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Vegetation Monitoring at Padre Island National Seashore: Results for 2023

grasses blooming on a gently undulating landscape
Seacoast Bluestem Grassland at Padre Island National Seashore

NPS/Jane Carlson

Why Monitoring Vegetation at Padre Island National Seashore?

Padre Island National Seashore boasts an incredible 66-miles of undeveloped beach on the south Texas coast. This long, sandy shoreline is popular for fishing, camping, swimming, birdwatching, and beachcombing. But there’s another side to the National Seashore that most visitors never see up close, just behind the dunes. There, you will find an immense coastal grassland, one of the largest of its kind in Texas.

These grasslands may not be the main attraction, but they are central to the island’s well-being. Outside of the limelight, native coastal grasslands keep the island together by stabilizing vast amounts of sand and providing habitat for wildlife. The intact island, in turn, acts as a buffer for the coastline against hurricanes and other storms, protecting life, resources, and property in areas such as the Laguna Madre, Corpus Christi, and Port Mansfield.

The island's native grasslands may bring Texas-sized ecological benefits, but their long-term survival isn’t guaranteed. Major hurricanes, prolonged droughts, and human activities can harm the park’s native plants. Loss of plant life can damage the ecosystem and make the park less enjoyable for visitors.

To help keep the island healthy, scientists from the Inventory and Monitoring Program (I&M) visit the park regularly to study the park’s grassland communities. Like doctors on your annual check-up, they track vital signs of ecological health and look for clues that plant communities are changing. Long-term studies like these help park managers make decisions that keep the native grasslands growing strong, which benefits the island, the public, and the coastline as a whole.

Factors Causing Plant Communities to Change and How Monitoring Helps

Environmental conditions at Padre Island National Seashore are often dry and windy, which creates drought and favors grassland fire. Native plants on the island are well-adapted to this, but their tolerances will depend on the plant species and the intensity of events. The island’s plant communities have also been shaped by human activities, such as historic cattle ranching and other uses of the island. For example, some areas of the island are burned far more often than others, due to a combination of prescribed fires, natural wildfires, and unintentional wildfires caused by people. Differences in the fire cycle across the park can lead to differences in plant communities over space and time.

With so much going on, it's no surprise that plant communities change. And not all changes are bad. To understand the park’s ecology and protect key resources, I&M scientists must learn to tell natural, cyclical change from changes that could lead to a permanent loss of biodiversity. To do this, they collect and analyze long-term data on the grasslands and their surrounding environment. When problematic declines are discovered, I&M data can help shape management goals and track progress. In these ways, long-term monitoring serves the park both now and into the future.

Plant Communities of the National Seashore

The island’s grasslands generally begin where the beach ends, over the rise of the dunes and into the gently rolling interior. A representative cross-section from dune ridge to eastern shore passes through several different communities (Figure 1). The back side of the fore-island dunes feature species like sea oats (Uniola paniculata) and bitter panicum (Panicum amarum) that have deep roots and help keep the dune intact. Blending into the dune edges, the expansive coastal grasslands are dominated by two main species: seacoast bluestem (Schizachyrium littorale) and gulfdune paspalum (Paspalum monostachyum). Approaching the island’s center, coastal grasslands intermix with freshwater and brackish wetlands. These wetlands range in size from small pocket wetlands in coastal grasslands, which are shallow and sometimes dry out, to larger swale wetlands, marshes or ponds that can hold water year-round. The park is widest at its northern end, and this section supports the park's largest ponds. This section is also home to unique umbrella sedge wetlands that are only known to occur in coastal Texas. Further south, grasslands intermix with vegetated salt flats that are sometimes flooded by sea water and contain salt marsh species. Collectively, these six broadly-defined communities encompass most of the park’s grasslands and are targets of this long-term monitoring project.

plant communities along a transect from East to West across the island
Figure 1. The main vegetation communities at Padre Island National Seashore and where they can be found along an example elevational profile, facing south and from the east to the west shore. This example profile is located 2 km north of Yarborough Pass, derived from a 2018 lidar dataset.

Photos NPS/Jane Carlson

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How is the Monitoring Done?

The Gulf Coast I&M Network monitors vegetation in 32 long-term plots spread throughout the park's interior grasslands. To establish these plots, the network first selected four focal monitoring areas, called panels, that were evenly-spaced between mile markers 0 and 40. These panels were named 08, 18, 28 and 38. Within each panel, the network assigned eight plot locations based on a random draw of points, excluding the beach, active dunes, wind-tidal flats, and permanently flooded ponds. In fall 2019, crews first visited the park to install plot markers and conduct the first round of sampling. Crews returned in fall 2023 to sample plots for the second time. Both sampling events followed the methods published in the protocol “Monitoring Terrestrial Vegetation in Gulf Coast Network Parks”. In short, botanists identified all plant species in each 400-meter2 (20 x 20 meter) plot, and they recorded the coverages and relative frequencies of plants in 1-m2 and 10-m2 sampling frames that were nested inside the full 400-meter2 plot (Figure 2).

two botanists identifying plants in a grassland, with plot sizes and boundaries superimposed
Figure 2. Vegetation plot at Padre Island National Seashore with colored lines superimposed over string-marked plot boundaries to highlight the 1-m^2 quadrat (teal) nested within a 10-m^2 microplot (green) in the corner of the 100-m^2 subplot (yellow). There are four 100-m^2 subplots per plot.

