Do Interspecific Differences in Sapling Growth Traits Contribute to the Co-dominance of Acer saccharum and Fagus grandifolia?

BACKGROUND AND AIMS: Acer saccharum and Fagus grandifolia are among the most dominant late-successional tree species in North America. The influence of sapling growth responses to canopy gaps on the co-dominance of the two species in an old-growth forest in southern Quebec, Canada was examined. Two predictions were evaluated: (a) F. grandifolia is more shade tolerant than A. saccharum due to greater sapling leaf area and net production per sapling in closed-canopy conditions; and (b) the height growth rate of A. saccharum in canopy gaps is greater than that of F. grandifolia due to increased net production per sapling. METHODS: Sapling crown allometry, net production and height growth rates were compared between and within the two species in closed canopy vs. canopy gaps. Standardized major axis regression was used to analyse differences in crown allometry. KEY RESULTS: F. grandifolia had greater crown projection, sapling leaf area and net production rate per sapling than A. saccharum in closed-canopy conditions. In response to canopy gaps, net production per sapling increased to the same degree in both species. The net production per sapling of F. grandifolia thus was much greater than that of A. saccharum in both canopy gap and closed-canopy conditions. The height growth rate of both species increased in canopy gaps, but the degree of increase was greater in F. grandifolia than in A. saccharum. CONCLUSIONS: F. grandifolia regenerated more successfully than A. saccharum in both closed-canopy conditions and canopy gaps, which indicates that the co-dominance of the two species cannot be maintained simply by interspecific differences in shade tolerance and growth in gaps. Previous research showed that although Fagus and Acer shared dominance at this site, their relative dominance shifted with edaphic conditions. This suggests that the widespread co-dominance of the two species in eastern North American forests is maintained by the joint influence of canopy disturbance and species-specific responses to the heterogeneity of moisture and fertility regimes within forested landscapes.     

Isolated Trees and Grass Removal Improve Performance of Transplanted Trema micrantha (L.) Blume (Ulmaceae) Saplings in Tropical Pastures

Cattle pastures established in areas previously covered by tropical rainforest can be abandoned in unproductive and degraded conditions. Transplanting native tree species to pastures is one common practice among rainforest restoration techniques, but several environmental barriers compromise transplant success. We assessed whether the presence of isolated trees and the removal of pasture grasses affect survival, growth, and allometry of transplanted saplings of the pioneer tree Trema micrantha (L.) Blume (Ulmaceae) into abandoned pastures in southeast Mexico. An isolated tree was selected in the center of each of four pasture sites of one hectare, and grass treatments were applied under the tree's canopy (0–10 m from the trunk) and in open pasture (15–48 m from the trunk). Grass removal treatments were control (grass present), cut with machete, herbicide application, and total grass removal with a gardening hoe. After 1 year, sapling survival was not different between the canopy and pasture areas (53%). Saplings showed higher survival probability (p < 0.05) in the hoe treatment (63 ± 9%) than in the control treatment (38 ± 9%). Height and crown cover growth rates were faster under the canopy of isolated trees compared with the open pasture. Saplings showed significantly greater crown area/height ratios under the canopy of isolated trees. Stressful environmental conditions restricted sapling growth in the open pasture. We conclude that complete removal of grasses and transplanting T. micrantha saplings in the vicinity of isolated trees can improve transplant success.     

