Grassland Ecology

A combined analysis of plant trait responses to the environment, and their effects on ecosystem properties has recently been proposed. In this study, we related the trait composition of plant communities to soil nutrients and disturbance as environmental drivers and to productivity, decomposition and soil carbon as ecosystem properties. We surveyed two sites, one comprising intensively grazed and fertilized grasslands, the other consisting of semi-natural grassland and open heathland. Species abundance and trait values of 49 species were recorded in 69 plots, as well as parameters describing soil resources, land-use disturbances,and ecosystem properties. Our main goal was to test whether the average or the diversity of the trait values of the vegetation had stronger effects on ecosystem properties (mass ratio vs.diversity hypothesis). Structural equation modeling was used to performa simultaneous analysis of trait responses and effects. Specific leaf area and leaf nutrient contents were always negatively correlated with stem dry matter content and canopy height, indicating greater investments in supportive and nutrient conserving tissue as plants increased in size. In the agricultural site, disturbance was the single most important factor decreasing plant height, while leaf traits such as specific leaf area and leaf nutrient contents increased with soil resources in heathlands. Productivity was directly or indirectly driven by leaf traits, and investments in structural tissue increased standing biomass and soil carbon.Different environmental drivers in the two sites produced opposing leaf trait effects on litter decomposition. Ecosystem properties were explained by the community mean trait value as predicted by the mass ratio hypothesis. Evidence for effects of functional diversity on productivity and other ecosystem properties was not detected, suggesting that diversity–productivity relationships depend on the length of the investigated environmental gradients. We conclude that changes in community composition and dominance hierarchies deserve the most attention when ecosystem properties must be maintained.

""Questions
During restoration of bare subsoil, are planted grassland communities with low species richness more susceptible to invasion by non-residents than communities augmented by additional species? What are the mechanisms of invasion resistance in early succession?

Location
Lexington, Virginia, USA (37.8°N, −79.4°W).

Methods
We planted 62 3 × 3 m plots on compacted clay subsoil with 20–21 replicates of three, 12 and 24 native species. Plots were sampled non-destructively using a stratified random point-intercept method each summer for 5 yr to determine number of species and percentage cover per species, which were classified as residents (planted), internal non-residents (planted in other plots) or external non-residents.

Results
A negative relationship between planted species richness and invasion developed by the fourth year after planting and strengthened thereafter. Plots consisting of a single, highly dominant, resident C3 grass species were vulnerable to invasion. With the exception of one species, which seems to have overcome resistance with a high seed rain, external non-resident species were less diverse and less abundant in higher diversity plots.

Conclusions
Increased planted species richness was correlated with increased resistance to invasion by non-residents, and we attribute this effect to a combination of the sampling effect and species complementarity, potentially augmented by high species dominance/low evenness in the low species richness plots.""

To further promote the excellent research that is being conducted by our ‘next generation’ of freshwater biologists, we are giving column space to current PhD students. Kevin Wood completed a Biology degree at the University of Bristol and a BSc thesis on guppy foraging behaviour, under the guidance of Prof. Julian Partridge. He then moved to Queen Mary, University of London for the Freshwater and Coastal Sciences MSc, supported by a NERC Masters Training Grant, and produced a thesis on interactions between invasive crayfish and native fish, part-sponsored by the Environment Agency and supervised by Dr Jonathan Grey. Below, he outlines his PhD focus, supervised by Dr Matthew O’Hare and supported by NERC.

Foragers typically attempt to consume food resources that offer the greatest energy gain for the least cost, switching between habitats as the most profitable food resource changes over time. Optimal foraging models require accurate data on the gains and costs associated with each food resource to successfully predict temporal shifts. Whilst previous studies have shown that seasonal changes in food quantity and quality can drive habitat shifts, few studies have shown the effects on habitat choice of seasonal changes in metabolic foraging costs. In this study we combined field and literature data to construct an optimal foraging model to examine the effect of seasonal changes in food quantity, food quality and foraging costs on the timing of a switch from terrestrial to aquatic habitat by non-breeding mute swans (Cygnus olor) in a shallow river catchment. Feeding experiments were used to quantify the functional response of swans to changes in aquatic plant biomasses. By sequentially testing alternative models with fixed or variable values for food quantity, food quality and foraging cost, we found that we needed to include seasonal variance in foraging costs in the model to accurately predict the observed habitat switch date. However, we did not need to include seasonal variance in food quantity and food quality, as accurate predictions could be obtained with fixed values for these two parameters. Therefore, the seasonal changes in foraging costs were the key factor influencing the behavioural decision to switch feeding habitats. These seasonal changes in foraging costs were driven by changes in water velocity; the profitability of aquatic foraging was negatively related to water velocity, as faster water required more energy to be expended in swimming. Our results demonstrate the importance of incorporating seasonal variation in foraging costs into our understanding of the foraging decisions of animals.

