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

Abstract

Crown displacement in trees is an adaptive response driven by neighbours that optimizes space use and reduces competition. But it can also be the result of wind force. Although morphological responses to neighbours have been well studied, the interplay between neighbours and wind in driving crown shape, and the implications for plant interactions remain poorly understood. However, it is crucial to predict such changes in vegetation structure and function under the scope of global change. We test the hypothesis that aboveground interactions are reduced with increasing soil stress and that wind becomes the main driver of crown shape in mangrove forests. We investigated the effect of neighbours and wind intensity and direction on crown displacement of mangrove canopy and below canopy trees along a salinity gradient, and assessed crown asymmetry for three mangrove tree species, as well as the contribution of crown displacement on reducing crown‐projected area overlap and thus neighbourhood competition. Results show that crown displacement of canopy trees is strongly influenced by winds at all salinities. At low salinities, competition for space accounted for 48% of crown displacement away from neighbours, compared to 49% found for the synthetized effects of wind and neighbours. While trees below the canopy displace their crowns away from their neighbours, no response to wind could be detected. This can be due to the wind protection conferred by a dense canopy stand related to bigger crowns that effectively reduce wind drag. At higher salinities, there was a reduction in canopy overlap due to crown displacement, which suggests reduced aboveground plant interactions with increasing soil stress. While neighbourhood avoidance is a fundamental strategy for optimal light foraging, this study shows that wind strength and directionality are main drivers of crown shape with increasing stress and highlights their potential influence in plant interactions and forest structure, pointing to an increased susceptibility of trees to disturbances that should be further studied. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13218/suppinfo is available for this article.

Roots of Cooperation: Can Root Graft Networks Benefit Trees Under Stress?

“…Root grafts can be either non-functional or functional (cambia and vascular tissues are fused 2 ). Non-functional grafts confer higher mechanical stability through shared anchoring systems 17 , which is particularly relevant for coastal wetland forests with shallow root systems 16,24 due to anoxic sediments, and exposure to strong winds 25 . Functional grafts can additionally facilitate resource exchange 21 and promote growth 7 by mitigating the adverse effects of defoliation and budworm outbreaks 10,26 and increasing the concentration of carbohydrates in shaded trees 21 .…”

“…), offer an ideal model system to study the ecological role of root graft networks. Avicennia germinans L. dominates forests on hypersaline mudflats with limited tree diversity, and strong salinity gradients offer particularly satisfactory conditions to study physiological responses 33 , tree architecture, and tree interactions 25,34,35 . These mangrove specificities provide ideal conditions to investigate in the field whether root grafting can benefit trees growing under environmental stress through an analysis of individual tree attributes and spatial root graft network structures.…”

“…Freshwater and marine water inputs into the lagoon come from extreme opposite directions: the connection to the GoM is located in the northern extreme, while a main riverine input is located in the southern extreme. This creates a year-round salinity gradient that increases northward, regardless of seasonality 25,49,50 . This is also reflected in the zonation of the mangrove species, where the northern and most saline For the pre-established plots, existing tree parameters were recovered from a publicly available database 52 , including a unique ID, species, x-and y-axis positions in the plot, stem diameter at 130 cm from the soil surface ( +,-) and height ( ).…”

“…A large index of neighbourhood asymmetry implies that the neighbours are large and in close proximity, potentially exerting higher competition pressure on a target tree than a small neighbourhood asymmetry would. The 5 m radius was chosen because it had been previously identified as the optimal radius for detecting the responses of trees to its neighbours at the same study site 25 . Neighbourhood asymmetry was only calculated for trees where their complete neighbourhood was within the limits of the sampling plots (187 trees) to avoid biased neighbourhood asymmetry sizes related to incomplete information for neighbouring trees located outside a plot.…”

Cooperative root graft networks benefit mangrove trees under stress

“…Still, even if grafts are non-functional, they can increase tree stability as compared to non-grafted trees by sharing anchoring systems 5 . The latter is particularly relevant for coastal wetland forests, which are regularly exposed to strong winds 15 and further characterised by shallow root systems due to anoxic sediments 3 , 16 .…”