Experimental study
Focal research questions
- Do insect herbivores, molluscs and fungal pathogens differ in their impact on plant communities? And do they interact with each other?
- When do invertebrate herbivores and fungal pathogens have the strongest effects on plant communities (productivity, community composition, diversity and functioning)?
BugNet Goals
Global research networks can rigorously test for general patterns and mechanisms and several, such as the NutNet or Drought-Net, have led to important advances. The goal of BugNet is to survey consumer and plant communities across sites and set-up identical insect herbivore, mollusc and fungal exclusion experiments in many parts of the world.
BugNet aims to implement a cross-site study requiring minimal investment of time and resources by each investigator. It involves an experimental study to quantify plant community and ecosystem responses to invertebrate herbivores and fungal pathogens in a wide range of herbaceous-dominated ecosystems, such as desert grasslands to arctic tundra, but also heathlands or Mediterranean shrublands.
Protocol
- 1.1. Selection of sites
- 1.2. Setting up the experiment
- 1.3. Treatment applications
- 1.4. Measurements per site – Baseline data (prior treatment application)
- 1.5. Annual measurements per plot
- 1.6 One-time measurement of plant traits
- 1.7 Special sampling campaigns per plot and additional measurements
- References
You can also download the protocol as a pdf here.
1.1. Selection of sites
The site should be relatively homogeneous, dominated by herbaceous or shrub vegetation. Natural disturbances, such as fire or browsing by vertebrates, do not need to be excluded from the site, but a record of the disturbance regime, and ideally a quantification of vertebrate herbivory, is required. It is preferable that the site is not heavily grazed by livestock. Grazed sites can be included if the plots are fenced, though.
During the experiment, the site should be managed as it is common in the area, i.e. if the grasslands are mown once or twice a year, then the experimental site should also be mown. In this case make sure to plan your measurements to take place at peak plant biomass.
1.2. Setting up the experiment
The experiment will be a randomized block design with three blocks, 8 treatments, and three replicates per treatment (N = 24 total experimental plots, Fig. 1). Each experimental plot will be 5 x 5 m in size, separated from the other plots by a 1m walkway. Each 25m2 plot will be subdivided into four 2.5 x 2.5 m subplots (A, B, C, D), with one dedicated to the core sampling, one to additional site-specific studies and two for future network-level research (e.g. exclusion of oomycetes, warming treatment, drought treatment…). The position of the treatments in the three blocks should be randomly assigned, and also the subplots should be randomly assigned to the different uses. The subplot dedicated to the core sampling will further be divided into four 1m x 1m small plots (i, ii, iii, iv), with the one located closest to the centre designated for the assessment of species composition (cover, i). The other three small plots will be designated for destructive sampling such as the assessment plant biomass, or herbivore and pathogen damage (only in some years, see Fig. 1).
To quantify the impact of different consumer groups, they will be excluded (reduced) using biocides. Treatments will involve the removal of consumer groups alone, i.e. insects, mollusc, and fungi, in all possible two-way combinations, all consumer groups together and a control, giving a total of 8 treatments (Fig. 1).
1.3. Treatment applications
To control insect herbivores, we will use Lambda-Cyhalotrhin (e.g. Karate Zeon, Syngenta), which is a broad spectrum, non-systemic insecticide frequently used in herbivore exclusion studies and with few non-target effects. It will be sprayed four times during the growing season. To control foliar fungi a combination of azoxystrobin and difenoconazole (azoxystrobin inhibits fungal mitochondrial respiration, Difenoconazole interrupts the synthesis of ergosterol, a fungal cell membrane component, e.g. a mix of Score Profi and Ortiva, Syngenta), will be spayed at the beginning of the growing season, and then every 4-6 weeks until the end of the growing season. To control molluscs, molluscicide pellets based on ferric phosphate (e.g. Limax Ferro, Maag, or any other product based on ferric phosphate (e.g. Sluggo), sometimes also called iron phosphate) will be applied at the same times as the other pesticides. The number of applications will be larger in regions with a very long growing season, but this is appropriate given that the consumers are active for longer and that we want to ensure that they are effectively reduced in abundance. However, do check the local regulations in your area and make sure you do not apply pesticides more than is allowed. It might also be that in some countries some biocides are not approved. If this is the case, please contact us and we will discuss alternative products that can be used. Biocides may not wipe out infestation, but they do significantly reduce enemy attack on plants and are so far the only experimental approach to assess the importance of invertebrate herbivores and pathogens in natural plant communities. Check out our detailed protocol on pesticide application.
