ABSTRACTS (as of October 4, 2006, ordered alphabetically by last name)

IUFRO Canopy Processes Workshop 6-13 October, 2006

No.	Presenter	Title
1	Billings, S.	Dendrochronological indicators of northern red oak susceptibility to red oak borer
2	Bradford, J.	Spatial variability of carbon pools and fluxes: consequences for landscape-level estimates.
3	Butnor, J.	Estimating Decay Volumes in Living Trees with Ground-Penetrating Radar
4	Carter, J.	Water use of an agroforestry system measured at different spatial scales
5	Chen, J.	Ecosystem Water Use of Forests - A New Concept for Understanding C&H2O Cycles
6	Chiba, Y.	Reconstruction of Canopy Profile Using DBH and Tree Height
7	Clark, D.	Landscape-scale leaf area distribution across a tropical rain forest land-use gradient
8	Cleverly, J.	A long-term, regional flux network for evaluating climate, canopy processes, and evapotranspiration along the Mid Rio Grande, NM
9	Crous, K.	Nutrient and CO2 Interactions in Tree Photosynthesis
10	Culvenor, D.	Advances in remote sensing for forest structure assessment
11	Daley, M.	Changes in Ecohydrological Function due to the Loss and Replacement of Eastern Hemlock in a New England Forest
12	Duursma, R.	Summary models for irradiance interception and light-use efficiency of non-homogenous canopies
13	Ellsworth, D.	Acclimation to light in a pine canopy under long-term elevated atmospheric CO2
14	Ensminger, I.	Spring recovery of photosynthesis in conifers is modulated by soil temperature and intermittent frost
15	Ewers, B.	Quantifying Spatial Patterns of Transpiration across Environmental Gradients using Plant Hydraulics and Geostatistics
16	Foster, R.	Environmental control of the onset of photosynthesis in spring in a balsam fir ecosystem
17	Griffin K.	Seasonal variation in the temperature response of leaf respiration in Quercus rubra at the Black Rock Forest
18	Hadley J.	Differences in carbon/water cycling between early-successional deciduous forest and late-successional conifer forests: Implications for long-term effects of invasive hemlock woolly adelgid
19	Han, Q.	Effect of leaf age on seasonal variability of photosynthesis parameters and leaf nitrogen within a Pinus densiflora crown
20	Hollinger D.	New frontiers in understanding canopy processes at the regional level and beyond: Eddy covariance data, model parameter estimation, and data assimilation.
21	Iverson, L.	Regional modeling of potential effects of climate change on tree species habitats
22	Jeon, S.	The Effects of Land-use Change on the Terrestrial Carbon Budgets of New England
23	Kitaoka S.	Seasonal changes of photosynthetic production of larch plantation in Northern Japan
24	Kull, O.	Leaf Level Acclimation to Light at Elevated CO2: Poplar Plantation in EUROFACE
25	Kull, O.	Reflection of Experimental Drought and Warming at European Shrublands
26	Lewis, JD	Indirect effects of the hemlock woolly adelgid on oak seedling growth through effects on mycorrhizal richness and abundance
27	Logan, B.	Physiological impacts of eastern dwarf infection and developmental responses of host white spruce
28	MacLean R	Abiotic immobilization of nitrite in forest soils: a double label approach
29	Martin, A.	Response of Tsuga canadensis photosynthetic rate to changes in temperature and N-form
30	McCarthy, H.	Interaction of ice storms and management practices on current carbon sequestration in forests with potential mitigation under future CO2 atmosphere
31	Mencuccini, M.	The Carbon Cycle of Forest Chronosequences
