Accurate estimation of terrestrial photosynthesis has broad scientific and societal impacts. Measurements of photosynthesis can be used to assess plant health, quantify crop yield, and determine the largest CO₂ flux in the carbon cycle. Long-term and continuous monitoring of vegetation optical properties can provide valuable information about plant physiology. Recent developments of the remote sensing of solar-induced chlorophyll fluorescence (SIF) and vegetation spectroscopy have shown promising results in using this information to quantify plant photosynthetic activities and stresses at the ecosystem scale. However, there are few automated systems that allow for unattended observations over months to years. Here we present FluoSpec 2, an automated system for collecting irradiance and canopy radiance that has been deployed in various ecosystems in the past years. The instrument design, calibration, and tests are recorded in detail. We discuss the future directions of this field spectroscopy system. A network of SIF sensors, FluoNet, is established to measure the diurnal and seasonal variations of SIF in several ecosystems. Automated systems such as FluoSpec 2 can provide unique information on ecosystem functioning and provide important support to the satellite remote sensing of canopy photosynthesis.

Anthropogenic and environmental pressures on wetland hydrology may trigger changes in carbon (C) cycling, potentially exposing vast amounts of soil C to rapid decomposition. We measured soil CO₂efflux (R_s) continuously from 2009 to 2010 in a lower coastal plain forested wetland in North Carolina, U.S., to characterize its main environmental drivers. To understand and quantify the spatial variation due to microtopography and associated differences in hydrology, measurements were conducted at three microsites along a microtopographic gradient. The seasonal hysteresis inR_sdiffered by microtopographic location and was caused by the transitions between flooded and nonflooded conditions. Because floodedR_swas small, we reportedR_sdynamics mainly during nonflooded periods. A nested model, modified from conventionalQ₁₀(temperature sensitivity) model with dynamic parameters, provided a significantly better simulation on the observed variation ofR_s. The model performed better with daily data, indicating that soil temperature (T_s) and water table depth (WTD) were the primary drivers for seasonal variation. The diel variation ofR_swas high and independent ofT_sand WTD, which both had small diel variations, suggesting the likely association with plant activity. Overall, the site‐average soil CO₂efflux was approximately 960–1103 g C m⁻² yr⁻¹in 2010, of which 93% was released during nonflooded periods. Our study indicates thatR_sis highly linked to hydroperiod and microtopography in forested wetlands and droughts in wetlands will accelerate soil C loss.

Sun-induced chlorophyll fluorescence (SIF) measurements have shown unique potential for quantifying plant physiological stress. However, recent investigations found canopy structure and radiation largely control SIF, and physiological relevance of SIF remains yet to be fully understood. This study aims to evaluate whether the SIF-derived physiological signal improves quantification of crop responses to environmental stresses, by analyzing data at three different spatial scales within the U.S. Corn Belt, i.e. experiment plot, field, and regional scales, where ground-based portable, stationary and space-borne hyperspectral sensing systems are used, respectively. We found that, when controlling for variations in incoming radiation and canopy structure, crop SIF signals can be decomposed into non-physiological (i.e. canopy structure and radiation, 60% ∼ 82%) and physiological information (i.e. physiological SIF yield, Φ_F, 17% ∼ 31%), which confirms the contribution of physiological variation to SIF. We further evaluated whether Φ_F indicated plant responses under high-temperature and high vapor pressure deficit (VPD) stresses. The plot-scale data showed that Φ_F responded to the proxy for physiological stress (partial correlation coefficient, r _p= 0.40, p< 0.001) while non-physiological signals of SIF did not respond (p> 0.1). The field-scale Φ_F data showed water deficit stress from the comparison between irrigated and rainfed fields, and Φ_F was positively correlated with canopy-scale stomatal conductance, a reliable indicator of plant physiological condition (correlation coefficient r= 0.60 and 0.56 for an irrigated and rainfed sites, respectively). The regional-scale data showed Φ_F was more strongly correlated spatially with air temperature and VPD (r= 0.23 and 0.39) than SIF (r= 0.11 and 0.34) for the U.S. Corn Belt. The lines of evidence suggested that Φ_F reflects crop physiological responses to environmental stresses with greater sensitivity to stress factors than SIF, and the stress quantification capability of Φ_F is spatially scalable. Utilizing Φ_F for physiological investigations will contribute to improve our understanding of vegetation responses to high-temperature and high-VPD stresses.

Soil respiration (R_s), the largest carbon emission flux in ecosystems, is usually modeled as an empirically parameterized function of temperature, and sometimes water availability. The likely contribution by other factors, such as carbohydrate substrate supply from photosynthesis, has been recognized, but modeling capacity to use this information is limited. Wavelet transformations of the residuals of a seasonal Q₁₀ temperature response model indicated structure at different temporal scales. We hypothesize that this indicates the lack of explicit representation of relevant processes in the models. Using cross‐spectral analysis, we found that time series of photosynthetically active radiation explained most of the diurnal variation, temperature, explained variability at multiple time scales (diurnal‐synoptic), whereas volumetric soil water content correlated with variability in R_s at scales 15‐30 days. The results suggest that the time domains of influence for different driving variables of R_s are discrete, and largely nonoverlapping, and represent functional relationships between soil biological activity and its constraints. Analysis of phase angles showed that R_s lagged photosynthetically active radiation by 1.5‐3.0 hr. As this time lag was the same in both young and mature trees, with more than fivefold difference in transport distances, we hypothesize that this finding adds to the body of literature that support the pressure‐concentration‐wave model of carbohydrate availability in plants.

The influence of aquaporin (AQP) activity on plant water movement remains unclear, especially in plants subject to unfavorable conditions. We applied a multitiered approach at a range of plant scales to (i) characterize the resistances controlling water transport under drought, flooding, and flooding plus salinity conditions; (ii) quantify the respective effects of AQP activity and xylem structure on root (K_root), stem (K_stem), and leaf (K_leaf) conductances; and (iii) evaluate the impact of AQP-regulated transport capacity on gas exchange. We found that drought, flooding, and flooding plus salinity reduced K_root and root AQP activity in Pinus taeda, whereas K_root of the flood-tolerant Taxodium distichum did not decline under flooding. The extent of the AQP control of transport efficiency varied among organs and species, ranging from 35–55% in K_root to 10–30% in K_stem and K_leaf. In response to treatments, AQP-mediated inhibition of K_root rather than changes in xylem acclimation controlled the fluctuations in K_root. The reduction in stomatal conductance and its sensitivity to vapor pressure deficit were direct responses to decreased whole-plant conductance triggered by lower K_root and larger resistance belowground. Our results provide new mechanistic and functional insights on plant hydraulics that are essential to quantifying the influences of future stress on ecosystem function.