1.4. Measurements per site – Baseline data (prior treatment application)
To characterize the different sites around the globe, several measurements of soil conditions, and a few characteristics of the plant community should be taken prior to the application of the treatments. This allows us to link consumer impact to several drivers (latitude, altitude, soil fertility, plant diversity and biomass), and to shed light at the context dependency of biotic interactions.
As baseline plot measures, estimate the percent plant cover per plant species in each of the 24 plots, in the subplot dedicated to the core sampling, in the small plot located closest to the centre (see Fig. 1, cover). Cover for each plant species rooted within the plot will be estimated to the nearest 1% (up to 20% cover) and the nearest 5% for cover 20-100%. Assign 0.5% to very rare species with less than 1% cover. Estimate also the percent cover for woody over storey, bryophytes, lichens, litter, bare soil, and rocks if present. Total cover will typically exceed 100% because species cover is estimated independently for each species (see cover datasheet). Take these measures before the application of the treatments.
To reduce bias in cover assessment, it is helpful to train yourself by placing differently sized pieces of paper on the plot: e.g. 10cm x 30cm = 3 %, 10cm x 10cm = 1 %, 3.1cm x 3.2cm = 0.1%, 31cm x 32cm = 10 % …
Aboveground biomass
As baseline measures of productivity in each of the 24 plots, clip the aboveground plant material to 2 cm above ground level, in two 10cm x 50cm strips (orange rectangles, Fig. 1) in one of the small plot dedicated for destructive sampling (ii, iii or iv). Collect the total aboveground biomass, dry it for 3 days at 70 °C and weigh it. Send a subsample of the dry biomass samples to the project coordinators. If you can grind the biomass to powder, that would be ideal, but if not you can cut the biomass sample in pieces and send us a well-mixed subsample. Please send us ca. 20 g of dry weight per plot (e.g. in a zip-block plastic bag or a jar, see here). We will use the samples to measure several leaf characteristics (leaf N and P, fibre content etc.), and to track plant quality in response to the treatments. We might also look at the phyllosphere microbiome at some point. Take these measures before the application of the treatments.
If you work in shrublands the biomass of the shrub species will be estimated using allometric equations. For each shrub species, measure the height and canopy diameter of 20 individuals of contrasting sizes outside of your experimental plots. Clip, dry and weigh them. If your shrubs form big patches and individuals are difficult to isolate, just take 20 “sampling units” with known height and canopy diameter, and collect the biomass of this sampling unit instead. To be able to obtain a measure of shrub green vs brown biomass, please separate the leaves from the woody biomass. For some species this might work better once the leaves are dry. Now measure the height and canopy diameter of all shrubs in the subplot dedicated to the core sampling, in the small plot in which you also assess the plant cover (Fig. 1, cover).
Soil samples
Soil cores will be collected to assess a range of soil characteristics. In each of the 24 plots, collect two soil cores (soil corer 2.5 x 10 cm) and homogenize the soil into a single sample per site. Please sieve the soil through a 2 mm mesh. Soils should be air-dried and send to the project coordinators (see labelling and mailing protocol sheet for more details). There, total organic C, total N and P stocks, as well as pH will be measured and will give information on soil characteristics at the site level.
Long term soil storage
In addition, we would like to obtain information of the soil microbial community at the plot level. The soil microbial community is likely to change in response to the pesticide treatments. To be able to analyse this change at some point in the future, it is necessary to have information on pre-treatment conditions. We therefore encourage you to store soil samples per plot prior to the application of the treatments in a freezer:
Please take five soil cores (e.g. 2.5 x 10 cm) from random locations per plot with a soil corer, and homogenize the soil into a single sample per plot. Make sure to carefully clean your soil corer between plots. We ask you to store 50g of soil per plot in a labelled zip block bag or any other container at -20°C, and 5g of soil per plot at -80°C (if you have access to such a freezer). Don’t forget to label your bags well, as they might sit in the freezer for a few years before we analyse the microbial community at some point in the future.
1.5. Annual measurements per plot
Plant species composition
Once annually, estimate the percent plant cover per plant species in the subplot dedicated to the core sampling, in the small plot located closest to the centre (see Fig. 1, i, cover). Cover measures follow the same protocol as for the baseline data. In systems in which species composition shifts strongly within the year or which have a two-times mowing regime, we recommend that species composition is assessed twice, once in spring/early summer and once in late summer. This allows us to account for differences in phenology and to capture the maximum cover of each species.
