Crown Construction, Leaf Dynamics, and Carbon Gain in Two Perennials with Contrasting Architecture

We used two co—occurring plant species of similar size and leaf morphology but with contrasting architecture to test if they would show specific patterns of leaf demography and leaf ecophysiology when they were grown in a range of experimental environments. Aster lanceolatus, a species with branched shoots, had exponentially increasing leaf populations. This led to high leaf turnover rates and to crowding of leaves at the top of the plants. Solidago canadensis, a species with unbranched shoots and rapid height growth, had linearly increasing populations of leaves, leading to a uniform vertical distribution of leaves and to lower leaf turnover rates. In comparison with the short—lived leaves of A. lanceolatus, the leaves of S. canadensis lived longer, contained more nitrogen and calcium per dry mass, and responded to changing light conditions within the plant crown by adjusting their angles towards the sun. Leaves of A. lanceolatus had an early peak in gas exchange activity followed by a rapid decline, whereas gas exchange activity in leaves of S. canadensis at the beginning of leaf development was lower than in A. lanceolatus but decreased only slightly with leaf age. Within the narrow vertical band in which leaves of A. lanceolatus were concentrated, young leaves at the top had higher mass per area, higher chlorophyll a/b ratios, and much higher rates of photosynthesis and conductance than the older leaves beneath. In S. canadensis, in which leaves were distributed over much of the vertical dimension along the plant, upper leaves, in contrast to lower (older) leaves, had lower mass per area, were held less perpendicular to the sun, and had higher rates of photosynthesis in high but not in low light. Because leaf populations grew exponentially in A. lanceolatus, assimilates could be re—invested; thus leaf number during the second half of the growing season was a good predictor of final biomass. In S. canadensis, with linearly growing leaf populations, the assimilates that were not re—invested in new leaves were allocated to the stem, and stem height or volume were good predictors of final biomass. Compared with the strong influence of architecture on the structure and dynamics of leaf populations, the effects of background species and fertilizer application were weak but nevertheless significant. Background species of similar aboveground mass but different stature (the tall Solidago altissima and the short Poa pratensis) affected leaf deployment (internode elongation, leaf orientation, and leaf survival), probably via competition for light, but not leaf quality (morphology, physiology) of the two target species A. lanceolatus and S. canadensis. Further, diameter—height allometries, which varied considerably among plants, tended to be flatter if the target species were grown in the tall than in the short background. Fertilizer application accelerated plant growth and phenological development. It increased light competition among leaves and leaf turnover; nevertheless, the photosynthetic water use efficiency of old leaves was higher in fertilized than in unfertilized plants. We suggest that the characteristic leaf dynamics of A. lanceolatus and S. canadensis are typical for the contrasting aboveground architectures these species represent. Based on the results of this and previous studies we suggest that an important driving force in the evolution of these complex adaptations is the degree of mixing within canopies of leaves of different plant genotypes. Leaves (and branches) are expected to be more autonomous if they frequently interact with leaves from other genotypes, as in the polyclonal patches of A. lanceolatus with its long and intermingling belowground rhizomes, than if they usually interact with leaves from the same genotype, as in the monoclonal patches of S. canadensis with its short rhizomes and compact belowground architecture.