2011 SOUTHEASTERN NATURALIST 10(1):25–38 Effects of Power-line Maintenance on Forest Structure in a Fragmented Urban Forest, Raleigh, NC Amanda S. Powell1 and Erin S. Lindquist2, * Abstract - With the increase in urban development, forest fragments are becoming more prevalent. In urban areas, there is a tendency to hide power-lines within or on the edges of these fragmented forests; however, it is unknown how the maintenance of vegetation under and along power-lines impacts the forest composition and structure of an adjacent fragmented, urban forest. An urban, fragmented maple-oak-hickory forest is located on the Meredith College campus, Raleigh, NC. A 1-ha plot with a hundred 10- x 10-m subplots was established in 2007 to initiate a long-term project supporting undergraduate research. An adjacent meadow is cut and maintained regularly up to the forest and plot edge for power-line clearance and access. We identified, tagged, and measured all of the trees with a diameter at breast height (DBH) ≥ 5 cm in this permanent plot, and compared the tree species richness (S), Shannon-Weiner diversity index (H), Sorenson’s similarity index (Ss), DBH, stem density, and basal area along the 100-m gradient from the forest edge. We also used a non-metric multidimensional scaling (NMS) analysis to describe how species composition changed along the gradient. Our findings showed that S, H, and Ss did not change along the 100-m gradient. The NMS confirmed that species composition was not different in the edge subplots (0–10 m from edge) compared to all other subplots and therefore was not impacted by continual, local disturbance along forest edges. However, we found that forest structure changed along the gradient with the exception of mean DBH; stem density and total basal area varied along the 100-m gradient. There was greater stem density along the edge of the forest (0–5 m and 10–20 m from edge) compared to the other interior subplots. Some of the interior subplots (10–20 m and 60–70 m from the edge) had a higher total basal area than the remaining plots. As expected, we also found that there was a negative linear relationship between DBH and stem density for all subplots. Our results confirm trends found in previous studies that community structure parameters (stand density and basal area) differ between forest edges and their respective forest interiors, but did not agree with previous research, which found species composition to be affected by edges. We believe the regular pruning of the forest edge adjacent to the power-lines explains our observed differences in forest structure, but tree species richness, diversity, similarity, and composition may be determined by the disturbance of larger-scale ecological processes. Our results show how power-line placement within a fragmented urban forest can affect the structure of the adjacent forest, and we recommend that the ecological effects of power-line corridors should be further investigated and incorporated into the larger body of literature on forest fragmentation. Introduction Fragmented edges are becoming more widespread in urban areas. In the United States, about 44% of trees in forests are located 90 m from the edge of the forest (McDonald and Urban 2004, Riitters et al. 2002). As forests become 1Department of Zoology, University of Wisconsin - Madison, Madison, WI 53706.2Department of Biological Sciences, Meredith College, 3800 Hillsborough Street, Raleigh, NC 27607-5298. *Corresponding author - erinlind@meredith.edu. 26 Southeastern Naturalist Vol. 10, No. 1 more fragmented and irregular in shape, edge habitat continues to increase and forest structure and composition are affected (Ries et al. 2004, Williams-Linera 1990). Forests become fragmented as power-line corridors, roads, gas lines, and other infrastructure are established (Benítez-López et al. 2010, Laurance et al. 2009, Luken et al.1991a). Although a significant number of studies have investigated how the creation of forest edges impact ecosystem function (Harper et al. 2005) and biodiversity (Fahrig 2003), there is a need for additional studies to elucidate how local factors such as climate, edge type, and forest type affect the changes in forest composition and structure (Harper et al. 2005). For example, Luken et al. (1991a) found that mature tree stem densities were higher along forest edges adjacent to pastures and clear-cuts compared to power-line corridors, but edges adjacent to power-line corridors had a higher density and basal area for saplings and seedlings. Forest habitat that is cleared for roads and electric or communication lines can significantly increase the exposure to wind and/or