Herr et al 1999 New For

New Forests 18: 219–230, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.

Effects of soil organic matter, moisture, shading and ash on white pine (Pinus strobus L.) seedling emergence D.G. HERR1, L.C. DUCHESNE2,∗ and R.J. READER1 1 Department of Botany, University of Guelph, Ontario, Canada N1G 2W1; 2 Canadian

Forest Service, P.O. Box 490, 1219 Queen Street East, Sault Ste. Marie, Ontario, Canada P6A 5M7 (∗ Author for correspondence) Received 15 May 1998; accepted 15 June 1998

Abstract. The effects of soil organic matter, soil moisture, shading and ash on white pine (Pinus strobus L.) emergence were investigated using soil monoliths in greenhouse experiments. White pine seedling emergence increased with soil organic matter removal, and levelled with the elimination of the litter and fermentation layers, where seedling emergence did not significantly increase further. Increased shade and soil moisture levels also improved seedling emergence whereas ash had no or little impact on white pine seedling emergence. The silvicultural implications of these findings are discussed with regard to white pine management. Key words: Pinus strobus, germination, fire, seeds, shading, ash

Introduction A major silvicultural problem in northern ecosystems is that the slowly decomposing organic material overlying mineral soil hinders seedling emergence of many conifer species (Smith 1951; Arnott 1974; Chrosciewicz 1974; Chrosciewicz 1978; Ahlgren and Ahlgren 1981). The success of many silvicultural programs depends largely on the reduction of this material either through prescribed burning or mechanical means (Logan 1951; Horton and Bedell 1960; Ahlgren 1976; Balmer and Williston 1983; Thomas and Wein 1985). Current guidelines for white pine (Pinus strobus L.) management involve a two to three cut shelterwood system (Hannah 1988). However, availability of suitable microsites, which are reduced by understorey vegetation or unsuitable seedbed conditions, for white pine germination is a factor in the success of management operations. Consequently, understorey prescribed burning and/or artificial seeding was proposed as a means to facilitate the regeneration of this species (McRae et al. 1994). Prescribed burning is an appealing management tool because of its relevance to pre-settlement wildfire disturbance regimes in northern ecosystems

220 (Duchesne 1994; Weber and Taylor 1992), particularly in white pine ecosystems (Methven and Murray 1974; Ahlgren 1976). However, under moist climate prescribed burns may not completely combust the wetter, lower, organic layers, leaving a range of, often undesirable, residual post-fire organic matter (Thomas and Wein 1985). Although the favourable properties of mineral soil seedbeds for early conifer regeneration are well documented (Smith 1951; Fraser and Farrar 1953; Arnott 1974; Hannah 1988), our understanding of conifer regeneration on a range of post-fire organic horizon thicknesses resulting from prescribed burning is incomplete (Thomas and Wein 1985; Johnson 1992). Additional studies are needed to determine the effect of partial removal of the organic horizon on conifer seedling emergence, simulating incomplete combustion of the organic horizon. Another potential problem associated with prescribed burning is that ash may be detrimental to conifer seedling emergence (Ahlgren 1976; Thomas and Wein 1990; Herr and Duchesne 1995, 1996). Ash-covered seedbeds have a lower albedo than seedbeds without ash, and therefore, under decreased shade levels, can be subject to extreme temperature and moisture fluctuations (Mallik et al. 1988). In addition, with a lower water retention capacity than mineral soil (Mallik et al. 1988), ash may dry faster and reduce conifer seedling emergence. The extent to which this detrimental effect of ash negates the benefits of organic matter removal needs to be examined. Lack of available soil water may contribute to poor conifer seedling emergence on unburned seedbeds as well as on burned seedbeds (Arnott 1974; Thomas and Wein 1985). Water retention capacity is lowest at the top of the organic horizon (Mallik et al. 1988), where wind-dispersed conifer seeds are initially deposited. The detrimental effect of low water retention capacities associated with thicker organic horizons on seedling emergence should be negated by either increasing water supply or by adding shade to reduce evaporation. Little information exists concerning conifer seedling emergence on seedbeds exposed to different shade levels and different organic horizon thicknesses. Because shading may alter seedbed conditions following understorey prescribed burning, there is a need for quantitative studies that address simultaneously the effects of shade and organic horizon thickness reduction on conifer seedling emergence. The role of organic matter in fire-prepared seedbeds on white pine seedling emergence remains unclear. While some authors maintain that early white pine regeneration (i.e. seed germination and seedling emergence) is best on mineral soil (Smith 1951; Hannah 1988), others suggest that white pine germinates and emerges on undisturbed litter, burned seedbeds, and mineral soil (Ahlgren 1976; Wendel and Smith 1990). Some authors suggest that white pine seedling emergence may be tolerant to ash seedbeds (Ahlgren

