Rodney A. Chimner

Factors controlling the establishment of Fremont cottonwood seedlings on the Upper Green River, USA

Declines in cottonwood (Populus spp.) recruitment along alluvial reaches of large rivers in arid regions of the western United States have been attributed to modified flow regimes, lack of suitable substrate, insufficient seed rain, and increased interspecific competition. We evaluated whether and how these factors were operating during 1993–1996 to influence demographics of Fremont cottonwood (P. deltoides Marshall subsp. wislizenii (Watson) Eckenwalder) along reaches of the Green and Yampa Rivers near their confluence in northwestern Colorado. We examined seedling establishment, defined as survival through three growing seasons, at three alluvial reaches that differed primarily in the level of flow regulation: a site on the unregulated Yampa, an upper Green River site regulated by Flaming Gorge Dam, and a lower Green River site below the Green–Yampa confluence. Seed rain was abundant in all sites, and led to large numbers of germinants (first-year seedlings) appearing each year at all sites. The regulated flow in the upper Green River reach restricted germination to islands and cut banks that were later inundated or eroded; no seedlings survived there. Mortality at the lower Green River site was due largely to desiccation or substrate erosion; 23% of 1993 germinants survived their first growing season, but at most 2% survived through their second. At the Yampa River site, germinants appeared on vegetated and unvegetated surfaces up to 2.5 m above base flow stage, but survived to autumn only on bare surfaces at least 1.25 m above base flow stage, and where at least 10 of the upper 40 cm of the alluvium was fine-textured. Our studies of rooting depths and the stable isotopic composition of xylem water showed that seedlings in the most favorable locations for establishment at the Yampa site do not become phreatophytic until their third or fourth growing season. Further, the results of experimental field studies examining effects of shade and competition supported the hypothesis that insufficient soil moisture, possibly in combination with insufficient light, restricts establishment to unvegetated sites. Collectively, the demographic and experimental studies suggest that, in arid regions, soil water availability is at least as important as light level in limiting establishment of Fremont cottonwood seedlings. We hypothesize that in cases where arid land rivers experience large spring stage changes, recruitment is further constrained within bare areas to those sites that contain sufficient fine-textured alluvium, saturated during the spring flood, to provide the flood-derived soil moisture normally necessary for late-summer seedling survival. Copyright

Nutrient and carbon dynamics in a replacement series of Eucalyptus and Albizia trees.

Tree plantations are an important component of tropical landscapes, providing wood, fuel, and perhaps carbon (C) sequestration. Primary production in wet tropical plantations is typically nutrient limited. In some Hawaiian Eucalyptus plantations, nitrogen (N) limitations to production are alleviated by intercropping N-fixing Albizia trees that may decrease available phosphorus (P). Thus, sustainable productivity and C sequestration may depend on species composition. We measured soil N and P availability and ecosystem N and C sequestration in a 17-yr-old replicated replacement series of Eucalyptus and Albizia in Hawaii. Species composition included pure plots of each species and four proportions of mixtures. Soil N availability increased with the proportion of Albizia in the plot, but soil P availability declined. Aboveground tree C accumulation showed a synergistic response to increasing percentage of Albizia, with the mixed stands having more tree C than pure stands of Eucalyptus or Albizia. In the top 50 cm of soil, total N and C increased linearly with percentage of Albizia. Stands with the highest percentage of Albizia had 230 g/m2 more soil N and 2000 g/m2 more soil C than stands without Albizia. Stable C isotope analyses showed that increased soil C resulted from differences in both tree-derived C and “old” sugarcane-derived C. Deeper soil C (50–100 cm) was a substantial fraction (0.36) of total soil C but did not vary among treatments. Our results demonstrate that tree species effects on nutrient and C dynamics are not as simple as monocultures suggest. Mixed-species afforestation increased tree and soil C accrual over 17 years, and N inputs may increase soil C storage by decreasing decomposition.

Using stable oxygen isotopes to quantify the water source used for transpiration by native shrubs in the San Luis Valley, Colorado U.S.A.

