PSNK Nature Trail Field Project Hone, Christopher M.

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1 Evidence of Allelopathic Influences of Common Spicebush (Lindera benzoin) on Canopy Tree Seedlings in an Upland, Mixed Deciduous, Pennsylvania Forest. Experimental Summary Christopher M. Hone Biology 220M, Spring 2007 Abstract Two areas on the PSNK nature Trail were observed to have very different populations of Common Spicebush (Lindera benzoin), and the presence of these Spicebush appeared to have an inhibitory effect on the regeneration of canopy trees. This experiment was set up to explore the possible reasons for the difference in the understories of the two sample sites and the possibility of allelopathic inhibition of tree seedlings by the Spicebush. The difference in apparent Spicebush densities was evaluated by an analysis of the abiotic variables in each site (soil temperature, ph, light exposure, humus-layer depth, and leaf litter composition) and by a census of all woody plants greater than 30 centimeters tall in both sites. The results of this phase of the experiment showed that there is no difference between the Spicebush populations of the two sites, and that the observed difference is a result of the average age and size of the specimens in each site. The site that appeared to lack an understory actually possessed an equal number of much smaller Spicebush. This supported the hypothesis that the difference between the sites is due to a different rate or origin of the succession process. This concludes the first phase of the experiment; the second will aim to determine if the observed disparity in forest floor seedling regeneration is due to allelopathy or some other mechanism. Introduction Certain portions of the Penn State New Kensington nature trail are conspicuous for their dense stands of understory shrubs and small trees, particularly the Common Spicebush (Lindera benzoin). Other portions of the trail seem to possess no appreciable understory of any kind, though there is no obvious difference in the environmental factors affecting these areas. One section of the trail in particular illustrates this disparity; the view to one side of the trail is obscured by the dense stands of Spicebush, while that on the other side (with an apparently similar environment) reveals a typical hardwood forest canopy with no visible woody understory. Further, the forest floor appeared relatively bare beneath the Spicebush, while casual observation showed limited American Elm (Ulmus Americana), Black Cherry (Prunus serotina), Red Maple (Acer rubrum), and American Beech (Fagus grandifolia) regeneration on the forest floor of the site without a shrub understory. Figures 1 and 2 illustrate the difference between the two sites. Biology 220M Page 1 of 9

2 Figure 1 Site A has a dense understory of Spicebush with yellow flowers. Figure 2 has no observable understory and has higher canopy seedling regeneration on the forest floor. Photos courtesy of Deborah Sillman, PSNK Biology 220M Page 2 of 9

3 A variety of factors could be hypothesized to account for the difference between these two sites. The Spicebush is an understory shrub with at least partial shade tolerance, often found beneath hardwood middle-succession canopies. Other shrubs that fulfill this ecological niche, such as the honeysuckles (Lonicera sp.), use allelopathy to inhibit the growth of competitors (Czarapata 2005). In addition, leafbased allelopathy has also been ascribed to other members of the Lauraceae, including Sassafras (Sassafras albidum), which shares many traits with the Spicebush, including chemical-laden leaves (Gant 1971). It is therefore a reasonable hypothesis that in an understory where Spicebush is present, the area occupied by Spicebush will steadily increase by allelopathy (either via roots or leaves) until it is the dominant species present, preventing regeneration of canopy species like Ulmus americana and perhaps increasing the probability that other Spicebush will be the only shrub able to survive. For the purposes of this experiment, the site with the observed understory will be referred to as Site A; that without it will be referred to as. Other hypotheses to explain the understory disparity could include the following. 1. The difference in Spicebush populations between these sites is due to differing depths of the A- Horizon (topsoil) layer in the sites. 2. The difference in Spicebush populations between these sites is due to differences in soil ph between the sites which favor different vegetation types. 3. The difference in Spicebush populations between these sites is due to differences in soil temperature between the sites. 4. The difference in Spicebush populations between these sites is due to a difference in the level of direct light exposure in the sites, stemming from the difference in their positions on the hillside. 5. The difference in Spicebush populations between the sites is due to a significant difference in the canopy species composition of the two sites. Site A possesses more species that are favorable to Spicebush, either through their effect on the soil or on light exposure. Evidence supporting any one of these hypotheses would not necessarily indicate no allelopathy has occurred, but would render this explanation unnecessary. Other hypotheses to explain the absence of canopy regeneration in Site A could include the following. 6. The critical factor influencing the presence of Spicebush is browsing by the Whitetail Deer (Odocoileus virginianus), which inhibits Spicebush growth in. 7. The critical factor influencing understory composition is the amount of light that enters through the summer canopy, because of a difference in density (i.e. a large hole) rather than species composition. 8. A predator is the main influence affecting one of the sites, and prevents whichever vegetation type would out-compete the other from maintaining a viable population. Possibilities include the Spicebush Swallowtail (Papilio troilus) or any of a number of insect pathogens. 9. The difference in the Spicebush populations between these sites is due to a differing rate or starting point of the understory succession process; will look like Site A when its understory has had time to develop. A variety of factors could account for this different successional stage. Because of the dual nature of this experiment, two experimental phases would be necessary to evaluate these hypotheses. The first phase would determine 1) that the observed difference in the two sites is actually present and quantifiable, and 2) that the perceived similarity between the two sites in terms of the abiotic environmental variables is in fact real. If the allelopathy hypothesis is true, we would expect this phase to reveal 1) that there is a significant difference between the numbers, size, and importance of the Spicebush in one site as compared to the other, and 2) that there are no statistically significant differences between the two sites in terms of soil temperature, ph, light exposure, humuslayer depth, and leaf litter composition. The purpose of the first phase was to determine that the difference was real and not due to any simple differences in habitat preference, as postulated by hypotheses 1-5. The second phase of the experiment would attempt to disprove the allelopathy Biology 220M Page 3 of 9

