Our results support previous work indicating that texture measures generally improve empirical models of LAI in forests when compared to using SVI alone (Wulder et al., 1996, 1998; Song and Dickinson, 2008). It has been hypothesized that this improvement is due to the textural features containing forest structural information. Indeed, this line of reasoning is supported by studies which have shown relationships between image texture and structural variables such as canopy diameter and stocking density (St-Onge and Cavayas, 1995; Song and Woodcock, 2003; Song, 2007; Song and Dickinson, 2008). Others have argued that the improvement is due to the textural information providing a pseudo-classification of the image (Colombo et al., 2003), which is likely the case in study-sites with many diverse vegetation communities. Our results support the former hypothesis more strongly than the latter by showing the greatest improvement in modeling efficiency for a single land cover type (deciduous stands), although a
0 1 2 3 4 5 6 9 33 57 81 113 145 177 209 241 273 305 337 LAI & Le 2009 DOY MCD15A2 Main MCD15A2 Backup Modeled A 1 1 1 1 1 1 1 1 1 1 1 1 1242 2 2 242 2 2 2 2 2 21 12 2121 1 1 1 1 1 1 1 1 1 1 1 0 1 2 3 4 9 33 57 81 113 145 177 209 241 273 305 337 LAI & Le 2009 DOY MCD15A2 Main MCD15A2 Backup Modeled B 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 14 41 1 1 141 141 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Figure 2.6: Comparison of estimatedLe and the MODIS LAI product (MCD15A2) for single
MODIS pixels centered on the Blackwoods study site (A) and a mostly developed pixel (B) over the 2009 calendar year. Numbers underneath each MODIS LAI estimate correspond to the five level confidence score (1: best result possible, 2: main algorithm with saturation, 3: main algorithm failed due to bad geometry, 4: main algorithm failed due to problems other than bad geometry, and 5: pixel not produced at all). Symbols surrounding this quality score indicate the cloud state (squares indicate that significant clouds were present, and circles indicate that mixed clouds were present on the pixel).
direct comparison between these studies is complicated by the use of different texture metrics and vegetation types.
Wulder et al. (1998) showed the greatest improvement in LAI prediction with texture measures among evergreen stands, but our results showed an insignificant relationship in these stands. In contrast, texture was the single best predictor of deciduousLein this investigation.
However, it is important to note that we used a single measure of texture and a single window- size to calculate this metric. While we believe we gain interpretability and simplicity with this approach, it is possible that different measures of image texture may contain complimentary information. For instance, the lack of a significant relationship between evergreen Le and
variance may be the result of a non-optimal window size for evergreen variance calculation. In fact, the deciduous crowns were much larger than the evergreens and a “one-size-fits-all” approach to variance calculation may have been inadequate. However, using multiple window sizes increases the reliance on image classification. The positive relationship between first-order image variance andLeobserved in this study may indicate that variance captures information
about canopy complexity which may be reasoned to increase with LAI as canopies mature. We have also demonstrated that multiple SVI can provide complimentary information and that multiple candidate models may be rigorously compared using AIC based model selection procedures.
We found that extensive filtering of MODIS SVI was required in order to obtain stable parameter estimates to the proposed phenological model in an automated fashion. Even af- ter restricting the data to pixels identified as reliable by the MOD13Q1 algorithm, there was still too much variability and the nonlinear least squares approach was unstable and sensi- tive to initial parameter estimates. We used a simple phenological model because it has been demonstrated to perform well for forested vegetation. It is inadequate, however, for multi-peak phenologies characteristic of multiple crop rotations, and an alternative model would be re- quired if capturing such vegetation changes was important in a given study area. We observed that there was no significant difference between deciduous and evergreen phenological models. We believe that this results from heterogeneity in pixels assumed to be pure deciduous or evergreen. The MODIS land-cover product used for constructing the time series has a spatial
resolution of 500 m which is likely too large for a study area as small and heterogeneous as ours. It should be noted, however, that the same lack of difference in phenologies was found when a much more restrictive pixel selection procedure was employed. Using the NLCD land-cover product we attempted to identify MODIS resolution pixels with at least 80% homogeneity in NLCD land-cover definition. This resulted in far fewer pixels available for the calculation of daily NDVI averages and necessitated that the area considered for phenological signal ex- traction be much larger. However, the lack of difference between evergreen and deciduous phenology under this more restrictive pixel selection procedure does not rule out the possi- bility that landscape heterogeneity is the underlying cause. Rather, it most likely indicates that there are very few homogeneous stands, and there is confusion in the classified map. This hypothesis is supported by field observations of abundant deciduous herbaceous and woody understory species in stands which would appear to be pure evergreen from a satellite view. Another possible source of phenological error is geolocation errors in the MODIS product. However, the difficulty in finding pure MODIS pixels using a finer scale product such as the NLCD lends stronger support to the hypothesis that subpixel heterogeneity is the dominant factor.
