Grass Canopy Research Articles

Spatial patterns of aeolian sediment deposition in vegetation canopies: Observations from wind tunnel experiments using colored sand

In environments affected by wind erosion, plants act as traps for aeolian sediment, which leads to a small-scale mosaic of depositional and erosional sediment transport regimes. This wind tunnel study used colored sand to visualize spatial patterns of sediment redistribution within grass canopies. Wind tunnel experiments were performed with high-, medium- and low-density canopies of Lolium perenne, corresponding to vegetation covers of 47%, 16% and 4%, respectively. In the low and medium-density canopies, the wake areas downstream of the tussocks were the primary locations of sediment deposition. In the medium-density canopy, these wedge-shaped wake deposits overlapped with the adjacent downstream tussocks, while in the low-density canopy they did not, indicating that these vegetation densities respectively represented wake-interference and isolated roughness flow. In the high-density canopy, very few sand grains were entrained by the wind, and were mostly deposited within the disturbed zones surrounding the tussocks. The deposited grains were evenly distributed around the tussocks in the high-density canopy without pronounced accumulations on their upstream, downstream or lateral sides. We interpret the high-density canopy as a skimming flow aerodynamic regime. The fraction of the sand surface which was exposed to erosion was substantially smaller than the area not covered by grasses. It accounted for 67–78% of the non-covered surface in the low-density canopy, and for 44–77% of the surface in the medium-density canopy. This finding indicates that wind erosion models overestimate the sediment source area if they assume the erodible area is the entire exposed surface not covered by roughness elements.

Demographic features, distribution, and habitat selection of the gray mouse opossum (Tlacuatzin canescens) in Colima, Mexico

In western Mexico, gray mouse opossums Tlacuatzin canescens typically are not only in tropical deciduous and semideciduous woodlands but also in croplands and orchards. We conducted mark-recapture studies in January 2003–2007 and 2010 in coastal, northern, and central Colima, Mexico. Each year, five grids, established in areas of thick vegetation within a mosaic of habitats, had 100 stations (10 × 10), each with two Sherman traps, one on the ground and another elevated 1–2 m. On 24 of 30 grids, 82 individuals were captured 126 times (85.7 % in elevated traps). Sex ratio did not deviate from 1:1; there was no sexual dimorphism in mass (average for males, 28.21 g; average for females, 25.64 g); 46.3 % of animals were adults, 27.3 % of adult females were reproductively active, and 77.5 % of males had scrotal testes. Distance from centroid of trap locations averaged 15.24 m, with mean minimum distance moved between captures being 33.19 m. Densities usually were low (0.67–8.03/ha), with the species widespread in habitats studied. We assessed 14 environmental characteristics for each station using ANOVA, logistic regression, and nonparametric multiplicative regression (NPMR) to characterize habitat selection. T. canescens was more likely found where percent grass was about 30 % and litter over 50 %, with height of canopy less than 10 m and about 40 % closed. NPMR, being able to recognize hump-shaped response curves where intermediate variable values are preferred, identified two variables (percent grass and percent canopy closed) not detected by other techniques as important in characterizing habitat selection of T. canescens.

Performance of supplemented heifers on Marandu grass pastures in the wet-to-dry transition and dry seasons

The objective of the present study was to evaluate the characteristics of the sward canopy of Marandu grass during the rainy season, the wet-to-dry transition and the dry seasons, between March and September 2004, under intermittent grazing, and to correlate those characteristics with the performance of crossbred heifers receiving mineral supplements ad libitum or protein supplements. The experiment consisted of a randomized block design with three blocks (set of 13 paddocks), each containing five crossbred heifers per experimental unit, totaling 15 replicates. The heifers were given protein supplements daily in individual stalls and received an average 4 g/kg/day of the supplement during the rainy season and 5 g/kg/day during the dry season. Their weight gain was assessed monthly. The pasture structure was assessed through destructive sampling, and the bromatological composition of esophageal extrusa samples was also assessed. Analysis of variance was used to assess performance, and regression analysis was used to evaluate the sward canopy characteristics in relation to the months of the year. A cluster procedure was used to determine the similarity between the months of the year under assessment. Two different groups were formed for pasture evaluation: one group including the months of March to July and another group including the months of August and September. The first group exhibited a better canopy structure than the second group. This fact was corroborated by the animal performance, which was lower during the months of the second group. Low-intake protein supplementation was effective in increasing the performance of the grazing heifers. Pasture structure is critical for animal performance in a grazing environment, regardless of the type of supplementation.