辽东栎林内不同小生境下幼树植冠构型分析

以黄土高原黄龙山林区辽东栎林内3个小生境（林下、林隙、林缘）下辽东栎天然更新幼树为研究对象,采用典型抽样法对辽东栎幼树侧枝、叶片和树冠的空间分布状况以及生物量分配状况进行调查分析,探讨微生境与幼树植冠构型特征的关系,明确辽东栎幼树对不同小生境的适应策略,为栎林经营和林分结构优化提供理论依据。结果显示：（1）3种生境下辽东栎幼树构型发生了可塑性变化,林下幼树树冠层次比较单一,林隙与林缘的幼树树冠层次更加丰富。（2）由林下至林缘,幼树的树高、枝下高呈逐渐减小的趋势,而地径变化趋势与之相反;幼树的冠幅、树冠面积、树冠率呈先增加后减小的趋势,并且林下与林隙、林缘的差异显著;幼树的总体分枝率、逐步分枝率、枝径比呈先增加后减小的趋势。（3）3种生境下,幼树的一级枝的枝长、直径与倾角随着树高的增加而呈减小的趋势,但3种生境的差异不显著;林下一级枝主要分布在冠层中上部,而林隙与林缘一级枝主要分布在冠层中下、中上部。（4）由林下至林缘幼树叶长、叶宽、单叶面积和比叶面积逐渐降低,而单株叶数、叶总面积、叶面积指数呈先增大后减小趋势;与其他2种生境相比林下叶片分布趋于冠层上部。（5）幼树地上部分生物量中林下主干生物量占83%,枝和叶生物量只占17%;而林隙与林缘虽然各部位生物量有所差异但比例基本一致,其中主干占66%左右,枝和叶生物量占34%左右。研究表明,林隙生境下幼树的构型优于林缘和林下生境,在今后栎林的经营中,可以通过适当间伐来增加林隙数量,为森林更新和结构的优化提供有利条件。     

Allometry, root/shoot ratio and root architecture in understory saplings of deciduous dicotyledonous trees in central Japan

Plant allometry that is related to plant architecture and biomass allocation strongly influences a plants ability to grow in shaded forest understory. Some allometric traits can change with plant size. The present study compared crown and trunk allometries, root/shoot biomass allometry, and root architecture among understory saplings (0.25--5m height, except for two trees > 5 < 7 m) of seven deciduous dicotyledonous species in central Japan. Associations of the crown and trunk allometries with several plant morphological attributes were analyzed. Branch morphology (plagiotropyvs orthotropy) and life size were correlated with sapling crown and trunk allometries. Both large leaves and orthotropic branches were associated with a narrow small crown and slender trunk. The root/shoot ratio decreased rapidly with increasing plant height for saplings shorter than about 1.5 m. Less shade-tolerant species tended to have smaller root/shoot ratios for saplings taller than 1.5 m. With an increase in plant height, the branch/trunk biomass ratio decreased for saplings with plagiotropic branches but increased for saplings with orthotropic branches. Four subcanopy species (Acer distylum, Carpinus cordata, Fraxinus lanuginosa and Acanthopanax sciadophylloides) had superficial root systems; a common understory species (Sapium japonica) had a deep tap root system; and a canopy species (Magnolia obovata) and a subcanopy species (Acer tenuifolium) had heart root systems of intermediate depth. The root depth was not related to shade tolerance. Among species of the same height, the difference in fine root length can be 30-fold.     

Vertical distribution of epiphytic bryophytes in an Indonesian rainforest

We studied species richness, composition and vertical distribution of epiphytic bryophytes in submontane rainforest of Central Sulawesi. Bryophytes were sampled on eight canopy trees and on eight trees in the forest understorey. Microclimate was measured at trunk bases and at crown bases. The total recorded number of 146 epiphytic bryophyte species is among the highest ever reported for tropical forests and underlines the importance of the Malesian region as a global biodiversity hotspot. Species composition differed significantly between understorey trees and canopy tree trunks on the one hand, and the forest canopy on the other. Fourty-five percent of the bryophyte species were restricted to canopy tree crowns, 12% to the understorey. Dendroid and fan-like species mainly occurred in the forest understorey whereas tufts were most species rich in the tree crowns. The findings reflect the different microclimatic regimes and substrates found in the understorey and in the forest canopy. The results indicate that assessments of the bryophyte diversity of tropical forests are inadequate when understorey trees and tree crowns are excluded.     

Ontogenetic changes in size, allometry, and mechanical design of tropical rain forest trees

Size, allometry, and mechanical design were measured for trees of three canopy species in a tropical rain forest in French Guiana. Mechanical design was expressed as the safety factor, using the elastic-stability model, and the wind resistance factor, using the constant-stress model. Changes with ontogeny were described as regressions using stem diameter as the independent variable, and they were compared between species. Height, crown size, and the wind resistance factor increased with ontogeny. The safety factor decreased to a minimum and then increased continuously in thicker trees. The crown width/height ratio did not change with ontogeny. Interspecific differences in allometry and mechanical design were related to the adult stature of the species, and not to shade tolerance. The short stature species (Vouacapoua americana) was less slender (height:DBH [stem diameter at 1.3 m] ratio) and had a higher crown width/height ratio than the tall stature species (Goupia glabra and Dicorynia guianensis). Vouacapoua had a higher safety factor, but a similar wind resistance factor. The safety factors of our study species were lower than those of two temperate tree species because of a higher slenderness. Differences in safety factors between tropical and temperate trees may result from unrealistic assumptions of the elastic-stability model, and may also be related to lower light levels and-or wind rates in the tropics.     

