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Recruitment success for mast year cohorts of sugar maple (Acer saccharum) over three decades of heavy deer browsing.

Introduction

Masting, or mass seeding, is the cyclic and synchronized production of atypically large seed crops between intervals where relatively little or even no seeds are produced. Plant biologists have long been interested in whether these highly concentrated episodes of fecundity, especially common in trees, can be interpreted in terms of adaptive strategies (Norton and Kelly, 1988; Sork and Bramble, 1993; Masaka and Maguchi, 2001; Politi et al., 2009; Overgaard et al., 2007). According to the popular 'seed predator satiation' hypothesis, seed predators are normally maintained at relatively moderate abundances during nonmast years (because of moderate seed abundance). In a mast year, they are generally satiated (because of the overabundance of seeds), leaving some or even many seeds uneaten and available to become established as seedlings (Janzen, 1971; Silvertown, 1980; Lalondc and Roitberg, 1992; Kelly, 1994; Koenig and Knops, 2000; Jensen et al, 2012). In some tree species, seed predation is normally so intense that seed crops produced in mast years may provide virtually the only opportunity for significant seedling recruitment success (Sork and Boucher, 1977; Hutchins and Lanner, 1982; Sork, 1983).

Offspring recruitment in trees, however, will also require survival in the face of several other mortality risks through many subsequent years of sapling and juvenile life stages, including those imposed by drought (Moles and Westoby, 2004), low soil nutrient availability (Frey et al., 2007), crowding effects of near neighbours (Curtis, 1959; Hett, 1971; Hett and Loucks, 1971; Taylor and Aarssen, 1989; Eliason and Allen, 1997), light limitation from overhead canopy (Bonser and Aarssen, 1994; Duchesneau and Morin, 1999; Aarssen and Franq, 2004; Collet and Chanost, 2006), and herbivory (Boerner and Brinkman, 1996; Gomez-Aparicio et al., 2005). Several studies have reported significant impact of white tailed deer browsing (Odocoileus virginianus (Boddaert)) on tissue loss and mortality rates in woodland plants (Tilghman, 1989; Inouye et al, 1994; Rooney, 2001; Rooney and Waller, 2003; Russel et al., 2001; White, 2012; Habeck and Schultz, 2015), including tree seedlings and young saplings, and especially for the species of interest in our study: sugar maple (Acer saccharum) (Switzenberg et al., 1955; Stoeckeler et al., 1957; Tierson et al., 1966; Jacobs, 1969; Horsley et al., 2003; Long et al., 2007; Matonis et al., 2011, Bradshaw and Waller, 2016).

Populations of white tailed deer have increased steadily in northeastern North America from the mid to late 20th century, primarily because of better habitat conditions, fewer predators, and reduction of hunting presssure (Alverson et al., 1988; Porter, 1992; Porter et al, 1994; Rooney, 2001; Horsley et al, 2003; Cote el al, 2004). Deer-overabundance was the subject of a 1994 conference hosted by the Smithsonian Institution (McShea et al, 1997) and was the subject of an entire issue of Wildlife Society Bulletin in 1997 (Vol. 25, No. 2). In the region of the present study (southeastern Ontario, Canada), deer numbers increased rapidly in the 1980's (Fryxell et al, 1991), with overabundance sustained into the late 2000's (S. Smithers and M. Charette, Regional Management Biologists, Ontario Ministry of Natural Resources and Forestry; pers. comm.), with striking effects on survival of seedlings and young saplings of sugar maple (L. Aarssen, pers. obs.). Similar nearby increases in deer density in the 1980's were reported for a Southwestern Ontario study (ca. 500 km from the present study site; Tanentzap et al., 2011), in an Ohio study (ca. 735 km from the present study site; Boerner and Brinkman, 1996), and in a study from northern Wisconsin (ca.1000 km west of the present study site), with numbers peaking there in 2002, having doubled since 1983 (Bradshaw and Waller, 2016).

In the present study, we revisited a sugar maple population studied earlier in connection with a 1984 masting event (Taylor and Aarssen, 1989). We asked whether offspring recruitment for this 1984 mast year cohort was conspicuous (relative to recruitment from other cohort years) after 30 y of potential impact from mortality risks posed by intense herbivory associated with the overabundance of white tailed deer during this time period. In other words, while masting (according to the 'seed predator satiation' hypothesis) may increase the likelihood of seedling recruitment in the face of intense seed predation (from small mammals; Hsia and Francl, 2009), we asked: might it also increase the likelihood of recruitment into the sapling stage when faced with unusually intense herbivore pressure from deer (on top of all the other mortality risks)? A mast year for sugar maple also occurred in 2013 for this same study population, with seedlings emerging for this cohort in 2014. Consequently, by sampling this population in 2014, we had an opportunity to construct an age frequency distribution for seedlings and saplings that spanned between the 2013 mast year cohort and the survivors of older cohorts, including the known 1984 mast year cohort.