other abiotic factors along forest fragment edges (Pohlman et al. 2009). For aesthetic purposes, there may be an attempt to hide the power-lines in urban forests and this may contribute to forest fragmentation and the development of the edge (Luken et al.1991a). Along forest edges where power-lines are located, the vegetation is maintained to keep the area under the lines clear for maintenance purposes and keep the vegetation out of the lines (Johnstone 1990, Luken et al. 1991a). Tree stems and branches along the forest edge are regularly pruned or removed and the adjacent herbaceous vegetation is mowed or removed by herbicidal treatments. Where the vegetation is mowed along forest edges, the root system of trees is often not killed (Johnstone 1990). Thus, mowing allows for multiple shoots to grow from the base (Johnstone 1990), and may lead to higher stem densities along the edge compared to the interior areas ( ≥10 m) of the forest (Luken et al. 1991b). Previous research indicates that forest edges located along power-line corridors have a higher stem density up to 10–15 m into the forests (Luken et al. 1991a). However, Luken et al. (1991a, 1991b, 1992) only investigated forest structure and composition 25 m into the forest from the power-line corridor, and therefore it is not known if edge effects are found further into the forest. In addition, research conducted by Luken et al. (1991a, 1991b, 1992) was done in a rural landscape with forest remnants. It is unknown how fragmented urban forests respond to power-line corridors. Our research was conducted in a southeastern deciduous forest to investigate how the composition and structure of a fragmented urban forest is affected by regular maintenance of a forest edge for power-lines. According to one study conducted in the Piedmont of North Carolina (McDonald and Urban 2006), which is in close proximity to our study site but not as urban as our study area, edge effects were found to affect the tree species composition up to 5 m from the forest edge. McDonald and Urban (2006) did not clarify how their study’s forest edges were created, but we assume that the majority were not along power-line corridors. At our study site, we predicted that tree species richness would be greater along the edge and that the composition of edge plots would be dissimilar when compared 2011 A.S. Powell and E.S. Lindquist 27 to the interior subplots. Furthermore, we expected to find lower diameter-atbreast height (DBH), higher stem density, and lower basal area along the edge when compared to the interior. Field-site Description We conducted this study in a fragmented urban forest on the Meredith College campus, Raleigh, in the piedmont of North Carolina (35°48'22.05"N, 78°41'31.39"W) (Fig. 1). Meredith College purchased the study area, which is approximately 22.26 ha in size, in the mid-1960s. Prior to the Meredith College acquisition, the study area was an alfalfa field when it was part of the Tucker Farm (Johnson 1972, Stenbuck and Waller 2002). The forest extends beyond the Meredith College property; its east–west axis varies in length from 150 to 400 m and its north–south axis ranges from 300 to 550 m (Fig.1). Figure 1. Aerial photo of the study area in Raleigh, NC. The 1-ha plot is located in a fragmented forest on the Meredith College property (white square; A). Power-lines (white line; B) are located along the western edge of the research plot (circles represent plot corners; B). 28 Southeastern Naturalist Vol. 10, No. 1 In the fragmented forest we set up a one-hectare (100- x 100-m) permanent plot on the north–south and east–west axes (Fig. 2). Power-lines and maintained grassland were located on the western boundary of the study plot which defined the forest edge in the study. The power-line right-of-way has a width of 30 m. Within the study plot, there were ninety 10- x 10-m subplots marked off with flags from 10 to 100 m from the western boundary or forest edge, and twenty 10- x 5-m subplots within the first 10 m of this edge. The edge subplots were located within the first 5 m from the western boundary of the plot, and the interior subplots were 5–100 m away from the western plot boundary. The southern boundary of the forest plot was located approximately 20 m from another forest edge. The slope and aspect of the plot were collected every 40 m throughout the plot using a clinometer and compass, respectively. Slope had a minimum of 1.5°, maximum of 22.5°, and a mean of 8.2° (n = 12). For aspect, the minimum was 218°, the maximum was 327°, and the mean was 270° (n = 12). Power-lines located along the western side of the forest were constructed in the early 