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Competitive effects of woody and herbaceous vegetation in a young boreal mixedwood stand Cosmin D. Man, Philip G. Comeau, and Douglas G. Pitt
Abstract: The influence of aspen (Populus tremuloides Michx.) and herbaceous (forb and grass) vegetation on resource availability and white spruce (Picea glauca (Moench) Voss) growth were examined as part of a long-term experiment established in 2002 near Whitecourt, Alberta, Canada. During the 2005 growing season, we examined the effects of herbicide treatments designed to control only woody (triclopyr ester) or both woody and herbaceous (glyphosate) vegetation on leaf area index (LAI) of both the woody and herbaceous components and relationships among LAI and light, soil moisture, air temperature, soil temperature, nitrogen availability, and spruce growth. Treatments reduced LAI and increased light, soil nitrogen availability, and white spruce growth. There were no apparent effects of the treatments on soil moisture in 2005. Both the woody and herb–grass layers appear to be competing for light and soil nitrogen in this young plantation. Controlling only woody vegetation resulted in an increase in herbaceous and total LAI (dominated by the grass Calamagrostis canadensis (Michx.) Beauv.). Spot treatment, involving control of vegetation within a 2 m radius of spruce seedlings while leaving 1 m of untreated ground between treated spots, may be a promising alternative to classical broadcast treatments for establishing spruce in a mixedwood stand. Spot treatments provided good growing conditions and reduced exposure of spruce seedlings to summer and winter frost injury during the first 3 years after planting. Re´sume´ : Nous avons e´tudie´ l’influence du peuplier faux-tremble (Populus tremuloides Michx.) et des plantes herbace´es (gramine´es et plantes herbace´es non gramine´ennes) sur la disponibilite´ des ressources et sur la croissance de l’e´pinette blanche (Picea glauca (Moench) Voss) dans le cadre d’une expe´rience a` long terme e´tablie en 2002 pre`s de Whitecourt, en Alberta, au Canada. Au cours de la saison de croissance 2005, nous avons e´tudie´ les effets de l’application d’herbicides visant a` maıˆtriser les plantes ligneuses seulement (ester de triclopyr) ou les plantes ligneuses et herbace´es (glyphosate) sur l’indice de surface foliaire (LAI) des composantes herbace´es et ligneuses et sur les relations entre LAI et la luminosite´, l’humidite´ du sol, la tempe´rature de l’air, la tempe´rature du sol, la disponibilite´ en azote et la croissance de l’e´pinette blanche. Les traitements ont re´duit LAI et augmente´ la luminosite´, la disponibilite´ d’azote dans le sol et la croissance de l’e´pinette blanche. Les traitements n’ont pas eu d’effet apparent sur l’humidite´ du sol en 2005. Les strates de ve´ge´tation ligneuse et herbace´e semblaient eˆtre en compe´tition pour la lumie`re et l’azote du sol dans cette jeune plantation. La maıˆtrise de la ve´ge´tation ligneuse seule a entraıˆne´ une augmentation de LAI des plantes herbace´es et de LAI total (domine´es par la plante herbace´e Calamagrostis canadensis (Michx.) Beauv.). Un traitement localise´, consistant a` maıˆtriser la ve´ge´tation dans un rayon de deux me`tres autour de semis d’e´pinette tout en laissant un me`tre de terrain non traite´ entre les surfaces traite´es, pourrait constituer une bonne solution de rechange aux traitements en plein classiques pour l’e´tablissement de l’e´pinette dans un peuplement mixte. Les traitements localise´s ont produit de bonnes conditions de croissance et re´duit l’exposition des semis d’e´pinette aux gele´es estivales et hivernales pendant les trois premie`res anne´es apre`s la plantation. [Traduit par la Re´daction]
Introduction Studies in various parts of the world indicate that herbaceous vegetation can be very competitive in young plantations (Richardson 1993; Rose et al. 1999; Bell et al. 2000; Kubner et al. 2000; Miller et al. 2003; Pitt and Bell 2005). Removal of woody vegetation in regenerating forests can often result in increased herb and grass cover, which can Received 10 May 2007. Accepted 29 February 2008. Published on the NRC Research Press Web site at cjfr.nrc.ca on 3 June 2008. C.D. Man and P.G. Comeau.1 Department of Renewable Resources, University of Alberta, 751 General Services Buildingg Edmonton, AB T6G 2H1, Canada. D.G. Pitt. Canadian Wood Fibre Centre, Canadian Forest Service, 1219 Queen Street East, Sault Ste. Marie, ON P6A 2E5, Canada. 1Corresponding
author (e-mail:
[email protected]).
