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The Influence of Nearest Seed Neighbors on Seed Removal in Deciduous Forests
Jaclyn L. Schnurr, Richard S. Ostfeld, and Charles D. Canham

Northeastern Naturalist, Volume 19, Issue 1 (2012): 43–48

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2012 NORTHEASTERN NATURALIST 19(1):43–48 The Influence of Nearest Seed Neighbors on Seed Removal in Deciduous Forests Jaclyn L. Schnurr*,1, Richard S. Ostfeld2, and Charles D. Canham2 Abstract - In the temperate deciduous forests of the northeastern United States, the majority of the dominant tree species disperse their seeds during the fall, causing a heterogeneous mixture of seeds to be present at a specific location at one time. Because these seeds vary in size and palatability to small mammals, some seed species, such as Quercus acorns, may alter the risk of removal of neighboring, less preferred species. The presence of a neighboring seed could attract seed predators, elevating the risk that a neighboring seed will be removed (apparent competition), or it could divert the attention of seed predators away from a neighboring seed (apparent mutualism). We evaluated the effects of nearest seed neighbors on the survival of 5 different tree species. Using logistic regression we determined whether the species or abundance of nearest neighbors caused changes in the risk of removal. Contrary to our expectations, we did not find any affect of neighboring seed species on the removal rates of other seeds, indicating that risk of removal of naturally occurring seeds, separated by at least 10 cm, is independent. Introduction The temperate deciduous forests of the northeastern United States are comprised of a heterogeneous mixture of canopy tree species, many of which disperse seeds during the fall of each year. This diversity causes spatial and temporal variation in the abundance and distribution of seed types present at a particular location on the forest floor (Ribbens et al. 1994, Schnurr et al. 2002). From the perspective of the tree species, it may be a favorable strategy to disperse seeds when others are as well, to potentially satiate any seed predators in the area (Koenig et al. 1994, Lalonde and Roitberg 1992, Shibata et al. 1998, Silvertown 1980). However, tree species produce seeds that are not nutritionally equivalent to the suite of seed predators found in these forests (J.L. Schnurr, unpubl. data). Therefore, seed predators may have preferences for certain seed types over others, which may increase, or decrease, the risk of removal of neighboring seeds. When survival of a specific seed type is influenced by the identity of its neighbors and mediated by the preference of its predator, then short-term apparent competition or apparent mutualism may be occurring (Holt and Kotler 1987; Veech 2000, 2001). Apparent competition is an indirect effect that occurs when species-specific seed survival decreases due to the identity of the neighboring 1Wells College, 170 Main St., Aurora, NY 13026. 2Cary Institute of Ecosystem Studies, 2801 Sharon Turnpike; PO Box AB, Millbrook, NY 12545. *Corresponding author - jschnurr@wells.edu. 44 Northeastern Naturalist Vol. 19, No. 1 seed, while apparent mutualism occurs when species-specific seed survival increases due to the other seed being preferred by the predator (Veech 2000). Veech (2000) found support for apparent competition among seeds of desert annuals, especially with those species that were preferred by their seed predators (see also Caccia et al. 2006). However, in another desert system, Garb et al. (2000) found support for short-term apparent competition that only occurred in patches with highly variable initial seed densities. Much research has been conducted on the effects of seed predation on survival of tree seeds in temperate deciduous forests (Schnurr et al. 2002, 2004), and many of these studies have indicated that acorns are a preferred food of the small mammals commonly found in these forests (Elkinton et al. 1996; McCracken et al. 1999; Ostfeld et. al 1996; Schnurr et al. 2002, 2004; Wolff 1996). Since seed rain is heterogeneous, seed species are mixed on the forest floor, such that the proximity of a fallen seed to an acorn may cause apparent competition or apparent mutualism to occur. Although a common method of measuring seed removal is to use a quadrat with seed species of varying identities found next to each other (e.g., Schnurr et al. 2002, 2004), there has been no research addressing the influences these seeds may have on each other. If apparent competition, or apparent mutualism, is occurring, measuring these processes would help predict seed survival and seedling recruitment in these communities. In this study, we asked: Does the identity of the nearest seed neighbor influence the chance of a particular seed being removed? We tested the hypothesis that proximity to one or more seeds of any species has an effect on the risk of removal of other seed species. We expected to find that only the proximity to Quercus rubra L. (Northern Red Oak) acorns would influence survival of the other seeds. Methods This study was conducted at the 2500-ha privately owned Great Mountain Forest (GMF), in Litchfield County, CT (41°57'N, 73°15'W). The major tree species there are Fagus grandifolia Ehrh. (American Beech), Tsuga canadensis (L.) Carr. (Eastern Hemlock), Acer saccharum Marsh. (Sugar Maple), Acer rubrum L. (Red Maple), Pinus strobus L. (White Pine), Red Oak, Prunus serotina Ehrh. (Black Cherry), and Fraxinus americana L. (White Ash), and the major small-mammal seed predators in this area are Peromyscus maniculatus (Wagner) (Deer Mouse), P. leucopus (Rafinesque) (White-footed Mouse), Myodes gapperi (Vigors) (Red-backed Vole), and Tamias striatus (L.) (Eastern Chipmunk). During the fall of 1994, we established fifty-two 1- x 1-m quadrats at 2 