Soil seed densities and emergence patterns in pastures in the seasonally dry tropics of northeastern Australia

Germinable seed densities in the surface (0–10 cm) soil of pasture communities growing at Lansdown, near Townsville, were measured during the late dry season before the first germinating rain and again during the following wet season after germination but before the input of new seed. Seedlings emerging in the field were counted at approximately weekly intervals during this period to determine emergence patterns. Twelve communities were sampled in 1980–81 and six were re-sampled in 1981–82. During the late dry season germinable seed densities ranged from 5000 to 40 000 seeds m^-2. Seeds of the introduced legume, Stylosanthes hamata, were present in all pastures. There were many seeds of annual grasses (Digitaria ciliaris and Brachiaria miliiformis) and sedges (Cyperus and Fimbristylis spp.) but only few seeds of perennial grasses (both native and introduced). Soil seed densities were much lower during the wet season than during the preceding dry season, particularly for the grasses. Emergence commenced and approximately 70% of all seedlings emerged on the first major rainfall of the wet season. The subsequent emergence pattern varied between years. In 1980–81 there was a gradual and continuous increase in seedling numbers under the continuously moist conditions which prevailed. In 1981–82 further emergence occurred in discrete events related to rainfall and intervening dry periods. Maximum seedling densities exceeded 34 000 seedlings m^-2 including 29 000 grass seedlings (mainly annual species). The implications of these results for species survival and pasture composition are discussed.     

Water source partitioning among trees growing on shallow karst soils in a seasonally dry tropical climate

The sources of water used by woody vegetation growing on karst soils in seasonally dry tropical regions are little known. In northern Yucatan (Mexico), trees withstand 4–6 months of annual drought in spite of the small water storage capacity of the shallow karst soil. We hypothesized that adult evergreen trees in Yucatan tap the aquifer for a reliable supply of water during the prolonged dry season. The naturally occurring concentration gradients in oxygen and hydrogen stable isotopes in soil, bedrock, groundwater and plant stem water were used to determine the sources of water used by native evergreen and drought-deciduous tree species. While the trees studied grew over a permanent water table (9–20 m depth), pit excavation showed that roots were largely restricted to the upper 2 m of the soil/bedrock profile. At the peak of the dry season, the δ¹⁸O signatures of potential water sources for the vegetation ranged from 4.1 ± 1.1‰ in topsoil to −4.3 ± 0.1‰ in groundwater. The δ¹⁸O values of tree stem water ranged from −2.8 ± 0.3‰ in Talisia olivaeformis to 0.8 ± 1‰ in Ficus cotinifolia, demonstrating vertical partitioning of soil/bedrock water among tree species. Stem water δ¹⁸O values were significantly different from that of groundwater for all the tree species investigated. Stem water samples plotted to the right of the meteoric water line, indicating utilization of water sources subject to evaporative isotopic enrichment. Foliar δ¹³C in adult trees varied widely among species, ranging from −25.3 ± 0.3‰ in Enterolobium cyclocarpum to −28.7 ± 0.4‰ in T. olivaeformis. Contrary to initial expectations, data indicate that native trees growing on shallow karst soils in northern Yucatan use little or no groundwater and depend mostly on water stored within the upper 2–3 m of the soil/bedrock profile. Water storage in subsurface soil-filled cavities and in the porous limestone bedrock is apparently sufficient to sustain adult evergreen trees throughout the pronounced dry season.     

More than meets the eye-a simulation of natural selection

One of the most easily understood examples of natural selection occurs when predators preferentially feed on conspicuous varieties in a prey population and thus confer a selective advantage on cryptic varieties. This effect can be demonstrated by using wild birds as predators, pastry ‘baits’ as prey, and coloured stones as backgrounds. We have carried out five experiments using two variations of this method and the results confirm its potential as an exercise in simulating natural selection.     

Impact of climate and land degradation on soil carbon fluxes in dry semiarid grasslands in SE Spain

Plant and Soil - This study investigates how precipitation, temperature and seasonality (as a proxy of plant productivity) affect the temporal and spatial variability of soil CO2 efflux in two dry...     

