Long-Term Growth and Succession in Restored and Natural Mangrove Forests in Southwestern Florida

We compared colonization, growth and succession from 1989 to 2000 in a restored mangrove site and in gap and closed canopy sites in a natural mangrove forest. The restored site was created in 1982 and planted with Rhizophora mangle (≈2 m⁻²) propagules. By 1989, Laguncularia racemosa, with densities up to 12.9 tree m⁻², was a dominant in all plots, although densities were greater at edge plots relative to inner plots, and near open water (west plots) relative to further inland (east plots), and in tall mangrove plots relative to scrub plots. Rhizophora mangle (1989 tree densities about 2 m⁻²) was a codominant in inner and scrub plots, while Avicennia germinans had the lowest densities (<1 tree m⁻²) in all plots. From 1989 to 2000 L. racemosa experienced reduced recruitment and apparent density-dependent mortality of canopy individuals in plots with high initial densities. Scrub plots experienced high rates of colonization by R. mangle and L. racemosa, rapid growth in height of all species (1989–1996), followed by a dieoff of L. racemosa in later years (1997–2000) as the canopy came to resemble that of tall mangrove plots. Colonization and growth rates were lower in gap and closed canopy regions of the natural forest relative to rates in the restored site. After 11 years, densities of L. racemosa were 10–20× lower and R. mangle slightly less in the gap relative to densities in tall mangrove plots in the restored site at the same age. Although the restored stand had converged with the natural forest by 2000 in terms of some factors such as species richness, vegetation cover, litterfall, and light penetration, trees were still much smaller and stem densities much higher. Full development of mature structure and ecological function will likely require decades more development.     

Mangroves found in Dominica (West Indies)

Mangroves has been described for the whole Caribbean are in their different habitats. The island of Dominica (West Indies) has always been excluded from the distribution area due to supposed inadequated conditions for mangrove growth. However, there are small areas with proper edaphic conditions for mangal in which black mangrove (Avicennia germinans L.) and white mangrove (Laguncularia racemosa L.) has been found. Even the occurrence of an additional Avicennia species is under discussion.     

Effect of climate on the growth of annual rings in the main roots of perennial forbs in an Inner Mongolian semi‐arid grassland,China

Question: Is there a pattern in growth of annual rings in roots of perennial forbs in relation to climate and climate extremes in grassland ecosystems? Location: Semi‐arid grassland in Duolun (42°27′N, 116°41′E, 1380 m a.s.l.), central Inner Mongolia, China. Methods: Main roots of three perennial species, Potentilla anserina L., Cymbaria dahurica L. and Lespedeza daurica Schindl., were sampled. Cross‐sections (10–15‐μm thick) were produced from the proximal end of sampled roots using a sledge microtome. Annual growth rings in the main roots were identified and measured by differentiating between earlywood and latewood in the secondary xylem. Relationships between annual growth rings and monthly mean temperature and total monthly precipitation were identified using correlation analysis. Differences in an annual ring width to the previous and following years were examined by calculating a distinctness score. Results: The three perennial forbs showed clearly demarcated annual growth rings in all individuals and the same fluctuation patterns. Their ring widths were generally positively correlated with precipitation from April to October (except for August) and with temperature from February to June (except June for L. daurica), September to October, and the annual mean. Strong deviations of annual ring widths from their neighbour rings were observed in 1998 and 2000. The trend of absolute distinctness scores (D_m) increased significantly from 1988 to 2003, indicating an increase in the frequency of annual ring width variation. Conclusions: Annual growth rings in the main roots of three perennial forb species can be used as an indicator of the influence of climate on below‐ground grassland growth. The change in below‐ground conditions and effects on the functioning of grassland should receive more attention in future studies.     

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.     

Development of microsatellite markers for the white mangrove (Laguncularia racemosa C.F. Gaertn., Combretaceae)

We developed nine new microsatellite markers for the white mangrove (Laguncularia racemosa C.F. Gaertn.), a mangrove species widely distributed in the Western Hemisphere. The loci were developed from a sample collected in Baja California Sur, Mexico, and were screened for polymorphism in 108 individuals from the Pacific Coast of Mexico and Costa Rica. The level of polymorphism was relatively low, from two to four alleles. These are the first set of polymorphic markers described for L. racemosa and should be of value in studying regional genetic differentiation among natural populations of this species.     

