How do leaf wetting events affect gas exchange and leaf lifespan of plants from seasonally dry tropical vegetation?

• Foliar uptake of dew is likely an important mechanism of water acquisition for plants from tropical dry environments. However, there is still limited experimental evidence describing the anatomical pathways involved in this process and the effects of this water subsidy on the maintenance of gas exchange and leaf lifespan of species from seasonally dry tropical vegetation such as the Brazilian caatinga.
• We performed scanning electron, bright‐field and confocal microscopic analyses and used apoplastic tracers to examine the foliar water uptake (FWU) routes in four woody species with different foliar phenology and widely distributed in the caatinga. Leaves of plants subjected to water stress were exposed to dew simulation to evaluate the effects of the FWU on leaf water potentials, gas exchange and leaf lifespan.
• All species absorbed water through their leaf cuticles and/or peltate trichomes but FWU capacity differed among species. Leaf wetting by dew increased leaf lifespan duration up to 36 days compared to plants in the drought treatment. A positive effect on leaf gas exchange and new leaf production was only observed in the anisohydric and evergreen species.
• We showed that leaf wetting by dew is relevant for the physiology and leaf lifespan of plants from seasonally dry tropical vegetation, especially for evergreen species.

     

Do photosynthetic metabolism and habitat influence foliar water uptake in orchids?

• Epiphytic and rupicolous plants inhabit environments with limited water resources. Such plants commonly use Crassulacean Acid Metabolism (CAM), a photosynthetic pathway that accumulates organic acids in cell vacuoles at night, so reducing their leaf water potential and favouring water absorption. Foliar water uptake (FWU) aids plant survival during drought events in environments with high water deficits. We hypothesized that FWU represents a strategy employed by epiphytic and rupicolous orchids for water acquisition and that CAM will favour increased water absorption.
• We examined 6 epiphyte, 4 terrestrial and 6 rupicolous orchids that use C3 (n = 9) or CAM (n = 7) pathways. Five individuals per species were used to evaluate FWU, structural characteristics and leaf water balance.
• Rupicolous species with C3 metabolism had higher FWU than other species. FWU (C_max and k) could be related to succulence, SLM and leaf RWC. The results indicated that high orchid leaf densities favoured FWU, as area available for water storage increases with leaf density. Structural characteristics linked to water storage (e.g. high RWC, succulence), on the other hand, could limit leaf water absorption by favouring high internal leaf water potentials.
• Epiphytic, rupicolous and terrestrial orchids showed FWU. Rupicolous species had high levels of FWU, probably through absorption from mist. However, succulence in plants with CAM appears to mitigate FWU.

     

Species trees,temporal divergence and historical biogeography of coastal rove beetles (Coleoptera: Staphylinidae) reveal their early Miocene origin and show that most divergence events occurred in the early Pliocene along the Pacific coasts

We investigated the species trees, temporal divergence and historical biogeography of Emplenota Casey and Triochara Bernhauer (ET clade hereafter), subgenera of the Aleochara coastal staphylinid beetles, to determine their place of origin. We explored a multilocus dataset to infer gene trees and species trees based on traditional concatenated and multispecies coalescent‐based approaches using both model‐ (maximum likelihood and Bayesian) and parsimony‐based methods. The multilocus dataset comprised DNA sequences from five nuclear genes (ArgK, CAD, EF1‐α, wg and 28S) and three mitochondrial genes (COI, COII and 16S). *BEAST analysis according to a coalescent‐based approach resolved the following phylogenetic relationships: ((A. trisulcata (A. nubis + A. zerchei)) (A. obscurella ((A. pacifica (A. curtidens + A. litoralis)) (A. hayamai (A. yamato (A. fucicola (A. segregata + A. puetzi))))))). Using a relaxed molecular clock, we reconstructed a time‐calibrated phylogeny for this group. Furthermore, to account for the historical biogeography of the ET clade at the ages of major divergences, we investigated the ancestral area based on a time‐calibrated phylogeny. Biogeographical analyses suggested that the ancestor of the ET clade was widely distributed along the eastern and western Palearctic and the western Nearctic coasts in the early Miocene. According to reconstruction of the ancestral area, one dispersal and three vicariance events were required, and the analyses indicated that vicariance events were important in shaping its current distribution patterns. Most of the divergence events occurred in the late Miocene and early Pliocene along the Pacific coasts, and the East Asian seacoasts harboured the most species‐rich fauna, including eight of the 13 species in the ET clade.     

