Niels P. R. Anten

Patterns of light and nitrogen distribution in relation to whole canopy carbon gain in C3 and C4 mono- and dicotyledonous species

An analytical model was used to describe the optimal nitrogen distribution. From this model, it was hypothesized that the non-uniformity of the nitrogen distribution increases with the canopy extinction rate for light and the total amount of free nitrogen in the canopy, and that it is independent of the slope of the relation between light saturated photosynthesis (Pm) and leaf nitrogen content (nL). These hypotheses were tested experimentally for plants with inherently different architectures and different photosynthetic modes. A garden experiment was carried out with a C3 monocot [rice, Oryza sativa (L.)], a C3 dicot [soybean, Glycine max (L.) Merr] a C4 monocot [sorghum, Sorghum bicolor (L.) Moensch] and a C4 dicot [amarantus, Amaranthus cruentus (L.)]. Leaf photosynthetic characteristics as well as light and nitrogen distribution in the canopies of dense stands of these species were measured. The dicot stands were found to have higher extinction coefficients for light than the monocot stands. Dicots also had more non-uniform N distribution patterns. The main difference between the C3 and C4 species was that the C4 species were found to have a greater slope value of the leaf-level Pm—nL relation. Patterns of N distribution were similar in stands of the C3 and C4 species. In general, these experimental results were in accordance with the model predictions, in that the pattern of nitrogen allocation in the canopy is mainly determined by the extinction coefficient for light and the total amount of free nitrogen.

Plant resistance to mechanical stress: evidence of an avoidance–tolerance trade-off

External mechanical forces resulting from the pressure exerted by wind or water movement are a major stress factor for plants and may cause regular disturbances in many ecosystems. A plants ability to resist these forces relies either on minimizing the forces encountered by the plant (avoidance strategy), or on maximizing its resistance to breakage (tolerance strategy). We investigated plant resistance strategies using aquatic vegetation as a model, and examined whether avoidance and tolerance are negatively correlated. We tested the avoidance-tolerance correlation across 28 species using a phylogenetically corrected analysis, after construction of a molecular phylogeny for the species considered. Different species demonstrated contrasting avoidance and tolerance and we demonstrated a significant negative relationship between the two strategies, which suggests an avoidance-tolerance trade-off. Negative relationships may result from costs that each strategy incurs or from constraints imposed by physical laws on plant tissues. The existence of such a trade-off has important ecological and evolutionary consequences. It would lead to constraints on the evolution and variation of both strategies, possibly limiting their evolution and may constrain many morphological, anatomical and architectural traits that underlie avoidance and tolerance.

Effects of Mechanical Stress and Plant Density on Mechanical Characteristics, Growth, and Lifetime Reproduction of Tobacco Plants

Plastic increases in stem elongation in dense vegetation are generally believed to be induced by canopy shading, but because plants protect each other from wind, shielding (reduced mechanical stress) could also play a role. To address this issue, tobacco Nicotiana tabacum plants were subjected to two levels of mechanical stress, 0 (control) or 40 (flexed) daily flexures, and grown solitarily, in a dense monostand (with plants of only one mechanical treatment), or in a mixed stand (flexed and control plants grown together). Flexed plants produced shorter and thicker stems with a lower Young’s modulus than control plants, while dense‐stand plants had relatively taller and thinner stems than solitary ones. Flexing effects on stem characteristics were independent of stand density. Growth, reproduction, and survival of solitary plants were not affected by flexing, while in the monostand growth was slightly reduced. But in the mixed stand, flexed plants were readily shaded by controls and had considerably lower growth, survival, and reproduction rates. These results suggest that wind shielding indeed plays a role in the plastic increase in stem elongation of plants in dense vegetation and that this response can have important consequences for competitive ability and lifetime seed production.

Defoliation and growth in an understory palm: Quantifying the contributions of compensatory responses

We analyzed to what extent and by what mechanisms plants of the tropical understory palm Chamaedorea elegans are able to mitigate the negative effects of defoliation on performance (i.e., plant size, total growth, leaf lamina growth, and reproduction) and how this is related to light availability. For this purpose we developed a new approach that allowed us to quantify the performance of defoliated plants relative not only to the performance of undamaged plants, but also relative to the estimated performance of hypothetical defoliated plants that do not exhibit any mechanisms of compensatory growth. The latter provides a way to quantify the adaptive value of compensation with reference to a hypothetical noncompensating alternative state. C. elegans plants were grown in a greenhouse at two light levels (5% and 16% of natural daylight) and subjected to five defoliation treatments (a control and four levels of defoliation). Defoliation was repeated every three months. Growth analysis revealed that defoliated...

