Stanley D. Smith

Elevated CO2 increases productivity and invasive species success in an arid ecosystem.

Arid ecosystems, which occupy about 20% of the earths terrestrial surface area, have been predicted to be one of the most responsive ecosystem types to elevated atmospheric CO2 and associated global climate change. Here we show, using free-air CO2 enrichment (FACE) technology in an intact Mojave Desert ecosystem, that new shoot production of a dominant perennial shrub is doubled by a 50% increase in atmospheric CO2 concentration in a high rainfall year. However, elevated CO 2 does not enhance production in a drought year. We also found that above-ground production and seed rain of an invasive annual grass increases more at elevated CO2 than in several species of native annuals. Consequently, elevated CO2 might enhance the long-term success and dominance of exotic annual grasses in the region. This shift in species composition in favour of exotic annual grasses, driven by global change, has the potential to accelerate the fire cycle, reduce biodiversity and alter ecosystem function in the deserts of western North America.

Photosynthesis in an invasive grass and native forb at elevated CO2 during an El Niño year in the Mojave Desert

Annual and short-lived perennial plant performance during wet years is important for long-term persistence in the Mojave Desert. Additionally, the effects of elevated CO2 on desert plants may be relatively greater during years of high resource availability compared to dry years. Therefore, during an El Niño year at the Nevada Desert FACE Facility (a whole-ecosystem CO2 manipulation), we characterized photosynthetic investment (by assimilation rate-internal CO2 concentration relationships) and evaluated the seasonal pattern of net photosynthesis (Anet) and stomatal conductance (gs) for an invasive annual grass, Bromus madritensis ssp. rubens and a native herbaceous perennial, Eriogonum inflatum. Prior to and following flowering, Bromus showed consistent increases in both the maximum rate of carboxylation by Rubisco (VCmax) and the light-saturated rate of electron flow (Jmax) at elevated CO2. This resulted in greater Anet at elevated CO2 throughout most of the life cycle and a decrease in the seasonal decline of maximum midday Anet upon flowering as compared to ambient CO2. Eriogonum showed significant photosynthetic down-regulation to elevated CO2 late in the season, but the overall pattern of maximum midday Anet was not altered with respect to phenology. For Eriogonum, this resulted in similar levels of Anet on a leaf area basis as the season progressed between CO2 treatments, but greater photosynthetic activity over a typical diurnal period. While gs did not consistently vary with CO2 in Bromus, it did decrease in Eriogonum at elevated CO2 throughout much of the season. Since the biomass of both plants increased significantly at elevated CO2, these patterns of gas exchange highlight the differential mechanisms for increased plant growth. The species-specific interaction between CO2 and phenology in different growth forms suggests that important plant strategies may be altered by elevated CO2 in natural settings. These results indicate the importance of evaluating the effects of elevated CO2 at all life cycle stages to better understand the effects of elevated CO2 on whole-plant performance in natural ecosystems.

Age and sex-specific rates of leaf regeneration in the Mojave Desert moss Syntrichia caninervis

The extremely skewed female-biased sex ratio in the desert moss Syntrichia caninervis was investigated by assessing the regeneration capacity of detached leaves. Juvenile, green, yellow-green, and brown leaves equating to approximately 0, 2, 6, and 12 yr of age, respectively, were detached from individuals of S. caninervis collected from 10 field populations and grown in a growth chamber for 58 d at a light intensity of 33-128 μmol · m(-2) · s(-1). Younger leaves (0-2 yr old) tended to have a greater viability, regenerate more quickly, extend their protonemal filaments farther, produce shoots (gametophores) more quickly, produce more shoots, and accumulate a greater biomass than older leaves (6 and 12 yr old). Among younger leaf classes, regenerating female leaves were more likely to produce a shoot than male leaves and produced more shoots than male leaves. The sexes did not differ significantly in time until protonemal emergence, linear extension of protonemata, or rate of biomass accumulation. However, protonemata of male leaves tended to emerge more quickly and produce a greater total biomass, ultimately consisting mostly of protonemata, than did female leaves. The more rapid proliferation of shoots by female leaf regenerants may help to explain the rarity of males in this species.