221 1976; Wendel and Smith 1990). It is unknown which of the presence of ash or the reduction of the organic horizon thickness has the most significant effect on white pine seedling emergence. A comprehensive study is required to simultaneously investigate the roles of organic horizon thickness, ash, water availability, and shade level on white pine seedling emergence. Recently, germination models for jack pine (Pinus banksiana Lamb.) and red pine (Pinus resinosa Ait.) were published (Herr and Duchesne 1995, 1996). However, such models are not applicable in white pine management because the latter species is an intermediate shade tolerance whereas red and jack pine are both shade intolerant. The objective of this investigation is to determine the role of organic matter, ash, water availability, and shade on white pine seedling emergence. Materials and methods Monolith collection Monoliths used in both experiments were collected from the Frontier Lake experimental site at latitude 46◦ 000 N and longitude 77◦ 330 W within the Middle Ottawa Section (L.4c) of the Great Lakes-St. Lawrence forest region (Rowe 1972). A detailed description of present vegetation in this stand is given by Herr et al. (1994). The main overstorey species in the stand include jack pine and eastern white pine; understorey white pine regeneration is present at a density of approximately 40,000 seedlings/saplings·ha−1 (Duchesne and McAlpine 1993). Mean organic horizon thickness (measured from the bottom of the Humus (H) horizon to the top of the litter (L) horizon of the monolith collection site was 7.2 cm with L: 1.3 cm; fermentation (F); 2.2 cm; and, H: 3.7 cm. Each 18 cm × 30 cm monolith (Herr et al. 1995) was first outlined by cutting into the soil with a serrated knife. Then the monolith was excavated to a depth of 25 cm using a tree planting spade to include the organic horizon, the A horizon, and a portion of the B horizon. A total of 155 monoliths were collected for this investigation. The monoliths were placed into plastic Rootrainer boxes (Spencer–Lemaire Industries Ltd., Edmonton, Alberta) lined with weed barrier nursery cloth (Geosynthetics Systems, Edmonton, Alberta), and transported to a greenhouse at the Petawawa National Forestry Institute (PNFI). Removal of organic matter To reduce soil organic matter the monoliths were turned on their sides and 0%, 25%, 50%, 75%, or 100% of the organic layers was mechanically

