Byron B. Lamont

The worldwide leaf economics spectrum

Bringing together leaf trait data spanning 2,548 species and 175 sites we describe, for the first time at global scale, a universal spectrum of leaf economics consisting of key chemical, structural and physiological properties. The spectrum runs from quick to slow return on investments of nutrients and dry mass in leaves, and operates largely independently of growth form, plant functional type or biome. Categories along the spectrum would, in general, describe leaf economic variation at the global scale better than plant functional types, because functional types overlap substantially in their leaf traits. Overall, modulation of leaf traits and trait relationships by climate is surprisingly modest, although some striking and significant patterns can be seen. Reliable quantification of the leaf economics spectrum and its interaction with climate will prove valuable for modelling nutrient fluxes and vegetation boundaries under changing land-use and climate.

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.

Leaf specific mass confounds leaf density and thickness

SummaryWe explored the relationship between leaf specific mass (LSM) and its two components, leaf density and thickness. These were assessed on the leaves of (a) the moderately sclerophyllous tree Arbutus menziesii distributed along a natural nutrient/moisture gradient in California, (b) eight sclerophyllous shrub species on four substrates in south-western Australia, and (c) seedlings of two morphologically contrasting Hakea species grown under varying soil nutrient, moisture and light regimes in a glasshouse experiment. Leaf area, mass, LSM, density and thickness varied greatly between leaves on the same plant, different species, and with different nutrient, moisture and light regimes. In some cases, variations in LSM were due to changes in leaf density in particular or thickness or both, while in others, density and thickness varied without a net effect on LSM. At lower nutrient or moisture availabilities or at higher light irradiances, leaves tended to be smaller, with higher LSM, density and thickness. Under increased stress, the thickness (diameter) of needle leaves decreased despite an increase in LSM. We concluded that, while LSM is a useful measure of sclerophylly, its separation into leaf density and thickness may be more appropriate as they often vary independently and appear to be more responsive to environmental gradients than LSM.

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.

Population fragmentation may reduce fertility to zero in Banksia goodii — a demonstration of the Allee effect

All individuals of all known populations of Banksia goodii were assessed for seed production. Small populations produced no or only a few seeds per unit canopy area. Effects of population size on seed production per unit area and seed production per plant were present over the whole range of population sizes, indicating that even in large populations seed production may still not be at its maximum. Resource differences could not explain this disproportionate decrease in seed production with decline in population size, because there were no differences in soil properties and understorey or overstorey cover between the small and large populations. Although plants in small and large populations were similar in size, seed production per plant was much lower in small populations. This was not because plants in small populations produced fewer cones but because the fraction of these cones that was fertile was much lower. Five of the nine smallest populations (<200 m2) produced no fertile cones over the last 10 years. The number of seeds per fertile cone did not depend on population size. The results are discussed in relation to pollination biology.

Mechanisms for enhancing nutrient uptake in plants, with particular reference to mediterranean South Africa and Western Australia