Above- and belowground biomass
To quantify consumer impact on productivity (top-down control), clip the aboveground plant material to 2 cm above ground level, in two 10cm x 50 cm strips of each core sampling subplot, in one of the small plots that are used for destructive sampling (ii, iii, iv). Each year, the biomass harvest should be done in a different small plot (see Fig. 1).
If you work in very unproductive systems which are not mown, and in which the removal of biomass might still be visible after three years, then try to not overlap the stripes for biomass harvest for as long as possible (see example of harvest positions, Fig. 2).
Fig. 2: In very unproductive systems, in which plants grow very slowly and the removal of biomass might still be visible after a few years try to not overlap the stripes for biomass harvests for as much time as possible.
Collect the total aboveground biomass, dry and weigh it (the two subsamples per plot can be combined). Sampling should be done at peak biomass production (the timing of peak biomass will vary between sites and will be defined by local researchers for their system). If your site has a two-times mowing regime, biomass should be collected twice per year to better estimate site productivity (before the cuts). Send the dry biomass samples to the project coordinators (Anne Kempel) every year. We will then measure several leaf characteristics (leaf N and P, fibre content etc.). Please send us ca. 20 g of dry weight per plot (e.g. in a zip-block plastic bag or a jar, the two strips per plot can be combined, see labelling protocol).
If you work in shrublands the biomass of the shrub species will be estimated using the allometric equations that you have developed for the baseline measures. Measure the height and canopy diameter for all shrubs in the subplot dedicated to the core sampling, in the small plot in which you also assess the plant cover (Fig. 1, cover, i).
1.6 One-time measurement of plant traits
At each site, several plant traits – plant height, specific leaf area (SLA) and leaf dry matter content (LDMC) – will be measured to characterize the plant communities. These traits are closely associated to two major axis of plant functional variation, the size of plants and their parts, and the resource economics spectrum (Wright et al. 2004, Díaz et al. 2016). You will measure the traits according to protocols in Garnier et al. 2001.
Height, SLA and LDMC of all plant species at a site should be measured. This is important to test whether the response of plants to enemy exclusions follow patterns predicted by defense-deployment strategies (e.g. growth defense-tradeoff). For each plant species present at a site, five individuals per site should be randomly sampled, and their height, SLA and LDMC assessed. The height can be directly measured in the field, for SLA and LDMC pick the five randomly selected individuals, bag them in a labelled plastic bags and place them in a cooler. If possible, your individuals have > 5 leaves without any damage symptoms, as ideally the leaf traits are measured on undamaged leaves (see detailed protocol on how to measure SLA and LDMC). These measurements can be done at any point up until the fourth year.
1.7 Special sampling campaigns per plot and additional measurements
At some point we will ask you to measure a few additional variables, such as root biomass, herbivore damage and fungal infection, or to assess the invertebrate communities per plot. We also plan to measure decomposition using the tea-bag approach. There will also be the possibility to propose additional measurements. We will hold several workshops and discussion groups where we can develop ideas together.
Root biomass
In year three of the experiment we will ask you to measure the standing root biomass. To do so, take a soil core of 5cm diameter, 30 cm deep, per plot, and sort to separate roots.
Herbivore damage and fungal infection
In some years we will ask you to measure herbivore damage and fungal infection per plot. If your system is very productive and will be mown regularly, damage and disease will be measured in the small plot dedicated for the plant cover (small plot i). If your system is very unproductive and removing individuals for damage assessment would have strong influences on the vegetation, then measure the herbivore damage and fungal infection in one of the other small plots dedicated for destructive sampling (ii, iii, iv), or alternatively, assess damage in the cover small plot without removing individuals. Damage does not need to be assessed every year and we will communicate the years in which the damage should be assessed.
Selection of species: We aim to measure the community weighted mean damage and disease per plot. Start with the species having the highest cover, followed by the species with the second highest cover, and so on, until the cover-sum of the species reaches 80% (relative cover excluding bare ground, rocks). However, do not assess damage on more than five plant species per plot. E.g. if Species A has a relative cover of 50%, Species B a relative cover of 20%, Species C a relative cover of 10% and Species D a relative cover of 8% assess damage on A, B and C. If Species A has already a relative cover of 90%, then only assess damage on species A. If Species A, B, C, D, and E have each a relative cover of 15% and species F, G, H, I… a cover of 5% then only assess damage on the five most abundant species (A,B,C,D and E) although the sum of their cover does not add up to 80%.