Soil carbon balance in a tropical grassland: Estimation of soil respiration and its partitioning using a semi-empirical model

In savannah and tropical grasslands, which account for 60% of grasslands worldwide, a large share of ecosystem carbon is located below ground due to high root:shoot ratios. Temporal variations in soil CO2 efflux (RS) were investigated in a grassland of coastal Congo over two years. The objectives were (1) to identify the main factors controlling seasonal variations in RS and (2) to develop a semi-empirical model describing RS and including a heterotrophic component (RH) and an autotrophic component (RA). Plant above-ground activity was found to exert strong control over soil respiration since 71% of seasonal RS variability was explained by the quantity of photosynthetically active radiation absorbed (APAR) by the grass canopy. We tested an additive model including a parameter enabling RS partitioning into RA and RH. Assumptions underlying this model were that RA mainly depended on the amount of photosynthates allocated below ground and that microbial and root activity was mostly controlled by soil temperature and soil moisture. The model provided a reasonably good prediction of seasonal variations in RS (R2=0.85) which varied between 5.4μmolm−2s−1 in the wet season and 0.9μmolm−2s−1 at the end of the dry season. The model was subsequently used to obtain annual estimates of RS, RA and RH. In accordance with results reported for other tropical grasslands, we estimated that RH accounted for 44% of RS, which represented a flux similar to the amount of carbon brought annually to the soil from below-ground litter production. Overall, this study opens up prospects for simulating the carbon budget of tropical grasslands on a large scale using remotely sensed data.

Effect of canopy structure on sun-induced chlorophyll fluorescence

We investigated the impact of canopy structure on chlorophyll fluorescence properties. For this purpose, we developed SpectroFLEX, an instrument for quantitative measurements of canopy fluorescence in O2A and O2B atmospheric absorption bands. The fluorescence emission of a natural grass canopy was compared with the leaf level fluorescence spectrum acquired simultaneously. It was found that the red-to-far-red fluorescence ratio decreased by a factor of two from the leaf to the canopy level. In addition, this ratio decreased under high light conditions. FluoSAIL simulations were conducted to study the impact of canopy density and geometry on this decrease. This effect has been attributed to a preferential re-absorption of red fluorescence emission during radiative transfer within the canopy compared to far-red emission.

Burning and mowing Wyoming big sagebrush: Do treated sites meet minimum guidelines for greater sage‐grouse breeding habitats?

AbstractWyoming big sagebrush (Artemisia tridentata wyomingensis) treatments are often implemented to improve breeding habitat for greater sage‐grouse (Centrocercus urophasianus), a species of considerable conservation concern. In 2008 and 2009, we measured vegetation structure available to sage‐grouse for breeding at 19 sites that were prescribed burned during 1990–1999 and 2000–2006, 6 sites that were mowed during 2000–2006, and 25 paired, untreated reference sites in the Bighorn Basin of north‐central Wyoming, USA. We compared minimum guidelines for canopy cover and height of Wyoming big sagebrush and perennial grass in arid greater sage‐grouse breeding habitat (Connelly et al. 2000b) to measurements at our sampling sites. Sagebrush canopy cover and height at reference sites met the minimum guidelines. Sagebrush canopy cover at burned and mowed sites did not meet the minimum guideline, except for sites mowed on aridic soils measured during 2009. Burned and mowed (3 of 4 cases) sagebrush did not meet minimum height for breeding up to 19 yr and 9 yr post‐treatment, respectively. Perennial grass canopy cover and height met the minimum guidelines for breeding habitat at reference, burned, and mowed sites. Burning increased grass canopy cover, but not height, compared to reference sites in 2 of 8 instances. Because burning, but not mowing, infrequently enhanced grass cover, but not height, and sagebrush structure was reduced by both practices for long periods, managers should consider how treatments may negatively affect Wyoming big sagebrush communities for sage‐grouse and consider other practices, including continued nontreatment and improved livestock grazing, to increase grass cover and height. © 2012 The Wildlife Society.

