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

Abstract

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.

Genetic and environmental factors in the trade‐off between sexual and asexual reproduction of a rare clonal angiosperm

“…A demographic study confirmed no resource or pollen limitations on fruit set in populations of M. deanei when flowering occurred (Hewitt et al ). However, viable seed loads per unit area were significantly lower in small populations of less than 100 stems (Hewitt et al ), possibly explained by unsuccessful selfing.…”

“…When flowering did occur, it was restricted to small numbers of plants per population. Further, significantly lower proportions of plants flower and set fruit within smaller populations, resulting in significantly lower canopy seed loads per square metre in smaller vs. larger populations (Hewitt et al ). While flowering by M. deanei is rare, it is noted to be absent from sites where fire is long absent and most intense across sites in the first two years following fire, or, in those plants heavily pruned under powerline easements or mechanically ‘trittered’ along track edges.…”

“…Adaptations of floral structure believed to promote outcrossing in M. deanei include protogyny (stigma presentation prior to anthers unfurling), herkogamy (spatial separation of stamens and stigma) and andromonoecy (staminate and hermaphrodite flowers produced on the same plant; Hewitt et al ). High levels of heterozygosity and very low inbreeding coefficients are evident in all populations studied, together with higher genetic variation within vs. between populations, providing additional evidence for an outcrossing breeding system (Hewitt et al ).…”

“…A series of nested, replicated field studies in large and small populations of M. deanei over five years established on‐ground areas of occupancy, abundance, density, size‐class distributions, flowering phenology, floral types, fire responses, seed:ovule ratios, fruit set, seed loads and patterns of seed release and seedling recruitment (see Hewitt et al and for details of data collection and study sites).…”