Grazing effects on the species‐area relationship: Variation along a climatic gradient in NE Spain

Questions: Does grazing have the same effect on plant species richness at different spatial scales? Does the effect of spatial scale vary under different climatic conditions and vegetation types? Does the slope of the species‐area curve change with grazing intensity similarly under different climatic conditions and vegetation types? Location: Pastures along a climatic gradient in northeastern Spain. Methods: In zones under different regimes of sheep grazing (high‐, low‐pressure, abandonment), plant species richness was measured in different plot sizes (from 0.01 to 100 m2) and the slope of the species‐area curves was calculated. The study was replicated in five different locations along a climatic gradient from lowland semi‐arid rangelands to upland moist grasslands. Results: Species richness tended to increase with grazing intensity at all spatial scales in the moist upland locations. On the contrary, in the most arid locations, richness tended to decrease, or remain unchanged, with grazing due to increased bare soil. Grazing differentially affected the slope (z) of the species‐area curve (power function S=c A^z) in different climatic conditions: z tended to increase with grazing in arid areas and decrease in moist‐upland ones. ß‐diversity followed similar pattern as z. Conclusions: Results confirm that the impact of grazing on plant species richness are spatial‐scale dependent. However, the effects on the species‐area relationship vary under different climatic conditions. This offers a novel insight on the patterns behind the different effects of grazing on diversity in moist vs. arid conditions reported in the literature. It is argued that the effect of spatial scale varies because of the different interaction between grazing and the intrinsic spatial structure of the vegetation. Variations in species‐area curves with grazing along moisture gradients suggest also a different balance of spatial components of diversity (i.e. a‐ and ß‐diversity).     

Plot sizes used for phytosociological sampling of European vegetation

Abstract. In European phytosociology, variable plot sizes are traditionally used for sampling different vegetation types. This practice may generate problems in current vegetation or habitat survey projects based on large data sets, which include relevés made by many authors at different times. In order to determine the extent of variation in plot sizes used in European phytosociology, we collected a data set of 41 174 relevés with an indication of plot size, published in six major European journals focusing on phytosociology from 1970 to 2000. As an additional data set, we took 27 365 relevés from the Czech National Phytosociological Database. From each data set, we calculated basic statistical figures for plot sizes used to sample vegetation of various phytosociological classes. The results show that in Europe the traditionally used size of vegetation plots is roughly proportional to vegetation height; however, there is a large variation in plot size, both within and among vegetation classes. The effect of variable plot sizes on vegetation analysis and classification is not sufficiently known, but use of standardized plot sizes would be desirable in future projects of vegetation or habitat survey. Based on our analysis, we suggest four plot sizes as possible standards. They are 4 m² for sampling aquatic vegetation and low‐grown herbaceous vegetation, 16 m² for most grassland, heathland and other herbaceous or low‐scrub vegetation types, 50 m² for scrub, and 200 m² for woodlands. It has been pointed out that in some situations, sampling in either small or large plots may result in assignment of relevés to different phytosociological classes or habitat types. Therefore defining vegetation and habitat types as scale‐dependent concepts is needed.     

Gas exchange and photosynthetic acclimation over subambient to elevated CO2 in a C3–C4 grassland

