More ecosystem-oriented considerations for heat wave, drought, flood and fire resilience

Compendium Volume 2 Number 2 January 2019 r.1

Hot days in the city? It’s all about location, NOAA 2018

In a project funded by National Oceanic and Atmospheric Association (NOAA), about two dozen citizen scientists measured temperatures in Baltimore and Washington DC on two of the hottest days of 2018. By measuring temperatures second by second with thermal sensors while driving prescribed routes through each city, the data collectors revealed a 17-degree temperature gap between the coolest and hottest parts of DC on the same day. The difference? Trees. The well-wooded areas of Natural Arboretum and Rock Creek Park were the coolest parts of the city. The results were similar in Baltimore, where the hottest places were neighborhoods covered in concrete and asphalt with little vegetation. These hotspots were 103 degrees, compared to areas with lots of big trees and parks, which were 16 degrees cooler on the same day.

“Major roadways and dense urban pockets are some of the warmest landscapes in both cities” [NOAA 2018], according to Jeremy Hoffman of the Science Museum of Virginia, one of the lead researchers on the study. “These are areas with little or no vegetation, more asphalt and concrete buildings, which can amplify a heat wave” [NOAA 2018].

Researchers used the data to create heat maps of both cities, which can pinpoint the neighborhoods most vulnerable to dangerous heat waves, and to help city officials identify cooling and resiliency strategies, namely bolstering the quantity and quality of green space, planting new trees and protecting existing trees.

Introduced annual grass increases regional fire activity across the arid western USA (1980–2009), Balch et al. 2013

Cheatgrass is an introduced annual grass that has spread everywhere throughout the western USA. It is among the first plants to emerge in the spring, after which it completes its life cycle, drying out in summer and thus creating a continuous, dry, fine fuel load across the landscape. This study examined the cheatgrass invasion’s effect on the fire regime of the Great Basin region of the western USA, finding that:

Fires were more likely to start in cheatgrass than in other vegetation types and that cheatgrass is associated with increased fire frequency, size, and duration [Balch 2013: 179-180].

Here, we have documented that cheatgrass-dominated areas, which currently cover ~40,000 km2, sustain increased fire probability compared with native vegetation types. As sites burn, more and more of them are likely to become cheatgrass grasslands thus increasing their future probability of burning. If future climate scenarios hold true, the combination of warmer temperatures and high water availability[7] could yield larger fire events that are carried between forested or shrubland areas by invasive grasses, thus perpetuating a novel grass-fire cycle across the western United States and ultimately reducing cover of woody species [Balch 2013: 182].

In native shrub and grassland ecosystems of the arid western United States, high antecedent precipitation has been shown to be one of the strongest predictors of government-registered burned area (1977–2003), even more so than current-year temperature or drought conditions. The oscillation between wet years that enable substantial grass growth and dry years that desiccate those built-up fuels may create ideal conditions for high fire years, but this hypothesis remains untested for cheatgrass rangelands [Balch 2013: 174].

Fire-driven conversion of shrubland to grassland has been linked to a loss of carbon storage and available soil water [Balch 2013: 174].

Adapt to more wildfire in western North American forests as climate changes, Schoennagel et al. 2017

Wildfires in the West have become larger and more frequent over the past three decades (globally, the length of the fire season increased by 19% from 1979 to 2013) and this trend will continue with global warming. Typical fire prevention strategy, centering on fuel reduction and fire suppression, has proved inadequate. Instead, society must accept the inevitability of fires and reorganize itself accordingly, according to this study. Specifically, an adaptive resilience approach would mean:

(i) recognizing that fuels reduction cannot alter regional wildfire trends; (ii) targeting fuels reduction to increase adaptation by some ecosystems and residential communities to more frequent fire; (iii) actively managing more wild and prescribed fires with a range of severities; and (iv) incentivizing and planning residential development to withstand inevitable wildfire [Schoennagel 2017: 4582].

Between 1990 and 2010, almost 2 million homes were added in the 11 states of the western United States, increasing the WUI [wild-urban interface] area by 24%. Currently, most homes in the WUI are in California (4.5 million), Arizona (1.4 million), and Washington (1 million). Since 1990, the average annual number of structures lost to wildfire has increased by 300%, with a significant step up since 2000. About 15% of the area burned in the western United States since 2000 was within the WUI, including a 2.4-km community protection zone, with the largest proportion of wildfires burning in the WUI zone in California (35%), Colorado (30%), and Washington (24%). Additionally, almost 900,000 residential properties in the western United States, representing a total property value more than $237 billion, are currently at high risk of wildfire damage. Because of the people and property values at risk, WUI fires fundamentally change the tactics and cost of fire suppression as compared with fighting remote fires and account for as much as 95% of suppression costs [Schoennagel 2017: 4583].

There often is a lack of political will to implement policies that incur short-term costs despite their long-term value or to change long-standing policies that are ineffective. For example, few jurisdictions have the will or means to restrict further residential development in the WUI, although modifying and curtailing residential growth in fire-prone lands now would reduce the costs and risks from wildfire in the long term. [Schoennagel 2017: 4585].

…modifying and curtailing residential growth in fire-prone lands now would reduce the costs and risks from wildfire in the long term [Schoennagel 2017: 4585].

Amplification of wildfire area burnt by hydrological drought in the humid tropics, Taufik et al. 2017

This study distinguishes between meteorological droughts (lower than average rainfall) and hydrological droughts, where rainfall shortage has eventually led to surface or groundwater levels falling, to predict area burnt from wildfires. By contrast, most studies consider only climate data when predicting wildfire, yet “these overlook subsurface processes leading to hydrological drought, an important driver” [Taufik 2017: 428].