Woody plants generally exhibit more conservative water-use characteristics than herbaceous perennials and annuals, including lower stomatal conductance, net photosynthesis, and transpiration, but higher photosynthetic water-use efficiency. Under soil moisture stress, stomatal conductance usually decreases more than net photosynthesis, resulting in lower ratios of internal to atmospheric CO2 concentrations and higher photosynthetic water-use efficiency. However, we found that shrubs of a semiarid grassland employed opportunistic water-use strategies, which likely evolved due to the short duration of soil moisture availability. We measured photosynthetic gas exchange, leaf reflectance, and foliar stable carbon isotope composition in Artemisia cana, Prunus virginiana, Rhus trilobata, and Ribes aureum in southern Alberta. Under moist conditions, net photosynthesis was very high in A. cana, R. trilobata, and R. aureum, but less so in P. virginiana. Declining volumetric soil moisture caused stomatal conductance to decrease in proportion with net photosynthesis in 3 of 4 species (P. virginiana, R. aureum, and R. trilobata). Consequently, ratios of internal to atmospheric CO2 were unaffected by soil moisture depletion, and photosynthetic water-use efficiency declined. At the leaf level, the normalized difference vegetation index did not vary with season in these species, while the chlorophyll index decreased only slightly and the photochemical reflectance index showed a small mid-season peak. A. cana had the lowest stable carbon isotope ratio, highest stomatal conductance, highest late-season net photosynthesis, and highest leaf nitrogen (% dry weight). As soil moisture decreased, A. cana exhibited a decline in ratios of internal to atmospheric CO2 concentrations but no change in photosynthetic water-use efficiency. This study shows that even long-lived, woody species of a semiarid grassland have evolved traits facilitating rapid water-use to maximize carbon gain during brief periods of soil moisture availability.

""Soil abiotic and biotic factors play key roles in plant community dynamics. However, little is known about how soil biota influence vegetation changes over time. Here, we
show that the effects of soil organisms may depend on both the successional development of ecosystems and on the successional position of the plants involved. In model systems of plants and soils from different successional stages, we observed negative plant–soil feedback for early-successional plant species, neutral feedback for
mid-successional species, and positive feedback for late-successional species. The negative feedback of early-successional plants was independent of soil origin, while latesuccessional plants performed best in late- and worst in early-successional soil. Increased performance of the subordinate, late-successional plants resulted in enhanced plant community diversity. Observed feedback effects were more related to soil biota than to abiotic conditions. Our results show that temporal variations in plant–soil interactions
profoundly contribute to plant community assemblage and ecosystem development.""

"Question

To what extent does the movement of animals between fragmented habitat patches provide functional connectivity via endozoochorous seed dispersal?

Location

The Stockholm archipelago, Sweden.

Methods

We followed all movements of livestock between islands during one grazing season. After each movement, manure was collected and its seed content assessed through seedling emergence. Seedling data were then compared to vegetation surveys from the grazed islands with regard to functional traits.

Results

Light- and nitrogen-demanding locally abundant species, and those with relatively small and persistent seeds were more likely to be moved between islands. For quantitative traits, only a subset of the available trait ranges were dispersed, with extreme values left behind. Species apparently specialized to other means of dispersal emerged from the manure samples. Neither dispersed traits nor seed density changed with timing of movement, but seed richness and diversity both increased throughout the season. The subsets of endozoochorously-dispersed species in the established vegetation were more similar than non-dispersed subsets between islands linked by livestock.

Conclusions

Grazing networks contribute to the connectivity of the core species in the system, and could provide useful tools for grassland management in fragmented landscapes."

"Past (landscape history, old management forms) and present (shrubbiness, weediness, forest cover) state of the wood - pasture between Pénzesgyõr and Hárskút villages (Bakony Mts., Hungary).
A wood - pasture (with old seed trees) is a highly complex and diverse method of land or landscape management, having a great importance in the past management systems. Past, original and previous management of the observed area was studied by the means of historical written and map sources, data of archives and information collected through interviews. Grazing and in parallel regular
caring for the pasture (that had been created cca. 100 years ago) were abandoned about 15 years
ago, resulting in a massive growth in the covering rate of shrubs and young trees on the pasture. Rate
of open grassy areas does not exceed even 20 % for this time, thus, only huge trees remained on the
area are witnesses of the one-time history of the pasture. Speed of succession and changes in shrubbiness
were observed by analysing aerial photographs, so thus it could be established that the time
remained for reconstructing the area or for further researches could be not more than 10 years, and
by passing it, without any intervention, the destiny of this wood - pasture will be the same as in case of
several ten or hundred other wood- pastures in Hungary: sinking into oblivion."

Delaying senescence as a response to tissue losses has been reported in some studies, but there is not information about its influence in growth compensation. We performed a first test of the relative contribution of delaying senescence after defoliation to growth compensation in Dactylis glomerata L. by means of an iterative growth analysis modified to estimate tissue losses to senescent leaves. Dactylis glomerata overcompensated for relative growth rate after defoliation, mainly reducing the amount of tissue that enters in senescence, and partially increasing the newly assimilated mass allocated to leaves.

Bees are a key component of biodiversity as they ensure a crucial ecosystem service: pollination. This ecosystem service is nowadays threatened, because bees suffer from agricultural intensification. Yet, bees rarely benefit from the measures established to promote biodiversity in farmland, such as agri-environment schemes (AES). We experimentally tested if the spatio-temporal modification of mowing regimes within extensively managed hay meadows, a widespread AES, can promote bees. We applied a randomized block design, replicated 12 times across the Swiss lowlands, that consisted of three different mowing treatments: 1) first cut not before 15 June (conventional regime for meadows within Swiss AES); 2) first cut not before 15 June, as treatment 1 but with 15% of area left uncut serving as a refuge; 3) first cut not before 15 July. Bees were collected with pan traps, twice during the vegetation season (before and after mowing). Wild bee abundance and species richness significantly increased in meadows where uncut refuges were left, in comparison to meadows without refuges: there was both an immediate (within year) and cumulative (from one year to the following) positive effect of the uncut refuge treatment. An immediate positive effect of delayed mowing was also evidenced in both wild bees and honey bees. Conventional AES could easily accommodate such a simple management prescription that promotes farmland biodiversity and is likely to enhance pollination services.