Does soil moisture availability explain liana seedling distribution across a tropical rainfall gradient?

Liana density tends to increase with decreasing rainfall and increasing seasonality. However, the pattern of liana distribution may be due to differences in soil water retention capacity, not rainfall and seasonality per se. We tested the effect of rainfall and soil substrate with respect to the distribution of liana seedlings in six sites across a rainfall gradient from the wet Atlantic to the dry Pacific in central Panama. Soils were either limestone, with low water‐holding capacity, or laterite, with higher water‐holding capacity. We sampled liana seedlings at each site using three 1 × 100 m transect. We found that relative liana seedling density was higher on limestone soils compared to laterite soils regardless of the amount of rainfall. Furthermore, liana community composition on limestone soils was more similar to dry forest sites than to adjacent wet and moist forest sites. Liana seedling species diversity relative to trees was significantly higher in a low‐fertility dry forest site compared to a high‐fertility forest, but did not differ from the other sites. Thus, liana seedling density and community structure may be driven more by soil type and thus by soil moisture availability than strictly by mean annual rainfall and the seasonality of rainfall.     

Effects of experimental sedimentation on the phenological dynamics and leaf traits of replanted mangroves at Gazi bay,Kenya

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北京山区干旱胁迫下侧柏叶片水分吸收策略

干旱与半干旱地区,水分是限制树木生长的重要影响因子。由于降水稀缺且分配不均,叶片吸收水分是此地区树木吸收和利用小量级降水和凝结水的主要方式。北京山区处于易旱少雨的生态脆弱地带,森林植被经常遭受干旱胁迫,所以对该地区的森林系统而言,叶片直接吸收利用截留的降雨是干旱时期树木获得水分的重要途径。基于野外对比控制试验和室内盆栽模拟试验,选取北京山区的主要造林树种侧柏为研究对象,进行利用天然降雨与模拟降雨试验,研究降雨前后侧柏叶片吸水特征,探究侧柏在干旱环境下如何通过叶片吸水缓解干旱胁迫。结果表明：当侧柏长期处于干旱胁迫状态时,叶片可以利用降雨,从中获益用来缓解树木的干旱胁迫状态;叶片的吸水能力与降雨强度呈正相关关系,与土壤含水率呈负相关关系;重度干旱下侧柏植株在降雨强度为15 mm/h时叶片吸水现象最明显,叶水势变化最大为(1.18±0.17) MPa,叶片含水率变化最大为(8.47±1.00)mg/cm~2;当土壤水率高于20.8%时,基本不发生叶片吸水现象。试验结果说明在干旱地区叶片吸水是树木除根系吸水外的重要水分来源方式,并且对干旱地区有效利用短缺水资源,减轻植物水分亏缺具有重要意义。     

Multi‐century tree‐ring precipitation record reveals increasing frequency of extreme dry events in the upper Blue Nile River catchment