32	Medhurst, J.	Spring photosynthetic recovery of boreal Norway spruce at the shoot- and tree-level under conditions of elevated [CO2] and air temperature
33	Megonigal, J. P.	Methane Cycling in Upland Forests: New Findings and Implications for Forest-Climate Interactions
34	Miller-Rushing, A	Effects of winter temperatures on flowering times in birch trees
35	Moore, G.	Nocturnal transpiration in Tamarix: A mechanism for temporal incongruence between sapflow and eddy covariance
36	O’Grady, A.	Constraints on transpiration in irrigated and rainfed Eucalyptus globulus trees in southern Tasmania
37	Olchev, A.	Responses of CO2 and H2O fluxes on land-use change in a tropical rain forest margin area in Central Sulawesi (Indonesia).
38	Ollinger, S.	Effects of multiple environmental stressors on northeastern forest carbon and nitrogen dynamics.
39	Pataki, D.	The isotopic composition of forest canopies: New issues and applications
40	Perämäki, M.	SPP - a model to estimate the photosynthetic production of forest stands applied to several pine stands across Europe
41	Peichl, M.	Carbon and water fluxes in a temperate pine forest chronosequence during a warm, dry summer in southern Ontario, Canada
42	Pettijohn, J. C.	A comparison of long-term irrigated and non-irrigated red maple transpiration
43	Phillips, N	Nocturnal Transpiration in Norway Spruce Trees is Consistent with the Nutrient Supply Hypothesis.
44	PulkkinenM.	Developing an empirical model of GPP with LUE approach: results from an analysis of eddy covariance data at five contrasting sites in Europe
45	Rodgers, V.	Impacts of Alliaria petiolata invasion on nutrient cycling and native plant diversity in southern New England forests
46	Rubino, L.	Hemlock woolly adelgid density affects net photosynthetic rates but not respiration rates or needle biochemistry in eastern hemlock
47	Ryan, M.G	Carbon Allocation in Forest Ecosystems
48	Sirulnik, A	Infestations of hemlock woolly adelgid are associated with changes in eastern hemlock ectomycorrhizal fungal communities and soil conditions
49	Smolander, S.	Current state of canopy spectral invariants in remote sensing
50	Stape, JL	Landscape-scale studies of ecosystem response to management: lessons for better interpretation of plot-level studies
51	Sterck, F.	Scaling up water relations in trees: Can we predict drought responses from underlying mechanisms and traits?
52	Templer, P.	Effect of calcium availability on nitrogen uptake by sugar maple and beech trees
53	Thomas, R.Q.	The importance of heterogeneity: integrating lidar remote sensing and height-structured ecosystem models to improve estimation forest carbon stocks and fluxes.
54	Tissue, D.	Spatial and temporal scaling of intercellular CO2 concentration in a temperate rainforest dominated by D. cupressinum in New Zealand
55	Turnbull, M.	Thermal Acclimation of Leaf Photosynthesis and Respiration in Populus Deltoides x Nigra
56	Way, D.	High growth temperatures reduce photosynthesis, respiration and growth in black spruce
57	Wharton, S.	The Impact of Water Stress on Net Carbon Exchange at the Wind River Old-growth Forest, Washington, USA
58	Whitehead, D.	Environmental regulation of ecosystem carbon exchange and water balance in a mature rainforest in New Zealand
59	Zhang, Q.	Large Area Forest Change Analysis Using Landsat Imagery and Generalized Methods
60	Zweifel, R.	Species-specific stomatal response of trees to microclimate – a functional link between climate change and vegetation dynamics