Long-term carbon exchange in a sparse, seasonally dry tussock grassland

Rainfall and its seasonal distribution can alter carbon dioxide (CO₂) exchange and the sustainability of grassland ecosystems. Using eddy covariance, CO₂ exchange between the atmosphere and a sparse grassland was measured for 2 years at Twizel, New Zealand. The years had contrasting distributions of rain and falls (446 mm followed by 933 mm; long‐term mean=646 mm). The vegetation was sparse with total above‐ground biomass of only 1410 g m^?2. During the dry year, leaf area index peaked in spring (November) at 0.7, but it was <0.2 by early summer. The maximum daily net CO₂ uptake rate was only 1.5 g C m^?2 day^?1, and it occurred before mid‐summer in both years. On an annual basis, for the dry year, 9 g C m^?2 was lost to the atmosphere. During the wet year, 41 g C m^?2 was sequestered from the atmosphere. The net exchange rates were determined mostly by the timing and intensity of spring rainfall. The components of ecosystem respiration were measured using chambers. Combining scaled‐up measurements with the eddy CO₂ effluxes, it was estimated that 85% of ecosystem respiration emanated from the soil surface. Under well‐watered conditions, 26% of the soil surface CO₂ efflux came from soil microbial activity. Rates of soil microbial CO₂ production and net mineral‐N production were low and indicative of substrate limitation. Soil respiration declined by a factor of four as the soil water content declined from field capacity (0.21 m³ m^?3) to the driest value obtained (0.04 m³ m^?3). Rainfall after periods of drought resulted in large, but short‐lived, respiration pulses that were curvilinearly related to the increase in root‐zone water content. Coupled with the low leaf area and high root : shoot ratio, this sparse grassland had a limited capacity to sequester and store carbon. Assuming a proportionality between carbon gain and rainfall during the summer, rainfall distribution statistics suggest that the ecosystem is sustainable in the long term.     

Biomass and carbon accumulation in a fire chronosequence of a seasonally dry tropical forest

Seasonally dry tropical forests (SDTF) are a widely distributed vegetation type in the tropics, characterized by seasonal rainfall with several months of drought when they are subject to fire. This study is one of the first attempts to quantify above- and belowground biomass (AGB and BGB) and above- and belowground carbon (AGC and BGC) pools to calculate their recovery after fire, using a chronosequence approach (six forests that ranged form 1 to 29 years after fire and mature forest). We quantified AGB and AGC pools of trees, lianas, palms, and seedlings, and BGB and BGC pools (Oi, Oe, Oa soil horizons, and fine roots). Total AGC ranged from 0.05 to nearly 72 Mg C ha⁻¹, BGC from 21.6 to nearly 85 Mg C ha⁻¹, and total ecosystem carbon from 21.7 to 153.5 Mg C ha⁻¹; all these pools increased with forest age. Nearly 50% of the total ecosystem carbon was stored in the Oa horizon of mature forests, and up to 90% was stored in the Oa-horizon of early successional SDTF stands. The soils were shallow with a depth of <20 cm at the study site. To recover values similar to mature forests, BGC and BGB required <19 years with accumulation rates greater than 20 Mg C ha⁻¹ yr⁻¹, while AGB required 80 years with accumulation rates nearly 2.5 Mg C ha⁻¹ yr⁻¹. Total ecosystem biomass and carbon required 70 and 50 years, respectively, to recover values similar to mature forests. When belowground pools are not included in the calculation of total ecosystem biomass or carbon recovery, we estimated an overestimation of 10 and 30 years, respectively.     

Partitioning of soil water among canopy trees in a seasonally dry tropical forest