1970s. Progress Energy maintained the vegetation area under the Figure 2. The grid represents the study plot located on Meredith College’s campus, Raleigh, NC; line on the western edge represents the power-line. The distance away from the power-line in meters is located along the northern side of the plot. 2011 A.S. Powell and E.S. Lindquist 29 power-lines by annual mowing until 2007 and applied herbicide (mixture of Accord ®, Arsenal Powerline®, and Milestone®) every three years (Henry Dickens, pers. comm.). They also pruned tree branches extending into the corridor every four to five years. During the study, the meadow under the lines was mowed and the edge cut on 7 November 2007. Methods We permanently tagged all living trees that had at least one stem with a diameter at breast height (DBH) ≥ 5 cm in the 1-ha plot with round aluminum tags on the north side of the trees. DBH was measured at 1.3 m for all stems of the tree. We collected leaf samples from all the tree species on the plot to create voucher specimens for the Meredith College herbarium, and indentified all individuals (E.C. Swab, environmental consultant, Raleigh, NC, pers. comm.; Swanson 1994). The field research and data collection was conducted August 2007–May 2008. To determine how species composition changed along the 100-m gradient from the edge of the forest adjacent to the power-line corridor to the interior of the forest (Fig. 2), we calculated tree species richness (S) for the twenty 5- x 10-m subplots (0–5 and 5–10 m from the forest edge) and the remaining ninety 10- x 10-m subplots (10–100 m from the forest edge), and the Shannon- Weiner diversity index (H) for each of the 10- x 10-m subplots, including the 0- to 10-m subplots. To assess how similar the edge subplots (0–10 m from edge) were to all other subplots in species composition, we calculated the Sorenson’s (Ss) similarity index for each of the 0–10 m plots contrasted with each of the interior subplots. We spatially represented the variation in the forest community composition with a non-metric multidimensional scaling (NMS) ordination analysis using PC-ORD software (McCune and Grace 2002). For the NMS ordination we used a matrix representing the species abundance of each 10- x 10-m subplot. Before the analyses, we used Beal’s Smoothing to transform the heterogeneous compositional data due to the high frequency of zeros (McCune and Grace 2002). For the analysis, we used the Sorenson distance measure recommended by McCune and Grace (2002) for NMS analysis of community data. A two-axis representation of the species composition in subplot space was found to minimize the stress of the model. We also assessed how our forest structure parameters varied relative to the 100-m gradient in the study plot by calculating the mean DBH (diameter-at-breast height, cm), stem density (ha-1), and total basal area (m2 ha-1) for the twenty 5- x 10-m subplots (0–5 and 5–10 m from the forest edge) and the remaining ninety 10- x 10-m subplots (10–100 m from the forest edge). Prior to testing how the species composition and forest structure parameters differed among the different subplots grouped by 5-m (0–5 m and 5–10 m for the forest structure parameters only) or 10-m intervals, we tested each parameter for normality using the Shapiro-Wilk test (JMP 8.0). All variables, with the exception of the Shannon-Weiner (H) diversity index, did not fit a normal distribution. 30 Southeastern Naturalist Vol. 10, No. 1 We used the Kruskal-Wallis (JMP 8.0) for all non-normal parameters, and an analysis of variance (ANOVA; JMP 8.0) for the Shannon-Weiner (H) index. Tukey-Kramer post-hoc tests (JMP 8.0) were calculated to obtain pair-wise comparisons for any parameter that showed differences in the mean values across the distance classes. To determine if mean DBH was related to stem density as we predicted, we used a linear regression (JMP 8.0). In all statistical tests, the null hypothesis was rejected if the P-value was ≤ 0.05. Results We tagged, identified, and measured 818 trees of 20 species (Table 1). The largest tree on the plot was a Quercus alba L. (White Oak), which had a DBH of 86.4 cm. Over 63% of the plot was made up of 3 species: Acer rubrum L. (Red Maple; 30.6% of stems), Oxydendrum arboreum (L.) DC. (Sourwood; 22.5% of stems), and White Oak (10.5% of stems). There were three species (Cornus florida L. [Flowering Dogwood], Juniperus virginiana L. [Eastern Redcedar], and Sassafras albidum (Nutt.) Nees [Sassafras]) that only occurred once in the plot. The mean DBH for each species ranged from 8.9 ± 3.4 cm (Nyssa sylvatica Marsh. [Blackgum]) to 34.8 ± 6.8 cm (Pinus echinata P. Mill. [Shortleaf