An understanding of the water source used by phreatophytic desert shrubs is critical for understanding how they function and respond to man-caused groundwater drawdowns. Shrubs can use primarily groundwater, precipitation recharged soil water, or a mixture of the two. If shrubs use primarily groundwater, a water table decline may reduce water availability and lead to high plant mortality. However, if shrubs can acquire precipitation recharged soil water, then groundwater decline could have less impact on plants. This study took place in the San Luis Valley, a large, arid, high elevation closed basin in south-central Colorado. We examined stable oxygen isotopes in precipitation, soil water from several depths, groundwater and plant xylem water to identify the likely water sources for the three most abundant shrubs in the valley: Sarcobatus vermiculatus (Hooker) Torrey, Chrysothamnus nauseosus (Pallas) Britton subsp. consimilis (Greene) Hall & Clements, and Chrysothamnus greenei (Gray) Greene. C. greenei is not known to be phreatophytic while S. vermiculatus and C. nauseosus may be phreatophytic. Mean annual San Luis Valley precipitation during the two years of study was 121 mm, with 67% occurring during the summer monsoon season of July through September. We found differences in water acquisition patterns by species, season, and along a depth to water table gradient. C. greenei only occurred in sites with a water table > 2.0 m deep, and utilized only soil water recharged by precipitation. At sites with a water table less then 2 m depth, S. vermiculatus and C. nauseosus utilized soil water from the top 0.5 m and shallow groundwater during the pre-monsoon and monsoon periods. A more complex water use pattern was found at sites with a water table deeper then 2 m. S. vermiculatus and C. nauseosus used both deep soil water and groundwater during 1996. During the pre-monsoon period in 1997, both shrubs utilized predominantly groundwater. However, during the 1997 monsoon season both species switched to utilize primarily precipitation recharged water acquired from the upper 0.3–0.4 m of soil. This is the first report that C. nauseosus can utilize summer precipitation. Our results support the hypothesis that plants utilize more summer rain recharged soil water in regions receiving a substantial proportion of annual precipitation during the summer.

Temperature and Microtopography Interact to Control Carbon Cycling in a High Arctic Fen

High arctic wetlands hold large stores of soil carbon (C). The fate of these C stores in a changing climate is uncertain, as rising air temperatures may differentially affect photosynthesis and ecosystem respiration (ER). In this study, open-top warming chambers were used to increase air and soil temperatures in contrasting microtopographic positions of a high arctic fen in NW Greenland. CO2 exchange between the ecosystem and the atmosphere was measured on 28 dates over a 3-year period. Measurements of the normalized difference vegetation index, leaf and stem growth, leaf-level gas exchange, leaf nitrogen, leaf δ13C, and fine root production were made to investigate the mechanisms and consequences of observed changes in CO2 exchange. Gross ecosystem photosynthesis (GEP) increased with chamber warming in hollows, which are characterized by standing water, and in hummocks, which extend above the water table. ER, however, increased only in hummocks, such that net ecosystem exchange (NEE) increased in hollows, but did not change in hummocks with chamber warming. Complementary measurements of plant growth revealed that increases in GEP corresponded with increases in C allocation to aboveground biomass in hummocks and belowground biomass in hollows. Our results and those of several recent studies clearly demonstrate that effects of climate change on the C balance of northern wetlands will depend upon microtopography which, in turn, may be sensitive to climate change.

SOIL RESPIRATION RATES OF TROPICAL PEATLANDS IN MICRONESIA AND HAWAII

There are very few published reports of soil respiration rates from tropical peatlands, despite their importance to global carbon cycling. This study quantified in situ soil respiration rates in a suite of tropical peatlands in Micronesia and Hawaii using a soil CO2 flux chamber connected to a LI-COR 6400 Portable Photosynthesis Infrared Gas Analyzer. Soil respiration rates were higher in the warmer Micronesian peatlands (2.15–2.54 umol m−2 s−1) than in the cooler Hawaiian montane peatlands (0.83–1.81 umol m−2 s−1). The lone exception was the taro-cultivated peatland in Micronesia that had low soil respiration rates likely due to low amount of litterfall, root biomass, and root production. Deep standing water decreased soil respiration rates, while lowered water levels had mixed effects on soil respiration rates. Surprisingly, measured soil respiration rates were lower than rates measured in temperate and boreal peatlands in the summer. However, soil respiration rates in tropical peatlands are not limited by large diurnal or seasonal changes and can continue respiring at the same rates, resulting in higher annual CO2 flux rates compared to other nontropical peatlands.

