Spatial Dynamics of Invasive Para Grass on a Monsoonal Floodplain, Kakadu National Park, Northern Australia
Abstract
:1. Introduction
2. Materials and Methods
2.1. Site and Data Descriptions
2.2. Native Vegetation Displaced by Para Grass from 1986 to 2006 (Site I)
2.3. Production and Accuracy Assessment of Para Grass Map Series from 2001 to 2010 (Site II)
2.3.1. Estimating Trends and Variability in Para Grass Cover (Site II)
3. Results and Discussion
3.1. Native Vegetation Displaced by Para Grass from 1986 to 2006 (Site I)
3.2. Production and Accuracy Assessment of Para Grass Map Series from 2001 to 2010 (Site II)
3.3. Measuring Distribution Trends and Inter-Annual Dynamics of Para Grass (Site II).
4. Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Site Analyses | Map Variable | Mapping Year(s) | Description | Estimated Scale/Accuracy |
---|---|---|---|---|
Broad scale, generalized, analyses (Site I) | Para grass cover | 2006 | Published vegetation map produced by object-based image analysis from 2006 Landsat multi-temporal (dry season) composite using the Nearest Neighbour classifier [20]. | Horizontal accuracy ± 30 m. Overall accuracy for para grass cover class = 96% |
Native vegetation cover | 1986 | Published vegetation map produced by aerial photo interpretation from 1:25000 images, in conjunction with georeferenced field knowledge. [83]. | Quantitative accuracy of the original map is unmeasured. Map digitized and spatially co-registered to 2006 map (Section 2.2). | |
Finer-scale, inter-annual, analyses (Site II) | Para grass cover | 2001, 2004,2006, 2008, 2010 | This map series was produce from high spatial resolution satellite imagery (Table 2) using a supervised, object-based, classification (Section 2.3). Map accuracies were estimated using separate image samples reserved for validation. | Imagery/maps spatially co-registered to an accurately georectified (2006 QuickBird imagery) with horizontal accuracy approximately ± 2.5 m. Overall classification accuracies were ≥96% (Table 3, results). |
Para grass Cover | 1991, 1996 | Map produced by aerial photo interpretation of 1:25000 images, with georeferenced ground surveys of vegetation used to validate interpretations. Published methods [19]. | Quantitative accuracy of the original imagery/maps is unmeasured. | |
Water Depth | 2006 | A depth model of site I, extracted for Site II analyses. Modeled by regression between a Landsat dry-season composite and georefereced floodplain depth records. Published methods [51]. | Horizontal accuracy ± 30 m. Depth prediction strength R2 = 0.67, p < 0.0001, n = 254. Mapped at a spatial resolution of 30 m. Predicted depths ranged from 0 to 1.85 m in increments of 0.1 m. | |
Fire Scar maps | 2000, 2003, 2005, 2007, 2009 | Maps produced by object-based image analysis of Landsat (available dry-season imagery) using the Nearest Neighbour classifier. Published methods [51]. | Horizontal accuracy ± 30 m pixels. Overall classification accuracies for map series: 98%, 99.6%, 93%, 99% and 99% respectively. |
Sensor | Spatial Resolution | Analysis Resolution | Spectral Characteristics | Acquisition Date(s) | Additional Notes |
---|---|---|---|---|---|
IKONOS | Pixel size: 0.8 m (pan) 4 m (MS) All bands were provided at 1 m | 0.6 m | Band 1: 445–516 nm (Blue) Band 2: 506–595 nm (Green) Band 3: 632–698 nm (Red) Band 4: 757–853 nm (Near-IR) Pan: 450–900 nm Dynamic range: 11 bit | 03-06-2001 | Data geo-rectified and resampled to 1 m [84] |
QuickBird | Pixel size: 0.6 m (pan and pan-sharped bands), 2.4 m (MS) | 0.6 m | Band 1: 450–520 nm (Blue) Band 2: 520–600 nm (Green) Band 3: 630–690 nm (Red) Band 4: 760–900 nm (Near-IR) Pan: 445–900 nm Dynamic range: 11 bit | 25-06-2004 | panchromatic + 4-band multispectral product |
23-06-2006 24-07-2006 | 4-band, UNB-pan-sharpened mosaic geo-rectified to ground control and used as the base image for spatial co-registration | ||||
15-06-2008 | UNB-pansharpened | ||||
WorldView-2 | Pixel size: 0.49 m (pan) 2.4 m (MS) | 0.6 m | Band 1 *: 400–450 nm (Coastal) Band 2: 450–510 nm (Blue) Band 3: 510–580 nm (Green) Band 4 *: 585–625 nm (Yellow) Band 5: 630–690 nm (Red) Band 6 *: 705–745 nm (Red-edge) Band 7: 770–895 nm (Near-IR-1) Band 8 *: 860–900 nm (Near-IR-2) Pan: 450–800 nm | 15-05-2010 | panchromatic + 8-band multispectral product. Two separate scenes (Region 1 and 2), |
Model Training Class | Sampled Vegetation Cover Types |
---|---|
Para grass | Urochloa mutica: dense cover, near mono-culture—wet and dry phases |
Non-para grass | Native perennial grasses and floating vegetation mats: Dense vegetative cover dominated by Hymenachne acutigluma or Leersi hexandra |
Annual grasses and sparse native perennial grasses, and ephemeral sedges: Oryza meriondalis, Pseudoraphis spinecens and E. dulcis (native rice, mud-grass and water chestnut) | |
Non-floodplain grasses and bare ground | |
Sedges: Dense vegetative cover of perennial and ephemeral sedges (e.g., Eleocharis sphacelata and E. dulcis) | |