Atmospheric CO₂ (C_a) has risen dramatically since preglacial times and is projected to double in the next century. As part of a 4‐year study, we examined leaf gas exchange and photosynthetic acclimation in C₃ and C₄ plants using unique chambers that maintained a continuous C_a gradient from 200 to 550 µmol mol^?1 in a natural grassland. Our goals were to characterize linear, nonlinear and threshold responses to increasing C_a from past to future C_a levels. Photosynthesis (A), stomatal conductance (g_s), leaf water‐use efficiency (A/g_s) and leaf N content were measured in three common species: Bothriochloa ischaemum, a C₄ perennial grass, Bromus japonicus, a C₃ annual grass, and Solanum dimidiatum, a C₃ perennial forb. Assimilation responses to internal CO₂ concentrations (A/C_i curves) and photosynthetically active radiation (A/PAR curves) were also assessed, and acclimation parameters estimated from these data. Photosynthesis increased linearly with C_a in all species (P < 0.05). S. dimidiatum and B. ischaemum had greater carboxylation rates for Rubisco and PEP carboxylase, respectively, at subambient than superambient C_a (P < 0.05). To our knowledge, this is the first published evidence of A up‐regulation at subambient C_a in the field. No species showed down‐regulation at superambient C_a. Stomatal conductance generally showed curvilinear decreases with C_a in the perennial species (P < 0.05), with steeper declines over subambient C_a than superambient, suggesting that plant water relations have already changed significantly with past C_a increases. Resource‐use efficiency (A/g_s and A/leaf N) in all species increased linearly with C_a. As both C₃ and C₄ plants had significant responses in A, g_s, A/g_s and A/leaf N to C_a enrichment, future C_a increases in this grassland may not favour C₃ species as much as originally thought. Non‐linear responses and acclimation to low C_a should be incorporated into mechanistic models to better predict the effects of past and present rising C_a on grassland ecosystems.     

北京山区河岸带植物群落种-面积关系

种-面积关系是群落生态学的基本问题之一,是了解植物群落结构的重要途径。为摸清北京山区河流河岸带植物群落调查最小样方面积,在北京市怀柔区怀九河河岸带沿线,采用基于河岸带立地条件逐步扩大样地面积的方法布设50个80m长样地,调查计算并拟合不同类型河岸带所需的最小样地面积。研究结果表明:北京市怀柔区怀九河河岸带植物种数255种,隶属于70科185属。通过聚类分析将怀九河河岸带分为自然河岸带、近自然河岸带、人工岸坡乔灌草河岸带、人工岸坡观赏性乔灌草河岸带、人工岸坡疏乔灌草干砌石河岸带和人工岸坡浆砌石河岸带6种类型。根据赤池信息量准则AIC可知自然河岸带、近自然河岸带、人工岸坡乔灌草河岸带和人工岸坡疏乔灌草干砌石河岸带优先选取S=c-ae~(-bA),人工岸坡观赏性乔灌草河岸带优先选取S=aA/(1+bA),人工岸坡浆砌石河岸带优先选取S=c/(1+ae~(-bA))。满足相同比例植物种调查,不同类型河岸带所需最小样地面积存在明显差异,当满足河岸带植物调查80%植物种时,人工岸坡浆砌石河岸带(84m~2)和自然河岸带(217m~2)所需样地面积较小,其次是人工岸坡疏乔灌草干砌石河岸带(362m~2),近自然河岸带(450m~2)和人工岸坡乔灌草河岸带(460m~2)所需样地面积相似,而人工岸坡观赏性乔灌草河岸带所需样地面积最大为571m~2。所得出的河岸带植物调查最小样地面积对于河岸带生物多样性保护和指导河岸带生态修复具有重要意义。     

Phytosociological data give biased estimates of species richness

Abstract. Large phytosociological data sets of three types of grassland and three types of forest vegetation from the Czech Republic were analysed with a focus on plot size used in phytosociological sampling and on the species‐area relationship. The data sets included 12975 relevés, sampled by different authors in different parts of the country between 1922 and 1999. It was shown that in the grassland data sets, the relevés sampled before the 1960s tended to have a larger plot size than the relevés made later on. No temporal variation in plot sizes used was detected in forest relevés. Species‐area curves fitted to the data showed unnatural shapes, with levelling‐off or even decrease in plot sizes higher than average. This distortion is explained by the subjective, preferential method of field sampling used in phytosociology. When making relevés in species‐poor vegetation, researchers probably tend to use larger plots in order to include more species. The reason for this may be that a higher number of species gives a higher probability of including presumed diagnostic species, so that the relevé can be more easily classified in the Braun‐Blanquet classification system. This attitude of phytosociologists has at least two consequences: (1) in phytosociological data bases species‐poor vegetation types are underrepresented or relevés are artificially biased towards higher species richness; (2) the suitability of phytosociological data for species richness estimation is severely limited.     

