Growth dominance varied substantially across similar plots in each treatment, and its mean values generally were at minuscule magnitudes and not significantly different from zero. This lack of a clear pattern in growth dominance does not agree with the hypothesis of Binkley (2004), who suggested that positive growth dominance occurs between canopy closure and old growth. The initially 15 to 74 years old stands in this study were expected to exhibit strong positive growth dominance as that reported in the mixed stands in coastal Oregon (Binkley 2004). Growth dominance generally increased, albeit remained largely negative, in relatively moist and dense western redcedar/western hemlock series but not in relatively dry grand fir and Douglas-fir series following fertilization in this study. Pronounced increases in growth dominance following fertilization and vegetation control have been reported in loblolly pine (Pinus taeda L.) stands in Florida (Martin and Jokela 2004). However, Tschieder et al. (2012) inferred that growth dominance should decrease in fertilized and irrigated young loblolly pine stands because the size-related differences in relative growth were diminished by higher resource availability for small trees.
The two types of young stands evaluated in Binkley et al. (2006) that were intended to support the hypothesis of Binkley (2004) also did not show significant positive growth dominance. Binkley et al. (2006) speculated that photosynthates shared through grafted roots of lodgepole pine and among ramets of aspen (Populus tremuloides Michx.) clones moderated potentially significant growth dominance. Natural root grafts occur in many tree species including most of those in this study (Graham and Bormann 1966). If this was the actual driving factor, it could be a cause of the lack of growth dominance observed in this study.
Alternative theories related the lack of growth dominance to wind stresses encountered by large lodgepole pine trees or their low growth efficiency (Binkley et al. 2006, Binkley and Kashian 2015), but mixed species in stands may have also played a role. Aspen stands mixed with understory conifers were found to have growth dominance around zero in pure aspen stands to negative values in mixed stands, where understory conifers kept high relative growth rates likely because of improved light interception and/or use efficiency (Binkley et al. 2006). Similarly in this study, shade-tolerant species generally started in lower crown positions with initial mean height at 8.8 m, while initial mean height for other species was 11.1 m. Shade-tolerant species remained in lower crown positions over the eight-year study period, and their relative growth apparently kept pace with large trees (Table 4, Figure 4). This may be a result of the large pine trees typically having clumped foliage arrangements that allow increased light to reach lower strata trees (Stenberg et al. 1994).
There may be species-specific disparities in developing growth dominance. For example, growth dominance was absent in lodgepole pine stands but accentuated in Eucalyptus saligna stands both across a wide range of stand ages (Binkley et al. 2006, Doi et al. 2010). McGown et al. (2015) found practically no growth dominance over a 35-year period in initially 40-70 years old ponderosa pine stands. Tschieder et al. (2012) concluded that competition generally is symmetrical in pine stands based on 21 years of observations on initially five years old loblolly pine stands, which resulted in null growth dominance. The abilities of species to develop growth dominance may also interact with site quality. The relatively small mid-tolerant Douglas-fir established growth dominance with improved proportions of stand volumes in moist western redcedar/western hemlock series but did not gain competitive advantages against intolerant pines in dry grand fir and Douglas-fir series (Table 4, Figure 5).
The phase of increasing growth dominance by large trees proposed by Binkley (2004) may apply to situations where light is the most limiting resource in tree growth, and large trees capture an increasingly larger proportion of light than small trees (Weiner 1990). Consequently, growth of large trees is sustained and accounts for an increasingly larger proportion of stand growth at the cost of the suppressed growth of small trees. This disproportionate growth of large trees will result in commensurate increases in transpiration and moisture demand (Maggard et al. 2017), but trees' abilities to compete for moisture are considered proportional to their sizes (Lieffers and Titus 1989, Weiner et al. 1997). In situations of limited moisture availability as may occur in grand fir and Douglas-fir series in this study, large trees would face relatively as much moisture deficiency as small trees, which would hamper the development of disproportionately large crowns and improved light interception. Competition for nutrients also is size-symmetric (Weiner et al. 1997, Nilsson et al. 2002), and nutrient deficiency is common in the Inland Northwest (Coleman et al. 2014). The alleviation of nutrient deficiency by fertilization generally did not improve growth dominance, especially in dry grand fir and Douglas-fir series. This may be an indication of the limiting role of soil moisture on mineralization of soil organic matter, release of nutrients, and nutrient transport in the soil-plant continuum (Cole et al. 1990).
