Simulated effects of tidal inundation and light reduction on Zostera muelleri flowering in seagrass nurseries.

Zostera muelleri is an abundant seagrass species distributed through intertidal and shallow subtidal waters on the subtropical coasts of Australia. The vertical distribution of Zostera is likely defined by tidal influences, particularly desiccation and light reduction stresses. These stresses were expected to affect the flowering of Z. muelleri; however, it is difficult to quantify the effects of tidal inundation with field studies due to multiple confounding environmental factors affecting flowering (e.g., water temperature, herbivory, nutrients). A laboratory aquarium experiment compared the effects of two levels of tidal height (intertidal and subtidal) and light intensity (shaded and unshaded) on flowering timing, abundance, the ratio between flowering shoots and vegetative shoots, the morphology and duration of flower development. The earliest and greatest flowering intensity was recorded in the subtidal-unshaded group, with no flowers observed in the intertidal-shaded group. Notably, the peak flowering time was the same across shaded and unshaded treatments. Shading prolonged the timing of the first flowering and reduced the density of flowering shoots and spathes, while tidal inundation had a more significant effect on the density of flowering shoots and the density of spathes. Results showed that Z. muelleri could flower under low light conditions or tidal stress but not when exposed to both stresses simultaneously in a laboratory 'nursery setting'. Therefore, applying subtidal-unshaded conditions appears to be beneficial for seagrass nurseries aimed at improved flower abundance despite the plants previously being collected from and adapted to intertidal meadows. Further studies that explore the suitable conditions for triggering and optimising the flowering will be beneficial in designing cost-effective seagrass nurseries.

Simulated megaherbivore grazing as a driver of seagrass flowering.

Seagrass meadows are an important habitat for Testudines (sea turtles) and Sirenia (dugong and manatee) megaherbivores. Megaherbivores can influence the structuring of seagrass meadows; for example, foraging patterns have been found to relate to seagrass phenological strategy. However, as these observations are derived from uncontrolled field studies, it is unclear whether grazing drives such changes or if the changes are related to other factors (e.g., temperature, tidal depth, light). In the present study, a mesocosm experiment was designed to test the impacts of grazing on metrics of flowering of Zostera muelleri over two consecutive flowering seasons. Prior to each flowering season, plants were cropped to 3 cm and 1 cm lengths to represent turtle and dugong grazing, respectively. This study measured the timing of flowering, the number of flowering shoots, the height of the flowering shoot, and the number of spathes (sheathing bracts containing seeds) per flowering shoot in each replicate (n = 5) weekly. Cropping had no significant influence on the timing of flowering (i.e., number of days to first and peak flowering) indicating that it is not a trigger for flowering. However, cropping significantly reduced the maximum density of flowering shoots and spathes, which was proposed to be due to resource allocation differences between clonal growth and flower production. A reduction in the flowering ratio was observed in both cropped plant groups and the relatively high density and the ratio of flowering observed in the 1 cm group indicate that the plant was adapting to cope with stress. Morphology of flowering (i.e., the maximum height of flowering shoot and the maximum number of spathes per flowering shoot) was not significantly affected by cropping and these two variables were strongly correlated. The results suggest that cropping can influence the overall flowering densities in a season but not the timing of flowering. This study demonstrated that cropping prior to the flowering season can reduce the expected production of spathes in seed nurseries and suggests it may be beneficial to consider megaherbivores in seed-based restoration activities.

Environmental drivers of the leaf nitrogen and phosphorus stoichiometry characteristics of critically endangered Acer catalpifolium.

Acer catalpifolium is a perennial deciduous broad-leaved woody plant, listed in the second-class protection program in China mainly distributed on the northwest edge of Chengdu plain. However, extensive anthropogenic disturbances and pollutants emissions (such as SO2, NH3 and NOX) in this area have created a heterogeneous habitat for this species and its impacts have not been systematically studied. In this study, we investigated the leaf nitrogen (N) and phosphorus (P) content of A. catalpifolium in the natural distribution areas, and a series of simulation experiments (e.g., various water and light supply regimes, different acid and N deposition levels, reintroduction management) were conducted to analyze responses of N and P stoichiometric characteristics to environmental changes. The results showed that leaf nitrogen content (LNC) was 14.49 ~ 25.44 mg g-1, leaf phosphorus content (LPC) was 1.29~3.81 mg g-1 and the N/P ratio of the leaf (L-N/P) was 4.87~13.93. As per the simulation experiments, LNC of A. catalpifolium is found to be relatively high at strong light conditions (80% of full light), high N deposition (100 and 150 kg N ha-1), low acidity rainwater, reintroduction to understory area or N fertilizer applications. A high level of LPC was found when applied with 80% of full light and moderate N deposition (100 kg N ha-1). L-N/P was high under severe shade (8% of full light), severe N deposition (200 kg N ha-1), and reintroduction to gap and undergrowth habitat; however, low L-N/P was observed at low acidity rainwater or P fertilizer application. The nutrient supply facilitates corresponding elements uptake, shade tends to induce P limitation and soil acidification shows N limitation. Our results provide theoretical guidance for field management and nutrient supply regimes for future protection, population rejuvenation of this species and provide guidelines for conservation and nutrient management strategies for the endangered species.