Growth rings, growth ring formation and age determination in the mangrove <i>Rhizophora mucronata</i>

Background and Aims The mangrove Rhizophora mucronata has previously been reported to lack annual growth rings, thus barring it from dendrochronological studies. In this study the reported absence of the growth rings was reconsidered and the periodic nature of light and dark brown layers visible on polished stem discs investigated. In addition, the formation of these layers in relation to prevailing environmental conditions, as well as their potential for age determination of the trees, was studied.Methods Trees of known age were collected and a 2.5-year cambial marking experiment was conducted to determine the periodic nature of the visible growth layers.Key Results Annual indistinct growth rings were detected in R. mucronata and are defined by a low vessel density earlywood and a high vessel density latewood. The formation of these growth rings and their periodic nature was independent from site-specific environmental conditions in two forests along the Kenyan coast. However, the periodic nature of the rings was seriously affected by slow growth rates, allowing accurate age determination only in trees with radial growth rates above 0.5 mm year(-1). The onset of the formation of the low vessel density wood coincided with the onset of the long rainy season (April-May) and continues until the end of the short rainy season (November). The high vessel density wood is formed during the dry season (December-March). Age determination of the largest trees collected in the two studied forests revealed the relatively young age of these trees (+/-100 years).Conclusions This study reports, for the first time, the presence of annual growth rings in the mangrove R. mucronata, which offers further potential for dendrochronological and silvicultural applications

Successive cambia development in <i>Avicennia marina</i> (Forssk.) Vierh. is not climatically driven in the seasonal climate at Gazi Bay, Kenya

This study is intended to provide early access to recent findings on the formation of the successive cambia of Avicennia marina (Forssk.) Vierh. in Kenya. The non-annual character of the growth layers was demonstrated by using three trees from a cambial marking experiment and three trees from a plantation of known age. The respective number of growth layers produced during one year was on average a half and three. Considering 28 stem disks of trees at three study sites, differing in local site conditions, growth layer development was shown to be strongly correlated with stem diameter (R²=0.84, pA. marina (from Kenya)

Influence of a salinity gradient on the vessel characters of the mangrove species <i>Rhizophora mucronata</i>

Although mangroves have been extensively studied, little is known about their ecological wood anatomy. This investigation examined the potential use of vessel density as a proxy for soil water salinity in the mangrove species Rhizophora mucronata (Rhizophoraceae) from Kenya. In a time-standardized approach, 50 wood discs from trees growing in six salinity categories were investigated. Vessel densities, and tangential and radial diameters of rainy and dry season wood of one distinct year, at three positions on the stem discs, were measured. A repeated-measures ANOVA with the prevailing salinity was performed. Vessel density showed a significant increase with salinity, supporting its use as a prospective measure of salinity. Interestingly, the negative salinity response of the radial diameter of vessels was less striking, and tangential diameter was constant under the varying environmental conditions. An effect of age or growth rate or the presence of vessel dimorphism could be excluded as the cause of the absence of any ecological trend. The clear trend in vessel density with salinity, together with the absence of a growth rate and age effect, validates the potential of vessel density as an environmental proxy. However, it can only be used as a relative measure of salinity given that other environmental variables such as inundation frequency have an additional influence on vessel density. With view to a reliable, absolute proxy, future research should focus on finding wood anatomical features correlated exclusively with soil water salinity or inundation frequency. The plasticity in vessel density with differing salinity suggests a role in the establishment of a safe water transport system. To confirm this hypothesis, the role of inter-vessel pits, their relationship to the rather constant vessel diameter and the underlying physiology and cell biology needs to be examined

Mangrove growth rings: fact or fiction?

The analysis of tree rings in the tropics is less straightforward than in temperate areas with a demarcated unfavourable winter season. But especially in mangroves, the highly dynamic intertidal environment and the overriding ecological drivers therein have been a reason for questioning the existence of growth rings. This study aimed at casting light on growth rings in mangroves. In six mangrove species growing in Gazi Bay, Kenya (Sonneratia alba, Heritiera littoralis, Ceriops tagal, Bruguiera gymnorrhiza, Xylocarpus granatum and Lumnitzera racemosa), the occurrence of growth rings was examined. Growth rate of each species was determined based on a 1-year period using the cambial marking technique. The effect of climate was furthermore considered by comparing the results with a number of wood samples originating from contrasting climatic regions. We can conclude that for growth rings to appear in mangroves more than one condition has to be fulfilled, making general statements impossible and explaining the prevalent uncertainty. Climatic conditions that result in a range of soil water salinity experienced over the year are a prerequisite for the formation of growth rings. For species with an anatomy characterized by indistinct ring boundaries, this should be combined with a growth rate of at least 0.3 mm/year. The use of growth rings for age or growth rate determinations should thus be evaluated on a case by case basis