Pine]). Table 1. Species composition and diameter at breast height (DBH) of a fragmented urban forest on the Meredith College campus, Raleigh, NC (35°48'22.05"N, 78°41'31.39"W). Over 63% of the plot is made up of 3 species: Red Maple, Sourwood, and White Oak. For each species, the sample size (n), minimum (min.), maximum (max.), mean, and standard deviation (SD) for the DBH are included. % of Stems DBH (cm) Common name Scientific name (n = 818) n Min. Max. Mean SD Red Maple Acer rubrum 30.6 250 2.9 50.4 11.4 8.0 Sourwood Oxydendrum arboreum 22.5 184 3.3 30.0 9.1 3.9 White Oak Quercus alba 10.5 86 4.1 86.4 33.9 16.0 Blackgum Nyssa sylvatica 7.5 61 3.7 19.4 8.9 3.4 Mockernut Hickory Carya tomentosa Nutt. 4.5 37 3.9 32.9 15.1 8.4 Sweetgum Liquidambar styraciflua L. 4.5 37 5.5 31.9 11.0 5.8 Scarlet Oak Quercus coccinea Muenchh. 3.9 32 4.3 55.9 20.5 13.9 Tuliptree Liriodendron tulipifera L. 3.4 28 4.3 59.9 20.8 14.1 American Beech Fagus grandifolia Ehrh. 2.7 22 3.9 44.7 13.0 9.7 Black Oak Quercus velutina Lam. 2.1 17 4.2 47.0 23.0 12.8 Shortleaf Pine Pinus echinata 2.1 17 17.0 43.6 34.8 6.8 Red Oak Quercus rubra L. 1.3 11 8.5 51.7 27.0 15.8 Loblolly Pine Pinus taeda L. 1.2 10 5.6 51.5 32.0 14.9 Southern Red Oak Quercus falcata Michx. 1.1 9 13.5 54.1 31.5 13.6 Pignut Hickory Carya glabra (Mill.) Sweet 0.9 7 13.5 34.2 22.5 8.3 Black Cherry Prunus serotina Ehrh. 0.5 4 5.6 15.5 10.0 4.5 Shagbark Hickory Carya ovata (Mill.) K. Koch 0.4 3 5.7 31.9 16.3 13.8 Flowering Dogwood Cornus florida 0.1 1 5.7 5.7 5.7 - Eastern Redcedar Juniperus virginiana 0.1 1 9.8 9.8 9.8 - Sassafras Sassafras albidum 0.1 1 11.1 11.1 11.1 - 2011 A.S. Powell and E.S. Lindquist 31 Figure 3. A. Tree species richness (S) (species/subplot) compared to the distance (m) from the forest edge of the 1-ha plot. B. Shannon-Weiner (H) compared to the distance (m) from the forest edge on the plot. C. Sorenson (Ss) index for the 0–10-m subplots (n = 10) contrasted with all remaining subplots (n = 99). The Ss was averaged for all subplots within any distance (m) from the forest edge category (n = 9 for 0–10 m, and n =10 for all other distances). For (A), (B), and (C): error bars = ± 1 standard deviation (stdev) (n = 10), and distance treatments which share the same letter are not significantly different from each other. (P ≤ 0.05). 32 Southeastern Naturalist Vol. 10, No. 1 We found that there was no difference in tree species richness (S) along the 100-m gradient from the forest edge next to the power-line corridor into the interior of the forest (χ2 = 13.54, P = 0.195; Fig. 3A). Tree species richness ranged from 3.6 to 5.8 species per subplot. We found that the mean species diversity (H) of each of the subplot was 1.28 ± 0.39, and the diversity index did not vary along the distance gradient from the forest edge (F = 1.425, P = 0.199; Fig. 3B). The calculated Ss between the edge plots (0–10 m) and all other subplots varied from 0.28 to 0.34, and we found that when we contrasted the Ss of the first 10 m from the edge (0–10 m) to each of the other distance classes, the Ss did not differ significantly (Fig. 3C, χ2 = 11.87, P = 0.221). The NMS ordination also showed that the species composition of the edge subplots (0–10 m) was not different than all other subplots (Fig. 4). Although the majority of the edge subplots clustered together in the NMS ordination space, and therefore were similar to each other in species composition, they also were similar in species composition to a large number of interior subplots (Fig. 4). There was no difference in mean DBH across the distances (χ2 = 13.76, P = 0.184; Fig. 5A). We found that there was a significant difference in stem density (ha-1) across the distances (χ2 = 26.77, P = 0.003; Fig. 5B). The 0–5-m (from the forest edge) subplots had a higher stem density (0.144 ± 0.0532 stems ha-1) than all the subplots located 20–100 m from the forest edge (0.066 to 0.104 stems ha-1), while the 5–20-m subplots had the same stem densities as the 0–5-m and 20–100-m subplots (Fig. 5B). We found that the total basal area for the subplots within 5 m (0.067 ± 416 m2 ha-1) of the forest edge was smaller than that of the 10–20-m (0.373 ± 1805 m2 ha-1) and 60–70-m (0.381 ± 1253 m2 ha-1) subplots Figure 4. Non-metric multidimensional scaling (NMS) ordination of species composition of the one hundred 10- x 10-m subplots. Subplots were grouped into edge (0–10 m from edge; gray squares) and interior (10–90 m from edge; open circles) categories. Subplots that are close together in Axes 1 and 2 are similar to each other in species composition. 