Abstract
Incoming solar radiation is the main determinant of terrestrial ecosystem processes, such as primary production, litter decomposition, or soil mineralization rates. Light in terrestrial ecosystems is spatially and temporally heterogeneous due to the interaction among sunlight angle, cloud cover and tree-canopy structure. To integrate this variability and to know light distribution over time and space, a high number of measurements are needed, but tools to do this are usually expensive and limited. An easy-to-use and inexpensive method that can be used to measure light over time and space is needed. We used two photodegrading fluorescent organic dyes, rhodamine WT (RWT) and fluorescein, for the quantification of light. We measured dye photodegradation as the decrease in fluorescence across an irradiance gradient from full sunlight to deep shade. Then, we correlated it to accumulated light measured with PAR quantum sensors and obtained a model for this behavior. Rhodamine WT and fluorescein photodegradation followed an exponential decay curve with respect to accumulated light. Rhodamine WT degraded slower than fluorescein and remained unaltered after exposure to temperature changes. Under controlled conditions, fluorescence of both dyes decreased when temperatures increased, but returned to its initial values after cooling to the pre-heating temperature, indicating no degradation. RWT and fluorescein can be used to measure light under a varying range of light conditions in terrestrial ecosystems. This method is particularly useful to integrate solar radiation over time and to measure light simultaneously at different locations, and might be a better alternative to the expensive and time consuming traditional light measurement methods. The accuracy, low price and ease of this method make it a powerful tool for intensive sampling of large areas and for developing high resolution maps of light in an ecosystem.
Highlights
Solar radiation is a key factor associated with terrestrial ecosystem processes
Given that our samples were subjected to the higher temperature fluctuations that occur on terrestrial surfaces compared to being placed instream, we investigated the relationship between rhodamine WT (RWT) and fluorescein concentrations and temperature by placing 10 replicate samples of each dye type into growth chambers (Percival Incubators, Perry IA, USA)
Rhodamine WT and fluorescein dyes degraded upon exposure to solar radiation but at different rates
Summary
The amount of exposure to sunlight can determine rates of primary production by photosynthesis [1,2] and litter decomposition due to photodegradation [3,4,5], which influences soil mineralization rates [6,7]. Light in terrestrial ecosystems is spatially and temporally heterogeneous [8,9]. Tree-canopy traits, such as height, leaf shape or density determine light regimes, and there are obvious differences in incoming light under closed canopies, open canopies such as grassland ecosystems, or canopy gaps where light is patchy. Light heterogeneity may be especially significant in canopies that contain gaps or other features such as sunflecks, resulting in patchy light patterns that are of great ecological importance given that available light at small scales can significantly affect whole-system photosynthesis or leaf decay [10]. Changes in the angle of sunlight, which occur daily and annually, and cloud cover, can interact with tree-canopy structure further increasing variability [9,11,12]
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