The Role of Wood Mass Density and Mechanical Constraints in the Economy of Tree Architecture

By applying engineering theory, we found that in order to achieve a certain degree of stem mechanical stability, trees with low wood dry‐mass density (ρD) need to produce thicker stems but invest less mass per unit stem length than those with high ρD. Mechanical stability was expressed as the ability of the vertical stem to either support a plant’s weight (i.e., the buckling safety factor) or resist wind forces without rupture. This contradicts the general notion that trees with low ρD are more prone to mechanical failure. Contrary to our results for stems, we predicted that high ρD can be more efficient than low ρD in terms of the mass needed to produce a branch of given length and resistance to rupture under its own weight. Such branches were also predicted to be more flexible. These predictions were generally in accordance with literature data for tropical tree species. This shows that differences in scaling rules associated with vertical self‐loading, resistance to external forces, and the production of stable horizontal branches have important implications for the way in which different crown traits determine the balance between economy of crown design and mechanical stability.

Leaf nitrogen distribution in relation to leaf age and photon flux density in dominant and subordinate plants in dense stands of a dicotyledonous herb

Abstract We studied the effects of photon flux density (PFD) and leaf position, a measure of developmental age, on the distribution of nitrogen content per unit leaf area (Narea) in plants of different heights, in dense stands grown at two nitrogen availabilities and in solitary plants of the erect dicotyledonous herb Xanthium canadense. Taller more dominant plants received higher PFD levels and experienced a larger difference in relative PFD between their youngest and oldest leaves than shorter subordinate plants in the stands. Differences in PFD between leaves of solitary plants were assumed to be minimal and differences in leaf traits, found for these plants, could thus be mainly attributed to an effect of leaf position. In the solitary plants, Narea decreased with leaf position while in the plants from the stands it decreased with decreasing relative PFD, indicating both factors to be important in determining the distribution of Narea. Due to the effect of leaf position on Narea, leaves of subordinate plants had a higher Narea than older leaves of dominant plants which were at the same height or slightly higher in the canopy. Consequently, the Narea distribution patterns of individual plants plotted as a function of relative PFD were steeper, and probably closer to the optimal distribution which maximizes photosynthesis, than the average distribution in the stand. Leaves of subordinate plants had a lower mass per unit area (LMA) than those of dominant plants. In the dominant plants, LMA decreased with decreasing relative PFD (and with leaf position) while in the subordinate plants it increased. This surprising result for the subordinate plants can be explained by the fact that, during the course of a growing season, these plants became increasingly shaded and newer leaves were thus formed at progressively lower light availability. This indicates that LMA was strongly determined by the relative PFD at leaf formation and to a lesser extent by the current PFD. Leaf N content per unit mass (Nmass) was strongly determined by leaf position independent of relative PFD. This indicates that Nmass is strongly ontogenetically related to the leaf-aging process while changes in Narea, in response to PFD, were regulated through changes in LMA.

Detect thy neighbor: Identity recognition at the root level in plants

Some plant species increase root allocation at the expense of reproduction in the presence of non-self and non-kin neighbors, indicating the capacity of neighbor-identity recognition at the root level. Yet in spite of the potential consequences of root identity recognition for the relationship between plant interactions and community structure and functioning, this phenomenon still remains poorly understood. We first critically assess the evidence for the existence of self/non-self and kin recognition at the root level in plants. While root identity recognition most likely exists to some degree, there remain valid points of criticism regarding experiments that have documented this, particularly concerning the effects of pot volume in self/non-self recognition experiments and the roles of size inequality and asymmetric competition in kin recognition studies. Subsequently we review and propose some plausible physiological mechanisms that may underlie these responses. Finally we briefly discuss the relation between under- and aboveground interactions and the potential consequences of root identity recognition for agriculture, and conclude with raising several questions for future studies.

Light partitioning among species and species replacement in early successional grasslands.