Biomass estimation of alpine grasslands under different grazing intensities using spectral vegetation indices

We measured and analyzed the spectral reflectance characteristics and biophysical properties such as biomass and vegetation cover of purple feather-grass (Stipa purpurea) alpine steppe grassland under different grazing intensities (no grazing, light, moderate, and heavy grazing) during the growing season. We investigated the effects of grazing intensities on the spectral reflectance and red edge parameters of S. purpurea alpine steppe grassland and established potential models of biomass estimation based on the relationship of spectral vegetation indices and measured alpine grassland biomass in Northern Tibet, China. The results indicated that the spectral reflectance of alpine grassland is similar to the spectral characteristics of typical green vegetation. Reflectance in the near-infrared region of the no grazing treatment was higher than the grazing treatments, while the reflectance of the no grazing treatment in the visible light region was weaker than the grazing treatments. With enhanced grazing intensity, the spectral reflectance of grass canopy followed an increasing trend; the red edge position moved to longer wavelengths, and the amplitude of the red edge decreased in alpine grassland. A significant relationship of measured aboveground biomass and vegetation indices was caculated based on spectral reflectance. Hence, the optimized soil adjusted vegetation index is prefered for the estimation of aboveground biomass of S. purpurea alpine steppe in Northern Tibet, China.

Modelling Ammonia Losses After Field Application of Biogas Slurry in Energy Crop Rotations

Over the past few years the number of biogas slurries, which are generally used as nitrogen fertilisers, have seen a steady increase in Germany. A mechanistic ammonia volatilisation model was developed to predict the ammonia losses of these slurries when applied to bare soil, maize, wheat and rye grass canopies. Data for model development were collected from several field measurements carried out at two locations in Northern Germany between the years of 2007 and 2008. Additionally, the behaviour of the slurries on and in the soil was investigated through the use of infiltration pot experiments. The model includes three main compartments: slurry, atmosphere and soil. The soil compartment model is relatively simple, as the slurry infiltration, nitrification and ploughing dislocation into the soil determined in the experiments showed quantitatively no significant differences between the tested slurries (mono-fermented, co-fermented and pig slurry) and soils (sand soil and loamy sand). Hence, instead of a complex soil model, stable reduction factors, as derived from the experiments, were implemented in the model. Simulated ammonia emissions were statistically compared (root mean square error (RMSE), modelling efficiency (ME), linear regression) to the observed emissions. All evaluations showed an acceptable model performance (RMSE = 1.80 kg N ha−1), although there were a few number of anomalies which could not be modelled in an adequate way. A model sensitivity analysis showed that temperature and slurry pH value are the main drivers of NH3 volatilization in the model. Following a change of +1°C or of +0.1 pH unit ammonia volatilization will increase by about 1% and 1.6% of the applied total ammoniacal nitrogen, respectively. We were able to show that a simple model approach could explain most factors of ammonia volatilization in biogas crop rotations.

Analysis of Microclimate Data Measured over Grass and Soybean Canopy and Their Impacts on Penman-Monteith Grass and Alfalfa Reference Evapotranspiration

The use of Penman-Monteith (PM)-type combination-based energy-balance equations to estimate reference evapotranspiration (ETref) requires climatic data measured over well-watered and well-maintained reference grass or alfalfa vegetation surfaces. However, establishing and maintaining reference weather stations for a long period of time is a very formidable and expensive process. Thus, expansion of the microclimate data available for use in the PM equation for estimating ETref is needed. In the absence of reference weather stations, one alternative is using microclimatic data measured over other well-watered vegetation surfaces as inputs to the PM equation. This study determines if weather data collected from a well-watered soybean surface in a semihumid climate can be used for this purpose. Measured and estimated microclimate variables, including net radiation (Rn), average air temperature (Tave), dew-point temperature (Td), average relative humidity (RHave), aerodynamic resistance (ra), and wind speed at...

