Fluctuations in abiotic conditions (e.g. temperature) and resources (e.g. nutrients) can modify the direction and strength of species interactions, shifting patterns of species abundance. Globally, estuaries are undergoing rapid environmental change at both local and larger scales. In temperate estuaries of southeast Australia, studies have documented landward encroachment of the mangrove Avicennia marina into salt marshes. Whether a salt marsh is invaded, and the degree of invasion, appears variable among marshes, suggesting that local processes, such as nutrient enrichment, might interact with geographic phenomena, such as warming or sea level rise.
Here we report the results of a three-year, field experiment investigating the combined effects of warming and nutrient enrichment on recruitment of the mangrove A. marina into coastal saltmarshes dominated by the chenopod Sarcocornia quinqueflora in southeast Australia. In plots in the open salt marsh at mid tidal elevations dominated by a monoculture (roughly 50% cover) of S. quinqueflora, we manipulated temperature using warming chambers and nutrients by adding fertilizer in an orthogonal design. We then added A. marinato plots and quantified propagule establishment and seedling growth and survival, as well as the shifting abundance of S. quinqueflora.
Results/Conclusions
Warming or nutrient enrichment on their own had similar positive effects on S. quinqueflora, with roughly 50% increases in cover relative to ambient conditions, whereas the combination of warming and nutrients doubled the cover of S. quinqueflora. Recruitment of A. marina seedlings into salt marsh was about 50% greater in areas warmed or enriched by nutrients relative to ambient conditions. In contrast, the combination of warming and enrichment reduced seedling abundance by about 80%. Consequently, there is a shift in the nature of the interaction between A. marina and S. quinqueflora that is mediated by the effects of warming and nutrients on the abundance of S. quinqueflora. Nutrient enrichment or warming alone appears to facilitate (buffering harsh conditions) mangrove recruitment, whereas a combination of warming and nutrients largely precludes mangrove recruitment (physical disturbance, competition).
These results demonstrate how warming temperatures and nutrient enrichment can lead to complex species interactions between mangroves and salt marsh plants. These observations may explain some of the local level variation in invasion of mangroves across salt marsh in southeast Australia. They also suggest that local process such as nutrient enrichment may interact with more regional processes such as warming temperatures to affect the future species composition of these salt marsh ecosystems.