Plant species thriving at habitats that differ in harshness or seasonality of climate show conspicuous phenotypic syndromes that demonstrate the adjustment of the plant body to contrasting thermal regimes. Comparisons of plant species living at low vs. high altitudes in temperate latitudes are illustrative examples. At canopy level, plant species from high altitudes tend to develop creeping and prostrate habits, with the living biomass close to the soil surface (Körner, 1999
). This habit confers more resistance to high seasonal and daily air temperature amplitudes and strong winds (Körner and Cochrane, 1983
). Also, the compaction of organs in condensed spaces protects against variable and harsh climates, modulating the microclimate to a point where the climate experienced by actual plant organs is buffered to a high degree (Körner, 1999
). Apart from thermal amplitudes, the shortness of the growth season is perhaps the most relevant constraint for life at high altitudes, so plants have developed long-lasting organogenetic periods that facilitate fast extension of pre-formed organs at the onset of the short growth season (Diggle, 1997
; Meloche and Diggle, 2003
; Cox, 2005
Considerable documentation on the above patterns contrasts with an almost total lack of knowledge on the evolutionary routes for a given genotype to generate a syndrome for adaptation to a short growth season. Architectural approaches may provide an insight in this regard. Simple architectural features impact on variation in pheno-morphology and in other life-history traits (Diggle, 1999
). For example, changes in the location of reproductive modules within shoots result in altered vegetative morphology (Wiltshire et al., 1994
; Comes and Kadereit, 1996
). Diggle (1997)
found that the architecture of high altitude plants and their ability to respond to environmental variation was highly dependent on pre-formation dynamics. She suggested that the pattern of development was crucial to understand plant adaptation to cold environments, and to gain knowledge of ecosystem processes in cold habitats.
The location and timing of bud differentiation with regard to reproduction and vegetative growth are poorly plastic as compared with continuous characters such as many leaf traits. It may be argued that this lack of plasticity downgrades their evolutionary role; however, the facts that (a
) variation in bud differentiation is of a genotypic nature, and is under the control of a set of organ identity genes (Zik and Irish, 2003
), and (b
) architecture indeed shows variation among genotypes of the same species, and is also responsive to the environment to a certain extent (McIntyre and Best, 1975
; Sachs and Novoplansky 1995
; Burgess et al., 2007
), support the potential evolutionary significance: inheritable variation occurs frequently enough to produce evolutionary divergences. Only the adaptive relevance of that variation needs to be unravelled.
The above has received close attention from an agronomic perspective, as a tool to select cultivars that evolved contrasting flowering phenologies or vegetative morphologies (see, for example, Jones, 1992
; Segura et al., 2008
). In addition, the location of reproductive and vegetative modules within the branch has significant consequences for fitness components as diverse as performance of offspring or light-capture efficiency (Fleming, 2005
; Lundgren and Sultan, 2005
; Prusinkiewicz et al., 2007
). Extensive knowledge of plant architecture and its developmental, morphological and genetic correlates has been accumulated since the middle of the 20th century (Hallé et al., 1978
; Sussex and Kerk, 2001
; Barthelemy and Caraglio, 2007
). Extending the developmental approach to the problem of the altitudinal segregation of wild species can provide insightful advances. To our knowledge, the use of the architectural approach to understand the segregation of closely related species across altitudes has not been explicitly addressed in detail.
In the present work, it is suggested that relatively subtle differences in branch design and vegetative vs. reproductive differentiation of buds can have far-reaching consequences for typical traits related to adaptation to altitude, such as plant compactness or bud pre-formation. This idea is explored making use of two series of Mediterraneo-Macaronesian Saxifraga
species within Sect. Saxifraga
, Subsect. Triplinervium
(namely ser. Ceratophyllae
and ser. Pentadactylis
). The five species of Ceratophyllae
and the 15 of ser. Pentadactylis
differ mainly in their shoot architecture (Vargas, 1991
species show determinate growth, with the apical meristem developing a terminate inflorescence, and vegetative new shoots developing from axillary meristems of the previous-year shoot. In contrast, in Ceratophyllae
species, the apical meristem develops a new vegetative shoot, while axillary meristems produce either lateral inflorescences or vegetative shoots. This architectural trait led to their classification in separate series within the subsection Triplinervium (see Fig. , and Vargas, 1991
Fig. 1. Typical reproductive branches of Saxifraga canaliculata (highland species) and Saxifraga trifurcata (lowland species) at the seed dispersal stage. Saxifraga canaliculata: 3-year-old branch with vegetative and reproductive branching, sampled at 1360 m (more ...)
The relevance of shoot architecture for altitudinal segregation and trait variation of these species is investigated in this study, making use of two distinct approaches. First, the hypothesis that the location of inflorescences within shoots can predict altitudinal segregation of Ceratophyllae and Pentadactylis species is tested. In order to do this, altitude data were collected for as many reliable and precise field records of Ceratophyllae and Pentadactylis specimens as possible. Secondly, the mechanistic causes of architectural segregation are explored by means of a selected case study within this genus. A detailed pheno-morphological study was carried out on two species of the Triplinervium subsection: Saxifraga trifurcata (ser. Ceratophyllae, lowland species with lateral inflorescences) and S. canaliculata (ser. Pentadactylis, highland species with terminal inflorescences), which are typical altitudinal vicariants in the north of the Iberian Peninsula. These species inhabit the same microhabitat, and share most of their morphological characters: both are evergreen, cushion-like chamaephytes, with helicoid phyllotaxy, acrotonic branching of non-reproductive axes, etc. (see Materials and Methods). The hypothesis is that plant architecture governs other structural and pheno-morphological derived traits that are well recognized to be adaptive in either lowland or highland environments. In Table , the particular predictions derived from this second hypothesis, their direct link with shoot architecture and the specific traits that were measured in this study are detailed.
Predictions and their relationship to branch architecture
If the above is supported, shoot architecture may help to understand plant species segregation patterns across altitudes beyond Saxifraga. To our knowledge, this idea has not been explicitly tested before.