Research Article |
Corresponding author: Paul W. Hacker ( paul.w.hacker@gmail.com ) Academic editor: Ingolf Kühn
© 2022 Paul W. Hacker, Nicholas C. Coops.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Hacker PW, Coops NC (2022) Using leaf functional traits to remotely detect Cytisus scoparius (Linnaeus) Link in endangered savannahs. NeoBiota 71: 149-164. https://doi.org/10.3897/neobiota.71.76573
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Identification of invasive plant species must be accurate and timely for management practices to be successful. Currently, Cytisus scoparius (Scotch broom) is expanding unmonitored across North America’s west coast, threatening established ecological processes and altering biodiversity. Remote detection of leaf functional traits presents opportunities to better understand the distribution of C. scoparius. This paper demonstrates the capacity for remotely sensed leaf functional traits to differentiate C. scoparius from other common plant species found in mixed grassland-woodland ecosystems at the leaf- and canopy-levels. Retrieval of leaf nitrogen percent, specifically, was found to be significantly higher in C. scoparius than each of the other 22 species sampled. These findings suggest that it may be possible to accurately detect introduced C. scoparius individuals using information collected from leaf and imaging spectroscopy at fine spatial resolutions.
Biodiversity, invasive plants, mixed grassland-woodland, partial least squares regression, Scotch broom, spectroscopy
The introduction of invasive plant species to an ecosystem can drastically alter diversity and threaten ecosystem processes, such as soil water dynamics and nutrient availability (
One such ecosystem exists along the west coast of North America. Defined as a mixed grassland-woodland, Garry oak (Quercus garryana Douglas ex Hook) savannahs provide habitat for several endangered plant and animal species (
C. scoparius presents a variety of challenges to native plant species (
Despite its damage to natural ecosystems, programs monitoring the spread of this species are not common. Initial analysis of multispectral satellite and airborne imagery has confirmed that large, dense C. scoparius patches can be observed during spring bloom; however a more reliable method of year-round identification at finer spatial scales is needed for realistic removal efforts (
Continuing improvements in both the platforms and sensors used for remote landscape classification present a variety of options for monitoring C. scoparius presence. The estimation of foliar functional traits across a site using remote sensing techniques presents an opportunity to identify invasive species, like C. scoparius, in mixed grassland-woodland ecosystems and has yielded success in a variety of other ecosystems (
Before air- or spaceborne analyses can be conducted, however, significant differences in both foliar functional traits and spectral reflectance between C. scoparius and other common mixed grassland-woodland plant species should be demonstrated at the leaf- and canopy-level. The aim of this study is to identify leaf functional traits of C. scoparius that are significantly different from other grassland-woodland species at the leaf- and canopy-levels through four hypotheses:
Leaf material for 23 plant species was collected in and around a mixed grassland – woodland savannah within the Cowichan Garry Oak Preserve (CGOP; 48°48'29.85"N, 123°37'54.34"W) between May 4–19, 2019 (Fig.
True color composite Imagery of a the Cowichan Garry Oak Preserve (CGOP) and b the extent of Quercus garryana (Little 1971) and locations of Cytisus scoparius (https://doi.org/10.15468/dl.dfdv48) individuals along North America’s west coast.
C. scoparius presents a unique challenge to Garry oak ecosystems due to its ecology. Labelled “invasive” due to profuse seed production and capacity for year round growth, this shrub faces limited competition from native plant species and is capable of altering soil chemistry through nitrogen fixation (
A total of 14 traits were measured across 23 unique plant species and four plant life forms. All leaf samples taken are considered to be from sunlit positions. Chemical evaluation of chlorophyll a + b (Chlab) and carotenoids (Car), as well as leaf dry matter content (LDMC), equivalent water thickness (EWT) and %N were conducted following standards presented by the Canadian Airborne Biodiversity Observatory (CABO) (
Individual leaf traits were modelled using partial least squares regression (PLSR), a statistical method well-suited for modelling datasets with high dimensionality, such as those created from spectroscopy. The data was split into training (70%) and test (30%) sets. This methodology models the relationship between spectral reflectance values recorded by leaf spectroscopy and measured leaf chemistry to enable the accurate prediction of leaf functional traits (
Radiative transfer models (RTM) are important methods of simulating the spectral reflectance of vegetation (
The four traits used as input arguments for the PROSAIL algorithm were Chlab, Car, LDMC and EWT. To determine the if canopy-level predicted traits react to changes in illumination geometry, such as different flight dates and latitudes, PROSAIL simulations were conducted at a variety of solar zenith angles spanning 20 – 70° at 1-degree intervals. The functional trait models derived from PLSR were then applied to these spectra to generate predicted trait values at the canopy-level. An independent 2-group Mann-Whitney test was used to determine if the predicted trait values of C. scoparius were significantly different from predicted trait values of the Site species.