NPS/GULN

What's the Latest News?

pink flowers in a grassland
Prairie agalinis (Agalinis heterophylla) is one of the 113 species recorded in vegetation monitoring plots in the park in 2023.

NPS/Jane Carlson

Here's a summary of Key Findings from vegetation monitoring at Padre Island National Seashore in 2023. You can find more details in the full report and appendices, linked further below.

  • Across all 32 plots sampled in 2023, I&M scientists recorded a total of 113 plant species. Bermudagrass was the only non-native species they found during monitoring, and it was present in just one corner of a single plot. These findings are consistent with our understanding that the park’s interior grasslands have very few non-native species. The grasslands also have relatively low species richness as compared to similar habitats on the mainland.

  • The most abundant community type in the sample was the Coastal Grasslands with Pocket Wetlands, representing 14 of 32 plots. This community type also had the two highest-richness plots in 2023, with 35-36 species each (class average=22 species per plot). With almost half of I&M plots featuring this community, it is clear that small, ephemeral wetlands are widespread throughout the park’s grasslands and they support distinct species within the dry grassland environment.
  • Of the remaining plots, there was an even split between plots in wetlands and plots in relatively dry uplands. For the nine upland plots, seven were in Coastal Grasslands and two were in Coastal Grasslands on Fore-Island Dunes. These plots had lower richness overall.
  • For the nine wetland plots, four were in Umbrella Sedge Wetlands, four were in Swale Wetlands, and one was in Vegetated Salt Flats. Among these, Umbrella Sedge Wetlands had the highest average species richness, at 29 species per plot. The unique Umbrella Sedge Wetland community contains at least one rare plant species, the coastal plain umbrella sedge (Fuirena longa).
  • When comparing findings from 2023 and 2019, I&M scientists found several key differences in plant species richness and cover. For example, most plots had fewer species in 2023 than in 2019, averaging four fewer species per plot. These between-year losses were most likely due to a minor drought in 2023. Similar non-permanent losses and gains were recorded during the park’s vegetation community mapping project several years earlier.
  • The recency of fire differed between the 2023 and 2019 sample years, and this may have caused some of the differences in plant communities over time as well. In 2019, all plots had burned once within the last two years, whereas in 2023, four or five years had passed since the last fire. Other studies have shown that plant species richness increases in the first few years after a fire, which is consistent with higher richness during the 2019 sampling event.

How is this Information Used?

The combined results of 2019 and 2023 sampling seasons create a picture of a relatively healthy, dynamic park. With few invasive plant species in the island's interior, fairly frequent fire, and almost no roads or buildings to interrupt the plains, the park’s grasslands seem intact and undisturbed. They contain many unique and specialized communities, such as Umbrella Sedge Wetlands. They are also home to the federally-listed black rail. The park’s grasslands and wetlands have not always been this pristine, however. Back when the island was used as a ranch, the grasslands were heavily overgrazed. With over 50 years since cattle were removed, today's grasslands and wetlands have largely recovered. Yet the recent arrival of feral hogs poses a new threat.

Even though the park's grasslands are relatively intact today, they still need active management. The work of I&M supports park decision-making in several ways. First, crews check for signs of feral hogs or other agents of disturbance in and around plots and then report findings to park staff. Second, I&M collects baseline data and analyzes changes in park plant communities to provide a clearer picture of the park’s natural ecological cycles. These studies help pinpoint when change is atypical or unnatural. They also can provide science-based recommendations for adapting to change.

Through regular park visits to collect data and share results, I&M scientists help National Parks thrive. They provide managers and stakeholders with crucial information on plant health, and by proxy, whole-park condition. Armed with timely and accurate information, the park can better protect these treasured American resources so they may continue to serve the public for generations to come.

Full 2023 Summary Report

The full summary for 2023 provides more complete descriptions of the study area and methods, including maps, imagery, and figures. It presents the full suite of results on species composition of plots, their richness, and which species dominate in each community type. It also explores spatial patterns, environmental effects, the role of fire, and, to a limited extent, change between 2019 and 2023. Appendices A-F include vegetation class assignment criteria, land use and fire history details per plot, I&M species lists, and plot level summaries for richness, example elevation profiles for each vegetation class, and remote sensing measures of vegetation greenness in relation to time since fire.

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Padre Island National Seashore

Last updated: December 16, 2025