Dendrochronological indicators of northern red oak susceptibility to red oak borer

S.A. Billings^1,2*, L.J. Haavik^1,2, F.M. Stephen³, M.K. Fierke³, V.B. Salisbury², and S.W. Leavitt^{4 1}Department of Ecology and Evolutionary Biology and ²Kansas Biological Survey, University of Kansas, Lawrence, KS 66047 ³Department of Entomology, Agri. 319, University of Arkansas, Fayetteville, AR 72701 ⁴Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ 85721 *Corresponding author, e mail: sharonb@ku.edu phone: (785) 864-1560 fax: (785) 864-1534

Oak-dominated forests in northwestern Arkansas have recently experienced an unprecedented outbreak of an endemic insect, the red oak borer, Enaphalodes rufulus, that has induced significant oak mortality across much of the region. We used a suite of dendrochronological parameters to determine whether northern red oak (Quercus rubra) most susceptible to E. rufulus attack experienced greater drought stress than apparently healthy trees. We sampled Q. rubra experiencing various levels of E. rufulus infestation from areas with distinct site quality differences and examined their tree rings from 1954 to 2002, a period encompassing four severe drought and three relatively wet periods. We hypothesized that δ13C of α-cellulose from tree rings, intrinsic water use efficiency (Wi), basal area increment (BAI), ring-width indices, early- and latewood widths, and lignin concentrations would reveal evidence of drought stress in trees most heavily attacked by E. rufulus. Five decades prior to oak mortality induced by E. rufulus, growth measurements exhibit reduced BAI and earlywood widths in Q. rubra most heavily infested with E. rufulus. This suggests that the most heavily attacked trees, though dominant in the canopy and apparently never suppressed compared to their counterparts, were at a competitive disadvantage for a significant time period. Lignin concentrations revealed no significant trends with level of E. rufulus infestation, water availability, or site quality. Parameters describing tree water relations (δ13C and Wi) responded to drought prior to the 1970s; after this time, all study trees ceased exhibiting a response to drought via these parameters. This time period is also associated with an increase in E. rufulus activity in a nearby forest, which later induced significant oak mortality similar to that observed in the current study. Latewood widths were sensitive to water availability, but exhibited no trend with level of E. rufulus infestation. These data imply that drought stress may not have been a dominant factor governing Q. rubra susceptibility to E. rufulus attack, though we cannot be sure due to lost sensitivity of water use parameters in later study years.

Developing a method to characterize the response of a mountainous ecosystem to variations in climate using stable carbon isotopes of respired CO₂ in nocturnal cold-air drainage

B.J. Bond¹, H. Barnard¹, D. Conklin¹, M. Hauck¹, Z. Kayler¹, A.C. Mix³, C. Phillips¹, T. Pypker³, E.W. Sulzman², W.D. Rugh³, M.H. Unsworth³

¹Dept. Forest Science, Oregon State University, Corvallis, OR, USA, 97331

²Dept. Crop and Soil Science, Oregon State University, Corvallis, OR, USA, 97331

³College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331

At least 20% of the terrestrial surface of the earth is covered by mountains, which contain many of the world’s most productive ecosystems. Do such mountainous ecosystems respond differently to climate change than flat ecosystems by virtue of their topography? There are many reasons to think they might; however, most of the tools used to measure and monitor metabolic processes in ecosystems are difficult or impossible to use in complex, mountainous terrain. In a project we call the “Andrews Airshed project”, located in western Oregon Cascades, we are measuring the stable carbon isotope composition of ecosystem-respired CO₂ (d¹³C_R-eco) in nocturnal cold-air drainage systems, with the eventual aim of “inverting” this understanding to monitor intra- and inter-annual responses of ecosystem metabolism to environmental variation on a basin scale. We are characterizing patterns of airflow, quantifying the CO₂ concentration in the flow, and measuring d¹³C_R-eco as well as the fluxes and isotopic composition of respiration from soils and foliage. We have found that the characteristics of the cold air drainage system are nearly ideal for our sampling. Nocturnal air drainage from the basin is very well-mixed and deep, and the air samples we collect appear to contain a well-mixed sample from the entire basin. We estimate that up to 100% of nocturnally-respired CO₂ exits the 96 ha basin advectively. Weekly samples (June-Sept, 2005; May-Sept 2006) of d¹³C_R-eco reveal more than 3 per mil variation seasonally; this variation was more closely associated with soil moisture depletion than with humidity, and unlike previous investigations we found no temporal lag between environmental conditions and d¹³C_R-eco. This may be due, in part, to a larger contribution of foliage, rather than soil, to total ecosystem respiration in this ecosystem. Soil-respired CO₂ was isotopically enriched compared with above-ground air, and also enriched on the north-facing relative to the south-facing slope. Soil samples also reveal seasonal variability, but not to the same degree as the air samples. Analyses of isotopes in foliar-respired CO₂ are currently underway. Our results to date show that d¹³C_R-eco can be measured with great precision and accuracy in cold-air drainage from a small, deeply-incised watershed, and that variations in d¹³C_R-eco are a sensitive indicator of metabolic change on the basin-scale in response to environmental variation.

Spatial variability of carbon pools and fluxes: consequences for landscape-level estimates.

John Bradford¹, Michael G. Ryan²

¹USDA Forest Service NCRS, 1831 Hwy. 169 E, Grand Rapids, MN 55744

²USDA Forest Service RMRS, 240 West Prospect Rd. Fort Collins, CO 80526

Terrestrial ecosystems hold substantial carbon which may impact atmospheric carbon concentrations, potentially influencing climatic conditions. Consequently, quantifying carbon dynamics in forest systems at large scales is a central goal for ecosystem ecologists. Previous studies have characterized carbon pools and fluxes at plot scales, and other work has linked these estimates to remote sensing or simulation modeling efforts. However, few studies have quantified large-scale carbon pools and directly from plot measurements. This study addressed two questions: 1) How much do forest structure, carbon pools and fluxes vary across landscapes? and 2) How many plots are necessary to accurately characterize these processes over landscapes?

