BigFoot Pilot Study: Test of the Vegetation Cover Component Characterization System (3CS)
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BigFoot Pilot Study: Test of the Vegetation Cover Component Characterization System (3CS) Warren B. Cohen, Polly Thornton and Tom K. Maiersperger Introduction BigFoot objectives require that we accurately characterize fractions of vegetation cover components at all four sites. Doing so facilitates several subsequent characterizations and analyses. Foremost among these is the provision of cover maps based on ETM+ data that provide building blocks for a variety of prescribed land cover classication systems. The BigFoot exible land cover classication system enables our cover data layers to be compared with many alternate cover classications developed by others for the same study sites. To accomplish the goal of providing fractional characterizations of cover components at each site we developed the Vegetation Cover Component Characterization System (3CS). In the summer and early autumn of 1999 we conducted a pilot study to test the system. Field sampling is expensive, especially over four 25 km2 sites. Consequently, for BigFoot, we had to devise a sampling strategy that would take full advantage of data from a limited number of plots. This meant utilizing the spatial domain, in combination with modeling and more classical statistical analyses involving the plot-level data. The sampling strategy we use (see Figure 1) involves measurements at 100 plots, 80 of which are arranged as a nested spatial series (Clinger and Van Ness 1976). Within each plot, several types of measurements, described in the BigFoot Field Manual are made at several points from which plot-level means are calculated. For most biophysical measurements (e.g., LAI, biomass, NPP) protocols were developed in other studies (Gower et al. 1999), but for characterizing cover fractions using 3CS there was no precedent. Critical to the accurate characterization of cover components within a plot is the number of photo samples required. An additional consideration for the sampling scheme was the apparent cost of misregistration. To develop the BigFoot ne-grained data surfaces of land cover, LAI, and NPP, it is imperative that the plot measurements be accurately aligned with the ETM+ imagery, and that any error associ- ated with the misregistration of these measurement sets be assessed. Over the relatively at terrain of the BigFoot sites, misregistration of ETM+ imagery, on average, will likely be on the order of one pixel or less. Each BigFoot plot is 25 m on a side and we are resampling the ETM+ data to that same resolution. To insure that the eld plots are geo- referenced as accurately as possible, they were positioned using a GPS with real-time correction and accurate to the nearest 0.1 m. To assess the apparent cost of misregistra- tion we focused on the vegetation cover component frac- tions. As these represent the most fundamental properties of the system, if misregistration is likely to be an issue, it should be expressed most prominently in this dataset. Figure 1, BigFoot Sampling Strategy Design The rst two sites to be characterized were NOBS (7-12 July) and AGRO (10 September). For these two sites, the design on the left side of Figure 2 below was used. For each cell in the 3 x 3 cell group (or test plot) we sampled four points spaced evenly across each cell within the test plot, as shown. Twelve Cohen et al. - 15 June 2000 - Page 1
additional points were sampled in the center cell; these were evenly spaced within the bounds of four corner points. At NOBS, one test plot was sampled in each of four cover types: 1) muskeg, 2) black spruce, 3) mixed black spruce, jack pine, and aspen, and 4) wetland. These cover types are roughly equivalent to the four cover types described in Appendix A (NOBS). At AGRO, one plot was sampled in corn and one in soybeans (see Appendix B (AGRO)). The other two sites, HARV (27-28 September) and KONZ (1-2 October), were sampled using the design on the right, in Figure 2 below. The only difference in the design was a more intense sampling density in the 8 external cells of each test plot. Figure 2, Pilot Study Sample Design At each point in the NOBS pilot study plots, sampling involved acquisition of one “ground” and one “canopy” photo. The tripod of the 3CS holds two cameras on a horizontal extension crossbar, with one camera facing up and one facing down (see Appendix E for tripod placement photos). The focal plane of the cameras was positioned roughly at 1.5 m above ground level. The ground camera was positioned on the crossbar, 0.85 m from the tripod center and placed between two legs of the tripod to minimize the occurrence of the legs in the pictures. The