Differences in sapling architecture betweenFagus crenata andFagus japonica

The ecological significance of architectural patterns for saplings ofFagus crenata andFagus japonica co-occurring in a secondary oak forest were evaluated by comparing the size and shape of leaves, trunks and crowns.Fagus japonica saplings were different fromF. crenata saplings in some architectural properties: (i) the leaf area and specific leaf area were larger; (ii) the ratio of sapling height to trunk length was lower, indicating greater leaning of the trunk; and (iii) the projection area of the crown was larger and the leaf area index lower indicating less mutual shading of leaves. These architectural features indicated thatF. japonica saplings were more shade tolerant thanF. crenata andF. crenata saplings were superior toF. japonica for growth in height and could, therefore, utilize sunlight in the upper layer. An erect trunk inF. crenata and a leaning trunk inF. japonica may be important characteristics associated with the regenerations patterns of each species; regeneration from seedlings under canopy gaps in the former and vegetative regeneration by sprouting in the latter.     

Plastic changes of leaf mass per area and leaf nitrogen content in response to canopy openings in saplings of eight deciduous broad-leaved tree species

Leaf nitrogen content per area (N_area) is a good indicator of assimilative capacity of leaves of deciduous broad-leaved trees. This study examined the degrees of increase in N_area in response to canopy openings as leaf mass per area (LMA) and leaf nitrogen content per mass (N_mass) in saplings of eight deciduous broad-leaved tree species in Hokkaido, northern Japan. Five of the species were well-branched species with a large number of small leaves (lateral-growth type), and the other three species were less-branched species with a small number of large leaves (vertical-growth type). The degrees of increase in N_area were compared between the two crown types. In closed-canopy conditions, leaves of the vertical-growth species tended to have a lower LMA and higher N_mass than those of the lateral-growth species, which resulted in similar N_area for both. LMA increased in canopy openings in the eight species, and the degrees of increase were not largely different between the lateral- and vertical-growth species. On the contrary, N_mass was unchanged in canopy openings in the eight species. As a result, N_area of each species increased in canopy openings in proportion to the increase in LMA, and the degrees of increase in N_area were similar in the lateral- and vertical-growth species. Therefore, this study showed that the degrees of increase in N_area were not correlated with the crown architecture (i.e., the lateral- and vertical-growth types).     

Developmental Patterns of Tree Dimensions in a Neotropical Deciduous Forest1

For 26 tree species in very dry tropical forest in Mexico, the developmental trends of relationships among trunk diameter, tree height, and crown diameter were inferred from a one‐time measurement of dispersed individuals across the size range from saplings to large, mature trees. On hillside sites in this high diversity forest, maximum dimensions were usually <10‐m height, 4‐m crown diameter, and 0.3‐m trunk diameter. The relationship of height to trunk diameter was characterized by an asymptotic, three‐parameter model. Crown diameter was a linear function of trunk diameter. The parameter values for both models varied widely among the species. In general, the dispersion among species of the height–crown diameter relationship increased linearly with trunk diameter (up to 0.2 m). Arborescent cacti were distant from other species at all sizes, although they were well modeled using the same equations. Empirical and theoretical features and limitations of the present and previous models, including mechanical buckling and water‐stress theories, are considered.     

Inter- and intraspecific relationships between shade tolerance and shade avoidance in temperate trees

