Macro-Climatic Distribution Limits Show Both Niche Expansion and Niche Specialization among C4 Panicoids

Grasses are ancestrally tropical understory species whose current dominance in warm open habitats is linked to the evolution of C₄ photosynthesis. C₄ grasses maintain high rates of photosynthesis in warm and water stressed environments, and the syndrome is considered to induce niche shifts into these habitats while adaptation to cold ones may be compromised. Global biogeographic analyses of C₄ grasses have, however, concentrated on diversity patterns, while paying little attention to distributional limits. Using phylogenetic contrast analyses, we compared macro-climatic distribution limits among ~1300 grasses from the subfamily Panicoideae, which includes 4/5 of the known photosynthetic transitions in grasses. We explored whether evolution of C₄ photosynthesis correlates with niche expansions, niche changes, or stasis at subfamily level and within the two tribes Paniceae and Paspaleae. We compared the climatic extremes of growing season temperatures, aridity, and mean temperatures of the coldest months. We found support for all the known biogeographic distribution patterns of C₄ species, these patterns were, however, formed both by niche expansion and niche changes. The only ubiquitous response to a change in the photosynthetic pathway within Panicoideae was a niche expansion of the C₄ species into regions with higher growing season temperatures, but without a withdrawal from the inherited climate niche. Other patterns varied among the tribes, as macro-climatic niche evolution in the American tribe Paspaleae differed from the pattern supported in the globally distributed tribe Paniceae and at family level.     

Climate, phylogeny and the ecological distribution of C4 grasses

'C4 photosynthesis' refers to a suite of traits that increase photosynthesis in high light and high temperature environments. Most C4 plants are grasses, which dominate tropical and subtropical grasslands and savannas but are conspicuously absent from cold growing season climates. Physiological attributes of C4 photosynthesis have been invoked to explain C4 grass biogeography; however, the pathway evolved exclusively in grass lineages of tropical origin, suggesting that the prevalence of C4 grasses in warm climates could be due to other traits inherited from their non-C4 ancestors. Here we investigate the relative influences of phylogeny and photosynthetic pathway in determining the ecological distributions of C4 grasses in Hawaii. We find that the restriction of C4 grasses to warmer areas is due largely to their evolutionary history as members of a warm-climate grass clade, but that the pathway does appear to confer a competitive advantage to grasses in more arid environments.     

The occurrence of C4 photosynthesis in Yunnan province, a tropical region in South-western China

Floristic composition, morphological functional types, and altitudinal distribution pattern for C₄ species were studied in Yunnan province, South-western China. 159 species, in 6 families and 60 genera, were identified with C₄ photosynthesis. 93 % of these C₄ species were found in Monocotyledoneae, e.g. Cyperaceae (18 species), Gramineae (129 species), and Commelinaceae (1 species), the other 7 % was in Dicotyledoneae, e.g. Amaranthaceae (5 species), Portulacaceae (4 species), and Chenopodiaceae (2 species). Hence C₄ plants mainly occurred in very few families in the tropical region. Compared with those in semi-arid grasslands and arid deserts in North China, more C₄ grasses and much less Chenopodiaceae C₄ species occurred in the tropical region. This indicates the physiological responses of C₄ plants from the two families are very different. Chenopodiaceae C₄ species may be more fit semi-arid and arid environments, while C₄ grasses are more fit the moist tropical conditions. There was a strong relationship between C₄ distribution and altitude in the tropical region. Altitudinal distribution pattern for C₄ species in the region was consistent with altitude, climate, and habitats.     

Global grass (Poaceae) success underpinned by traits facilitating colonization,persistence and habitat transformation