camera is a Canon PowerShot Pro70. The angle of view was set to 53 degrees, which yielded a ground image area of approximately 1.5 m by 1 m. The images were acquired using standard compression in high resolution mode (1.573 million pixels), and a remote switch. The canopy camera was a Sony Mavica FD91, and had its angle of view set at 20 degrees. Standard compression and high resolution mode (0.786 million pixels) were used. This camera was situated on the opposite end of the crossbar from the ground camera, about 0.25 m from the tripod center. Assuming an average tree canopy height of 10 m, the canopy projection area imaged was approximately 3 m by 2.2 m. An accessory set of measurements were made at each of the four points of the external cells of each plot and at each of the four corner points of the center cell. These included: tree basal area, understory basal area, and ocular proportional estimates of major ground cover components. Descriptions of these measurements are given in BigFoot Field Measurements. At AGRO there was no overstory, thus only ground photos were acquired. One test plot was sampled in each of the two major cover types, corn and soybeans. The corn was approximately 3 m tall, thus the horizontal crossbar had to be placed on a vertical extension bar to acquire the images (3CS). At HARV we sampled two test plots, one in a mixed-hardwood conifer site, and the other at a pure hardwood site (see Appendix C (HARV)). The 3CS was set up the same as at NOBS. At the mixed site, the predominant overstory species were deciduous hardwoods, whereas the understory was largely coniferous. At KONZ we sampled a grassland cover type with no overstory and the gallery forest cover type (see Appendix D (KONZ)). For all ground photos at these three sites, the camera was situated at a height above the vegetation that would yield an approximate coverage of 1.5 m by 1 m at the surface height of the vegetation (the same as at NOBS). At the gallery forest site the trees were approximately 30 m tall, which yielded an imaged canopy surface area of roughly 10 m by 7.5 m. Accessory measurements were acquired at AGRO, but not at KONZ or HARV. Cohen et al. - 15 June 2000 - Page 2
Analysis & Results Each digital photograph was analyzed by laying a mylar sheet over a printed copy about 20 cm by 26 cm in size. The mylar had a grid with 99 line intersections spaced 2.5 cm apart. The vegetation over component lying directly under each grid intersection was noted, and the proportion of total cover in each component cover type was calculated. At NOBS (see Appendix A), the cover components that we could characterize from the photos included conifer, hardwood, and dead tree canopy components, and moss, lichen, herbaceous, shrub, ne litter, tree regeneration, coarse woody debris (CWD), water , and unknown ground components. The unknown component is a result of either glare or deep shadow, because of which, the true component could not be identied. At AGRO (see Appendix B) the component set included corn, soybeans, and open ground. HARV plots (see Appendix C) included canopy trees, and tree regeneration, shrub, fern, herbaceous, coarse woody debris, rock, moss, water, and ne litter ground cover components. There were essentially no dead canopy trees in the plots and we could not condently separate conifer from hardwood, due to a complex vertical mixing of these components. In the grassland KONZ test plot (see Appendix D), the separable components were grass and herbaceous plants; no bare ground was seen. In the gallery forest KONZ test plot, we could characterize canopy tree cover, and the following ground cover components: grass, herbaceous plants, ne litter, coarse woody debris, understory tree regeneration, shrub, and unknown. A basic characterization of each test plot (n=48 for NOBS and AGRO; n=88 for KONZ and HARV) reveals that at NOBS conifer cover was highly variable among test plots, with Plots 4 and 1 having the lowest (18% and 23% respectively) and Plots 3 and 2 having the highest (35% and 52% respectively). Hardwood canopy cover was low in all plots (