The authors hypothesize that periods with low groundwater recharge will create conditions for a greater area burnt. They found that massive wildfires in Borneo over the past two decades coincided with years when there were large areas of hydrological drought.

Statistical modelling evidence shows amplifying wildfires and greater area burnt in response to El Niño/Southern Oscillation (ENSO) strength, when hydrology is considered. [Taufik 2017: 428]

Hydrological drought stems from a lack of rain, but also depends on the ability of the land to store water. Thus, land use can exacerbate a hydrological drought.

Human activities through land-use change and associated drainage and land clearing immediately following deforestation or long fallow periods create favourable conditions for the fires and amplify the hydrological drying processes in the aboveground fuels and the underlying organic soil [Taufik 2017: 428].

Human activities through land-use change and associated drainage and land clearing immediately following deforestation or long fallow periods create favourable conditions for the fires and amplify the hydrological drying processes in the aboveground fuels and the underlying organic soil [Taufik 2017: 428].

Tall Amazonian forests are less sensitive to precipitation variability, Giardina et al. 2018

Our results demonstrate that in the Amazon, forest height and age regulate photosynthesis interannual variability and are as relevant as mean precipitation. In particular, tall, old and dense forests are more resistant to precipitation variability. Tree size and age directly impact forest structure and thus the carbon cycle in the Amazon. This is especially significant given the importance of the Amazon rainforest, not only for the global carbon cycle, but also for global atmospheric circulation, which is closely connected to the evapotranspiration process of this area. Forest height, age and biomass have a role equivalent to mean precipitation in the regulation of forest photosynthesis response to interannual climate variability [Giardina 2018: 4].

Subordinate plant species enhance community resistance against drought in semi-natural grasslands, Mariotte et al. 2013

This study examines how subordinate species[8] influence community insurance against drought in semi-natural grasslands of the Swiss Jura. The insurance hypothesis proposes that an increase in community diversity corresponds to an increase in the range of potential species responses to environmental stress. The authors tested the role of subordinate species in community resistance to drought, recovery and resilience, and on productivity. They induced summer drought conditions for two months by covering the test plants with raincovers.

The drought simulation reduced soil water content by 67%, relative to comparable watered land plots. Drought, removal of subordinate species, and their interaction, all had dramatic adverse impacts on community resistance. In contrast to dominant and transient species, subordinate species showed significantly stronger resistance in drought plots than in control plots. Additional findings supported the conclusion that the plant community was more resistant and produced more biomass after drought when containing high biomass of subordinate plants.  

Plant community resilience was not affected by drought but was decreased by the subordinate removal treatment. Species composition was also affected by drought and removal conditions; most dominant and transient species[9] were associated with watered plots. Some transient species (such as the ox-eye daisy) were associated with plots in which subordinate removal had occurred.

The authors conclude that, in general, dominant species fared poorly in response to drought, whereas the proportion of subordinate and transient species increased under these conditions.  They also noted that the decline in resistance was about 10 times higher in plots where subordinates had been removed than in plots without removal. Thus, the subordinates facilitated the regrowth of dominants and transients during drought. They proposed that the reduced competition among dominants during drought conditions afforded the subordinates the opportunity to accumulate more biomass.

The authors demonstrate that: “in species-rich grassland communities, subordinate species, a key component of plant diversity, are a main driver of community resistance to drought. Our findings show the importance of ecosystem-level impacts of these low abundant plants” [Mariotte 2013: 771]. They further speculated that the role of subordinates in resisting drought for the whole community may lie in their ability to increase water availability through greater interaction with the soil microbial community, such as mycorrhizal fungi. This article adds credence and specificity to our understanding of the key role of biodiversity in ecosystem functioning.

Balch, Jennifer K., Bethany A. Bradley, Carla M. D'Antonio & José Gómez‐Dans, 2013, Introduced annual grass increases regional fire activity across the arid western USA (1980-2009), Global Change Biology 19, https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.12046.  

Giardina, Francesco, Alexandra G. Konings, Daniel Kennedy, et al., 2018, Tall Amazonian forests are less sensitive to precipitation variability, Nature Geoscience, https://www.nature.com/articles/s41561-018-0133-5.

Mariotte, Pierre, Charlotte Vandenberghe, Paul Kardol, et al. 2013, Subordinate plant species enhance community resistance against drought in semi-natural grasslands, Journal of Ecology 101, https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.12064.

NOAA (National Oceanic and Atmospheric Administration), 2018, Hot days in the city? It’s all about location, https://www.noaa.gov/news/hot-days-in-city-it-s-all-about-location.

Schoennagel, Tania, Jennifer K. Balch, Hannah Brenkert-Smith, et al. 2017, Adapt to more wildfire in western North American forests as climate changes, PNAS 114:18, http://www.pnas.org/content/114/18/4582.  

Taufik, Muh, Paul J. J. F. Torfs, Remko Uijlenhoet, et al., 2017, Amplification of wildfire area burnt by hydrological drought in the humid tropics, Nature Climate Change 7, https://www.nature.com/articles/nclimate3280.

[7] “In the northern Great Basin, precipitation is projected to increase during the winter and early spring months most critical for cheatgrass growth” [Balch 2013: 182].

[8] Among grassland plants, subordinate species, as distinguished from dominants, “are smaller, grow under the canopy of dominants and account for a low proportion of the total community biomass” [Mariotte 2013: 764].

[9] “Species that generally do not persist over time and appear only briefly as seedlings that fail to survive are defined as transient species” [Mariotte 2013: 764].

For the full PDF version of the compendium issue where this article appears, visit Compendium Volume 2 Number 2 January 2019 r.1