Local–regional boundary shifts in oribatid mite (Acari: Oribatida) communities: species–area relationships in arboreal habitat islands of a coastal temperate rain forest, Vancouver Island, Canada

Aim This study investigates the species–area relationship (SAR) for oribatid mite communities of isolated suspended soil habitats, and compares the shape and slope of the SAR with a nested data set collected over three spatial scales (core, patch and tree level). We investigate whether scale dependence is exhibited in the nested sampling design, use multivariate regression models to elucidate factors affecting richness and abundance patterns, and ask whether the community composition of oribatid mites changes in suspended soil patches of different sizes. Location Walbran Valley, Vancouver Island, Canada. Methods A total of 216 core samples were collected from 72 small, medium and large isolated suspended soil habitats in six western redcedar trees in June 2005. The relationship between oribatid species richness and habitat volume was modelled for suspended soil habitat isolates (type 3) and a nested sampling design (type 1) over multiple spatial scales. Nonlinear estimation parameterized linear, power and Weibull function regression models for both SAR designs, and these were assessed for best fit using R² and Akaike's information criteria (ΔAIC) values. Factors affecting oribatid mite species richness and standardized abundance (number per g dry weight) were analysed by anova and linear regression models. Results Sixty‐seven species of oribatid mites were identified from 9064 adult specimens. Surface area and moisture content of suspended soils contributed to the variation in species richness, while overall oribatid mite abundance was explained by moisture and depth. A power‐law function best described the isolate SAR (S = 3.97 × A^0.12, R² = 0.247, F_1,70 = 22.450, P < 0.001), although linear and Weibull functions were also valid models. Oribatid mite species richness in nested samples closely fitted a power‐law model (S = 1.96 × A^0.39, R² = 0.854, F_1,18 = 2693.6, P < 0.001). The nested SAR constructed over spatial scales of core, patch and tree levels proved to be scale‐independent. Main conclusions Unique microhabitats provided by well developed suspended soil accumulations are a habitat template responsible for the diversity of canopy oribatid mites. Species–area relationships of isolate vs. nested species richness data differed in the rate of accumulation of species with increased area. We suggest that colonization history, stability of suspended soil environments, and structural habitat complexity at local and regional scales are major determinants of arboreal oribatid mite species richness.     

Pre‐dawn stomatal opening does not substantially enhance early‐morning photosynthesis in Helianthus annuus

Most C₃ plant species have partially open stomata during the night especially in the 3–5 h before dawn. This pre‐dawn stomatal opening has been hypothesized to enhance early‐morning photosynthesis (A) by reducing diffusion limitations to CO₂ at dawn. We tested this hypothesis in cultivated Helianthus annuus using whole‐shoot gas exchange, leaf level gas exchange and modelling approaches. One hour pre‐dawn low‐humidity treatments were used to reduce pre‐dawn stomatal conductance (g). At the whole‐shoot level, a difference of pre‐dawn g (0.40 versus 0.17 mol m^?2 s^?1) did not significantly affect A during the first hour after dawn. Shorter term effects were investigated with leaf level gas exchange measurements and a difference of pre‐dawn g (0.10 versus 0.04 mol m^?2 s^?1) affected g and A for only 5 min after dawn. The potential effects of a wider range of stomatal apertures were explored with an empirical model of the relationship between A and intercellular CO₂ concentration during the half‐hour after dawn. Modelling results demonstrated that even extremely low pre‐dawn stomatal conductance values have only a minimal effect on early‐morning A for a few minutes after dawn. Thus, we found no evidence that pre‐dawn stomatal opening enhances A.     