components. The clearest result from this analysis was that, with the exception of Plot 4, at NOBS, conifer canopy proportion is the greatest source of variation in the data set. At AGRO and at the grassland KONZ plot, the PC analyses were not performed because of the within- plot lack of cover component variation. For the gallery forest at KONZ, PC1 was essentially a function of the variation in the amount of ground cover that was grass, whereas the second PC axis was a function of the amount of tree canopy and litter. Together, PC1-PC3 accounted for 80% of the variation in the data set, with PC3 being a function of contrast between the amount of herbaceous and litter components. At HARV , the rst two PC axes from Test Plot 1 accounted for 86% of the variation in the data set, with PC1 being a contrast of litter and tree regeneration and PC 2 being a contrast of tree and litter with fern. In Test Plot 2, PC1 was a contrast between litter and shrub, PC2 was a contrast between shrub and tree regeneration, and PC3 was a contrast between fern and regeneration and shrub cover. The rst three PC axes accounted for 93% of the variation in Test Plot 2. The lack of variation in tree canopy cover at this is evident in its lack of importance in the PC analyses. For the next set of analyses, the data from all photographs within a cell of a given test plot (n=16 for center cell; n=4 or n=9 for surrounding cells) were averaged to provide a set of mean cover component proportions for that cell. From these data, Analysis of Variance (ANOVA) tests were run by cover component to examine the variation among the 9 cells as compared to the variation within each of the 9 cells of each test plot (see Figure 3 below). Greater variation within cells would suggest the larger test plot area is homogenous and 1-2 pixel misregistration should not be an important concern. Greater variation among cells would suggest the test plot neighborhood is not homogenous and misregistration could be a problem. At the NOBS site, in Test Plot 1 (muskeg), only litter was different among cells (p
different among cells. No cover components were different among cells in either Test Plot 3 or 4. It is important to note that, for test plots 1 and 2, conifer canopy cover is the most important variable in the PC analysis and it is not different among cells. Neither corn nor soybean cover was different among cells at the AGRO site. In the grassland test plot at KONZ no cover components were different among cells. In the gallery forest, all ground cover components were different among cells, but canopy cover was not. Since this is a closed canopy environment, variation in ground cover will not be detected by the ETM+ sensor and we need not be concerned with that variation for mapping purposes. Similar results are seen at the HARV site; i.e., ground cover components were different among cells and canopy cover was not. This is also a closed canopy site, and ground components will not be detected. From these ANOVA results misregistration does not appear to be an important problem at any of the four sites. We experimented with two different sample sizes, since we had no previous experience using the 3CS system and did not know what sample size was needed and logistics, particularly at NOBS, were a major concern. It is not feasible to choose an adequate sample size for this project based only on statistics, as cost and logistics are too constraining. However, we want to have an estimate of the error as a function of sample size. Using the same data as for the ANOVA tests, i.e., cover proportions from each photo, we calculated the standard error associated with varying sample sizes, from Standard Error = population standard deviation / sqrt(n) where n=sample size This required an estimate of the true population standard deviation, for which we used the pooled variance, or Mean Square Within, from the ANOVA results. At the NOBS site, conifer cover had the largest standard error of all cover components, approximately 15% with a sample size of 4 at each of the four test plots. At the AGRO site, soybean cover was 100% in all photos so the standard error at any sample size is 0. For corn the standard error at a sample size of 4 is less than 1.5%. In the grassland at KONZ standard error for both grass and herb is less than 3% with n=4. In the gallery forest standard errors for all cover components are less than 10% with n=4. At HARV canopy standard error is less than 15% at both test plots with n=4. Understory standard errors are higher in the mixed forest than in the hardwood forest, but both are less than 5% with a sample size of 4 points. The shapes of the curves indicate that an n of 4 is too small. Logistics and cost allow for a maximum n of 9. Fortunately, an n of 9 is near the asymptote in the graphs of the standard error as a function of n and provides a reasonably acceptable standard error for all components. Conclusions We wanted an in-depth characterization of the canopy and ground cover components in each cover type at all sites, which we now have. In addition, we have examined the potential problem of satellite image pixels not being accurately aligned with the ground. This was done by comparing the variance within 25 m cells to the variance among 25 m cells for each cover component. At NOBS, some cover components did vary more among cells than within cells. However, they were not the