Materials and Methods

STUDY SPECIES

Acer saccharum Marsh, (sugar maple) is one of the most important native hardwood trees in eastern North America (Fowells, 1965). The species flowers in the spring and seeds fall in late September to early October, where they overwinter before germinating in the following spring, depending on adequate soil moisture and soil temperature (Hough, 1936; Hett and Louks, 1971). Sugar maple is extremely shade tolerant and individuals often survive for many years under die deep shade of overhead canopy trees (Fowells, 1965; Marks and Gardescu, 1998). Suppressed individuals as old as 15 y can be under 30 cm (all without ever having produced a single branch (Bonser and Aarssen, 1994). Masting is very common in sugar maple (Houle, 1999), with some reports showing intervals of between 2 and 5 y (Graignic et al., 2014; Rapp and Crone, 2015), but records of mast years are not well documented and even fewer studies have recorded seedling / sapling recruitment succcss of sugar maple from a mast year cohort (Boerner and Brinkman, 1996).

STUDY SITE

The study was conducted in 2014 within a 10 ha woodland bordered by Darling Farm Road and Opinicon Road, al Queen's University Biology Station (QUBS; 44[degrees]32'19.9"N, 76[degrees]22'03.2"W) in eastern Ontario. The woodland is known locally as "The Sugarbush" and was used for maple syrup production in the first half of the last century, but the site has been free of major human disturbances since 1976 when the property was acquired by Queen's University as part of QUBS (Raleigh Robertson, pers. comm.). The understory is deeply shaded in the summer by a mostly closed canopy of large Acer saccharum (sugar maple) trees, with less abundant canopy species including Fraxinus americana L. and Carya cordiformis (Wang.) K. Koch, and with Ostiya virginiana (Mill.) K. Koch, as the most abundant subcanopy tree. The same population was used in an earlier study of a 1984 mast year cohort of sugar maple seedlings (Taylor and Aarssen, 1989).

FIELD SAMPLING

Because most sugar maple seedlings become established beneath or near parent trees, 25 of the largest canopy ('target') trees within the population were selected for sampling. The shape of the study site is long and relatively narrow; therefore, target trees were selected toward the approximate center of the site, along the long axis, avoiding trees whose canopies were within 10 m of a road or hiking trail. For each target tree, the furthest extent of the canopy at the north, south, east, and west corners was located using a hand compass and visually projected onto the forest floor and marked with a flag. A four-sided polygon was created using a meter tape connecting these four corners, and then a 1.0 m by 0.5 m quadrat was placed randomly at 1, 2, or 3 m intervals along the outside of the perimeter. If quadrat positioning was prevented by the presence of a large sapling or tree, or a boulder, the quadrat was placed on the inside of the perimeter line.

Within each 1.0 m by 0.5 m quadrat, all sugar maple individuals [less than or equal to] 50 cm tall were aged by Counting annual terminal bud scale scars. In addition percent ground cover (by visual estimation) of other woody species ([less than or equal to] 50 cm tall) was recorded, as well as percent cover of sedge (Carex) species and total percent cover of other herbaceous species. A total of 346 quadrats were surveyed within the polygons of the selected trees between 14 July and 5 September, 2014.

A larger sample plot was used for aging larger sugar maple saplings. Beginning at the first sampled target tree at one end of the long axis of the site, a transect line was drawn from the base to the next closest target tree, and continuing to the next closest target tree, and the next in turn until all target trees were similarly connected by transect lines. Along these successive transect lines, 5 m by 10 m pic its were laid out using a meter tape with the corner of each plot placed at randomly chosen positions between 5 and 10 m along the line between pairs of target trees and with the 5 m side of the plot directly along the transect line. A coin toss decided which side of the line each plot would lay.

