Response of mangroves to drought and non-tidal conditions in St Lucia Estuary,South Africa

The effect of a prolonged closed-mouth state on the condition of mangrove habitats was studied in May 2010 at St Lucia Estuary, KwaZulu-Natal. The mouth had been closed to the sea since 2002 as a result of artificial mouth closure and drought, which had resulted in non-tidal, dry and hypersaline aquatic conditions. Sediment characteristics, mangrove population structure, and snail and crab abundance, were measured at four sites of differing salinity and water level to determine if there was a relationship between environmental characteristics and biotic response. A site fringing the main channel had the lowest elevation and the highest density of mangrove seedlings and saplings at the water's edge, whereas a dry site at the highest elevation had the lowest density of mangroves with mostly tall adult trees and few saplings. At these two sites the salinity (>34.5) was significantly higher and the surface sediment water content (<30%) was significantly lower than at other sites. Highest tree density was found at a freshwater seepage site, where all size classes were represented, while the permanently flooded site between the Mfolozi River and St Lucia Estuary had the tallest trees but no seedlings or saplings. At the freshwater seepage and permanently flooded sites, porewater and sediment salinity (<10) were significantly lower, while sediment moisture (>55.5%) was significantly higher, than at the fringing and dry sites. Low sediment moisture was unfavourable for mangrove growth and recruitment. Long-term monitoring is needed to document the response of the mangroves to closed-mouth, non-tidal conditions.     

Competition and facilitation may lead to asymmetric range shift dynamics with climate change

Forecasts of widespread range shifts with climate change stem from assumptions that climate drives species' distributions. However, local adaptation and biotic interactions also influence range limits and thus may impact range shifts. Despite the potential importance of these factors, few studies have directly tested their effects on performance at range limits. We address how population‐level variation and biotic interactions may affect range shifts by transplanting seeds and seedlings of western North American conifers of different origin populations into different competitive neighborhoods within and beyond their elevational ranges and monitoring their performance. We find evidence that competition with neighboring trees limits performance within current ranges, but that interactions between adults and juveniles switch from competitive to facilitative at upper range limits. Local adaptation had weaker effects on performance that did not predictably vary with range position or seed origin. Our findings suggest that competitive interactions may slow species turnover within forests at lower range limits, whereas facilitative interactions may accelerate the pace of tree expansions upward near timberline.     

Relationships between height and girth of mangroves and soil-water conditions in the Mary and Hawkesbury River estuaries,eastern Australia

Abstract The height and diameter at breast height (DBH) of mangroves on the Hawkesbury River, New South Wales and the Mary River, Queensland were related to soil-water salinity, soil-water content and distance from the mouth of the estuary. On the Hawkesbury River, both Avicennia marina and Aegiceras corniculatum declined in height and DBH with increasing soil-water salinity and soil-water content, and increased in height with distance from the mouth of the estuary. Both species showed an extensive range of all variables. On the Mary River, where species diversity was higher, no relationship was found between the height and DBH of A. marina and A. corniculatum and soil-water salinity and soil-water content. Both species increased in height with increasing distance from the mouth of the estuary, a characteristic shared with all other species studied (Excoecaria agallocha, Ceriops tagal var. australis, Rhizophora stylosa). The results suggest that growth characteristics of mangroves are not a simple response to salinity gradients in diverse systems and that other variables such as nutrient availability may be important controls on mangrove growth.     

Subtidal macrozoobenthic community resilience to change following natural mouth breaching of the temporarily open/closed East Kleinemonde Estuary,South Africa

The East Kleinemonde Estuary, on the south-east coast of South Africa, typically opens for only short periods, usually <10 days. During a nine-month study in 2006, marine influence persisted for a near-continuous period of three months. This provided an opportunity to explore potential changes in the structure and pattern of the subtidal estuarine macrozoobenthic community. Salinity changes were minimal after mouth opening, decreasing by c. 4 and 8 in the lower and upper estuary, respectively. Salinity decrease was due to river dominance over tidal influence. Two biotic assemblages, structured by sediment characteristics, typified the community throughout the study: a sand-associated group near the mouth and a mud-associated group upstream. Amphipods were proportionally the most abundant group within each assemblage and on each sampling occasion. At the species level, high variability typified the community, with no trends or patterns observed. However, species richness declined from 27–30 to 23 after mouth opening. The number of species returned to former levels at the time of the final survey and just before final mouth closure. An analysis of the subtidal macrozoobenthos in the East Kleinemonde and other temporarily open/closed estuaries (TOCEs) of similar size in the bioregion indicated that communities were typically composed of relatively few, broadly tolerant euryhaline species with no species unique to TOCEs.     