Patterns of physiological and architectural adaptation and acclimation to decreasing light availability were investigated along a light gradient for saplings of 12 common species of temperate deciduous trees in southeastern Ontario, Canada. Physiological adaptation and acclimation (shade tolerance physiology) were quantified at the leaf level by measuring leaf mass per unit area (LMA), dark respiration per unit leaf nitrogen, chlorophyll per unit leaf nitrogen and the chlorophyll a:b ratio for the newest fully expanded leaf on the leader. Architectural adaptation and acclimation (shade avoidance) were quantified by measuring branching intensity and side shoot:main shoot length ratios for the most recent three years of growth on the leader and selected side branches. Within species, increases in LMA, chlorophyll a:b ratio and respiration per unit nitrogen and decreases in chlorophyll per unit nitrogen indicated that shade tolerance physiology generally increased with decreasing canopy openness. Increases in the branching intensity and side shoot:main shoot ratios of the leader and side branches indicated that shade avoidance also increased with decreasing canopy openness for the majority of species; however, in some species, stem bending under deep shade resulted in lateral growth. Interspecific variation in shade tolerance physiology was minimal when species were compared under equal amounts of canopy openness. In contrast, interspecific variation in shade avoidance variables was relatively high under equal canopy openness, with saplings of shade tolerant canopy species exhibiting higher shade avoidance than saplings of shade tolerant understory species.     

Influence of canopy tree size on stand basal area may reflect uncoupling of crown expansion and trunk diameter growth

Abstract A recent article by Midgley and colleagues suggests that large trees give rise to inordinately high stand basal areas because they pack canopy space more efficiently than smaller trees. We argue that this phenomenon bears more relation to the fact that diameter increment is not necessarily accompanied by significant crown expansion during all stages of a tree's life. Using data from a canopy tree population in an old‐growth temperate forest, we found that crown area scaled as roughly the 3/5 power of trunk basal area. Rather than reflecting fixed scaling laws, we suggest that this pattern arises because of limited opportunities for crown expansion in dense stands. Old canopy trees in dense stands can thus accumulate large basal areas without occupying a commensurately large canopy area.     

Growth traits of juvenile American chestnut and red oak as adaptations to disturbance

American chestnut (Castanea dentata) was a dominant species in eastern North America prior to the importation of chestnut blight. In light of recent efforts to restore viable populations of chestnut in eastern forests, an increased understanding of its association with other co‐occurring, disturbance‐adapted oak species is necessary. We evaluated crown architecture and leaf morphology in juvenile chestnut and red oak (Quercus rubra) to assess potential differences in establishment strategies of both species. We also investigated differences in nonstructural carbohydrate reserves and whole tree biomass partitioning between species. Seedlings of both species were planted in forest stands treated either with midstory removal or small patch cuts, simulating potential restoration plantings. After 5–7 years, chestnut's allocation to its root system was lower than red oak's, with chestnut saplings instead diverting resources to branches and foliage. Chestnut had lower leaf area index, greater crown projection area, and higher specific leaf area than red oak, indicating the species may have an advantage in shaded understories. There were only minor differences in nonstructural root carbohydrate reserves, between red oak and American chestnut, indicating that chestnut may respond similarly to oak by resprouting after disturbances topkill young saplings. We suggest that American chestnut has morphological and physiological attributes that allow it to function as an opportunistic and plastic species that can utilize gaps to facilitate its canopy recruitment, yet still persist after occasional surface fire. This knowledge can guide restoration strategies for this iconic species of the eastern temperate forest region.     

Branch architecture,light interception and crown development in saplings of a plagiotropically branching tropical tree,Polyalthia jenkinsii (Annonaceae)

To investigate crown development patterns, branch architecture, branch-level light interception, and leaf and branch dynamics were studied in saplings of a plagiotropically branching tree species, Polyalthia jenkinsii Hk. f. & Thoms. (Annonaceae) in a Malaysian rain forest. Lengths of branches and parts of the branches lacking leaves ('bare' branches) were smaller in upper branches than in lower branches within crowns, whereas lengths of 'leafy' parts and the number of leaves per branch were larger in intermediate than in upper and lower branches. Maximum diffuse light absorption (DLA) of individual leaves was not related to sapling height or branch position within crowns, whereas minimum DLA was lower in tall saplings. Accordingly, branch-level light interception was higher in intermediate than in upper and lower branches. The leaf production rate was higher and leaf loss rate was smaller in upper than in intermediate and lower branches. Moreover, the branch production rate of new first-order branches was larger in the upper crowns. Thus, leaf and branch dynamics do not correspond to branch-level light interception in the different canopy zones. As a result of architectural constraints, branches at different vertical positions experience predictable light microenvironments in plagiotropic species. Accordingly, this pattern of carbon allocation among branches might be particularly important for growth and crown development in plagiotropic species.     

