Summary
1 The invasion of habitats by non-native plant and animal species is a global phenomenon with potentially grave consequences for ecological, economic, and social systems. Unfortunately, to date, the study of invasions has been primarily anecdotal and resistant to generalization.

2 Here, we use insights from experiments and from long-term monitoring studies of vegetation to propose a new theory in which fluctuation in resource availability is identified as the key factor controlling invasibility, the susceptibility of an environment to invasion by non-resident species. The theory is mechanistic and quantitative in nature leading to a variety of testable predictions.

3 We conclude that the elusive nature of the invasion process arises from the fact that it depends upon conditions of resource enrichment or release that have a variety of causes but which occur only intermittently and, to result in invasion, must coincide with availability of invading propagules.

https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1046/j.1365-2745.2000.00473.x

Fluctuating resources in plant communities: a general theory of invasibility

Contents

 Summary 1

I. What is FACE? 2
II. Materials and methods 2
III. Photosynthetic carbon uptake 3
IV. Acclimation of photosynthesis 6
V. Growth, above-ground production and yield 8
VI. So, what have we learned? 10

 Acknowledgements 11


 References 11


Appendix 1. References included in the database for meta-analyses  14


 Appendix 2. Results of the meta-analysis of FACE effects 18



Summary
Free-air CO2 enrichment (FACE) experiments allow study of the effects of elevated [CO2] on plants and ecosystems grown under natural conditions without enclosure. Data from 120 primary, peer-reviewed articles describing physiology and production in the 12 large-scale FACE experiments (475–600 ppm) were collected and summarized using meta-analytic techniques. The results confirm some results from previous chamber experiments: light-saturated carbon uptake, diurnal C assimilation, growth and above-ground production increased, while specific leaf area and stomatal conductance decreased in elevated [CO2]. There were differences in FACE. Trees were more responsive than herbaceous species to elevated [CO2]. Grain crop yields increased far less than anticipated from prior enclosure studies. The broad direction of change in photosynthesis and production in elevated [CO2] may be similar in FACE and enclosure studies, but there are major quantitative differences: trees were more responsive than other functional types; C4 species showed little response; and the reduction in plant nitrogen was small and largely accounted for by decreased Rubisco. The results from this review may provide the most plausible estimates of how plants in their native environments and field-grown crops will respond to rising atmospheric [CO2]; but even with FACE there are limitations, which are also discussed.

What have we learned from 15 years of free-air CO2 enrichment (FACE)? A meta-analytic review of the responses of photosynthesis, canopy properties and plant production to rising CO2.

The effects of land use change on soil carbon stocks are of concern in the context of international policy agendas on greenhouse gas emissions mitigation. This paper reviews the literature for the influence of land use changes on soil C stocks and reports the results of a meta analysis of these data from 74 publications. The meta analysis indicates that soil C stocks decline after land use changes from pasture to plantation (−10%), native forest to plantation (−13%), native forest to crop (−42%), and pasture to crop (−59%). Soil C stocks increase after land use changes from native forest to pasture (+ 8%), crop to pasture (+ 19%), crop to plantation (+ 18%), and crop to secondary forest (+ 53%). Wherever one of the land use changes decreased soil C, the reverse process usually increased soil carbon and vice versa. As the quantity of available data is not large and the methodologies used are diverse, the conclusions drawn must be regarded as working hypotheses from which to design future targeted investigations that broaden the database. Within some land use changes there were, however, sufficient examples to explore the role of other factors contributing to the above conclusions. One outcome of the meta analysis, especially worthy of further investigation in the context of carbon sink strategies for greenhouse gas mitigation, is that broadleaf tree plantations placed onto prior native forest or pastures did not affect soil C stocks whereas pine plantations reduced soil C stocks by 12–15%.