4 hypothesis by finding alternative explanations for this disparity, such as any evidence supporting hypotheses 6-9. Method This experiment was set up to explore the possible reasons for the difference in the understories of the two sample sites and the possibility of allelopathic inhibition of tree seedlings by the Spicebush. Two sites circular plots of approximately 163 square meters (approximately 10 meters apart) were identified for each area. Nine random sample locations were identified in each plot by shooting a bearing every 45 degrees from the midpoint and following it for either 7.2 or 3.6 meters all around the midpoint, including the midpoint itself. At each of these sample locations, soil temperature, soil ph, soil A- horizon depth, and leaf litter composition were measured. Light exposure was also measured with a light meter at each midpoint. A census was then taken of each woody plant taller than 30 centimeters in either site. Measurements and observations were taken over a two-week period in March Variables measured for each plant included species (where possible), circumference at breast height, area of the biggest stem, apparent health, and height when the specimen was lower than breast height. The soil variables were compared using 95% confidence intervals, and a comparison of the woody plants in each site was then made with the data from the census. Evidence of activity of the Whitetail Deer (Odocoileus virgianus) was also noted in the form of scat, browsed twigs, tracks, or buck rubs. Results Analysis of the abiotic components of each site yielded no statistically significant difference in the environment. The final 95% confidence intervals yielded statistical overlap with regards soil ph, soil temperature, and A-horizon depth between the two sites, ruling out hypotheses 1-3. Measurements of the light exposure (in March) based on the two sites position on the hill were very similar, and this, together with the proximity of the sites, allowed us to rule out hypothesis 4. Table 1 illustrates some of the findings. Pin samples of the leaf litter were also taken at each sample site and used to generate a description of the importance of each species in the leaf litter. Results were not definitively conclusive because differing rates of leaf decomposition or retention based on species or site could affect the proportion of each species present in the leaf litter. However, no evidence was found to support hypothesis 5. Table 2 and Figures 3 and 4 illustrate the findings. Biology 220M Page 4 of 9

5 Table 1 - Abiotic Site Variables Data Note - Distribution of samples was randomized by distributing the 8 samples outside of the midpoint at 45 degree intervals around the midpoint, and by alternating the distance from the midpoint between 7.0 and 3.5 meters. Samples were also taken from each midpoint. Declination was not taken into consideration in the compass bearings. Samples Distance from Midpoint (meters) Light Exposure (candles) A-Horizon Depth (centimeters) Soil Temperature (Fahrenheit) Site A Bearing (degrees) Soil ph Midpoint A Mean Variance Standard Deviation Standard Error Times T Final 95% Confidence Interval 14.2 ± ± ± 0.5 Distance from Midpoint (meters) Light Exposure (candles) A-Horizon Depth (centimeters) Soil Temperature (Fahrenheit) Bearing (degrees) Soil ph Midpoint B Mean Variance Standard Deviation Standard Error Times T Final 95% Confidence Interval 12.4 ± ± ± 0.3 Biology 220M Page 5 of 9

6 Site A Table 2 - Pin Samples of Leaves on Forest Floor PSNK Nature Trail Field Project Hone, Christopher M. White Oak Northern Red Oak Shagbark Hickory American Beech Red Maple Tulip Poplar Spicebush White Ash Seeds American Elm Other (mostly Spicebush and Sweet Birch) Total Numbers Percent of Whole 20.0% 20.0% 3.3% 10.0% 6.7% 6.7% 3.3% 0.0% 6.7% 23.3% Samples Total numbers Percent of Whole 14.8% 21.3% 0.0% 14.8% 8.2% 8.2% 4.9% 1.6% 4.9% 21.3% Biology 220M Page 6 of 9

7 Site A White Oak Northern Red Oak Shagbark Hickory American Beech Red Maple Tulip Poplar Spicebush White Ash Seeds American Elm Other (mostly Spicebush and Sweet Birch) White Oak Northern Red Oak Shagbark Hickory American Beech Red Maple Tulip Poplar Spicebush White Ash Seeds American Elm Other (mostly Spicebush and Sweet Birch) Figures 3 and 4 - A Comparison of the Pin Sample Data from the Sites Biology 220M Page 7 of 9