MODELING CARBON ACCUMULATION IN ROCKY MOUNTAIN FENS

Despite the importance of peatlands in the global carbon cycle, no widely applicable ecosystem model exists for peatlands. Simulations of three montane fens in Colorado, USA were conducted to test the capabilities of the CENTURY ecosystem model to simulate 1) long-term carbon accumulation and 2) short-term changes in carbon accumulation due to hydrologic changes. The CENTURY model was calibrated to simulate long-term carbon accumulation in two fens for up to 10,000 years by adjusting three variables that represent anaerobic soil conditions. CENTURY was unable to simulate long-term carbon accumulation in a third fen using settings for the two calibrated fens. However, CENTURY correctly simulated total carbon storage by adjusting two of the three anaerobic variables. A sensitivity analysis revealed that carbon accumulation in CENTURY is highly sensitive to anaerobic soil conditions. CENTURY predicted that half of the fen peat is composed of structural root material. The majority of the remaining peat was composed of recalcitrant slow and passive soil organic matter. Precipitation levels were altered to determine if CENTURY could predict the change in carbon accumulation rates due to periodic drier conditions. The simulated drying scenario predicted an average carbon loss of 70 g C m−2 yr−1 during the 100-year simulation. The loss of carbon occurred despite plant production increasing from an average of 249 g C m−2 yr−1 to 391 g C m−2 yr−1. Slightly more than 90% of the carbon lost was from the structural root pool and slow organic matter pool, while there was no carbon loss or a slight net carbon gain in the passive organic matter pool and above-ground structural and metabolic pools. Despite several shortcomings, our results indicate that an ecosystem model, such as CENTURY, can be useful for simulating carbon dynamics in peatlands.

Differences in carbon fluxes between forested and cultivated micronesian tropical peatlands

Taro is a staple crop that is often grown in wetlands throughout the Indo-Pacific, but the long-term impacts of its cultivation on wetland ecosystem functions are unknown. The objective of this study was to determine how cultivating taro affects carbon cycling by comparing key pathways in a forested peatland and an adjacent cultivated taro patch. Leaves decomposed rapidly at both sites with roughly 73% remaining after 2 weeks, 53% after 8 weeks, 38% after 17 weeks, and 17% after 36 weeks. Root decomposition proceeded much more slowly with roughly 93% remaining after 2 weeks, 80% after 8 weeks, 71% after 17 weeks, and 66% after 36 weeks. Annual litterfall was 1181 g m−2 year−1 and 849 g m−2 year−1 for the forested and cultivated sites, respectively. For the two sites combined, litterfall consisted of 78% leaves, 10% reproductive material, 3% branches, and 9% miscellaneous material. Fine root biomass was greater in the forested site than the cultivated site, averaging 205 g m−2 and 34 g m−2, respectively. Fine root production was much greater in the forested than the cultivated site, averaging 226 g C m−2 year−1 and 48 g C m−2 year−1, respectively. Soil respiration averaged 99 mg C m−2 h−1 and 55 mg C m−2 h−1 at the forested and cultivated sites, respectively. We found that the major change to carbon fluxes in the cultivated site was less carbon was entering the peatland, particularly less root production. Alterations to the carbon cycle caused by cultivation would probably not be permanent, because taro patches are periodically abandoned and allowed to regenerate naturally.

