We propose a working framework for future studies of net carbon exchange (NCE) in disturbed landscapes at broad spatial scales based on the central idea that landscape-level NCE is determined by the ...land mosaic, including its age structure. Within this framework, we argue that the area-of-edge-influence (AEI), which is prevalent in many disturbed, fragmented landscapes, should constitute a distinct ecosystem type since numerous studies have indicated unique ecological properties within these areas. We present and justify four working hypotheses currently being tested in northern Wisconsin, based on this framework: (1) the area of an ecosystem that is influenced by structural edges (e.g., AEI) has NCE that is significantly different from the ecosystem interior; (2) age structure and composition of an ecosystem play critical roles in determining the ecosystem's contribution to the cumulative net ecosystem production (NEP) of the landscape mosaic; (3) the relative importance of different structural and biophysical controls of carbon exchange is ecosystem dependent; and (4) the frequency and intensity of disturbances in time and space control the cumulative NCE of the land mosaic through alteration of ecosystems that vary in age, structure, physical environment, and interactions. In addition, we describe five different research approaches to quantify NCE at broad scales, including biometric estimations, ecophysiological methods, micrometeorological methods, applications of remote sensing and GIS, and ecosystem models.
Changes in climate and land use affect soil respiration rates (SRR) significantly, but studies of these effects across entire landscapes are rare. We simulated responses of landscape mean SRR (LMSRR) ...to such changes from May to October over a 30 yr period in a managed, predominantly forested landscape in northern Wisconsin, USA, using: (1) 6 satellite-derived land-cover maps (1972, 1978, 1982, 1987, 1992, and 2001); (2) monthly air temperature data in the corresponding years of the cover maps; and (3) SRR models driven by soil temperature (T
s) at 5 cm depth. LMSRR seemed to increase linearly by 77% from 0.625 in May to 1.104 g CO₂ m⁻² h⁻¹ in July, and then decreased at an increasing rate to 0.411 g CO₂ m⁻² h⁻¹ in October. LMSRR was more sensitive to an increase of minimum temperature than that of mean or maximum temperature, suggesting that future climate change might impact SRR in high-latitude forests more than other biomes. LMSRR in September over the study period was similar to that of June but with 92% higher variation, while both landscape mean air temperature and precipitation in September had lower variation than in June. This indicates that the topsoil layer functions differently during soil warming and cooling phases. Changes in land cover composition from 1972 to 2001 increased LMSRR by 2.8 to 3.1% while 2°C differences in growing season mean air temperature increased the SRR by 6.7 to 7.0%. The combined effects of both variables on the SRR are more complex, varying from 3.8 to 10.0%.
The interaction of elevated carbon dioxide and/or ozone on the occurrence and extent of aspen leaf rust on trembling aspen was investigated. The role of changes in leaf surface properties associated ...with such exposures in this host-pathogen interaction was also explored. Increases in rust infection index levels of three- to five-fold were documented in two consecutive years after growing season exposures with either O sub(3) alone or O sub(3) plus CO sub(2). Marked leaf surface property changes were also evident. Elevated O sub(3) appears to be altering leaf surfaces in such a way as to predispose plants to increased infection by aspen leaf rust.
Elevated CO
2 did not ameliorate all of the adverse effects of O
3.
To determine whether elevated CO
2 reduces or exacerbates the detrimental effects of O
3 on aspen (
Populus tremuloides Michx.), ...aspen clones 216 and 271 (O
3 tolerant), and 259 (O
3 sensitive) were exposed to ambient levels of CO
2 and O
3 or elevated levels of CO
2, O
3, or CO
2+O
3 in the FACTS II (Aspen FACE) experiment, and physiological and molecular responses were measured and compared. Clone 259, the most O
3-sensitive clone, showed the greatest amount of visible foliar symptoms as well as significant decreases in chlorophyll, carotenoid, starch, and ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) concentrations and transcription levels for the Rubisco small subunit. Generally, the constitutive (basic) transcript levels for phenylalanine ammonia-lyase (
PAL) and chalcone synthase (
CHS) and the average antioxidant activities were lower for the ozone sensitive clone 259 as compared to the more tolerant 216 and 271 clones. A significant decrease in chlorophyll
a,
b and total (
a+
b) concentrations in CO
2, O
3, and CO
2+O
3 plants was observed for all clones. Carotenoid concentrations were also significantly lower in all clones; however,
CHS transcript levels were not significantly affected, suggesting a possible degradation of carotenoid pigments in O
3-stressed plants. Antioxidant activities and
PAL and 1-aminocyclopropane-1-carboxylic acid (ACC)-oxidase transcript levels showed a general increase in all O
3 treated clones, while remaining low in CO
2 and CO
2+O
3 plants (although not all differences were significant). Our results suggest that the ascorbate-glutathione and phenylpropanoid pathways were activated under ozone stress and suppressed during exposure to elevated CO
2. Although CO
2+O
3 treatment resulted in a slight reduction of O
3-induced leaf injury, it did not appear to ameliorate all of the harmful affects of O
3 and, in fact, may have contributed to an increase in chloroplast damage in all three aspen clones.