222 removed using a serrated knife. Controls (0% reduction) consisted of monoliths in which no organic material was removed; 25% reduction led to the removal of the L horizon; 50% reduction led to the removal of the L and F horizons; 75% removal led to the elimination of the L and F horizons and half the H horizon; and 100% removal led to the elimination of the L, F, and H horizons. Effect of ash on seedling emergence The organic matter removed from each monoliths was oven-dried at 60–80 ◦ C for 48 h and burned ex situ using a propane torch in a fume hood until no further combustion was attainable. This method was found to be more practical and precise than burning organic matter in situ (unpublished results). Half the ashes generated by burning the organic material from a monolith was then placed as a single layer on half the original monolith and separated from the other half of the monolith using plastic dividers, hence creating a split-plot design. Ash from the 100% removal treatment made up a layer of approximately 5 mm on the monoliths. Effect of soil moisture on seedling emergence The effect of organic matter, ash, and water availability on white pine seedling emergence were investigated. Five monoliths from each of the organic horizon reduction levels (0%, 25%, 50%, 75%, or 100%) were subjected to one of three watering regimes for a total of 75 monoliths (n = 5). The watering treatments consisted of 50%, 100%, and 200% of the 30 year average daily June rainfall (i.e. 1/2× normal, 1× normal, and 2× normal average daily June rainfall) from the Bransted weather station of the Petawawa National Forestry Institute (latitude 46◦ 000 N and longitude 77◦ 330 W). The weather station is located 15 km east of the monolith collection site. The average daily June rainfall of 2.7 mm rain/day was adjusted for the surface area of the monolith, yielding a 100% water regime of 145 mL water per day per monolith. The 50% and 200% water regimes resulted in an application of 73 and 290 mL of deionized water per day respectively. The monoliths were watered using a plastic bottle with a perforated lid. Effect of shading on seedling emergence The effect of organic matter, ash, and shade on white pine seedling emergence were investigated. Five monoliths from each of the organic horizon reduction levels (0%, 25%, 50%, 75%, or 100%) were subjected to one of three watering regimes for a total of 75 monoliths (n = 5). The three shade levels

223 consisted of ground level shade from the monolith collection site (26.5% of maximum PAR), decreased shade (39% of maximum PAR) and, increased shade (14.5% of maximum PAR). Shade in the collection site was based on the light intensity level at the forest floor of the monolith collection site measured using a Li-Cor LI-190SZ Quantum sensor (Li-Cor, Lincoln, Nebraska, USA). Inside the greenhouse, the shade level was controlled by supporting a wooden lattice 25 cm above the monoliths. The number of slats in the lattice were adjusted until the desired PAR level was obtained. Each monolith received 100% of the average daily June rainfall (145 mL deionized water per day). Monolith incubation and seedling emergence White pine seeds were cold-stratified using the method of Creasey and Myland (1992). Seeds used in this investigation were obtained from the National Forest Genetics Resource Centre Seed Bank (NFGRCSB) of PNFI (Watering experiment: seed lot No 8930585, 90% viability in vitro; Shading experiment: seedlot 8930576.0, 98% viability in vitro). Following coldstratification, percent viability was retested in vitro using optimal conditions, and found to be identical to seed viability reported by the NFGRCSB. Based on the reported viability of the seedlots, the number of seeds required to achieve 50 viable seeds per treatment was calculated. Seeds were randomly broadcast on the monolith surface. The monoliths were kept in a greenhouse at ambient air temperature at monolith surface (without shade) of 23.8±4.3 ◦ C standard deviation. It was not possible to control relative humidity inside the greenhouse. Every second day, the number of seed coats that had lifted off the monolith substrate (i.e., seedling emergence) of each treatment combination was counted and removed. At the end of the 7 weeks, after which no new seedlings emerged, total percent seedling emergence was calculated for each treatment combination. Statistical analyses Effects of treatments on seedling emergence were analysed using analysis of variance (ANOVA). Total percent emergence data was arcsine-transformed and tested for homoscedasticity prior to ANOVA using SAS (SAS Institute Inc. 1991). When ANOVA results indicated a significant treatment effect, Tukey’s HSD test was performed on each reduced data set to determine which levels of the treatments differed significantly (α = 0.05) (Sokal and Rohlf 1981).

