Compendium Vol. 2 No. 2: Compilation of article summaries on resilience through eco-restoration

The following articles were selected and summarized by Bio4Climate’s Compendium editors and writers. The purpose of this collection is to highlight the scientific evidence and argumentation showing healthy restored and protected ecosystems as a powerful (albeit under-recognized) tool for managing the weather extremes wrought by climate change.  

Floodplains and wetlands: making space for water

Sustainable floodplains through large-scale reconnection to rivers, Opperman et al. 2009

The area of floodplains allowed to perform the natural function of storing and conveying floodwaters must be expanded by strategically removing levees or setting them back from the river. Floodplain reconnection will accomplish three primary objectives: flood-risk reduction, an increase in floodplain goods and services, and resiliency to potential climate change impacts [Opperman 2009: 1487].

Floodplain reconnection reduces flood risk by: (1) replacing vulnerable land uses with flood-tolerant uses, thereby reducing damages, and (2) giving the water somewhere to go, thereby sparing downstream and other nearby communities. Furthermore, by storing and conveying water, floodplains alleviate pressure on upstream dams/reservoirs for flood control and water supply, increasing the resilience of this infrastructure. Finally, by restoring biological activity and diversity, floodplain restoration activates ecosystem services, including carbon sequestration and water quality improvement and groundwater recharge.

The authors note that agricultural lands would be less expensive than densely populated residential areas to reconnect and should be prioritized. Furthermore, agricultural land could remain as such by switching to production of flood-tolerant crops, such as timber and pasture. Furthermore, floodplain reconnection has proven popular among farmers, who requested more than ten times the amount of land be enrolled in a one-time floodplain easement program than the USDA could afford to support with American Recovery and Reinvestment Act funds.

Multifunctionality of floodplain landscapes: relating management options to ecosystem services, Schindler et al. 2014

Human societies tend to value the potential benefits that a landscape might provide in a limited way, adjusting management practices towards desired outputs by maximizing the benefits gained from one or some of the services (often the provision of goods) leading to the loss of multifunctionality and the degradation of natural capital at the expense of human welfare. As a result of this biased valuation, the opportunity costs of biodiversity conservation have been perceived as too high [Schindler 2014: 230].

Using a lens of landscape multifunctionality, this study evaluates 38 potential interventions (ranging from mining, agriculture and residential development to dam removal, natural habitat creation and hiking trail maintenance) in European floodplain ecosystems for their potential to provide multiple ecosystem services (ESS). “Most ESS arise from living organisms and the interaction of biotic and abiotic processes, and refer specifically to the ‘final’ outputs from landscapes that provide benefits to humans”  [Schindler 2014: 230].

Each intervention was evaluated to determine whether its effect on a given ecosystem service was positive, negative or neutral. The more ecosystem services an intervention was considered to positively affect (such as pollination, water purification, flood mitigation, providing for farming, fishing, drinking water, or recreation), the greater its contribution to landscape multifunctionality.

Interventions with the most positive effects were related to the creation of natural habitat, dike relocation, lateral floodplain reconnection, creation of channels, oxbows and ponds, whereas the interventions [related to] terrestrial settlement and transportation infrastructure, navigational infrastructure, and intensive forms of agriculture, forestry and fisheries are rather problematic when preserving multifunctionality in floodplains [Schindler 2014: 238].

Thus the authors found that:

Restoration and rehabilitation measures strongly improved the multifunctionality of the landscape and caused win–win situations for enhancing overall ESS supply for all regulation/maintenance and cultural services, but also for provisioning services [Schindler 2014: 242].

In short, a multifunctional approach allows for ecosystem services and goods that we depend on yet often take for granted, such as clean, abundant drinking water, clean air, pollination, and productive wild fish populations, for example, to be considered in economic evaluations of sites and landscapes such as floodplains.

