Phenology and Climate Change: Understanding Nature’s Language through Data and Community Science

• Jalyn Gearries, Scientists in Parks Fellows Intern, Grand Canyon National Park, National Park Service (currently PhD Student, School of Informatics – Computing and Cyber Systems, Northern Arizona University)
• Lonnie Pilkington, Vegetation Program Manager, Grand Canyon National Park, National Park Service
• Annie Kilby, Training Specialist Education Ranger, Grand Canyon National Park, National Park Service (currently Park Ranger, Big Cypress National Preserve, National Park Service)

Abstract

Phenology, the study of seasonal life-history stages in plants and animals, serves as a critical indicator of ecosystem responses to environmental changes. This study explores the relationship between phenology and climate change through the lens of the Local Touchstones: Phenology and Climate Change monitoring protocol at Grand Canyon National Park. Initiated in 2013, this long-term project engages community scientists in recording phenological data for Gambel oak (Quercus gambelii) across five sites on the South Rim of the Grand Canyon. Over nine years, park staff documented various phenophases for 26 oak trees, providing a robust dataset for ecological analysis.

The analysis involved linking phenophase onset with climate drivers, and the findings reveal a significant delay in phenophase onset correlated with increased temperature accumulation and precipitation, with the maximum leaf cover phase showing the most pronounced response. Additionally, the interaction between temperature accumulation and precipitation suggests that higher precipitation levels can mitigate some of the delays caused by increased temperatures, emphasizing the complexity of climate impacts in arid environments like Grand Canyon National Park. This study underscores the value of long-term phenology monitoring in understanding and predicting ecological responses to climate change, highlighting the crucial role of ecological monitoring in our national parks to inform conservation strategies and ensure ecosystem resilience.

Jalyn Gearries, Scientists in Parks Fellows intern, leading a community science discussion describing the phenology of Gambel oak at Grand Canyon National Park, Arizona.

Photo courtesy of Northern Arizona University

Phenology and Climate Change

Just as Indigenous people in the Southwest have developed a culture and connection to their land and patterns since time immemorial, the record of nature’s calendar can be considered a biological artifact. Phenology is the study of seasonal life history stages in plants and animals and can be thought of as one “language” scientists have constructed to understand biological phenomena. The timing of nesting birds, dispersing pollen, and falling leaves are all examples of phenological events that are tools we value and monitor in our everyday lives. If we take a close look at the patterns of the timing of these events, we can use them to connect the relationships that biotic and abiotic features share in an ecosystem. Specifically, phenology can be an excellent indicator of ecosystem response to rising temperatures, extreme weather events, and climate change-induced drought.

The Southwest United States (U.S.) has the hottest and driest environments in the country and has been observed to be shifting toward more arid conditions throughout the last 50 years (Gonzalez et al. 2018). At Grand Canyon National Park (GCNP), scientists and resource managers closely monitor observed and projected impacts from climate change that could have significant effects on the park’s vegetation, wildlife, and water supply (Tillman et al. 2020). One of the ways that the park monitors these impacts is through a phenology observational study for the plant life at the Grand Canyon.

In this case study, we’ll explore an example of understanding nature by analyzing the statistics that ecological observations provide by connecting community science, phenology, and data analysis.

Local Touchstones: Phenology and Climate Change

In 2013, Laura Shultz, the first George Melendez Wright Climate Change Intern, developed the Local Touchstones: Phenology and Climate Change monitoring protocol. This protocol established a long-term phenology dataset for community scientists to contribute to and learn from. Over the nine-year observation period, GCNP staff from the Interpretation Division and Vegetation Program recorded weekly to biweekly phenological observations for 26 Gambel oak (Quercus gambelii) trees at five sites on the South Rim of Grand Canyon National Park.