外来红树植物拉关木对乡土种桐花树和正红树的化感作用研究

为了探究外来红树植物拉关木对乡土红树植物的化感作用,该研究观察了不同浓度(0.1、0.3、0.5g·mL~(-1))的拉关木根、叶水浸提液对乡土红树植物桐花树和正红树的胚轴(种子)萌发、幼苗生长及叶片抗氧化酶活性的影响。结果表明:(1)拉关木水浸提液对桐花树种子的成苗率、萌发指数和根长均存在抑制作用,其中对根长的抑制作用随水浸提液浓度的提高而增强。(2)根水浸提液对桐花树幼苗的根长、苗高、生物量等生长指标的影响总体上均表现为低浓度促进,高浓度抑制。(3)拉关木水浸提液对正红树胚轴的萌发率、萌发指数、生长指标均表现为促进作用,且根水浸提液0.1、0.3 g·mL~(-1)处理组的芽长以及根、叶水浸提液0.1、0.3 g·mL~(-1)处理组的生物量显著大于对照组;拉关木水浸提液对正红树幼苗的生物量也表现为促进作用。(4)抗性生理方面,随着拉关木水浸提液浓度的升高,桐花树和正红树幼苗SOD活性降低,正红树幼苗POD活性在根水浸提液0.3 g·mL~(-1)和叶水浸提液0.1 g·mL~(-1)处理组显著高于对照组。以上结果表明,不同乡土植物对拉关木化感作用的敏感性不同,拉关木水浸提液抑制了桐花树的生长,而对正红树的生长则表现出一定程度的促进作用。     

Selective feeding by larval dytiscids (Coleoptera:Dytiscidae) and effects of fish predation on upper littoral zone macroinvertebrate communities of arctic lakes

Carbon and nitrogen stable isotopic data from the primary producers in mangrove ecosystems are needed to investigate trophic links and biogeochemical cycling. Compared with other mangrove species (e.g. Rhizophora mangle) very few measurements have been conducted on the white mangrove, Laguncularia racemosa. The carbon and nitrogen stable isotopic and elemental compositions of L. racemosa were analyzed and compared from Florida and Belize. ¹³C values of L. racemosa from Florida (mean = –26.4) were slightly higher than those from Twin Cays, Belize (mean = -27.4), which may be due to higher salinity in some parts of the Florida site. There was no difference between the ¹⁵N values from L. racemosa from these two sites (Florida mean = 0.6; Belize mean = 0.3), which are indicative of nitrogen derived from nitrogen fixation in a planktonic marine system. However, higher ¹⁵N values from L. racemosa at Man of War Cay in Belize (11.4 and 12.3), which is fertilized by roosting marine birds (14.0), illustrate that L. racemosa can sensitively reflect alternative nitrogen sources. Although the isotopic data could not distinguish between Avicennia germinans, R. mangle and L. racemosa in Belize the L. racemosa had considerably higher C/N ratios (46.5 – 116.1) compared with the Florida samples (42.2 – 76.0) or the other mangrove species. Unlike some previous findings from R. mangle, substrate characteristics (e.g. salinity, NH₄ ⁺, and H₂S) were not related to the isotopic or elemental composition of L. racemosa. ¹³C, ¹⁵N and C/N were analyzed for ecosystem components from L. racemosa habitats at Twin Cays, including other plants (e.g. R. mangle, A. germinans and seagrass), detritus, microbial mats and sediments. Results from mass-balance calculations show that mangrove detritus composes very little of the sediment, which is principally composed of microbial biomass (80 – 90%). Detritus at some sites is also influenced by sources other than that from L. racemosa, including seagrass leaves.     

Nutrient and salt relations of <Emphasis Type="Italic">Pterocarpus officinalis</Emphasis> L. in coastal wetlands of the Caribbean: assessment through leaf and soil analyses

Pterocarpus officinalis L. is a dominant tree of freshwater coastal wetlands in the Caribbean and the Guiana regions. It is frequently associated with mangroves in areas with high rainfall and/or surface run-off. We hypothesized that P. officinalis is a freshwater swamp species that when occurring in association with mangroves occupies low-salinity soil microsites, or alternatively that it possesses mechanisms preventing accumulation of salt in photosynthetic tissues. To test this we compared the mineral composition of soils and leaves of several species in two Pterocarpus forests in Puerto Rico associated with coastal mangroves, Sabana Seca and Punta Viento. Results indicate that (1) Sabana Seca has low soil salinity values even in the P. officinalis and Laguncularia racemosa mixed zone. In Punta Viento, salinity in the mixed zone was higher than in the Pterocarpus forest, but much lower than in the mangrove area; (2) In both forests, leaves of P. officinalis showed much lower Na concentrations than mangrove species. The K/Na ratios were 16–20 times higher in P. officinalis, indicating preferential absorption of K against Na. The mangrove fern (Acrostichum aureum) growing side by side with P. officinalis and L. racemosa in Punta Viento also revealed high Na exclusion capacity. We found an asymmetric distribution of cations in the blade and rachis of the P. officinalis compound leaves in both sites. The rachis accumulates more Na and Ca, but less Mg than the leaf blade. This sequestration of Na in the rachis prevents salt damage of photosynthetic tissue in the leaf blade.     