Reply to comment by Zhi-Hua Wang and Elie Bou-Zeid on “Impact of wave phase difference between soil surface heat flux and soil surface temperature on soil surface energy balance closure”

[1] We thank Wang and Bou-Zeid [2011] (hereinafter referred to as WB) for their comments about the phase difference between soil temperature and soil heat flux in our paper [Gao et al., 2010] (hereinafter referred to as GHL). In our previous work [Gao et al., 2008], we gave lots of derivations about the phase difference mentioned above. The main difference between WB's comments and our work (GHL) is the third term of their equation (11), which denotes the transient disturbance caused by starting the oscillation of surface temperature from the initial condition. In micrometeorological experimental research, this term is usually neglected because it is small. For soil surface heat flux, the transient term vanishes. [2] Our assurance (see GHL) that all of Rn, H and LE have phases identical to that of soil surface temperature may be more complicated for vegetation (such as dense grassland) surfaces because there are complicated physiological and physical processes [e.g., Sellers et al., 1996a, 1996b]. In the database used by WB, the sensible heat flux and latent heat fluxes partly come from grass canopy and partly come from ground soil surface, and the proportion depends on leaf area index, vegetation type and canopy height, etc. From the Web site WB provided, we failed to find how to decide the soil heat flux G0 which WB used in their comments. For grassland surface energy analysis, we should account for the heat storages in both grass and soil into soil surface heat flux. The same principle applies to soil surface temperature, and it should represent the canopy temperature and ground soil surface temperature [e.g., Sellers et al., 1996a, 1996b]. We suggest WB provide more information around soil surface heat flux and soil surface temperature determinations in their comments for further discussion. We found that WB's data of LE and H varied quite a lot (big changes from time to time, particularly around 1000, 1100, and 1200 EST) in their Figure 1b. Since Rn is quite smooth, G0 should have varied from time to time based on G0 = Rn-H-LE. The net radiation (Rn) shown in WB's Figure 1b was derived from four components of radiation budget: incoming and outgoing components of short- and long-wave radiation. I suggest WB insert a figure to show the diurnal variation courses of these four components to see whether the surface temperature measured by a thermal infrared radiometer has a phase identical to the outgoing long-wave radiation. Furthermore, there are three essential issues WB should address for their Figure 1b. (1) Rn suddenly decreased for a while at 1315 and 1415 (EST), and it was most probably caused by the intermittent cloudiness. However, both H and LE courses have a nick around 1200 (EST). As is well known, both H and LE should respond to change in Rn accordingly. (2) Both H and LE reached their peaks of 120 W m−2 and 160 W m−2, respectively, around 1115 (EST) when Rn and G0 were 260 W m−2 and 20 W m−2, respectively. It is obvious that Rn < H+LE under the condition that G0 > 0. This result may be caused by the different footprints of Rn, H and LE. (3) WB computed the eddy covariance fluxes over 5 min periods. Why select 5 min rather than 20 min or 30 min for average as is usual? [3] This study was supported by the National Program on Key Basic Research Project of China (973) under grant 2010CB428502, by National Natural Science Foundation of China under grants 40975009 and 40906023, by China Meteorological Administration under grant GYHY201006024, and by the Centurial Program sponsored by the Chinese Academy of Sciences. I am grateful to Heping Liu (from Department of Civil and Environmental Engineering, Washington State University) for his valuable comments.