Poaceae (the grasses) is arguably the most successful plant family, in terms of its global occurrence in (almost) all ecosystems with angiosperms, its ecological dominance in many ecosystems, and high species richness. We suggest that the success of grasses is best understood in context of their capacity to colonize, persist, and transform environments (the “Viking syndrome”). This results from combining effective long‐distance dispersal, efficacious establishment biology, ecological flexibility, resilience to disturbance and the capacity to modify environments by changing the nature of fire and mammalian herbivory. We identify a diverse set of functional traits linked to dispersal, establishment and competitive abilities. Enhanced long‐distance dispersal is determined by anemochory, epizoochory and endozoochory and is facilitated via the spikelet (and especially the awned lemma) which functions as the dispersal unit. Establishment success could be a consequence of the precocious embryo and large starch reserves, which may underpin the extremely short generation times in grasses. Post‐establishment genetic bottlenecks may be mitigated by wind pollination and the widespread occurrence of polyploidy, in combination with gametic self‐incompatibility. The ecological competitiveness of grasses is corroborated by their dominance across the range of environmental extremes tolerated by angiosperms, facilitated by both C₃ and C₄ photosynthesis, well‐developed frost tolerance in several clades, and a sympodial growth form that enabled the evolution of both annual and long‐lived life forms. Finally, absence of investment in wood (except in bamboos), and the presence of persistent buds at or below ground level, provides tolerance of repeated defoliation (whether by fire, frost, drought or herbivores). Biotic modification of environments via feedbacks with herbivory or fire reinforce grass dominance leading to open ecosystems. Grasses can be both palatable and productive, fostering high biomass and diversity of mammalian herbivores. Many grasses have a suite of architectural and functional traits that facilitate frequent fire, including a tufted growth form, and tannin‐like substances in leaves which slow decomposition. We mapped these traits over the phylogeny of the Poales, spanning the grasses and their relatives, and demonstrated the accumulation of traits since monocots originated in the mid‐Cretaceous. Although the sympodial growth form is a monocot trait, tillering resulting in the tufted growth form most likely evolved within the grasses. Similarly, although an ovary apparently constructed of a single carpel evolved in the most recent grass ancestor, spikelets and the awned lemma dispersal units evolved within the grasses. Frost tolerance and C₄ photosynthesis evolved relatively late (late Palaeogene), and the last significant trait to evolve was probably the production of tannins, associated with pyrophytic savannas. This fits palaeobotanical data, suggesting several phases in the grass success story: from a late Cretaceous origin, to occasional tropical grassland patches in the later Palaeogene, to extensive C₃ grassy woodlands in the early–middle Miocene, to the dramatic expansion of the tropical C₄ grass savannas and grasslands in the Pliocene, and the C₃ steppe grasslands during the Pleistocene glacial periods. Modern grasslands depend heavily on strongly seasonal climates, making them sensitive to climate change.     

C4 Photosynthesis Promoted Species Diversification during the Miocene Grassland Expansion

Identifying how organismal attributes and environmental change affect lineage diversification is essential to our understanding of biodiversity. With the largest phylogeny yet compiled for grasses, we present an example of a key physiological innovation that promoted high diversification rates. C₄ photosynthesis, a complex suite of traits that improves photosynthetic efficiency under conditions of drought, high temperatures, and low atmospheric CO₂, has evolved repeatedly in one lineage of grasses and was consistently associated with elevated diversification rates. In most cases there was a significant lag time between the origin of the pathway and subsequent radiations, suggesting that the ‘C₄ effect’ is complex and derives from the interplay of the C₄ syndrome with other factors. We also identified comparable radiations occurring during the same time period in C₃ Pooid grasses, a diverse, cold-adapted grassland lineage that has never evolved C₄ photosynthesis. The mid to late Miocene was an especially important period of both C₃ and C₄ grass diversification, coincident with the global development of extensive, open biomes in both warm and cool climates. As is likely true for most “key innovations”, the C₄ effect is context dependent and only relevant within a particular organismal background and when particular ecological opportunities became available.     

Evolutionary innovations driving abiotic stress tolerance in C4 grasses and cereals

Grasslands dominate the terrestrial landscape, and grasses have evolved complex and elegant strategies to overcome abiotic stresses. The C₄ grasses are particularly stress tolerant and thrive in tropical and dry temperate ecosystems. Growing evidence suggests that the presence of C₄ photosynthesis alone is insufficient to account for drought resilience in grasses, pointing to other adaptations as contributing to tolerance traits. The majority of grasses from the Chloridoideae subfamily are tolerant to drought, salt, and desiccation, making this subfamily a hub of resilience. Here, we discuss the evolutionary innovations that make C₄ grasses so resilient, with a particular emphasis on grasses from the Chloridoideae (chloridoid) and Panicoideae (panicoid) subfamilies. We propose that a baseline level of resilience in chloridoid ancestors allowed them to colonize harsh habitats, and these environments drove selective pressure that enabled the repeated evolution of abiotic stress tolerance traits. Furthermore, we suggest that a lack of evolutionary access to stressful environments is partially responsible for the relatively poor stress resilience of major C₄ crops compared to their wild relatives. We propose that chloridoid crops and the subfamily more broadly represent an untapped reservoir for improving resilience to drought and other abiotic stresses in cereals.