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have substantial competitive effects on the conifers. The competitive effects of woody and herbaceous vegetation and of different species are expected to differ because of differences in resource requirements and utilization (Goldberg and Werner 1983; Balandier et al. 2006). Variation in equivalence or inequality of competitive effects of different species or life forms may occur because of differences in climate, site, microsite, species, and other factors (Balandier et al. 2006). Bell et al. (2000) found strong differences between the competitive effects of woody and herbaceous vegetation on jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana (Mill.) BSP) growth when evaluated on the basis of percent cover. In contrast, Comeau et al. (1993) reported no differences in the competitive influences of different shrub, forb, and grass species on Engelmann spruce (Picea engelmannii Parry ex Engelm.) in southern British Columbia. Balandier et al. (2006) provide a detailed review and discussion of the competitive effects of various functional groups and indicate that perennial grasses, such
doi:10.1139/X08-032
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as Calamagrostis spp. and Deschamsia spp., which develop dense cover and dense root systems, can be very competitive with small seedlings. They suggest that competition for water and nutrients from graminoid species may be of greater significance than competition for light. In addition, Balandier et al. (2006) suggest that forbs are generally less competitive than graminoids. In western boreal forests, trembling aspen (Populus tremuloides Michx.) and Canada blue-joint grass (Calamagrostis canadensis (Michx.) Beauv.) are widely recognized as competitors of white spruce (Picea glauca (Moench) Voss). Following harvesting, trembling aspen regenerates vigorously and typically dominates the early stages of subsequent stand development (Thorpe 1992). In mixed stands, white spruce grows more slowly during the early stages of development and usually does not become dominant in the canopy until after age 70 years (Lieffers and Beck 1994). Because mixedwood stands are a common and natural component of the boreal forest landscape and because the presence of an aspen component in a spruce stand may contribute to stand health, productivity, amelioration of frost, biodiversity, and longterm sustainability (Man and Lieffers 1999; Comeau et al. 2005), there is substantial interest in finding effective methods for regenerating mixedwood stands and in treatments that have the potential to accelerate development of mixedwood stand characteristics. Although aspen can grow well with little or no intervention, rapid early growth of white spruce appears to occur only under ideal circumstances. Competition for light, water, and nutrients has been observed in these mixedwood ecosystems. During the first years after planting, competition appears to be most problematic, with the best growth of conifer seedlings occurring when trees are maintained free of competition from an early age (Wagner et al. 1999). Competition for light is considered to be of primary importance in these regenerating conifer stands (Comeau and Heineman 2003; Balandier et al. 2006). As light availability declines, survival, height, and diameter growth of juvenile spruce also declines (Comeau et al. 1993; Comeau et al. 1999; Lieffers et al. 2002; Filipescu and Comeau 2007a, 2007b). Survival of white spruce is poor when light levels are less than 10%–15% of full sunlight (Eis 1981; Lieffers and Stadt 1994; Chen 1997). Wright et al. (1998) report that between 60% and 85% light is required for these seedlings to grow at about 70% of their maximum rate, and optimal height growth may be realized when light levels exceed 40% (Lieffers and Stadt 1994). Competition for water may also occur on some boreal sites, especially during dry summers (Brand 1991; Coopersmith et al. 2000; Voicu and Comeau 2006). Climatic and site factors influence the intensity, duration, and temporal pattern of competition for water, with competition during periods of moisture deficit increasing with increases in vegetation density and leaf area index (LAI; Petersen et al. 1988). Growth is reduced if seedlings are unable to maintain a favourable internal water balance, especially when soil volumetric water content drops below 20% (Grossnickle 2000). Other factors that can negatively affect spruce include cold wet soils, summer frost, winter (Chinook) injury, and hare damage. Delayed soil warming resulting from shade and litterfall from Calamagrostis, aspen, and other vegetation can significantly shorten the growing season (approxi-
Can. J. For. Res. Vol. 38, 2008
