Backup_JSC_Vulnerability of Central TX Urban Forests_20210526 — original pdf

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Vulnerability of Central Texas Urban Forests to Climate Change Wendy Gordon, PhD Ecologia Consulting Climate Action Texas May 25, 2021 Impacts of Climate Change on Texas • Physical impacts from changing temperature and precipitation • Hotter days, warmer nights, more extreme precipitation, wildfire • Physiological impacts on fauna and flora • Hitting upper tolerances for processes like photosynthesis • Phenological impacts • Longer growing season, mismatches between predator and prey lifecycles • Range shifts • Disrupted bird migration, changing habitat distributions, “new” plant hardiness zones Today we’re going to talk about a specific project that recently examined the vulnerability of the Central Texas landscape to changing climate Goal of the Urban Forestry Climate Change Response Framework To ensure that urban forests will continue to provide benefits to the people that live in urban communities as the climate changes. We define the urban forest as all publicly and privately-owned trees within an urban area— including individual trees along streets and in backyards, as well as stands of remnant forest. The trees, developed green spaces, and natural areas within the City of Austin’s 400,882 acres will face direct and indirect impacts from a changing climate over the 21st century. This assessment evaluates the vulnerability of urban trees and natural and developed landscapes within the City of Austin to a range of future climates. The Vulnerability Assessment Used scientific projections of future changes in climate, such as differences in seasonal temperature and precipitation, to set boundary conditions Reviewed results from the latest research to determine how urban forests and natural areas around Austin may respond to changes in climate, disturbance, and management Drew from local expertise - scientists and forest managers - to synthesize results and identify key vulnerabilities within the urban forest and natural ecosystems Described the implications that future changes will have on a wide variety of ecological, social, and economic factors Vulnerability is the susceptibility of a system to the adverse effects of climate change. It is a function of potential climate change impacts and the adaptive capacity of the system. A system is vulnerable if it is at risk for no longer being recognizable as that community type, or if the system is anticipated to suffer substantial declines in health or productivity. To assess vulnerability, a panel of experts on the ecology and management of Austin’s urban forest, including developed and natural areas, met for a two-day workshop. Areas in the Austin region tended to be rated in the moderately vulnerable range. Key Vulnerability Assessment Findings: Climate Trends & Projections • Austin has been warming at a rate of about 0.4°F per decade since 1948 and is expected to warm by 5 to 10 degrees by the end of this century compared to the most recent 30-year average. • Since 2000, all years have been hotter than the 1961-1990 average – a standard baseline for examining climate trends. • Eight of the top 10 years with the most 100°F days have occurred this century. • Six of the ten hottest years in Austin have occurred between 2000 and 2019. Key Vulnerability Assessment Findings: Climate Trends & Projections • Austin has been getting slightly wetter on average, but precipitation can vary widely within and between years, and future projections of precipitation are uncertain. • Overall, the balance of precipitation and temperature may shift Austin’s climate to be more similar to the arid Southwest. • It is highly probable there will be both an increase in heavy rain events and severe droughts in the future decades, which will stress the area’s trees. • Changes in temperature and precipitation may exacerbate current stressors such as non-native invasive plants, insect pests, and pathogens. Austin’s Contemporary Landscape: 2014 Urban Forest Inventory and Analysis • Austin is composed of two ecoregions: Edwards Plateau to the west and Blackland Prairie to the east. These areas support different tree species that are uniquely adapted to each ecoregion. • Austin’s urban forest is made up of approximately 34 million trees with a tree canopy covering about 31% of the city. • The majority (92%) of trees are native to Texas, and the 10 most common trees account for 84% of all trees. Austin’s Contemporary Landscape – Continued • The most