simulated and real data sets are used to explore the ability of these methods to identify different components of spatial pattern (e.g. departure from randomness, regularity vs. aggregation, scale and strength of pattern). First-order tests suffer from their inability to characterise pattern at distances beyond those at which local interactions (i.e. nearest neighbours) occur. Nevertheless, the tests explored (first-order nearest neighbour, Diggle’s G and F) are useful first steps in analysing spatial point patterns, and all seem capable of accurately describing patterns at these (shorter) distances. Among second-order tests, a density-corrected form of the neighbourhood density function (NDF), a non-cumulative analogue of the commonly used Ripley’s K-function, most informatively characterised spatial patterns at a range of distances for both univariate and bivariate analyses. Although Ripley’s K is more commonly used, it can give very different results to the NDF because of its cumulative nature. A modified form of the K-function suitable for inhomogeneous point patterns is discussed. We also explore the use of local and spatially-explicit methods for point pattern analysis. Local methods are powerful in that they allow variations from global averages to be detected and potentially provide a link to recent spatial ecological theory by taking the ‚plant’s-eye view’. We conclude by discussing the problems of linking spatial pattern with ecological process using three case studies, and consider some ways that this issue might be addressed.

Resprouting as a key functional trait: how buds, protection and resources drive persistence after fire

Resprouting as a response to disturbance is now widely recognized as a key functional trait among woody plants and as the basis for the persistence niche. However, the underlying mechanisms that define resprouting responses to disturbance are poorly conceptualized. Resprouting ability is constrained by the interaction of the disturbance regime that depletes the buds and resources needed to fund resprouting, and the environment that drives growth and resource allocation. We develop a buds-protection-resources (BPR) framework for understanding resprouting in fire-prone ecosystems, based on bud bank location, bud protection, and how buds are resourced. Using this framework we go beyond earlier emphases on basal resprouting and highlight the importance of apical, epicormic and below-ground resprouting to the persistence niche. The BPR framework provides insights into: resprouting typologies that include both fire resisters (i.e. survive fire but do not resprout) and fire resprouters; the methods by which buds escape fire effects, such as thick bark; and the predictability of community assembly of resprouting types in relation to site productivity, disturbance regime and competition. Furthermore, predicting the consequences of global change is enhanced by the BPR framework because it potentially forecasts the retention or loss of above-ground biomass.

Seed banks, fire season, safe sites and seedling recruitment in five co-occurring Banksia species

Quantity of seed stored in the canopy of 5 co-occurring Banksia species varied by nearly 2 orders of magnitude. The 3 species which resprout vegetatively after fire, produced less seeds and retained smaller seed banks than 2 non-sprouting species. Contiguous patches of scrub-heath were burned in spring and autumn. The spring fire was cooler, and both sets of seed released subsequently did not germinate until the following (common) winter. Rate of seed release was higher after the autumn fire. The non-sprouting species released more seeds after each fire and yielded more seedlings per parent than the resprouting species. Percentage (field) germination of the non-sprouters was not consistently different from that of the resprouters. Seeds exposed on the soil surface during summer soon lost viability compared with buried seeds. The large number of seedlings established up to 8 months after the spring fire resulted from many seeds escaping exposure by dispersal into litter-covered safe" sites. Although litter microsites covered only 30% of the ground surface after the spring burn, they accounted for 80% of seedlings both before and after the summer drought. Litter microsites covered only 14% of the autumn-burned site but accounted for 60% of seedlings before summer and 40% after summer, suggesting density-dependent thinning of seedlings. The autumn-burned site supported more than twice the density of seedlings than the spring-burned site by the end of the 1st winter. Summer mortality of seedlings was 32% in the spring-burned site and 65% in the autumn-burned site, equalizing the seedling:parent ratio at both sites. Net recruitment of 1-yr-old seedlings varied from 0 for 2 resprouters to >100 per parent for a non-sprouter.

POST-FIRE LITTER MICROSITES: SAFE FOR SEEDS, UNSAFE FOR SEEDLINGS'

We explore the effect of post—fire microsites on seed and seedling distribution and hence their potential role in community restoration. A summer wildfire and control burn in a sclerophyll shrubland in mediterranean Australia produced mosaics of physically and chemically contrasting microsites of litter and sand. Most seeds (>75%) of all species released from the burnt canopies fell, or were redispersed by wind, into the litter patches after both fires. Data on microsite characteristics and wind exposure (fire intensity), height of fruits, time of release, and seed properties were required to interpret relative distribution between the litter and sand patches. Seeds remained equally viable (up to 100%) over summer—autumn in the litter and sand and had equally high rates and levels (up to 100%) of subsequent winter germination. However, seedlings were 2—3 times less likely to survive in the litter and survivors were 35% smaller than those in the sand by the end of the first summer. Banksia hookeriana was particularly vulnerable to microsite properties, whereas the needle—leaved Hakea polyathema showed only minor responses. Pre—summer thinning of seedlings in the litter increased survival of the remainder by 2 times and size of the survivors by 31%. The fire—sensitive, small—seeded B. hookeriana had 17 times more seeds in the backburn litter than the resprouting, larger—seeded B. attenuata, which more than compensated for its 3 times greater seedling mortality levels over the dry summer. Recruitment of species prone to density—dependent mortality in the litter was enhanced by the retention of some seeds in the sand where competition for water was minimal, as indicated by the 2.2 times greater stomal conductance of their seedlings in early summer.