All data manipulation was conducted in R (
An independent 2-group Mann-Whitney test determined that 11 of the 14 measured traits exhibited a significant difference between C. scoparius and the 22 Site species (Table
Resulting p-values from Mann-Whitney tests comparing measured Cytisus scoparius nitrogen percent with the 22 Site species. The difference in %N between C. scoparius and each of the 22 other species is significantly different (p < 0.05). The number of individuals sampled per species is included in parentheses under their names.
Species | Nitrogen (%) | Species | Nitrogen (%) |
---|---|---|---|
Berberis aquifolium Pursh (10) | 1.08E-05 | Lomatium utriculatum (Nuttall ex Torrey & A. Gray) J.J. Coulter & Rose (10) | 1.08E-05 |
Bromus sitchensis var. carinatus (Hooker & Arnott) R.E. Brainerd & Otting (10) | 1.08E-05 | Oemleria cerasiformis (Torrey & A. Gray ex Hooker & Arnott) J.W. Landon (10) | 1.08E-05 |
Bromus sterilis Linnaeus (6) | 2.50E-04 | Plectritis congesta (Lindley) de Candolle (10) | 1.08E-05 |
Camassia leichtlinii (Baker) S. Watson (10) | 5.67E-06 | Poa pratensis Linnaeus (10) | 2.50E-04 |
Camassia quamash (Pursh) Greene (10) | 1.08E-05 | Polystichum munitum (Kaulfuss) C. Presl (7) | 1.03E-04 |
Claytonia perfoliata Donn ex Willdenow (10) | 1.08E-05 | Quercus garryana Douglas ex Hooker (10) | 1.08E-05 |
Crataegus monogyna Jacquin (10) | 2.50E-04 | Rosa nutkana C. Presl (10) | 1.08E-05 |
Dactylis glomerata Linnaeus (10) | 1.08E-05 | Sanicula crassicaulis Poeppig ex de Candolle (10) | 1.08E-05 |
Festuca idahoensis Elmer (6) | 2.50E-04 | Sericocarpus rigidus Lindley (3) | 0.007 |
Holodiscus discolor (Pursh) Maximowicz (10) | 1.08E-05 | Symphoricarpos albus Poeppig ex de Candolle (10) | 1.08E-05 |
Lathyrus sphaericus Retzius (6) | 2.50E-04 | Vicia sativa Linnaeus (10) | 4.33E-05 |
Comparisons of 14 functional traits between C. scoparius and Site species. Boxplots depicting the differences between C. scoparius (yellow) and 22 other “Site” plants (green) for 14 leaf functional traits using a Mann-Whitney test, 11/14 of which are significantly different. The level of significance is denoted in the banner of each facet (* <= 0.05, ** <= 0.01, *** <= 0.001).
PLSR prediction plot. Comparison of the measured and predicted leaf nitrogen percent (%N) for 23 plant species at the Cowichan Garry Oak Preserve.
The use of the %N PLSR model to predict foliar %N from leaf spectral signatures determined that the leaf-level predicted %N values of C. scoparius and the 22 Site species were significantly different (W = 1910, p-value = 1.02e-07) (Fig.
Partial Least Squares Regression model evaluation. Functional traits selected for hypothesis testing and their associated model performance metrics (R2, Root mean squared error of the predictor (RMSEP), normalized-RMSEP (NRMSEP) and the number of components, or latent variables).
