The correlation between the attainment of a certain minimal size and/or
complexity and the process of phase change has frequently been reported in the literature (e.g. Davidson and Remphrey, 1 990; Bostrack 1 993 ; Evans and Poethig,
1 993; D ay et al. 1 997; Sachs, 1 999; Sachs and N ovoplansky, !995; Eysteinsson and Greenwood, 1 993 ; Gilmore et aI., 1 995; Hatta et aI., 1 999; Honda et aI., 1 997; Huhn and Kleinshchmit, 1 993; McGowran et aI., 1 998; Nicolini et aI. , 1 993; Powell, 1 987; 1 992; Powell et aI., 1 995; Snowball et aI., 1 994; Takemoto and Greenwood, 1 993 . However, while these studies are generally in agreement that a correl ation between plant size and/or complexity and phase change exists, this conclusion tends to be intuitively based. Only limited biometrics and statistical proofs have been produced, as noted by Snowball ( 1 989).
Because the juvenile and adult plant states were often clearly distinguishable, many earlier studies (e.g. Libby and Hood, 1 976; Borchert, 1 976) described these architecture parameters, such as the canopy shape and long/short shoot
production, in colloquial terminology. However, this terminology was often appropriate only for a particular species, and was not always suitable for general descriptions of architectural differences associated with phase change (Borchert, 1 976). Poethig ( 1 990) concluded that tree architectural differences between ontogenetic plant states were often described in 'conventional' terminology, owing to the lack of methods to quantify tree architecture.
In the following review, a number of studies are noted that attempted to quantify plant architecture parameters so that statistical analysis could be applied to experimental data. However, the majority of these studies did not use common units to express plant size, often did not define rigorously the units used, and failed to address the complexity parameters of plant architectural structures. Various studies are reviewed with respect to their focus on examining the
reproductive plant state, consciously disregarding constraints of botanical classification.
Among the studies that attempted to quantify plant architecture, as opposed to giving qualitative descriptions of parameters, is that of Hanzawa and Kal isz ( 1 993). These authors examined the relationship between age, size and reproduction in the herbaceous p lant Trillium grand�jlorum . Leaf area and
rhizome volume were measured as parameters of plant size. It was concluded that these two size parameters were better predictors of plant reproductive status than plant age. Interestingly, these authors found that some plants that were above the reproductive size threshold did not flower. The authors maintained that while there was evidence of a certain size threshold for reproduction, other factors such as age or growth rate also affected the reproductive status of the plant. Young ( 1 985) arrived at a simil ar concl usion while studying size, growth rate and reproduction in Lobelia telekii.
Comparable conclusions were also reported by Garcia and Antor ( 1 995), who examined the age, size and reproductive status in populations of the long-lived Borderea pyrennaica. In this case, dry weight of tubers was used as a size parameter. The results indicated that j uvenile plants lacked flowers because they were too small to reproduce, and not because they were too young to reproduce. Bostrack ( 1 993) compared morphological features of adult leaves and leaves on sucker (juvenile) branches in four deciduous trees. He found significantly greater leaf area on juvenile branches than on canopy (adult) branches.
Bostrack ( 1 993) hypothesised that the position of sucker branches in relation to the root system, and thus the probable difference in water balance, affected the development of a large surface area of leaves on j uvenile branches. Also, total branch length, internode length and number of nodes per branch was
significantly greater for juvenile branches than for adult branches.
Examining the trade-off between reproduction and growth in Oenothera biennis, Reekie et al. ( 1 998) used the mass of harvested plants as a size parameter for correlation with reproduction. They concluded that mass was positively correl ated
with reproduction. However, at high densities, stem elongation, another possible size parameter, appeared to be an equally important factor. Reekie ( 1 998) believed reproductive allocation in plants to be strongly correlated with size, and quoted others who supported this view ( Lotz, 1 990; Mendez and Obeso, 1 993). In
perennial herbaceous plants of Plantago major, Reekie ( 1 998) used the dry weight of vegetative and reproductive organs as expressions of plant size. He found that reproductive outputs showed no relationship with size, while reproductive allocation ratio decreased with size. He pointed out that this decrease might be a consequence of the increase in reproductive cost with size. In this study,
controlled environment and photoperiod were used to manipulate the onset of reproduction.
Welham and Setter ( 1 998) in their study of size-dependent reproduction in dandelion ( Taraxacum officinale) used vegetative biomass as an expression of size. They found a positive linear relationship between vegetative mass and reproduction. Similarly, Reekie et at ( 1 997) categorised the canopies of Oenothera biennis into closed versus open, or of a small , medium and large biomass in their search for the trade-off between reproductive and vegetative growth.
Plant size and ontogenetic state was found to be correlated by Schmidt and Zotz (200 1 ) in a study of a heteroblastic bromeliad ( Vriesea sanguinoleta), which these authors interpreted as a size-related management of water storage. Other changes in anatomical, morphological and physiological properties were emphasised in relation to the increase in size, change from juvenile to adult state and associated heteroblastic changes.