广西山口红树林保护区互花米草扩散动态及其驱动力

互花米草在红树林湿地中扩散对生境与生态系统造成严重影响,已成为生物入侵问题研究的焦点之一。然而目前关于互花米草扩散动态及其驱动力的研究成果仍为鲜见。论文以广西山口红树林保护区为研究对象,以2009、2013、2019年遥感影像为数据源,采用人机互译、野外勘查结合的方法识别互花米草布局,利用土地类型转移矩阵、质心的变化、景观指数以及灰色关联度等方法分析互花米草扩散特征及其驱动力。结果表明：（1）2009-2019年间,互花米草面积、斑块数量呈现增长趋势,但面积增幅下降,2009-2013年间年均增长率为7.60%,2013-2019年间年均增长率则为1.99%;互花米草面积年均增长率大于红树林,光滩转化为互花米草的面积是其转化为红树林面积的1.507倍;互花米草、红树林均有破碎化趋势;（2）2009-2019年间,互花米草的质心坐标都位于丹兜海潮滩,2009-2013年互花米草斑块质心向西北方向偏移,2013-2019年向东南方向偏移;（3）2009-2019年间,互花米草动态变化受到人为因素与自然因素共同影响。其中,赶海人口比例与最大斑块指数、斑块密度、斑块数相关性最大;年均最高温与互花米草分维数、破碎化指数、面积相关性最大。（4）影响互花米草面积变化的因素依次为：年均最高温、年均最低温、赶海人口比例、地区生产总值;互花米草面积与年均最高、年均低温均呈正相关,与赶海人口比例呈负相关。研究结果将为互花米草监控提供科学借鉴,为红树林保护提供理论依据。     

五种红树植物叶片结构的比较研究

以木榄(Bruguiera gymnorrhiza)、桐花树(Aegiceras corniculatum)、拉关木(Laguncularia racemosa)、海芒果(Cerbera manghas)、杨叶肖槿(Thespesia populnea)五种红树植物为材料,采用常规石蜡切片法对它们的叶片横切面结构进行显微观察,比较真红树植物和半红树植物叶片结构的特点及变化规律,研究红树植物叶对盐浸环境的适应性。结果表明:除杨叶肖槿为异面叶、掌状网脉外;拉关木为等面叶、羽状脉,其它三种植物为异面叶、羽状网脉;五种材料具4级叶脉,3级、4级脉具明显维管束鞘。木榄、桐花树、拉关木、杨叶肖槿1级脉为半周韧无限维管束,海芒果1级脉为外韧无限维管束。五种材料叶肉具有分泌腔,除海芒果外,具有含晶体细胞;木榄、桐花树有内皮层,拉关木有贮水组织;桐花树、海芒果有含单宁细胞;桐花树、拉关木、杨叶肖槿有盐腺。这五种植物的叶片结构体现出不同植物对盐浸环境适应性的特征。相比较而言,真红树植物的特化结构较半红树植物多。     

红树植物功能性状对盐和铜胁迫的响应

目前,室内通过人工控制方式开展红树林对环境胁迫的响应研究,忽视了不同红树植物物种的生长习性,且以单一胁迫为主。通过野外生长与人工控制相结合方式,研究5个时期(2018.6-2020.8)盐和铜胁迫对3种红树植物(秋茄、拉关木和木榄)11种功能性状的影响,使用单因素和双因素方差分析研究不同时期不同树种下盐和铜胁迫对叶片功能性状的影响,回答红树植物功能性状对盐和铜单一及复合胁迫响应的科学问题。结果表明,不同时期下,三种红树植物功能性状具有明显差异。随着时间增长,三种植物的碳含量和冠层高度均呈升高趋势,其他功能性状因物种不同而表现出不同趋势;与乡土红树植物(秋茄和木榄)相比,外来入侵物种(拉关木)具有更快的生长速度。不同时期下,拉关木比秋茄和木榄具有更好耐盐性,三个物种的植物生长发育对铜胁迫的响应均不敏感。大部分时期下,单一盐胁迫对拉关木的植物功能性状无显著影响(P < 0.05),对秋茄和木榄的碳含量、冠层高度和比叶面积有显著影响(P < 0.05);单一铜胁迫对三个物种的绝大多数功能性状无显著影响。另外,盐-铜复合胁迫对秋茄叶片碳、磷、氮磷比及冠层高度有显著影响(P < 0.05),对拉关木叶片碳、铜含量、冠层高度、比叶面积有显著影响(P < 0.05),仅对木榄SPAD值有显著影响(P < 0.05)。在红树林保护修复工作中,研究成果可为红树林退化原因和机理阐释、红树林生长监测评估、生境治理以及树种选择提供一定的理论依据。     

