Using drone technology to investigate topographic effects on plant phenology at small spatial scales across multiple elevation gradients

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Zachariah Madsen, Rocky Mountain Biological Labratory, Gothic, CO

Background/Question/Methods
Phenotypic plasticity in plant phenology is a key mediator of biological responses to a rapidly changing climate. In temperate mountain ecosystems, climate change has increased temperatures, reduced snowpack, and accelerated snowmelt, inducing earlier reproduction in a diversity of organisms. These ecosystems are spatially heterogeneous, and the effects of accelerated snowmelt potentially could be modified by microtopography and other sources of small-scale environmental variation. In this study, we used drone-based remote sensing to examine if microtopography affects phenotypic plasticity in flowering time of Boechera stricta (Brassicaceae) in five common gardens located in the Elk Mountains (Colorado, USA). We used photogrammetry techniques to reconstruct 1-cm resolution Digital Elevation Models (DEMs) and Digital Surface Models (DSMs) from drone imagery of each garden, as well as a variety of topographic indices, including topographic wetness, slope, aspect, solar radiation, and catchment area. We compared these values to over 3000 observations of reproductive phenology in B. stricta transplants exposed to control conditions and early snow removal treatments. Specifically, we modeled flowering phenology (the number of days to first flower after snowmelt) as a function of microtopographic variables and snow removal treatment using Bayesian Generalized Linear Mixed Models.
Results/Conclusions
Microtopography influenced flowering time differently at high vs. low elevations. For example, a negative correlation between topographic wetness and days-to-flower at the higher elevations suggested that wet conditions induced earlier flowering in these mesic locales. However, in arid lower elevation gardens the relationship was reversed, suggesting that plants in drier microsites flowered earlier. These results highlight the complexity and context dependency of the relationship between microtopography and flower phenology. In addition, our results suggest that drone-based remote-sensing techniques, which can effectively measure environmental characteristics at centimeter scales, can provide important new insights when applied to traditional ecological experiments.