The Local Touchstones: Phenology and Climate Change project describes a biweekly monitoring protocol in which participants record visual observations of the presence and strength of select phenophases, the developmental life stages that the tree is experiencing at the time. The phenophases being monitored for the Gambel oak include breaking leaf buds, leaf cover, changing leaf color, falling leaves, fruit, ripe fruit, and falling fruit (Figure 1). Monitoring the timing of phenophase onset and duration provides insight into the health of the individual as well as the ecosystem it inhabits (USA-NPN 2022).

Figure 1. Monitored phenophases for Gambel oak. Top row (from left to right): Breaking leaf buds, leaf-out, colored leaves, fallen leaves. Bottom row (from left to right): Fruit, ripe fruit, fallen fruit.

NPS/Laura Shultz

A Gambel oak tree at the South Rim, Grand Canyon National Park.

NPS/Laura Shultz

Gambel oak is a deciduous broadleaf tree distributed throughout the Southwest U.S., often growing between 10-40 feet (3-12 m; Kauffman 2016). At the South Rim of Grand Canyon National Park, Gambel oak is commonly found among the midstory of ponderosa pine (Pinus ponderosa), Utah juniper (Juniperus osteosperma), and Colorado pinyon (Pinus edulis) forests. These small, scrubby oaks may not catch one’s eye the way that the towering ponderosa pines do, but their value as an ecological indicator should not be underestimated. Compared to coniferous trees in this ecosystem, Gambel oak trees undergo more visible change through the seasons, making the species an excellent candidate for phenology monitoring.

For the first few years of the project, staff integrated this monitoring protocol into educational trail walks where park visitors could learn about phenology, ecology, and climate change while recording observations for the long-term dataset. Further, these observations were recorded in Nature’s Notebook, a program developed by the U.S.A. National Phenology Network (USA-NPN) that allows anyone to become a “backyard observer” and contribute phenology observations for over 1,500 plant and animal species to a national phenology database (USA-NPN 2022).

Visual observation of one of the Gambel oak trees as part of the biweekly monitoring protocol.

NPS/Jalyn Gearries

Currently, data from the USA-NPN and Nature’s Notebook have been used in over 185 peer-reviewed publications, including studies that specifically look at phenology in National Park Service (NPS) units. Monahan and others (2016) found that 76% of parks within the NPS are experiencing early spring onset, with Grand Canyon National Park in the “extreme early” category. “This means that recent ‘springs’ are at the far early end of the distribution of all springs at the park since 1900,” explains Alyssa Rosemartin, the USA-NPN Partner and Application Specialist. This study is an excellent example of phenology’s power as an ecological indicator that can be used at many spatial scales, from the boundaries of a single park to ecosystems that span multiple states across the country.

In the summer of 2022, a National Park Service Scientists in Parks Fellows Program intern Jalyn Gearries was tasked with interpreting the nine-year dataset to determine if there is any relationship between Gambel oak phenology and climate change at Grand Canyon National Park. The following section summarizes the methods and results of this statistical analysis, which is just one example of how scientists can work with observational data to understand nature’s complex interactions.

Ecological Data Analysis - Methods

In order to analyze how phenology changes over time, the timing of phenophase onset was computed for three select phenophases based on visual observation availability and reliability: breaking leaf buds, maximum leaf cover, and fruiting. For all observations, the average day of onset for breaking leaf buds was April 25 (116^th day of the year [DOY]), the average day of onset for maximum leaf cover was May 23 (143^rd DOY), and the average day of onset for fruiting was July 22 (204^th DOY).

Next, phenophase day of onset was linked to two climate drivers, temperature and precipitation, to assess the relationship between phenology and the changing climate. Heat accumulation over the course of a year is an effective predictor of phenological transitions and is typically reported in terms of growing degree days (Schwartz et al. 2013). The USA National Phenology Network defines growing degree days as “the number of degrees the mean daily temperature exceeds a base temperature, below which an organizsm will remain developmentally active” (USA-NPN 2022). Using methods described in Herms (2004), the number of accumulated growing degree days (AGDD) were computed at each phenophase day of onset as the temperature-based climate driver. Further, the amount of accumulated precipitation through both rain and snowmelt was computed at each phenophase day of onset as the precipitation-based climate driver. Daily maximum temperature and precipitation data were derived from the PRISM Daily Spatial Climate Dataset AN81d, produced by the PRISM Climate Group at Oregon State University.