224 Results Effect of soil moisture Mean white pine seedling emergence ranged widely from a minimum of 16.4% on the 25% organic matter reduction with ash under the 50% watering regime (Figure 1A) to a maximum of 94% on the 50% organic matter reduction without ash under the 200% watering regime (Figure 1B). Variation around mean white pine seedling emergence was relatively large, averaging 26.7% standard deviations (SD) for all treatment combinations. The overall effect of organic matter reduction on seedling emergence accounted for 29% of the total variation in seedling emergence among treatments (P < 0.0001). Mean white pine seedling emergence improved significantly with increasing organic matter removal, regardless of the presence or absence of ash, or the watering regime (Figure 1A–B). The only exception was under the 2× watering regime, where seedling emergence was not significantly affected by organic matter reduction without ash (Figure 1A). Seedling emergence increased the most between 0% and 50% organic matter reduction. Additionally, at the 0% organic horizon reduction level, white pine seedling emergence increased by greater than 40% in response to the watering regime (Figure 1A–B). No significant differences in seedling emergence were found among the 50%, 75%, and 100% organic reduction levels (Figure 1A–B). The overall effect of water regime on seedling emergence accounted for 27% of the total variation in seedling emergence among treatments (P < 0.0001). For most cases, the difference between the 50% water regime and the 100% water regime was greater than the difference between 100% water regime and 200% water regime (Figure 1A–B). Ash had no effect on seedling emergence (P = 0.44). Effect of shading Mean white pine seedling emergence ranged widely from a minimum of 4% on the 0% organic horizon reduction treatment under decreased shade (Figure 2A), to a maximum of 97% on the 100% organic horizon reduction with ash treatment under increased shade (Figure 2A–B). Variation around mean white pine seedling emergence was relatively large, averaging ±36.3% SD for all treatment combinations. The effect of organic matter reduction on seedling emergence accounted for the largest portion of the total variation (40%) in seedling emergence among treatments (P < 0.001). Mean seedling emergence of white pine increased as increasing portions of the organic horizon were removed, regard-

225

Figure 1. Effect of water regime and organic horizon reduction on white pine seedling emergence A. With ash. B. Without ash Means followed by different letters are significantly different α = 0.05. Error bars indicate ± one standard error. n = 5.

226

Figure 2. Effect of shading and organic horizon reduction on white pine seedling emergence. A. with ash. B. without ash. Means followed by different letters are significantly different α = 0.05. Error bars indicate ± one standard error. n = 5.

227 less of the presence or absence of ash, or the shade level (Figure 2A–B). Seedling emergence was significantly greater than the control (0% organic horizon reduction) on 50%, 75% and 100% organic horizon reduction treatments, but not on the 25% organic horizon reduction treatment. Mean seedling emergence did not differ significantly among the 50%, 75%, and 100% organic horizon reduction treatments. The effect of shade on seedling emergence explained 14% of the total variation in seedling emergence among treatments (P < 0.003). There was no light-organic removal interaction. Mean white pine seedling emergence was lowest under the decreased shade level (39% of maximum PAR), regardless of the organic horizon reduction treatment or the presence or absence of ash. As shade level increased from 39% to 15% of maximum PAR, mean white pine seedling emergence generally increased. The significant increase in seedling emergence with decreased light intensities was not related consistently to either organic horizon reduction or to ash addition. The effect of ash on seedling emergence was significant in the analysis of variance, and accounted for 0.4% of the total variation in seedling emergence among treatments (P < 0.05). Discussion In this investigation we showed that white pine seedling emergence is affected by light, organic matter, soil moisture and ash. Others also reported the beneficial effect of removing soil organic matter on white, jack and red pine germination (Logan 1951; Wendel and Smith 1991; Thomas and Wein 1985; Hannah 1988; Herr and Duchesne 1995, 1996). Contrasting results also suggest that white pine is tolerant to various seedbed conditions and that seedlings emerge equally well on all seedbed types (Ahlgren 1976; Horton and Bedell 1960; Kozlowski and Ahlgren 1974). However, our results are first to indicate that white pine is not uniformly tolerant of all seedbeds and that germination on unsuitable seedbeds can be improved by increasing soil water supply and shading. Indeed, while seedling emergence was increased by the removal of the L and F horizons (50% reduction), further reduction of the H layer (75% and 100% reduction) did not improve emergence (Figure 1). Because the L and F horizons were more detrimental to white pine seedling emergence than the H layer, we speculate that the loosely arranged L and F horizons are subjected to wide changes in moisture content, which should expose germinating and emerging seedling to less favourable conditions as compared to the wetter, more compact, H horizon. Increased water supply was beneficial to white pine seedling emergence. By doubling the amount of water applied to the monolith surface, we were

