Lucie Lepine

Decreased water flowing from a forest amended with calcium silicate

Acid deposition during the 20th century caused widespread depletion of available soil calcium (Ca) throughout much of the industrialized world. To better understand how forest ecosystems respond to changes in a component of acidification stress, an 11.8-ha watershed was amended with wollastonite, a calcium silicate mineral, to restore available soil Ca to preindustrial levels through natural weathering. An unexpected outcome of the Ca amendment was a change in watershed hydrology; annual evapotranspiration increased by 25%, 18%, and 19%, respectively, for the 3 y following treatment before returning to pretreatment levels. During this period, the watershed retained Ca from the wollastonite, indicating a watershed-scale fertilization effect on transpiration. That response is unique in being a measured manipulation of watershed runoff attributable to fertilization, a response of similar magnitude to effects of deforestation. Our results suggest that past and future changes in available soil Ca concentrations have important and previously unrecognized implications for the water cycle.

Variation in foliar nitrogen and albedo in response to nitrogen fertilization and elevated CO2

Foliar nitrogen has been shown to be positively correlated with midsummer canopy albedo and canopy near infrared (NIR) reflectance over a broad range of plant functional types (e.g., forests, grasslands, and agricultural lands). To date, the mechanism(s) driving the nitrogen–albedo relationship have not been established, and it is unknown whether factors affecting nitrogen availability will also influence albedo. To address these questions, we examined variation in foliar nitrogen in relation to leaf spectral properties, leaf mass per unit area, and leaf water content for three deciduous species subjected to either nitrogen (Harvard Forest, MA, and Oak Ridge, TN) or CO2 fertilization (Oak Ridge, TN). At Oak Ridge, we also obtained canopy reflectance data from the airborne visible/infrared imaging spectrometer (AVIRIS) to examine whether canopy-level spectral responses were consistent with leaf-level results. At the leaf level, results showed no differences in reflectance or transmittance between CO2 or nitrogen treatments, despite significant changes in foliar nitrogen. Contrary to our expectations, there was a significant, but negative, relationship between foliar nitrogen and leaf albedo, a relationship that held for both full spectrum leaf albedo as well as leaf albedo in the NIR region alone. In contrast, remote sensing data indicated an increase in canopy NIR reflectance with nitrogen fertilization. Collectively, these results suggest that altered nitrogen availability can affect canopy albedo, albeit by mechanisms that involve canopy-level processes rather than changes in leaf-level reflectance.

Nitrogen cycling, forest canopy reflectance, and emergent properties of ecosystems

In Ollinger et al. (1), we reported that mass-based concentrations of nitrogen in forest canopies (%N) are positively associated with whole-canopy photosynthetic capacity and canopy shortwave albedo in temperate and boreal forests, the latter result stemming from a positive correlation between %N and canopy near infrared (NIR) reflectance. This finding is intriguing because a functional link between %N and NIR reflectance could indicate an influence of nitrogen cycling on surface energy exchange, and could provide a means for estimating %N using broad-band satellite sensors.

Evapotranspiration and water use efficiency in relation to climate and canopy nitrogen in U.S. forests

Understanding relations among forest carbon (C) uptake and water use is critical for predicting forest-climate interactions. Although the basic properties of tree-water relations have long been known, our understanding of broader-scale patterns is limited by several factors including: 1) incomplete understanding of drivers of change in coupled C and water fluxes, and water use efficiency (WUE); 2) difficulty in reconciling WUE estimates obtained at different scales; and 3) uncertainty in how evapotranspiration (ET) and WUE vary with other important resources such as nitrogen (N). To address these issues, we examined ET, gross primary production (GPP) and WUE at eleven AmeriFlux sites across North America. Our analysis spanned leaf and ecosystem scales, and included foliar δ13C, δ18O and %N measurements, eddy covariance estimates of GPP and ET, and remotely sensed estimates of canopy %N. We used flux data to derive ecosystem WUE (WUEe) and foliar δ13C to infer intrinsic WUE (iWUE). We found that GPP, ET and WUEe scaled with canopy %N, even when environmental variables were considered, and discuss the implications of these relationships for forest-atmosphere-climate interactions. We observed opposing patterns of WUE at leaf and ecosystem scales, and examined uncertainties to help explain these opposing patterns. Nevertheless, significant relationship between C isotope-derived ci/ca and GPP indicates that δ13C can be an effective predictor of forest GPP. Finally, we show that incorporating species functional traits - wood anatomy, hydraulic strategy and foliar %N -into a conceptual model improved the interpretation of Δ13C and δ18O vis-a-vis leaf to canopy water-carbon fluxes.

Use of waveform lidar and hyperspectral sensors to assess selected spatial and structural patterns associated with recent and repeat disturbance and the abundance of sugar maple (Acer saccharum Marsh.) in a temperate mixed hardwood and conifer forest

Waveform lidar imagery was acquired on September 26, 1999 over the Bartlett Experimental Forest (BEF) in New Hampshire (USA) using NASAs Laser Vegetation Imaging Sensor (LVIS). This flight occurred 20 months after an ice storm damaged millions of hectares of forestland in northeastern North America. Lidar measurements of the amplitude and intensity of ground energy returns appeared to readily detect areas of moderate to severe ice storm damage associated with the worst damage. Southern through eastern aspects on side slopes were particularly susceptible to higher levels of damage, in large part overlapping tracts of forest that had suffered the highest levels of wind damage from the 1938 hurricane and containing the highest levels of sugar maple basal area and biomass. The levels of sugar maple abundance were determined through analysis of the 1997 Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) high resolution spectral imagery and inventory of USFS Northern Research Station field plots. We found a relationship between field measurements of stem volume losses and the LVIS metric of mean canopy height (r2 = 0.66; root mean square errors = 5.7 m3/ha, p < 0.0001) in areas that had been subjected to moderate-to-severe ice storm damage, accurately documenting the short-term outcome of a single disturbance event.

Reply to Smith and Shortle: Lacking evidence of hydraulic efficiency changes

After calcium silicate amendment to an entire watershed at the Hubbard Brook Experimental Forest, evapotranspiration (ET) increased by ∼20% for 2 y, broadly attributed to a fertilization of tree physiology (1). We suggested that the increase in ET most likely arose from enhanced transpiration due to increased stomatal conductance (gs) associated with increased photosynthesis. Smith and Shortle (2) point out that enhanced xylem conductivity due to increased soil water ionic strength could help account for increased stomatal conductance because of the role calcium plays in the construction and efficiency of xylem water transport. We accept that this may be a relevant mechanism due to the importance of the entire hydraulic architecture of a tree to stomatal function (3). However, although we agree that enhanced xylem conductivity could have contributed to the enhanced ET response after calcium silicate amendment, we have no evidence that this mechanism was active during the enhancement. Our data were consistent with increased photosynthetic capacity [a major control on stomatal conductance (4)], which led to a general stimulation of primary production (tree and leaf biomass) during and after the enhanced ET (1). Thus, the available information leads us to conclude that increased gs was related to increased photosynthesis.