Researchers led by Cecilia Martin of Red Sea Global tagged 96 mature Avicennia marina trees across 12 sites spanning the northern Saudi Red Sea coastline. On each tree, three permanent terminal branches were marked and monitored monthly from August 2021 to October 2022, with the team counting bud clusters, flowers and propagules at every visit. The trees, averaging just 2.2 meters in height, reflect the naturally dwarfed growth habit that characterizes mangroves in this nutrient-poor, freshwater-starved region, where a one-meter tree is already considered reproductively mature.

The monitoring revealed a striking seasonal choreography. Buds and flowers increased in abundance during the hot summer months and declined through winter, while propagules followed the exact opposite pattern, peaking when temperatures fell. Using Generalized Additive Models, the team confirmed that these patterns were statistically significant rather than random fluctuations. Cross-correlation analysis showed that rising air temperatures prompted bud and flower production with a lag of about one month, while falling temperatures triggered propagule formation with a lag of zero to one month. Solar radiation also played a role, with buds and flowers responding positively to increased irradiation after a three-month delay, and propagules showing a negative correlation with a two-month lag.

Perhaps most surprising was what did not matter. Precipitation showed no significant correlation with any reproductive phase, and sea surface salinity showed no link to propagule abundance. In a hyper-arid environment where annual rainfall is less than 50 millimeters and fell almost entirely in just two months of the study period, this makes ecological sense. The species’ renowned salt tolerance appears to render sea surface salinity irrelevant to its reproductive timing, although the authors caution that soil pore-water salinity, which can reach far higher levels during hot, dry summers when evaporation concentrates salts in the sediment, may still suppress propagule development and germination.

When the team normalized propagule counts across sites to remove the influence of naturally more productive forests, a second, previously hidden reproductive season emerged. Beyond the dominant winter season running from December to April and peaking in February, a secondary flush of propagule production occurs at the end of summer, around August to October. Cluster analysis then revealed a clear latitudinal gradient: the three northernmost sites peaked around September, the five central sites peaked from February to March, and the three southernmost sites peaked earliest, from December to January. This south-to-north progression mirrors patterns documented across Australia, New Zealand and Papua New Guinea, where mangrove reproduction is consistently delayed at higher, cooler latitudes.

The ecological logic behind this temperature-driven schedule is compelling. Studies of the related species Avicennia germinans have shown that high temperatures inhibit propagule rooting and germination, suggesting that mangroves maximize recruitment success by releasing their offspring during cooler months. In the northern Red Sea, where winter temperatures can drop low enough to threaten less hardy species, Avicennia marina’s well-documented cold tolerance allows it to exploit this window. The correlation between propagule production and seawater level further supports the idea that higher water levels aid the floating dispersal mechanism by which mangrove propagules spread.

These findings arrive at a critical moment. Saudi Arabia has made mangrove restoration a centerpiece of its Vision 2030 strategy and the Saudi Green Initiative, with Red Sea Global committing to plant or enhance 50 million mangrove trees in the region. Until now, propagule harvesting has been based on local observations at a handful of forests. The new study provides the region-wide, quantitative foundation needed to plan harvesting operations scientifically, identifying exactly where and when propagules can be collected at their peak abundance across the entire study area.

The latitudinal gradient offers a practical advantage: by targeting different forests during their respective peak months, harvesters can spread collection across multiple months of the year, reducing pressure on any single stand while maximizing the genetic diversity of collected propagules. To safeguard natural regeneration, the researchers applied a Maximum Sustainable Yield framework, capping harvests at 50 percent of mature propagules per tree. Under this framework, the 12 assessed forests, which together represent roughly one third of the region’s total mangrove area of 2,400 to 2,700 hectares, could sustainably supply around 680,000 propagules per year during peak season, or an estimated 2 million across the full region. Including shoulder months, availability could reach approximately 4 million propagules annually, meaning the 50-million-tree goal could theoretically be met in about 13 years, though the authors stress this idealized scenario ignores accessibility constraints and germination failures.

The dual-season reproductive pattern carries an additional operational bonus for nurseries. Avicennia marina propagules germinate within days of harvesting and seedlings are typically grown for six to eight months before reaching the roughly 50-centimeter size suitable for outplanting. Propagules collected in December would therefore be ready to leave the nursery just as the second propagation season begins in August, allowing nursery space to be reused twice a year and minimizing the physical footprint of propagation operations.

The study also sounds a note of caution for a warming world. Rising temperatures could extend the flowering season at this northern range limit, potentially improving reproductive completion, yet the same warming is expected to impair propagule germination and early recruitment, which favor cooler conditions. Conversely, milder winters may boost seedling survival where cold damage currently constrains regeneration. These opposing feedbacks make the fate of range-edge mangrove populations genuinely uncertain. What is clear is that understanding the fine-grained, site-specific reproductive biology of these remarkable trees is no longer an academic luxury. As mangrove ecosystems worldwide continue to decline, losing roughly 3.4 percent of their extent between 1996 and 2020, the success of the restoration projects meant to reverse that trend will depend on precisely this kind of ground-truthed ecological knowledge, harvested one tagged branch at a time.

Subject of Research: Reproductive phenology of the grey mangrove Avicennia marina at its northwestern range limit in the northern Red Sea

Article Title: Understanding the reproductive phenology of Avicennia marina at its northwestern range in the Arabian Peninsula supports improved afforestation planning

Article References: Martin, C., Rossbach, S., Alansari, A., & El-Bana, M. (2026). Understanding the reproductive phenology of Avicennia marina at its northwestern range in the Arabian Peninsula supports improved afforestation planning. Discover Oceans, 3(1), Article 22. https://doi.org/10.1007/s44289-026-00135-3

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00135-3

Keywords: mangroves, Avicennia marina, Red Sea, phenology, propagules, afforestation, restoration, Saudi Arabia, hyper-arid climate, temperature, Maximum Sustainable Yield, coastal ecosystems