Need for ecosystem management of large rivers and their floodplains: these phenomenally productive ecosystems produce fish and wildlife and preserve species, Sparks 1995

In their natural state, rivers are not separate or separable from surrounding lands. Rather, a river channel is just one integral part of a larger river-floodplain ecosystem. Annual flood pulses and larger flooding events connect river channels to their floodplains, driving the cycles of life for the particularly diverse ensemble of species that live in floodplain ecosystems. For example, fish use floodplain lakes and backwaters for spawning, shelter, feeding and nurseries. Plants on the floodplain depend on nutrients supplied by sediment deposited during flooding. Due to their geological age, size, habitat complexity, and variability, large river ecosystems – such as the Amazon basin – are among the more biodiverse ecosystems on Earth.

Building levees to contain river water eliminates annual flood pulses, thereby fracturing an ecosystem dependent on these processes. Therefore, for example, “in both tropical and temperate rivers, fish yield per acre is considerably greater in rivers with flood pulses and floodplains than in nearby impoundments where flood pulses are reduced or absent” [Sparks 1995: 172]. In addition,

On land, the natural nutrient-replenishment system once provided by the flood must be replaced with commercial fertilizer. Some societies practice a flood-adapted form of agriculture or harvest both fish and a compatible crop, such as rice, but intensive, high-yield agriculture often conflicts with fisheries, particularly if pesticides are used that can contaminate fish through biomagnification [Sparks 1995: 172].

To at least partially reconnect rivers with floodplains, the author recommends modifying existing structures to divert some flow to create or maintain side channels into the floodplains and restore the annual flood pulse.

The Value of Coastal Wetlands for Flood Damage Reduction in the Northeastern USA, Narayan et al. 2017

The authors address the lack of high-resolution, large-scale assessments of the value of coastal wetlands for reducing property damages from flooding. In the first part of this paper, they assess Hurricane Sandy-induced damages to wetlands. The second part examines the risk reduction benefits of salt marshes in Ocean County, NJ, in terms of average annual economic flood losses. This study involved over 2000 synthetic storm events in Ocean County. The storm events were matched in frequency with actual storms that occurred between 1900 and 2011.  

Wetland extent was positively correlated with damage reduction in all but one of 12 states impacted by Hurricane Sandy. The authors used a hydrodynamic model that calculated the propagation of storm surges from the coastal shelf on to land.  The average amount of damage reduction was slightly over 1%; however, four states with extensive wetlands experienced flood damage reduction of 20-30%.

Losses were less for areas with salt marshes than for those without. On average, salt marshes reduced flood-related damages by 18%. Higher elevations were also correlated with damage reductions.

The authors noted that damage reduction was also apparent at locations several kilometers upstream of affected wetlands. A few areas, however, showed increased storm damage because of their proximity to wetlands. These areas often were dammed, or had their stream channel redirected. Based on their findings, the authors advocate for the increased use of flood risk models  by the insurance industry and small businesses.

The second warning to humanity – providing a context for wetland management and policy, Finlayson et al. 2018

The authors of this article note that prior agreements to halt wetland degradation, such as the Ramsar Convention of 1971, have been largely unsuccessful. They advocate for both a re-emphasis on how wetlands help mitigate climate change, and how to protect existing wetlands from the damaging effects of climate change. They had previously authored the Second Warning to Humanity and Wetlands, which urged the Society for Wetland Scientists (SS) and other organizations to raise the profile of wetlands. Doing so can lead to policy changes which would attenuate the deleterious actions that humans currently apply to wetlands.

The authors then provide 11 recommendations for preserving and renewing wetlands. These recommendations include halting the conversion of wetlands to other land uses, rewilding wetlands with native species, and reducing the wastage of wetland-derived foods. Other recommendations are increasing wetland education, adopting renewable energy sources that don’t impact wetlands, prioritizing the enactment of connected, well-funded and well-managed networks of protected wetland areas, and supporting ecologically sound financial investments.  

Wetlands in a changing climate: science, policy and management, Moomaw et al. 2018

This article emphasizes the global importance of protecting and restoring wetlands in the context of climate change and outlines policy strategies for wetland protection and restoration.

