Non‐fire induced seed release ina weakly serotinous pine: climatic factors,maintenance costs or both?

The advantages of canopy seed retention (serotiny) for plants inhabiting fire‐prone ecosystems are well documented. However, very few species are completely serotinous and non‐fire induced opening of serotinous fruits is commonly observed (weak serotiny). Two non‐mutually exclusive causes are envisaged to contribute to this process: mechanical changes in serotinous fruits mediated by climatic conditions (e.g. drought) or the costs of maintenance for the plant of these long‐lasting structures. However, their relative contribution to the spontaneous opening of serotinous fruits remains elusive as well as the consequences for the build‐up of the canopy seed bank and inter‐individual differences in serotiny. In this study we monitored the dynamics of cone production and cone opening in the weakly serotinous Pinus halepensis for five years (2004–2008), including two severe drought episodes (2005, 2006). Drought decreased the production of conelets, increased the abortion of immature cones, reduced the seed quality in the cohorts of cones produced during these years, and increased the opening of serotinous cones. During the first drought episode, a higher proportion of serotinous cones opened in those pines bearing a larger crop of younger cones. This suggests that not only passive changes induced by drought but also competition among cones for resources (e.g. water) might be involved in this process. The opening of serotinous cones in pines bearing more cones made inter‐individual differences in the size of the canopy cone bank to narrow or even to reverse from 2004 to 2008. These results may help to understand the decrease in serotiny when pines grow and accumulate more cones and the large inter‐individual variability in the degree of serotiny observed in P. halepensis forests. In addition, the negative effects of drought episodes for the size of the canopy cone bank and the seeds contained can be an unexplored cause of post‐fire regeneration constraint.     

Fire temperatures and follicle-opening requirements in 10 Banksia species

Temperature requirements for follicle rupture were assessed in 10 Banksia species from south-western Australia. Species examined varied in degree of serotiny, persistence of dead florets, height, follicle size and number, and dependence on seed for regeneration. When treated for 2 min at temperatures over the range 100–500°C, follicles of some species ruptured at quite low temperatures (e.g. Banksia tricuspis with 50% opening at 145°C). Others required substantially higher temperatures (e.g. Banksia hookeriana with 50% opening at 390°C). There was a significant correlation between temperature requirements for follicle rupture and persistence of the highly combustible florets. However, there was no overall correlation between degree of serotiny and temperature requirements, nor with the other biological attributes assessed. Cone temperatures and follicle rupture in five co-occurring Banksia species were monitored for an experimental spring burn in Banksia- dominated shrubland. Temperatures recorded at follicle surfaces were higher than those needed for follicle rupture in the four species with persistent florets. Critical temperatures were rarely reached for the weakly serotinous Banksia menziesii which possesses bare cones and an open canopy.     

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.     

Anatomic basis and insulation of serotinous cones in Pinus halepensis Mill

Pinus halepensis Mill., a widespread, low elevation conifer common in Mediterranean Basin, shows a dual reproductive strategy: post-fire obligate seeder (from serotinous cones) and an early coloniser (from non-serotinous cones). Release of seeds encased in serotinous cones is induced either by fire (pyriscence, serotiny or bradychory) or by drying (xeriscence). Morphological differences in serotinous and non-serotinous cones in natural populations of P. halepensis in Southeastern Italy were analyzed. Relationships between tree size (diameter class) and serotiny were checked by counting and sampling serotinous and non-serotinous cones. The macro and microscopic characteristics that could affect cones’ opening were measured in sampled cones. Protection against high temperatures offered by wood scales was also evaluated by applying different temperatures and time exposures, and following the inner thermal raise. Results showed that non-serotinous cones had bigger resin ducts and more separate scales. Also it was highlighted that ovuliferous scales of serotinous cones were bigger and thicker. These scales had more lamellated (multilayered) sclereid cells, and were significantly thinner with a shorter lumen diameter. Continuous temperature-monitoring heat tests inside cones showed that temperatures close to the cone axis were rather low, so seed germination was not influenced. Results confirm that serotinous cones are more compact, rigid and consistent than non-serotinous cones. These characteristics explain the lower insulation, seed protection and the ease opening of non-serotinous cones as well. In conclusion, opening mechanism of pinecone scales under the effect of fire or dry conditions seem related to anatomic differences and it provides seeds with an efficient protection against heat.     