The major constraints to nutrient uptake by vascular plants in mediterranean South Africa and Western Australia are: very infertile soils, relatively low temperatures when water availability is high, and hot, dry summers. These constraints are partly overcome through increased efficiency of uptake, tapping novel sources of nutrients, and prolonging water uptake. Absorptive area per unit “cost” may be enlarged directly through increased fineness of the root system and proliferation of long root hairs. This reaches its greatest development in the root clusters of the Proteaceae (proteoid roots), Restionaceae (“capillaroid” roots) and Cyperaceae (dauciform roots). Absorptive area is increased indirectly through fungal hyphae which extend from hairless rootlets into the soil. Two major groups can be recognised: general (VA mycorrhizas) and host-specific (ericoid, orchid and sheathing mycorrhizas). Mycorrhizas are the most widespread specialised modes of nutrition and are probably universal in such major taxa here asPodocarpus, Acacia, Fabaceae, Poaceae, Asteraceae, Rutaceae, terrestrial orchids, Ericales and Myrtaceae. General mycorrhizas are the least drought-adapted of mechanisms for maximising absorptive area. All have been implicated in enhancing P uptake through increasing access to inorganic P, solubilisation and shortening the diffusion path. However, selective uptake of other nutrients, especially N, by host-specific mycorrhizas may be equally important.Included under novel sources of nutrients are free N2 (utilised by N2-fixing nodules), small-animal prey (carnivorous leaves) and persistent leaf bases (aerial roots ofKingia australis). Both legume and non-legume N2-fixing species are well-represented in these two regions, with stands of individual species in southwestern Australia estimated to contribute 2–19 kg N/ha/yr to the ecosystem. Free nitrogen fixation requires additional nutrients, especially Mo and Co, but is enhanced following fires and by supplementary uptake mechanisms, especially VA mycorrhizas. Southwestern Australia is particularly rich in carnivorous species. Nitrogen, P, K and S are important nutrients absorbed, with digestion aided by enzymes provided by bacteria and the glands. Parasitic plants both tap novel sources of nutrients and capitalise on any efficient water and nutrient uptake mechanisms of the hosts. Root parasites are better represented than stem parasites in mediterranean South Africa and Western Australia. Phosphorus and K in particular are absorbed preferentially by the haustoria, but much remains to be known about their modes of operation.Maximum activity of all uptake mechanisms, except those attached to some deep-rooted plants, is restricted to winter-spring. Most new seasons’s rootlets and specialised roots are confined to the uppermost 15 cm of soil, especially in or near the decomposing litter zone. Nutrient uptake is further enhanced by the tendency for the rootlets to cluster, trapping water by capillary action and prolonging nutrient release. As an early product of decomposition, N tends to be available as NH4 (rather than NO3) and it is absorbed preferentially by almost all specialised modes of nutrition. Microorganisms are required in the formation and/or functioning of all these structures, except haustoria. Uptake mechanisms which are optional to the plant reach their peak contribution to the root system at soil nutrient levels well below those required for greatest plant growth, when they may be absent altogether. It is only over the narrow range of nutrient availability, where shoot content of a nutrient is greater in the presence of the mechanism than in its absence (other factors remaining constant), that specialised modes can be termed nutrient-uptake “strategies.”For all specialised modes of nutrition, the component genera are better represented in these two regions than in the surrounding more fertile, arid to subtropical regions of much greater area. Endemism of species with each mode exceeds that for the two floras overall (75%). This is taken as preliminary evidence that specialised modes of nutrition are best represented in nutrient-poor soils. While they serve to limit nutrient loss from the ecosystem, their proliferation is therefore not necessarily a response to increasing “leaks” in the system.A hierarchical scheme of the functional/structural relationships between the various mechanisms is presented, starting with the rootless, VA-mycorrhizal plant as the most primitive condition. Taxa with many of the specialised modes of nutrition at present in southwestern South Africa and Western Australia have been evident in the pollen record since the early Tertiary Period. The absence of ectomycorrhizal forests in mediterranean South Africa, in marked contrast to Western Australia, can be traced to differences in their paleohistory. In both regions, the combination of fluctuating, but essentially diminishing, nutrient and water availability that began with the first mediterranean climate < 5 million years ago resulted in decimation of the less-tolerant rainforest ancestors on the one hand, and remarkable rates of speciation of the pre-adapted sclerophyll nucleus on the other.AbstraktDie Haupthindernisse der Nährstoffaufnahme der Kormophyten des Mittelmeerklimas Südafrikas und Westaustraliens sind sehr nahrungsarme Böden, relativ niedrige Temperaturen, wenn genügend Bodenwasser zur Verfügung steht und heisse, trockene Sommer. Diese Hindernisse werden zum Teil durch erhöhte Leistungsfähigkeit von Nährstoffaufnahme, Anzapfung verborgener Quellen von Nährstoffen und Erhöhung und Verlängerung der Wasseraufnahme überwunden. Die Absorptionsfläche kann direkt durch die Feinheit des Wurzelsystems und die Entwicklung langer Wurzelhaare vergrössert werden. Diese Situation ist am besten durch die Wurzelbüschel der Familie Proteaceae (proteoid Wurzeln), die Kapillarwurzeln der Familie Restionaceae und die dauciform Wurzeln der Familie Cyperaceae repräsentiert. Indirekte Erhöhung der Absorptionsfläche ist durch Pilzfäden, die sich von haarlosen Wurzeln im Boden ausbreiten, gewährleistet. Hierbei können zwei Hauptgruppen beobachtet werden: allgemeine (VA Mykorrhizen) und wirt-spezifische (Ericales-, Orchideen- und Hüllmykorrhizen). Mykorrhizen sind die am weitesten verbreiteten, spezialisierten Arten erhöhter Nährstoffaufnahme und sind wahrscheinlich universal inPodocarpus, Acacia, Fabaceae, Poaceae, Asteraceae, Rutaceae, Land Orchideen, Ericales und Myrtaceae. Der Nährstoffaufnahmemechanismus der VA Mykorrhyzen ist der am wenigsten trockenresistente. Alle Mykorrhyzen haben die Fähigkeit entwickelt, grössere Mengen von Phosphor durch vergrösserten Zugang, erhöhte Auflösung und Verkürzung des Aufnahmeweges von inorganischem Phosphor aufzunehmen. Im Falle der wirt-spezifischen Mykorrhyzen ist jedoch bevorzugte Aufnahme anderer Nährstoffe, vor allem Stickstoff, gleichgalls wichtig.Andere Quellen der Nährstoffaufnahme sind freier Stickstoff (ausgenutzt von N2-Bakterien in Wurzelknollen), Kleintierbeute in Blättern von Carnivoren und beharrende Blattbasen (Luftwurzeln vonKingia australis). Beide Formen von Legume- und Nichtlegume-Fixierung von N2 sind in diesen beiden Gegenden gut vertreten. In Südwestaustralien können einzelne Formen zwischen 2–19 kg N/ha/Jahr dem Ökosystem zuführen. N2-Fixierung benötigt zusätzliche Nährstoffe, vor allem Mo und Co. Es ist erhöht nach Busch (Wald) bränden und durch spezielle Ergänzungsaufnahme, vor allem in VA Mykorrhizen.Südwestaustralien im besonderen ist reich an Carnivoren Spezies: N, P, K und S sind wichtige Nährstoffe, die aufgenommen werden. Die Verdauung von Kleintieren wird durch Enzyme bewerkstelligt, die von Bakterien und Drüsen ausgeschieden werden. Parasitische Pflanzen zapfen neue Quellen von Nährstoffen an und werten auch alle Vorrichtungen des Wirtes in Bezug auf erhöhte Wasser- und Nährstoffaufnahme aus. In den Mittelmeerklimaten Südafrikas und Westaustraliens sind Wurzelparasiten häufiger als Stammparasiten. Besonders P und K werden von den Haustorien bevorzugt aufgenommen, jedoch mehr Forschung ist nötig, um den Aufnahmemechanismus zu verstehen.Mit der Ausnahme von tief-wurzelnden Pflanzen, optimale Nährstoffaufnahme ist auf die Winter-Frühlingszeit beschränkt. Dabei entwickeln sich Fein- und Spezialwurzeln innerhalb der oberen 15 cm-Bodenschicht, vorzugsweise innerhalb oder nahebei der Verwitterungszone des Laubes. Nährstoffaufnahme ist weiterhin durch Büschelformation der Feinwurzeln—wobei Wasser durch Kapillaraktion festgehalten und die Dauer der Nährstoffaufnahme verlängert wird—gesteigert. Ein zeitiges Produkt der Verwesung ist NH4, welches von bald allen spezialisierten Formen eher aufgenommen wird als NO3. Mit Ausnahme der Haustorien der Parasiten alle oben erwähnten Aufnahmeformen von Nährstoffen benötigen die Gegenwart von Mikroorganism. Nährstoffaufnahmemechanismen, die nicht unbedingt für die Pflanze notwendig sind, erreichen ihre grösste Verbreitung in der Bodenschicht, die weniger Nährstoffe enthält. In Bodenschichten mit einem hohen Nährstoffgehalt sind diese Mechanismen oft abwesend. In bezug auf die Verfügbarkeit von Nährstoffen ist es nur ein enger Bereich, in dem der Stengelnährstoffgehalt in der Gegenwart eines Spezialaufnahmemechanismus grösser ist als in der Abwesenheit eines solchen (wenn andere Faktoren gleich sind). In solchen Fällen kann man von Nährstoff aufnahme ‘Strategien’ sprechen.Alle Spezialnährstoffaufnahmemechanismen sind in beiden Gegenden wohlvertreten. Dies steht im Gegensatz zu den umgebenden fruchtbareren ariden und subtropischen Gegenden. Endemismus von Spezies mit diesen spezialen Aufnahmeeinrichtungen übertrifft die anderen Spezies (75%). Diese Feststellung mag wohl zeigen, dass Spezialformen der Nährstoffaufnahme am besten in nährstoffarmen Böden gedeihen. Während diese dazu dienen, den Verlust von Nährstoffen vom Ökosystem zu vermindern, ist ihre Verbreitung innerhalb des Ö