Climate‐related environmental and humanitarian crisis are important challenges in the Great Horn of Africa (GHA). In the absence of long‐term past climate records in the region, tree‐rings are valuable climate proxies, reflecting past climate variations and complementing climate records prior to the instrumental era. We established annually resolved multi‐century tree‐ring chronology from Juniperus procera trees in northern Ethiopia, the longest series yet for the GHA. The chronology correlates significantly with wet‐season (r = .64, p < .01) and annual (r = .68, p < .01) regional rainfall. Reconstructed rainfall since A.D. 1811 revealed significant interannual variations between 2.2 and 3.8 year periodicity, with significant decadal and multidecadal variations during 1855–1900 and 1960–1990. The duration of negative and positive rainfall anomalies varied between 1–7 years and 1–8 years. Approximately 78.4% (95%) of reconstructed dry (extreme dry) and 85.4% (95%) of wet (extreme wet) events lasted for 1 year only and corresponded to historical records of famine and flooding, suggesting that future climate change studies should be both trend and extreme event focused. The average return periods for dry (extreme dry) and wet (extreme wet) events were 4.1 (8.8) years and 4.1 (9.5) years. Extreme‐dry conditions during the 19th century were concurrent with drought episodes in equatorial eastern Africa that occurred at the end of the Little Ice Age. El Niño and La Niña events matched with 38.5% and 50% of extreme‐dry and extreme‐wet events. Equivalent matches for positive and negative Indian Ocean Dipole events were weaker, reaching 23.1 and 25%, respectively. Spatial correlations revealed that reconstructed rainfall represents wet‐season rainfall variations over northern Ethiopia and large parts of the Sahel belt. The data presented are useful for backcasting climate and hydrological models and for developing regional strategic plans to manage scarce and contested water resources. Historical perspectives on long‐term regional rainfall variability improve the interpretation of recent climate trends.     

Contrasting physiological responsiveness of establishing trees and a C4 grass to rainfall events,intensified summer drought,and warming in oak savanna

Climate warming and drought may alter tree establishment in savannas through differential responses of tree seedlings and grass to intermittent rainfall events. We investigated leaf gas exchange responses of dominant post oak savanna tree (Quercus stellata and Juniperus virginiana) and grass (Schizachyrium scoparium, C₄ grass) species to summer rainfall events under an ambient and intensified summer drought scenario in factorial combination with warming (ambient, +1.5 °C) in both monoculture and tree‐grass mixtures. The three species differed in drought resistance and response of leaf gas exchange to rainfall events throughout the summer. S. scoparium experienced the greatest decrease in A_area (?56% and ?66% under normal and intensified drought, respectively) over the summer, followed by Q. stellata (?44%, ?64%), while J. virginiana showed increased A_area under normal drought (+13%) and a small decrease in A_area when exposed to intensified summer drought (?10%). Following individual rainfall events, mean increases in A_area were 90% for S. scoparium, 26% for J. virginiana and 22% for Q. stellata. The responsiveness of A_area of S. scoparium to rainfall events initially increased with the onset of drought, but decreased dramatically as summer drought progressed. For Q. stellata, A_area recovery decreased as drought progressed and with warming. In contrast, J. virginiana showed minimal fluctuations in A_area following rainfall events, in spite of declining water potential, and warming enhanced recovery. J. virginiana will likely gain an advantage over Q. stellata during establishment under future climatic scenarios. Additionally, the competitive advantage of C₄ grasses may be reduced relative to trees, because grasses will likely exist below a critical water stress threshold more often in a warmer, drier climate. Recognition of unique species responses to critical global change drivers in the presence of competition will improve predictions of grass–tree interactions and tree establishment in savannas in response to climate change.     