Robust methods for detecting a small island effect

Aim The small island effect (SIE), i.e. the hypothesis that species richness below a certain threshold area varies independently of island size, has become a widely accepted part of the theory of island biogeography. However, there are doubts whether the findings of SIEs were based on appropriate methods. The aim of this study was thus to provide a statistically sound methodology for the detection of SIEs and to show this by re‐analysing data in which an SIE has recently been claimed ( Sfenthourakis & Triantis, 2009 , Diversity and Distributions, 15 , 131–140). Location Ninety islands of the Aegean Sea (Greece). Methods First, I reviewed publications on SIEs and evaluated their methodology. Then, I fitted different species–area models to the published data of area (A) and species richness (S) of terrestrial isopods (Oniscidea), with log A as predictor and both S (logarithm function) and log S (power function) as response variables: (i) linear; (ii) quadratic; (iii) cubic; (iv) breakpoint with zero slope to the left (SIE model); (v) breakpoint with zero slope to the right; (vi) two‐slope model. I used non‐linear regression with R²_adj., AIC_c and BIC as goodness‐of‐fit measures. Results Many different methods have been applied for detecting SIEs, all of them with serious shortcomings. Contrary to the claim of the original study, no SIE occurs in this particular dataset as the two‐slope variants performed better than the SIE variants for both the logarithm and power functions. Main conclusions For the unambiguous detection of SIEs, one needs to (i) include islands with no species; (ii) compare all relevant models; and (iii) account for different model complexities. As none of the reviewed SIE studies met all these criteria, their findings are dubious and SIEs may be less common than reported. Thus, conservation‐related predictions based on the assumption of SIEs may be unreliable.     

Elevated CO2 affects photosynthetic responses in canopy pine and subcanopy deciduous trees over 10 years: a synthesis from Duke FACE

Leaf responses to elevated atmospheric CO₂ concentration (C_a) are central to models of forest CO₂ exchange with the atmosphere and constrain the magnitude of the future carbon sink. Estimating the magnitude of primary productivity enhancement of forests in elevated C_a requires an understanding of how photosynthesis is regulated by diffusional and biochemical components and up‐scaled to entire canopies. To test the sensitivity of leaf photosynthesis and stomatal conductance to elevated C_a in time and space, we compiled a comprehensive dataset measured over 10 years for a temperate pine forest of Pinus taeda, but also including deciduous species, primarily Liquidambar styraciflua. We combined over one thousand controlled‐response curves of photosynthesis as a function of environmental drivers (light, air C_a and temperature) measured at canopy heights up to 20 m over 11 years (1996–2006) to generate parameterizations for leaf‐scale models for the Duke free‐air CO₂ enrichment (FACE) experiment. The enhancement of leaf net photosynthesis (A_net) in P. taeda by elevated C_a of +200 μmol mol^?1 was 67% for current‐year needles in the upper crown in summer conditions over 10 years. Photosynthetic enhancement of P. taeda at the leaf‐scale increased by two‐fold from the driest to wettest growing seasons. Current‐year pine foliage A_net was sensitive to temporal variation, whereas previous‐year foliage A_net was less responsive and overall showed less enhancement (+30%). Photosynthetic downregulation in overwintering upper canopy pine needles was small at average leaf N (N_area), but statistically significant. In contrast, co‐dominant and subcanopy L. styraciflua trees showed A_net enhancement of 62% and no A_net–N_area adjustments. Various understory deciduous tree species showed an average A_net enhancement of 42%. Differences in photosynthetic responses between overwintering pine needles and subcanopy deciduous leaves suggest that increased C_a has the potential to enhance the mixed‐species composition of planted pine stands and, by extension, naturally regenerating pine‐dominated stands.     