Within each 5 m by 10 m plot, all of the sugar maple saplings taller than 50 cm with a stem diameter of 6 cm or less (at 10 cm above ground), were harvested as stem sections cut at ground level and at 10 cm above the ground. The upper limit of 6 cm stem diameter was chosen so that the upper age limit would be about 70-80 y (based on preliminary sampling), therefore ensuring that the age distribution of samples would include an approximately equal range on either side of the 1984 mast year cohort (30 y old in 2014). A total of 35 of these larger plots were surveyed, with 149 saplings collected, labeled and brought back to the lab for aging. lab processing

LAB PROCESSING

Sapling stem sections were air dried for 10 wk and then sanded to a smooth surface using four successive grades of fine sand paper. The average stem diameter was recorded for the top of each stem section based on two measurements, one at the widest extent of the sample and another at the widest point perpendicular to the first measurement. Each sample was then aged by counting annual growth rings using a dissecting microscope. Using a sample with the center most core (pith) intact and clearly defined, an average measurement of one annual ring width = 0.1mm was obtained using a stage micrometer and applied to estimate central core ring numbers for a few larger samples that had deteriorated central piths. For samples with poorly defined annual rings throughout the face of the sanded side, growth rings were made more distinct by using a phloroglucinol solution, that stains the lignin of the wood red and leaves cellulose unstained. The samples were first soaked for 2 min in a solution of 1% phloroglucinol in 95% ethanol and then placed in a solution of 50% aqueous hydrochloric acid until the rings began to turn red (between 1 and 2 min). The samples were then rinsed with cold water and allowed to dry before examination under the microscope.

Annual rings for each sample were counted twice, once from the center to the outer bark and once from the outer bark to the center. If the two numbers corresponded, that number was recorded for the age of the sample. If the two numbers did not correspond, additional counts were made until two counts of the same age were obtained. The standard deviation for each sample requiring multiple counts was calculated and the average of these standard deviations (1.69 y) was used to represent a reasonable estimate of the measurement error for all samples. One sample could not be aged because of rotted wood throughout the majority of the stem section.

Results

Canopy 'target' trees ranged in size from 43.4 to 91.7 (mean: 58.2) cm in diameter at breast height. Beneath these 25 canopy trees, a total of 28,859 sugar maple individuals were aged; 28,711 were seedlings 50 cm in height or less and 148 were saplings greater than 50 cm in height but [less than or equal to] 6 cm in diameter at 10 cm above ground. Older saplings had generally larger stem diameter but with wide variability, typical of sugar maple and commonly associated with its strong shade tolerance (Bonser and Aarssen, 1994) (Fig. 1).

The overall age frequency distribution is roughly bimodal with zero to very few individuals recorded for ages 9 through 29 y (Fig. 2). The 1 y old seedlings produced from the 2013 mast year represent 99.3% of the total sugar maple individuals sampled (Fig. 2), with a mean density of 166 individuals per [m.sup.2] across all of the 1.0 by 0.5 m quadrats (n = 346). For each of the three recorded groups of neighboring species, the vast majority of the 1.0 by 0.5 m quadrats had 0% cover and 90% of the quadrats had less than 5% cover. As a result Type 1 linear regressions failed the normality test. Based on quasi-poisson regressions (Zeileis et al., 2008), 1 y old sugar maple seedling counts per quadrat showed no significant relationship with percent cover of sedge (Carex species; P = 0.221), or with percent cover of other woody species less than 50 cm tall (P = 0.907), but there was a significant negative relationship with percent cover of other (nonsedge) herbaceous species (P < 0.001).

The mean density of 2 y old seedlings (from the 2012 cohort) was only 0.26 individuals per [m.sup.2] (405 per 1750 [m.sup.2] Fig. 2) (compared with 166 per [m.sup.2] for the 1 y old mast year cohort from 2013). In other words, in their first year as seedlings, the 2013 mast year cohort outnumbered the 2 y old seedlings, the cohort from the year prior to the mast year (2012), by a 638-fold margin (Fig. 2).

Discussion

As a tree grows and passes through different life stages (seed, seedling, sapling, juvenile, reproductive adult) and undergoes associated changes in exposure to the physical environment (on the forest floor, in the understory, and in the open canopy), individuals encounter a shifting regime of mortality risks, and older trees of course have had more time to incur cumulative effects of these mortality risks. Accordingly, if age/stage specific mortality rates have been roughly constant over time, a static age frequency distribution is expected to approximate an 'inverse-J' shape for a strongly shade tolerant climax tree like sugar maple (Hett and Loucks, 1976; Whipple and Dix, 1979; Marks and Gardescu, 1998; Worbes et al, 2003; Frey et al., 2007). This is represented by the dashed line in Figure 2, which contrasts sharply with the distinctly bimodal age-frequency distribution for our population sample.