Parameterizing canopy resistance using mechanistic and semi‐empirical estimates of hourly evapotranspiration: critical evaluation for irrigated crops in the Mediterranean

AbstractIn this paper two models are presented for calculating the hourly evapotranspiration λE (W m−2) using the Penman–Monteith equation. These models were tested on four irrigated crops (grass, soya bean, sweet sorghum and vineyard), with heights between 0·1 and 2·2 m at the adult growth stage. In the first model (Katerji N, Perrier A. 1983. Modélisation de l'évapotranspiration réelle ETR d'une parcelle de luzerne : rôle d'un coefficient cultural. Agronomie 3(6): 513–521, KP model), the canopy resistance rc is parameterized by a semi‐empirical approach. In the second model (Todorovic M. 1999. Single‐layer evapotranspiration model with variable canopy resistance. Journal of Irrigation and Drainage Engineering—ASCE 125: 235–245, TD model), the resistance rc is parameterized by a mechanistic model. These two approaches are critically analysed with respect to the underlying hypotheses and the limitations of their practical application. In the case of the KP model, the mean slope between measured and calculated values of λE was 1·01 ± 0·6 and the relative correlation coefficients r2 ranged between 0·8 and 0·93. The observed differences in slopes, between 0·96 and 1·07, were not associated with the crop height. This model seemed to be applicable to all the crops examined. In the case of the TD model, the observed slope between measured and calculated values of λE for the grass canopy was 0·79. For the other crops, it varied between 1·24 and 1·34. In all the situations examined, the values of r2 ranged between 0·73 and 0·92. The TD model underestimated λE in the case of grass and overestimated it in the cases of the other three crops. The under‐ or overestimation of λE in the TD model were due: (i) to some inaccuracies in the theory of this model, (ii) to not taking into account the effect of aerodynamic resistance ra in the canopy resistance modelling. Therefore, the values of rc were under‐ or overestimated in consequence of mismatching the crop height. The high value of air vapour pressure deficit also contributed to the overestimation of λE, mainly for the tallest crop. The results clarify aspects of the scientific controversy in the literature about the mechanistic and semi‐empirical approaches for estimating λE. From the practical point of view the results also present ways for identifying the most appropriate approach for the experimental situations encountered. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Uncertainties in fPAR estimation of grass canopies under different stress situations and differences in architecture

The fraction of intercepted photosynthetic active radiation (fPAR) is a key variable used by the Monteith model to estimate the net primary productivity (NPP). This variable can be assessed by vegetation indices (VIs) derived from spectral remote sensing data but several factors usually affect their relationship. The objectives of this work were to analyse the fPAR dynamics and to describe the relationships between fPAR and several indices (normalized difference vegetation index (NDVI), optimized soil adjusted vegetation index (OSAVI), Green NDVI (GNDVI), visible atmospherically resistant index (VARI) green, VIgreen and red edge position (REP)) under different water and nutrient treatments for two species with different canopy architectures. Two C3 grass species with differences in leaf orientation (planophile and erectophile) were cultivated from seeds in pots. Four treatments were applied combining water and nitrogen availability. Every week, canopy reflectance and fPAR were measured. Aerial biomass was clipped to estimate final above-ground production for each species and treatment. Starting from reflectance values, the indices were calculated. Planophile species have a steeper (but not significantly) slope in VIs–fPAR relationships than the erectophile species. Water and nutrient deficiencies treatment showed no relationship with fPAR in any spectral index in the erectophile species. In the other species, this treatment showed significant relationship according to the index used. Analysing each species individually, treatments did not modify slopes except in one case (planophile species between both treatments with high nitrogen but differing in water availability). Among indices, GNDVI was the best estimator of fPAR for both species, followed by NDVI and OSAVI. Inaccurate results may be obtained from commonly reported spectral relationships if plants' stress factors are not taken into account.