mately 20 weeks) for spruce in boreal forests (Hogg and Lieffers 1991). Mechanical site preparation and herbicide application can be effective for reducing competition from aspen and other broadleaved trees and increasing growth of white spruce (e.g., Biring and Hays-Byl 2000; Jobidon 2000; Boateng et al. 2006). In spruce plantations, the most common treatments used for site preparation are mounding or disk-trenching. Vegetation-management treatments include use of herbicides (triclopyr ester (Release1), glyphosate (Vision1)), or cutting (‘‘brushing’’) treatments. Triclopyr ester is used to control aspen and other woody species, and glyphosate is used to control woody species, grasses, and herbaceous vegetation. However, there are concerns about the potential effects of broadcast herbicide treatments on stand composition and biodiversity. Ten years after broadcast herbicide treatment, Biring and Hays-Byl (2000) found that deciduous densities were less than 1500 stems/ha, and that the deciduous component was dominated by white birch (Betula papyrifera Marsh.) and balsam poplar (Populus balsamifera L.). Pitt and Bell (2005) also indicate that application of herbicide treatments may result in conifer-dominated rather than mixedwood stands. Spot application of herbicides, involving treatment of only a portion of the ground surface, while allowing aspen to develop on the remaining areas, offer a promising method for maintaining mixedwood characteristics in young forests. Such treatments can be used to reduce competition around planted spruce, while retaining the surrounding vegetation to provide protection from summer frosts and winter injury. Yang (1991) reports that growth can be increased substantially following treatments that remove aspen around individual spruce saplings. The objectives of this study were to examine the effects of treatments used to control only the woody vegetation and both woody and herbaceous vegetation on (i) development of vegetation LAI and root surface area; (ii) major factors and resources influencing spruce growth (light, soil moisture, soil nitrogen (N) availability, air temperature, and soil temperature); and (iii) growth of planted white spruce. In this paper, we use results obtained during the third growing season (2 years after planting) to test the following hypotheses: (i) aspen and bluejoint are equivalent in their effects on resource availability and white spruce growth; (ii) resource availability and spruce growth increase with increasing level and duration of vegetation control; and (iii) spot treatments result in reduced resource availability and growth of spruce compared to broadcast treatments.
Materials and methods The study was conducted on a subset of plots and treatments in a long-term field installation known as the Judy Creek Mixedwood Experiment, located 30 km northeast of Whitecourt, Alberta, Canada, on lands licensed to Blue Ridge Lumber (1981) Ltd (54803’N, 115836’W; elevation 1000 m). The establishment of this experiment is documented by Pitt et al. (2004). A brief synopsis follows. History and characteristics of the site Prior to establishment of the underlying experiment, the #
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parent stand was dominated by trembling aspen, with small components of lodgepole pine (Pinus contorta Dougl. ex. Loud.) and balsam poplar scattered throughout. Aspen in the stand were approximately 75 years old and likely originated from a fire, as suggested by charred stumps and fire scars on the lodgepole pine. Mean basal area of the stand was 33 m2/ha, with a mean diameter at breast height (DBH) of 26.5 cm and a stand height of 23.5 m. Full-tree harvest took place during March and April of 2002 on snow cover. Harvest records showed that 26 m3ha–1 of conifer and 191 m3ha–1 of deciduous were removed from the site. Soils across the site tend to be luvisols, with parent material consisting of ablation till. There is a rounded cobble layer at about 25–40 cm, indicating postglacial fluvial action. Soils are generally mesic and fine textured, with texture ranging between silty loam and clay, with a bulk density for the first 20 cm of 1.14 gcm–3 (lower in the first 10 cm and higher in the next 10 cm). Some plots have a sandy loam veneer over silty clay loam. In the adjacent, uncut portion of the stand, the ecosite phase was determined to be an E2 (low-bush cranberry Aw) with a plant community type of E2.3 (Aw/ green alder) (Beckingham et al. 1996). This suggests site indices of 17.6 m for trembling aspen and 17.5 m for white spruce. Mensurational data collected preharvest indicated a site index of 18.6 m for aspen and 16.3 m for lodgepole pine. Both aspen and Calamagrostis were present on the site following the first growing season in 2002. Aspen cover (visually estimated percent ground cover) ranged from a plot-mean low of 2% to a high of 15%. Aspen densities were relatively high, with plot means ranging from just over 44 000 stemsha–1 to 140 000 stemsha–1. Calamagrostis cover was low but occurred consistently across the site, ranging from a plot mean of 0.2% to just over 2%. In 2005, height