Nutrient inputs and herbivores interactively influence black mangrove tolerance to damage
In Florida, the black mangrove (Avicennia germinans) has recently expanded its geographic range to higher latitudes due to fewer winter cold snaps, leading to its colonization of herbaceous salt marsh habitats. Some marsh-associated insects feed on black mangroves, but their impact on the mangrove range expansion has not been studied. Although herbivores are known to regulate plant populations and facilitate or deter the spread of plant species into novel geographic areas, abiotic factors such as light, nutrient, and water availability may also define range limits and expansion. In order to understand how herbivory and abiotic factors interact to influence the success and spread of mangroves, we used a factorial experiment to impose various types of tissue damage and nutrient availability on lab-grown seedlings. Damage treatments included mechanical damage to apical meristems or leaves, actual feeding damage from marsh insects, or no damage (controls). These treatments were crossed with high or low nutrient availability, defined by field studies of mangrove habitats isolated from or greatly impacted by anthropogenic nutrient inputs. We quantified the growth of seedlings by measuring their biomass and changes in height and number of leaves sixteen weeks after treatments were imposed.
The effects of nutrient availability on mangrove growth depended on damage treatment: nutrient levels did not affect the growth of plants damaged by herbivores, but higher nutrient concentrations increased the growth of control and mechanically damaged plants. Plants exhibited lower tolerance for all damage treatments in high nutrient conditions compared to low nutrient conditions. In high nutrient conditions, mechanically damaged plants compensated for damage such that their growth was similar to that of undamaged control plants, while the herbivore-damaged plants grew significantly less than undamaged plants. However, in low nutrient conditions, mechanically damaged plants overcompensated for damage and grew more than the undamaged control plants, while the herbivore damaged plants only compensated for damage since their growth was not different from undamaged plants. The difference in mangrove tolerance to similar amounts of mechanical damage versus herbivore damage was evident when comparing above-ground biomass produced during the experiment and changes in height and leaf number, suggesting that the loss of photosynthetic area is not the only detriment of herbivory. Our results indicate that anthropogenic nutrient inputs and herbivory interactively influence black mangrove seedling growth, and perhaps the expansion of this species’ range.