Response of foundation macrophytes to near‐natural simulated marine heatwaves

Marine heatwaves have been observed worldwide and are expected to increase in both frequency and intensity due to climate change. Such events may cause ecosystem reconfigurations arising from species range contraction or redistribution, with ecological, economic and social implications. Macrophytes such as the brown seaweed Fucus vesiculosus and the seagrass Zostera marina are foundation species in many coastal ecosystems of the temperate northern hemisphere. Hence, their response to extreme events can potentially determine the fate of associated ecosystems. Macrophyte functioning is intimately linked to the maintenance of photosynthesis, growth and reproduction, and resistance against pathogens, epibionts and grazers. We investigated morphological, physiological, pathological and chemical defence responses of western Baltic Sea F. vesiculosus and Z. marina populations to simulated near‐natural marine heatwaves. Along with (a) the control, which constituted no heatwave but natural stochastic temperature variability (0HW), two treatments were applied: (b) two late‐spring heatwaves (June, July) followed by a summer heatwave (August; 3HW) and (c) a summer heatwave only (1HW). The 3HW treatment was applied to test whether preconditioning events can modulate the potential sensitivity to the summer heatwave. Despite the variety of responses measured in both species, only Z. marina growth was impaired by the accumulative heat stress imposed by the 3HW treatment. Photosynthetic rate, however, remained high after the last heatwave indicating potential for recovery. Only epibacterial abundance was significantly affected in F. vesiculosus. Hence both macrophytes, and in particular F. vesiculosus, seem to be fairly tolerant to short‐term marine heatwaves at least at the intensities applied in this experiment (up to 5°C above mean temperature over a period of 9 days). This may partly be due to the fact that F. vesiculosus grows in a highly variable environment, and may have a high phenotypic plasticity.     

Entamoeba marina n. sp.; a New Species of Entamoeba Isolated from Tidal Flat Sediment of Iriomote Island,Okinawa, Japan

The genus Entamoeba includes anaerobic lobose amoebae, most of which are parasites of various vertebrates and invertebrates. We report a new Entamoeba species, E. marina n. sp. that was isolated from a sample of tidal flat sediment collected at Iriomote Island, Okinawa, Japan. Trophozoites of E. marina were 12.8–32.1 μm in length and 6.8–15.9 μm in width, whereas the cysts were 8.9–15.8 μm in diam. and contained four nuclei. The E. marina cells contained a rounded nucleus with a small centric karyosome and uniformly arranged peripheral chromatin. Although E. marina is morphologically indistinguishable from other tetranucleated cyst‐forming Entamoeba species, E. marina can be distinguished from them based on the combination of molecular phylogenetic analyses using SSU rDNA gene and the difference of collection sites. Therefore, we propose E. marina as a new species of the genus Entamoeba.     

Proximate weather patterns and spring green‐up phenology effect Eurasian beaver (Castor fiber) body mass and reproductive success: the implications of climate change and topography

Low spring temperatures have been found to benefit mobile herbivores by reducing the rate of spring‐flush, whereas high rainfall increases forage availability. Cold winters prove detrimental, by increasing herbivore thermoregulatory burdens. Here we examine the effects of temperature and rainfall variability on a temperate sedentary herbivore, the Eurasian beaver, Castor fiber, in terms of inter‐annual variation in mean body weight and per territory offspring production. Data pertain to 198 individuals, over 11 years, using capture‐mark‐recapture. We use plant growth (tree cores) and fAPAR (a satellite‐derived plant productivity index) to examine potential mechanisms through which weather conditions affect the availability and the seasonal phenology of beaver forage. Juvenile body weights were lighter after colder winters, whereas warmer spring temperatures were associated with lighter adult body weights, mediated by enhanced green‐up phenology rates. Counter‐intuitively, we observed a negative association between rainfall and body weight in juveniles and adults, and also with reproductive success. Alder, Alnus incana, (n = 68) growth rings (principal beaver food in the study area) exhibited a positive relationship with rainfall for trees growing at elevations >2 m above water level, but a negative relationship for trees growing <0.5 m. We deduce that temperature influences beavers at the landscape scale via effects on spring green‐up phenology and winter thermoregulation. Rainfall influences beavers at finer spatial scales through topographical interactions with plant growth, where trees near water level, prone to water logging, producing poorer forage in wetter years. Unlike most other herbivores, beavers are an obligate aquatic species that utilize a restricted ‘central‐place’ foraging range, limiting their ability to take advantage of better forage growth further from water during wetter years. With respect to anthropogenic climate change, interactions between weather variables, plant phenology and topography on forage growth are instructive, and consequently warrant examination when developing conservation management strategies for populations of medium to large herbivores.     