Biomass and leaf‐level gas exchange characteristics of three African savanna C4 grass species under optimum growth conditions

C₄ savanna grass species, Digitaria eriantha, Eragrostis lehmanniana and Panicum repens, were grown under optimum growth conditions with the aim of characterizing their above‐ and below‐ground biomass allocation and the response of their gas exchange to changes in light intensity, CO₂ concentration and leaf‐to‐air vapour pressure deficit gradient (D_l). Digitaria eriantha showed the largest above‐ and below‐ground biomass, high efficiency in carbon gain under light‐limiting conditions, high water use efficiency (WUE) and strong stomatal sensitivity to D_l (P = 0.002; r² = 0.5). Panicum repens had a high aboveground biomass and attained high light saturated photosynthetic rates (A_sat, 47 μmol m^?2 s^?1), stomatal conductance, (g_sat, 0.25 mol m^?2 s^?1) at relatively high WUE. Eragrostis lehmanniana had almost half the biomass of other species, and had similar A_sat and g_sat but were attained at lower WUE than the other species. This species also showed the weakest stomatal response to D_l (P = 0.19, r² = 0. 1). The potential ecological significance of the contrasting patterns of biomass allocation and variations in gas exchange parameters among the species are discussed.     

Soil carbon changes under Miscanthus driven by C4 accumulation and C3 decompostion – toward a default sequestration function

Bioenergy has to meet increasing sustainability criteria in the EU putting conventional bioenergy crops under pressure. Alternatively, perennial bioenergy crops, such as Miscanthus, show higher greenhouse gas savings with similarly high energy yields. In addition, Miscanthus plantations may sequester additional soil organic carbon (SOC) to mitigate climate change. As the land‐use change in cropland to Miscanthus involves a C₃‐C₄ vegetation change (VC), it is possible to determine the dynamic of Miscanthus‐derived SOC (C₄ carbon) and of the old SOC (C₃ carbon) by the isotopic ratio of ¹³C to ¹²C. We sampled six croplands and adjacent Miscanthus plantations exceeding the age of 10 years across Europe. We found a mean C₄ carbon sequestration rate of 0.78 ± 0.19 Mg ha^?1 yr^?1, which increased with mean annual temperature. At three of six sites, we found a significant increase in C₃ carbon due to the application of organic fertilizers or difference in baseline SOC, which we define as non‐VC‐induced SOC changes. The Rothamsted Carbon Model was used to disentangle the decomposition of old C₃ carbon and the non‐VC‐induced C₃ carbon changes. Subsequently, this method was applied to eight more sites from the literature, resulting in a climate‐dependent VC‐induced SOC sequestration rate (0.40 ± 0.20 Mg ha^?1 yr^?1), as a step toward a default SOC change function for Miscanthus plantations on former croplands in Europe. Furthermore, we conducted a SOC fractionation to assess qualitative SOC changes and the incorporation of C₄ carbon into the soil. Sixteen years after Miscanthus establishment, 68% of the particulate organic matter (POM) was Miscanthus‐derived in 0–10 cm depth. POM was thus the fastest cycling SOC fraction with a C₄ carbon accumulation rate of 0.33 ± 0.05 Mg ha^?1 yr^?1. Miscanthus‐derived SOC also entered the NaOCl‐resistant fraction, comprising 12% in 0–10 cm, which indicates that this fraction was not an inert SOC pool.     

Comparative analysis of thylakoid protein complexes in the mesophyll and bundle sheath cells from C3, C4 and C3–C4 Paniceae grasses