Enhanced seed defenses potentially relax selection by seed predators against serotiny in lodgepole pine

Serotiny, the retention of seeds in a canopy seed bank until high temperatures cause seeds to be released, is an important life history trait for many woody plants in fire‐prone habitats. Serotiny provides a competitive advantage after fire but increases vulnerability to predispersal seed predation, due to the seeds being retained in clusters in predictable locations for extended periods. This creates opposing selection pressures. Serotiny is favored in areas of high fire frequency, but is selected against by predispersal seed predators. However, predation also selects for cone traits associated with seed defense that could reduce predation on serotinous cones and thereby relax selection against serotiny. This helps explain the elevated defenses in highly serotinous species. However, whether such interactions drive variation in seed defenses within variably serotinous populations has been studied rarely. We investigated the effects of phenotypic selection exerted by red squirrel (Tamiasciurus hudsonicus) predation on Rocky Mountain lodgepole pine (Pinus contorta latifolia) seeds. Squirrels preferentially harvested cones with more and larger seeds, indicating a preference for a higher food reward. We found evidence for stronger selection on trees with serotinous cones, which presumably accounts for the elevated defenses of and lower predation on serotinous compared to non‐serotinous cones. Lower levels of predation on serotinous cones in turn lessen selection against serotiny by squirrels. This has important implications because the frequency of serotiny in lodgepole pine has profound consequences for post‐fire communities and ecosystems widespread in the Rocky Mountains.     

Increased canopy seed-storage in post-fire pine invaders suggests rapid selection mediated by fire

Exotic species storing seeds in the canopy (serotinous species) can experience a clear advantage in fire-prone communities that lack native taxa with such fire-resistant traits. In addition, selection in the new environment can potentially increase the frequency of fire-adapted characteristics such as serotiny. We studied the potential role of fire favoring the serotinous, non-native conifer Pinus radiata in NW Patagonia. We characterized the degree of serotiny (percentage of serotinous cones) and the size of the canopy seed bank in the unburned plantation and in stands of trees recruited after a fire 30 years ago as a proxy for invasion potential. Fire had a positive effect, increasing serotiny in post-fire P. radiata stands. Post-fire recruited cohorts showed higher serotiny levels and a larger canopy seed bank compared with plantations. Our study suggests that fire-linked traits like serotiny may be subjected to a rapid, fire-driven selection process in fire-adapted species such as P. radiata invading fire-prone ecosystems. Thus, increased serotiny can lead to higher postfire invasion densities, which in turn create a positive feedback loop in invaded areas under recurrent fires.     

Demography of a non-sprouting and resprouting Hakea species (Proteaceae) in fire-prone Eucalyptus woodlands of southeastern Australia in relation to stand age,drought and disease

Population size-structures, seed production, canopy seed storage (serotiny), and recruitment were investigated in relation to fire, drought and disease for a pair of co-occurring resprouting and non-sprouting shrub species from the genus Hakea (Proteaceae) in fire-prone Eucalyptus woodlands in western Victoria, Australia. The non-sprouter species, Hakea decurrens, showed faster height growth, higher seed production and higher seed viability than the resprouter, Hakea rostrata. Population size structures in stands up to 24 years since last fire showed no evidence of inter-fire recruitment for either species. Following a fire in 1990 in a mixed species stand 15–20 years old, the estimated number of viable seeds released from canopy-stored seed banks was approximately equal for both species. However, the rate of seedling establishment in the first year was about 10 times higher, and seedling suvivorship over the first 5 years was seven times higher, for the non-sprouter. Seedlings of Hakea decurrens and resprouts of Hakea rostrata began to produce seeds within three years of the last fire, while the few surviving seedlings of Hakea rostrata showed no evidence of reproductive maturity after six years. Inter-fire recruitment was recorded for the non-sprouter, Hakea decurrens, in the oldest stand (burned in 1967) between 24 and 28 years since last fire. This was associated with an increased rate of seed release from serotinous fruits due to the onset of high rates of adult plant mortality. High adult mortality and increased seed release correlated with increasing stand age, the occurrence of severe drought, and the likely presence of Phytophthora cinnamomi, a fungal pathogen which damages the root system, reducing water and nutrient uptake. There were no new recruits for the resprouter Hakea rostrata in this stand, but old plants continued to resprout from basal lignotubers and no mortality was observed. While recruitment of strongly serotinous shrub species is commonly described as being restricted to the immediate post-fire period, the present study illustrates that other events (e.g., senescence, drought, disease) can lead to recruitment of serotinous non-sprouters and may be important in the maintenance of populations during unusually long periods without fire.     