Increased mortality following fertilization in Inland Northwest has been reported (e.g., Shen et al. 2001), although this mortality occurs more often among small trees mainly because improved stand growth following fertilization intensifies self-thinning (Barclay and Brix 1985, Elfving 2010). As a result, tree size distribution narrowed (CV decreased) following fertilization especially in western redcedar/western hemlock series of relatively high initial density (Table 1). Mortality noticeably increased among relatively large trees in dry grand fir and Douglas-fir series likely because of large trees facing similar moisture deficiency in the size-symmetric competition with small trees (Table 4, Figure 4; Weiner et al. 1997). This mortality had a potential effect to extend DBH distribution of remaining trees. Consequently, CV of DBH showed more decreases in western redcedar/western hemlock series than in the other series (Table 2). This observed pattern in CV followed its general trend during stand development (McGown et al. 2016), but Harrington and Devine (2011) found that fertilization broadened tree size distribution in western redcedar dominated stands. Harrington and Devine (2011) considered the cause to be the greatly improved growth of dominant trees and some small redcedar trees having survived the intensified competition but had stagnated growth. Both of these effects were not observed in this study.
Growth dominance does not only result from changes in relative growth of various-sized trees (Binkley et al. 2006). Since it is a statistic computed on survivor trees over a period (e.g., West 2014), mortality during this period directly affects growth dominance. For example, in a stand where large trees have initial growth dominance (i.e., account for a larger proportion of growth than volume), the death of a large tree will remove relatively more growth than volume and hence reduce the growth dominance of large trees. The lack of positive growth dominance in grand fir and Douglas-fir series may partly result from mortality observed among relatively large trees. This pattern occurred across all treatments including the control. The minuscule and not significant growth dominance may also be a result of the distribution of growth. It clearly differed between large and small trees in some previous studies (e.g., Martin and Jokela 2004, Fernández et al. 2011), but both the largest and smallest 25% trees generally accounted for ≥25% of stand growth in this study (Table 3), which had a neutralizing effect on growth dominance. This pattern was consistent across fertilization treatments and vegetation series where site quality and stand age clearly differed. Improved nutrient availability may not be the sole cause since this pattern also was observed in unfertilized plots (Tschieder et al. 2012). Species-specific potentials in developing growth dominance and limited moisture availability may also be contributing factors.
The metric of growth dominance has been considered a useful tool to quantitatively assess the effectiveness of silvicultural treatments such as thinning (Bradford et al. 2010, Keyser 2012). It was found that thinning eased competition and favored growth of small rather than large trees, although the goal of thinning often is to concentrate growth on large crop trees (Nyland 2016). The general effect of fertilization found in this study is similar to that of thinning. The largely negative growth dominance across fertilization treatments indicates that small trees maintained higher relative growth rates than large trees, even if absolute growth and size was concentrated in large trees. In the case fertilization does improve stand growth, a significant part of this improved growth will be lost in density-dependent mortality over time if not captured through biomass removals.
A caveat is that growth dominance found in this study and some previous studies (e.g., Binkley et al. 2006, Bradford et al. 2010, Tschieder et al. 2012, McGown et al. 2015) is of very small magnitudes. Growth dominance is computed based on volume, biomass, or other dimensions of trees, which generally are not directly measurable and are predicted using various empirical models. The minuscule growth dominance, regardless positive or negative, likely falls within the uncertainties of these models, and may not be able to provide quantitative and even qualitative information to assess the efficacy of silvicultural treatments as proposed by Bradford et al. (2010). In the case of patterns existing in these uncertainties across, e.g., stand structure, composition, and site quality, growth dominance statistics may also be biased according to these patterns, and silvicultural prescriptions based on growth dominance statistics may be misguided. Finally, growth dominance may be evaluated with improved accuracy using data from large sample plots. Otherwise, growth dominance may be overly affected by rare mortality observations. The relatively small plots with an average of ~30 trees likely caused growth dominance to vary in wide ranges across plots in this study.