Shift from short-term competition to facilitation with drought stress is due to a decrease in long-term facilitation

Disentangling short- and long-term neighbour effects, using both removal and observational methods within a single experiment, has strongly improved our understanding of the driving mechanisms of plant–plant interactions. However, there has been no attempt to assess two important underlying processes of their changes along gradients, either environmental-severity (changes in target performance without neighbours) or neighbour-traits (changes in performance with neighbours) effects, the former previously shown in alpine communities to be involved in competition and the latter in facilitation. We addressed this goal in an experiment conducted in continental saline depressions (sebkhas) from the Mediterranean arid climate of central Tunisia. We quantified short- and long-term effects of dominant shrubs, transplanting three target grass species in open, nurse and removed-nurse microhabitats of two habitats of different salinity levels in height sebkhas. The design extended greographically from central Tunisia to the Libyan border, 500 km southeastward. We used the relative interaction index to calculate short- and long-term effects before and after the dry summer seasons and environmental-severity and neighbour-trait effects. Short-term effects were slightly negative and long-term effects strongly positive before the dry summer season in the two habitats. Short-term effects switched to positive with increasing drought stress, due to an environmental-severity effect, whereas long-term effects decreased due to a neighbour-trait effect. Salinity did not affect neither short- nor long-term shrub effects. Soil moisture measurements showed that both changes were due to vanishing shrub soil engineering-effects during the summer drought. We conclude that an increase in short-term facilitation with increasing drought stress through time, apparently supporting the stress gradient hypothesis, might be due to a decrease in long-term facilitation. Thus, we recommend using, as much as possible, both the removal and observational methods in experiments assessing changes in plant–plant interactions along stress gradients to avoid wrong conclusions.     

Evaluating,predicting and mapping belowground carbon stores in Kenyan mangroves

Despite covering only approximately 138 000 km², mangroves are globally important carbon sinks with carbon density values three to four times that of terrestrial forests. A key challenge in evaluating the carbon benefits from mangrove forest conservation is the lack of rigorous spatially resolved estimates of mangrove sediment carbon stocks; most mangrove carbon is stored belowground. Previous work has focused on detailed estimations of carbon stores over relatively small areas, which has obvious limitations in terms of generality and scope of application. Most studies have focused only on quantifying the top 1 m of belowground carbon (BGC). Carbon stored at depths beyond 1 m, and the effects of mangrove species, location and environmental context on these stores, are poorly studied. This study investigated these variables at two sites (Gazi and Vanga in the south of Kenya) and used the data to produce a country‐specific BGC predictive model for Kenya and map BGC store estimates throughout Kenya at spatial scales relevant for climate change research, forest management and REDD+ (reduced emissions from deforestation and degradation). The results revealed that mangrove species was the most reliable predictor of BGC; Rhizophora muronata had the highest mean BGC with 1485.5 t C ha^?1. Applying the species‐based predictive model to a base map of species distribution in Kenya for the year 2010 with a 2.5 m² resolution produced an estimate of 69.41 Mt C [±9.15 95% confidence interval (C.I.)] for BGC in Kenyan mangroves. When applied to a 1992 mangrove distribution map, the BGC estimate was 75.65 Mt C (±12.21 95% C.I.), an 8.3% loss in BGC stores between 1992 and 2010 in Kenya. The country‐level mangrove map provides a valuable tool for assessing carbon stocks and visualizing the distribution of BGC. Estimates at the 2.5 m² resolution provide sufficient details for highlighting and prioritizing areas for mangrove conservation and restoration.     