8 Site Comparison Total Site Area (square meters) Total Specimens Total Species present (Richness) Average dbh (meters) Percent Healthy Average Number of Stems Total Area occupied by Woody Plants (square meters) Percentage of Forest Area Wooded Site A % % % % Neither site richness nor observed specimen health nor the number of specimens present differed significantly between the sites. The main difference between the sites appeared to be a difference in the size and stem habits of the woody plants, rather than in species or number. This can be seen in the average number of stems per woody plant; Site A has slightly more than twice as many stems per plant as. In spite of the difference in average dbh, the total area occupied by woody plants at Table 3 A comparison of the data obtained from each site. breast height in Site A was 1.35 square meters, as opposed to 0.44 in. This is because of the large portion of specimens that were not breast height. The percentage of the site occupied by these woody plants also showed a difference, from 0.83% in Site A to 0.27% in. It might be justifiable to state that Site A provides a slightly more favorable habitat to the vegetation present, because of the slightly higher values in richness, health, and area occupied by the main stems at breast height. Spicebush Importance Comparison Total number of Woody Plants Total number of Spicebush Spicebush Percentage of Forest Woody Plants Average Number of Stems per Spicebush Average dbh of those Breast Height (meters) Percent Not Breast Height Percentage Healthy Area covered by Woody Plants (m 2 ) Site A % 91.11% % 95.65% 0.44 A comparison of the importance of Spicebush in the two sites provides a slightly more informative picture. The total population of the Spicebush in each site was almost identical although Spicebush comprised a slightly higher percentage of the forest in Site A. A significant difference can be seen in the average number of stems per specimen, which was 6.48 in Site A and 2.55 in. About 31% of Area covered by Spicebush (m 2 ) those in Site A could be considered below breast Table 4 A comparison of the site data specific to Lindera benzoin height for measuring purposes, while 87% of those in were in this category. Although this is a measurement in which a high level of precision was not possible, this is still a staggering difference, probably accounting for most of the observed disparity in the sites. These findings suggest that the real difference between the sites is that the Spicebush of Site A are larger specimens with more stems than those in. Those Biology 220M Page 8 of 9

9 in site B are still numerous but small, with few stems per plant and few specimens above breast height. It seems logical to infer that such a large size differential is indicative of an age difference, and that the specimens in Site A constitute a mature, successful population while those in are relatively new. Although a definitive answer may not be possible until further research has been performed, this data would appear to support hypothesis #9; that both sites are in a state of ecological succession in which Spicebush comes to dominate the understory, and Site A is further along than. No direct sightings of the Whitetail Deer occurred during these observations. I found some evidence of their presence, including two samples of scat in Site A and one buck rub in, but I did not find any evidence to support the idea that they are more active in one site than in the other. Discussion This experiment found no evidence to suggest that the difference in the understory populations of the two sites was due to Soil temperature, ph, A-horizon depth, light exposure, or canopy composition. No evidence supported the hypothesis that deer browsing influences one site more than the other. The fact that the difference between the Spicebush in each site is based on specimen size rather than number suggests that the best explanation for this disparity is a differing rate or origin of ecological succession in the two sites. is Site A in its infancy. We can speculate on possible causes for this succession stage difference. Some possibilities include lingering allelopathic influence from the Sassafras specimens in, a higher amount of realized sunlight during the summer in Site A due to a hole in the canopy caused by a windthrown Oak, or the presence of some predator, such as the Spicebush Swallotail (Papilo troilus), which is active but hard to observe in the early spring. Although canopy regeneration appears to be occurring in, almost all the seedling specimens are less than 30 centimeters tall. This would seem to indicate that either the seedlings too have only recently been able to begin growth, or something is preventing them from living past a certain age or size. In Site A, the smallest saplings appear to be as big as the Spicebush themselves, and are relatively few. Both sites appear to participate in a much larger trend that can be observed throughout the surrounding area. In certain sections of the trail, limited Tulip Poplar (Liriodendron tulipifera) regeneration can be seen, and American Beech (Fagus grandifolia) has formed thickets of saplings in some areas, but for the most part the understory of the surrounding forest does not hold any significant number of canopy seedlings. Most likely a variety of factors are to blame, but based on my observations I would suggest that in areas where Spicebush is dominant canopy regeneration is effectively blocked at germination, and that in other areas it is allowed to begin but cannot continue due to the uniform influence of deer browsing. Further speculation before the next phase of research would be unsupported and probably counterproductive. If definitive results can be realized from this study, the potential applications for these results are enormous. Few things are more important to understand about PA forests than the stresses present on the canopy trees, especially as we head into an era that will probably characterized by ecological destabilization (Irland). I thought, and still think, that this experiment may prove a microcosm in which the changing species composition of Pennsylvania forests may be seen. Let us hope it will be utilized according to its potential. References Czarapata, Elizabeth J. Invasive Plants of the Upper Midwest: An Illustrated Guide to Their Identification and Control. Madison: U. of Wisconsin, Gant, Robert Edward. The Allelopathic Influences of Sassafras Albidum in Old Field Succession In Tennessee. MS thesis. U. of Tennessee, Irland, Lloyd C. The Northeast's Changing Forest. Petersham, Massachusetts: Havard U., Biology 220M Page 9 of 9