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Progress report April 2013
- 1. A contrast of direct and indirect nurse-plant effects on annual plant and seedbank dynamics in two Californian Deserts Filazzola, Alessandro 1st Progress report
- 2. Nurse Plants
- 3. Chapter Title Purpose Status 1 The relative frequency and Identify current Data collection importance of facilitation literature on nurse- complete, write-up mechanisms in nurse plants plant mechanisms pending 2 Nurse plants as seed Summarized literature Papers collected, magnets: meta analysis on seed trapping by extract data and nurses write up 3 Regional contrast of plant Compare nurse effect Two surveys and seedbank by nurses on seedbanks and complete, transient annual community seedbank collected 4 Initial community density Determine density Upcoming field approach dependence on season annual communities under shrub 5 Importance of nurse plants Do nurse-plants On-going as seed traps function as seed traps and is there a cost of granivory
- 4. Nurse-plant mechanisms Hypothesis: There is a gap in the scientific literature regarding seed- trapping of nurse plants Expect nurse plants and pollinator studies to be understudied The Mojave Desert is an understudied region C1
- 5. Nurse-plant Studies Web of Knowledge: “nurse*” + “plant” = ~700 articles C1
- 6. C1
- 7. 120 100 Meta-mechanisms Number of Studies 80 60 40 20 0 Substrate Seed Trap Pollinator Moisture Reduced Anti-herbivore Evolution Spatial stress C1
- 8. Seed-trapping meta Conduct meta-analysis on ~30 papers Hypothesis: nurse plants function as seed traps by increasing seed arrival Papers are already found, data still needs to be extracted C2
- 9. Regional contrast of nurse plants Hypothesis: Nurse plants increase seed arrival and facilitate annual community production Determine study sites ◦ Northern Mojave Ridgecrest (NMR) ◦ Mojave National Preserve (MJP * 5) ◦ Panoche Hills Management Area (PAN * 3) Vegetation sampling ◦ Measure shrub ◦ Quadrat sampling (diversity, abundance, cover) ◦ Soil Cores C3
- 10. Mojave Grand Mean Panoche Mean calculated Rii with standard error between 12-15 shrub/open pairs C3
- 11. Census 2 Grand Mean Census 1 Mean calculated Rii with standard error between 12-15 shrub/open pairs C3
- 12. Mojave Grand Mean Panoche Mean calculated Rii with standard error between 12-15 shrub/open pairs C3
- 13. Upcoming surveys Conduct two additional surveys (C3 - Peak biomass and C4 – just prior to recession) Identify environmental gradient Soil cores for transient seed bank C3
- 14. ICD: Estimating Density Dependence H1: Higher seed density under shrub results in greater competition between seedlings Determine cost of nurse-association from competition Competition greater under shrub C4
- 15. Density dependence experiment Shrub ¼x ½x 1x 2x Open ¼x ½x 1x 2x C4
- 16. 80 70 60 # Germinate Plants 50 40 Shrub 30 Open 20 10 0 0.25X 0.5X 1X 2X Density
- 17. Nurse effects on seedbank H1: Higher seed densities present under shrubs than in open H2: Increased seed rain results in higher granivory rates. Determine cost of nurse-association from granivory C5
- 18. Modified Community Shrub C S 0 A Open C S 0 A C5
- 19. C5
- 20. Granviory experiment Mammal/Bird Insect Control - Control + Exclusion Exclusion No Exclusion Full Exclusion C5
- 21. 90 80 70 60 Seeds Remaining 50 Shrub 40 Open 30 20 10 0 No Mammal No Ant Control - Control + C5
- 22. Chapter Title Timeline 1 The relative frequency and Write up – end of year importance of facilitation mechanisms in nurse plants 2 Nurse plants as seed magnets: meta Extract data – end of year analysis 3 Regional contrast of plant and April 15-17 (C3), May 1-5 (C4), seedbank by nurses collect transient 4 ICD April 18-26 at Mojave/Panoche 5 Importance of nurse plants as seed Collect cups (May 18), set up traps granivory exclusions (April 26- 30), Collect exclusions (May 20)
- 23. Thank you!