Successive Cambia in the mangrove <i>Avicennia</i>: A study on the three-dimensional structure of the Cambia and the functioning of the internal phloem tissue

Mangrove forests grow in conditions which must be considered extreme for woody angiosperms: high and changing salinity, frequent inundation with associated hypoxia, low relative humidity of the air and high temperatures. As ‘marine formations’, mangrove ecosystems are characteristic for the intertidal area of estuaries, creeks, sheltered bays and coastlines in tropical and sub-tropical areas worldwide. The genus Avicennia has been shown to be eurytopic as regards the above mentioned environmental conditions. Locally, Avicennia can often be found at the seaward as well as at the landward side (Disjunct zonation pattern) of the mangrove forest, sites with highly contrasting environmental conditions, while globally it has the largest latitudinal range in both the Eastern and the Western biogeographic mangrove regions (i.e. Indo-West Pacific and East Africa vs. America and West Africa respectively). The question is how Avicennia copes with this large and varying range of environmental conditions? It already has been proven that the wood anatomy of Avicennia is especially adapted to harsh environmental conditions. Properties of the wood (transport tissues) were suggested to be adapted to reduce cavitation events, defined as air bubble formation in the xylem sap. Inside vessels, those air bubbles can enlarge and therewith block the water transport (i.e. vessel embolism) so that cavitation and subsequent embolism is highly influencing the functionality of the hydraulic system. Amongst mangrove trees, the genus Avicennia L. (Acanthaceae) stands out by its successive cambia (i.e. having not one cambial layer but subsequent active cambia possibly conferring many sites of active growth in the stem). Secondary growth by successive cambia can offer Avicennia ecological advantages since (1) the internal phloem tissue can store water that could be used in refilling air-filled vessels with water and (2) the special, patchy growth can offer the tree woody tissue that is well-adapted to the conditions of the moment. In this study we investigate the organization of the successive cambia in Avicennia in three dimensions in order to complete the already investigated three dimensional network of transport tissues (phloem and xylem). Small stems (max. diameter: 3 cm) and branches of Avicennia marina (Forssk.) Vierh. and A. officinalis L. were sampled from the Rekawa lagoon in Sri Lanka, where the two species encounter spatially and temporally varying conditions as regards salinity, inundation. Serial sections and microscopic observation of the samples allow the reconstruction and visualization of the three-dimensional organization of the successive cambia. The working hypothesis of the research is: ‘successive cambia are important for Avicennia to survive in extreme high environmental conditions and explains the genus eurytopic nature as compared to other mangrove genera’

Comparative anatomy of intervessel pits in two mangrove species growing along a natural salinity gradient in Gazi Bay, Kenya

Background and Aims: According to the air-seeding hypothesis, embolism vulnerability in xylem elements is linked directly to bordered pit structure and functioning. To elucidate the adaptive potential of intervessel pits towards fluctuating environmental conditions, two mangrove species with a distinct ecological distribution growing along a natural salinity gradient were investigated. Methods: Scanning and transmission electron microscopic observations were conducted to obtain qualitative and quantitative characteristics of alternate intervessel pits in A. marina and scalariform intervessel pits in Rhizophora mucronata. Wood samples from three to six trees were collected at seven and five sites for A. marina and R. mucronata, respectively, with considerable differences between sites in soil water salinity. Key Results: Vestured pits without visible pores in the pit membrane were observed in A. marina, the mangrove species with the widest geographical distribution on global as well as local scale. Their thick pit membranes (on average 370 nm) and minute pit apertures may contribute to reduced vulnerability to cavitation of this highly salt-tolerant species. The smaller ecological distribution of R. mucronata was in accordance with wide pit apertures and a slightly higher pitfield fraction (67 % vs. 60 % in A. marina). Nonetheless, its outer pit apertures were observed to be funnel-shaped shielding non-porous pit membranes. No trends in intervessel pit size were observed with increasing soil water salinity of the site. Conclusions: The contrasting ecological distribution of two mangrove species was reflected in the geometry and pit membrane characteristics of their intervessel pits. Within species, intervessel pit size seemed to be independent of spatial variations in environmental conditions and was only weakly correlated with vessel diameter. Further research on pit formation and function has to clarify the large variations in intervessel pit size within trees and even within single vessels