To better understand the coordination between dark and light reactions during the transition from C₃ to C₄ photosynthesis, we optimized a method for separating thylakoids from mesophyll (MC) and bundle sheath cells (BSCs) across different plant species. We grew six Paniceae grasses including representatives from the C₃, C₃–C₄ and C₄ photosynthetic types and all three C₄ biochemical subtypes [nicotinamide adenine dinucleotide phosphate‐dependent malic enzyme (NADP‐ME), nicotinamide adenine dinucleotide‐dependent malic enzyme (NAD‐ME) and phosphoenolpyruvate carboxykinase (PEPCK)] in addition to Zea mays under control conditions (1000 μmol quanta m^?2 s^?1 and 400 ppm of CO₂). Proteomics analysis of thylakoids under native conditions, using blue native polyacrylamide gel electrophoresis followed by liquid chromatography‐mass spectrometry (LC‐MS), demonstrated the presence of subunits of all light‐reaction‐related complexes in all species and cell types. C₄ NADP‐ME species showed a higher photosystems I/II ratio and a clear accumulation of the NADH dehydrogenase‐like complexes in BSCs, while Cytb₆f was more abundant in BSCs of C₄ NAD‐ME species. The C₄ PEPCK species showed no clear differences between cell types. Our study presents, for the first time, a good separation between BSC and MC for a C₃–C₄ intermediate grass which did not show noticeable differences in the distribution of the thylakoid complexes. For the NADP‐ME species Panicum antidotale, growth at glacial CO₂ (180 ppm of CO₂) had no effect on the distribution of the light‐reaction complexes, while growth at low light (200 μmol quanta m^?2 s^?1) promoted the accumulation of light‐harvesting proteins in both cell types. These results add to our understanding of thylakoid distribution across photosynthetic types and subtypes, and introduce thylakoid distribution between the MC and BSC of a C₃–C₄ intermediate species.     

Species richness, environmental heterogeneity and area: a case study based on land snails in Skyros archipelago (Aegean Sea, Greece)

Aim To test the performance of the choros model in an archipelago using two measures of environmental heterogeneity. The choros model is a simple, easy‐to‐use mathematical relationship which approaches species richness as a combined function of area and environmental heterogeneity. Location The archipelago of Skyros in the central Aegean Sea (Greece). Methods We surveyed land snails on 12 islands of the archipelago. We informed the choros model with habitat data based on natural history information from the land snail species assemblage. We contrast this with habitat information taken from traditional vegetation classification to study the behaviour of choros with different measures of environmental heterogeneity. R² values and Akaike's information criterion (AIC) were used to compare the choros model and the Arrhenius species–area model. Path analysis was used to evaluate the variance in species richness explained by area and habitat diversity. Results Forty‐two land snail species were recorded, living in 33 different habitat types. The choros model with habitat types had more explanatory power than the classic species–area model and the choros model using vegetation types. This was true for all islands of the archipelago, as well as for the small islands alone. Combined effects of area and habitat diversity primarily explain species richness in the archipelago, but there is a decline when only small islands are considered. The effects of area are very low both for all the islands of the archipelago, and for the small islands alone. The variance explained by habitat diversity is low for the island group as a whole, but significantly increases for the small islands. Main conclusions The choros model is effective in describing species‐richness patterns of land snails in the Skyros Archipelago, incorporating ecologically relevant information on habitat occupancy and area. The choros model is more effective in explaining richness patterns on small islands. When using traditional vegetation types, the choros model performs worse than the classic species–area relationship, indicating that use of proxies for habitat diversity may be problematic. The slopes for choros and Arrhenius models both assert that, for land snails, the Skyros Archipelago is a portion of a larger biogeographical province. The choros model, informed by ecologically relevant habitat measures, in conjunction with path analysis points to the importance of habitat diversity in island species richness.     

Seed bank dynamics and tree‐fall gaps in a northwestern Mexican Quercus‐Pinus forest

Questions: How does the seed bank respond to different types of tree‐fall gaps and seasonal variations? How does the soil seed bank influence recovery of the standing vegetation in the mature forest and tree‐fall gaps? Location: 1800 — 2020 m a.s.l., Quercus‐Pinus forest, Baja California Sur, Mexico. Methods: Seed size, species composition and germination were estimated under different environmental conditions during dry and rainy seasons: a mature forest plot and gaps created by dead standing trees, snapped‐of f trees and uprooted trees. The soil seed bank was investigated using direct propagule emergence under laboratory conditions, from soil cores obtained during both seasons. Results: 21 species, 20 genera and 14 families constitute the seed bank of this forest community. Fabaceae, Asteraceae, Euphorbiaceae and Lamiaceae were the most frequently represented families in the seed bank. Floristic composition and species richness varied according to the different modes of tree death. Species composition of seed banks and standing vegetation had very low similarity coefficients and were statistically different. Seed bank sizes varied between 164 and 362 ind.m^‐2 in the mature forest plot for the dry and rainy seasons, respectively, while soil seed bank sizes for gaps ranged between 23–208 ind.m^‐2 forthe dry season and between 81–282 ind.m^‐2 for the rainy season. Conclusions: Seed bank sizes and germination response were always higher in the rainy season under all the environmental conditions analysed. Results suggest that timing responses to gap formation of the soil seed bank could be more delayed in this temperate forest than expected.     