Linking Plant Invasions to Global Environmental Change

Biotic invasions have been recognized as an important element of global change (Vitousek 1994). Introductions of alien species into novel habitats have increased in tandem with travel and international trade (McNeely 2001). Many species have been introduced accidentally (e.g., in water ballast, in soil, or as crop seed “contaminants”), but some have been intentionally introduced as ornamentals, food, or fiber products. The introduction of alien species can have many ecological impacts, and contribute to the homogenization of biological systems worldwide (Lockwood and McKinney 2001). Biotic invasions, along with alterations in land-use patterns and disturbance regimes, are among the major causes of biodiversity loss worldwide (Soulé 1991). In cases where alien species have quantitative or qualitative trait differences from native species, invasions can also alter ecosystem processes such as nutrient cycling dynamics and disturbance regimes (Vitousek et al. 1987; D’Antonio and Vitousek 1992; D’Antonio and Corbin 2003; Levine et al. 2003). There are some well known cases of devastating effects of invasive plants on ecosystems such as the invasion of annual grasses in western U.S. (Mack 1981) or the invasion of pines in South-African shrublands (Le Maitre et al. 1996). These dramatic invasions emphasize that invaders often parallel environmental changes that are taking place at the regional scale. Therefore, research on the links between invasions and environmental changes is urgent and timely. Biotic invasions are capable of interacting with other anthropogenic changes in the environment to alter biodiversity and ecosystem processes in invaded habitats. For example, there is evidence from a variety of ecosystems that N inputs favor alien plant species (Huenneke et al. 1990; Vinton and Burke 1995; Maron and Connors 1996). Furthermore, land-use changes such as clearing for agriculture, road building (Gelbard and Belnap 2003), or alteration of disturbance regimes (Mack and D’Antonio 1998; D’Antonio et al. 1999) have been shown to facilitate plant invasions. The aim of this chapter is to present evidence of the interactions between several components of global environmental change and plant invasions. We focus on the effects of increasing atmospheric CO2 concentrations ([CO2]), climate change, terrestrial eutrophication, and changes in land-use/cover on the distribution and performance of plant invasions. We focus on plant invasions because in terrestrial ecosystems plants interact most dramatically with environmental and landscape changes, often reverberating to higher tropic levels. We do not consider the interactions between changes in disturbance regimes “per se”, such as wildfires and invasions, because they have been extensively reviewed elsewhere (Mack and D’Antonio 1998; D’Antonio et al. 1999).

Long-term response of a Mojave Desert winter annual plant community to a whole-ecosystem atmospheric CO2 manipulation (FACE)

Desert annuals are a critically important component of desert communities and may be particularly responsive to increasing atmospheric (CO2 ) because of their high potential growth rates and flexible phenology. During the 10-year life of the Nevada Desert FACE (free-air CO2 enrichment) Facility, we evaluated the productivity, reproductive allocation, and community structure of annuals in response to long-term elevated (CO2 ) exposure. The dominant forb and grass species exhibited accelerated phenology, increased size, and higher reproduction at elevated (CO2 ) in a wet El Niño year near the beginning of the experiment. However, a multiyear dry cycle resulted in no increases in productivity or reproductive allocation for the remainder of the experiment. At the community level, early indications of increased dominance of the invasive Bromus rubens at elevated (CO2 ) gave way to an absence of Bromus in the community during a drought cycle, with a resurgence late in the experiment in response to higher rainfall and a corresponding high density of Bromus in a final soil seed bank analysis, particularly at elevated (CO2 ). This long-term experiment resulted in two primary conclusions: (i) elevated (CO2 ) does not increase productivity of annuals in most years; and (ii) relative stimulation of invasive grasses will likely depend on future precipitation, with a wetter climate favoring invasive grasses but currently predicted greater aridity favoring native dicots.

Maintenance of C sinks sustains enhanced C assimilation during long-term exposure to elevated [CO2] in Mojave Desert shrubs

During the first few years of elevated atmospheric [CO2] treatment at the Nevada Desert FACE Facility, photosynthetic downregulation was observed in desert shrubs grown under elevated [CO2], especially under relatively wet environmental conditions. Nonetheless, those plants maintained increased Asat (photosynthetic performance at saturating light and treatment [CO2]) under wet conditions, but to a much lesser extent under dry conditions. To determine if plants continued to downregulate during long-term exposure to elevated [CO2], responses of photosynthesis to elevated [CO2] were examined in two dominant Mojave Desert shrubs, the evergreen Larreatridentata and the drought-deciduous Ambrosiadumosa, during the eighth full growing season of elevated [CO2] treatment at the NDFF. A comprehensive suite of physiological processes were collected. Furthermore, we used C labeling of air to assess carbon allocation and partitioning as measures of C sink activity. Results show that elevated [CO2] enhanced photosynthetic performance and plant water status in Larrea, especially during periods of environmental stress, but not in Ambrosia. δ13C analyses indicate that Larrea under elevated [CO2] allocated a greater proportion of newly assimilated C to C sinks than Ambrosia. Maintenance by Larrea of C sinks during the dry season partially explained the reduced [CO2] effect on leaf carbohydrate content during summer, which in turn lessened carbohydrate build-up and feedback inhibition of photosynthesis. δ13C results also showed that in a year when plant growth reached the highest rates in 5xa0years, 4% (Larrea) and 7% (Ambrosia) of C in newly emerging organs were remobilized from C that was assimilated and stored for at least 2xa0years prior to the current study. Thus, after 8xa0years of continuous exposure to elevated [CO2], both desert perennials maintained their photosynthetic capacities under elevated [CO2]. We conclude that C storage, remobilization, and partitioning influence the responsiveness of these desert shrubs during long-term exposure to elevated [CO2].