ABSTRACT
Leaf gas exchange parameters and the content of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2‐year‐old aspen (
Populus tremuloides
Michx.) clones (no. 216, ...ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO
2
and/or O
3
. The plants were exposed either to ambient air (control), elevated CO
2
(560 p.p.m.) elevated O
3
(55 p.p.b.) or a mixture of elevated CO
2
and O
3
in a free air CO
2
enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light‐saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age‐related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO
2
and decreased by O
3
at both control and elevated CO
2
. The relative stomatal limitation to photosynthesis (
l
s
) was in both clones about 10% under control and elevated O
3
. Exposure to elevated CO
2
+ O
3
in both clones and to elevated CO
2
in clone 259, decreased
l
s
even further – to about 5%. The corresponding changes in Rubisco content and the stability of
C
i
/
C
a
ratio suggest that the changes in photosynthesis in response to elevated CO
2
and O
3
were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O
3
tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable
C
i
under the given treatment, that indicates close coupling between stomatal and mesophyll processes.
Various leaf antioxidant activities and gene-expression patterns were measured in aspen trees growing in a Free Air Carbon Dioxide and/or Ozone Enrichment (FACE) experiment. Aspen clones were used ...that differed in O sub(3) sensitivity. The FACE facility is described. Four treatments were employed: control, elevated O sub(3), elevated CO sub(2), and elevated O sub(3)+CO sub(2). In all three clones, O sub(3) exposure caused significantly smaller base diameters, average number of leaves per branch, and mean leaf sizes compared to control trees, and similar results were obtained for average leaf sizes and leaves per branch for CO sub(2)+O sub(3)-treated plants. Activities of ascorbate peroxidase, glutathione reductase, and catalase were generally highest in O sub(3)-treated plants and lowest for CO sub(2)- and CO sub(2)+O sub(3)-treated plants. While Rubisco content was not affected appreciably, all three treatments affected chlorophyll a and b contents, decreasing significantly in all treated clones. The overall results suggested that elevated CO sub(2) could have exacerbated the harmful effects of O sub(3) by suppressing the ascorbate glutathione and phenylalanine ammonia-lyase pathways, resulting in increased cellular damage.
Genomic and cDNA clones, corresponding to an ozone-induced cytosolic copper–zinc superoxide dismutase, were isolated from quaking aspen (
Populus tremuloides Michx.). The cytosolic superoxide ...dismutase (SOD) appears to be part of a multi-gene family in aspen and is interrupted by five introns in the coding region. Northern blot analysis with a gene-specific probe revealed an increase in the expression of this gene in response to ozone in the leaves of an ozone-tolerant aspen clone, compared with an ozone-sensitive clone. Cytosolic SOD transcript expression levels in leaves were also found to increase significantly within 6 h of mechanical wounding, after which the level of the transcript decreases. Under normal growing conditions, immature male and female aspen floral bud tissues contained the highest levels of the cytosolic SOD gene transcript, whereas transcript levels were almost undetectable in older leaves.
1. The impacts of elevated atmospheric CO2and/or O3have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional ...groups (the indeterminate, pioneer, O3-sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera, and the determinate, late successional, O3-tolerant species Sugar Maple, Acer saccharum). 2. The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O3-tolerant as well as O3sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34-36 nL L-1during the summer daylight hours) O3offsets or moderates the responses induced by elevated CO23. After 3 years of exposure to 560 μmol$\text{mol}^{-1}\ \text{CO}_{2}$, the above-ground volume of Aspen stands was 40% above those grown at ambient CO2, and there was no indication of a diminishing growth trend. In contrast, O3at 1 5 × ambient completely offset the growth enhancement by CO2, both for O3-sensitive and O3-tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO2, and global models of forest productivity and climate change are presented.
The impacts of elevated atmospheric CO sub(2) and/or O sub(3) have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different ...functional groups (the indeterminate, pioneer, O sub(3)-sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera, and the determinate, late successional, O sub(3)-tolerant species Sugar Maple, Acer saccharum). The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O sub(3)-tolerant as well as O sub(3)-sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1.5 x ambient, with ambient averaging 34-36 nL L super(-1) during the summer daylight hours), O sub(3) offsets or moderates the responses induced by elevated CO sub(2). After 3 years of exposure to 560 mu mol mol super(-1) CO sub(2), the above-ground volume of Aspen stands was 40% above those grown at ambient CO sub(2), and there was no indication of a diminishing growth trend. In contrast, O sub(3) at 1.5 x ambient completely offset the growth enhancement by CO sub(2), both for O sub(3)-sensitive and O sub(3)-tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO sub(2), and global models of forest productivity and climate change are presented.
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