Salt Tolerance and Canopy Reflectance of Kentucky Bluegrass Cultivars

Six cultivars or selections of kentucky bluegrass (Poa pratensis L.) were grown outdoors from vegetative clones in a gravelly sand medium from Apr. to Sept. 2005 in Riverside, CA, at soil water salinities ranging from 2 to 22 dS·m−1. Cultivars Baron, Brilliant, Cabernet, Eagleton, Midnight, and the selection A01-856, a ‘Texas’ × kentucky bluegrass hybrid (P.· arachnifera × P. pratensis), were evaluated for salt tolerance based on relative and absolute cumulative biomass production, growth rates, leaf chloride concentration, and hyperspectral ground-based remote sensing (RS) canopy reflectance measurements. Remotely sensed indices were linearly correlated with absolute biomass production. Three variations of a Normalized Difference Vegetation Index (NDVIred, NDVIprotein, and NDVIinfra) decreased with increasing salinity-induced changes in grass canopies. An index based on the red-edge inflection point increased (became less negative) with increasing salinity. A Floating Water Band Index decreased with decreased leaf moisture content related to increasing salinity but did not discriminate between cultivars. Shoot spreading rate and NDVIinfra were both related to shoot chloride concentration differences among the kentucky bluegrass (Poa pratensis L) (KBG) cultivars or selections. In theory, non-destructive RS monitoring of above-ground turf development, including NDVIinfra, coupled with measurement of leaf chloride concentrations could be useful in turf salt tolerance breeding programs. Salt tolerance rankings among the KBG cultivars varied depending on the evaluation methods and selection criteria used. Based on absolute and relative biomass, growth rate, and RS, cultivars Baron, Brilliant, and Eagleton were rated as more salt-tolerant than ‘Cabernet’, ‘Midnight’, and AO1-856.

Resilience of a high‐conservation‐value, semi‐arid grassland on fertile clay soils to burning, mowing and ploughing

AbstractIn grassland reserves, managed disturbance is often necessary to maintain plant species diversity. We carried out experiments to determine the impact of fire, kangaroo grazing, mowing and disc ploughing on grassland species richness and composition in a nature reserve in semi‐arid eastern Australia. Vegetation response was influenced by winter–spring drought after establishment of the experiments, but moderate rainfall followed in late summer–autumn. Species composition varied greatly between sampling times, and the variability due to rainfall differences between seasons and years was greater than the effects of fire, kangaroo grazing, mowing or disc ploughing. In the fire experiment, species richness and composition recovered more rapidly after spring than autumn burning. Species richness and composition were similar to control sites within 12 months of burning and mowing, suggesting that removal of the dominant grass canopy is unnecessary to enhance plant diversity. Two fires (separated by 3 years) and post‐fire kangaroo grazing had only minor influence on species richness and composition. Even disc ploughing caused only a small reduction in native richness. The minor impact of ploughing was explained by the small areas that were ploughed, the once‐off nature of the treatment, and the high degree of natural movement and cracking in these shrink‐swell soils. Recovery of the composition and richness of these grasslands was rapid because of the high proportion of perennial species that resprout vegetatively after fire and mowing. There appears to be little conservation benefit from fire, mowing or ploughing ungrazed areas, as we could identify no native plant species dependent on frequent disturbance for persistence in this grassland community. However, the ability of the Astrebla‐ and Dichanthium‐dominated grasslands to recover quickly after disturbance, given favourable seasonal conditions, suggests that they are well adapted to natural disturbances (e.g. droughts, fire, flooding and native grazing).

The Influence of Spatial Patterns of Soil Moisture on the Grass and Shrub Responses to a Summer Rainstorm in a Chihuahuan Desert Ecotone

The cycling of surface water, energy, nutrients, and carbon is different between semiarid grassland and shrubland ecosystems. Although differences are evident when grasslands are compared to shrublands, the processes that contribute to this transition are more challenging to document. We evaluate how surface redistribution of precipitation and plant responses to the resulting infiltration patterns could contribute to the changes that occur during the transition from grassland to shrubland. We measured soil water potential under grasses (Bouteloua eriopoda), shrubs (Larrea tridentata) and bare soil and changes in plant water relations and gas exchange following a 15 mm summer storm in the grassland–shrubland ecotone at the Sevilleta National Wildlife Refuge in central New Mexico USA. Following the storm, soil water potential (Ψs) increased to 30 cm depth beneath both grass and shrub canopies, with the greatest change observed in the top 15 cm of the soil. The increase in Ψs was greater beneath grass canopies than beneath shrub canopies. Ψs under bare soil increased only to 5 cm depth. The substantial redistribution of rainfall and different rooting depths of the vegetation resulted in high Ψs throughout most of the rooting volume of the grasses whereas soil moisture was unchanged throughout a large portion of the shrub rooting volume. Consistent with this pattern, predawn water potential (ΨPD) of grasses increased more than 5 MPa to greater than −1 MPa whereas ΨPD of shrubs increased to −2.5 MPa, a change of less than 2 MPa. Transpiration increased roughly linearly with ΨPD in both grasses and shrubs. In grasses, assimilation was strongly correlated with ΨPD whereas there was no relationship in shrubs where assimilation showed no significant response to the pulse of soil moisture following the storm. These data show that preferential redistribution of water to grass canopies enhances transpiration and assimilation by grasses following large summer storms. This process may inhibit shrubland expansion at the ecotone during periods without extreme drought.