INFLUENCE OF NEIGHBORS ON TREE FORM: EFFECTS OF LATERAL SHADE AND PREVENTION OF SWAY ON THE ALLOMETRY OF LIQUIDAMBAR STYRACIFLUA (SWEET GUM)

Liquidambar styraciflua (sweet gum) saplings were open grown (control), guy-wired to prevent wind-induced sway (constrained), or guy-wired within open-topped shade cloth cylinders to simulate shading by neighboring trees (shaded). Mechanical stability was assessed after 2 years of growth by field measurement of whole-tree flexibility and with the critical buckling height predictions of five allometrical models. Data from this experiment are supplemented with measurements of sweet gum trees growing in a 9-year-old pine plantation; individuals crowded but not overtopped by pines are compared with similar-aged trees growing in openings within the plantation. Changes in aboveground architecture were stimulated by both prevention of sway and lateral shade but the effect of the latter was most pronounced; shaded saplings approached or exceeded their calculated critical height and buckled under their own weight when the shade cloth cylinders were removed. Rates of aboveground biomass acquisition, total leaf area, wood density, and wood elastic moduli did not differ among the three treatments; changes in stem allometry accounted for most of the observed differences in tree stability. Sweet gum trees growing in a pine plantation responded similarly to neighbors; crowded trees had larger height: diameter ratios and more closely approached their calculated critical heights than did trees growing in the open. Sweet gum saplings growing in dense stands of similar-sized plants may surmount their neighbors by allocating relatively more wood to height growth than to girth increment.     

Leaf size, sapling allometry, and Corner's rules: phylogeny and correlated evolution in maples (Acer)

We studied the evolution of leaf size, sapling canopy allometry, and related traits in 17 Acer species growing in the understory of temperate deciduous forests, using parsimony methods, randomization tests, and independent contrasts calculated on idena phylogeny inferred from nuclear ribosomal internal transcribed spacer (ITS) sequences. Bivariate correlations and multivariate analyses indicated two independent suites of coevolving traits, and the results were robust over a range of alternative phylogenies. The first suite consisted of strong positive correlations among twig thickness, leaf size, inflorescence length, and branch spacing (Corner's rules). Seed size and mature height were also weakly corre- lated with these traits. The second suite reflected aspects of sapling crown allometry, including crown size, stem diameter, and total leaf area, which appear to be related to shade tolerance. There was a weak negative correlation between sapling crown size and mavegetative ture height, but no correlation with leaf or seed size. Most correlattion were similar in magnitude for ahistorical and independent contrasts analyses, and discrepancies between these two measures were greater in traits with lower levels of convergent evolution. The evolutionary correlations among twig, leaf, seed, inflorescence, and canopy dimensions emphasize the need for integrated theories of evolution and function of these disparate traits.     

How liana loads alter tree allometry in tropical forests

Intense competition with lianas (wood climbers) can limit tree growth, reproduction, and survival. However, the negative effects of liana loads on tree allometry have not yet been addressed. We investigated the hypothesis that liana loading on tree crown alters tree’s allometry, expressed through slenderness (height–diameter ratio). The relationship between trunk slenderness and percentage of tree crown covered by lianas was investigated for 12 tree species from 10 fragments of the Semideciduous Seasonal Forest in Southeastern Brazil. We also tested whether the relationship between slenderness and wood density differ between trees without lianas and trees heavily infested. Liana loads significantly altered tree allometry by decreasing slenderness, even when lianas covered less than 25% of tree crown. Heavy-wood species decreased their trunk slenderness in a greater ratio than light-wood species. Our findings indicate that liana infestation shifts tree allometry, and these effects are stronger on heavy-wood tree species.     

Age‐related Crown Thinning in Tropical Forest Trees

Gap dynamics theory proposes that treefall gaps provide high light levels needed for regeneration in the understory, and by increasing heterogeneity in the light environment allow light‐demanding tree species to persist in the community. Recent studies have demonstrated age‐related declines in leaf area index of individual temperate trees, highlighting a mechanism for gradual changes in the forest canopy that may also be an important, but less obvious, driver of forest dynamics. We assessed the prevalence of age‐related crown thinning among 12 tropical canopy tree species sampled in lowland forests in Panama and Puerto Rico (total N = 881). Canopy gap fraction of individual canopy tree crowns was positively related to stem diameter at 1.3 m (diameter at breast height) in a pooled analysis, with 10 of 12 species showing a positive trend. Considered individually, a positive correlation between stem diameter and canopy gap fraction was statistically significant in 4 of 12 species, all of which were large‐statured canopy to emergent species: Beilschmiedia pendula, Ceiba pentandra, Jacaranda copaia, and Prioria copaifera. Pooled analyses also showed a negative relationship between liana abundance and canopy gap fraction, suggesting that lianas could be partially obscuring age‐related crown thinning. We conclude that age‐related crown thinning occurs in tropical forests, and could thus influence patterns of tree regeneration and tropical forest community dynamics.     