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.

Structure, ecology and physiology of root clusters - a review

Hairy rootlets, aggregated in longitudinal rows to form distinct clusters, are a major part of the root system in some species. These root clusters are almost universal (1600 species) in the family Proteaceae (proteoid roots), with fewer species in another seven families. There may be 10–1000 rootlets per cm length of parent root in 2–7 rows. Proteoid roots may increase the surface area by over 140× and soil volume explored by 300× that per length of an equivalent non-proteoid root. This greatly enhances exudation of carboxylates, phenolics and water, solubilisation of mineral and organic nutrients and uptake of inorganic nutrients, amino acids and water per unit root mass. Root cluster production peaks at soil nutrient levels (P, N, Fe) suboptimal for growth of the rest of the root system, and may cease when shoot mass peaks. As with other root types, root cluster production is controlled by the interplay between external and internal nutrient levels, and mediated by auxin and other hormones to which the process is particularly sensitive. Proteoid roots are concentrated in the humus-rich surface soil horizons, by 800× in Banksia scrub-heath. Compared with an equal mass of the B horizon, the A1 horizon has much higher levels of N, P, K and Ca in soils where species with proteoid root clusters are prominent, and the concentration of root clusters in that region ensures that uptake is optimal where supply is maximal. Both proteoid and non-proteoid root growth are promoted wherever the humus-rich layer is located in the soil profile, with 4× more proteoid roots per root length in Hakea laurina. Proteoid root production near the soil surface is favoured among hakeas, even in uniform soil, but to a lesser extent, while addition of dilute N or P solutions in split-root system studies promotes non-proteoid, but inhibits proteoid, root production. Local or seasonal applications of water to hakeas initiate non-proteoid, then proteoid, root production, while waterlogging inhibits non-proteoid, but promotes proteoid, root production near the soil surface. A chemical stimulus, probably of bacterial origin, may be associated with root cluster initiation, but most experiments have alternative interpretations. It is possible that the bacterial component of soil pockets rich in organic matter, rather than their nutrient component, could be responsible for the proliferation of proteoid roots there, but much more research on root cluster microbiology is needed.