Do environmental factors affect the male frequency of exotic mangrove species <Emphasis Type="Italic">Laguncularia racemosa</Emphasis> (Combretaceae) along the southeast coast of China?

The exotic mangrove species Laguncularia racemosa (Combretaceae) is fast-growing and was used for forestation in recent years along the southeast coast of China. The breeding system of L. racemosa is variable among populations, such as hermaphroditism, gynodioecy, and androdioecy. To determine whether androdioecy is widespread in L. racemosa, 19 planted populations were surveyed along the southeast coast of China. To determine whether local environmental factors could affect the sex ratio in androdioecious populations, the observed male frequency of different populations was compared to local average annual temperature, rainfall, and salinity. The results showed that the 19 L. racemosa populations along the southeast coast of China were androdioecious. The male frequencies of these populations varied from 31.0 to ?88.9%. Partial correlation analysis showed that average annual salinity explained 74.7% of the male frequency (p?=?0.001). It is reasonable to note that the male frequency followed a general trend, presenting peak that coincided with the low salinity. The average annual rainfall explained only 30.4% of the male frequency (p?=?0.403). And the average annual temperature explained only 20.2% of the male frequency (p?=?0.206). The variable male frequency in different androdioecious L. racemosa populations may presumably be caused by ecological or genetical processes; these hypotheses will be tested in future studies.     

Reproductive phenology of a northeast Brazilian mangrove community: Environmental and biotic constraints

Brazil has the third largest area of mangrove in the world, which is widely threatened by anthropogenic pressures. We carried out the first long-term phenological study investigating whether environment and competition for pollinators shape the reproduction of a western mangrove community in Brazil, and provide new information for mangrove conservation. We monitored monthly the flowering and fruiting of Avicennia schaueriana, Conocarpus erectus, Laguncularia racemosa and Rhizophora mangle, the only species composing this mangrove community. We applied circular statistics to detect seasonal trends, null models to test for aggregated, staggered or random flowering patterns, performed correlations between phenophases and climate, and calculated intra-specific phenological synchrony. Each species presented a different flowering pattern, from brief annual to continuous and from regular to irregular, resulting in a bimodal pattern at community level. Fruiting was annual or continuous and seasonally unimodal at community level. Precipitation showed the strongest correlation with reproduction for all species, except L. racemosa. Flowering was randomly distributed among species sharing pollinators and each species presented high intra-specific synchrony. The studied mangrove showed a diversity of flowering patterns despite the low number of species. Annual to sub-annual sequential flowering were prevalent, sustaining the pollinators of species all the year long, while the wind-pollinated species flowered continuously. We provide strong evidence that daylength, rainfall and temperature are driving the flowering and fruiting rhythm of these mangrove species.     

Deep phylogeographic splits and limited mixing by sea surface currents govern genetic population structure in the mangrove genus Lumnitzera (Combretaceae) across the Indonesian Archipelago

The Indonesian Archipelago accommodates the largest mangrove area in Southeast Asia and possesses the world's richest composition of mangrove species. The archipelago comprises areas of the biogeographic regions Sunda and Wallacea, separated by Wallace's line. Here, we used the true mangrove species Lumnitzera littorea and Lumnitzera racemosa as a study case for understanding the effects of phylogeographic history, sea surface currents, and geographical distance on genetic diversity and genetic structure. We sampled 14 populations of L. littorea (N = 106) and 21 populations of L. racemosa (N = 152) from Indonesia and used 3122 and 3048 SNP loci, respectively, genotyped using the ddRADseq approach. We assessed genetic diversity, genetic structure, and effective dispersal of the populations and related them to geographical distance and sea surface currents. Our study revealed low levels of genetic variation at the population level in Lumnitzera. Pronounced genetic differentiation between populations indicated two phylogroups in both species. While in L. littorea the two phylogroups were largely separated by Wallace's line, L. racemosa showed a northwest vs. southeast pattern with strong mixture in Wallacea. Our findings provide novel insights into the phylogeography of the mangrove genus Lumnitzera and the role of sea surface currents in the Indonesian Archipelago.     