Individuals greater than 60 y in our sample are few in number not just because they have had more years to incur mortality risks, but also in part because we sampled only trees that were [less than or equal to] 6 cm in stem diameter (Fig. 1). Although larger unsampled individuals were much less abundant, some of them were likely within the 60-80 v range, which is probably underrepresented in our age frequency distribution (Fig. 2). Nevertheless, the analysis here concerns only the expected relative representation of much younger age groups associated with known mast year seed production. In this respect our distinctly biomodal distribution indicates the age specific mortality risks for the older cohorts in our sample, pre-1974 (i.e., greater than approximately 40 y old in 2014; Fig. 2), were not as intense as those experienced by the younger half of the age frequency distribution (i.e., less than 40 y old, where the distribution starts resembling more of a bimodal shape). Two conspicuous cohorts were produced in 2006 (8 y of age) and in 2000 (14 y of age) but there is a complete absence of individuals in the 5 y period (2001-2005) involving ages 9-13 y (Fig. 2). This is followed by a large spread (from 1986-1999) of very low densities between 15 and 28 y of age and then another relatively conspicuous cohort indicated for 29/30 y of age (coinciding with the [+ or -] 1.69 y margin of error) for the known 1984 mast year (Fig. 2).

Three explanations seem plausible for these distribution gaps. First, we cannot discount the possibility that these years are associated with unusually high seed predation levels from small mammals (e.g. chipmunks and squirrels), although there are no records of this from wildlife biologists working locally at QUBS. Second, these years may have had unusually low precipitation levels, not conducive therefore to seed germination and seedling establishment success. However, an examination of annual precipitation data for 1961-2007 from the weather station (at Kingston Ontario) nearest to (30 km from) the study site, does not support this (Fig. 3). Most probably, we suggest, the frequency gaps in the younger half of the cohorts in Figure 2, is accounted for by a period of unusually intense deer browsing pressure, corresponding with the known time period of regional overabundance of white tailed deer (Fryxell et al, 1991; Boerner and Brinkman, 1996; Tanentzap et al, 2011; Bradshaw and Waller, 2016), including in eastern Ontario (S. Smithers and M. Charette, Regional Wildlife Biologists, Ontario Ministry of Natural Resources and Forestry, pers. comm.), This involved sugar maple cohorts that were between about 34 and 6 y of age in the year of our study (2014) (Fig. 2).

Importantly, the 1984 mast year cohort (represented by 30 [+ or -] 1.69 y in Fig. 2) became established as seedlings in 1985, just as local deer numbers were ramping up; therefore, they would have been particularly vulnerable to mortality from intense browsing pressure over the subsequent two decades. Older saplings in 1985 would also have been vulnerable to these browsing effects, but those taller than 2.2 m would have exceeded the estimated maximum browsing reach of white-tailed deer (Ross et al, 1970; Huot, 1974).

In this study our central question was whether we could detect any evidence suggesting that the high seed production for sugar maple at our study site in the 1984 mast year was associated with relatively high recruitment success for this cohort three decades later, in 2014. The evidence is inconclusive. Survivors of the cohort produced in the year prior to the 1984 mast year, 31 y old in our sample (aged in 2014), number no less than 3 per 1750 [m.sup.2] (Fig. 2), which makes them out-numbered by the 1984 mast year survivors by, at most, a margin of only 3-fold. This seems like a negligible difference given that mast year seed production typically outnumbers seed production in the year prior to masting by several hundred fold (as indicated in the above comparison between the 2013 and 2012 cohorts (Fig. 2) (see Results). In addition the density of the 1984 mast year cohort in its second year of growth (1986) was 66 individuals per [m.sup.2] (Taylor and Aarssen, 1989), whereas in our 2014 survey, at 30 y of age, the survivors numbered only about 0.006 individuals per [m.sup.2] (no more than 10 per 1750 [m.sup.2]; Fig. 2). This represents a loss of 99.99% of the 1984 mast year cohort within 28 y (from 1986-2014), most of which coincides with the period of time involving very high deer numbers in the study region. Nevertheless, without masting in 1984, the number of survivors in 2014 would likely have been far less than 10 per 1750 [m.sup.2]. This becomes significant when recognizing that a parent tree will leave, on average, only one descendant, representing genetic fitness that is greater than the vast majority of other resident plants within the population, which leave no descendants at all (Harper, 1977).