Chloridoid grasses have evolved unique adaptations to adverse environments and represent an untapped reservoir for improving resilience to drought and other abiotic stresses in cereals.     

Dynamic photo-inhibition and carbon gain in a C4 and a C3 grass native to high latitudes

C₄ plants are rare in the cool climates characteristic of high latitudes and altitudes, perhaps because of an enhanced susceptibility to photo‐inhibition at low temperatures relative to C₃ species. In the present study we tested the hypothesis that low‐temperature photo‐inhibition is more detrimental to carbon gain in the C₄ grass Muhlenbergia glomerata than the C₃ species Calamogrostis Canadensis. These grasses occur together in boreal fens in northern Canada. Plants were grown under cool (14/10 °C day/night) and warm (26/22 °C) temperatures before measurement of the light responses of photosynthesis and chlorophyll fluorescence at different temperatures. Cool growth temperatures led to reduced rates of photosynthesis in M. glomerata at all measurement temperatures, but had a smaller effect on the C₃ species. In both species the amount of xanthophyll cycle pigments increased when plants were grown at 14/10 °C, and in M. glomerata the xanthophyll epoxidation state was greatly reduced. The detrimental effect of low growth temperature on photosynthesis in M. glomerata was almost completely reversed by a 24‐h exposure to the warm‐temperature regime. These data indicate that reversible dynamic photo‐inhibition is a strategy by which C₄ species may tolerate cool climates and overcome the Rubisco limitation that is prevalent at low temperatures in C₄ plants.     

The age of the grasses and clusters of origins of C4 photosynthesis

At high temperatures and relatively low CO₂ concentrations, plants can most efficiently fix carbon to form carbohydrates through C₄ photosynthesis rather than through the ancestral and more widespread C₃ pathway. Because most C₄ plants are grasses, studies of the origin of C₄ are intimately tied to studies of the origin of the grasses. We present here a phylogeny of the grass family, based on nuclear and chloroplast genes, and calibrated with six fossils. We find that the earliest origins of C₄ likely occurred about 32 million years ago (Ma) in the Oligocene, coinciding with a reduction in global CO₂ levels. After the initial appearance of C₄ species, photosynthetic pathway changed at least 15 more times; we estimate nine total origins of C₄ from C₃ ancestors, at least two changes of C₄ subtype, and five reversals to C₃. We find a cluster of C₄ to C₃ reversals in the Early Miocene correlating with a drop in global temperatures, and a subsequent cluster of C₄ origins in the Mid‐Miocene, correlating with the rise in temperature at the Mid‐Miocene climatic optimum. In the process of dating the origins of C₄, we were also able to provide estimated times for other major events in grass evolution. We find that the common ancestor of the grasses (the crown node) originated in the upper Cretaceous. The common ancestor of maize and rice lived at 52 ± 8 Ma.     

Photosynthetic responses of native and introduced C4 grasses from Venezuelan savannas

Summary Introduced African grasses are invading the grasslands of the Venezuelan savannas and displacing the native grasses. This work, which is part of a program to understand the reasons for the success of the African grasses, specifically investigates whether introduced and native grasses differ in some photosynthetic characteristics.The responses to photon flux density, leaf temperature, leaf-air vapour pressure difference and leaf water potential of leaf photosynthetic rate of two introduced African C₄ grasses (Hyparrhenia rufa and Melinis minutiflora) and of a lowland and a highland population of a native Venezuelan grass (Trachypogon plumosus) grown under controlled conditions were compared. These responses in all three species were typical of tropical C₄ pasture grasses. The introduced grasses had higher maximum leaf conductance, net photosynthetic rates, and optimum temperature (H. rufa only) for photosynthesis than T. plumosus. However, T. plumosus was able to continue photosynthesis to lower leaf water potentials than the two introduced grasses, and the efficiency which it utilized water, light and mineral nutrients to fix carbon were similar to those of the introduced grasses.The higher rates of leaf photosynthesis of the introduced grasses contributed to, but only partially explained, the higher growth rates compared to T. plumosus. The higher growth rates and nutrient concentration of the introduced grasses are consistent with their ability to establish rapidly, compete successfully for resources, and displace T. plumosus from moist, fertile sites. Conversely, the slower growth rate, lower nutrient concentrations, and superior water relations characteristics are consistent with the capacity of T. plumosus to resist invasion by introduced grasses in poorer sites.     