Little is known about partitioning of soil water resources in species-rich, seasonally dry tropical forests. We assessed spatial and temporal patterns of soil water utilization in several canopy tree species on Barro Colorado Island, Panama, during the 1997 dry season. Stable hydrogen isotope composition (δD) of xylem and soil water, soil volumetric water content (θ_v), and sap flow were measured concurrently. Evaporative fractionation near the soil surface caused soil water δD to decrease from about –15‰ at 0.1 m to –50 to –55‰ at 1.2 m depth. Groundwater sampled at the sources of nearby springs during this period yielded an average δD value of –60‰. θ_v increased sharply and nearly linearly with depth to 0.7 m, then increased more slowly between 0.7 and 1.05 m. Based on xylem δD values, water uptake in some individual plants appeared to be restricted largely to the upper 20 cm of the soil profile where θ_v dropped below 20% during the dry season. In contrast, other individuals appeared to have access to water at depths greater than 1 m where θ_v remained above 45% throughout the dry season. The depths of water sources for trees with intermediate xylem δD values were less certain because variation in soil water δD between 20 and 70 cm was relatively small. Xylem water δD was also strongly dependent on tree size (diameter at breast height), with smaller trees appearing to preferentially tap deeper sources of soil water than larger trees. This relationship appeared to be species independent. Trees able to exploit progressively deeper sources of soil water during the dry season, as indicated by increasingly negative xylem δD values, were also able to maintain constant or even increase rates of water use. Seasonal courses of water use and soil water partitioning were associated with leaf phenology. Species with the smallest seasonal variability in leaf fall were also able to tap increasingly deep sources of soil water as the dry season progressed. Comparison of xylem, soil, and groundwater δD values thus pointed to spatial and temporal partitioning of water resources among several tropical forest canopy tree species during the dry season. Received: 5 October 1998 / Accepted: 23 June 1999     

气候变化情景下中国自然植被净初级生产力分布

基于国际上较通用的Lund-Potsdam-Jena(LPJ)模型,根据中国自然环境特点对其运行机制进行调整,并重新进行了参数化,以B2情景气候数据作为主要的输入数据,以1961-1990年为基准时段,模拟了中国1991-2080自然植被净初级生产力(NPP)对气候变化的响应.结果表明:1961-1990年,中国自然植被的NPP总量为3.06 Pg C·a-1;1961-2080年,NPP总量呈波动下降趋势,且下降速度逐渐加快.在降水相对变化不大的条件下,平均温度的增加对我国植被生产力可能会产生一定的负面影响.NPP的空间分布从东南沿海向西北内陆呈逐渐递减趋势,在气候变化过程中,该格局基本没有太大变化.在东部NPP值相对较高地区,NPP值以减少为主,东北地区、华北东部和黄土高原地区的减少趋势尤为明显;在西部NPP值相对较低地区,NPP以增加趋势为主,青藏高原地区和塔里木盆地的表现尤为突出.随着气候变化的深入,东西部地区这种变化趋势的对比将越发明显.     

Respiration acclimation contributes to high carbon-use efficiency in a seasonally dry pine forest

Predictions of warming and drying in the Mediterranean and other regions require quantifying of such effects on ecosystem carbon dynamics and respiration. Long‐term effects can only be obtained from forests in which seasonal drought is a regular feature. We carried out measurements in a semiarid Pinus halepensis (Aleppo pine) forest of aboveground respiration rates of foliage, R_f, and stem, R_t over 3 years. Component respiration combined with ongoing biometric, net CO₂ flux [net ecosystem productivity (NEP)] and soil respiration measurements were scaled to the ecosystem level to estimate gross and net primary productivity (GPP, NPP) and carbon‐use efficiency (CUE=NPP/GPP) using 6 years data. GPP, NPP and NEP were, on average, 880, 350 and 211 g C m^?2 yr^?1, respectively. The above ground respiration made up half of total ecosystem respiration but CUE remained high at 0.4. Large seasonal variations in both R_f and R_t were not consistently correlated with seasonal temperature trends. Seasonal adjustments of respiration were observed in both the normalized rate (R₂₀) and short‐term temperature sensitivity (Q₁₀), resulting in low respiration rates during the hot, dry period. R_f in fully developed needles was highest over winter–spring, and foliage R₂₀ was correlated with photosynthesis over the year. Needle growth occurred over summer, with respiration rates in developing needles higher than the fully developed foliage at most times. R_t showed a distinct seasonal maximum in May irrespective of year, which was not correlated to the winter stem growth, but could be associated with phenological drivers such as carbohydrate re‐mobilization and cambial activity. We show that in a semiarid pine forest photosynthesis and stem growth peak in (wet) winter and leaf growth in (dry) summer, and associated adjustments of component respiration, dominated by those in R₂₀, minimize annual respiratory losses. This is likely a key for maintaining high CUE and ecosystem productivity similar to much wetter sites, and could lead to different predictions of the effect of warming and drying climate on productivity of pine forests than based on short‐term droughts.     

