Morphological plasticity in mangrove trees: salinity-related changes in the allometry of Avicennia germinans

Abstract

Key Message Morphological plasticity helps plants to cope to environmental conditions. Allometric responses of the mangrove Avicennia germinans to increasing salinity are easily detectable when focusing on the top height trees. Abstract Several studies show that mangrove trees possess high species-and site-related trait allometry, suggesting large morphological plasticity that might be related to environmental conditions, but the causes of such variation are not clearly understood and systematic quantification is still missing. Both aspects are essential for a mechanistic understanding of the development and functioning of forests. We analyzed the role of salinity in the allometric relations of the mangrove Avicennia germinans, using: (1) the top height trees (trees with the largest diameters at breast height, which reflect forest properties at the maximum use of resources); (2) the slenderness coefficient (which indicates competition and environmental conditions); and (3) the crown to DBH ratio. These standard tools for forest scientists dealing with terrestrial forests are suitable to analyze the plastic responses of mangroves to salinity. First, the top height trees help to recognize structural forest properties that are not detectable when studying the whole stand. Second, we found that at salinities above 55 %, trees are less slender and develop wider crowns in relation to DBH than when growing at lower salinities. Our results suggest a significant change in allometric traits in relation to salinity, and reflect the plastic responses of tree traits in response to environmental variation. Understanding the plastic responses of plants to their environment can help to better model, predict, and manage forests in changing environments.

Change in drivers of mangrove crown displacement along a salinity stress gradient

“…The observed directionality of crown displacement towards SW and angles of alignment, best explained by the synthesized effect of wind and neighbours, points to a greater influence of wind directionality on crown displacement at higher salinities and suggests an increased susceptibility to wind dynamic loading, presumably as a consequence of salinity‐induced reduction in growth rate (Ball, ; Nguyen, Stanton, Schmitz, Farquhar, & Ball, ). This is ultimately expressed by smaller canopies (Vovides et al, ) with lower wind damping (James, Haritos, & Ades, ; MacFarlane & Kane, ). Within MFs, the observed reduced canopy cover and reduced crown overlap related to smaller trees could explain the greater effect of wind at higher salinities; smaller tree sizes and crowns could render greater canopy openness.…”

“…The IBs flank the main waterways that drain fresh water from higher lands towards the lagoon, contributing to high accumulation of organic matter and keeping low salinity throughout the year (Méndez‐Alonzo et al, ). Mean height of top‐height trees (20% of largest trees in the stand) is 14.65 m for IBs and 12.70 m for MFs, and trees in IBs are more slender ( D 130 /H ), having stems 20% narrower and crown areas 10% narrower than in MFs (Vovides et al, ).…”

“…To estimate crown‐projected area overlap in each plot, we mapped the crown polygons and computed the total canopy cover (area covered by at least one crown) and the total overlapping area (area covered by at least two overlapping crowns). Because the area of two sampling plots differed from the rest, and also because crown diameters decrease with increasing salinities (Vovides et al, ), we calculated relative canopy cover (%) and relative crown overlap (%) with the observed asymmetry and crown displacement ( in %, Figure c). Further, was used to virtually place crown centroids () over the basal stem locations (), and the resulting mapped overlapped areas were quantified (, Figure d).…”

“…The Rayleigh tests were performed separately for trees below the canopy and for canopy trees, and 95% confidence intervals were estimated by a bootstrap for a von Mises distribution on 1,000 iterations. Given that allometric relations change between the two habitats (Vovides et al, ), the average height of top‐height trees from each habitat was taken as the splitting point to separate canopy and below canopy trees: 13 m for MFs and 15 m for IBs, to ensure that the responses of below canopy trees are not confounded with dominant trees at the canopy level.…”

“…Mangrove plastic responses to saline stress are remarkable. At individual level, trees become smaller and with larger stem diameters in relation to height, allowing trees to maintain an optimal water balance at higher salinities (Robert, Koedam, Beeckman, & Schmitz, 2009;Vovides et al, 2014), and at stand level, size-asymmetry is reduced, potentially reducing aboveground competition (Méndez-Alonzo, Hernández-Trejo, & López-Portillo, 2012;Naidoo, 2009). Additionally, seasonal northtrade winds with speeds up to 20.8 m/s dominate during seven months of the year (end of September to early May).…”