Salt tolerance evolves more frequently in C4 grass lineages

Salt tolerance has evolved many times in the grass family, and yet few cereal crops are salt tolerant. Why has it been so difficult to develop crops tolerant of saline soils when salt tolerance has evolved so frequently in nature? One possible explanation is that some grass lineages have traits that predispose them to developing salt tolerance and that without these background traits, salt tolerance is harder to achieve. One candidate background trait is photosynthetic pathway, which has also been remarkably labile in grasses. At least 22 independent origins of the C₄ photosynthetic pathway have been suggested to occur within the grass family. It is possible that the evolution of C₄ photosynthesis aids exploitation of saline environments, because it reduces transpiration, increases water‐use efficiency and limits the uptake of toxic ions. But the observed link between the evolution of C₄ photosynthesis and salt tolerance could simply be due to biases in phylogenetic distribution of halophytes or C₄ species. Here, we use a phylogenetic analysis to investigate the association between photosynthetic pathway and salt tolerance in the grass family Poaceae. We find that salt tolerance is significantly more likely to occur in lineages with C₄ photosynthesis than in C₃ lineages. We discuss the possible links between C₄ photosynthesis and salt tolerance and consider the limitations of inferring the direction of causality of this relationship.     

Variations in nitrogen use efficiency reflect the biochemical subtype while variations in water use efficiency reflect the evolutionary lineage of C4 grasses at inter‐glacial CO2

C₄ photosynthesis evolved multiple times in diverse lineages. Most physiological studies comparing C₄ plants were not conducted at the low atmospheric CO₂ prevailing during their evolution. Here, 24 C₄ grasses belonging to three biochemical subtypes [nicotinamide adenine dinucleotide malic enzyme (NAD‐ME), phosphoenolpyruvate carboxykinase (PCK) and nicotinamide adenine dinucleotide phosphate malic enzyme (NADP‐ME)] and six major evolutionary lineages were grown under ambient (400 μL L^?1) and inter‐glacial (280 μL L^?1) CO₂. We hypothesized that nitrogen‐related and water‐related physiological traits are associated with subtypes and lineages, respectively. Photosynthetic rate and stomatal conductance were constrained by the shared lineage, while variation in leaf mass per area (LMA), leaf N per area, plant dry mass and plant water use efficiency were influenced by the subtype. Subtype and lineage were equally important for explaining variations in photosynthetic nitrogen use efficiency (PNUE) and photosynthetic water use efficiency (PWUE). CO₂ treatment impacted most parameters. Overall, higher LMA and leaf N distinguished the Chloridoideae/NAD‐ME group, while NADP‐ME and PCK grasses were distinguished by higher PNUE regardless of lineage. Plants were characterized by high photosynthesis and PWUE when grown at ambient CO₂ and by high conductance at inter‐glacial CO₂. In conclusion, the evolutionary and biochemical diversity among C₄ grasses was aligned with discernible leaf physiology, but it remains unknown whether these traits represent ecophysiological adaptation.     

Differences in drought sensitivities and photosynthetic limitations between co-occurring C3 and C4 (NADP-ME) Panicoid grasses

Background and Aims

The success of C₄ plants lies in their ability to attain greater efficiencies of light, water and nitrogen use under high temperature, providing an advantage in arid, hot environments. However, C₄ grasses are not necessarily less sensitive to drought than C₃ grasses and are proposed to respond with greater metabolic limitations, while the C₃ response is predominantly stomatal. The aims of this study were to compare the drought and recovery responses of co-occurring C₃ and C₄ NADP-ME grasses from the subfamily Panicoideae and to determine stomatal and metabolic contributions to the observed response.

Methods

Six species of locally co-occurring grasses, C₃ species Alloteropsis semialata subsp. eckloniana, Panicum aequinerve and Panicum ecklonii, and C₄ (NADP-ME) species Heteropogon contortus, Themeda triandra and Tristachya leucothrix, were established in pots then subjected to a controlled drought followed by re-watering. Water potentials, leaf gas exchange and the response of photosynthetic rate to internal CO₂ concentrations were determined on selected occasions during the drought and re-watering treatments and compared between species and photosynthetic types.

