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Desert lab report...

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Desert Cactus: Do They Use Nurse Plants for better Survival Abstract Our study was done on the nursing plant hypothesis, for cacti in the desert. The purpose of our study was to determine whether or not seedling cacti used the shade of other plants in the desert to protect themselves from the harsh environment, in order to grow better. We hypothesized that the seedling cacti did in fact use the shade of the other plants in order to thrive in the desert. The methods of testing that we used, were simple field observation. My partner and I walked in a straight line, out in the desert, and counted every saguaro and barrel cactus that was under 30cm. We then put them into either the associated or not associated categories based off if they appeared to be using another plant for shade or not. We also counted triangle- leaf bursage to show that it is only a phenomenon shown in cacti. Some important results we found are 100 of the 144 saguaro cacti were associated with a nursing plant, 127 of the 189 barrel cacti we found were associated, and only 179 of the 583 triangle- leaf bursage we found were associated. In conclusion the study allowed us to strongly support our nursing plant hypothesis, while at the same time see that it is only a phenomenon that happens to cacti not all desert plants. Introduction We did our study on two types of cacti. The barrel cactus which is scientifically known as Ferocactus wislizenii, and saguaro cactus which are scientifically known as Carnegiea gigantea. The purpose of the study was to determine whether or not young cacti used other plants as nursing plants. This means that the cacti thrive better when under the shade of other plants in the desert, which would explain why we can find more cacti than expected under such plants. We hypothesized that the young cacti did in fact use other larger plants to protect them from the harsh sun in the desert, allowing them to have a better survival rate. We predicted that if our hypothesis was correct we would find more of the young cacti underneath larger nurse plants than would be expected if everything was based merely on chance. This predication does not mean that more young cacti had to be associated to nursing plants, only that more than would be expected had to be associated in order to support our hypothesis. Methods The methods used for our study were rather simple, and consisted of only two lab partners and a meter stick. You go into the desert and stand about 5 meters away from your partner. You both pick a point on the horizon, directly in front of you, and then you begin walking towards it. While you are walking you count every study plant (barrel or saguaro cactus) that you see. You then determine whether that plant is associate or not associated with a nursing plant. You also count the first ten triangle- leaf bursage that you see and also say whether they are associated or not. While doing this make sure to walk in a straight line, not crossing paths with any other groups, and not counting any plants twice. While this is going on another group should be calculating the percent of shade cover. In order to do this they must take a tape measure that is 50 meters long. One partner should hold one end while the other stands with the other end 50 meters away. A third partner should then walk along the line and measure every shading plant crossed by the line and then divide it by 5000 centimeters to get the average shade cover for the desert at that time.

The methods used for this study were rather basic, consisting only of people and meter sticks. The first step is to split up into several groups. We had about 11 or 12 groups of 2 people and one group of three people. Then each 2-person group stood about 5 meters from each other, picked a direction, chooses a point on the horizon, and started walking straight at that point. While they walked the groups counted every test plant (saguaro or barrel cactus) that was under 30cm. The groups then decided if those plants were associated or unassociated by determining their distance from possible nurse plants. At the same time each of the 2-person groups counted the first 10 triangle-leaf bursage that they saw and record whether or not they are associated with a nurse plant. While all this is going on the 3-person group was measuring the shade coverage for the day. To do this they had a long tape measure. This group started by making a 50m line through the desert and then measuring all the width of the shading plants on that line. They then took that number and divided it by 50m (the length of the line). The 3-person group continued to do this several times in multiple different directions in the desert. Averaging all the totals that they got gave us a good estimate for the average shade cover at that time. Results The results of our study showed that there were 144 saguaro cactus spotted, of which 100 were associated with nursing plants, and 44 were not. There were 189 barrel cactus spotted, of which 127 were associated to a nursing plant and 62 were not. Finally 583 triangle-leaf bursage were spotted, 179 of which were associated, leaving 404 unassociated. Our last results had to do with the average shade cover. There was a total of 8589 cm of shade plants measured, with a total intercept distance of 30000 cm. This leaves us with an estimated plant cover of 28.6 percent. Figures and Tables Species

Associated NOT ASSOCIATED Total

Saguaro

100

44

144

Barrel cactus

127

62

189

Triangle-leaf bursage

179

404

583

Table 1. Plant association table. Table includes count of all cacti and triangle-leaf bursage associated and not associated with a nurse plant along with a total number counted for each plant.

Total Plant Distance (cm) Intercept Distance (cm) Estimated Plant Cover

8589 30000 0.286

Table 2. Desert shade cover

Table includes the total distance of shading plants along with a total distance in which those plants were measured, used to calculate the estimated plant cover percentage Species

Expected Associated

Expected Not Associated 102.816 134.946 416.262

Total

Saguaro 41.184 144 Barrel Cactus 54.054 189 Triangle-Leaf 166.738 583 Bursage Table 3. Expected plant association table Table includes the expected number of plants to be associated and not associated given the measured shading conditions during the experiment. This dates shows us that the deviation from the expected results if the null hypothesis was true is: X2 = 182.438, d.f.= 1, p p > 0.20. Discussion In our study we hypothesized that young desert cacti used other plants, as nursing plants, to get shade from the harsh desert sun. In order to support our hypothesis and disprove the null hypothesis we predicted that we would see more than the expected number of young cacti, under 30 cm tall, associated with nursing plants. After running our Chi square test on the observed and expected values the probability of our results occurring by random chance turned out to be less than one percent. This allowed us to disprove the null hypothesis. We then ran a Chi square test on the numbers of the triangle leaf bursage, and got a probability of between twenty and fifty percent. This means that we can say that triangle leaf bursage do not use nursing plants and were found pretty much as expected in the desert. These results allow us to support the hypothesis that young cacti in the desert do in fact use nursing plants to protect them from the harsh sun. This is a hypothesis that is strongly supported and widely documented, especially in the Sonoran Desert (Withgott 2000). The nurse plant theory says that seedlings that are sheltered by nursing plants, as opposed to those that are exposed, have a significantly increased chance of survival, given the protection of the harsh desert sun (Franco 2003). Some follow up studies that could be useful would be doing the same study in multiple other sites, as well as including other desert species. This would allow for us to eliminate the possibility that nurse plants only occur in the one section of desert in which we studied. This follow up experiment would also allow us to see that cacti are the only species that have the nurse plant phenomenon, eliminating other desert species. One last follow up study would be to do the same study at different times in the day to see in the sun position and shade coverage has any effects on the results.

References

Franco, A.C. "Effect of Nurse Plants on the Microhabitat and Growth of Cacti." STOR (1989): n.pag. UCLA, 17 Mar. 2003. Web. 27 Mar. 2017. Withgott, Jay. "Botanical NursingFrom Deserts to Shorelines, Nurse Effects Are Receiving Renewed Attention." BioScience. Oxford University Press, 01 June 2000. Web. 27 Mar. 2017. (2016) Introductory Laboratory II Lab Manual. Ecology and Evolutionary Biology....