Hinweis der Redaktion
- The desert, as beautiful as it is, is a difficult environment for plants to grow in. It is extremely arid, with most plant life entirely dependent on water abundance. In hot deserts, such as the Mojave this is even more so the case.
- However! Plants have adapted. Nurse plants, such as the one displayed here, benefit the annual community underneath through various methods such as water stress amelioration. There are also alternate mechanisms which facilitate plant growth such as herbivore protection, substrate modification, pollinator visitation, etc. One or more of these mechanisms can combine to produce a favourable environment under the shrub that is better than the adjacent open areas. * here you can seen what is referred to as the bulls-eye effect where the greenest vegetation is present directly beneath the shrub.
- The currently literature is expanding greatly on how nurse plants can reduce abiotic stress of an annual community, but not enough attention has addressed other mechanisms such as seed trapping. This presents a gap in the literature to expand on additional ways nurse plants can facilitate plant establishment and growth. * Moreover, the majority of studies that have been conducted have looked at Mediterranean climates and less so desert environments. *What is even more interesting is that the Mojave Desert has been, for the most part, neglected from most studies. I have therefore decided to conduct my research in the Mojave and looking at the effects of a nurse plant on seed bank dynamics.
- Here is a map with each red dot representing a location where a study on nurse plants has occurred. While at first glance it might appear that the Mojave has a number of studies, * all these dots actually part of the Sonoran desert. California generally has a gap in nurse studies until you reach the Coastal Mediterranean climate. Globally, there has been a vast amount of studies in the Mediterranean areas of Southern Spain and the Andes of Chile.
- This is a clock I made that categorizes all the nurse plant mechanisms. I used previous review papers as a guide to determine what factors can affect an annual plant throughout its life-stages (from seed arrival right to reproduction and spatial patterns). I derived mechanisms: substrate modification, seed trapping, pollinator visitation, onotgenic shifts, nurse-mediate distribution, soil modification, abiotic stress reduction and herbivore protection.
- Here we have a graph representing the number of studies associated with each nurse plant mechanism. You can see that * abiotic stress amelioration and * soil moisture modification are among the highest number studied. The last two categories, * evolution and altered spatial patterns, are what I classified as meta-mechanisms. Nurse plants through another mechanism alter the annual community to change variation in plasticity leading to evolution, or causing a dependent relationship between shrub-protege. This last category called substrate modification may need to be altered. My original definition of it is one where the ground cover is physically modified favourably to allow for plant establishment. However, it could loosely include changes to soil microbes or nutrients, etc. That still needs to be sorted out. Lastly, my two least studied topics are seed trapping and pollinator visitation. While pollinator visitation appears to desperately need of some work, my focus is more seedbank dynamics.
- I have acknowledged that a research gap exists on seed trapping mechanisms of nurse plants. I intend on conducting a meta-analysis on the roughly 30 papers I found which specifically examine seed trapping. The good thing about scouring the literature for nurse plant papers and categorizing them is that I have already collected all my seed trapping papers. When I expect to begin a write up later in the year once I have extracted all the data from the papers.
- In January I travelled through California looking for the ideal study sites to conduct my research. I would end up dropping the Ridgecrest site because there was not any germination. At the Mojave Preserve, there were 5 sub-sites located along a road that spanned an elevation gradient. For Panoche, there were three sites each separated by different directional aspect (North facing, South facing and plateau). At all of these sites I sampled the diversity, abundance and cover under a shrub and an adjacent open area. I would also measure the size of the shrub. Using these values I would calculate the Rii interaction that the shrub produces. The mean effect size for each of these sizes were then taken and plotted with the standard error. Lastly, I collected two soil cores under each shrub and open pair to bring back for analysis. I hope to see that the persistent seedbank varies between shrub and open conditions, to tie in with my seed trapping hypothesis.