variables that contributed the most to the overall variation in the dataset. Canopy conifer was the most important variable, and it was not different among cells, suggesting that misregistration will not be a problem at NOBS. The other three sites had less variability then NOBS, and again, the most important variables did not vary among cells. Small amounts of misregistration should not be a problem at any of the sites. Our primary goal was to determine if the Vegetation Cover Component Characterization System would provide data we needed to develop continuous elds of cover components at each site, and if so, what the appropriate sample size should be. AGRO is a simple system, consisting largely of corn, soybeans, and fallow elds. Each eld is quite compositionally homogenous. As such, there is no need to characterize cover components in any greater detail than to note the crop type present. At KONZ, the situation is similar, in there is either grassland or gallery forest. There is spatial variability in grass species present and in the mix of herbaceous species present (in relatively small quantities). However, biomass measurements made at each plot will be sufcient to characterize spatial variations in the proportions of these cover components. The gallery forest at KONZ is a relatively uniform, closed Cohen et al. - 15 June 2000 - Page 5
canopy of mixed hardwood species. Plot-level biomass measurements alone should be sufcient to characterize relative proportions of canopy species present. Understory components will not be visible to the ETM+ sensor, thus we need not concern ourselves with that level of detail for mapping with ETM+. The forest at HARV is also closed canopy and there is little in the way of understory component variability. The overstory canopy, however, contains considerable variability in hardwood and conifer species that is important to characterize. Hardwood species appear always to be in the upper canopy layer (dominant), but conifer species occupy the full range of relative canopy positions from dominant and co-dominant to intermediate and suppressed. We found it difcult to characterize the relative canopy positions of trees using 3CS, and this measure can be easily and accurately assessed for all “in” trees when making the plot-level biomass measurements. Also the satellite will not detect vegetation in the understory of these closed canopy forests. Using the 3CS in these environments will not further our goal of developing land cover maps. Therefore, we will not use this system at HARV, KONZ, or AGRO. NOBS is a very compositionally complex site, consisting of a relatively open, even- aged forest overstory with a spectrally diverse and highly variable understory and set of ground cover components. Without use of 3CS, or some comparable system, we could not adequately characterize vegetation cover at this site. From the graphs of standard error versus sample size, and having a eld season of experience working at the site, we have chosen to sample nine points in each 25 m cell. Cohen et al. - 15 June 2000 - Page 6
APPENDIX A - (NOBS) Major Cover Types at Northern Old Black Spruce Major Cover Types Encountered Cover Type qualiers 1.Muskeg (open-canopy black spruce) 1.Burned 2.Black spruce (closed-canopy black spruce) 2.Unburned 3.Aspen 4.Wetlands 5.Jack pine Cover Type Descriptions Muskeg Acronym: MSKG Overstory: dominated by black spruce often mixed with eastern larch Understory: sparse to heavy cover Labrador tea, Vaccinium spp., and willow spp. Ground Cover: predominately sphagnum with feathermoss and reindeer lichen Vegetation Structure: ground cover hummocky; canopy sparse; trees often stunted (1-6 m tall) Land Form: at, low-lying occasionally ooded Comments: this cover type is very abundant in NOBS; there exists a gradual transition between muskeg and closed canopy black spruce- feathermoss forests; demarcation unavoidably arbitrary. Black spruce Acronym: BLSP Overstory: dominated by black spruce occasionally mixed with eastern larch; low level occurrence of balsam poplar, and jack pine Understory: sparse coverage of Labrador tea, Vaccinium spp. Ground Cover: predominately feathermoss Vegetation Structure: ground cover at (not hummocky); canopy closed; trees not stunted (6-9 m tall). Land Form: at, low-lying, but never ooded Comments: this cover type is very abundant in NOBS; transition between muskeg and closed canopy black spruce-feathermoss forests is gradual; demarcation is unavoidably arbitrary Aspen Acronym: ASPN Overstory: dominated by trembling aspen; low level occurrence of white spruce, balsam poplar, black spruce, and jack pine Understory: green alder and hazel spp. Ground Cover: very little moss or forbs present Vegetation Structure: canopy closed, trees often tall (12-15 m tall), hazel and alder often forming second closed canopy at 1-2 m Land Form: uplands Comments: several patches occur at NOBS but they are small and infrequent Wetland Acronym: WTLD Overstory: scattered bog birch and eastern larch Understory: open water lined with willow, Labrador tea, and marsh grasses Ground Cover: mosses Land Form: ooded lowlands, creek margins, and beaver ponds Comments: this is a difcult community to describe because it includes both ooded peatlands Cohen et al. - 15 June 2000 - Page 7