Soil carbon stocks and land use change: a meta analysis

SUMMARY When agricultural land is no longer used for cultivation and allowed to revert to natural vegetation or replanted to perennial vegetation, soil organic carbon can accumulate by processes that essentially reverse some of the effects responsible for soil organic carbon losses from when the land was converted from perennial vegetation. We discuss the essential elements of what is known about soil organic matter dynamics that may result in enhanced soil carbon sequestration with changes in land-use and soil management. We review literature that reports changes in soil organic carbon after changes in land-use that favor carbon accumulation. This data summary provides a guide to approximate rates of SOC sequestration that are possible with management, and indicates the relative importance of some factors that influence the rates of organic carbon sequestration in soil. There is a large amount of variation in rates and the length of time that carbon may accumulate in soil that are related to the productivity of the recovering vegetation, physical and biological conditions in the soil, and the past history of soil organic carbon inputs and physical disturbance. Maximum rates of C accumulation during the early aggrading stage of perennial vegetation growth, while substantial, are usually much less than 100 g C m y . Average rates of accumulation are similar for forest or grassland establishment: 33.8 g C m y and 33.2 g C m y respectively. These observed rates of soil organic C accumulation, when combined with the small amount of land area involved, are insufficient to account for a significant fraction of the missing C in the global carbon cycle as accumulating in the soils of formerly agricultural land.

Soil carbon sequestration and land‐use change: processes and potential

https://www.ipcc-nggip.iges.or.jp/public/gpglulucf/gpglulucf_files/0_Task1_Cover/Cover_TOC.pdf

Good Practice Guidance for Land Use, Land-Use Change and Forestry

The land use change from cropland to perennial grass cover associated with the Conservation Reserve Program (CRP) may sequester atmospheric CO2 back into the soil carbon pool, thereby changing formerly cultivated soils from sources to sinks for atmospheric carbon. To evaluate the effect of CRP on soil organic carbon (SOC) levels, samples from adjacent cropland, native pasture, and five year old CRP sites in Texas, Kansas, and Nebraska were analyzed. Across all locations, SOC levels for cropland, CRP, and native pasture were 59.2, 65.1, and 90.8 metric tons C ha−1 in the surface 300 cm, respectively. CRP lands gained an average of 1.1 tons C ha−1 yr1 suggesting that the 17 million hectares of land enrolled in CRP may have the potential to sequester about 45% of the 38.1 million tons of carbon released annually into the atmosphere from U.S. agriculture. These findings illustrate that agricultural CO2 emissions may be effectively controlled through changes in land use and management systems.

The CRP increases soil organic carbon

ATMOSPHERIC CO2 concentration was 160 to 200 μmol mol−1 during the Last Glacial Maximum (LGM; about 18,000 years ago)1, rose to about 275 (μmol mol−1 10,000 years ago2,3, and has increased to about 350 μmol mol−1 since 1800 (ref. 4). Here we present data indicating that this increase in CO2 has enhanced biospheric carbon fixation and altered species abundances by increasing the water-use efficiency of biomass production of C3 plants, the bulk of the Earth's vegetation. We grew oats (Avena sativa), wild mustard (Brassica kaber) and wheat (Triticum aes-tivum cv. Seri M82 and Yaqui 54), all C3 annuals, and selected C4 grasses along daytime gradients of Glacial to present atmospheric CO2 concentrations in a 38-m-long chamber. We calculated parameters related to leaf photosynthesis and water-use efficiency from stable carbon isotope ratios (13C/12C) of whole leaves. Leaf water-use efficiency and above-ground biomass/plant of C3 species increased linearly and nearly proportionally with increasing CO2 concentrations. Direct effects of increasing CO2 on plants must be considered when modelling the global carbon cycle and effects of climate change on vegetation.