Composition and Dynamics of Functional Groups of Trees During Tropical Forest Succession in Northeastern Costa Rica

We compared the functional type composition of trees ≥10 cm dbh in eight secondary forest monitoring plots with logged and unlogged mature forest plots in lowland wet forests of Northeastern Costa Rica. Five plant functional types were delimited based on diameter growth rates and canopy height of 293 tree species. Mature forests had significantly higher relative abundance of understory trees and slow-growing canopy/emergent trees, but lower relative abundance of fast-growing canopy/emergent trees than secondary forests. Fast-growing subcanopy and canopy trees reached peak densities early in succession. Density of fast-growing canopy/emergent trees increased during the first 20 yr of succession, whereas basal area continued to increase beyond 40 yr. We also assigned canopy tree species to one of three colonization groups, based on the presence of seedlings, saplings, and trees in four secondary forest plots. Among 93 species evaluated, 68 percent were classified as regenerating pioneers (both trees and regeneration present), whereas only 6 percent were classified as nonregenerating pioneers (trees only) and 26 percent as forest colonizers (regeneration only). Slow-growing trees composed 72 percent of the seedling and sapling regeneration for forest colonizers, whereas fast-growing trees composed 63 percent of the seedlings and saplings of regenerating pioneers. Tree stature and growth rates capture much of the functional variation that appears to drive successional dynamics. Results further suggest strong linkages between functional types defined based on adult height and growth rates of large trees and abundance of seedling and sapling regeneration during secondary succession.
Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp     

山西濒危植物翅果油树植冠的构型分析

应用分形理论及方法,从分枝格局和冠幅扩展2个方面对山西翼城和乡宁的35株翅果油树(Elaeagnus mollis Diels)个体的植冠构型进行统计分析。结果表明,幼树和成树的总体分枝率和逐步分枝率有显著变化;一级枝的平均枝长和枝径有显著差异,但枝和叶的方位角及叶倾角差异不明显。不同发育阶段个体的冠幅变化较大,幼树冠幅的分形维数(2.0026)小于成树(2.2694)。翅果油树在不同生长发育阶段对生境变化有不同的构型策略。     

Above-ground biomass investments and light interception of tropical forest trees and lianas early in succession

BACKGROUND AND AIMS: Crown structure and above-ground biomass investment was studied in relation to light interception of trees and lianas growing in a 6-month-old regenerating forest. METHODS: The vertical distribution of total above-ground biomass, height, diameter, stem density, leaf angles and crown depth were measured for individual plants of three short-lived pioneers (SLPs), four long-lived pioneers (LLPs) and three lianas. Daily light interception per individual Phi(d) was calculated with a canopy model. The model was then used to estimate light interception per unit of leaf mass (Phi(leaf mass)), total above-ground mass (Phi(mass)) and crown structure efficiency (E(a), the ratio of absorbed vs. available light). KEY RESULTS: The SLPs Trema and Ochroma intercepted higher amounts of light per unit leaf mass (Phi(leaf mass)) because they had shallower crowns, resulting in higher crown use efficiency (E(a)) than the other species. These SLPs (but not Cecropia) were also taller and intercepted more light per unit leaf area (Phi(area)). LLPs and lianas had considerably higher amounts of leaf mass and area per unit above-ground mass (LMR and LAR, respectively) and thus attained Phi(mass) values similar to the SLPs (Phi(mass)=Phi(area)xLAR). Lianas, which were mostly self-supporting, had light interception efficiencies similar to those of the trees. CONCLUSIONS: These results show how, due to the trade-off between crown structure and biomass allocation, SLPs, and LLPs and lianas intercept similar amount of light per unit mass which may contribute to the ability of the latter two groups to persist.