We established 36 nested plots (circular with 8-meter radius) in each of three subalpine rocky mountain forests. Within each plot, we quantified stand structure (height, leaf area index, basal area and density), carbon pools (aboveground live, aboveground dead, forest floor and mineral soil) and carbon fluxes (live biomass increment, litterfall, forest floor decomposition and net ecosystem carbon balance).

Our results indicate that stand structure displays the least variability whereas carbon fluxes display the most variability. These differences imply that stand structure requires the least effort to quantify (24 plots for 1 km2), carbon fluxes require the greatest effort (39 plots for 1 km2) and carbon pool require an intermediate sampling level (29 plots for 1 km²). These results suggest that characterizing carbon fluxes at landscape scales may require greater sampling intensity that traditionally used for forest inventories. In addition, we found that site history and vegetation structure influence the magnitude and scale of spatial variability and consequently impact the required sampling intensity and optimal spatial sampling design. These results have implications for numerous studies of regional to global scale carbon dynamics, which often rely on extremely limited field plots.

Estimating Decay Volumes in Living Trees with Ground-Penetrating Radar

John R. Butnor¹, Michele L. Pruyn², David C. Shaw³, Mark E. Harmon⁴ and Michael G. Ryan⁵

¹Southern Research Station, USDA Forest Service, Research Triangle Park, NC

²Biological Sciences, Plymouth State University, Plymouth, NH

³Department of Forest Science, Oregon State University, Corvallis, OR

⁴Forest Science Dept., Oregon State University, Corvallis, OR

⁵Rocky Mountain Exp. Station, USDA Forest Service, Fort Collins, CO

Decomposition by saprophytic organisms of wood in living tree stems contributes substantially to disease loss in U.S. forests, causing destruction or decreased value of usable timber and the depletion of carbon reserves in forest stands. Precise methods that predict rot volume are lacking. Estimating rot volume via external visual cues, (i.e. conks, broken branches, stem bulges, or stem discoloration) is often restricted to a certain species and site. Also, visual cues of rot in stems usually do not appear until the decay process is well underway. Our objective was to test and apply ground penetrating-radar (GPR) to non-destructively estimate active and inactive rot volumes in living trees. GPR is a geophysical tool which uses an antenna to propagate short bursts of electromagnetic energy in solid materials and measure the two-way travel time and amplitude of the reflected signals. When the transmitted energy contacts a layer of material with different electromagnetic properties (i.e. resistivity) a portion of the energy is reflected back to the antenna and discriminated. For this study we used the Tree Radar Unit (TRU) and TreeWin software developed by TreeRadar Inc. (Silver Spring, MD). The TRU consists of a commercially available SIR-3000 radar unit and a specially configured 900 MHz antenna (GSSI, North Salem, NH). In March 2005, with the assistance of a USDA Forest Service, hazard tree felling crew, we selected 10 trees of three different species for evaluation, followed by destructive verification: Pseudotsuga menziesii, Thuja plicata, and Tsuga heterophylla. Prior to felling, the circumference of each tree bole was scanned at several heights with the TRU (0-2 m). Once on the ground, partial circumferential scans (~50%) were collected every 2 – 5m, depending on the length of the bole. Cross-sectional “cookies” were sawed from each scanned location. Each cookie was photographed to preserve any visual information, and two perpendicular strips were sampled from each cross-section. In the laboratory, specific gravity and water content were measured at regular intervals from bark to pith. Using TreeWin we estimated rot volume at each elevation and compared it to physical and photographic data. GPR successfully detected abrupt changes in moisture content caused by decay, voids and wetwood. We found that near-surface decay and air-filled voids had unique electromagnetic signatures, which could be separated from other defects. Detection of incipient decay was possible, but it was difficult to grade levels of decay. We are currently working to understand non-target detections of water gradients and wood density in healthy material and separate it from actual defects. This work should allow more precise estimation of rot volumes in forest ecosystems and enable better prediction of losses to harvestable timber and carbon reserves.