Wetlands play a major though under-appreciated role in climate change mitigation and adaptation. Wetlands enhance local resilience to climate change by providing: “flood storage, buffering of storm damage, protecting water quality by filtering pollutants and sediment out of runoff generated by severe storm events, groundwater recharge and provision of water supply during drought, provision of wildlife refuges and corridors and maintenance of biodiversity” [Moomaw 2018: 192], as well as “direct harvests of fish, animals, and plants” [Moomaw 2018: 188]. Furthermore, wetlands/peatlands store massive amounts of carbon, drawing it out of the atmosphere.  

Peatlands and vegetated coastal wetlands are among the most carbon rich sinks on the planet sequestering approximately as much carbon as do global forest ecosystems [Moormaw 2018: 183].

 

Wetland conditions are critical for C accumulation and storage since decomposition in these systems is limited by a lack of oxygen due to water saturation. Therefore, when plant productivity exceeds decomposition there is an accumulation of soil C. This process eventually develops deep peat deposits, which may accumulate for thousands of years [Moomaw 2018: 187].

By the same token wetlands can become major GHG sources when damaged or destroyed by land use change, fire or climate change.

Altering wetlands can increase the vulnerability of the organic C pool by weakening the self-regulating feedbacks that exist in many peatland systems. Land use change that affects wetland hydrology has had substantial impacts on wetland structure and function. Draining wetlands decreases CO2 uptake and increases rates of microbial decomposition and CO2 release. Soil C is also lost by peat extraction, drainage and other disturbance. The hydrologic changes can be so large that they result in massive losses of C to the atmosphere, such as occurred during the fires in tropical peatlands in Southeast Asia [Moomaw 2018: 187].

 

Many land-use practices in or near wetlands reduce wetlands’ resilience to any further stress, such as hotter, drier weather wrought by climate change.

Unfortunately, many of the world’s freshwater wetlands are already stressed by increased land-use pressure, so that additional hydrological alteration can contribute to an overall decrease in resilience to climate change. Human alteration is commonplace throughout river corridors, challenging management as the impacts of upstream alterations accumulate along the waterway. As demands for river resources increase, such problems are expected to worsen. Flowing water is compromised by river re-engineering practices, even though moving water generally improves oxygenation and plant health. Also, upriver freshwater extraction in tidal freshwater wetlands coupled with sea level rise can cause the salinification of surface and ground water, with accompanying stress and even the collapse of tidal vegetation in the freshwater reaches of estuaries [Moomaw 2018: 188].

On the other hand, wetland resilience can be bolstered through proper land management.

The effects of climate changes on wetland C storage will be determined largely by the extent to which the wetlands have been modified through land-use change [Moomaw 2018: 187].

 

One opportunity to decrease the amount of saltmarsh loss that is likely to occur with sea level rise is to actively plan for future inland marsh migration now [Moomaw 2018: 191].

The authors express concern that wetlands are overlooked in policy discussions on climate change, noting that climate scientists tend to sideline the role of wetlands, while wetlands science and management have often failed to acknowledge the outsized role of wetlands as a carbon sink. Thus:

To play a more effective role in climate change mitigation and adaptation/resiliency, wetland scientists need to clearly communicate the significance of wetlands to the wellbeing of society and the economy. Communicating with policy makers and the public requires aligning wetland science and specific climate mitigation and adaptation/resiliency ecosystem services with the concerns and mindset of the audience [Moomaw 2018: 198].

A handful of policy structures at international, national and subnational levels aim to better account for and protect wetlands. For example, the International Panel on Climate Change (IPCC) has since 2013 provided guidance (through the Wetlands Supplement) to countries about including wetlands in national GHG inventories, thus moving “closer to requiring countries to account for the substantial emissions from these ecosystems when they are disturbed or destroyed” [193]. The 1975 Ramsar Agreement establishes an international framework for wetland management, but lacks adequate guidance on how to best protect wetlands from the stressors of climate change. At the local level, decisions about wetlands are often made by land managers.

Thinking globally and acting locally, wetland managers can incorporate carbon management and climate resiliency science into project-level work (including developing a body of climate-related Best Management Practices), whether or not governing policies and regulations exist. As noted earlier in this article, avoidance of impacts to wetlands, and associated carbon stocks and processes, is likely to be the most effective management practice for preventing increases in GHG emissions from wetlands, protecting climate resiliency functions, and protecting traditional wetland ecosystem services, and it is therefore important for managers to understand the underlying science [Moomaw 2018: 197].