Impacts of the invasive fungus Austropuccinia psidii (myrtle rust) on three Australian Myrtaceae species of coastal swamp woodland

Exotic fungal pathogens can substantially affect individuals and populations of susceptible native plant species, potentially resulting in changes in community structure and composition. Austropuccinia psidii (myrtle rust) is a pathogenic fungus native to South America that affects species in the plant family Myrtaceae. The pathogen was introduced accidentally to Australia and first detected in NSW in April 2010. Ecological impacts have been poorly studied in the native range of A. psidii and even less in its Australian introduced range. In order to assess the potential impact of A. psidii on coastal swamp woodland, two glasshouse experiments were conducted using three co‐occurring species: Melaleuca quinquenervia, Leptospermum laevigatum and Baeckea linifolia. Plants of each species were grown individually (Experiment 1) and in mixed species assemblages (Experiment 2), with half inoculated with A. psidii and the other half remaining as controls. Infection level was assessed and impact on seedling survival and growth recorded. In both experiments L. laevigatum and M. quinquenervia seedlings were heavily infected and showed high degrees of susceptibility with negative effects on growth (height, biomass and number of leaves). In contrast, no B. linifolia seedling presented visible symptoms of disease, although seedlings showed reduced growth. Melaleuca quinquenervia seedlings had greater infection levels and suffered greater growth reductions than L. laevigatum in both experiments. However, there was no significant difference in the relative abundance of the three species in the mixed‐species experiment. This study provides a better understanding of the potential impacts of A. psidii in this vegetation community and has significant implications for the conservation and management of Australian Myrtaceae‐dominated plant communities generally.     

First report of Avicennia germinans (Acanthaceae) as a larval host of Junonia litoralis (Nymphalidae) causing higher foliar damage in seedlings than trees

Herbivory is considered an important biotic interaction in mangroves. Nevertheless, detailed information on specific plant–herbivore interactions that might have considerable influence in ecological and evolutionary processes is still very poor and fragmented. Herbivory damage was quantified during December 2015 in seedlings and trees in monospecific stands of Avicennia germinans in Laguna Madre of Tamaulipas, Mexico. The use of this mangrove species as a larval host is well documented; however, this is the first report for Junonia litoralis. These lepidopteran larvae consumed, on average, 10.2% of the leaf area; but herbivory was higher on seedlings (mean 15%) than on trees (mean 2.3%). These values are comparable to estimations of 10% mean herbivory damage in other field studies. This indicates that herbivores cause greater damage in seedlings than in trees. Herbivore activity by J. litoralis only occurred in A. germinans and did not affect associated herbs or shrubs of other species. The close interaction between J. litoralis and A. germinans may have profound implications for ecological and evolutionary processes of mangroves and enables a better understanding of ecosystem function and its conservation. Further studies are needed to investigate such interactions and their implications including long-term monitoring of interstitial salinity and leaf chemistry at different stages of growth and maturity of the host species.     

SH3‐class Ras guanine nucleotide exchange factors are essential for Aspergillus fumigatus invasive growth

Proper hyphal morphogenesis is essential for the establishment and progression of invasive disease caused by filamentous fungi. In the human pathogen Aspergillus fumigatus, signalling cascades driven by Ras and Ras‐like proteins orchestrate a wide variety of cellular processes required for hyphal growth. For activation, these proteins require interactions with Ras‐subfamily‐specific guanine nucleotide exchange factors (RasGEFs). Although Ras‐protein networks are essential for virulence in all pathogenic fungi, the importance of RasGEF proteins is largely unexplored. A. fumigatus encodes four putative RasGEFs that represent three separate classes of RasGEF proteins (SH3‐, Ras guanyl nucleotide‐releasing protein [RasGRP]–, and LTE‐class), each with fungus‐specific attributes. Here, we show that the SH3‐class and RasGRP‐class RasGEFs are required for properly timed polarity establishment during early growth and branch emergence as well as for cell wall stability. Further, we show that SH3‐class RasGEF activity is essential for polarity establishment and maintenance, a phenotype that is, at least, partially independent of the major A. fumigatus Ras proteins, RasA and RasB. Finally, loss of both SH3‐class RasGEFs resulted in avirulence in multiple models of invasive aspergillosis. Together, our findings suggest that RasGEF activity is essential for the integration of multiple signalling networks to drive invasive growth in A. fumigatus.     