Threats to mangroves from climate change and adaptation options: A review

Mangrove ecosystems are threatened by climate change. We review the state of knowledge of mangrove vulnerability and responses to predicted climate change and consider adaptation options. Based on available evidence, of all the climate change outcomes, relative sea-level rise may be the greatest threat to mangroves. Most mangrove sediment surface elevations are not keeping pace with sea-level rise, although longer term studies from a larger number of regions are needed. Rising sea-level will have the greatest impact on mangroves experiencing net lowering in sediment elevation, where there is limited area for landward migration. The Pacific Islands mangroves have been demonstrated to be at high risk of substantial reductions. There is less certainty over other climate change outcomes and mangrove responses. More research is needed on assessment methods and standard indicators of change in response to effects from climate change, while regional monitoring networks are needed to observe these responses to enable educated adaptation. Adaptation measures can offset anticipated mangrove losses and improve resistance and resilience to climate change. Coastal planning can adapt to facilitate mangrove migration with sea-level rise. Management of activities within the catchment that affect long-term trends in the mangrove sediment elevation, better management of other stressors on mangroves, rehabilitation of degraded mangrove areas, and increases in systems of strategically designed protected area networks that include mangroves and functionally linked ecosystems through representation, replication and refugia, are additional adaptation options.     

Synergistic effects associated with climate change and the development of rocky shore molluscs

Global climate change and ozone layer thinning will simultaneously expose organisms to increasingly stressful conditions. Early life stages of marine organisms, particularly eggs and larvae, are considered most vulnerable to environmental extremes. Here, we exposed encapsulated embryos of three common rocky shore gastropods to simultaneous combinations of ecologically realistic levels of ultraviolet radiation (UVR), water temperature stress and salinity stress to identify potential interactions and associated impacts of climate change. We detected synergistic effects with increases in mortality and retardation in development associated with the most physiologically stressful conditions. The effects of UVR were particularly marked, with mortality increasing up to 12‐fold under stressful conditions. Importantly, the complex outcomes observed on applying multiple stressors could not have been predicted from examining environmental variables in isolation. Hence, we are probably dramatically underestimating the ecological impacts of climate change by failing to consider the complex interplay of combinations of environmental variables with organisms.     

广西北部湾典型海岛红树林的结构特征、空间格局及种间种内关联性

为研究广西北部湾茅尾海自然保护区海岛红树林群落的空间点格局和演替规律,在典型红树林海岛设置调查样地,以样地内的桐花树(Aegiceras corniculatum)、秋茄(Kandelia obovata)、白骨壤(Avicennia marina)等红树植物为研究对象,构建单变量函数的完全空间随机模型、异质泊松模型与双变量函数的环形转变模型,分析3种红树林种群的结构特征、空间格局及种内种间关联性。结果发现：(1)桐花树种群的小树和中树的个体数目较多,种群年龄结构呈金字塔型,种群处于增长状态;秋茄种群的中树个体数目较多,种群结构呈钟型,种群数量处于稳定状态;白骨壤个体数相对较少,且幼树较少老树较多,种群更新受阻,种群结构呈纺锤型,属于衰退型种群。(2)红树林群落整体上呈现出空间集聚的分布特征,从不同种群的集聚程度来看：桐花树>白骨壤>秋茄。(3)随着空间尺度的增大,红树林群落在空间分布上依次表现出聚集、随机和均匀3种特征。(4)桐花树与秋茄、白骨壤的种间关系随着尺度增加均表现为空间无关联性-空间负关联性-空间无关联性的演变特征,秋茄与白骨壤的种间关系则为全尺度的空间无关联性。(5)桐花树种内不同龄级之间存在空间正关联性、空间无关联性和空间负关联性,秋茄种内不同龄级之间均表现出空间无联性,白骨壤种群则在不同龄级之间均出现空间正关联性。(6)红树植物种群的空间格局受到自身繁殖特性、生境异质性、种内和种间竞争、病虫害等多种因素的影响。红树林人工种植和恢复需要结合沿海地区的自然生态条件,按照红树林种群的适应形态学特点、群落类型和自然演替规律,种植乡土红树植物,适当引进外来物种,研制出近似天然的人工红树林林分结构,提高群落的生物多样性和群落稳定性,发挥红树林湿地生态系统的生态环境效应。     