Open water Lilies dominated by Nymphaea or Nymphoides spp. | |
Nelumbo nucifera (red lily) | |
Melaleuca (paperbark trees) | |
Deeper open water with no emergent vegetation cover |
Group | Variable | Description | Derivation |
---|---|---|---|
Para Grass | Cumulative Persistence Score (CPS) | The persistence of para grass at any one location, over time (2001 to 2010). | The sum of binary map layers for para grass presence using maps 2001, 2004, 2006, 2008 and 2010 (i.e., present = 1, or not present = 0). |
(a) Cell-density, and (b) change in cell-densities | (a) The percentage of para grass cover measured within each 0.24 ha, hexagonal, sample cell of each map layer (2001, 2004, 2006, 2008, and 2010); and (b) the negative or positive change in cell-densities, calculated for each image-difference pair in series: 2001-04, 2004-06, 2006-08 and 2008-10. | Percentage cover calculated based on the number of para grass pixels as a proportion of the total number of pixels within a hexagonal cell. Change in cover then calculated by subtraction for image-difference pair (please refer to Equations (1) and (2), below). | |
Distance to patch and change in patch distance | The Euclidean distances (m) to nearest discrete para grass ‘patch’ over time, 2001 to 2010. Changes in patch distances were also measured for each image-pair in series: 2001-04, 2004-06, 2006-08 and 2008-10 and denoted as either an increase (+) or decrease (–) in distance. | The Euclidean distance function applied at 1 m resolution to each map. Zone statistics were then derived for each layer from the hexagonal sample matrix. Refer to Equation (3), below for the ‘change in distance’ calculation. | |
Distance to ‘Permanent’ Patch | The Euclidean distances (m) to the nearest ‘Permanent’ patch, defined as patches with a possible maximum cumulative persistence score (CPS) of 5 | Euclidean distance function applied at 1 m resolution. The spatial analyst ‘Reclass’ function was used to generate the CPS map layer. | |
Patch Size | Contiguous areas classified as para grass. Patch sizes (ha) were calculated for each classification layer: 2001, 2004, 2006, 2008 and 2010. | Patch areas (ha) calculated from polygon layers generated for all classifications. Georeferenced zone statistics (mean and maximum) were calculated for each respective layer using the hexagonal sample lattice. | |
Other | Depth habitat | Used in the analysis of variance (ANOVA) of para grass change in relation to three depth categories: ‘Shallow’, ‘Moderate’ and ‘Deep’. Selection of the depth ranges of each depth category were based on the distribution of para grass in relation mapped depth [51}. | |
Previous dry-season fire | The burnt (or unburnt) areas mapped in the first dry-season period between each image- difference pair (i.e., 2001-04, 2004-06, 2006-08 and 2008-10.) | The fire-scar maps derived from Landsat representing years 2003, 2005, 2007 and 2009 [51]. |
Class | Number of Pixels | Accuracy | Kappa Statistic | Error Rate (%) | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Image | Reference | Classified | Correct | Producers | Users | Overall | Producers | Users | Overall | Omission | Commission |
IKONOS (2001) | 410897 | 413727 | 377619 | 92 | 91 | 96 | 0.90 | 0.89 | 0.89 | 8 | 9 |
QuickBird (2004) | 707180 | 756735 | 664166 | 94 | 88 | 96 | 0.92 | 0.84 | 0.88 | 6 | 12 |
QuickBird (2006) | 617425 | 643414 | 590333 | 96 | 92 | 96 | 0.94 | 0.88 | 0.91 | 4 | 8 |
QuickBird (2008) | 625118 | 700247 | 618718 | 99 | 88 | 97 | 0.99 | 0.88 | 0.91 | 1 | 12 |
WorldView (2010, R1*) | 86461 | 89831 | 74124 | 86 | 83 | 99 | 0.85 | 0.82 | 0.83 | 14 | 17 |
WorldView (2010, R2*) | 273762 | 279851 | 264567 | 96.6 | 95 | 97.6 | 0.95 | 0.93 | 0.94 | 3.4 | 5 |
Cover Measurement | Depth Range (m) | Linear Regression Results | ||||
---|---|---|---|---|---|---|
Slope (b) | R | Adjusted R2 | p | Sig. | ||
Year by year totals | Shallow | 18 | 0.84 | 0.61 | 0.074 | * |
Moderate | 37 | 0.75 | 0.42 | 0.141 | * | |
Deep | 11 | 0.59 | 0.13 | 0.297 | ns | |
Cumulative ‘footprint’ overtime | Shallow | 37 | 0.99 | 0.98 | 0.001 | *** |
Moderate | 95 | 0.99 | 0.98 | 0.001 | *** | |
Deep | 46 | 0.99 | 0.98 | 0.001 | *** |
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Boyden, J.; Wurm, P.; Joyce, K.E.; Boggs, G. Spatial Dynamics of Invasive Para Grass on a Monsoonal Floodplain, Kakadu National Park, Northern Australia. Remote Sens. 2019, 11, 2090. https://doi.org/10.3390/rs11182090
Boyden J, Wurm P, Joyce KE, Boggs G. Spatial Dynamics of Invasive Para Grass on a Monsoonal Floodplain, Kakadu National Park, Northern Australia. Remote Sensing. 2019; 11(18):2090. https://doi.org/10.3390/rs11182090
Chicago/Turabian StyleBoyden, James, Penelope Wurm, Karen E. Joyce, and Guy Boggs. 2019. "Spatial Dynamics of Invasive Para Grass on a Monsoonal Floodplain, Kakadu National Park, Northern Australia" Remote Sensing 11, no. 18: 2090. https://doi.org/10.3390/rs11182090