Carbon isotope discrimination and growth response of old Pinus ponderosa trees to stand density reductions

Stand density reductions have been proposed as a method by which old‐growth ponderosa pine (Pinus ponderosa) forests of North America can be converted back to pre‐1900 conditions, thereby reducing the danger of catastrophic forest fires and insect attacks while increasing the productivity of the remaining old‐growth individuals. However, the duration of productivity response of individual trees and the physiological mechanisms underlying such a response remain speculative issues, particularly in old trees. Tree‐ring measurements of carbon isotope ratios (δ¹³C) and basal area increment (BAI) were used to assess the response of intrinsic water‐use efficiency (the ratio of photosynthesis, A to stomatal conductance, g) and growth of individual> 250‐year‐old‐ponderosa pine trees to stand density reductions. It was hypothesized that reductions in stand density would increase soil moisture availability, thus decreasing canopy A/g and increasing carbon isotope discrimination (Δ). Cellulose‐δ¹³C of annual tree rings, soil water availability (estimated from pre‐dawn leaf water potential), photosynthetic capacity, stem basal growth and xylem anatomy were measured in individual trees within three pairs of thinned and un‐thinned stands. The thinned stands were treated 7 to 15 years prior to measurement. The values of δ¹³C and BAI were assessed for 20 consecutive years overlapping the date of thinning in a single intensively studied stand, and was measured for 3 years on either side of the date of thinning for the two other stands to assess the generality of the response. After thinning, Δ increased by 0.89‰ (± 0.15‰). The trees in the un‐thinned stands showed no change in Δ (0.00‰ ± 0.04‰). In the intensively studied trees, significant differences were expressed in the first growing season after the thinning took place but it took 6 years before the full 0.89‰ difference was observed. BAI doubled or tripled after disturbance, depending on the stand, and the increased BAI lasted up to 15 years after thinning. In the intensively studied trees, the BAI response did not begin until 3 years after the Δ response, peaked 1 year after the Δ peak, and then BAI and Δ oscillated in unison. The lag between BAI and Δ was not due to slow changes in anatomical properties of the sapwood, because tracheid dimensions and sapwood‐specific conductivity remained unchanged after disturbance. The Δ response of thinned trees indicated that A/g decreased after thinning. Photosynthetic capacity, as indexed by foliar nitrogen ([N]) and by the relationship between photosynthesis and internal CO₂ (A–C_i curves), was unchanged by thinning, confirming our suspicion that the decline in A/g was due to a relatively greater increase in g in comparison with A. Model estimates agreed with this conclusion, predicting that g increased by nearly 25% after thinning relative to a 15% increase in A. Pre‐dawn leaf water potential averaged 0.11 MPa (± 0.03 MPa) less negative for the thinned compared with the un‐thinned trees in all stands, and was strongly correlated with Δ post‐thinning (R² = 0.91). There was a strong relationship between BAI and modelled A, suggesting that changes in water availability and g have a significant effect on carbon assimilation and growth of these old trees. These results confirm that stand density reductions result in increased growth of individual trees via increased stomatal conductance. Furthermore, they show that a physiological response to stand density reductions can last for up to 15 years in old ponderosa pines if stand leaf area is not fully re‐established.     