It is important to note that by relying on NDVI time series to fit the function f(t) there is an implicit assumption of linearity in the relationship between SVI and NDVI that is likely violated in some cases, particularly in densely vegetated pixels. However, whereas this leads to underestimation of LAI in simple linear empirical relationships, the most likely effect on our model is a seasonal trajectory which reaches peak Le too early in the growing season. In
reality, some very densely vegetated pixels may continue to accumulate leaf area throughout the summer. This could possibly be avoided by utilizing a conversion function to convert MODIS NDVI observations to LAI, or by using the MODIS LAI product directly to fit the phenological function. In our case, using MODIS LAI to fit f(t) would have exacerbated the mixed pixel problem due to the coarser spatial resolution of that product in comparison to NDVI, and would not have permitted the decompositing process used to obtain a finer temporal resolution time series. Nevertheless, this is probably a viable option in areas with larger, more heterogeneous forested areas, and a smoothing approach such as the one employed
by Gao et al. (2008) would be useful in obtaining a reasonable time series from MODIS LAI estimates.
The lack of time series of LAI ground observations prevented a rigorous validation, but comparison of predicted Le at the two sites where time series LAI data was available are en-
couraging. The green-up period and time to peak LAI was well predicted for both deciduous and evergreen sites. Peak LAI for deciduous was under predicted, but the magnitude of the error is within the limits that are generally accepted among other studies. The senescence pattern for the deciduous vegetation is not well matched by our model. This is most likely the result of contamination of the phenological signal due to pixel heterogeneity as previously discussed. At the MODIS scale, our modeled estimates compared favorably over the pre- dominately deciduous Blackwoods pixel, but not as well over the urban pixel where MODIS maximum and minimum LAI exceeded our estimates by two and one unit, respectively. There is a large amount of variability in the single pixel time series of MODIS LAI estimates, and although this product is not intended to be used at the scale of an individual pixel, it has important implications for studies utilizing this product for fine spatial scale ecosystem simu- lations. Our algorithm does not rely on instantaneous surface reflectance for estimatingLeand
therefore produces a much smoother temporal trend of Le. This more realistic trend, along
with the finer spatial resolution and greater flexibility in land-cover designation are the main advantages of our algorithm over the MODIS product for local to regional-scale ecosystem simulation applications.
The accuracy of Le estimates produced with our algorithm are determined primarily by
the accuracy of the maps of minimum and maximum Le. To overcome the problem of signal
saturation using spectral information alone, we included image texture information based on findings from other studies (Colombo et al., 2003; Song and Dickinson, 2008; Song et al., 2010). It appears that this method works, at least for deciduous vegetation. However, the use of texture measures introduces its own limitations such as increased computational and data requirements. Texture measures also increase the dependency on land-cover maps because they are significant predictors ofLe only in closed canopy forest stands. The dependence on a
is only capable of fitting SVI time series with a single yearly maximum, making it unsuitable for fitting time series of agricultural areas with multiple rotations in a year. Second, we must rely on coarser spatial resolution SVI estimates from MODIS in order to achieve the necessary temporal resolution for time series construction. As previously discussed, the main limitation of this approach is that it is difficult to fit vegetation specific phenological models in areas with few land-cover patches that are homogeneous at the MODIS SVI spatial resolution. In this investigation, this was evident in the lack of significant difference in models fitted to evergreen and deciduous SVI time series, and the prolonged senescence period evident in the estimated Le time series. An additional issue associated with this approach is that relatively coarse land
cover schema (e.g. “deciduous” as opposed to “oak”) obscure phenological differences between individual species. With our approach, a single phenology is prescribed for all deciduous species. However, since the coarse spatial resolution SVI represent the bulk contribution to greenness from all species, the effect is likely not severe except in extreme cases where there are strong sub-pixel species gradients. A more serious limitation of this approach is that a single, land cover specific phenology is prescribed for the entire study-area which results in an insensitivity to environmental gradients affecting phenology. However, our method is primarily aimed at improving vegetation representation at spatial scales where these differences are small; in larger study areas where this effect is not small, existing moderate to coarse resolution vegetation representations are likely adequate and finer scale representations would be computationally infeasible.
This investigation combined complimentary information from IKONOS, Landsat, and MODIS sensors across a wide range of spatial resolutions. Our approach to using images from multiple sensors at a variety of temporal and spatial resolutions might best be thought of as information fusion rather than image fusion. The distinguishing feature is that we have combined complimentary information from multiple images without resorting to the produc- tion of synthetic images. The main application of this technique in this study was the fusing of phenological information from temporally dense MODIS observations with vegetation pattern information from higher spatial resolution Landsat imagery. Our approach is applicable to all naturally vegetated landscapes, particularly forested areas with high LAI, but is not applicable
in landscapes dominated by agricultural crops with multiple rotations due to the phenological model used.