Jeremy J. Midgley

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.

The Evolutionary Ecology of Sprouting in Woody Plants

Woody plants may be killed by severe disturbance or resprout from vegetative tissue. Sprouters can persist at a site through several generations of nonsprouters. Differences in sprouting behavior are therefore important for understanding vegetation dynamics, extinction risks, and woody plant management. Although sprouting appears not to be uniquely correlated with many other intrinsic attributes, such as specific leaf area or breeding systems, a clear correlate is reduced seedling aboveground growth rates from sprouters allocating more to belowground structures. Consequently, sprouters tend to have low seedling recruitment rates, and saplings take longer to reach maturity. Sprouters also tend to have lower seed output than nonsprouters, but comparative studies have seldom taken other trait differences such as plant size into account. Added to these trade‐offs between persistence and recruitment, sprouters are often multistemmed and shorter than related nonsprouters and may be outcompeted by them when disturbances are rare. Since sprouters tend to have long generation times, damped demographic trends, and gene flow across generations, it has been suggested that their speciation rates would be low. The available data, primarily from fire‐prone Gondwanan shrublands in South Africa, show no strong differences in speciation rates of related sprouters versus seeders. This indicates that ecological factors are important determinants of the evolution of fire life histories. Analysis of disturbance regimes indicates a fundamental ecological correlate: sprouters are favored where disturbance regimes are frequent and severe in comparison to regrowth rates. To bridge the gap between ecology and evolution, data are needed on the genetical differences between related species with contrasting sprouter life histories.

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.

Kill thy neighbour: an individulalistic argument for the evolution of flammability

The idea that flammability has evolved in many fire-prone communities has been criticised for being group-selectionist. However flammability may enhance inclusive fitness if the resulting fires kill neighbouring less flammable individuals and also open recruitment possibilities. We modelled the evolution of flammability using cellular automata to simulate neighbourhood effects of burning. For plants that survive fire only from unbumt canopies (non-sprouters), increased flammability would cause them to burn to death. Flammability traits can evolve in such species only if they result in fire preading to kill less flammable neighbours and only if they carry additional fitness benefits. In species that resprout from roots or stem bases after fire, flammability can evolve merely by increasing neighbour mortality but is more likely to do so if the associated traits had other benefits. Most flammability-enhancing traits, both structural and biochemical, are likely to have such additional benefits. We predict that flammability traits will be associated with dense populations where the effect on neighbours is most marked and suggest several tests. Fire has been a key, but neglected, evolutionary force. Alteration of the fire regime through the evolution of flammability, even in a single species contribution heavily to fuel loads, would result in the selective exclusion or admission of other species to an ecosystem depending on the compatibility of their pre-existing traits with fire

Savanna woody plant dynamics: the role of fire and herbivory, separately and synergistically

Althoughthedemographyofwoodyplantsinsavannashaslongbeenshowntobeduetomanyfactors,therestill is no consensus as to the relative importance of the top-down processes of fire and herbivory, nor on how fire and herbivory affect plant demography. We review the recent literature and suggest that further progress depends on the following: (i) a demographic framework with clear terminology and which focuses on recruitment, transitions and mortality, (ii) an understandingofmechanismsofhow fireactuallydamagesplantsandhowplantssurviveandout-growthisdamage,mainly throughheight,barkthicknessordiametergrowth,(iii)anunderstandingofhowlossesinbiomassduetoherbivorymayaffect plant demography and, (iv) a consideration of interactions between fire and herbivory. Our synthesis suggests (i) strong recruitment limitation as well as some evidence of transition limitation by both fire and herbivory, (ii) that in some cases herbivory alone, notably by elephants and impala, can be more significant than fire alone, on woody plant population size, (iii) that fire and herbivory together are a lethal combination for woody plants and, (iv) that differences in strategies and responses of savanna plants to fire and herbivory are poorly explored.

A synthesis of the demography of African acacias

Although acacias are ecologically and economically important, their demography is poorly known. In part this is because few field experiments have been undertaken. Also, a bewildering diversity of factors have been suggested to determine their demography. These factors include disease, fire, large and small browsers and grazers, climatic variation, competition with grass, seed predation and anthropogenic impacts. This synthesis concentrates on the life-history period from seed to adult plant and indicates that important hurdles have been demon- strated for some species in some habitats. Seed and seedling limitation have not been clearly demonstrated, nor has the value of dispersal been quantified. In con- trast, the impacts of herbivores and fire in affecting the escape or release of resprouts has been repeatedly demonstrated. Whether fire and herbivory merely slow down the rate of promotion through size-classes or actually prevent it, needs further work. We note that very little comparative work amongst Acacia species has taken place. Most studies have concentrated on single aspects of their life history (especially seed predation), and have not been analysed in relation to popu- lation growth. Few field experiments, especially concerning seedling biology, have been performed. We conclude with some suggestions of a framework for interpret- ing acacia demography.