A positive correlation between phenophase day of onset and each of the climate drivers can be seen in Figure 2, indicating that increased heat and precipitation accumulation causes a delay in the onset of some phases. To explore these interactions further, phenophase day of onset, accumulated growing degree days, and accumulated precipitation were analyzed using a statistical model that accounts for variation of climate driver relationships between each phenophase.

Figure 2. Scatterplots depicting the positive relationships between accumulated growing degree days (AGDD; panel 1) and accumulated precipitation (panel 2) and the response variable, phenophase day of onset (DOY).

Ecological Data Analysis – Results

The analysis of the nine-year dataset revealed significant insights into how climate drivers influence the Gambel oak trees at Grand Canyon National Park. Using a flexible generalized linear model, we assessed the effects of heat and precipitation accumulation on the day of year for phenophase onset. The results showed that both AGDD and accumulated preciptation significantly influence the timing of phenophase events by delaying the onset of the three selected phases. Among the three selected phases, the maximum leaf-out (ML) phase showed the most pronounced response, indicating a greater delay to other phases.

Further, we explored the interaction between AGDD and accumulated precipitation and found that the combined effect of the two climate drivers is just as significant as the individual effect of temperature or precipitation. The interaction effect implies that higher levels of precipitation can buffer the delaying effect of increased AGDD in phenophase onset.

Decoding Climate Impacts: Insights from Long-Term Phenology Monitoring

Long-term phenology monitoring protocols such as the Local Touchstones: Phenology and Climate Change project provide unique insights into the health and response of ecosystems as we face rising temperatures, increased aridification, and more extreme weather patterns.

The results of our analysis have important ecological implications, particularly in the context of trophic mismatch and water stress. Trophic mismatch occurs when the timing of phenological events in producers (e.g., plants) and consumers (e.g., herbivores, pollinators) becomes out of sync due to differential responses to climate change (Renner and Zohner 2018). Our findings indicate that increased accumulated growing degree days (AGDD) and precipitation delay the onset of key phenophases such as leaf-out and fruiting. Such delays can lead to a mismatch between the availability of resources, like leaves and fruits, and the life cycles of species that depend on them. For example, herbivores and pollinators may emerge based on historical climate patterns, only to find that their food sources are not yet available, which can lead to reduced survival and reproductive success (Lawrence et al. 2024).

Additionally, the interaction between AGDD and accumulated precipitation suggests that higher precipitation levels can mitigate some of the delays caused by increased temperatures. This buffering effect is crucial in arid environments like Grand Canyon National Park, where water stress is a significant concern. Delays in phenophase onset due to higher temperatures can exacerbate water stress by extending the period during which plants require water for growth and development, potentially leading to reduced water availability later in the season. This interaction effect underscores the complexity of climate change impacts and highlights the need for integrated water and temperature management strategies to support ecosystem resilience.

Using statistical methods to analyze long-term phenology data allows us to read nature’s history books by uncovering patterns and trends that might not be immediately apparent. In this study, statistical models enabled us to quantify the combined effects of temperature and precipitation on phenological events. By transforming complex observational data into interpretable results, we can gain insights into the mechanisms driving ecological changes. This approach not only enhances our understanding of how climate change affects ecosystems but also provides valuable information for developing conservation strategies. The use of community science data further emphasizes the importance of public involvement in scientific research, demonstrating how collective efforts can lead to significant advancements in our understanding of ecological processes.

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Cite this article as:

Gearries, J., L. Pilkington, and A. Kilby. 2024. Phenology and climate change: Understanding nature's language through data and community science. National Park Service, Intermountain Park Science 2(1), 2024.