Grassland canopy management and native tallgrass species composition effects on C and N in grass canopies and soil

Grassland canopy management (spring burn, mowing and residue removal in late-summer, or no management) and native tallgrass species composition (cool season mixture, warm season mixture, or combined cool and warm mixture) effects on C and N in aboveground biomass and soil were investigated at Brookings SD on a previously-plowed Barnes clay loam (fine-loamy, superactive, frigid Calcic Hapludoll). During the last 2 yr of the 9-yr experiment, shoot biomass was affected by canopy management with the burn (2,730 kg ha−1) and mow (3,421 kg ha−1) treatments containing less than no management (4,655 kg ha−1). Burn treatment biomass contained 1,189 kg ha−1 and 25 kg ha−1 of C and N, mow contained 1,433 kg ha−1 and 33 kg ha−1 of C and N, while no management contained 2,014 kg ha−1 and 39 kg ha−1 of C and N, respectively. Soil C accumulation was independent of grass species composition. Soil C accumulation rates, which increased in strong linear fashion (r2 of 0.89 to 0.92) after initial grass establishment, were 387 kg C ha−1 yr−1, 503 kg C ha−1 yr−1, and 711 kg C ha−1 yr−1 for burn, mow, and no management treatments, respectively. Thus, grassland management methods used after conversion of cropland to grassland have important effects on grass biomass and soil C accumulation.

Barriers to tree seedling emergence on human‐induced grasslands in Sri Lanka

Summary 1. Colonization by woody plants is often very slow or absent on grasslands occupying degraded land in the tropics. Seed dispersal limitation is widely reported, but the constraints to forest succession imposed by barriers to seedling establishment are poorly understood. We tested the hypotheses that seedling emergence of woody plants is limited by interactions of anthropogenic fire, vertebrate herbivory and competition with the dominant grass sward in human‐induced montane grasslands in Sri Lanka. 2. Seedling emergence was determined fortnightly for 18 months in response to experimental manipulation of fire regimes, access by vertebrate herbivores and competition from the dominant grass canopy at the forest/grassland edge and at 10, 20 and 40 m from the edge into four blocks of grassland. Seedling emergence was also monitored in the absence of any experimental manipulation at 10, 20 and 40 m into adjacent blocks of lower montane rainforest. 3. Emergence of seedlings of woody plants was much lower in the grassland (mean &lt;0·1 seedlings m−2 year−1) than in natural forest patches (mean 6·0 seedlings m−2 year−1), but was maximal at the forest/grassland edge (mean 9·5 m−2 year−1). In the grassland, fire reduced seedling emergence by 36% in fenced vertebrate exclosures but increased seedling emergence by 68% in unfenced plots. Exclusion of vertebrate herbivores had no impact on seedling emergence at the forest/grassland edge. Removal of the grass canopy by clipping or tilling increased seedling emergence at the edge, but had no effect in the grassland. Seedlings of the woody pioneer species Macaranga indica dominated the community of emergents in the grassland and at the edge. 4. Synthesis and applications. Although dispersal limitation represents the primary constraint to forest succession on degraded montane grasslands in Sri Lanka, management of fire regimes, vertebrate herbivory and competition from the dominant grasses would influence the abundance and composition of tree seedling emergents. Tilling in narrow strips, 2–5 m wide and within 10 m of the forest edge, would facilitate the emergence and establishment of early successional trees in the grassland, but the strips would require protection from fire for at least 2 years. Within the interior of grassland patches, a combination of controlled burning and short‐term access to vertebrate dispersers would promote tree seedling emergence, but the burned patches would then require long‐term protection from fire and grazing subsequently to allow the emergents to establish and catalyse forest succession.