When is an island not an island? Insular effects and their causes in fynbos shrublands

SummaryAccording to the equilibrium theory of island biogeography, insularisation will lead to species loss from habitat remnants. Extinctions will continue untill species number equilibrates at a level appropriate for the size and isolation of the island remnants. We tested whether insularisation leads to species loss by comparing plant species numbers on islands of fynbos shrublands surrounded by Afrotemperate evergreen forest with extensive “mainland” tracts of fynbos. Species area curves for islands and subsamples of mainland had significantly different slopes (zisland=0.43, zmainland=0.16). Small islands had the fewest species (less than one fifth) relative to mainland samples of similar size. The species area curves intersect at 590 ha so that reserve sizes of this order of magnitude are needed to avoid species losses relative to extensive areas of fynbos.We compared traits of species on islands and mainlands to determine processes most affected by insularisation. Island floras did not differ from the mainland in the mix of dispersal types, pollinator syndromes or proportion of dioecious species. Islands did have significantly fewer species of low stature and significantly more species that survive fire only as seed and not by resprouting. We infer that the main cause of species loss is change in disturbance frequency. Islands have fewer fires and lose species dependent on frequent fires. We predict that island effects could be reduced by judicious fire management of small reserves.

Fly pollination of Gorteria diffusa (Asteraceae), and a possible mimetic function for dark spots on the capitulum.

We investigated the functional significance of raised black spots on the ray florets of Gorteria diffusa (Asteraceae) in South Africa. Field observations showed that G. diffusa is pollinated by a small bee-fly, Megapalpus nitidus (Bombyliidae), which is strikingly similar to the raised spots that occur on some of the ray florets. Removal of the spots resulted in a significant decrease in the rate of fly visits to capitula, but did not significantly affect seed set. Replacement of the spots with simple ink spots also significantly reduced the rate of pollinator visits, suggesting that flies respond to details in the structure of the spots. Investigations using scanning electron microscopy showed that the spots of G. diffusa consist of a complex of different cell types. Differences in epidermal sculpturing may partly explain the UV reflectance pattern of these spots, which is similar to that of the flies. Male flies are strongly attracted to the spots, as well as to other flies sitting in the capitula, although female flies also visit the capitula. We conclude that the spots of G. diffusa mimic resting flies, thereby eliciting mate-seeking and aggregation responses in fly pollinators. Similar dark spots have evolved in unrelated South African Gazania, Dimorphotheca, and Pelargonium species pollinated by bee-flies.

Bark thickness determines fire resistance of selected tree species from fire-prone tropical savanna in north Australia

We investigated the fire resistance conferred by bark of seven common tree species in north Australian tropical savannas. We estimated bark thermal conductance and examined the relative importance of bark thickness, density and moisture content for protecting the cambium from lethal fire temperatures. Eucalypt and non-eucalypt species were contrasted, including the fire-sensitive conifer Callitris intratropica. Cambial temperature responses to bark surface heating were measured using a modified wick-fire technique, which simulated a heat pulse comparable to surface fires of moderate intensity. Bark thickness was a better predictor of resistance to cambial injury from fires than either bark moisture or density, accounting for 68% of the deviance in maximum temperature of the cambium. The duration of heating required to kill the cambium of a tree (τc) was directly proportional to bark thickness squared. Although species did not differ significantly in their bark thermal conductance (k), the thinner barked eucalypts nevertheless achieved similar or only slightly lower levels of fire resistance than much thicker barked non-eucalypts. Bark thickness alone cannot account for the latter and we suggest that lower bark moisture content among the eucalypts also contributes to their apparent fire resistance. Unique eucalypt meristem anatomy and epicormic structures, combined with their bark traits, probably facilitate resprouting after fire and ensure the dominance of eucalypts in fire-prone savannas. This study emphasises the need to take into account both the thermal properties of bark and the mechanism of bud protection in characterising the resprouting ability of savanna trees.

Leaf size and inflorescence size may be allometrically related traits

Summary“Corners rules” for plant form relate the degree of branching to branch diameter, and branch diameter to leaf or inflorescence size. We report the first interspecific test of these rules for inflorescence size and branch diameter. We derived a simple corollary of Corners rules; since leaf size and inflorescence size are both correlated to branch thickness, they may be correlated to each other. This corollary holds for Leucadendron and Protea (Proteaceae), and in certain other taxa in the Asteraceae, Bruniaceae and Pinaceae which also have leaves and reproductive structures on the same shoot. For such taxa this implies that selection for aspects of floral display (inflorescence size, pollination type) may also be expressed at the level of leaf size and vice versa. This has implications for many aspects of botany and also points to the importance of the co-ordinating role of plant architecture for aspects of plant form.