Ring width and vessel features of the mangrove Excoecaria agallocha L. depend on salinity in the Sundarbans, Bangladesh

Highlights

• Excoecaria agallocha forms distinct annual rings that characterized by a band of radially flattened fibres.

• Radial growth of this species is mainly driven by salinity that is modulated by freshwater inputs through precipitation during the monsoon.

• Using a combination of ring width and vessel features allows better understanding of hydrology-growth relationships in E. agallocha.

Abstract

The Bangladesh Sundarbans is the largest continuous mangrove in the world that providing crucial environmental services, particularly related to coastal protection and livelihoods of millions of people. However, anthropogenic disturbances, diseases infestation and environmental changes including sea level rise (SLR) and fresh-water flux into the delta are threatening the Sundarbans and other mangrove ecosystems worldwide. Protection of mangrove ecosystems requires knowledge on factors that mainly drive growth and vitality of tree species to evaluate which consequences can be expected from, mainly hydrology-related, environmental changes. In this study, we assessed the nature and periodicity of tree rings in Excoecaria agallocha, a wide spread mangrove species in the Bangladesh Sundarbans. We also analysed the influence of climatic factors, such as precipitation, temperature and vapor pressure deficit (VPD), and river discharge, as a proxy of salinity on ring width (RW) and vessel features, such as mean vessel area (MVA) and mean vessel density (MVD). E. agallocha forms distinct tree-ring boundary that characterized by a narrow (2–4 cells wide) band of radially flattened fibres. The RW as well as the MVA and MVD are crossdatable. The RW is mainly driven by salinity which is influenced by freshwater inputs through precipitation during monsoon along with river discharge January to April. The MVA and MVD responded to similar seasons and months as RW, but mostly with opposite signs in MVD. The results suggest that fresh water inputs through precipitation and river discharge positively influence the radial growth of E. agallocha in the Sundarbans. The RW and vessel features can be used as proxies to explore the growth dynamics of this species, especially in relation to global environmental changes.

Introduction

Globally mangroves are very important ecosystems that providing coastal protection, coastal fisheries, land formation, nursery and breeding ground to marine organisms, and ecotourism (Lee et al., 2014; Himes-Cornell et al., 2018; Dasgupta et al., 2019), as well as sustaining human lives and livelihoods (Abdullah et al., 2016). In addition, mangroves are imperative for carbon sequestration (Ragavan et al., 2019; Rahman et al., 2021). Sundarbans (10014 km²), the largest single block of mangrove in the world, is located along the coast of Bangladesh (59%) and India. Considering its unique ecosystem value, in terms of biological diversity and ecosystem services, the Bangladesh Sundarbans was declared as Ramsar site in 1992 and part of the forest (23%) as World Heritage Site in 1997 (Gopal and Chauhan, 2006). However, this ecosystem is threatened by increasing anthropogenic disturbances, diseases infestation and environmental changes like many other mangroves in the world (Gilman et al., 2008; Mukherjee et al., 2014). Besides, climate change and sea level rise (SLR) are influencing the growth of many tree species in the Sundarbans (Chowdhury et al., 2016a; Maxwell et al., 2018). Anticipated climate change and SLR may alter population structure and species composition in this ecosystem in future (Ghosh et al., 2019; Sarker et al., 2021). Studying the driving factors behind long-term mangrove growth provides important information on ecosystem functioning and the vulnerability of mangrove species. Based on this direct management options to mitigate the impact of future environmental changes can be developed.

Long-term data on tree growth is essential for understanding changes in natural forests and useful for forest management (Worbes and Schöngart, 2019). Detection of tree age and estimates of radial growth rate for woody species are usually carried out by applying indirect methods, including periodic measurements of tree diameter in forest inventories usually from permanent sample plots (PSPs, Groenendijk et al., 2014). In the Sundarbans, forest inventories have been carried out periodically since 1926 to quantify forest stocks and for understanding forest dynamics (Iftekhar and Saenger, 2008). However, these inventories were not conducted at the same plots, and therefore estimations of species-specific tree age and growth rate is problematic. Since 1986, 120 PSPs have been established in the Sundarbans (Rahman, 1994) to obtain growth and demographic data e.g., age, growth rate, natural regeneration and survival. Even though these PSPs are effective in forest stock and diversity assessment (Sarker et al., 2016, 2019), accurate tree age and growth rate estimation from the PSPs is still challenging due to lack of systematic monitoring, such as all trees are not properly tagged and periodic diameter measurements are not documented for the respective trees (Chowdhury et al., 2016b). Therefore, retrospective studies of tree growth using tree-ring analysis is an option for accurate and faster estimation of tree age and growth rate for understanding growth dynamics, in response to changing environmental factors.