Key Results

Leaves of C₄ species of grasses maintained their photosynthetic advantage until water deficits became severe, but lost their water-use advantage even under conditions of mild drought. Declining C₄ photosynthesis with water deficit was mainly a consequence of metabolic limitations to CO₂ assimilation, whereas, in the C₃ species, stomatal limitations had a prevailing role in the drought-induced decrease in photosynthesis. The drought-sensitive metabolism of the C₄ plants could explain the observed slower recovery of photosynthesis on re-watering, in comparison with C₃ plants which recovered a greater proportion of photosynthesis through increased stomatal conductance.

Conclusions

Within the Panicoid grasses, C₄ (NADP-ME) species are metabolically more sensitive to drought than C₃ species and recover more slowly from drought.     

Quo vadis C4? An ecophysiological perspective on global change and the future of C4 plants

C₄ plants are directly affected by all major global change parameters, often in a manner that is distinct from that of C₃ plants. Rising CO₂ generally stimulates C₃ photosynthesis more than C₄, but C₄ species still exhibit positive responses, particularly at elevated temperature and arid conditions where they are currently common. Acclimation of photosynthesis to high CO₂ occurs in both C₃ and C₄ plants, most notably in nutrient-limited situations. High CO₂ aggravates nitrogen limitations and in doing so may favor C₄ species, which have greater photosynthetic nitrogen use efficiency. C₄ photosynthesis is favored by high temperature, but global warming will not necessarily favor C₄ over C₃ plants because the timing of warming could be more critical than the warming itself. C₃ species will likely be favored where harsh winter climates are moderated, particularly where hot summers also become drier and less favorable to C₄ plant growth. Eutrophication of soils by nitrogen deposition generally favors C₃ species by offsetting the superior nitrogen use efficiency of C₄ species; this should allow C₃ species to expand at the expense of C₄ plants. Land-use change and biotic invasions are also important global change factors that affect the future of C₄ plants. Human exploitation of forested landscapes favors C₄ species at low latitude by removing woody competitors and opening gaps in which C₄ grasses can establish. Invasive C₄ grasses are causing widespread forest loss in Asia, the Americas and Oceania by accelerating fire cycles and reducing soil nutrient status. Once established, weedy C₄ grasses can prevent woodland establishment, and thus arrest ecological succession. In sum, in the future, certain C₄ plants will prosper at the expense of C₃ species, and should be able to adjust to the changes the future brings. This revised version was published online in August 2006 with corrections to the Cover Date.     

Physiological advantages of C4 grasses in the field: a comparative experiment demonstrating the importance of drought

Global climate change is expected to shift regional rainfall patterns, influencing species distributions where they depend on water availability. Comparative studies have demonstrated that C₄ grasses inhabit drier habitats than C₃ relatives, but that both C₃ and C₄ photosynthesis are susceptible to drought. However, C₄ plants may show advantages in hydraulic performance in dry environments. We investigated the effects of seasonal variation in water availability on leaf physiology, using a common garden experiment in the Eastern Cape of South Africa to compare 12 locally occurring grass species from C₄ and C₃ sister lineages. Photosynthesis was always higher in the C₄ than C₃ grasses across every month, but the difference was not statistically significant during the wettest months. Surprisingly, stomatal conductance was typically lower in the C₃ than C₄ grasses, with the peak monthly average for C₃ species being similar to that of C₄ leaves. In water‐limited, rain‐fed plots, the photosynthesis of C₄ leaves was between 2.0 and 7.4 μmol m^?2 s^?1 higher, stomatal conductance almost double, and transpiration 60% higher than for C₃ plants. Although C₄ average instantaneous water‐use efficiencies were higher (2.4–8.1 mmol mol^?1) than C₃ averages (0.7–6.8 mmol mol^?1), differences were not as great as we expected and were statistically significant only as drought became established. Photosynthesis declined earlier during drought among C₃ than C₄ species, coincident with decreases in stomatal conductance and transpiration. Eventual decreases in photosynthesis among C₄ plants were linked with declining midday leaf water potentials. However, during the same phase of drought, C₃ species showed significant decreases in hydrodynamic gradients that suggested hydraulic failure. Thus, our results indicate that stomatal and hydraulic behaviour during drought enhances the differences in photosynthesis between C₄ and C₃ species. We suggest that these drought responses are important for understanding the advantages of C₄ photosynthesis under field conditions.     