mixedcanopy sites that were separated by approximately 2 km. Within each quadrat, we randomly placed 1 seed (technically fruit propagules containing the seed) of 5 different species every 10 cm on a grid—which gave us varying sample sizes for the number and identities of neighbors for each seed. The species used were Red Oak (average seed weight 2.6 g), Black Cherry (36.4 mg dry wt), Sugar 2012 J.L. Schnurr, R.S. Ostfeld, and C.D. Canham 45 Maple (27.4 mg), White Ash (≈16 mg), and Red Maple (9.3 mg). The propagules were placed directly on the ground and marked with a wooden coffee stirrer. We returned after 1 week and recorded their fate. The numbers of neighbors within 10 cm of the focal seed and their identities were calculated in every quadrat (maximum possible n = 8). We used logistic regression to determine if seed status (live, dead) was influenced by the numbers and species of nearest neighbors. If a seed was missing it was considered “dead”. Figure 1. The influence of Red Oak acorns on the average survival of A) Red Maple seeds, B) Sugar Maple seeds, C) White Ash seeds, D) Black Cherry seeds, and E) Red Oak acorns. Error bars are one standard error. 46 Northeastern Naturalist Vol. 19, No. 1 Results Independent of the identity of the neighboring seed, Red Oak acorns were most likely to be removed, but there was no significant effect of proximity and density of acorns on the removal of any other seed species (Table 1, Fig. 1). Sugar Maple seeds were the only species that showed a significantly higher removal rate with conspecific neighbors (Table 1). There were no patterns of seed survival based on numbers and identities of neighboring species (Table 1), and seed removal rates were constant for each species, independent of the numbers and the identities of a seed’s neighbors (Fig. 1). Discussion Our results do not support our hypothesis: risk of seed removal was independent of the identity of neighboring seeds at a local scale. Red Oak acorns were consistently found and removed, supporting the general research consensus that Quercus is a preferred food of small-mammal seed predators in temperate forests (Elkinton et al. 1996; McCracken et al. 1999; Ostfeld et. al 1996; Schnurr et al. 2002, 2004; Wolff 1996). However, it appears that animals remove acorns as they discover them and do not concentrate their foraging in the areas where acorn density is higher. A confounding effect may occur due to the species of small mammals that are removing the seed. Although the major seed predators at GMF (White-footed Mice, Deer Mice, Red-backed Voles, and Eastern Chipmunks) are all considered generalists, it is possible that these different mammal species were differentially removing certain species of our seeds. Schnurr et al. (2002) found that Peromyscus species preferred Red Oak acorns, while Red-backed Voles appeared to prefer Red Maple samaras. If these species are foraging independently within our quadrats, it may appear that there is Table 1. Results of the logistic regression of seed survival (live, dead) of the focal seed species (Red Oak, Black Cherry, Sugar Maple, White Ash, or Red Maple) found with conspecific or interspecific neighbors. Significant P-value shown in bold. Focal With neighbors of seed species Red Oak Black Cherry Sugar Maple White Ash Red Maple Red Oak t = 0.174, t = 1.003, t = -1.318, t = -0.682, t = -0.35, P = 0.862 P = 0.316 P = 0.188 P = 0.495 P = 0.726 Black Cherry t = -1.246, t = 0.035, t = 0.211, t = -0.111, t = 1.648, P = 0.213 P = 0.726 P = 0.833 P = 0.912 P = 0.099 Sugar Maple t = -1.028, t = 0.826, t = 2.704, t = -0.567, t = -1.242, P = 0.304 P = 0.409 P = 0.007 P = 0.571 P = 0.214 White Ash t = 0.699, t = -1.661, t = 0.437, T = -0.763, t = 0.45, P = 0.484 P = 0.097 P = 0.662 P = 0.446 P = 0.653 Red Maple t = 0.626, t = 1.897, t = -1.726, t = -1.366, t = -1.326, P = 0.532 P = 0.058 P = 0.084 P = 0.172 P = 0.185 2012 J.L. Schnurr, R.S. Ostfeld, and C.D. Canham 47 no effect of neighboring seeds on focal seed removal, while the real effect may be caused by the mammal species removing the seeds: White-footed and Deer Mice may be preferentially removing acorns while ignoring maple seeds, while Red-backed Voles might be removing maple seeds. However, these differing foraging strategies do not affect the eventual survival and germination of seeds. Much current research is focused on understanding the importance of indirect effects in the maintenance of temperate forests (Ostfeld and Keesing 2000, Ostfeld et al. 1996). This study demonstrates that the patterns and processes associated with seedling recruitment can be understood through seed dispersal and survival alone since seed survival is independent of neighboring seeds, at least simplifying one aspect of community dynamics. Acknowledgments The authors wish to acknowledge the field help of Erika Latty and Sue Bookhout. The manuscript benefited by comments from Brian Hough and two anonymous reviewers. Literature Cited Caccia, F.D., E.J. Chaneton, and T. Kitzberger. 2006. Trophic and non-trophic pathways mediate apparent competition through post-dispersal seed predation in a Patagonian mixed forest. Oikos 113:469–480. Elkinton, J.S., W.M. Healey, J.P. Buonaccorsi, G.H. Boettner, A.M. Hazzard, H.R. Smith and A.M. Liebold. 1996. Interactions among Gypsy Moths, White-footed Mice, and acorns. Ecology 77:2332–2342. Garb, J., B.P. Kotler, and J.S. Brown. 2000. Foraging and community consequences of seed size for coexisting Negev Desert granivores. Oikos 88:291–300. Holt, R.D., and B.P. Kotler. 1987. Short-term apparent competition. American Naturalist 130:412–430. Koenig, W.D., R.L. Mumme, W.J. Carmen, and M.T. Stanback. 1994. Acorn production by oaks in central coastal California: Variation within and among years. Ecology 75:99–109. Lalonde, R.G., and B.D. Roitberg. 1992. 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