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Terrestrial Lidar and Chlorophyll Fluorescence Reveal Structure-to-Function Relationships of Spruce Saplings at the Forest-Tundra Ecotone
Andrew
Maguire, University of Idaho, magu7563@vandals.uidaho.edu
(Presenter)
Jan
Eitel, University of Idaho, jeitel@uidaho.edu
Lee
A.
Vierling, University of Idaho, leev@uidaho.edu
Daniel
Johnson, University of Idaho, danjohnson@uidaho.edu
Kevin
Griffin, Columbia University, griff@ldeo.columbia.edu
Natalie
Boelman, Lamont-Doherty Earth Observatory, Columbia Univ., nboelman@ldeo.columbia.edu
Johanna
Jensen, Columbia University, jej2141@columbia.edu
Arjan
Meddens, University of Idaho, ameddens@uidaho.edu
Purpose and Methods
The effect of climate – and microclimate – on treeline position at the latitudinal forest-tundra ecotone (FTE) is poorly understood. While the Arctic FTE is expansive (~13000 km), understanding relationships among climate, tree location, and tree function may depend on very fine scale processes, therefore tools are needed to appropriately characterize the leading (northernmost) edge of the FTE. We hypothesized that microstructural metrics obtainable from lidar remote sensing can explain variation in the physical growth environment that governs sapling photosynthetic processes. To test our hypothesis, we used terrestrial laser scanning (TLS) to collect highly spatially resolved (<1 cm) 3-D structural information of white spruce (Picea glauca) saplings and their aboveground growth environment at the leading edge of a FTE in northern Alaska. Coordinates of sapling locations were extracted from the 3-D TLS data along with sapling heights. Dark-adapted chlorophyll fluorescence was measured using a handheld fluorometer on three needle bundles per sapling. Digital terrain models (DTMs) and digital surface models (DSMs) were interpolated based on ground- and canopy-classified laser returns of the TLS data, respectively. Five terrain attributes were modeled from interpolated DTMs: average aspect, average slope, average curvature, wind-shelter index, and solar insolation. Two canopy attributes were modeled from interpolated DSMs: wind-shelter index and solar insolation. Average canopy depth was calculated as the height difference between DTMs and DSMs. Ground roughness and canopy roughness were calculated as the standard deviation of the heights of ground- and canopy-classified laser return. Each microstructural metric was extracted within variable search radii (0.1 – 2.0 m) from the sapling coordinates.
Results and Discussion
Random-Forest statistical modeling was conducted to determine which microstructural metrics were most strongly related to chlorophyll fluorescence (Fv/Fm, the maximum quantum yield of primary photochemistry). Wind-shelter index from the dominant wind direction (south-southeast) and solar insolation from the seven day period prior to sampling within 2.0 m radius from the saplings explained the most variance in Fv/Fm. Both terrain attributes were found to be moderate predictors of Fv/Fm using simple linear regression (r2 = 0.40; RMSE = 0.024). The observed Fv/Fm values were low (0.67 – 0.79), potentially indicating chronic stress commonly observed at other ecotones. The solar radiation environment and the wind environment, as characterized based on TLS data, both constrain sapling photosynthesis. These findings corroborate broader research on the importance of the Arctic light environment (360 solar azimuth and 24 h daylight during growing season) and treeline wind environment on plant function. Our results indicate that high resolution structural information from TLS remote sensing may provide valuable insights on sapling function at the FTE. In particular, the observed negative relationship with wind-shelter and Fv/Fm is surprising given ample evidence of the importance of positive sheltering effects for immature trees at treeline. The coupling of wind and light environments suggests that fine-scale exposure plays a critical role in the condition of spruce trees during vulnerable development stages. These results reveal a sensitivity of the photosynthetic performance of spruce saplings to fine-scale exposure, with respect to both wind and light, suggesting that temperature regulation may be a critical process for photosynthesis at the FTE. Characterizing the relationships between the remotely sensible structural growth environment and ecological responses is of great interest to broader research endeavors of ecological vulnerability at the FTE.
Associated Project(s):
Poster Location ID: 68
Session Assigned: Vegetation Dynamics and Distribution
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