common species are Ashe juniper, cedar elm, live oak, sugarberry, and Texas persimmon. • Trees with diameters <5” account for 62% of the tree population. • Large-diameter trees >15” are only 3% of the total population, but 18% of total leaf area and provide many ecosystem services. Upland Forest Ashe juniper, Texas red oak, Texas/escarpment live oak, white shin oak, cedar elm, sugarberry, post oak, blackjack oak, Arizona walnut, Escarpment black cherry, Texas ash, gum bumelia, Texas redbud, Carolina buckthorn, rusty blackhaw, red buckeye, Mexican buckeye, Mexican plum, Texas madrone Upland Woodland Ashe juniper, Texas/escarpment live oak, cedar elm, sugarberry, post oak, white shin oak, blackjack oak, Shumard oak, southern live oak, mesquite, eastern redcedar, gum bumelia Upland Mixed Shrubland Texas/escarpment live oak, Ashe juniper Floodplains and Terraces Sugarberry, cedar elm, Texas/escarpment live oak, green ash, pecan, American elm, American sycamore, little walnut, western soapberry, Texas oak/Buckley oak, black walnut, eastern cottonwood, Ashe juniper, chinaberry, bald cypress, boxelder, Texas ash, Vitex, Chinese elm, wafer ash, mesquite, black willow, mulberry sp., eastern redcedar, gum bumelia Large Versus Small Tree Species • The most common large-diameter (>15”) trees are Ashe juniper, live oak, cedar elm, pecan, sugarberry, Texas red oak, honey mesquite, chinaberry, and cottonwood. • The most common small diameter (<5”) species: Ashe juniper, cedar elm, Texas persimmon, sugarberry, live oak, yaupon holly, Texas mountain laurel, glossy privet (ligustrum), chinaberry, and green ash. • If current large trees are not replaced by other large trees, this may reduce the future canopy cover of Austin. East Versus West Austin • Ashe juniper and live oak make up 80% of species composition in West Austin versus four species (Ashe juniper, cedar elm, honey mesquite, and live oak) making up 60% of in East Austin. • In West Austin, Ashe juniper covers 68% of the area compared to 20% in East Austin. Cedar elm is more common in East Austin at 18% compared to just 2% in West Austin. Live oak is similar across the city at 12% to 14%. • West Austin contains 11 unique species, while East Austin contains a 29 unique species. Climate Projections • Climate projections were retrieved from Climate Mapper using 2 emissions scenarios and 20 climate models (averaged) downscaled to 4km. Scenario 1: greenhouse gas emission rates are dramatically reduced. Scenario 2: “business as usual” in which emissions keep growing at the current rate. • At 4km resolution we can look at climate trends across the Austin area, but cannot identify microclimates that could be hotter or cooler, such as urban heat islands and north-facing slopes. • The project bounded results by pairing a model that tends to be cooler and wetter than average projections with the reduced emissions scenario and using a model that tends to be hotter and drier than average with the high emissions scenario. Climate Projections • Hotter on the order of 5-10˚F by the end of the century with the “business as usual” scenario generating larger increases • Mixed message on precipitation; regardless, increasing heat will drive increased soil evaporation and drying. Evidence of storms becoming more intense (e.g., Atlas14). Note: Forecasting precipitation in Central Texas is notoriously challenging to meteorologists using present-day weather models due to the atmospheric dynamics of this region. Those same challenges hamper climate modeling of this region. End of Century Climate Modeling Envelope Modeling Projections of Species Distributions Scientists can project future habitat suitability using species distribution models (SDMs). SDMs establish a statistical relationship between the current distribution of a species or ecosystem and key attributes of its habitat. This relationship is used to make projections about how the range of the species will shift as climate change affects those attributes. Species Distribution Modeling Results - Decline Species Distribution Modeling Results - Stable Additional Results from Species Distribution Modeling Gain blackjack oak pecan sugarberry water oak Uncertain Change cedar elm gum bumelia honey locust Alternatively, we projected changes to species distributions from heat and hardiness zone shifts. Cold Hardiness Zones Cold Hardiness Zone – Scenario 1 Cold Hardiness Zone – Business As Usual Heat Tolerance, Cold Hardiness, and Growing Season Length in the Austin Area through 2099 We examined species’ current ranges by county