Nitrous oxide fluxes in estuarine environments: response to global change

Nitrous oxide is a powerful, long‐lived greenhouse gas, but we know little about the role of estuarine areas in the global N₂O budget. This review summarizes 56 studies of N₂O fluxes and associated biogeochemical controlling factors in estuarine open waters, salt marshes, mangroves, and intertidal sediments. The majority of in situ N₂O production occurs as a result of sediment denitrification, although the water column contributes N₂O through nitrification in suspended particles. The most important factors controlling N₂O fluxes seem to be dissolved inorganic nitrogen (DIN) and oxygen availability, which in turn are affected by tidal cycles, groundwater inputs, and macrophyte density. The heterogeneity of coastal environments leads to a high variability in observations, but on average estuarine open water, intertidal and vegetated environments are sites of a small positive N₂O flux to the atmosphere (range 0.15–0.91; median 0.31; Tg N₂O‐N yr^?1). Global changes in macrophyte distribution and anthropogenic nitrogen loading are expected to increase N₂O emissions from estuaries. We estimate that a doubling of current median NO₃^? concentrations would increase the global estuary water–air N₂O flux by about 0.45 Tg N₂O‐N yr^?1 or about 190%. A loss of 50% of mangrove habitat, being converted to unvegetated intertidal area, would result in a net decrease in N₂O emissions of 0.002 Tg N₂O‐N yr^?1. In contrast, conversion of 50% of salt marsh to unvegetated area would result in a net increase of 0.001 Tg N₂O‐N yr^?1. Decreased oxygen concentrations may inhibit production of N₂O by nitrification; however, sediment denitrification and the associated ratio of N₂O:N₂ is expected to increase.     

Patterns of persistence and isolation indicate resilience to climate change in montane rainforest lizards

Globally, montane tropical diversity is characterized by extraordinary local endemism that is not readily explained by current environmental variables indicating a strong imprint of history. Montane species often exist as isolated populations under current climatic conditions and may have remained isolated throughout recent climatic cycles, leading to substantial genetic and phenotypic divergence. Alternatively, populations may have become contiguous during colder climates resulting in less divergence. Here we compare responses to historical climate fluctuation in a montane specialist skink, Lampropholis robertsi, and its more broadly distributed congener, L. coggeri, both endemic to rainforests of northeast Australia. To do so, we combine spatial modelling of potential distributions under representative palaeoclimates, multi‐locus phylogeography and analyses of phenotypic variation. Spatial modelling of L. robertsi predicts strong isolation among disjunct montane refugia during warm climates, but with potential for localized exchange during the most recent glacial period. In contrast, predicted stable areas are more widespread and connected in L. coggeri. Both species exhibit pronounced phylogeographic structuring for mitochondrial and nuclear genes, attesting to low dispersal and high persistence across multiple isolated regions. This is most prominent in L. robertsi, for which coalescent analyses indicate that most populations persisted in isolation throughout the climate cycles of the Pleistocene. Morphological divergence, principally in body size, is more evident among isolated populations of L. robertsi than L. coggeri. These results highlight the biodiversity value of isolated montane populations and support the general hypothesis that tropical montane regions harbour high levels of narrow‐range taxa because of their resilience to past climate change.     

Research challenges to improve the management and conservation of subtropical reefs to tackle climate change threats

This paper reports on a workshop conducted in Australia in 2010, entitled ‘Management, Conservation, and Scientific Challenges on Subtropical Reefs under Climate Change’. The workshop brought together 26 experts actively involved in the science and management of subtropical reefs. Its primary aim was to identify the areas of research that need to be most urgently addressed to improve the decision‐making framework for managers of subtropical reefs. The main findings of the workshop were a sustainable subtropical reefs declaration that highlights seven research priorities for subtropical reefs. These are to (i) conduct research and management activities across local government, state and bioregion borders; (ii) understand natural variability of environmental conditions; (iii) quantify socio‐economic factors and ecosystem services; (iv) benchmark cross‐realm connectivity; (v) know marine population connectivity; (vi) habitat mapping and ecological research; and (v) determine refugia. These findings are hoped to form a basis for focussing research efforts, leveraging funds and assisting managers with allocation of resources.     

Nonlinearity in interspecific interactions in response to climate change: Cod and haddock as an example

Climate change has profound ecological effects, yet our understanding of how trophic interactions among species are affected by climate change is still patchy. The sympatric Atlantic haddock and cod are co‐occurring across the North Atlantic. They compete for food at younger stages and thereafter the former is preyed by the latter. Climate change might affect the interaction and coexistence of these two species. Particularly, the increase in sea temperature (ST) has been shown to affect distribution, population growth and trophic interactions in marine systems. We used 33‐year long time series of haddock and cod abundances estimates from two data sources (acoustic and trawl survey) to analyse the dynamic effect of climate on the coexistence of these two sympatric species in the Arcto‐Boreal Barents Sea. Using a Bayesian state‐space threshold model, we demonstrated that long‐term climate variation, as expressed by changes of ST, affected species demography through different influences on density‐independent processes. The interaction between cod and haddock has shifted in the last two decades due to an increase in ST, altering the equilibrium abundances and the dynamics of the system. During warm years (ST over ca. 4°C), the increase in the cod abundance negatively affected haddock abundance while it did not during cold years. This change in interactions therefore changed the equilibrium population size with a higher population size during warm years. Our analyses show that long‐term climate change in the Arcto‐Boreal system can generate differences in the equilibrium conditions of species assemblages.     