Abstract We studied canopy structure, shoot architecture and light harvesting efficiencies of the species (photon flux captured per unit above-ground plant mass) in a series of exclosures of different age (up to 4.5 yr) in originally heavily grazed grassland in N Japan. Vegetation height and Leaf Area Index (LAI) increased in the series and Zoysia japonica, the dominant in the beginning, was replaced by the much taller Miscanthus sinensis. We showed how this displacement in dominance can be explained by inherent constraints on the above-ground architecture of these two species. In all stands light capture of plants increased with their above-ground biomass but taller species were not necessarily more efficient in light harvesting. Some subordinate species grew disproportionally large leaf areas and persisted in the shady undergrowth. Some other species first grew taller and managed to stay in the better-lit parts of the canopy, but ultimately failed to match the height growth of their neighbours in this early successional series. Their light harvesting efficiencies declined and this probably led to their exclusion. By contrast, species that maintained their position high in the canopy managed to persist in the vegetation despite their relatively low light harvesting efficiencies. In the tallest stands ‘later successional’ species had higher light harvesting efficiencies for the same plant height than ‘early successional’ species which was mostly the result of the greater area to mass ratio (specific leaf area, SLA) of their leaves. This shows how plant stature, plasticity in above-ground biomass partitioning, and architectural constraints determine the ability of plants to efficiently capture light, which helps to explain species replacement in this early successional series. Nomenclature: Makino (1962); Ohwi (1965). Abbreviations: LAI = Leaf area index; LAR = Leaf Area Ratio; LMR = Leaf Mass Ratio; PPFD = Photosynthetically active photon flux density; SLA = Specific Leaf Area.

Differences in the compensatory growth of two co-occurring grass species in relation to water availability

We compared the potential for compensatory growth of two grass species from the Mongolian steppe that differ in their ability to persist under grazing: the rhizomatous Leymus chinensis and the caespitose Stipa krylovii, and investigated how this ability might be affected by drought. Plants were grown in a greenhouse under wet and dry conditions and subjected to a clipping treatment (biweekly removal of 75–90% of the aerial mass). Leymus exhibited a much stronger compensatory growth after clipping than Stipa. Leymus showed a significant increase in its relative growth rate (RGR) after clipping, while for Stipa RGR was negatively affected. Clipped Leymus plants maintained leaf productivity levels that were similar to undamaged individuals, while leaf-productivity in clipped Stipa dropped to less than half of that of the controls. In Leymus, there was less compensatory growth under dry than under wet conditions, while in Stipa the compensation was increased under drought. This difference probably reflects the fact that Stipa is more drought-tolerant than Leymus. The greater compensatory growth of Leymus compared to Stipa mainly resulted from a greater stimulation of its net assimilation rate (NAR), and its greater capacity to store and reallocate carbohydrates by clipping. The greater increase in NAR was probably the result of a stronger reduction in self-shading, because Leymus shoots were much denser than those of Stipa, which resulted in a higher increase in light penetration to remaining leaves after clipping. The results of this study suggest that the greater ability of Leymus to persist under grazing is the result of its larger capacity for compensatory growth.

Limitations on photosynthesis of competing individuals in stands and the consequences for canopy structure

The canopy structure of a stand of vegetation is determined by the growth patterns of the individual plants within the stand and the competitive interactions among them. We analyzed the carbon gain of individuals in two dense monospecific stands of Xanthium canadense and evaluated the consequences for intra-specific competition and whole-stand canopy structure. The stands differed in productivity, and this was associated with differences in nitrogen availability. Canopy structure, aboveground mass, and nitrogen contents per unit leaf area (Narea) were determined for individuals, and leaf photosynthesis was measured as a function of Narea. These data were used to calculate the daily carbon gain of individuals. Within stands, photosynthesis per unit aboveground mass (Pmass) of individual plants increased with plant height, despite the lower leaf area ratios of taller plants. The differences in Pmass between the tallest most dominant and shortest most subordinate plants were greater in the high-nitrogen than in the low-nitrogen stand. This indicated that competition was asymmetric and that this asymmetry increased with nitrogen availability. In the high-nitrogen stand, taller plants had a higher Pmass than shorter ones, because they captured more light per unit mass and because they had higher photosynthesis per unit of absorbed light. Conversely, in the low-nitrogen stand, the differences in Pmass between plants of different heights resulted only from differences in their light capture per unit mass. Sensitivity analyses revealed that an increase in Narea, keeping leaf area of plants constant, increased whole-plant carbon gain for the taller more dominant plants but reduced carbon gain in the shorter more subordinate ones, which implies that the Narea values of shorter plants were greater than the optimal values for maximum photosynthesis. On the other hand, the carbon gain of all individual plants, keeping their total canopy N constant, was positively related to an increase in their individual leaf area. At the same time, however, increasing the leaf area for all plants simultaneously reduced the carbon gain of the whole stand. This result shows that the optimal leaf area index (LAI), which maximizes photosynthesis of a stand, is not evolutionarily stable because at this LAI, any individual can increase its carbon gain by increasing its leaf area.