Historical climate change and speciation: neotropical seasonally dry forest plants show patterns of both tertiary and quaternary diversification

Historical climate changes have had a major effect on the distribution and evolution of plant species in the neotropics. What is more controversial is whether relatively recent Pleistocene climatic changes have driven speciation, or whether neotropical species diversity is more ancient. This question is addressed using evolutionary rate analysis of sequence data of nuclear ribosomal internal transcribed spacers in diverse taxa occupying neotropical seasonally dry forests, including Ruprechtia (Polygonaceae), robinioid legumes (Fabaceae), Chaetocalyx and Nissolia (Fabaceae), and Loxopterygium (Anacardiaceae). Species diversifications in these taxa occurred both during and before the Pleistocene in Central America, but were primarily pre-Pleistocene in South America. This indicates plausibility both for models that predict tropical species diversity to be recent and that invoke a role for Pleistocene climatic change, and those that consider it ancient and implicate geological factors such as the Andean orogeny and the closure of the Panama Isthmus. Cladistic vicariance analysis was attempted to identify common factors underlying evolution in these groups. In spite of the similar Mid-Miocene to Pliocene ages of the study taxa, and their high degree of endemism in the different fragments of South American dry forests, the analysis yielded equivocal, non-robust patterns of area relationships.     

Temporal coherence of aboveground net primary productivity in mesic grasslands

Synchrony in ecological variables over wide geographic areas suggests that large‐scale environmental factors drive the structure and function of ecosystems and override more local‐scale environmental variation. Described also as coherence, this phenomenon has been documented broadly in the ecological literature and has recently received increasing attention as scientists attempt to quantify the impacts of global changes on organisms and their habitats. Using a mesic grassland site in North America, we assessed coherence in ecosystem function by quantifying similarity in aboveground net primary production (ANPP) dynamics in 48 permanent sampling locations (PSLs) over a 16‐yr period. Our primary objective was to characterize coherence across a broad geographic region (with similar ecosystem structure and function), and we hypothesized that precipitation and a similar fire frequency would strengthen coherence between PSLs. All 48 PSLs at our site (Konza Prairie Biological Station, Manhattan, KS, USA; KPBS) were exposed to a similar regional driver of ANPP (precipitation); however, local drivers (including differences in fire frequency and soil depth at different topographic positions) varied strongly among individual PSLs. For the purpose of this assessment, the watershed‐level experimental design of KPBS was considered a model, which represented different fire management strategies across the Great Plains Region. Our analyses revealed a site‐level (KPBS) coherence in ANPP dynamics of 0.53 for the period of 1984–1999. Annual fire enhanced coherence among PSLs to 0.76, whereas less frequent fire (fire exclusion or a 4‐yr fire return interval) failed to further increase coherence beyond that of the KPBS site level. Soil depth also strongly influenced coherence among PSLs with shallow soils at upland sites showing strong coherence across fire regimes and annually burned uplands closely linked to annual precipitation dynamics. The lack of coherence in ecosystem function in PSLs with deep soils and low fire frequencies suggests that conservation and management efforts will need to be more location specific in such areas where biotic interactions may be more important than regional abiotic drivers.     

Synchronized fruiting between trees of Ficus thonningii in seasonally dry habitats

Abstract. The strategy of flowering available to monoecious fig-trees ( Ficus spp., Moraceae) is restricted by the obligate mutualistic relationship they have with their pollinating wasps (Hymenoptera, Agaonidae). Flowering should be synchronous within a tree, asynchronous between trees, random in the yearly cycle, and irregular from year to year. Exceptions have been predicted to occur in a population that inhabits both seasonally dry and adjacent riparian habitats. Ficus thonningii B1 meets these criteria in Harare, Zimbabwe. The vegetative and reproductive phenologies of twenty-five mature trees growing close together but in three habitats (watered, non-watered and intermediate) were studied over 32 months. Trees were generally deciduous between April and August in all habitats. In contrast fruiting phenology was habitat dependant. Watered trees conformed to the general model, but half the trees in the seasonally dry habitat flowered synchronously in the cool dry season. This strategy may result in these trees having relatively superior female reproductive function.     