<Emphasis Type="Italic">Daphnia galeata</Emphasis> in the deep hypolimnion: spatial differentiation of a “typical epilimnetic” species

One of the most serious threats to tropical mangrove ecosystems caused by shrimp farming activities is the poor management of pond waste materials. We hypothesise that mangroves can tolerate chemical residues discharged from shrimp farms and can be used as biofilters, but the capability of mangroves to cope with solid sediments dredged from shrimp ponds is limited. Our study in Pak Phanang, Thailand, confirmed that the excess sediments discharged from nearby shrimp ponds reduced mangrove growth rates and increased mortality rates. A series of transformed multi-temporal satellite images was used in combination with the field data to support this claim. In addition, a comparison between four dominant mangrove species revealed that Avicennia marina could tolerate sedimentation rates of >6 cm year⁻¹, while Bruguiera cylindrica tolerated sedimentation rates of 5 cm year⁻¹ (total sediment depth = 25 cm) before dying, while Excoecaria agallocha and Lumnitzera racemosa performed intermediate. This outcome implied that in our situation A. marina and to lesser extent E. agallocha and L. racemosa could be more effective as biofilters than B. cylindrica, as they may survive the sedimentation longer in the disposal areas. Further studies on the impact of sedimentation and chemical pollution of shrimp farm wastes on mangrove mortality and growth are required.     

Increasing fluctuations of soil salinity affect seedling growth performances and physiology in three Neotropical mangrove species

Background

Micro-tidal wetlands are subject to strong seasonal variations of soil salinity that are likely to increase in amplitude according to climate model predictions for the Caribbean. Whereas the effects of constant salinity levels on the physiology of mangrove species have been widely tested, little is known about acclimation to fluctuations in salinity.

Aims and methods

The aim of this experiment was to characterize the consequences of the rate of increase in salinity (slow versus fast) and salinity fluctuations over time versus constant salt level. Seedling mortality, growth, and leaf gas exchange of three mangrove species, Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle were investigated in semicontrolled conditions at different salt levels (0, 685, 1025, and 1370 mM NaCl).

Results

Slow salinity increase up to 685 mM induced acclimation, improving the salt tolerance of A. germinans and L. racemosa, but had no effect on R. mangle. During fluctuations between 0 and 685 mM, A. germinans and R. mangle were not affected by a salinity drop to zero, whereas L. racemosa took advantage of the brief freshwater episode as shown by the durable improvement of photosynthesis and biomass production.

Conclusions

This study provides new insights into physiological resistance and acclimation to salt stress. We show that seasonal variations of salinity may affect mangrove seedlings’ morphology and physiology as much as annual mean salinity. Moreover, more severe dry seasons due to climate change may impact tree stature and species composition in mangroves through higher mortality rates and physiological disturbance at the seedling stage.     

Climate–growth relationships of the dominant tree species from semi-arid savanna woodland in Ethiopia

Long-term climate–growth relationships, were examined in tree rings of four co-occurring tree species from semi-arid Acacia savanna woodlands in Ethiopia. The main purpose of the study was to prove the presence of annual tree rings, evaluate the relationship between radial growth and climate parameters, and evaluate the association of El Niño and drought years in Ethiopia. The results showed that all species studied form distinct growth boundaries, though differences in distinctiveness were revealed among the species. Tree rings of the evergreen Balanites aegyptiaca were separated by vessels surrounding a thin parenchyma band and the growth boundary of the deciduous acacias was characterized by thin parenchyma bands. The mean annual diameter increment ranged from 3.6 to 5.0 mm. Acacia senegal and Acacia seyal showed more enhanced growth than Acacia tortilis and B. aegyptiaca. High positive correlations were found between the tree-ring width chronologies and precipitation data, and all species showed similar response to external climate forcing, which supports the formation of one tree-ring per year. Strong declines in tree-ring width correlated remarkably well with past El Niño Southern Oscillation (ENSO) events and drought/famine periods in Ethiopia. Spectral analysis of the master tree-ring chronology indicated occurrences of periodic drought events, which fall within the spectral peak equivalent to 2–8 years. Our results proved the strong linkage between tree-ring chronologies and climate, which sheds light on the potential of dendrochronological studies developing in Ethiopia. The outcome of this study has important implications for paleoclimatic reconstructions and in restoration of degraded lands.     