Phylogenetic signals in the realized climate niches of Chinese grasses (Poaceae)

Explaining relationships between species richness and biogeographical patterns over a broad geographic scale is a central issue of biogeography and macroecology. We document the realized climate niches for grasses in China’s nature reserves and discuss its formation mechanism using grass richness data combined with climatic, physiological, and phylogenetic data. Our results suggest that climate niche structure of grasses is phylogenetically conservative for BEP (Bambusoideae, Ehrhartoideae, and Pooideae) and PACMAD (Panicoideae, Arundinoideae, Chloridoideae, Micrairoideae, Aristidoideae, and Danthonioideae) clades along temperature gradients and for Chloridoideae and Panicoideae along precipitation gradients. At the national scale, the divergence patterns of climate niches between two major clades are more distinguishable than between C₃ and C₄ grasses. High rates of climate niche evolution are found in C₄ clades in the subtropical forest region. There appears to be a strong association between elevation gradients and grass diversity: the specific environmental conditions (e.g. energy) and the rapid shifts of climate conditions drive high grass diversification. Evolutionary conservatism of climate niches may be influenced by the specific adaptive ability to changing environmental conditions within NAD-ME/NADP-ME clades. Our results indicate that adaptations to major climate changes may be accomplished by C₄ grass nodes of high climate niche evolutionary rates in China’s nature reserves.     

Photosynthetic capacity and leaf nitrogen decline along a controlled climate gradient in provenances of two widely distributed Eucalyptus species

Climate is an important factor limiting tree distributions and adaptation to different thermal environments may influence how tree populations respond to climate warming. Given the current rate of warming, it has been hypothesized that tree populations in warmer, more thermally stable climates may have limited capacity to respond physiologically to warming compared to populations from cooler, more seasonal climates. We determined in a controlled environment how several provenances of widely distributed Eucalyptus tereticornis and E. grandis adjusted their photosynthetic capacity to +3.5°C warming along their native distribution range (~16–38°S) and whether climate of seed origin of the provenances influenced their response to different growth temperatures. We also tested how temperature optima (T_opt) of photosynthesis and J_max responded to higher growth temperatures. Our results showed increased photosynthesis rates at a standardized temperature with warming in temperate provenances, while rates in tropical provenances were reduced by about 40% compared to their temperate counterparts. Temperature optima of photosynthesis increased as provenances were exposed to warmer growth temperatures. Both species had ~30% reduced photosynthetic capacity in tropical and subtropical provenances related to reduced leaf nitrogen and leaf Rubisco content compared to temperate provenances. Tropical provenances operated closer to their thermal optimum and came within 3% of the T_opt of J_max during the daily temperature maxima. Hence, further warming may negatively affect C uptake and tree growth in warmer climates, whereas eucalypts in cooler climates may benefit from moderate warming.     

Photosynthetic innovation broadens the niche within a single species

Adaptation to changing environments often requires novel traits, but how such traits directly affect the ecological niche remains poorly understood. Multiple plant lineages have evolved C₄ photosynthesis, a combination of anatomical and biochemical novelties predicted to increase productivity in warm and arid conditions. Here, we infer the dispersal history across geographical and environmental space in the only known species with both C₄ and non‐C₄ genotypes, the grass Alloteropsis semialata. While non‐C₄ individuals remained confined to a limited geographic area and restricted ecological conditions, C₄ individuals dispersed across three continents and into an expanded range of environments, encompassing the ancestral one. This first intraspecific investigation of C₄ evolutionary ecology shows that, in otherwise similar plants, C₄ photosynthesis does not shift the ecological niche, but broadens it, allowing dispersal into diverse conditions and over long distances. Over macroevolutionary timescales, this immediate effect can be blurred by subsequent specialisation towards more extreme niches.     

Implications of quantum yield differences on the distributions of C3 and C4 grasses

Summary The implications of a reduced quantum yield (initial slope of the photosynthetic light response curve) in C₄ plants and temperature dependence of quantum yield in C₃ plants on total canopy primary production were investigated using computer simulations. Since reduced quantum yield represents the only known disadvantage of the C₄ photosynthetic pathway, simulations were conducted with grass canopies (high LAI and hence photosynthesis in most leaves will be light-limited) to see if quantum yield is a significant factor in limiting the primary production and thus distributions of C₄ grasses. Simulations were performed for three biogeographical or environmental conditions: the Great Plains region of North America, the Sonoran Desert of North America, and shade habitats. For all three cases, the simulations predicted either spatial or temporal gradients in the abundances of C₄ grasses identical to the abundance patterns of C₄ grasses observed in the field. It is thus concluded that while the C₄ photosynthetic mechanism may be highly advantageous in specific environments, it may be disadvantageous in others.C.I.W.-D.P.B. Publication No. 598