The species–area relationship derived from species-specific incidence functions

The species–area relationship (SAR), describing the increase in species number (S) with increasing area (A), is one of the most robust patterns in ecology with great significance for conservation. The SAR is generally formulated as a power function, S = kA^z, although the semilogarithmic form S = a + b log A has often been used by botanists. Here we unite the two forms by deriving SARs from the incidence functions of the species that make up the community. We show how the decisive scaling parameters z and b relate to the properties of individual species, and highlight why the biological interpretation of SARs has been so enigmatic.     

Mesophyll cell properties for some C3 and C4 species with high photosynthetic rates

Leaves of twelve C₃ species and six C₄ species were examined to understand better the relationship between mesophyll cell properties and the generally high photosynthetic rates of these plants. The CO₂ diffusion conductance expressed per unit mesophyll cell surface area (g_CO2^cell) cell was determined using measurements of the net rate of CO₂ uptake, water vapor conductance, and the ratio of mesophyll cell surface area to leaf surface area (A^mes/A). A^mes/A averaged 31 for the C₃ species and 16 for the C₄ species. For the C₃ species g_CO2^cell ranged from 0.12 to 0.32 mm s^-1, and for the C₄ species it ranged from 0.55 to 1.5 mm s^-1, exceeding a previously predicted maximum of 0.5 mm s^-1. Although the C₃ species Cammissonia claviformis did not have the highest g_CO2^cell, the combination of the highest A^mes and highest stomatal conductance resulted in this species having the greatest maximum rate of CO₂ uptake in low oxygen, 93 μmol m^-2 s^-1 (147 mg dm^-2 h^-1). The high g_CO2^cell of the C₄ species Amaranthus retroflexus (1.5 mm s^-1) was in part attributable to its thin cell wall (72 nm thick).     

Relationship between topographic heterogeneity and vegetation patterns in a Californian salt marsh

Questions: Are species richness and species abundances higher in the presence of tidal creeks? Do species richness and species abundances vary with plot size? Location: Intertidal plain of Volcano Marsh, Bahia de San Quintin, Mexico. Methods: We analysed vegetation patterns in large areas (cells) with tidal creeks (+creek) and without (‐creek). We surveyed vegetation cover, microtopography, habitat type, and distance to creeks in nested plots of five sizes, 0.1, 0.25, 1, 2.5, and 10 m². Results: Species richness, frequency, cover, and assemblages differed between ±creek cells. Richness tended to be higher in +creek cells, and cover and frequency of individual species differed significantly between ±creek cells. We found consistent patterns in vegetation structure across plot sizes. We encountered 13 species that occurred in 188 unique assemblages. The most common assemblage had six species: Batis maritima, Frankenia salina, Salicornia bigelovii, S. virginica, Salicornia spec. and Triglochin concinna. This assemblage occurred in ±creek cells and at all spatial scales. Of the most common assemblages all but one were composed of multiple species (3–9 species/plot). Conclusions: The persistence of vegetation patterns across a 100‐fold range in spatial scale suggests that similar environmental factors operate broadly to determine species establishment and persistence. Differences in assemblage composition result from variation of frequency and cover of marsh plain species, particularly Suaeda esteroa and Monanthochloe littoralis. The recommendation for restoration of Californian salt marshes is to target (and plant) multi‐species assemblages, not monocultures.     