EFFECT OF ATMOSPHERIC CO2 ENRICHMENT ON ROOT GROWTH AND CARBOHYDRATE ALLOCATION OF PHASEOLUS SPP.

A glasshouse experiment was conducted with plants of Phaseolus grown in liquid culture. Root growth parameters (biomass, diameter, length, growth rate, zone of cell division), root rheological components (wall extensibility, water potential yield threshold, water potential), shoot growth, carbon allocation, and abscisic acid (ABA) concentration were measured in Phaseolus acutifolius A. Gray at ambient (550 &mgr;mol mol−1) and elevated (700 &mgr;mol mol−1) atmospheric CO2 concentrations. For contrast, measurements of above‐ and belowground growth were conducted on Phaseolus vulgaris L. in the same treatments. Under nonlimiting conditions of water and nutrients, elevated CO2 increased root and shoot growth of P. acutifolius but not P. vulgaris. While root mass was increased by nearly 60% in P. acutifolius, there was no effect of atmospheric CO2 on any of the rheological components measured. In contrast, starch and ABA accumulated in roots of P. acutifolius. The concentration of starch in roots of P. acutifolius increased by 10‐fold, while root concentrations of ABA doubled. From the data it is concluded that CO2 enrichment is favorable for root growth in some species in that more carbon is allocated to belowground growth. In addition, ABA may play a role in growth responses and/or allocation of photosynthates at elevated CO2 in P. acutifolius.

North American Deserts: Environments and Vegetation

North American deserts range from subtropical thorn scrub to high-latitude steppe to extreme barren desert. The deserts of North America are significantly smaller than those in the Old World, and generally not arid enough to be considered true deserts; North America is thus largely typified by semideserts (Shmida and Whittaker 1979; West 1988). The main determinant of arid climates in western North America is the presence of local mountain ranges creating rain shadows in the Great Basin and Mojave Deserts (the Sierra Nevada-Cascades and Rockies), the Sonoran Desert (the Peninsular Ranges and Sierra Madre Occidental), and the Chihuahuan Desert (the Sierra Madre Oriental and Sierra Madre Occidental). In contrast, most of the world’s great arid deserts are created by predictable descending high pressure systems at subtropical latitudes (Shmida 1985). Although a multitude of desert definitions proliferate in the literature, the 120–150 mm isohyet of annual precipitation is generally recognized as the boundary between deserts and semideserts (Shmida 1985) and of arid and semiarid climates (Meigs 1953). Based on this classification, only the western extent of North America’s deserts, which lie adjacent to the primary rain shadow of the Sierra Nevada and associated cordillera, can be considered true arid deserts. Shmida (1985) made a further distinction at the 70 mm isohyet between true desert and extreme desert, which corresponds roughly to the boundary between diffuse and contracted vegetation.

Plant Processes and Responses to Stress

All three photosynthetic pathways (C3, C4, and CAM) occur among North American desert plants. C3 photosynthesis is the most common type and predominates among winter-active taxa (Table 4). The C4 pathway is most often found in summer ephemerals, short-lived summer active perennials, and halophytic shrubs. Its predominance in hot and/or saline desert environments supports the purported advantages of the C4 pathway: (1) high photosynthetic temperature optima; (2) high light-saturation points; and (3) high water-use efficiencies (WUEs) (Ehleringer and Monson 1993). Crassulacean acid metabolism (CAM) is abundant in leaf- and stem-succulents of the warm deserts (see Chap. 5), but CAM plants are rare in the cold deserts. The overall distribution of CAM plants in North America is highly correlated with aridity (Teeri et al. 1978). This pattern reflects the high photosynthetic WUEs that are possible with nocturnal stomatal opening and CO2 assimilation of CAM plants.

Canopy volume–aboveground biomass relationships of desert perennials and the effects of elevated CO2

Abstract Known allometric relationships between aboveground plant biomass and canopy volume allow standing biomass and net primary productivity (NPP) to be estimated non-destructively. Canopy volume–aboveground biomass relationships are published for many ecosystems, but not for most desert species. It is also unknown if elevated atmospheric carbon dioxide [CO2] affects canopy volume–aboveground biomass relationships in desert perennials, limiting efforts to model NPP in an elevated CO2 environment. We measured canopy volume and aboveground biomass for perennial plants in the Mojave Desert, USA, at the Nevada Desert FACE Facility (NDFF): four species before treatment and 22 species after 10 years (1997–2007) of elevated CO2. Canopy volume to aboveground biomass allometry was estimated for each of the nine most common species individually; the remaining 13 species and unidentified dead shrubs combined to form a single “other” group. The resulting slopes and intercepts, which were estimated using a robust v...