Water use of an agroforestry system measured at different spatial scales

Jenny Carter^1,2, Phil Ward^1,3 and Don White^1,2

¹CRC for Plant-Based Management of Dryland Salinity, University of Western Australia, Crawley, Western Australia.

²Ensis

³CSIRO Plant Industry, CSIRO Centre for Environment and Life Sciences, Floreat, Western Australia.

Following the clearing of deep-rooted native vegetation for agriculture in southern Australia, trees are now being planted back into the landscape in an attempt to control rising groundwater and protect against dryland salinity. We measured the water use of widely-spaced belts of Eucalyptus kochii trees and inter-bays of annual crop, as part of a project parameterising models of tree growth and water use to predict impacts on hydrology. Tree transpiration and response to the environment was measured by leaf gas exchange and heat pulse sap flow sensors, while crop and whole system evapotranspiration was measured through water balance calculations and with eddy covariance.

Eucalyptus kochii had high rates of transpiration, and stomata were relatively insensitive to vapour pressure deficit, particularly when trees had access to groundwater. On a projected canopy area basis, trees with access to groundwater had transpiration rates that were similar to Priestley-Taylor potential evapotranspiration, which was approximately five-fold greater than evapotranspiration of the annual crop. Trees without access to groundwater had transpiration rates approximately double that of the annual crop.

Despite these differences in water use of trees and crop, eddy covariance measurements did not detect differences between parts of the landscape where trees did or did not have access to groundwater, or even between the crop (or bare soil) alone and the crop plus the tree belts. This may have been partly due to the fact that tree belts only occupied 5% of the landscape, with annual crop occupying the remainder. In the part of the system where trees had access to groundwater this proportion of tree planting was sufficient to prevent recharge, with an increase in whole-system evapotranspiration of 25%. However, over the entire study area integrating trees with and without access to groundwater, the belts increased the whole-system evapotranspiration by less than 10%. These results demonstrate the need for multi-scale measurements of water use when designing agricultural system aimed at incorporating hydrological benefits.

Ecosystem Water Use of Forests - A New Concept for Understanding C&H2O Cycles

Jiquan Chen, University of Toledo, jiquan.chen@utoledo.edy; 419-530-2664

Compared to ecosystem carbon exchange, the dynamics and controls of ecosystem water balance have received relatively less attention. Yet, these two fluxes are tightly interdependent, with water availability having a direct effect on the carbon budget. Using direct measurements (eddy flux towers) of net ecosystem exchange (NEE) of water and carbon from over several ecosystem types in North America, I examined the magnitude and rate of change in ecosystem water use (WUEe, defined as the mass ratio of NEE of carbon to NEE of water). In addition to constructing the empirical relationships for estimating WUEe that incorporate disturbance and stand age, as well as the biophysical regulations such as vapor pressure deficit (VPD), solar radiation, and soil moisture conditions, I will demonstrate the use of this new concept in understanding ecosystem water consumption and needs. WUEe varied little throughout the growing season (June through September) in both regions, but there were notable differences in WUe with stand age. In northern Wisconsin, WUe was higher in mature stands (4.5 mg C g H2O) than in the younger, recently disturbed stands (2.1 mg C g H2O); yet WUe was much higher in 20 and 40 year-old stands (0.34-5.57) than in a 450-year-old stand (0.38-1.59) in the Pacific Northwest. The lowest WUe was found in a recent clearcut, where leaf area index was also the lowest. VPD is found to be a significant predictor of WUe in many ecosystems. Given the broad network of eddy flux towers currently collecting NEE data across the United States (e.g., Fluxnet), we expect that in the future we will be able to make further generalizations about WUe in ecosystems of different ages and under various disturbance regimes.

Reconstruction of Canopy Profile Using DBH and Tree Height

Yukihiro Chiba Forestry and Forest Products Research Institute, Tsukuba, Ibaraki, 305-8687 Japan

Forest stands respond to environmental changes through physiological processes: such as energy capture, carbon gain, and resource allocation. Canopy components of foliage and branches contribute to such physiological processes to a great extent, so that one should quantitatively describe the 3D distribution of a forest canopy to understand forest dynamics and productivity in relation to canopy and stand structure. However, it would be laborious and time-consuming to get through tree harvest and data processing for reconstructing canopy structure. The objective of this study is to develop a simple and useful method to reconstruct vertical distributions of individual tree crowns in a forest stand, and then a canopy profile of the stand by summing up all of the trees. Several architectural tree form models provided theoretical bases for developing a new method.