Trait | R2 | RMSEP (NRMSEP) | Components |
---|---|---|---|
Chlorophyll a (mg/g)* | .54 | 3.25 (31%) | 7 |
Chlorophyll b (mg/g)* | .56 | 1.16 (33%) | 8 |
Carotenoids (mg/g) | .36 | 0.68 (31%) | 4 |
Nitrogen (%)* | .70 | 0.5(17%) | 4 |
Carbon (%) | .48 | 0.99 (2%) | 6 |
C:N* | .71 | 2.98 (18%) | 4 |
Leaf mass per area (g/m2)* | .67 | 10.34 (25%) | 6 |
Leaf dry matter content (mg/g)* | .69 | 48.64 (22%) | 7 |
EWT* | .85 | 0.002 (16%) | 4 |
Solubles (%) | .41 | 9.78 (16%) | 4 |
Hemicellulose | .36 | 6.43 (40%) | 4 |
Cellulose* | .59 | 3.96 (27%) | 4 |
Lignin | .46 | 3.64 (55%) | 4 |
Recalcitrants | .28 | 0.12 (56%) | 4 |
Measured and predicted leaf %N. Comparison of measured and predicted leaf N% of C. scoparius (yellow) and 22 Site species of various lifeforms (Site; green) sampled at CGOP.
Measured leaf %N and predicted canopy N%. Comparison of the measured leaf-level and predicted canopy-level %N for C. scoparius (yellow) and 22 other plant species (Site; green) sampled at the CGOP in May 2019. Note that the y-axis scale varies, with the relative %N values predicted by PROSAIL being negative. This occurs as a result of using the relatively lower reflectance values generated by PROSAIL with a PLSR model developed using leaf-level spectra.
Predicted relative %N compared between C. scoparius and Site species using various solar zeniths. Boxplots demonstrating the difference between the PROSAIL predicted relative %N for C. scoparius (yellow) and Site species (green) using different solar zeniths (20 degrees, 45 degrees and 70 degrees).
Mapping the spatial extent of invasive plant species is a key component of managing biodiversity at any scale. In North America, the invasion of C. scoparius populations is destabilizing the traditional species composition of plant communities, especially in mixed grassland-woodland ecosystems (
This paper demonstrated that C. scoparius is distinguishable from other common grassland-woodland plants based on leaf functional traits, rather than bloom color. Multiple C. scoparius leaf traits were significantly different from those of 22 other plant species evaluated, with %N proving the most different. This is unsurprising as C. scoparius is a nitrogen-fixing legume and is likely to have leaves that are relatively nitrogen-rich (
The leaf-level PLSR model used to predict leaf %N explained 70% of the total variance between measured and predicted values while demonstrating a normalized error of 17%. The use of only four components suggests that this model is well fitted. Differences in measured and predicted leaf %N between C. scoparius and Site species promoted testing whether leaf %N was scalable from the leaf- to canopy-level. It is interesting to note that similar differences existed for C:N, suggesting that this trait could potentially be used to differentiate C. scoparius from Site species. This would, however, require the measurement of two traits, rather than one.
The RTM canopy model PROSAIL was used to simulate canopy reflectance of C. scoparius and Site species, and determined that significant differences in %N scale from the leaf to canopy. This scalability suggests that this method could be used for the detection of individuals that have recently been introduced. There are currently no civilian satellite programs capable of providing this type of data at the required spectral and spatial resolution, meaning that the imagery must be acquired from airborne sensors. Some studies have demonstrated that imagery collected from drone-based sensors can accurately map shrubland vegetation (
The significant differences in measured and predicted leaf %N between C. scoparius and 22 other plant species common in Canadian mixed woodland-grassland savannahs suggest that remote detection of C. scoparius is possible. This concept is supported by the up-scaling of leaf traits using the radiative transfer model PROSAIL, which demonstrated the aforementioned differences in leaf %N scale from the leaf- to the canopy-level. Successful scaling, in turn, proves that C. scoparius could be detected based on its relatively high leaf %N, given that remote sensing technologies have the required spectral and spatial resolutions to identify small, individual plants.
Technological advances have made RPAS more affordable, allowing them to become a common platform used for the collection of imagery with fine spatial resolution in a variety of ecosystems (
The authors would like to thank the Nature Conservancy of Canada (NCC) for allowing this research to take place at the Cowichan Garry Oak Preserve. Special thanks to Irv Banman for facilitating field work and providing relevant ecological and botanical knowledge. We also acknowledge Sabrina Demers-Thibeault, Rosalie Beauchamp-Rioux, Florence Blanchard, Maria Juliana Pardo Losada, Antoine Mathieu, Madeleine Trickey-Massé, Sabine St-Jean, Guillame Tougas, Charlotte Taillefer, Chris Mulverhill, Sam Grubinger, Chris Colton and Francois du Toit for their efforts in conducting field work. This research is funded by a NSERC Discovery Frontiers grant 509190-2017 supporting the Canadian Airborne Biodiversity Observatory (CABO).