Regeneration of<Emphasis Type="Italic"> Rhizophora mangle</Emphasis> in a Caribbean mangrove forest: interacting effects of canopy disturbance and a stem-boring beetle

Current theory predicts that in low-density, seed-limited plant populations, seed predation will be more important than competition in determining the number of individuals that reach maturity. However, when plant density is high, competition for microsites suitable for establishment and growth is expected to have a relatively greater effect. This dichotomous perspective does not account for situations in which the risk of seed predation differs inside versus outside recruitment microsites. We report the results of a field experiment and sampling studies that demonstrate such an interaction between microsite quality and the risk of propagule predation in mangrove forests on the Caribbean coast of Panama, where it appears to play a key role in shaping the demography and dynamics of the mangrove, Rhizophora mangle. Rhizophora's water-borne propagules establish wherever they strand, but long-term sampling revealed that only those that do so in or near lightning-created canopy gaps survive and grow to maturity. These microsites afford better growth conditions than the surrounding understory and, as importantly, provide a refuge from predation by the scolytid beetle, Coccotrypes rhizophorae. This refuge effect was confirmed with a field experiment in which Rhizophora seedlings were planted at different positions relative to gap edges, from 5 m inside to 20 m outside the gap. Mortality due to beetle attack increased linearly from an average of 10% inside a gap to 72% at 20 m into the forest. The interaction between canopy disturbance and propagule predation may be having a large impact on the composition of our study forests. Being shade-tolerant, Rhizophora seedlings that escape or survive beetle attack can persist in the understory for years. However, the high rate of beetle-induced mortality effectively eliminates the contribution of advance regeneration by Rhizophora saplings to gap succession. This may explain why the shade-intolerant mangrove, Laguncularia racemosa, is able to co-dominate the canopy in low intertidal forests at our study sites.     

Dendrochronological potential of <Emphasis Type="Italic">Millettia stuhlmannii</Emphasis> in Mozambique

Key message

This study demonstrates that Millettia stuhlmannii produces annual growth rings responsive to seasonal climate and should be useful for dendrochronology.

Abstract

Millettia stuhlmannii is a highly valuable and potentially overexploited timber species indigenous to southeastern Africa. It is of particular economic importance in Mozambique though relatively little is known about its growth rate or response to climate. This study investigates whether M. stuhlmannii is potentially useful for dendrochronology—that is whether this species forms annual growth rings that are responsive to external forcing such as climate. Five methods were used to determine whether M. stuhlmannii growth rings are indeed annual in nature, including analysis of ring anatomy, dating trees of known age, cambial wounding, classical cross-dating, and comparison of annual growth to climate variables. Growth rings of Millettia stuhlmannii are distinct and well formed, young trees from plantations of known age formed an appropriate number of distinct annual rings, trees showed distinct wood reaction to cambial wounding, adding exactly one complete ring in one calendar year, cross-dating within and between trees was somewhat successful, and annual growth is significantly correlated with wet season precipitation. Results of this study indicate that M. stuhlmannii is a potentially useful species for dendrochronology. These findings should allow a better understanding of this species’ growth dynamics and ecology, as well as its response to climate variability in the past and potentially to future climate change.

     

Relation of water transport to leaf gas exchange properties in three mangrove species

Mangrove species more tolerant to salinity may function with less efficient water transport, which may be related to more conservative water use. To test the hypothesis, we investigate the gas exchange and hydraulic properties of three mangrove species: Rhizophora mangle L., Laguncularia racemosa Gaert and Avicennia germinans (L.)L. Experiments were performed with adult plants growing naturally in the field under a salinity of 35‰. Gas exchange parameters showed that A. germinans had significantly higher photosynthetic rates, and lower stomatal conductance and transpiration rates, compared to the other two mangroves. In concert with this, instantaneous water use efficiency was significantly high in A. germinans, intermediate in L. racemosa and lowest in R. mangle. The hydraulic parameters of the three mangrove species were in the lowest end of the range reported for tropical trees. However, the three mangrove species exhibited measurable differences in hydraulic parameters related to the control of water requirements for maintenance of carbon gain. L. racemosa and A. germinans showed less efficient water transport at shoot level but were the more efficient species in water use at the leaf level in comparison to R. mangle. Received: 7 April 1999 / Accepted: 25 July 1999