We also note the conspicuous cohorts from 2006 (8 y of age in 2014; Fig. 2)) and from 2000 (14 y of age in 2014; Fig. 2), correspond with mast years for sugar maple in Vermont (Rapp and Crone, 2015), approximately 300 km from our study site, and also in Quebec (for 2006; Graignic et al., 2014), approximately 140 km from our study site, although we have no direct confirmation that the trees at our study site also had seed masting in these years. Masting in some North American trees, including sugar maple, is known to occur synchronously over areas as large as hundreds to thousands of square kilometers (Koenig and Knops, 1998, 2000; Graignic et al, 2014). We suggest, therefore, (hat the striking contrast in density for the 2006 cohort (age 8 yr) and 2000 cohort (age 14 y), 31 and 20 individuals per 1750 [m.sup.2] respectively (Fig. 2), compared with cohorts for the 5 y both before and after 2000, with a total of only three trees recorded across all ten of these years (Fig. 2), provides strong inference that masting in sugar maple (in 2006 and 2000) at the study site improved the probability of recruitment for these cohorts over this time period (1995-2005) when local deer numbers are known to have been particularly high. [Rapp and Crone (2015) also report a mast year in 2011 and Graignic et al. 2014 identified mast years also in 1996, 2002, and 2003].

In conclusion our results indicate something caused an unusually massive limitation on seedling/sapling recruitment success in sugar maple, extending across approximately three decades in our temperate mixed hardwood forest site. We can also conclude, based on available evidence, the most probable cause is from mortality due to intense browsing pressure resulting from an atypicallv large local deer population size over this same time period. Our results also point to evidence suggesting seed masting in sugar maple can bolster cohort recruitment success that otherwise would virtually (or completely) fail when severe impact from deer browsing is combined with other typical early life stage mortality risks, e.g., from soil water/nutrient deprivation, neighborhood crowding/competition, and persistent overhead canopy shade.

Acknowledgments.--Amanda Tracev, John Serafmi, and Erika Irwin assisted with field data collection, and Dale Kristensen, Tice Post, Ron Kerr, and Ryan Danby provided technical assistance. Dale Kristensen also provided helpful comments on the manuscript. This project was supported by a research grant to LWA from the Natural Sciences and Engineering Research Council of Canada, and by an Alexander and Cora Munn Research Award to JM from Queen's University.

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Submitted 1 July 2016

Accepted 27 January 2017

JENNIFER MACMILLAN and LONNIE W. AARSSEN (1)

Department of Biology, Queen's University, Kingston, ON, Canada, K7L 3N6

(1) Corresponding author: e-mail: aarssenl@queensu.ca

Caption: Fig. 1.--Relationship between number of annual growth rings (age in years) and average stem section diameter (mm) at 10 cm above ground for saplings of Acer saccharum ((Product moment correlation: r = 0.689, P < 0.001, n = 148). Sampled stems were greater than 50 cm in height but [less than or equal to] 6 cm in stem diameter, measured (at 10 cm above ground) in the field. Data include all stems harvested from within the 5m x 10m plots at the study site in 2014

Caption: Fig. 2.--Relationship between cohort age (years) and density (individuals per 1750 [m.sup.2]) of all individuals of Acer saccharum sampled at the Darling Farm forest site in 2014. Adjacent bars are 1 v apart, and age 1 refers to seedlings in their first year of growth. The density of 2 v old individuals (405 per 1750 [m.sup.2]) is indicated by a short horizontal line above the break. Samples aged at 30 y ([+ or -] 1.69 y, taking account of measurement error for annual growth ring counts) are the survivors of the 1984 mast Near recorded by Taylor and Aarssen (1989). The dashed line illustrates a hypothetical 'inverse J' curve characteristic of populations with a uniform rate of survivorship through time

Caption: Fig 3.--Environment Canada precipitation reports based on data collected daily al Kingston Ontario (44[degrees]14'38.052"N, 76[degrees]28'50.040"W) from 1961 to 2007. (A) total annual precipitation: (B) total spring precipitation (March 1st-June 30th); (C) total summer precipitation (July 1st-October 31st). Bars with white fill correspond with two conspicuous cohort gaps (2005-2001 and 1999-1986) in the age-frequency distribution in Figure 2. Source: http://climate.weather.gc.ca/
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Author:Macmillan, Jennifer; Aarssen, Lonnie W.
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1CONT
Date:Jul 1, 2017
Words:5629
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