Canopy seed bank structure in relation to: fire,tree size and density

To assess the canopy seed bank structure of Pinus halepensis, we measured the level of serotiny and the seed bank size and density of trees in unburned stands and post-fire regenerated stands in Israel. We analysed the effects of tree size, tree density and fire history on the level of serotiny. The level of serotiny decreased with an increase in tree height. The high level of serotiny in short trees could be explained by selection to increase regeneration chances after burning at pre-mature age. Also, limitation of long-distance seed dispersal opportunities in short trees may favour high serotiny levels. The level of serotiny was higher in post-fire stands than in unburned stands, suggesting a fast selection for serotiny by fire. Unburned stands had a higher total stand seed density than post-fire regenerated stands, but the proportion of seeds in serotinous cones of the total stand seed density was higher in post-fire regenerated stands. The fact that P. halepensis bears simultaneously serotinous and non-serotinous cones reflects its dual strategy as both a post-fire obligate seeder, mainly from serotinous cones and an early coloniser during fire-free periods, mainly from non-serotinous cones. The relative investment in these strategies is dependent on fire history and varies with tree height. Furthermore, mature brown cones can contribute to post-fire regeneration in case of spring fires, and serotinous cones are known to open partially also in dry spell events. Thus, post-fire regeneration and invasion are strategies, which seem to complement each other.     

Fire, soil fertility and delayed seed release: a community analysis of the degree of serotiny

Delayed seed release (serotiny) is a convergent plant trait in fire-prone regions of the world but explaining the degree of serotiny has remained elusive because of the paucity of community data. Selective forces involving seed predators, fire and soil nutrients have been suggested as factors influencing serotiny. We tested whether protection of seeds and/or synchronized dispersal were associated with different levels of serotiny and if resprouting ability influences selection for strong serotiny. We compared the numbers and abundance of 146 woody species with delayed dispersal among five community types varying in combinations of fire severity, fire frequency, soil fertility and seed predators. The strength of the relationship between levels of serotiny and environmental factors was tested among community types ranging from rainforests to heathlands. Highest levels of serotiny were recorded in low nutrient shrublands with intermediate fire return intervals that burn at high severity, while the lowest were recorded in high nutrient, low flammability forests. Both protection of seeds and synchronized seed release were related to fire effects in nutrient-limited environments. Strong serotiny is prominent in species killed by fire whereas weak serotiny is more common in resprouting species. Recruitment failure in the inter-fire interval appears to drive selection for strong maternal care of seeds and synchronized seed dispersal in fire-prone environments. Weak serotiny is proposed as a bet-hedging strategy that relies on resprouting after fire for population persistence and higher probability of inter-fire recruitment. The spectrum of serotiny (weak to strong) in these communities is proposed to be driven by the interactive effect of both fire and soil nutrients on the selection for delayed seed dispersal.     