Increase in C3 plant water-use efficiency and biomass over Glacial to present C0 2 concentrations

Carbon sequestration in soil organic matter may moderate increases in atmospheric CO(2) concentrations (C(a)) as C(a) increases to more than 500 micromol mol(-1) this century from interglacial levels of less than 200 micromol mol(-1) (refs 1 6). However, such carbon storage depends on feedbacks between plant responses to C(a) and nutrient availability. Here we present evidence that soil carbon storage and nitrogen cycling in a grassland ecosystem are much more responsive to increases in past C(a) than to those forecast for the coming century. Along a continuous gradient of 200 to 550 micromol mol(-1) (refs 9, 10), increased C(a) promoted higher photosynthetic rates and altered plant tissue chemistry. Soil carbon was lost at subambient C(a), but was unchanged at elevated C(a) where losses of old soil carbon offset increases in new carbon. Along the experimental gradient in C(a) there was a nonlinear, threefold decrease in nitrogen availability. The differences in sensitivity of carbon storage to historical and future C(a) and increased nutrient limitation suggest that the passive sequestration of carbon in soils may have been important historically, but the ability of soils to continue as sinks is limited.

Nonlinear grassland responses to past and future atmospheric CO 2

An investigation to determine whether stomatal acclimation to [CO 2 ] occurred in C 3 /C 4 grassland plants grown across a range of [CO 2 ] (200‐550 µ mol mol − − − 1 ) in the field was carried out. Acclimation was assessed by measuring the response of stomatal conductance ( g s ) to a range of intercellular CO 2 (a g s ‐ C i curve) at each growth [CO 2 ] in the third and fourth growing seasons of the treatment. The g s ‐ C i response curves for Solanum dimidiatum (C 3 perennial forb) differed significantly across [CO 2 ] treatments, suggesting that stomatal acclimation had occurred. Evidence of non‐linear stomatal acclimation to [CO 2 ] in this species was also found as maximum g s ( g smax ; g s measured at the lowest C i ) increased with decreasing growth [CO 2 ] only below 400 µ mol mol − 1 . The substantial increase in g s at subambient [CO 2 ] for S. dimidiatum was weakly correlated with the maximum velocity of carboxylation ( V cmax ; r 2 = 0·27) and was not associated with CO 2 saturated photosynthesis ( A max ). The response of g s to C i did not vary with growth [CO 2 ] in Bromus japonicus (C 3 annual grass) or Bothriochloa ischaemum (C 4 perennial grass), suggesting that stomatal acclimation had not occurred in these species. Stomatal density, which increased with rising [CO 2 ] in both C 3 species, was not correlated with g s . Larger stomatal size at subambient [CO 2 ], however, may be associated with stomatal acclimation in S. dimidiatum . Incorporating stomatal acclimation into modelling studies could improve the ability to predict changes in ecosystem water fluxes and water availability with rising CO 2 and to understand their magnitudes relative to the past.

/pdf/stomatal-acclimation-over-a-subambient-to-elevated-co2-1rwdnteave.pdf

Stomatal acclimation over a subambient to elevated CO2 gradient in a C3/C4 grassland

The abundance of woody plants on grasslands and savannas often is controlled by the availability of water and its location in soil. Water availability to plants is limited by precipitation, but the distribution of soil water and period over which it is available in these ecosystems are influenced by the transpiration rates of grasses. We discuss implications of recent and projected increases in atmospheric CO2 concentration for transpiration, soil water availability, and the balance of grasses and shrubs. An increase in CO2 concentration often reduces potential transpiration/leaf area by reducing stomatal conductance. On grasslands where effects of stomatal closure on transpiration are not negated by an increase in leaf temperature and leaf area, rising CO2 concentration should slow the depletion of soil water by grasses and potentially favor shrubs and other species that might otherwise succumb to water stress. Predicted effects of CO2 are supported by results from CO2-enrichment studies in the field and are compatible with recent models of interactions between resource levels and vegetation pattern and structure.

/pdf/viewpoint-atmospheric-co2-soil-water-and-shrub-grass-ratios-5bookjjd8p.pdf

Hyrum B. Johnson

Papers

The CRP increases soil organic carbon

Increase in C3 plant water-use efficiency and biomass over Glacial to present C0 2 concentrations

Nonlinear grassland responses to past and future atmospheric CO 2

Stomatal acclimation over a subambient to elevated CO2 gradient in a C3/C4 grassland

Viewpoint: atmospheric CO2, soil water, and shrub/grass ratios on rangelands.