(oligotriphic fens dominated by aquatic sphagnum spp, Vaccinium, and Labrador tea) as well as the marshy borders of creeks and beaver ponds (marshes containing willows and sedges); despite the range of plant communities in this cover type they are grouped together because of their similar structure Jack pine Acronym: JKPN Overstory: dominated by Jack pine; low level occurrence of white spruce, balsam poplar, black spruce, and trembling aspen Understory: sparse coverage of Labrador tea, vaccinium spp. and occasional patches of green alder Ground Cover: sparse to complete coverage by reindeer lichen, sparse coverage by feathermoss Vegetation Structure: canopy closed, trees often tall (10-12 m tall) Land Form: uplands, sandy soils Comments: very rare at NOBS except for regeneration stands in 1981 burn at southern edge of site Cover Type Qualiers and Additional Comments A large re burned a 150 km2 area on the southern boundary of the NOBS BigFoot study area in 1981. A few of the extensive plots on the south end of the 5x5 km grid occur in this burn. These plots are classied according to their current plant community (i.e., MSKG, BLSP, WTLD, ASPN, or JKPN) but their status as burned will also be recognized as a cover type qualier since the burn inuences the species composition, LAI and NPP A cover type map including the NOBS BigFoot study area was constructed from aerial photography by the Manitoba Department of Natural Resources (MDNR) in 1988 and is available as raster map from the BORIS data base. This is a high quality map that recognizes over 100 cover types. Based on our own on-ground experience, the map is accurate. Cohen et al. - 15 June 2000 - Page 8
NOBS Test Plot 1: Muskeg Figure 4 Correlation for NOBS Test Plot 1: Muskeg Figure 5 Cohen et al. - 15 June 2000 - Page 10
NOBS Test Plot 2: Black Spruce Figure 6 Correlation for NOBS Test Plot 2: Black Spruce Figure 7 Cohen et al. - 15 June 2000 - Page 11
NOBS Test Plot 3: Mixed Forest Figure 8 Correlation for NOBS Test Plot 3: Mixed Forest Figure 9 Cohen et al. - 15 June 2000 - Page 12
NOBS Test Plot 4: Wetland Figure 10 Correlation for NOBS Test Plot 4: Wetland Figure 11 Cohen et al. - 15 June 2000 - Page 13
Principal Component Analysis for NOBS Test Plot 1: Muskeg Figure 12 Test Plot 2: Black Spruce Figure 13 Cohen et al. - 15 June 2000 - Page 14
Principal Component Analysis for NOBS Test Plot 3: Mixed Forest Figure 14 Test Plot 4: Wetland Figure 15 Cohen et al. - 15 June 2000 - Page 15
Standard Error Estimates for NOBS Muskeg Black Spruce Figure 16 Figure 17 Mixed Forest Wetland Figure 18 Figure 19 Cohen et al. - 15 June 2000 - Page 16
APPENDIX B - (AGRO) Major Cover Types at Midwestern Cropland Major Cover Types Encountered 1.Corn 2.Soybean 3.Fallow Cover Type Descriptions Corn Acronym: CORN Species: corn Architecture: closed canopy row crop growing >2 m tall by late summer Comments: roughly half of the row crops planted in the site will be corn Soybean Acronym: SOYB Species: soybean Architecture: closed canopy row crop growing 50-75 cm tall by late summer Comments: roughly half of the row crops planted in the site will be soybean Fallow Acronym: FALO Species: hay grasses Architecture: grassland of variable height Comments: only a small proportion of the site (
AGRO Ground Cover Image Samples Cohen et al. - 15 June 2000 - Page 18
AGRO Corn and Soybean Test Plots Figure 20 AGRO Standard Error Estimates Corn Test Plot Soybean Test Plot There was no variance in soybean cover, so the SE is 0 Figure 21 Cohen et al. - 15 June 2000 - Page 19