of untreated aspen averaged 260 cm, height of untreated Calamagrostis averaged 60 cm, and height of spruce (May 2005) averaged 25.5 cm. For this location, climatic normals estimated using ClimateAB version 3.0 software (Wang et al. 2006) for the period between 1991 and 2000 are mean annual precipitation, 529 mm; summer season precipitation, 369 mm; mean annual temperature, 1.9 8C; degree-days >5 8C, 1120; number of frost-free days, 157; and frost-free period, 98 days. Precipitation measured on site during the period from May to September was 930 mm in 2004 and 690 mm in 2005, with precipitation being well distributed during the growing season. Experimental design The study reported in this paper was conducted on selected replicated plots and treatments of the underlying experiment that pertain to a factorial arrangement of the method of vegetation control (spot vs. broadcast) and type of vegetation control (woody only vs. complete) (Table 1). Spot vegetation control plots were established as 45 m 45 m plots, with spruce planting locations at square, 5 m spacing. Vegetation control in these plots was focused in the areas defined by a 2 m radius around each of the planting locations. Broadcast control plots were established as 35 m 35 m plots, with spruce planted at 2.5 m square spacing, and vegetation control was applied to the entirety
1819 Table 1. Treatment names and codes and number of plots selected. Treatment Broadcast complete control (BCC) Spot complete control (first 2 years only) (SCC2) Spot complete control (SCC) Broadcast woody-only control (BWC) Spot woody-only control (SWC) No control (untreated) (NC)
No. of plots 3 3 4 3 4 3
of these plots. The wider spacing of planted white spruce in the spot vegetation control treatments was designed to create a mixedwood condition, with herbicide treatments being applied to approximately 50% of the ground area in each plot. Woody-only vegetation control was completed in August 2002 using basal bark treatments (streamline method) with triclopyr (butoxyethyl ester; Release1) mixed with mineral oil at 25:75 (v/v). In subsequent years, woody regeneration in treated areas in these plots was removed by manual clipping. Complete vegetation control was achieved through foliar applications (in August 2002) of glyphosate (Vision1) at a rate of 3204 g a.i.ha–1, mixed with Sylgard1 309 surfactant at 0.375% volume. Vegetation control was maintained in each subsequent year via directed foliar applications of glyphosate (Vision1, 2% solution) as needed. For comparison with the above treatments, a set of untreated 35 m 35 m plots, each with 2.5 m planting spacing, were also established. In addition, a set of plots with 2 years of spot complete vegetation control treatment was established for comparison with the plots receiving spot complete control for four growing seasons. The site was planted on 27–28 June 2003, with 2+0 PSB 412 white spruce container stock. At the time of planting, these trees were 18.6 ± 3.5 cm in height (mean ± SD), 3.5 ± 0.6 mm in stem diameter, 7.5 ± 1.6 cm in crown diameter, and 0.62 ± 0.27 cm3 in stem volume (assuming conical form). No differences were detected among treatments with respect to initial tree size (p > 0.47). For the study reported in this paper, data were collected from eight trees systematically selected from the measurement trees in each plot (the inner 25 trees in 45 m 45 m plots and the inner 100 trees in 35 m 35 m plots). Measurements LAI-2000 plant canopy analyzers (LI-COR Inc., Lincoln, Neb.) were used to measure LAI and light (DIFN; diffuse non-interceptance) every 2 weeks during the 2005 growing season. These measurements were taken at 5 cm above the ground (with matching open-sky readings) for each of the eight selected seedlings in all plots. LAI-2000 measurements were taken with a 1808 view restrictor on the sensor, and with the sensor head being oriented west in the morning and east in the afternoon to avoid having the sensor pointing towards the sun. The LAI-2000 sensor was positioned outside of the dripline of the seedling and pointing away from the seedling (to avoid including the seedling leaf area in the measurement). In midsummer (26 July 2005), LAI of overstory and understory was measured by taking an additional sensor reading above the shrub–forb–grass understory layer. (Most of the times this was equal to the top of the herbaceous layer.) This reading was used as a measurement of as#
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Can. J. For. Res. Vol. 38, 2008 Table 2. Summary of orthogonal contrasts used in this experiment. Spot treatment Contrast Method Type Interaction Treated vs. untreated 2 years vs. 3 years
SCC2 0 0 0 1 1
SCC 1 1 1 1 –1
Broadcast treatment SWC 1 –1 –1 1 0
BCC –1 1 –1 1 0
BWC –1 –1 1 1 0
NC 0 0 0 –5 0
Note: For each contrast, comparsions were made among the variables with positive and negative values. See Table 1 for treatment abbreviations.