- From the resulting graphs we can see that the middle elevations of the Mojave had the greatest facilitation effects, and the lowest as well as the highest had very little positive effects. Another good point is that all the Mojave sites are well above positive. Unfortunately, the Panoche sites, while positive are not significant, however I feel if I change my resolution I will get a better representation. In the second graph there appears to be a larger difference in facilitation effect during the second census than the first one. This is potentially because as the season continues on, percipitation stops and nurses may provided moist soil.
- This next graph is the mean effect size of plant interactions after compensating for nurse shrub size. We can see that the trends all dissolve. Although all the means are positive, none could be considered significant. This is interesting because it suggests nurse size plants a dominate role in the strength of facilitation effects. * You can see that nurse size results in a greater density in an almost logrithmic manner. I still need to find an appropriate model to fit the graph below, especially since there are mixed effects. One another thing we can take out of this data is that nurse shrubs are largest at the middle elevations, potentially where stress is the least dominate.
- When I return in a week, I will conduct a third survey immediately which should hopefully represent peak biomass. I will also conduct a fourth and final survey at the beginning of May after the seeding period has occurred and the plants are beginning to go into recession. I will also identify the environmental gradient using Hobos and spots measurements. I will of course have to collect my cups to bring back and measure, as well as re-sample the seed bank using soil corers to determine the transient seed bank. I also intend on conducting two additional experiments to further examine seedbank dynamics under shrubs.
- The second experiment I intent o set up examines density dependence under shrubs. While more seeds may be present under shrubs, greater competition may occur during germination. By modifying the seedbank density I will estimate competition under shrubs and in open areas. I expect competition to occur at a higher rate under nurses than in open areas.
- Similar to my previous modified cup-community experiment, I will bury four cups under shrub and open micro-environments. I will then sieve the sand to modify the seedbank to one-quarter natural, one-half natural, natural, and two-times natural densities. The cups will be re-buried and germination will be measured the following season. This approach was used by Goldberg and Estabrook. There is a potentially additional step where source densities are crossed between shrub/open environments.
- Although nurse plants may present an ideal environment for seeds to grow, they may act as a feeding cache for granivores. Using modified seed trays I will estimate granivore rates under shrubs and in open areas. I expect granivory to occur at a higher rate under nurses than compared to open areas.
- A second experiment I was running in tandem specifically looked at the seed trapping effects of nurse plants. I took four cups and buried them underneath a nurse plant and open adjacent area. I tend filled one cup with the identical community originally found underneath. *This would be my control. *The next cup I did the same, but removed the annual community on top so that it was natural seeds only. The third community I sieved the sand so that there was no seeds placed back into the cup. The last community, I sieved the sand to remove any seeds, and placed an artifical plant made from carpetting, to decouple the annual facilitation effects on seed arrival from the nurse effects.
- Here is a picture of what that looked like in the field. I would randomize each cup between shrubs but keep them the same between shrub/open pairs. I also put in 8” framing nails with labels that will hopefully be there when I get back.
- Using cut PVC pipe I would create rings in the soil to mark an exclusion area. I would cover the pipe with a wire mesh to exclude mammals and birds from accessing seeds. To prevent ant access, I will coat the exterior with a substance, potentially Fluon which will prevent the ants from accessing the seeds. This exclusion will still allow mammal access. Two controls will also be set up, one without an any exclusion methods and a second with a full exclusion to determine the effectiveness of our anti-granviory methods. These four treatments, like the cups, will be paired under shrubs and in open areas. A pre-determined set of seeds will be buried in sieved sand at different densities.
- In summary those are the five chapters of my thesis. The first is an analysis of nurse mechanisms and identification of gaps in the literature. The second is a meta-analysis, specifically examing the direct and indirect facilitation effects of nurse plants on seed arrival. The 3rd chapter is a field experiment comparing nurse shrubs to the annual seedbank and plant community in arid environments. These will be examined along an elevation gradient and aspect differences. The 4th chapter will look at density dependence of an annual community under shrubs and open conditions. Lastly, the 5th chapter will examine the facilitation effects of nurses on seed arrival and the cost of association as a result of granivory.