Vertical distribution of leaves of a forest tree would strongly depend on branching architecture, as is pointed out in the pipe model (Shinozaki et al. 1964, Chiba 1990). Thus tree trunks that consist of branches flowing into should be affected by stand density. Let the weight densities of leaves, branches, and stems at position z along the stem be G(z), B(z) and S(z), respectively. The mutual relationship can be formulated as

dS/dz = 1/b (G(z) + B(z) + S(z)), (1)

where b is a constant (Chiba 1991). Employing this equation, stem form can be modeled by a hyperbolic function with two asymptotes of exponential functions. Examining the parameters included in these equations as related to the stem form and crown architecture, strong interdependencies were found with tree size: i.e. DBH and tree height. Using the stem form model, therefore, the vertical profile of leaves and branches (G(z) + B(z)) of an individual tree can be reconstructed in combination with Equation (1).

The interrelationships among tree height H, height at crown base HB, and diameter at breast height DBH can easily be expressed by empirical formulae whose parameters are specific to the stand. Employing these formulations mentioned above, it is possible to reconstruct canopy structure (or profile) representing the vertical distributions of leaves and branches in a forest stand as related with stand structure.

Landscape-scale leaf area distribution across a tropical rain forest land-use gradient

David B. Clark¹, Paulo Olivas², Steven F. Oberbauer², Michael G. Ryan³, Deborah A. Clark¹ and Harlyn Ordoñez⁴

¹La Selva Biological Station, Puerto Viejo de Sarapiquí, COSTA RICA and Department of Biology, University of Missouri-St. Louis, St. Louis, MO USA dbclark@sloth.ots.ac.cr

²Department of Biological Sciences, Florida International University, Miami, FL, USA.

³USDA Forest Service, Rocky Mountain Research Station, Ft. Collins, CO, USA.

⁴Organization for Tropical Studies, La Selva Biological Station, Puerto Viejo de Sarapiquí, COSTA RICA.

Current estimates of tropical rain forest (TRF) carbon stocks and fluxes are poorly constrained due to a variety of technical issues. The TOWERS project is using multiple approaches to develop independent estimates of GPP, NPP, and NEE over a TRF landscape with multiple land use histories at the La Selva Biological Station, Costa Rica. A critical need for these estimates is the distribution of leaf area and biomass across the landscape. We focused on upland old-growth tropical rain forest using a random design stratified by soil nutrients and slope conditions. A GIS was used to map 515 ha of old growth into 10 x 10 m blocks of high, medium, or low conditions of slope and total soil P, resulting in a 9 x 9 matrix with approximately 5700 blocks in each cell. Within each of the 9 categories five sites were selected using random coordinates, and a walk-up canopy tower was erected. Within the 1.86 x 2.45 m tower footprint all plant biomass was harvested, stratified by tower section (height above ground) and plant functional group. Secondary forests of 17, 27 and 44 years were also sampled using the same tower-based approach.

In old growth mean landscape-level Leaf Area Index (LAI) was 5.99 + 0.33 (1 SEM). Trees (55%), palms (22%) and lianas (13%) accounted for 90% of total LAI, while herbaceous epiphytes and climbers, understory herbs and ferns accounted for the remainder. At the tower footprint scale (ca. 5 m²), LAI was not related to soil P or slope and forest height explained only 12% of the variation in total LAI. The weak relation of forest height to total LAI is partially explained by the rarity of low canopy sites in a random sample. In a sample of additional sites selected to span canopy heights from 0 - 20 m, total LAI was highly correlated with canopy height (r²=0.70), reaching the landscape average LAI at 17 m.

In secondary forests LAI increased from 4.21 in the 17 year-old abandoned pastures to levels comparable to old growth in the 27 and 44 year-old forests (6.37 and 6.45 respectively).

We discuss the implications of these results for analyses of TRF landscape canopy structure at large spatial scales and across gradients of different land use. We also show how we will incorporate the results into carbon exchange modeling and on-going canopy research using high-resolution remotely-sensed data.