Photosynthetic performance of invasive Pinus ponderosa and Juniperus virginiana seedlings under gradual soil water depletion

Changes in climate, land management and fire regime have contributed to woody species expansion into grasslands and savannas worldwide. In the USA, Pinus ponderosa P.&C. Lawson and Juniperus virginiana L. are expanding into semiarid grasslands of Nebraska and other regions of the Great Plains. We examined P. ponderosa and J. virginiana seedling response to soil water content, one of the most important limiting factors in semiarid grasslands, to provide insight into their success in the region. Photosynthesis, stomatal conductance, maximum photochemical efficiency of PSII, maximum carboxylation velocity, maximum rate of electron transport, stomatal limitation to photosynthesis, water potential, root‐to‐shoot ratio, and needle nitrogen content were followed under gradual soil water depletion for 40 days. J. virginiana maintained lower L_s, higher A, g_s, and initial F_v/F_m, and displayed a more gradual decline in V_cmax and J_max with increasing water deficit compared to P. ponderosa. J. virginiana also invested more in roots relative to shoots compared to P. ponderosa. F_v/F_m showed high PSII resistance to dehydration in both species. Photoinhibition was observed at ～30% of field capacity. Soil water content was a better predictor of A and g_s than Ψ, indicating that there are other growth factors controlling physiological processes under increased water stress. The two species followed different strategies to succeed in semiarid grasslands. P. ponderosa seedlings behaved like a drought‐avoidant species with strong stomatal control, while J. virginiana was more of a drought‐tolerant species, maintaining physiological activity at lower soil water content. Differences between the studied species and the ecological implications are discussed.     

Variable rainfall has a greater effect than fire on the demography of the dominant tree in a semi‐arid Eucalyptus savanna

Rainfall, fire and competition are emphasized as determinants of the density and basal area of woody vegetation in savanna. The semi‐arid savannas of Australia have substantial multi‐year rainfall deficits and insufficient grass fuel to carry annual fire in contrast to the mesic savannas in more northern regions. This study investigates the influence of rainfall deficit and excess, fire and woody competition on the population dynamics of a dominant tree in a semi‐arid savanna. All individuals of Eucalyptus melanophloia were mapped and monitored in three, 1‐ha plots over an 8.5 year period encompassing wet and dry periods. The plots were unburnt, burnt once and burnt twice. A competition index incorporating the size and distance of neighbours to target individuals was determined. Supplementary studies examined seedling recruitment and the transition of juvenile trees into the sapling layer. Mortality of burnt seedlings was related to lignotuber area but the majority of seedlings are fire resistant within 12 months of germination. Most of the juveniles (≤1 cm dbh) of E. melanophloia either died in the dry period or persisted as juveniles throughout 8.5 years of monitoring. Mortality of juveniles was positively related to woody competition and was higher in the dry period than the wet period. The transition of juveniles to a larger size class occurred at extremely low rates, and a subsidiary study along a clearing boundary suggests release from woody competition allows transition into the sapling layer. From three fires the highest proportion of saplings (1–10 cm dbh) reduced to juveniles was only 5.6% suggesting rates of ‘top‐kill’ of E. melanophloia as a result of fire are relatively low. Girth growth was enhanced in wet years, particularly for larger trees (>10 cm dbh), but all trees regardless of size or woody competition levels are vulnerable to drought‐induced mortality. Overall the results suggest that variations in rainfall, especially drought‐induced mortality, have a much stronger influence on the tree demographics of E. melanophloia in a semi‐arid savanna of north‐eastern Australia than fire.     

The significance of episodic rains for reproductive phenology and productivity of trees in semiarid regions of northwestern Venezuela