Post-floral perianth functionality: contribution of persistent sepals to seed development in Helleborus foetidus (Ranunculaceae)

The perianth persists after flowering in many plants, yet its post-floral functionality has been little investigated, and the few studies available provide ambiguous evidence. This paper tests the hypothesis that the green persistent sepals of the perennial herb, Helleborus foetidus, contribute to the plant's fitness by enhancing seed number and/or size. Post-floral contribution of the calyx to fruit and seed development was evaluated by manipulating sepal number and measuring the effect on follicle size, seed set, seeds per follicle, and mean seed mass. The allometric relationship between calyx size and follicle mass was examined for flower buds, open flowers, and immature fruits differing in follicle number. Calyx manipulation had no significant effect on follicle size, seed set, or number of seeds per follicle, but it did have a significant influence on the size of individual seeds. Calyx size and mean seed mass were positively, linearly related. The calyx mass/seed number ratio declined with increasing number of follicles per fruit. The persistent sepals of Helleborus do contribute resources to the development of seeds, although there is not evidence of post-floral allometric adjustment of the calyx mass/follicle number ratio that could compensate for variations in seed number per fruit.     

Herbivory, serotiny and seedling defence in Western Australian Proteaceae

We examined how acceptability characteristics displayed by 28-day-old seedlings of 12 species of Western Australian Proteaceae affect the likelihood of seedling herbivory in the field. The seedling attributes quantified were cotyledon phenolic, cyanide and nitrogen concentrations, and cotyledon area, thickness and specific leaf area. Only phenolic content was significantly correlated (negatively) with field rates of herbivore attack. This finding shows that the phenomenon of selective herbivore attack on seedlings may be influenced by a specific plant life-history trait, (in this case cotyledon phenolic concentration). In addition, we also studied the interaction between fire, serotiny and herbivory in matched burned and unburned plots. Although herbivore activity was greater in unburned plots, weakly serotinous species were as prone to defoliation as congeneric, strongly serotinous species, even though their seedlings recruit successfully in the absence of fire. This result suggests that seedlings of species able to establish between fires are not better defended against the higher levels of herbivory normally associated with unburned vegetation.     

Comments on relationships between native seed preferences of shrub-steppe granivores and seed nutritional characteristics

Summary Kelrick et al. (1986) argued that seed preferences of desert granivores are strongly influenced by soluble carbohydrate contents of native seed species. They assumed that bitterbrush (Purshia tridentata) seeds are eaten in their entirety by rodents, but in fact these granivores eat only embryos of bitterbrush seeds. Embryos have a much lower percentage of soluble carbohydrate than whole bitterbrush seeds, and the correlation between preference and soluble carbohydrate content of seeds for six native species becomes non-significant when embryo values are substituted for whole-seed values.     

Variation in serotiny of three Banksia species along a climatic gradient

The degree of serotiny (i.e. the proportion of follicles remaining closed in each year's crop of cones since the last fire) was measured in Bank-sia attenuata, B. menziesii and B. prionotes at five sites along a climatic gradient extending 500 km north of Perth, Western Australia. The decrease in annual rainfall and increase in average temperature along the gradient paralleled a decrease in plant height and an increase in the degree of serotiny of all species. Extreme serotiny was recorded in the scrub-heath at the xeric end of the gradient whereas two species were essentially non-serotinous in the low woodland at the most mesic site. It is concluded that degree of serotiny is related to the fire characteristics of the site which depend on plant height. In xeric scrub-heath, the entire canopies of the Banksia spp. are consumed by fire which promotes massive release of seed. This facilitates recruitment in an otherwise unpredictable and unreliable seedbed. In mesic woodland, where cones rarely come into contact With flames, seeds are released spontaneously and site conditions are more conducive to recruitment in the inter-fire period.     

Genome-wide association genetics of an adaptive trait in lodgepole pine

Pine cones that remain closed and retain seeds until fire causes the cones to open (cone serotiny) represent a key adaptive trait in a variety of pine species. In lodgepole pine, there is substantial geographical variation in serotiny across the Rocky Mountain region. This variation in serotiny has evolved as a result of geographically divergent selection, with consequences that extend to forest communities and ecosystems. An understanding of the genetic architecture of this trait is of interest owing to the wide-reaching ecological consequences of serotiny and also because of the repeated evolution of the trait across the genus. Here, we present and utilize an inexpensive and time-effective method for generating population genomic data. The method uses restriction enzymes and PCR amplification to generate a library of fragments that can be sequenced with a high level of multiplexing. We obtained data for more than 95,000 single nucleotide polymorphisms across 98 serotinous and nonserotinous lodgepole pines from three populations. We used a Bayesian generalized linear model (GLM) to test for an association between genotypic variation at these loci and serotiny. The probability of serotiny varied by genotype at 11 loci, and the association between genotype and serotiny at these loci was consistent in each of the three populations of pines. Genetic variation across these 11 loci explained 50% of the phenotypic variation in serotiny. Our results provide a first genome-wide association map of serotiny in pines and demonstrate an inexpensive and efficient method for generating population genomic data.     