A long-term, regional flux network for evaluating climate, canopy processes, and evapotranspiration along the Middle Rio Grande, New Mexico

James R. Cleverly^1*, Clifford N. Dahm¹, James R. Thibault¹, Kiyoshi Hattori², and Lawrence E. Hipps²

¹Department of Biology, University of New Mexico, Albuquerque, New Mexico, 87154

²Department of Plants, Soils, and Biometeorology, Utah State University, Logan, UT, 84322

*Correspondence: cleverly@sevilleta.unm.edu, 505-277-9341, 505-277-5355 (FAX)

The Middle Rio Grande (MRG) in New Mexico provides habitat for a wide diversity and of vegetation: from low-stature grasslands and xeroriparian shrublands to dense forests and thickets of native and non-native species. This paper presents the long-term record of riparian canopy flux processes using eddy covariance measurements of water, energy, and carbon fluxes collected from representative riparian forests: (1) native Populus deltoides ssp. wislizeni (Rio Grande cottonwood), (2) native P. deltoides with a dense non-native understory of Tamarix chinensis (saltcedar) and Elaeagnus angustifolia

(Russian olive), (3) T. chinensis-Distichlis spicata (saltgrass) mosaic woodland, (4) dense, monospecific T. chinensis, and (5) early successional E. angustifolia and Salix exigua (coyote or sandbar willow). Energy balance closure (75-90%) was invariant from year to year and did not improve following standard flux corrections. Evapotranspiration (ET), ranging from 69 to 134 cm/yr, was not different between T. chinensis, P. deltoides, and E. angustifolia and the highest seasonal ET rates were measured from a mixed stand of all three species. On a daily basis, average ET fluxes were higher in P. deltoids forests (5.5 mm/day) than in T. chinensis thickets (4.8 mm/day), reflecting the shorter growing season observed in the southern reaches of the MRG due to local topographic effects. Daytime carbon dioxide flux (-0.33 ± 0.01 mg/m2 s) was decoupled from water fluxes during flooding at the T. chinensis site due to delayed leaf-out, and the response of nighttime CO2 flux to cessation of flooding was dependent upon the local history of

soil saturation during flooding. Because of the episodic nature of meteorological and hydrological events like flooding and drought, long-term monitoring of fluxes provides a more complete understanding of canopy-atmosphere interactions than typical short-term studies.

Key words: Drought, canopy water use, micrometeorology, eddy covariance,

evapotranspiration, energy balance, Middle Rio Grande, saltcedar, cottonwood, Russian olive, Tamarix chinensis, Populus deltoides ssp. wislizeni, Elaeagnus angustifolia

Nutrient and CO2 Interactions in Tree Photosynthesis

Kristine Y. Crous, School of Natural Resources & Environment, University of Michigan 440 Church St. ,Ann Arbor, MI 48109, USA

Theory and measurements suggest that when tree growth is significantly limited by resources like nutrients, then the CO₂-induced enhancement of net photosynthesis is reduced. In a mature loblolly pine forest at a N-limited site under elevated atmospheric CO₂ (560 ml l^-1) in FACE, I previously observed photosynthetic adjustments to long-term CO₂ enrichment (FACE) in one-year old needles of Pinus taeda. I hypothesized that with N addition at the Duke FACE experiment, the reduction in net photosynthesis in one-year old needles would be alleviated. The experimental treatments at this site are three replicates of elevated CO₂ (560 ml l^-1) exposure treatments in FACE, which has been ongoing for ten years. Within each CO₂ replicate, half of each plot received N addition at 110 kg N ha^-1 in a split-plot design with ambient and elevated levels of N. Gas-exchange measurements and CN elemental analysis were done for each leaf sample in upper and lower canopy of Pinus taeda. We found that the slope of the photosynthesis-nitrogen relationship was significantly reduced by long-term elevated CO₂. The slopes of photosynthesis vs. nitrogen and carboxylation vs. N were both significantly reduced in elevated CO₂ in one-year old needles but not in current year needles, suggesting a reduction in nitrogen-use efficiency in long-term elevated CO₂. In the second year of N fertilization, there was a significant effect of N addition, particularly in the elevated CO₂ treatment. The nitrogen content of needles support this finding. The re