Proximate causes and possible adaptive functions of mast seeding and barren flower shows in spinifex grasses (Triodia spp.) in arid regions of Australia

Abstract

Mast seeding, the intermittent production of large synchronised seed crops among plant populations, is a phenomenon that occurs at exceptionally long intervals in spinifex grasses (Triodia spp.) from arid regions of Australia. This is despite the reliance of these fire-sensitive plants on seeds for post-fire regeneration, and that spinifex grasslands rate among Australia’s most flammable ecosystems. The proximate causes and possible adaptive functions of masting in seven species of spinifex from arid regions within the 350-mm rainfall isohyet were investigated. Specifically, the seed set percentages of 79 specimens collected between 1947 and 2012 were related to the following environmental covariates: antecedent rainfall over 6, 12 and 36 months, relative humidity, and the number of days above 40°C and below 0°C during anthesis. Given the potential importance of seeding events for post-fire regeneration, it was also investigated whether masting in Triodia could represent a fire-related form of environmentally predictive masting, by testing whether high-yield years corresponded to years of increased fire occurrence. Examination of the dataset showed that 43% of specimens contained completely aborted inflorescences (0% seed fill), while seed set ranged from 2 to 69% in the remaining specimens. High levels of insect activity were also found, with 42% of specimens showing evidence of insect occupation. Statistical analyses showed that the main environmental driver of seed set was high precipitation over the previous 12 months, and that high-yield years were strongly related to years of increased fire likelihood. The number of days over 40°C was a weakly significant driver of yield, while the remaining covariates were not significant. It is hypothesised that intermittent reproduction by Triodia is a fire-related form of environmentally predictive masting, which maximises chances of post-fire regeneration by satiating seed predators during flammable periods (i.e. after heavy rain years). Furthermore, it is suggested that non-viable flower crops after initial low rainfalls may have an adaptive function, by diluting pre-dispersal seed predator densities with ‘decoy’ ovules that do not mature and lead to the starvation of developing larvae.

Interactions between rainfall, fire and herbivory drive resprouter vital rates in a semi‐arid ecosystem

“…Yet, long‐term rainfall accumulation had a stronger effect upon floral initiation, whereby if rainfall in the 13–24 month period preceding the census was below average, then flowering was suppressed. Rainfall accumulated over multiple years is important for promoting seed set in arid zone Triodia species (Wright, Zuur & Chan ) and although our findings are an indirect measure of reproductive effort they suggest a similar response in this semi‐arid species.…”

“…, ; Chesson et al . ; Davies & Kenny ; Wright, Zuur & Chan ). Here, we have advanced research on this topic by showing that relationships between plant vital rates and rainfall can differ markedly between life‐history stages.…”

“…The proportion of hummocks flowering per plot was modelled as a function of antecedent rainfall using a logit link function and a random intercept to account for extra variation among sites. Consistent with the response of other perennial grasses (Jacobs 1973;Wright, Zuur & Chan 2014), we assumed that above-average antecedent rainfall triggers mast flowering in T. scariosa within 1 or 2 years of the rainfall event, depending on its timing in relation to T. scariosa's late spring/early summer flowering. Therefore, we tested the relative and combined influence of long-term rainfall accumulation on floral initiation over two time frames: 0-12 months and 13-24 months prior to the census.…”

“…In this study, we test three hypotheses relating to the effects of climate, fire and herbivory on three fundamental population processes; recruitment, survival of seedlings and survival of established plants. First, because pulses of precipitation markedly change soil water availability in arid ecosystems (Noy-Meir 1973;Enright et al 2014;Wright, Zuur & Chan 2014), we hypothesise that rainfall will be a major driver of floral initiation, post-fire recruitment and post-fire survival of seedlings and resprouting plants. Second, since herbivores preferentially target recently burnt areas to exploit higher quality forage (Fuhlendorf & Engle 2004) we propose that herbivory will reduce post-fire recruitment and regeneration and this will be accentuated in the substantially smaller prescribed fires (Cohn & Bradstock 2000).…”

Overcoming physiological dormancy in seeds of Triodia (Poaceae) to improve restoration in the arid zone

“…Numerous technical reports have identified a multitude of issues surrounding the reestablishment of Triodia species from seed (Jacobs ; Atkins ; Adkins et al ; Meissner ; Wells & Dixon ; Wells et al ). Some of the challenges identified include the sourcing of sufficient quantities of germinable seed due to low fecundity and the sporadic nature of seeding events in arid systems (Jacobs ; Westoby et al ; Wright et al ), and the erratic germination achieved when seeds are sown at restoration sites.…”