2011 A.S. Powell and E.S. Lindquist 33 (χ2 = 25.42, P = 0.0046; Fig. 5C). There was a large amount of variability in the total basal area among the subplots at any given distance (Fig. 5C). Figure 5. A. Mean DBH (diameter-at-breast height; cm) compared to the distance (m) from the forest edge of the 1-ha plot. B. Mean stem density (trees/ha) compared to the distance (m) from the forest edge of the plot. C. Total tree basal area (m2 ha-1) compared to the distance (m) from the forest edge of the plot. For (A), (B), and (C): error bars = ± 1 standard deviation (stdev) (n = 10), and distance treatments which share the same letter are not significantly different from each other. (P ≤ 0.05). 34 Southeastern Naturalist Vol. 10, No. 1 We found that as the stem density increased in the subplots, the mean DBH did not decrease significantly, but a trend was apparent (n =110, r2 = 0.034, P = 0.053; Fig. 6). To investigate this trend further, we made a second regression model where we excluded a subplot with a mean DBH (38.7 cm, 90–100 m; Fig. 6) larger than two standard deviations from the mean DBH of all subplots. With this outlier removed, as stem density increased in the subplots, the mean DBH decreased (n =109, r2 = 0.036, P = 0.038). Discussion Previous literature has shown that forests generally have higher species richness and stem densities along their disturbed edges (Fraver 1994, Matlack 1994). The extent to which these edge effects penetrate the forest interior differed from site to site. In an assortment of tropical and temperate zone forests, there was a higher stem density and basal area within 20 m of the forest edge (Murcia 1995). Other results have shown edge effects to extend 15 m to 5 km, depending on habitat and taxa (Laurance and Yensen 1991). In order to generalize beyond one study site, we argue that it is critical to focus on the type of edge when determining how the forest composition and structure is affected by fragmentation. Our results show that the placement of power-lines along a fragmented urban forest affects the forest structure but not species composition. Furthermore, we found that only the first 5 m of forest differs in structure when compared to the forest interior (up to 100 m from edge). Our results are contrary to some previous studies investigating other types of edges, which found species compositional changes along forest edges (McDonald and Urban 2006), but in support of Luken et al.’s (1991a, 1991b) findings investigating forests adjacent to power-lines in a rural landscape. Luken et al. (1991a, 199b) found that there was no difference in tree species richness and diversity (H) between the forest edge (<10 m from edge) and interior. The continued mowing or removal by Figure 6. Linear regression of the mean DBH (diameter-at-breast height; cm) and the stem density (trees/m2) of each subplot (n = 100) within the 1-ha plot. (r2 = 0.037, P = 0.045). 2011 A.S. Powell and E.S. Lindquist 35 herbicides of vegetation below the power-lines may explain why there was no difference in species richness relative to the edge in both study sites. Likewise, species richness has been found to increase with the age of the fragmented patch (Jacquemyn et al. 2001). In our study, we found that the mean diameterat- breast height (DBH) did not differ in relation to the distance from the forest edge. We believe that as the interior of the forest continues to mature, the mean DBH will be lower along the edge compared to the interior because the edge trees will not be able to increase significantly in size due to the continued maintenance of the power-line corridor. Based on our stem density and basal area findings, edge effects on forest structure clearly extend 5 m into the forest and may extend up to 20 m away from the forest edge. We found that stem density was higher in plots along the forest edge (0–5 m from edge) than in plots in the forest interior (≥20 m from edge). Because the edge of our study plot was routinely disturbed, smaller stems were more abundant along the edge. In addition, smaller stems were able to grow along the edge better because the immature, open canopy allows more sunlight to reach the ground. Our results are in agreement with Luken et al. (1991b); they found that routine cutting along the edge of the forest leads to higher stem densities. Forest edges adjacent to power-line corridors may have a higher stem density when compared to forest edges adjacent to fields and clear-cuts because of the continual clearing and trimming of the edge vegetation. Furthermore, forests along power-line corridors may be affected further into the forests due to the continual human disturbance. In contrast to the higher stem density along the forest edge, we found that total basal