pen LAI, with understory LAI being calculated as the difference between the reading taken at 5 cm height and the aspen LAI. All five rings of the LAI-2000 sensor were used to calculate the final values for LAI and light. Some restrictions were applied to avoid ‘‘bad readings:’’ (i) avoid measuring between 11:30 and 14:00 when the sun is high in the sky; (ii) avoid measuring in foggy or rainy weather; and (iii) avoid measuring when there is a thick, low cloud layer. To characterize vegetation (total) root surface area and root dry mass, soil cores were collected using a SS heavy duty short soil sampler with foot assist (Star Quality Samplers, Edmonton, Alta.) (2.54 cm diameter) (approximately 100 cm3 each soil sample collected) next to one-half of the spruce (four seedlings) sampled in each plot. The core samples were extracted to 20 cm depth, 1 m west from the selected spruce. In broadcast-treated plots, trees adjacent to the datalogger and sensor installations (described below) were sampled. The soil samples were sealed in plastic bags, labelled, and frozen for later processing. In the laboratory, roots were extracted, using the following technique: (i the soil sample was washed through a 1 mm sieve with a gentle stream of warm water; (ii) roots were separated from the remaining materials (e.g., charcoal, gravel, twigs, and pieces of decaying wood) using tweezers; (iii) the surface area of the resulting root sample was measured by scanning (Epson Expression 1680 scanner) and analysis of the images using WinRhizo version 2002c (Regent Instruments Inc., Montreal, Que.) computer software; and (iv) dry masses were obtained after drying the samples at 70 8C for 48 h. Microclimate variables were monitored using sensors for air and soil temperature and volumetric water content, attached to dataloggers starting in May 2004. In each of the 20 selected plots (Table 1), one CS616 TDR soil moisture probe (Campbell Scientific Inc., Edmonton, Alta.) was installed 1 m south of one randomly selected spruce. Air temperature sensors (unshielded chromel–constantan thermocouples) were installed at 1.5 m and 0.3 m height, 1 m east of the one selected spruce in each of these plots. A soil temperature sensor was installed at 20 cm depth, 1 m north of the same seedling. Campbell Scientific CR10x 2Mb dataloggers were attached to all sensors. Soil moisture sensors (CS616 probes) were read once each hour and hourly samples and daily minima and maxima values were stored. Air and soil temperature sensors were scanned at 300 s (5 min) intervals and hourly minima, maxima, and mean and daily minima and maxima values were stored. To characterize treatment effects on soil N availability, PRSTM probes (Western AG Innovations Inc., Saskatoon,
Sask.) were used. The PRSTM probes consist of an ion-exchange resin membrane, which facilitates the measurement of inorganic N (NH4+-N and NO3–-N) and other nutrients. Before insertion into soil, the probes (both anion and cation exchange) were regenerated as described in Hangs et al. (2004): (i) shaken three successive times in 0.5 molL–1 NaHCO3 for 4 h to be saturated with sodium (Na+) and bicarbonate (HCO3–), respectively; (ii) shaken in 0.01 molL–1 ethylene-diaminetetraacetate for 4 h to allow the adsorption of micronutrients, in particular polyvalent metal cations such as aluminum, iron, manganese, copper, and zinc; and (iii) rinsed with deionized water. In each of the 20 plots monitored during the 2005 growing season (Table 1), four anion and four cation probes were installed (1 m west from selected spruce) to measure the amount of available N during 4 week periods starting in early May, early June, late June and late July. After 4 weeks, the probes were removed from the ground, collected, washed with deionized water, sealed in Ziploc1 bags, and sent to Western AG Innovations Inc., Saskatoon, Sask., for further analysis, as detailed by Hangs et. al. (2004): (i) elution of adsorbed ions for analytical measurement of N, using 1 mol/L KCl for 1 h to remove ‡95% of the adsorbed ions from the ion-exchange resin membrane; (ii) determine inorganic N (NH4+-N and NO3–-N) colorimetrically using a Technicon autoanalyzer II; and (iii) NO3–-N slightly modified by addition of NaOH to NH4Cl reagent (bringing the pH at 8.5) to neutralize the sample solution before its entry into the Cd-reduction column. Statistical analysis All statistical analyses were conducted using SAS software (SAS Institute Inc., Cary, N.C.). Treatment effects on all measured response variables (midsummer LAI; light; total soil nitrogen availability; root surface area and dry mass; and spruce root collar diameter (RCD), height, volume, and height/diameter ratio (HDR)), and all microclimate variables (air and soil temperature and volumetric water content) were analyzed using analysis of variance (ANOVA) based on a factorial arrangement of treatments (method of control type of control) in a completely randomized design with at least three replications. Planned orthogonal (a priori) contrasts (Table 2) were used to compare (i) method of control (spot vs. broadcast treatment), averaging over type of control; (ii) type of control (woody only vs. complete), averaging over method of control; (iii) the interaction between the type of vegetation control and the method of control; (iv) the mean treated response to the mean untreated response; and (v) two growing seasons of complete spot control to three growing seasons of complete spot control. In all cases, #
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Fig. 1. Seasonal trends of total vegetation leaf area index (above 5 cm height) between 5 May and 19 September 2005. BCC, broadcast complete control; SCC2, spot complete control 2 years; SCC, spot complete control; NC, no vegetation control; BWC, broadcast woodyonly control; SWC, spot woody-only control.