Maintenance costs of serotiny do not explain weak serotiny

Considerable variation in the duration of serotiny exists among species of both Australian and South African Proteaceae. ‘Weak’ serotiny (pre‐fire loss after <3 years) could be dictated by the costs (water or carbon) of cone/fruit retention or by benefits accruing from pre‐fire seed establishment. We determined that cones/fruits of a range of Australian and south western Cape Proteaceae species (Leucadendron xanthoconus, Aulax umbellata, L. linifolium, L. gandogeri, Hakea drupacea, H. sericea) are not sealed dead wood, but that they continuously lose H₂O and CO₂. Water loss from cones/fruits was poorly controlled, occurring in both light and dark. The rates of both H₂O and CO₂ loss from mature cones/fruits were negatively correlated with the degree of serotiny (r² = 0.59 and 0.18, respectively, P < 0.001 both). However, the amounts of H₂O and CO₂ lost per weight were small relative to the fluxes from leaves (13–29% for H₂O and 3–10% for CO₂). The [N] and [P] in the cones/fruits and seeds was substantial. Despite 25% of N and 38% of P being recovered from the cones/fruits following maturation, the loss of the cones/fruits and seeds would still incur a substantial nutrient cost. The seed [P] was positively correlated with the degree of serotiny (r² = 0.24, P = 0.001). We suggest that maintenance costs (water and carbon) of serotiny, although exceeding those of soil stored seeds, are relatively low. The correlation between the degree of serotiny and seed [P] indicates that stronger serotiny is required, much like sclerophylly, for survival under low nutrient availability in frequently burnt vegetation.     

Evolution of serotiny in maritime pine (Pinus pinaster) in the light of increasing frequency of fires

Wildfire frequency and intensity in the Mediterranean region are predicted to increase with climate and anthropogenic changes in the following decades. Pines species often posses fire-embracing and fire-avoiding strategies that increase the probability of persistence and performance in fire-prone habitats. One such strategy is serotiny, i.e., the capacity to retain seeds in long-closed cones within the plant canopy; serotinous cones release seeds only when either a fire or a heat shock occurs. In this work, we used a simulation approach and Pinus pinaster populations as a model system to investigate how (i) an increased frequency of fire, (ii) genetic characteristics of serotiny, and (iii) observed differences in life histories interact to determine (a) risk of local population extinction and (b) temporal changes in the prevalence of serotiny in the modeled population. In addition, we tested whether the contemporary evolution of serotiny in the face of increased probability of occurrence of fires increased the probability of population persistence with respect to a scenario in which serotiny was not allowed to evolve. Our simulations showed that over the 300 years of simulated time, the evolution of serotiny did not substantially contribute to the persistence of populations. Extinction risk increased with the increasing probability of occurrence of fire and slightly decreased with (i) higher gene flow from outside the modeled population, and (ii) higher prevalence of serotiny at the beginning of the simulation. The prevalence of serotiny at the end of simulation time was difficult to predict and mostly driven by stochasticity.     

Long-term variation in seed mass and seed production in populations of Paris quadrifolia

Seed production of the perennial herb Paris quadrifolia L. (Liliaceae) was investigated in five populations in northern Poland. The long-term seed production per square metre differed significantly among populations and years. Moreover, throughout the 7-year study period, 30% of both whole ripe fruits and seeds alone were predated. Variation in seed mass per fruit in both space and time was significant. Throughout the 7-year study, nearly all the marked individuals produced fruits once every 2 years. The most frequent break between fruiting was 2 years and the longest was 5 years. Only the mean seed mass in fruits of the same individual varied significantly over subsequent years. In the five populations, the number of ovules, number of seeds in the fruit and seed mass varied significantly between populations. However seed/ovule ratio did not differ in fruits in the five populations. The seed mass/number trade-off in fruits was strongly partially correlated when the effect of total seed mass was considered. Breeding experiments suggest that P. quadrifolia has a substantial capacity for both inbreeding and outbreeding. There were no significant differences in the seed/ovule ratio, seed number or seed mass in fruits produced from bagged or control flowers. However, both seed/ovule ratio and number of seeds were significantly lower in fruits from emasculated flowers.     