Anthropogenic disturbance promotes hybridization between Banksia species by altering their biology

Abstract Putative hybrids between Banksia hookeriana and B. prionotes were identified among 12 of 106 populations of B. hookeriana located at or near anthropogenically disturbed sites, mainly roadways, but none in 156 undisturbed populations. Morphometrics and AFLP markers confirmed that a hybrid swarm existed in a selected disturbed habitat, whereas no intermediates were present where the two species co‐occurred in undisturbed vegetation. Individuals of both species in disturbed habitats at 12 sites were more vigorous, with greater size and more flower heads than their counterparts in undisturbed vegetation. These more fecund plants also showed a shift in season and duration of flowering. By promoting earlier flowering of B. hookeriana plants and prolonging flowering of B. prionotes, anthropogenic disturbance broke the phenological barrier between these two species. We conclude that anthropogenic disturbance promotes hybridization through increasing opportunities for gene flow by reducing interpopulation separation, increasing gamete production and, especially, promoting coflowering.

Long‐distance seed dispersal in a metapopulation of Banksia hookeriana inferred from a population allocation analysis of amplified fragment length polymorphism data

There is currently a poor understanding of the nature and extent of long‐distance seed dispersal, largely due to the inherent difficulty of detection. New statistical approaches and molecular markers offer the potential to accurately address this issue. A log‐likelihood population allocation test (aflpop) was applied to a plant metapopulation to characterize interpopulation seed dispersal. Banksia hookeriana is a fire‐killed shrub, restricted to sandy dune crests in fire‐prone shrublands of the Eneabba sandplain, southwest Australia. Population genetic variation was assessed for 221 individuals sampled from 21 adjacent dune‐crest populations of B. hookeriana using amplified fragment length polymorphism. Genetic diversity was high, with 175 of 183 (96%) amplified fragment length polymorphism markers polymorphic. Of the total genetic diversity, 8% was partitioned among populations by amova and FST. There was no relationship between genetic diversity within populations and population demographic parameters such as population size and sample size. A population allocation test on these data unambiguously assigned 177 of 221 (80.1%) individuals to a single population. Of these, 171 (77.4% of total) were assigned to the population from which they were sampled and 6 (2.7% of total) were assigned to a known population other than the one from which they were sampled. A further 9 (4.1% of total) were assigned to outside the sampled metapopulation area, and 35 individuals (15.8%) could not be assigned unambiguously to any particular population. These results suggest that both the extent [15 of 221 (6.8%) individuals originating from a population other than the one in which they occur] and distance (1.6 to > 2.5 km), of seed dispersal between dune‐crest populations is greater than expected from previous studies. The extent of long‐distance interpopulation seed dispersal observed provides a basis for explaining the survival of populations of the fire‐killed B. hookeriana in a landscape experiencing frequent fire, where local extinctions and recolonizations may be a regular occurrence.

Comparing plant life histories using elasticity analysis: the importance of life span and the number of life-cycle stages

AbstractRecent studies have used transition matrix elasticity analysis to investigate the relative role of survival (L), growth (G) and fecundity (F) in determining the estimated rate of population increase for perennial plants. The relative importance of these three variables has then been used as a framework for comparing patterns of plant life history in a triangular parameter space. Here we analyse the ways in which the number of life-cycle stages chosen to describe a species (transition matrix dimensionality) might influence the interpretation of such comparisons. Because transition matrix elements describing survival (“stasis”) and growth are not independent, the number of stages used to describe a species influences their relative contribution to the population growth rate. Reduction in the number of stages increases the apparent importance of stasis relative to growth, since each becomes broader and fewer individuals make the transition to the next stage per unit time period. Analysis of a test matrix for a hypothetical tree species divided into 4–32 life-cycle stages confirms this. If the number of stages were defined in relation to species longevity so that mean residence time in each stage were approximately constant, then the elasticity of G would reflect the importance of relative growth rate to λ. An alternative, and simpler, approach to ensure comparability of results between species may be to use the same number of stages regardless of species longevity. Published studies for both herbaceous and woody species have tended to use relatively few stages to describe life cycles (herbs: n=45,

Survival, growth and water relations of Banksia seedlings on a sand mine rehabilitation site and adjacent scrub-heath sites

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