Remote sensing of solar-induced chlorophyll fluorescence (SIF) provides great potential for estimating gross primary production (GPP) of terrestrial ecosystems. A strong relationship between SIF and GPP has been observed at the seasonal scale from both ground-based and satellite observations. However, variations of SIF due to changes in plant growth stages appear to influence the SIF-GPP relationship. It remains unclear how this relationship is affected by plant growth-related changes, especially for C4 plants such as maize. In this study, continuous in situ measurements for canopy far-red SIF retrieval and GPP calculation were made in maize during the growing season of 2017. Diurnal and seasonal variations of canopy SIF and its yield (SIFyield) were analyzed over different growth stages of maize to understand how they affect the relationship with GPP. The results show that the relationship between SIF and GPP varies with the growth stages of maize during the growing season, indicating that canopy structure has a strong impact on the seasonal variations of canopy SIF and its relation to GPP. Furthermore, we found that SIFyield is significantly correlated with canopy photosynthetic light use efficiency (LUE) at the canopy level throughout the season. However, it is almost uncorrelated with LUE after adjusting for the effects of canopy structure with the structural vegetation index MTVI2. This finding highlights the importance of canopy structure in the relationship between SIFyield and LUE, complicating the use of canopy SIF for tracking vegetation physiological activity. Overall, our observation-based findings show that canopy structure affects the SIF-GPP relationship, strengthening our understanding of the mechanistic link between SIF and photosynthesis.

Forest mortality is accelerating due to climate change and the largest trees may be at the greatest risk, threatening critical ecological, economic, and social benefits. Here, we combine high-resolution airborne LiDAR and optical data to track tree-level mortality rates for ~2 million trees in California over 8 years, showing that tree height is the strongest predictor of mortality during extreme drought. Large trees die at twice the rate of small trees and environmental gradients of temperature, water, and competition control the intensity of the height-mortality relationship. These findings suggest that future persistent drought may cause widespread mortality of the largest trees on Earth.

Traditional methods of carbon monitoring in mountainous regions are challenged by complex terrain. Recently, solar‐induced fluorescence (SIF) has been found to be an indicator of gross primary production (GPP), and the increased availability of remotely sensed SIF provides an opportunity to estimate GPP across the Western United States. Although the empirical linkage between SIF and GPP is strong, the current mechanistic understanding of this linkage is incomplete and depends upon changes in leaf biochemical processes in which absorbed sunlight leads to photochemistry, heat (via nonphotochemical quenching [NPQ]), fluorescence, or tissue damage. An improved mechanistic understanding is necessary to leverage SIF observations to improve representation of ecosystem processes within land surface models. Here we included an improved fluorescence model within the Community Land Model, Version 4.5 (CLM 4.5), to simulate seasonal changes in SIF at a subalpine forest in Colorado. We found that when the model accounted for sustained NPQ, this provided a larger seasonal change in fluorescence yield leading to simulated SIF that more closely resembled the observed seasonal pattern (Global Ozone Monitoring Experiment‐2 [GOME‐2] satellite platform and a tower‐mounted spectrometer system). We found that an acclimation model based on mean air temperature was a useful predictor for sustained NPQ. Although light intensity was not an important factor for this analysis, it should be considered before applying the sustained NPQ and SIF to other cold climate evergreen biomes. More leaf‐level fluorescence measurements are necessary to better understand the seasonal relationship between sustained and reversible components of NPQ and to what extent that influences SIF.