The semiarid regions of northwestern Venezuela have extremely low and highly unpredictable precipitation, yet these conditions support species with contrasting phenology and leaf longevity. Episodic rains significantly increased leaf water potential (from –5 to –2.5 MPa) in several species and, in some cases, triggered flowering, leading us to hypothesize that the coexistence of species with contrasting phenology is due to differences in their ability to utilize small rainfall events. Irrigation treatments were used to simulate brief rainfall events, and the response of three species (Erythrina velutina [deciduous], Croton heliaster [semideciduous], and Capparis odoratissima [evergreen]) was monitored over a period of 14 months. To partition the effects of water reaching the canopy versus the soil, irrigation was supplied either in the form of mist to the canopy or by minisprinklers near the base of the trees. Nonirrigated trees were used as controls. Productivity (estimated as aboveground litter production) and water potential were enhanced by soil irrigation in two species. However, in the evergreen species canopy irrigation had a greater effect on water relations and productivity than soil irrigation, as indicated by higher predawn water potential, higher total annual flower (40 g m^–2 year^–1) and fruit (5 g m^–2 year^–1) production, and longer leaf longevity (410 days in control trees versus 520 days in canopy-irrigated trees). Canopy irrigation augmented flower and fruit production in all three species. Our findings suggest that reproductive phenology in these species is driven by episodic rains and that evergreen species may sustain productivity by their ability to make use of water deposited on leaf surfaces.     

Climate‐driven substitution of habitat‐forming species leads to reduced biodiversity within a temperate marine community

Aim

In marine ecosystems, habitat‐forming species (HFS) such as reef‐building corals and canopy‐forming macroalgae alter local environmental conditions and can promote biodiversity by providing biogenic living space for a vast array of associated organisms. We examined community‐level impacts of observed climate‐driven shifts in the relative abundances of two superficially similar HFS, the warm‐water kelp Laminaria ochroleuca and the cool‐water kelp Laminaria hyperborea.

Location

Western English Channel, north‐east Atlantic

Methods

We compared algal and invertebrate assemblages associated with kelp stipes and holdfasts, across multiple sites and sampling events. Significant differences were recorded in the structure of assemblages between the host kelp species at each site and event.

Results

Assemblages associated with stipes of the cool‐water HFS were, on average, >12 times more diverse and supported >3600 times more biomass compared with the warm‐water HFS. Holdfast assemblages also differed significantly between species, although to a lesser extent than those associated with stipes. Overall, assemblages associated with the warm‐water HFS were markedly impoverished and comprised far fewer rare or unique taxa.

Main conclusions

While previous research has shown how climate‐driven loss of HFS can cause biodiversity loss, our study demonstrates that climate‐driven substitutions of HFS can also lead to impoverished assemblages. The indirect effects of climate change remain poorly resolved, but shifts in the distributions and abundances of HFS may invoke widespread ecological change, especially in marine ecosystems where facilitative interactions are particularly strong.     

Life history and population dynamics of a tree species in tropical semi‐arid climate: A case study with Cordia oncocalyx

In semi‐arid climates, plant population dynamics are strongly influenced by the amount and temporal distribution of rainfall. We monitored a population of the tree species Cordia oncocalyx (Boraginaceae) for 24 months in the dry thorny woodland of semi‐arid northeastern Brazil, to investigate which life‐history traits allow this tree to be locally dominant. We used horizontal life tables and a Lefkovitch matrix and tested for relationships among demographic parameters of seedling, infant, juvenile, immature, virginile and reproductive ontogenetic stages with rainfall and canopy openness. Germination and recruitment occurred in the rainy months, and dry‐season mortality occurred only in seedlings (76% and 100%, first and second years, respectively) and infants (3% and 6%). Juveniles showed greater height growth under more open canopies (Spearman correlation coefficient = 0.24), suggesting that light availability influences growth. The population growth rate was λ = 1.0336, and the highest sensitivity occurred in the infant‐juvenile transition. Our results show light as a restrictive growth factor for plants in the juvenile stage and confirm the strong influence of rainfall on the dynamics of trees in a seasonally dry environment. The formation of a persistent seed bank with germination concentrated at the rainfall onset but spreading over the rainy season are strategies that hedge bets before establishment. The formation of a bank of infants, which can resume growth as soon as there is water, hedges bets after establishment. We attribute the positive population growth rate of Cordia oncocalyx to survival strategies allowing bet‐hedging both before and after establishment.