APPENDIX C - (HARV) Major Cover Types at Harvard Forest Major Cover Encountered Cover type qualiers 1.Eastern hardwoods 1.disturbed (clearcut) 2.Eastern hemlock 2.undisturbed 3.Red pine 4.Oldeld meadow Cover Type Descriptions Eastern hardwood Acronym: EHWD Overstory: dominated by sugar maple mixed with red oak, ash, basswood, and beech Understory: saplings of shade tolerant tree species, and Vaccinium Ground Cover: grasses and forbs belonging to the “Canadian Carpet” community Land Form: uplands Comments: additional visits to HARV will allow us to better describe this community Eastern hemlock Acronym: HEML Overstory: eastern hemlock with remnant red oak Understory: hemlock saplings Ground Cover: sparse cover of grasses and forbs belonging to the “Canadian Carpet” community Land form: uplands to lowlands Comments: additional visits to HARV will allow us to better describe this community Red pine Acronym: RDPN Overstory: red pine Understory: red pine saplings Ground Cover: sparse cover of grasses and forbs and shrubs Land Form: uplands Comments: additional visits to HARV will allow us to better describe this community Oldeld meadow Acronym: OLDF Overstory: none Understory: grasses, shrubs Comments: this cover type is largely the result of anthropogenic disturbance; additional visits to HARV will allow us to better describe this community Cover Type Qualiers and Additional Comments A clearcut in 1999 removed the forest from a portion of the private land occurring within the Harvard Forest study area, affecting one or more of the extensive plots. These plots will be classied according to their current plant community but their status as clearcut will also be recognized as a cover type qualier since cutting inuences the vegetation structure and function. Cohen et al. - 15 June 2000 - Page 20
HARV Canopy Image Samples HARV Ground Cover Image Samples Cohen et al. - 15 June 2000 - Page 21
HARV Test Plot 1: Mixed Forest Figure 22 Correlation for HARV Test Plot 1: Mixed Forest Figure 23 Cohen et al. - 15 June 2000 - Page 22
HARV Test Plot 2: Hardwood Forest Figure 24 Correlation for HARV Test Plot 2: Hardwood Forest Figure 25 Cohen et al. - 15 June 2000 - Page 23
Principal Component Analysis for HARV Test Plot 1: Mixed Forest Figure 26 Test Plot 2: Hardwood Forest Figure 27 Cohen et al. - 15 June 2000 - Page 24
Standard Error Estimates for HARV Test Plot 1: Mixed Forest Figure 28 Test Plot 2: Hardwood Forest Figure 29 Cohen et al. - 15 June 2000 - Page 25
APPENDIX D - (KONZ) Major Cover Types at Konza Prairie Major Cover Types Encountered Cover type qualiers 1.Tallgrass prairie 1.Cattle grazed 2.Shortgrass prairie 2.Bison grazed 3.Shrub community 3.Ungrazed 4.Gallery forest 4.Burn frequency Cover Type Descriptions Tallgrass prairie Acronym: TGPR Species: Big bluestem, Indian grass, little bluestem, switchgrass, and other forbs Architecture: 1-1.5 m tall at full ush Land Form: bottomlands, deep soils, unexposed aspects Comments: a wide, poorly dened, gradient exists between the tallgrass and shortgrass prairies Shortgrass prairie Acronym: SGPR Species: blue-grama, hairy grama, xeric forbs Architecture: 10-20 cm tall at full ush Land Form: exposed ridge tops, shallow claypan soils Comments: a wide, poorly dened, gradient exists between the tallgrass and shortgrass prairies Shrub community Acronym: SHRB Species: smooth sumac and Cornus spp Architecture: 1-2 m tall, very dense, thin stems, closed canopy Land Form: exposed ridge tops, shallow claypan soils Comments: forms patches in drainage gulches and seeps; shrub communities also occur adjacent to creeks and as a transition between prairie and forest Gallery forest Acronym: GALF Species: oaks, elm, hackberry, walnut, and hickory Architecture: 15-20m tall closed canopy but lots of edge supports signicant understory with open canopy at 3-5m Land Form: lowlands, largely riparian Comments: this is a diverse community that transitions into prairie either by way of open savanna or by shrub communities; about 6% of Konza is gallery forest Cover Type Qualiers and Additional Comments Konza is divided into over 60 managed experimental watersheds. The management practices vary with respect to grazing regime and re frequency. Grazing treatments include cattle grazing, bison grazing, and no grazing. Fire regimes vary by frequency (1, 2, 4, 10, or 20 year re cycles) and timing (winter, summer, fall, and spring burning). While not all combinations of burning and grazing regimes are practiced, many are making the Konza landscape very diverse. The BigFoot design can not sample each of these management areas. The management history of each study plot will be recognized as a cover type qualier since the management practice inuences species composition, vegetation structure and function. Cohen et al. - 15 June 2000 - Page 26
KONZA Canopy Image Samples KONZA Ground Cover Image Samples Cohen et al. - 15 June 2000 - Page 27
KONZA Test Plot 1: Grasslands Figure 30 Cohen et al. - 15 June 2000 - Page 28
KONZA Test Plot 2: Gallery Forest Figure 31 Correlation for KONZA Test Plot 2: Gallery Forest Figure 32 Cohen et al. - 15 June 2000 - Page 29
Principal Component Analysis for KONZA Test Plot 2: Gallery Forest Figure 33 Standard Error Estimates for KONZA Test Plot 2: Gallery Forest Figure 34 Cohen et al. - 15 June 2000 - Page 30
APPENDIX E 3CS in Action Cohen et al. - 15 June 2000 - Page 31
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