## 4 Discussion  
The broad, comparative analysis of the vegetation dynamics across the Triple Crown trail system revealed changing dominant land covers, hotspots of deforestation, and significant impacts of wildfire. The land cover change and the transition from needleleaf forests to grasslands along the western PCT and CDT likely resulted from a combination of wildfires, dry climatic conditions, and invasive species [41]. Prior studies indicate wildfire occurrence and transition to shrub and herbaceous land cover increased more significantly in warmer areas with temperatures above 17.5 \({}^{\circ}\)C [102]. As the climate continues to warm, the land cover shift away from forested habitats will likely continue along these trails. The tree cover loss documented in southern regions such as the Southern and Baja California Pine-Oak Mountains in this study may also result from the impact of invasive species. Invasive species such as the Western Pine Beetle and Spruce Beetle cause widespread die-off of Ponderosa pine forests along the PCT and disturb ecological dynamics [6]. The full impact of these invasive species may not be captured using MODIS or Landsat imagery as the shortcomings of optical spectral indices in identifying invasive species using satellites are documented [5]. In the Appalachian Mountains, acid rain and Hemlock decline from the invasive Woolly Adelgid have significantly transformed the forest structure [3], but these impacts are best identified with high-resolution imagery and often require manual digitization and ground-truthing to quantify the understory dynamics [7]. Seasonality highly influenced terrestrial productivity across the Triple Crown network throughout the 35 years of analysis. On the AT, the highest median GPP values occurred in recent years, potentially a result of warmer temperatures and increased atmospheric carbon levels from anthropogenic climate change [103]. The PCT showed a slight dip in GPP after 2017, likely related to wildfire events. Even after ecosystem conditions improve to the initial state, there is often a delay in vegetation biomass recovery [104]. In the state of California, the large fire burn area and increased fire severity in recent years have decreased the carbon uptake in affected areas [105]. The increase in the annual NPP over the 35-year time period across all three trails is consistent with warmer temperatures and longer growing seasons associated with climate change [106]. While this analysis is intentionally broad in scale, it must be noted that ecosystem productivity is highly dependent on microclimate and environmental variables and may differ greatly across the study area based on local factors and elevation [107]. Figure 11: Boxplots and whisker plots showing the monthly NDVI for the PCT, CDT, and AT. The lower correlation between the PhenoCCam GCC and Landsat-derived GPP at some sites is likely explained by the different vegetation covers in arid environments. The GCC data for the PhenoCCam locations in the Southern and Baja California Pine-Oak Mountains section of the PCT showed significantly lower accuracy than other sites, potentially due to the drought and wildfire conditions in the region. Some ecoregions within the Triple Crown network are better suited for vegetation monitoring with remote sensing, as was indicated by the weaker correlation between the PhenoCCam data and Landsat GPP in Northwestern Mountainous Forest [73]. There are also possible discrepancies between the PhenoCCam GCC and Landsat GPP due to the large spatial resolution accompanying Landsat products. In addition, ground-based PhenoCCam estimates are unavailable across most of the trail system and the eleven PhenoCCam stations used here are not representative of all the ecosystems. Vegetated areas such as the PCT Klamath Mountains and Eastern Cascades Slopes and Foothills and the CDT Canadian Rockies and Idaho Batholith do not have nearby PhenoCCam data within 15 km of the study site.