Selection of individuals per species: If your species has less than 10 individuals, select all individuals. If your species has between 10 and 20 individuals per plot, mark all individuals in the 1m2 with grill sticks numbered from 1-20. Use a random number generator (or quickly select 10 numbers in your head) to select 10 individuals (it is important that the individuals are selected randomly, and that there is no bias towards particularly damaged or undamaged ones). If your species has more than 20 individuals, divide your 1m2 plot into four quadrants, and estimate the proportion of individuals in each of the four quadrants. Randomly select individuals per quadrant in proportion to their numbers of individuals, e.g., if quadrant 1 contains 80% of all individuals, and quadrant 3 20%, then randomly select eight individuals of quadrant 1 and two of quadrant 3. Particularly if the distribution of your species is very patchy (e.g. one large patch with many individuals, and 3 isolated individuals) this methods prevents that you will select isolated individuals with a higher likelihood (see Fig. 3).
Fig. 3: Selection of individuals per plant species.For each selected plant species, select 10 individuals on which you will assess the presence/absence of damage signs of the different damage categories (incidence). Of these 10 individuals you will select five individuals which you will cut at ground level (respectively a subset of them), bag, and bring to the lab for the assessment of % leaf damage.
For more guidance – this youtube video gives a tutorial on how to select species and individuals for damage incidence and damage % measurements.
Measurements: On each of the selected individuals per species (max. of 10), record the presence or absence (0,1) of damage signs by chewing, mining, galling and sucking/rasping herbivores, and pathogen disease symptoms of the categories downy mildews, powdery mildews, rusts and leaf spots (see damage gallery). This will give us an estimation of damage incidence.
In addition, randomly pick five of the ten individuals (select them using the grill sticks with the first five random numbers from your random-number list), and measure the maximal height (stretch out if necessary, see section on plant traits below). Then, cut them at ground level. If your individual is very large, or builds large tussocks as is often the case for grasses, then take a subsample from the middle of the individual which contains at least five leaves. If you selected only one plant species per plot because this species had a relative cover of 80% or more, then cut all 10 individuals. On these individuals you will estimate the leaf area (%) that has been damaged. You can either do this directly in the field, or you can bag the plants in labelled plastic bags, place them in a cooler, bring them to the lab, and do the % damage assessment there. In any case, visually survey five random, mature, and non-senescing leaves (or leaflets if your leaves are very small) per individual for damage and disease symptoms. For easier inspection you can use hand lenses to better assign damage types. On each of the five leaves estimate the leaf area (%) that has been removed by chewing herbivores, mining, galling and sucking/rasping herbivores, and the leaf area that is covered by pathogenic disease symptoms of the categories downy mildews, powdery mildews, rusts and leaf spots. Some plant individuals will have fewer than 5 leaves, and for these all leaves should be surveyed (but leave out senescent leaves). Note that in some cases, damage is present on only the underside of leaves, so remember to check both sides of the leaf for damage. You can check out our detailed protocol on how to assess damage and also our photo gallery of the most common damage types. We also highly recommend that you train your damage assessment with the ZAX Herbivory Trainer app by Zoe Xirocostas and Angela Moles!
With this method you assess damage on a minimum of 10 individuals (if one species had a cover of 80% or more) or on up to 25 individuals (if you selected 5 plant species with each 5 individuals) per plot.
Shrublands: If you work in shrublands, you may not find 10 individuals of a species in your 1m2 plot but very likely will have only one or two individuals. You may also have spreading shrubs where it can be difficult to distinguish individuals. In this case, randomly pick 50 leaves from 10 different branches per shrub species throughout the plot (5 leaves per branch). The five leaves per branch should always originate from the same leaf position, i.e. from the tip of each branch, count the first ten leaves and start sampling from there (leaf 11 to 15). Make sure that the 10 branches are from either as many shrub individuals as possible, or else take branches from different positions and orientations within the shrub patch. Assess how many of those 50 leaves are damaged. This allows us to assess the proportion of leaves that are damaged by the different damage categories per shrub individual (incidence). In addition, assess the % leaf area damaged on at least 25 randomly chosen leaves per shrub species.
If your shrub species is leafless and instead has photosynthetic stems (e.g. Retama ssp.), instead of picking leaves, randomly cut 5 cm pieces from 10 different branches throughout the plot. On these, assess how many of those pieces show damage signs of the different damage categories (incidence). In addition, on five of those branches, assess the % photosynthetic area that is damaged. You can assess % damage either in the field or collect the leaves to assess damage in the lab.
References
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