Reproductive biology and success of invasive Australian acacias in Portugal

Abstract

Reproductive traits are crucial for the establishment and maintenance of populations in new areas, and therefore for the invasion process. This work aimed to study the reproductive biology of four aggressive invasive Australian Acacia spp. in Portugal. Fruit and seed set, seed weight and germinability, and seedling growth were assessed for self-and outcross treatments in invasive populations of A. dealbata, A. longifolia, A. melanoxylon and A. saligna. Acacia spp. showed different investments in the production of reproductive units and in natural reproductive success, with A. dealbata, the most aggressive species, having the highest investment and reproductive success. Acacia melanoxylon showed a different reproductive strategy, andromonoecy, contrasting with the other hermaphroditic species. Acacia spp. were shown to be predominantly self-incompatible, but a low level of spontaneous selfing enabled the production of viable offspring. Acacia dealbata and A. longifolia suffered pollen limitation. Self-progeny had lower viability than progeny from outcrossing for A. dealbata and A. melanoxylon. Acacia spp. did not show higher compatibility rates in comparison with the native area. They had low fruit set but, as a result of their massive flower production, their realized reproductive success was high and could have contributed to the invasion.

Why so many flowers? A preliminary assessment of mixed pollination strategy enhancing sexual reproduction of the invasive <i>Acacia longifolia</i> in Portugal

“…Since Acacia species are usually self-infertile, we planned exclusion experiments to verify the relative contribution of wind and insects to pollination. Pods from crosspollination have previously been found to have more seeds per pod than those from self-pollination (Correia et al, 2014), as expected. Correia et al (2014) performed pollination experiments in Portugal that took into account open pollination, supplementary pollination, spontaneous autogamy, and self-pollination.…”

“…Pods from crosspollination have previously been found to have more seeds per pod than those from self-pollination (Correia et al, 2014), as expected. Correia et al (2014) performed pollination experiments in Portugal that took into account open pollination, supplementary pollination, spontaneous autogamy, and self-pollination. But no study has yet verified the relative contribution of wind and insects to acacia seed set.…”

“…Acacias match the above characteristics defined for anemophilous species. They form dense inflorescences and show numerous flowers per flower head, abundant stamens per flower, and functionally male flowers (Correia et al, 2014;Giuliani et al, 2016). Pollen diameter is 30-70 µm in most Australian acacias (Guinet, 1981), and even if rarely taken into account, acacia pollen was recorded in aerobiological surveys of South America, Jordan, and Italy (Sanchez-Medina and Fernandez, 1966;Al-Eisawi and Dajani, 1988;Romano, 1988;Hurtado and Alson, 1996) confirming that wind is able to conveniently handle the polyad (composite pollen grains).…”

“…On A. longifolia, insect activity is expected to be low, considering that flowering occurs at the end of winter. Honeybees have seldom been collected or recorded (Silva, 2012;Correia et al, 2014) and records lack any behavioural observation demonstrating the active involvement of the honeybee in pollen transfer at this time of the year. In fact, honeybees resuming activity after winter dormancy may be unable to keep the constancy needed for successful pollination, due to the unsuitable weather conditions.…”

Two invasive acacia species secure generalist pollinators in invaded communities

“…¼ Acacia s.s.], are two native Australian trees which have become invasive in several parts of the world, including Portugal where they are considered the most problematic and widespread invasive species in the country (Almeida and Freitas, 2012;. Although both acacia species have the ability to successfully self-fertilize, the efficiency of selfpollination is low (below 0.5% of seed:ovule ratio), thus both species require pollination vectors to produce significant seed yields, and both are considered predominantly outcrossing species (Correia et al, 2014). In their native range both species sustain mutualistic interactions essential for the successful maintenance of populations and for the colonization of new areas (Berg, 1975;Gibson et al, 2011;O'Dowd and Gill, 1986;Rodríguez-Echeverría et al, 2011).…”