## 4 Discussion
### Effects of Ecosite  
We found steeper slopes for the iAPAR to ANHP relationship in ecosites with taller structured vegetation, which aligns with previous results from the same site [12]. There could be at least two mechanisms driving this finding. First, taller structured herbaceous vegetation may be allocating more resources aboveground per unit of APAR. This seems reasonable considering that the shorter structured herbaceous vegetation in this area predominantly consists of drought-tolerant C\({}_{4}\) shortgrasses, which allocate a considerable amount of resources to belowground root mass [38]. By contrast, the taller structured herbaceous vegetation consists of mid-height cool-season grasses and warm-season saltgrasses, which devote more resources to leaves and seeds. Indeed, cool-season grasses in this system  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Source** & **Ecosite** & **MAE (kg/ha\({}^{-1}\))** & **MAE\%** & **MPE\%** & **MAPE\%** & **R\({}^{2}\)** & \(r\)** \\ \hline iAPAR & \multirow{2}{*}{All} & 268.51 & 25.60 & \(-\)14.97 & 32.64 & 0.62 & 0.78 \\ RAP & & 343.08 & 32.71 & \(-\)28.57 & 49.59 & 0.40 & 0.72 \\ \hline \multirow{3}{*}{iAPAR} & Loamy Plains & 249.21 & 28.00 & \(-\)17.16 & 35.51 & 0.49 & 0.70 \\  & Sandy Plains & 290.58 & 25.32 & \(-\)13.94 & 31.60 & 0.58 & 0.76 \\  & Salt Flats & 295.75 & 19.92 & \(-\)8.14 & 22.86 & 0.67 & 0.82 \\ \hline \multirow{3}{*}{RAP} & Loamy Plains & 301.77 & 33.90 & \(-\)47.98 & 59.67 & 0.40 & 0.72 \\  & Sandy Plains & 354.08 & 30.85 & \(-\)12.08 & 39.76 & 0.36 & 0.74 \\ \cline{1-1}  & Salt Flats & 494.03 & 33.27 & 13.40 & 31.02 & 0.07 & 0.80 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparing predictions from RAP and the empirical iAPAR model. Model metrics used included mean absolute error (MAE; absolute value of observed minus predicted value), MAE percent (MAE%; the percentage of the mean absolute error relative to the mean of the response variables), mean percent error (MPE; mean of the observed value minus the predicted value divided by the observed value), mean absolute percent error (MAPE; mean of the absolute values of observed minus predicted divided by the observed values), R\({}^{2}\) (coefficient of determination), and \(r\) (Pearson correlation coefficient) to evaluate model fit. The metrics were calculated for all ecosites together and for individual ecosites. To see a breakdown of error metrics by ecosite and grazing management, see Supplemental Table S1.  
are known to produce more aboveground biomass per unit of received precipitation than warm-season grasses [39].  
Second, taller structured vegetation may have overlapping leaf canopies, in which case the satellite would not see the entire leaf surface area and may be underestimating APAR. This seems less likely in our semi-arid study area since even in the tall structured communities, plant height rarely exceeds 50 cm, and the dominant species do not have vertically complex leaf structures. Regardless of the mechanism, our study reinforces prior work suggesting that aboveground plant structure can be an important modifier of satellite-based iAPAR-ANHP relationships [3; 12].