model residuals were examined to ensure that the ANOVA assumptions of homogeneity of variance and normality were met. Data transformations were not necessary. Regression analyses were used to explore relationships between soil moisture and LAI, between soil nitrogen availability and LAI, between root surface area or dry mass and LAI, and between the variables previously mentioned and spruce growth. Nonlinear regression analyses were used to examine relationships between spruce growth and environmental factors (soil moisture, soil N, light, air temperature, and soil temperature) or LAI and to examine relationships between environmental conditions and LAI. The NLIN procedure (SAS Institute Inc., Cary, N.C.) was used to fit ordinary least squares nonlinear models using the default Gauss– Newton iterative method with RHO set to 0.1. The relative offset convergence measure of Bates and Watts (the default) was used to determine convergence. Power and exponential models were tested for these relationships after examination of scatterplots of the data and based on other previous studies (Voicu and Comeau 2006; Filipescu and Comeau 2007a, 2007b). Scatterplots of residuals and approximated R2 values were used to determine appropriateness of selected models.
Results Leaf area index Mean total LAI values ranged from 0.15 to 3.12 m2m–2, with the highest values observed for no vegetation control (NC) and broadcast woody-only control (BWC) treatments (Fig. 1). On 9 July 2005, a hail storm resulted in a substantial reduction in overstory (mostly aspen) leaf area (approximately one-third of the leaf area was lost). Following the hailstorm, aspen leaf area did not fully recover during the growing season. LAI values of spot-applied complete vegetation control (first two years only, SCC2, and all four years,
SCC) were higher than broadcast complete vegetation control (BCC) becauseof the influence of vegetation beyond the 2 m treatment radius on LAI-2000 sensor readings (using LAI-2000 sensors with all five sensor rings active results in measurements to within 168 of the horizon). There was an overall treatment effect on midsummer (28 July 2005) total LAI (LAIt) (Table 3) (p < 0.01). LAIt in NC was higher than the mean LAIt across treated plots (p < 0.01). However, among treatments, LAIt levels among methods (spot vs. broadcast) were not consistent across the type of treatment (woody vs. complete control) (p < 0.01). This is indicated by the significance (p < 0.01) of the interaction term. Where woody vegetation was removed in a broadcast fashion (BWC), LAI reached the highest levels observed (3.12) because of increased cover of Calamagrostis canadensis. On the other hand, the BCC plots had the lowest LAIt values (0.15). Woody-only control generally resulted in increased LAIt; the extent of the increase was dependent on the type of control applied (5-fold for spot treatments and 20-fold for broadcast). Statistical results for LAI separated by vegetation components (LAIo, overstory, represented mostly by aspen, and LAIh, understory, represented mostly by herbaceous layer; Table 3) also suggested that treatments generally reduced LAI (p < 0.01). Overstory LAI was higher for NC than in all other treatments, while understory LAI was highest for BWC (p £ 0.01). The interaction for the herbaceous layer (p < 0.01) reflects the increase in LAI of the grass layer resulting from the removal of the woody component being higher for broadcast than for spot treatments, as described above for LAIt. No clear differences between 2 and 3 years of spot complete vegetation control were evident in 2005 (p ‡ 0.09). Roots Complete vegetation control reduced root surface area #
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Note: See Table 1 for treatment abbreviations. Contrasts are as follows: method, spot versus broadcast; type, complete versus woody; interaction, treated versus untreated (no vegetation control vs. all treatments), 2 years versus 3 years of complete spot vegetation control. LAIt, total leaf area index; LAIo, overstory leaf area index; LAIh, herbaceous leaf area index; HDR, height to diameter ratio.
0.49 0.74 0.25 0.51 0.48 0.11 1.33 0.43