Influence of rainfall and grazing on herbage dynamics in a seasonally dry tropical savanna

Species composition and herbage dynamics in relation to rainfall variability and cattle grazing were studied in permanently protected, grazed, and temporarily fenced treatments on three sites in a seasonally dry tropical savanna. Permanently protected sites, established between 1979 and 1984, were 55–79% similar with each other in species composition, and 14–25% similar with grazed sites during the period 1986–1988. Similarity among grazed sites was only 36–43%. Number of species was greater in the grazed treatment than in the permanently protected treatment. The percentages of annual grasses and non-leguminous forbs were greater in grazed savanna than in permanently protected savanna. Species diversity was higher in grazed savanna than in the corresponding permanently protected savanna. Species the two annual cycles studied, peak live shoot biomass was 614 g m^-2 in permanently protected savanna, 109 g m^-2 in grazed savanna, and 724 g m^-2 in temporarily fenced savanna. Live shoot biomass in temporarily fenced savanna was 18 to 44% greater than in permanently protected savanna. Peak canopy biomass ranged from 342 to 700 g m^-2 in permanently protected savanna. It was related with total rainy season rainfall, and was particularly sensitive to late rainy season rainfall. On the other hand, peak canopy biomass in grazed savanna ranged from 59 to 169 g m^-2 and was related to grazing intensity rather than either total rainy season rainfall or late rainy season rainfall. Coefficient of variation of green biomass in permanently protected savanna was related with rainfall variability indicating it to be a pulsed system which responds quickly to rainfall events. Biomass of woody species ranged from 2466 to 5298 g m^-2 in permanently protected savanna and from 744 to 1433 g m^–2 in the grazed savanna. Green foliage biomass was 3.7 to 6.4% of the woody biomass in permanently protected and 5.6 to 5.9% in grazed savanna, and supplements substantially the fodder resource during the dry periods of the year.     

More stable structure of wheat germ lipase at low pH than its native state

Wheat germ lipase is a cereal lipase which is a monomeric protein. In the present study we sought to structurally characterize this protein along with equilibrium unfolding in solution. Conformational changes occurring in the protein with varying pH, were monitored by circular dichroism (CD) spectroscopy, fluorescence emission spectroscopy, binding of hydrophobic dye, 1-anilino 8-naphthalenesulfonic acid (ANS) and dynamic light scattering (DLS). Our study showed that acid denaturation of lipase lead to characterization of multiple monomeric intermediates. Native protein at pH 7.0 showed far-UV spectrum indicating mixed structure with both alpha and beta-type of characteristics. Activity of lipase was found to fall on either sides of pH 7.0–8.0. Acid-unfolded state was characterized at pH 4.0 with residual secondary structure, disrupted tertiary spectrum and red-shifted fluorescence spectrum with decreased intensity. Further decrease in pH lead to formation of secondary structure and acid-induced molten globule state was found to be stabilized at pH 1.4, with exposed tryptophan residues and hydrophobic patches. Notably, interesting finding of this study was characterization of acid-induced state at pH 0.8 with higher secondary structure content than native lipase, regain in tertiary spectrum and induction of compact conformation. Although enzymatically inactive, acid-induced state at pH 0.8 was found to be structurally more stable than native lipase, as shown by chemical and thermal denaturation profiles.     

Overwhelming effects of autumn-time drought during seedling establishment impair recovery potential in sown and semi-natural pastures in Portugal

Plant Ecology - Climate change-induced alterations to precipitation may affect the regeneration dynamics of plant species, with the occurrence of drought influencing germination and seedling...     

Flooded meadow communities an analysis of productivity in a dry year

In the year 1967, a study was made of the production of the following four selected plant communities arranged in an ecological series according to moisture:Serratuleto-Festucetum commutatae, Gratiola officinalis Carex praecox-suzae subass.Galium boreale, Gratiola officinalis-Carex praecoxsuzae subass.Rorippa silvestris; andGlycerietum maximae, the most humid member, partly left unmowed. Attention was paid to the rate of aerial and root production, root: shoot ratio, weight ratio of various plant groups to the dominant, vertical structure of the vegetation, and ash content. Primary dry matter production was used for determining the values of growth characteristics: relative growth rateR and productivityC.