Mangrove species have received least momentum in dendrochronological studies due to the assumption that no distinct annual rings are formed in the highly dynamic intertidal growing conditions (Rao et al., 1987; Sun and Suzuki, 2000; Srivastava and Suzuki, 2001). However, meanwhile annual ring formation has been proved for some important mangrove species (e.g., Rhizophora mangle L., R. mucronata Lam., Laguncularia racemosa (L.) C.F. Gaertn, Heritiera fomes Buch.-Ham. and Sonneratia apetala Buch.-Ham. (Menezes et al., 2003; Verheyden et al., 2004; Estrada et al., 2008; Robert et al., 2011; Chowdhury et al., 2008, 2016a, 2016b; Souza et al., 2016; Maxwell et al., 2018; Rahman et al., 2020). Like in most species in (sub)tropical regions, the formation of distinct tree rings in mangrove species is attributed to the occurrence of a seasonal climate, characterized by one distinct dry and wet season, and also the seasonal variation in soil salinity (Chowdhury et al., 2008; Robert et al., 2011). The impact of soil salinity, which is related to freshwater flux into the system, leads to complex hydrological conditions in mangrove ecosystems which can affect growth of trees, even in the same area in different ways (Rahman et al., 2020). Despite influencing distinctness of the tree rings, also the timing of tree-ring formation can be affected by local differences in driving factors which in turn can lead to a reduced common signal in ring-width series even within a species (Menezes et al., 2003; Verheyden et al., 2004; Groenendijk et al., 2014). Thus, working with a mangrove species from different areas requires anatomical characterization of ring boundaries using a standard protocol (IAWA Committee, 1989; Tarelkin et al., 2016), and testing the periodicity of ring formation.

Including time series of wood anatomical features, i.e., mean vessel (MVA), vessel area and/or vessel density (MVD), can lead to additional information on growth determining factors if these features are influenced by other, e.g. more seasonal environmental factors than ring width (RW) (Verheyden et al., 2005; Ohashi et al., 2014; Islam et al., 2019). Vessel features are known to be sensitive intra-annual indicators of seasonal changes in hydrology (Verheyden et al., 2005; Schmitz et al., 2006), which in turn affects the hydraulic conductivity in trees. In mangroves, long-term changes in MVA and MVD across the radius can reflect associated temporal variations in soil-water salinity (Verheyden et al., 2005), as salinity determines the possibility of the tree to absorb sufficient water to support physiological processes (Jiang et al., 2017). Thus, understanding variation in vessel features in tree rings of mangroves can accomplish information on climate-growth relationships as gained from RW only especially to study hydrology driven tree responses.

Excoecaria agallocha L. (Euphorbiaceae) is a widely distributed mangrove species in the Indo-Malaysian and Indo-West Pacific region (Reef and Lovelock, 2015), but it occurs at high densities and considered as the second major species in the Sundarbans (Hossain et al., 2015; Sarker et al., 2016). Its wide distribution can be relevant for long-term assessment of the ecological amplitude of this important mangrove species through studying its performance under different environmental conditions. Earlier studies reported that this species produces distinct tree rings (InsideWood, 2004; Wheeler, 2011). Here, we present an evaluation of dendrochronological potential of E. agallocha in the Sundarbans, Bangladesh. In this study, we first characterize the tree-ring structure and test their periodicity along with statistical quality of ring width (RW) and vessel features (MVA and MVD) of E. agallocha. In a second step, we analyse the influence of climatic factors, such as precipitation, temperature and vapor pressure deficit (VPD) and river discharge on time series of RW and vessel features to evaluate which factors mainly drive the growth of E. agallocha in its complex environment.