Robinson and Wareing ( 1 969) believed that phase change was correlated with, but not dependent on, the attainment of a certain size, expressed as the d istance of a shoot meristem from the root system. Later, Wareing and Frydman ( 1 976) speculated that with the increasing distance of the apex from the roots, the effect of plant hormones produced in the roots declines, allowing for maturation to progress. Similarly, Snowball ( 1 989) and Snowball et al. ( 1 994a,b) measured the
number of vegetative nodes from the root, as a size parameter in single-stem Citrus plants, and found a correlation between size and phase change.
Poethig ( 1 990) posed the question of how plants maintain the coexistence of different developmental phases in a system that is constantly increasing in size and complexity. In this respect, Westwood ( 1 978) had envisaged tree growth and development and the coexistence of ontogenetic states within the tree structure as shown in Figure 1 .
/\<1 . . 11
. r ranMtl() n
- SCIOn bud
B
Figure 1.2 Representation of the coexistence of ontogenetic states within a tree structure A. A seedling tree with a j uvenile zone at the base and lower part of the tree, a transition zone in the mid-region, and an adult zone at top and periphery. B. A grafted or budded n ursery tree, which is entirely adult above the bud union. (Reproduced from Westwood, 1978).
Borchert ( 1 976) made conclusions relating directly to tree architecture and phase change. Addressing the shoot growth patterns of j uvenile and adult trees, he suggested that the reduction in shoot growth in adult trees could be a consequence of the increased complexity due to a larger numbers of shoots per tree. Focusing on the root/shoot ratio effect, he assumed that growth flushes would become
shorter and less frequent with increasing complexity, thus decreasing the vigour of adult trees.
Later, Borchert and Tomlinson ( 1 984) studied crown geometry and other architecture parameters in the bifurcating Tabebuia rosea, examining shoot phenology, crown symmetry and shape. Erect, narrow, and multi-tiered crowns, usually associated with the juvenile state were more efficient than broad open crowns, which were associated with the adult state. These suggestions were based on computer simulations of tree crowns that illustrated higher mechanical support costs in broad open crowns. With respect to shoot phenology the authors observed that in Tabebuia, growth flushing was markedly asynchronous among individual young trees, in contrast to synchronised flushes in adult plants. Moreover, the number of flushes appeared to be negatively correlated with tree size. Both descriptive and quantitative methods were used to observe changes in tree
architecture (Borchert and Tomlinson, 1 984). This early report appeared to be the most comprehensive approach to the issue of recording the dynamics of crown architecture, and addressed directly the phenomenon of phase change through study of architecture parameters. Interestingly, the authors also noted that
' Quantitative data on the changes in efficiency occurring with increasing tree size are hard to obtain without destructive sampling, and few, if any experimental
studies of these changes appear to exist ' (Borchert and Tomlinson 1 984, p. 964). Some studies of crown architecture, show a correlation between phase change and tree crown characteristics indirectly, using parameters such as the loss of apical dominance, flower initiation, loss of vigour, or apical abortion (Rackett, 1 985). For example, in an attempt to address both size and complexity in adult male and female green ash trees (Fraxinus pennsylvanica var. subintegerrima), Remphrey and Davidson ( 1 992) examined the relationship between branch length, branch order in relation to the trunk, and shape of the whole crown. They found that there was a negative correlation between branch length and increasing order, and that trees with sympodial growth (weak apical control) developed broader crowns than those with monopodial growth. They also elaborated on the correlation between
followed by loss of apical dominance. The authors of this study loosely correlated the effect of tree architecture on bud conversion to the floral state. However, no direct conclusion with respect to phase change was reached. Apart from dealing with the general characteristics associated with phase change, thi s study also represents a rare attempt to include the parameter of crown complexity.
Earlier, Davidson and Remphrey ( 1 990) used architecture parameters such as shoot length, numbers, diameter, and angle for comparisons between male and female trees of green ash. It was concluded that there are strong interrelationships between these architecture parameters and shoot tip abortion. Shoot tip abortion in M. excelsa is a distinct characteristic of the adult state (Dawson, 1 968a), and thus quantification of these features would be desirable in ontogenetic studies in this (but also other) species.
Similarly, Leakey and Longman ( 1 986) studied branching patterns as a function of apical dominance using decapitated 1 m tall Triplochiton scleroxylon trees. The authors concluded that branching, whilst not well understood, was a function of apical dominance. Two 'phases' of growth were identified: the 'dominance phase', in which uppermost shoots dominated and suppressed growth, and the 'sprouting phase' when many buds were released from inhibition. While their study was not directly related to ontogenesis, the growth type descriptions fitted some of the general characteristics of j uvenile and adult phases of tree growth. Some of these characteristics cover those of M. excelsa, and thus relate to the issue of size and complexity expression in trees.
Similarly, Baltunis and Greenwood ( 1 999) examined shoot elongation and phenology in larch (Larix spp.), and concluded that vigour, often also associated with juvenility (Hackett, 1 985), seemed to be partly a function of late growth cessation and/or increased duration of shoot elongation.