area was lower in plots along the forest edge (0–10 m from edge) than some of the more interior plots (10–20 m and 60–70 m from edge). Luken et al. (1991a) found that the edge had a higher basal area for saplings and seedlings (DBH < 10 cm) but not for larger trees (DBH ≥10 cm). We believe our results may be due to a lower mean DBH (7.48 ± 2.01 cm) in the edge plots caused by the continual maintenance of the forest edge next to the power-lines. Now that the mowing treatments have stopped, and the vegetation under the power-lines will be maintained by herbicidal treatment allowing woody vegetation to grow, we predict that the basal area of the forest edge will increase as the trees grow larger. In addition to edge type, the differences in the extent of edge effects may be related to forest type, age (Jacquemyn et al. 2001), area and shape (Jacquemyn et al. 2001, Laurance and Yensen 1991), or orientation (Matlack 1993, Sherich et al. 2007). The smaller and more irregular in shape the fragment is, the stronger the edge effect may be (Laurance and Yensen 1991). Given that our forest fragment is small in size (>22 ha) relative to those used in Laurance and Yensen’s (1991) models (0–500 ha), fairly regular in shape (Fig. 1), approximately 40 years old, and has a western-facing edge and a north-facing slope, we expect that the edge effects due to these factors were not as strong as documented in these other studies (Jacquemyn et al. 2001, Laurance and Yensen 1991, Matlack 1993, Sherich et 36 Southeastern Naturalist Vol. 10, No. 1 al. 2007). Because our forest edge is disturbed on a regular basis for power-line maintenance, we believe that the changes in abiotic and biotic conditions due to this regular disturbance override any variations in edge effects due to age, area, shape or orientation of the forest. Based on our findings in an urban, fragmented forest in the piedmont of North Carolina, we conclude that forest structure (stem density and total basal area) was altered at least 5 m from a routinely disturbed forest edge due to power-line right-of-way maintenance. However, we found no changes in mean tree DBH, tree species richness, or diversity relative to distance from the forest edge. Further research at our study site should be conducted to determine if the routine maintenance of the forest edge by herbicidal treatments continues to affect the structure of the forest as it ages. Furthermore, we encourage additional studies investigating how power-lines affect forest composition and structure because our results highlight how their maintenance causes the edge effects to differ from those found adjacent to other types of infrastructure or causes of forest fragmentation. Beyond the documented effects on forest structure and composition, we recommend that further research is conducted under power-lines to investigate how the managed corridors affect the spread of invasive species, alteration of small mammal habitat, and the dynamics of herbaceous vegetative communities. Our findings confirm that power-line right-of-way maintenance can magnify the ecological effects of forest fragmentation in urban settings. Based on our research we recommend that, if possible, power companies minimize the placement of new power-lines in intact forest habitats. This would be beneficial to power companies given that direct contact between power-lines and trees or tree branches can cause power outages (Appleton 2006). In addition, managing vegetation along utility lines costs between two to ten billion dollars in North America (Appleton 2006), with much of this expense due to pruning tree branches. Therefore, in order to minimize the expense of managing woody vegetation, we recommend that when possible, power-lines are routed through disturbed grasslands or other low-lying vegetated areas where tree cutting and pruning is avoided. Acknowledgments We would like to thank the Meredith College Undergraduate Research Committee for funding and supporting us throughout the project. The publication of this paper was supported by a grant to Meredith College from the Margaret A. Cargill Foundation. We would like to thank B. Powell, J. Powell, and L. Racz for their field assistance, and E.C. Swab for his assistance with tree identification. Literature Cited Appleton, B.L. 2006. Designing and implementing utility-line arboreta. Arboriculture and Urban Forestry 32:80–85. Benítez-López, A., R. Alkemade, and P.A. Verweij. 2010. The impacts of roads and other infrastructure on mammal and bird populations: A meta-analysis. Biological Conservation 143:1307–1316. 2011 A.S. Powell and E.S. 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