Artificial drainage and associated carbon fluxes (CO2/CH4) in a tundra ecosystem

AbstractEcosystem flux measurements using the eddy covariance (EC) technique were undertaken in 4 subsequent years during summer for a total of 562 days in an arctic wet tundra ecosystem, located near Cherskii, Far‐Eastern Federal District, Russia. Methane (CH4) emissions were measured using permanent chambers. The experimental field is characterized by late thawing of permafrost soils in June and periodic spring floods. A stagnant water table below the grass canopy is fed by melting of the active layer of permafrost and by flood water. Following 3 years of EC measurements, the site was drained by building a 3 m wide drainage channel surrounding the EC tower to examine possible future effects of global change on the tundra tussock ecosystem. Cumulative summertime net carbon fluxes before experimental alteration were estimated to be about +15 g C m−2 (i.e. an ecosystem C loss) and +8 g C m−2 after draining the study site. When taking CH4 as another important greenhouse gas into account and considering the global warming potential (GWP) of CH4 vs. CO2, the ecosystem had a positive GWP during all summers. However CH4 emissions after drainage decreased significantly and therefore the carbon related greenhouse gas flux was much smaller than beforehand (475 ± 253 g C‐CO2‐e m−2 before drainage in 2003 vs. 23 ± 26 g C‐CO2‐e m−2 after drainage in 2005).

Effect of Manipulated Rainfall on Root Production and Plant Belowground Dry Mass of Different Grassland Ecosystems

A field experiment was established to quantify the effects of different amounts of rainfall on root growth and dry mass of belowground plant parts in three types of grassland ecosystems. Mountain (Nardus grassland), highland (wet Cirsium grassland), and lowland grassland (dry Festuca grassland) ecosystems were studied in 2006 and 2007. Roofs constructed above the canopy of grass stands and gravity irrigation systems simulated three climate scenarios: (1) rainfall reduced by 50%, (2) rainfall enhanced by 50%, and (3) the full natural rainfall of the current growing season. Experimentally reduced amounts of precipitation significantly affected both yearly root increments and total root dry mass in the highland grassland. Dry conditions in 2007 resulted in considerable reduction of total belowground dry mass in highland and mountain grasslands. Although not all differences in root biomass of studied grasslands were statistically significantly, some also showed a decrease in root increment and in the amount of belowground dry mass in dry conditions.

Influence of clouds and diffuse radiation on ecosystem‐atmosphere CO2 and CO18O exchanges

This study evaluates the potential impact of clouds on ecosystem CO2 and CO2 isotope fluxes (“isofluxes”) in two contrasting ecosystems (a broadleaf deciduous forest and a C4 grassland) in a region for which cloud cover, meteorological, and isotope data are available for driving the isotope‐enabled land surface model (ISOLSM). Our model results indicate a large impact of clouds on ecosystem CO2 fluxes and isofluxes. Despite lower irradiance on partly cloudy and cloudy days, predicted forest canopy photosynthesis was substantially higher than on clear, sunny days, and the highest carbon uptake was achieved on the cloudiest day. This effect was driven by a large increase in light‐limited shade leaf photosynthesis following an increase in the diffuse fraction of irradiance. Photosynthetic isofluxes, by contrast, were largest on partly cloudy days, as leaf water isotopic composition was only slightly depleted and photosynthesis was enhanced, as compared to adjacent clear‐sky days. On the cloudiest day, the forest exhibited intermediate isofluxes: although photosynthesis was highest on this day, leaf‐to‐atmosphere isofluxes were reduced from a feedback of transpiration on canopy relative humidity and leaf water. Photosynthesis and isofluxes were both reduced in the C4 grass canopy with increasing cloud cover and diffuse fraction as a result of near‐constant light limitation of photosynthesis. These results suggest that some of the unexplained variation in global mean δ18O of CO2 may be driven by large‐scale changes in clouds and aerosols and their impacts on diffuse radiation, photosynthesis, and relative humidity.