228 able to almost completely negate the detrimental effect of the L and F horizon on white pine seedling emergence (Figure 2A–B). This pattern of increased seedling emergence in response to added water indicates that water availability is a key factor in determining the effect of organic material on white pine seedling emergence. The importance of water supply may also help clarify some discrepancies among past studies concerning the role of organic matter on white pine seedling emergence. It is possible that water availability was unusually high in studies (e.g., Ahlgren 1976; Horton and Bedell 1960; Kozlowski and Ahlgren 1974) that suggest white pine is tolerant of a range of seedbeds. Our results show that both the physical seedbed characteristics (e.g., organic horizon thickness and type) and the environmental conditions to which the seedbeds are exposed (e.g., water availability) are important determinants of white pine seedling emergence. As well, because organic horizon thickness and water availability accounted for similar levels of variation in our experiment, we conclude that these two factors may have similar degrees of influence on white pine germination. The beneficial effect of increasing shade by manipulating the overstorey has been suggested by previous authors (e.g., Smith 1951; Fraser and Farrar 1953). However, our results present the first study comparing the effect of shade and other environmental factors on white pine seedling emergence. Shading had a significant effect on white pine seedling emergence for the extremes tested in this study (i.e., 14.5% vs. 39% of maximum PAR), although its influence is less important (under the limitations imposed by our experimental conditions) than organic matter and water availability. We speculate that increasing shade has an indirect effect on seedling emergence, and operates by reducing evaporation at the seedbed surface (Thomas and Wein 1985). However, the results from this study indicate that a relatively large change in shade at the seedbed surface would be required to increase seedling emergence significantly for white pine (i.e., at the very least, a reduction from 39% to 14.5% of maximum PAR). Nevertheless, this does not take into account the effect of increased plant competition by herbs and shrubs which is normally associated with canopy reduction. Additionally, manipulating shade had less effect on seedling emergence than manipulating water regime within the range of shade levels tested here. This is particularly true for the lower organic horizon reduction levels (i.e., 0% and 25% organic matter reduction). While increased water supply had a greater effect than decreased shade on seedling emergence of white pine, manipulation of available water forest stands would be more difficult or impossible to achieve. Since shade is more easily manipulated through a modification of the canopy structure or by adding slash to cover the ground, it represents a more feasible way of maximizing seedling regeneration.

229 Ash had much less effect on seedling emergence than the other factors tested in this investigation. Our results indicate that white pine seedling emergence is not reduced by potential harsh conditions associated with ash seedbeds. Surprisingly, ash increased seedling emergence in some cases (e.g. 50% and 75% organic horizon reduction levels with decreased water regime). Overall, white pine was at least tolerant to, if not benefiting, from the presence of ash. These results are consistent with previous findings for white pine (Ahlgren 1976; Wendel and Smith 1990). In contrast, germination of both jack and red pine was affected by ash (Herr and Duchesne 1995, 1996) suggesting different tolerance to ash among these three pine species. Our observations have implications in the use of prescribed burning as a site-preparation tool for white pine regeneration. We have shown that a mineral seedbed is not required for adequate white pine seedling emergence; rather, a reduction of the L and F horizons should be satisfactory for seedling emergence. Then, prescribed burns at low fire intensity can be used to achieve increased white pine seedling emergence, thereby avoiding damaging high intensity fires. Also, our results on the relationship of organic horizon removal, shading, and watering regime provide a partial explanation why the shelterwood system is best suited for white pine management (Horton and Bedell 1960). Indeed, under this silvicultural system, seedling emergence is favoured by shading from the residual canopy, and a partial scarification of the forest floor through logging activities.

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