The effect of long‐term water table manipulations on dissolved organic carbon dynamics in a poor fen peatland

Dissolved organic carbon (DOC) production, consumption, and quality displayed differences after long-term (~55 years) hydrological alterations in a poor fen peatland in northern Michigan. The construction of an earthen levee resulted in areas of a raised and lowered water table (WT) relative to an unaltered intermediate WT site. The lowered WT site had greater peat aeration and larger seasonal vertical WT fluctuations that likely elevated peat decomposition and subsidence with subsequent increases in bulk density, vertical hydraulic gradient, decreased hydraulic conductivity (Ksat), and a greater pore water residence time relative to the unaltered site. The raised WT site displayed a decreased Ksat combined with seasonal upwelling events that contributed to a longer residence time in comparison to the unaltered site. These differences are potentially contributing to elevated DOC concentrations at the lowered and raised WT site relative to the unaltered site. Additionally, spectrophotometric indices and chemical constituent assays indicated that the lowered site DOC was more aromatic and contained elevated concentrations of phenolics compared to the intermediate site. The raised site DOC was less aromatic, less humified, and also had a greater phenolic content than the intermediate site. Furthermore, an incubation experiment showed that DOC in the raised site contained the greatest labile carbon source. Based on our results, long-term WT alterations will likely impose significant effects on DOC dynamics in these peatlands; however, WT position alone was not a good predictor of DOC concentrations, though impoundment appears to produce a more labile DOC whereas drainage increases DOC aromaticity.

Photosynthetic traits of Sphagnum and feather moss species in undrained, drained and rewetted boreal spruce swamp forests

In restored peatlands, recovery of carbon assimilation by peat-forming plants is a prerequisite for the recovery of ecosystem functioning. Restoration by rewetting may affect moss photosynthesis and respiration directly and/or through species successional turnover. To quantify the importance of the direct effects and the effects mediated by species change in boreal spruce swamp forests, we used a dual approach: (i) we measured successional changes in moss communities at 36 sites (nine undrained, nine drained, 18 rewetted) and (ii) photosynthetic properties of the dominant Sphagnum and feather mosses at nine of these sites (three undrained, three drained, three rewetted). Drainage and rewetting affected moss carbon assimilation mainly through species successional turnover. The species differed along a light-adaptation gradient, which separated shade-adapted feather mosses from Sphagnum mosses and Sphagnum girgensohnii from other Sphagna, and a productivity and moisture gradient, which separated Sphagnum riparium and Sphagnum girgensohnii from the less productive S. angustifolium, S. magellanicum and S. russowii. Undrained and drained sites harbored conservative, low-production species: hummock-Sphagna and feather mosses, respectively. Ditch creation and rewetting produced niches for species with opportunistic strategies and high carbon assimilation. The direct effects also caused higher photosynthetic productivity in ditches and in rewetted sites than in undrained and drained main sites.

Influence of grazing and precipitation on ecosystem carbon cycling in a mixed-grass prairie

Grasslands sequester and store large amounts of soil carbon, which is primarily controlled by herbivory and precipitation. However, few studies have examined the combined effects of these two factors and quantified how they control carbon cycling in temperate grasslands. The objective of this study was to quantify how grazing intensity affects the magnitudes and patterns of net CO2 exchange in the mixed-grass prairie, the largest native grassland ecosystem in North America. The study was conducted during two contrasting precipitation years (dry vs. wet summer), which allowed investigation of the interaction between precipitation and grazing intensity on the magnitudes and patterns of net CO2 exchange. Our three grazing regimes have been in place for 20 years and consist of light and heavy grazing and ungrazed exclosures. Ecosystem CO2 exchange rates were strongly influenced by changes in summer precipitation. Decreasing summer precipitation reduced ecosystem respiration (RE) by 45%, gross ecosystem production (GEP) by 75%, and net ecosystem exchange (NEE) by 70%. The lightly grazed pastures had the greatest rates of RE, GEP, and NEE during the wet summer; however, NEE did not differ between grazing treatments in the dry summer. These results indicate that grazing intensity and precipitation interact to influence carbon cycling on mixed-grass prairie ecosystems.