using the Biota of North America (Kartesz, 2015). The climate of Austin is projected to become similar to areas south and west over the coming decades. Species presently found south and west of Austin may be best suited to future climate conditions. A species currently at the northern and/or eastern extent of its range in Travis County (more common to the southwest), is likely to be positively affected by climate change. A species at the southern and/or western extent of its range (more common to the northeast), is likely to be negatively affected. Based on this method, 23 species may benefit from milder winters (indicated by a shift in hardiness zone) over the next century including southern live oak (Q. virginiana), Texas mountain laurel, loquat, Mexican (Berlandier) ash, Jerusalem thorn (retama), Mexican white oak, and sweet acacia (huisache). 60 species had either hardiness zone, heat zone, or range limits (or a combination thereof) that suggests a negative impact from an increase in temperature. Many of Austin’s most common species are included in this category including Ashe juniper, cedar elm, sugarberry/ hackberry, yaupon, green ash, Texas red oak (Q. buckleyi), boxelder, bastard/white shin oak (Q. sinuata), pecan, western soapberry, crapemyrtle, winged elm, American sycamore, and Texas live oak (Q. fusiformis). 21 species are not anticipated to be affected by temperature. Species include Texas persimmon, honey mesquite, and Texas ash. Sample Results from Species Distribution Models Sample Results from Hardiness & Heat Zone Shift Evaluation Evaluating Adaptive Capacity of Species • Species were scored according to modification factors to reflect their adaptive capacity. • Modification factors include fire or drought tolerance, dispersal ability, shade tolerance, site specificity, and susceptibility to insect pests and diseases. • A species with a large number of positive modification factors = high adaptive capacity. • 104 species and varieties were ranked as having high, medium, and low adaptive capacity based on modification factor scores. Highly Adaptable Species Many of the most adaptable species are non-native invasive species, such as Chinese tallow, Chinese elm, glossy privet, chinaberry, paper and white mulberry, mimosa/silktree, Chinese pistache, and goldenrain tree. Native species with high adaptability scores include bald cypress, cedar elm, eastern redbud, gum bumelia, sumac species, yaupon, Texas persimmon, live oak species, possumhaw, and sugarberry. Native species with low adaptability scores include black and little walnut, black hickory, Escarpment black cherry, pecan, and lacey oak. Vulnerability = Susceptibility to Climate Change x Adaptability Summary of Impacts, Adaptive Capacity, and Vulnerability for Evaluated Areas Key Findings Both natural and developed areas in the Austin region are vulnerable to changes in climate. Natural and developed upland areas in West Austin are also vulnerable to drought, erosion, and wildfire and have less tree diversity than East Austin. Natural and developed areas in East Austin are more vulnerable to precipitation extremes due to shrink-swell soils and low elevation but have a greater potential for a diverse tree canopy than West Austin. The urban core and other highly developed areas will experience stress not only from changes in climate but also from compounding effects of drought, heat, and localized runoff-induced flooding. Summary • Species distribution modeling of native trees suggests that suitable habitat may decrease for 14 primarily northern species. Suitable habitat was expected to increase for 4 species. Habitat was stable for 10. • Adaptive capacity of 104 species was evaluated using scoring systems for planted and natural environments, with many non-native invasive species among those with the highest capacity to adapt to a range of stressors. Management Considerations Maintaining species diversity and selecting appropriate species for the projected changes in habitat suitability will become more of a challenge for everyone, from land managers to the nursery industry. Given the uncertainties around the effects of climate change it will be important for land managers to continue to observe and document impacts on tree species and refine models and management strategies. So how do you decide what to plant next? Are you planting for 2025, 2055 or 2095? Resources Climateactiontexas.org • https://www.climatehubs.usda.gov/index.php/hubs/northern- forests/topic/vulnerability-assessment-austins-urban-forest-and- natural-areas