适应气候变化的海岸带韧性研究综述——基于文献计量分析

海岸带是人口经济高度集聚且生态敏感区域,提升其适应气候变化扰动的韧性能力是促进沿海地区可持续治理的重要手段。研究从适应气候变化风险出发,在辨析海岸带韧性概念、内涵及特征的基础上,运用软件分析法和归纳总结法梳理海岸带韧性相关研究的热点结构分布、演进脉络特征和热词主题类别,指出当前海岸带韧性研究聚焦于韧性作用机制解析、韧性表征测度、韧性提升路径和韧性管理策略4大方面。研究表明灾害影响范围界定、多灾耦合的链式演变、复杂系统时空韧性测度和人-地-海弹性匹配尚存薄弱之处,提出我国海岸带气候韧性未来研究需关注理论体系建构、空间规划实践和多学科交叉融合。依据"原则-目标-路径"逻辑从治理对象、治理主体、治理空间和治理机制4个维度展望海岸带气候韧性治理框架,即辨识治理对象,以风险链评估为韧性治理起点;协同治理主体,探索跨权益治理的韧性平衡;统筹治理空间,建构韧性空间优化治理模式;创新治理机制,完善韧性治理能级长效路径。     

Seedling recruitment, survival and facilitation in alpine Pinus uncinata tree line ecotones. Implications and potential responses to climate warming

Aims Alpine tree line ecotones are harsh environments where low temperatures constrain tree regeneration and growth. However, the expected upward shift of tree line ecotones in response to climate warming has not been ubiquitous. The lack of coupling between tree line dynamics and climate warming might be explained by factors other than climate variation that determine seedling recruitment in these ecotones. We want to assess how the availability of suitable habitat for establishment and the effects of facilitation on seedling survival and growth affect tree recruitment within tree line ecotones and modulate their responses to climate. Location We evaluate the relevance of these factors for Pinus uncinata tree line ecotones in the Catalan Pyrenees (north‐east Spain) and Andorra. Methods We analysed the microhabitat of naturally established seedlings in rectangular plots at the tree line ecotone, assessing the habitat type and the proximity to potentially protective elements that may improve microsite conditions. We tested whether krummholz individuals influence regeneration at the tree line by performing a transplantation field experiment to evaluate the extent of facilitation on seedling survival and growth in height. A total of 820 seedlings were transplanted at different distances and orientations (resulting in 12 positions) from krummholz mats and monitored over 2 years. Results Safe sites for P. uncinata recruits consisted of sparse vegetation covering bare soil, gravel or litter, and close to protective elements that may ameliorate microsite conditions. The field experiment showed that directional positive interactions enhance seedling survival and growth, altering the spatial patterns of recruit survivorship, especially during harsh winter conditions (shallow and irregular snowpack). Main conclusions Our results suggest that scarce availability of safe sites and uneven facilitation by krummholz control seedling recruitment patterns within alpine tree line ecotones. Such constraints may distort or counter the response of tree line ecotones to climate warming at local and regional scales.     

Response of a rocky intertidal ecosystem engineer and community dominant to climate change

To evaluate how climate change might impact a competitively dominant ecological engineer, we analysed the growth response of the mussel Mytilus californianus to climate patterns [El Niño-Southern Oscillation, Pacific Decadal Oscillation (PDO)]. Mussels grew faster during warmer climatic events. Growth was initially faster on a more productive cape compared to a less productive cape. Growth rates at the two capes merged in 2002, coincidentally with a several year-long shift from warm to cool PDO conditions. To determine the mechanism underlying this response, we examined growth responses to intertidal sea and air temperatures, phytoplankton, sea level and tide height. Together, water temperature (32%) and food (12.5%) explained 44.5% of the variance in mussel growth; contributions of other factors were not significant. In turn, water temperature and food respond to climate-driven variation in upwelling and other, unknown factors. Understanding responses of ecosystem engineers to climate change will require knowing direct thermal effects and indirect effects of factors altered by temperature change.