Delayed seed dispersal in Pinus torreyana (Torrey pine)

Summary Torrey pine exhibits a pattern of seed release intermediate between an open and closed-cone species. It was found that the cones open at maturity, but that seed fall from some cones continues for up to 13 years. Approximately 77% of the total seed crop was present in age classes one or more years past the time of seed maturity.Serotiny is thought to evolve in response to fire, and partial serotiny at the population level has been attributed to circumstances in which the strong selective pressure of fire is relaxed. We hypothesize that delayed seed dispersal in Torrey pine is a stable trait which is the result of a pattern of large relatively rare crown fires separated by intervals longer than one generation. The reduced serotiny observed in other conifers on headlands and islands in California seems consistent with our hypothesis.     

Germination response to fire-related factors of seeds from non-serotinous and serotinous cones

Pinus halepensis, a Mediterranean pine tree, is a partially serotinous species: individual trees of this species carry both non-serotinous and serotinous cones. Serotinous cones open mainly after fire, whereas non-serotinous cones open in absence of fire. In this study we addressed the question, whether or not this cone response is linked with the germination response of seeds to fires. Two main factors associated with fire are heating of seeds and soil pH. A combination of high heat and high pH simulates a scenario with fire, whereas low heat and low pH simulates a scenario without fire. We assessed the separate and combined effects of heat and pH on the germination rate and the percentage of germination of seeds from non-serotinous cones and two age classes of serotinous cones of P. halepensis. Heat had no effect on the percentage of germination of seeds from any of the cone types, but did positively affect the germination rates of seeds from both age-classes of serotinous cones. High pH negatively affected the germination rate of seeds from all cone types as well as the percentage of germination of seeds from non-serotinous cones. The combinations of heat and pH had different effects on the three cone types: percentage of germination and rate of germination of seeds from non-serotinous cones was higher in the combination high heat-high pH than in the combination low heat-low pH. In the combination high heat-high pH, seeds from serotinous cones germinated better than seeds from non-serotinous cones. The different germination responses of seeds from non-serotinous and serotinous cones could not be attributed to differences in cone age. Our results indicate that the cone response is linked to the germination response of the seeds in P. halepensis, with seeds from serotinous cones being more tolerant to fire related factors. This revised version was published online in June 2006 with corrections to the Cover Date.     

Serotiny and seed germination in three threatened species of Mammillaria (Cactaceae)

Individuals of some species of Mammillaria (Cactaceae) store some seeds on the plant over periods exceeding 1 year (serotiny). We examined the phenomenon of serotiny and germination behaviour of three rare and endangered Mammillaria species that occur in central Mexico. The species with the highest seed retention was Mammillaria solisioides, whose individuals kept on average 24% of their total seed crop throughout their observable lifetime. Individuals of Mammillaria napina and Mammillaria hernandezii did not differ in their degree of seed retention (about 5%). In M. solisioides and M. hernandezii, seed germination declined significantly with seed age, whereas in M. napina germination increased slightly. In all three species, over 70% of retained seeds were still alive after 8 years. Increasing fractions of dormant seeds were observed with seed age in M. solisioides and M. hernandezii, whereas in M. napina this fraction followed the opposite trend. All three species showed strict light dependence for germination. Serotiny was positively correlated with the harshness of the environment when species and populations were assumed independent. However, these correlations were not significant at the 5% level when the degree of relatedness of species and populations was taken into account using phylogenetically independent constrasts. We hypothesise that serotiny in these species represents a mechanism by which they can cope with a harsh, unpredictable environment. To our knowledge, this is the first assessment of serotiny in cacti.