Variation in Stomatal Characteristics of <i>Bursera simaruba</i>(L.) Sarg., a Dominant Tree Species of Tropical Hardwood Hammock Forest across a Habitat Gradient in the Florida Keys

Tree species in coastal forests may exhibit specialization or plasticity in coping with drought through changes in their stomatal morphology or activity, allowing for a balance between gas exchange and water loss in a periodically stressful environment. To examine these responses, we sought to answer two primary research questions: a) how is variation in B. simaruba’s stomatal traits partitioned across hierarchical levels, i.e., site, tree, and leaf;and b) is variation in stomatal traits an integrated response to physiological stress expressed across the habitat gradient of Florida Keys forests? At eight sites distributed throughout the Keys, five leaves were collected from three mature trees for stomatal analysis. Leaf carbon stable isotope ratio (δ13C) was determined to infer the changes in water use efficiency caused by physiological stress experienced by each tree. The results showed that substantial proportions of the total variance in three traits (stomatal density, stomatal size, and δ13C) were observed at all levels, suggesting that processes operating at each scale are important in determining trait values. A significant negative correlation between stomatal density and size across scales was observed. Path model analysis showed that environmental variables, distance to ground water and ground water salinity, affect leaf δ13C indirectly, via its effects on stomatal traits, not directly to leaf δ13C. Therefore, the combination of small and densely distributed stomata seems to represent a strategy that allows B. simaruba to conserve water under conditions of physiological drought induced by either higher ground water salinity or flooding stress at very low elevation.

Progress in the study of mercury methylation and demethylation in aquatic environments

Mercury(Hg) and its compounds are a class of highly toxic and pervasive pollutants.During the biogeochemical cycling of Hg,methylmercury(MeHg),a potent neurotoxin,can be produced and subsequently bioaccumulated along the food chain in aquatic ecosystems.MeHg is among the most widespread contaminants that pose severe health risks to humans and wildlife.Methylation of inorganic mercury to MeHg and demethylation of MeHg are the two most important processes in the cycling of MeHg,determining the levels of MeHg in aquatic ecosystems.This paper reviews recent progress on the study of Hg methylation and demethylation in aquatic environments,focusing on the following three areas:(1) sites and pathways of Hg methylation and demethylation,(2) bioavailability of Hg species for methylation and demethylation,and(3) application of isotope addition techniques in quantitatively estimating the net production of MeHg.

Mobility of toxic metals in sediments: Assessing methods and controlling factors

Metals and metalloids（termed as metals in this article）are important constituent elements of the earth＇s crust.A number of metals,if present in excess,are toxic to organisms and therefore they are usually defined as toxic metals.

Special issue on toxic metal pollution

Metals and metalloids are an integral part of the Earth’s crust.Some of these elements are of great concern due to their high toxicity and threat to humans,wildlife,and the ecosystem.For simplicity,the term toxic metals is adopted to represnt

Refining mercury emission estimations to the atmosphere from iron and steel production

Due to its long-range transport in the atmosphere,mercury is a pollutant of global concern with health risks to humans and ecosystems worldwide（Li et al.,2012;Lindqvist et al.,1991;Liu et al.,2012a;Tang et al.,2016;Wang et al.,2015;Shao et al.,2016）.Atmospheric mercury,mainly from emission from various natural and anthropogenic sources and re-emission of previously released mercury built up in surface soils and oceans,

The effects of habitat fragmentation on the reproduction and abundance of Angadenia berteroi

Aims The pine rocklands of southern Florida are a fire-dependent forest associated with outcrops of limestone.Pine rockland plants have sev-eral adaptations to fire,and for many species,burning increases plant growth,flowering and seedling establishment.The pine rockland forest has been reduced and fragmented in recent decades.Outside of Everglades National Park,only 2%of the original pine rocklands remain,and they are in the form of small fragments.Our objective is to investigate the effects of fragmentation and habitat quality on abun-dance and plant reproductive fitness of Angadenia berteroi(A.D.C.)Miers,a threatened species of the southern Florida pine rockland.Methods We estimated the density of plants using a stratified random sam-pling design,and reproductive fitness(in terms of percentage of plants with flowers and fruit)by walking transects in an array of habitat fragments of different sizes and degrees of isolation(distance to the nearest fragment)as well as in continuous habitat.Structural equation modeling(SEM)was employed to investigate how A.berteroi reproductive fitness was affected by fragmentation and habitat quality.Important Findings Habitat fragment size was correlated with the density of A.berteroi,but did not have a great impact on its reproductive success.However,habitat quality represented by litter depth and subcan-opy cover had strong negative effects on the reproductive fitness of A.berteroi,suggesting that increased light availability and low litter cover resulting from recent fires may favor reproduction.