Reconciling the apparent difference between mass- and area-based expressions of the photosynthesis-nitrogen relationship

The relationship between photosynthetic carbon assimilation (A _max) and leaf nitrogen content (N _leaf) can be expressed on either a leaf area basis (A _area vs N _area) or a leaf mass basis (A _mass vs N _mass). Dimensional analysis shows that the units for the slope of this relationship are the same for both expressions (μmol [CO₂] g⁻¹ [N] s⁻¹). Thus the slope measures the change in CO₂ assimilation per gram of nitrogen, independent of leaf mass or leaf area. Although they have the same units, large differences between the area and mass-based slopes have been observed over a broad range of taxonomically diverse species. Some authors have claimed that regardless of these differences, the fundamental nature of the A _max-N _leaf relationship is independent of the units of expression. In contrast, other authors have claimed that the area-based A _max-N _leaf relationship is fundamentally different from the mass-based relationship because of interactions between A _max, N _leaf, and leaf mass per area (LMA, g [leaf] m⁻² [leaf]). In this study we consider the mathematical relationships involved in the transformation from mass- to area-based expressions (and vice versa), and the implications this transformation has for the slope of the A _max-N _leaf relationship. We then show that the slope of the relationship is independent of the units of expression when the effect of LMA is controlled statistically using a multiple regression. The validity of this hypothesis is demonstrated using 13 taxonomically and functionally diverse C₃ species. This analysis shows that the slope of the A _max-N _leaf relationship is similar for the mass- and area-based expressions and that significant errors in the estimate of the slope can arise when the effect of LMA is not controlled. Received: 7 May 1998 / Accepted: 19 October 1998     

On the general dynamic model of oceanic island biogeography

Aim To investigate the biological meaning of equations used to apply the general dynamic model (GDM) of oceanic island biogeography proposed by R. J. Whittaker, K. A. Triantis and R. J. Ladle. Location Analyses are presented for 17 animal groups living on the Aeolian Islands, a volcanic archipelago in the central Mediterranean, near Sicily. Methods In addition to the mathematical implementation of the GDM proposed by Whittaker, Triantis and Ladle, and termed here logATT² (, where S is species number or any other diversity metric, t is island age, A is island area, and a, b, c and d are fitted parameters), a new implementation based on the Arrhenius equation of the species–area relationship (SAR) is investigated. The new model (termed powerATT²) is: . For logATT² and powerATT² models, equations were developed to calculate (1) the expected number of species at equilibrium (i.e. when the island has reached maturity) per unit area (S_eq), and (2) the time required to obtain this value (t_eq). Whereas the intercept in the Gleason model (S = C + z log A) or the coefficient of the Arrhenius power model (S = CA^z) of the SAR can be considered measures of the expected number of species per unit area, this is not the case for the parameter a of the ATT² models. However, values of S_eq can be used for this purpose. The index of ‘colonization ability’ (CAB), calculated as the ratio , may provide a measure of the mean number of species added per unit area per unit time. Results Both ATT² models fitted most of the data well, but the powerATT² model was in most cases superior. Equilibrial values of species richness (S_eq) varied from c. 3 species km^?2 (reptiles) to 100 species km^?2 (mites). The fitted curves for the powerATT² model showed large variations in d, from 0.03 to 3. However, most groups had values of d around 0.2–0.4, as commonly observed for the z‐values of SARs modelled by a power function. Equilibration times ranged from about 170,000 years to 400,000 years. Mites and springtails had very high values of CAB, thus adding many more species per unit area per unit time than others. Reptiles and phytophagous scarabs showed very low values, being the groups that added fewest species per unit area per unit time. Main conclusions Values of equilibrial species richness per unit area are influenced by species biology (e.g. body size and ecological specialization). Theoretical and empirical evidence suggests that higher immigration rates should increase the z‐values of the Arrhenius model. Thus, in the same archipelago, groups with larger z‐values should be characterized by higher dispersal ability. Results obtained here for the parameter d conform to this prediction.