Future response of global coastal wetlands to sea-level rise, Schuerch et al. 2018

The vulnerability of coastal wetlands to sea-level rise is disputed, with some researchers predicting most will be flooded out of existence by the end of the 21st Century. Coastal wetlands provide critical ecosystem services, including protection from storm surges, water quality improvement, fisheries habitat and carbon sequestration. By accounting for the enhancement of sediment build-up when storms are more frequent and more severe and for the possibility of “accommodation space” for coastal wetlands to move inland, however, these authors reach a more optimistic conclusion. (Sediment build-up, or accretion, allows coastal wetlands to grow vertically, potentially remaining at a higher elevation than sea-level.) They estimate that:

Rather than losses, wetland gains of up to 60 per cent of the current area are possible, if more than 37 per cent (our upper estimate for current accommodation space) of coastal wetlands have sufficient accommodation space, and sediment supply remains at present levels [Schuerch 2018: 231].  

This is an important ecosystems restoration message because it means humans can directly influence the persistence of coastal wetlands, and thus the continuation of the essential ecosystem services they provide.

This is an important ecosystems restoration message because it means humans can directly influence the persistence of coastal wetlands, and thus the continuation of the essential ecosystem services they provide.

Our simulations suggest that the resilience of global wetlands is primarily driven by the availability of accommodation space, which is strongly influenced by the building of anthropogenic infrastructure in the coastal zone and such infrastructure is expected to change over the twenty-first century. Rather than being an inevitable consequence of global sea-level rise, our findings indicate that large-scale loss of coastal wetlands might be avoidable, if sufficient additional accommodation space can be created through careful nature-based adaptation solutions to coastal management [Schuerch 2018: 231].

The authors describe specific solutions to protect coastal wetlands, which they recommend be implemented at a large, regional or landscape scale.

Existing nature-based adaptation solutions that allow coastal wetlands to migrate inland include the inland displacement of coastal flood defenses (typically along highly engineered coastlines) or the designation of nature reserve buffers in upland areas surrounding coastal wetlands. These schemes, however, are currently implemented as local-scale projects only; strategically upscaling such projects, for example, as suggested by the shoreline management plans in England and Wales or the coastal master plan in Louisiana, may help coastal wetlands adapt to SLR [sea level rise] at the landscape scale and protect rapidly increasing global coastal populations [Schuerch 2018: 234].

Partnering with beavers to restore ecosystems

Beaver dams and overbank floods influence groundwater–surface water interactions of a Rocky Mountain riparian area, Westbrook et al. 2006

This study provides empirical evidence that beavers influence hydrologic processes in riparian areas. Conducted at the headwaters of the Colorado River in the Rocky Mountains, the study examines patterns from two beaver dams of surface inundation, groundwater flow, and groundwater level dynamics. The authors observe that :

Beaver dams on the Colorado River caused river water to move around them as surface runoff and groundwater seepage during both high- and low-flow periods. The beaver dams attenuated the expected water table decline in the drier summer months for 9 and 12 ha of the 58 ha study area [Westbrook 2006: 1] … by providing a constant supply of water to the riparian area via surface and subsurface flow paths [Westbrook 2006: 10].

In both cases [both dams], water left the Colorado River, flowed across the floodplain and terrace, and then back to the river far downstream of the dams [Westbrook 2006: 11].

Noting that the current beaver population is but a small fraction of what it was before Europeans settled the west, the authors state that:

If the results of our intensive study were extrapolated to a time of more abundant beaver then the magnitude of their hydrologic effects may have encompassed nearly the entire study area. It is easy to visualize abundant beaver as key drivers of hydrologic processes in mountain valleys and other unconfined stream valleys throughout North America [Westbrook 2006: 10].

The significance of this study is that beaver dams can maintain the water table in forests, creating resilience to drought. Beaver dams do this by causing water to overflow the banks of the river and spread over a greater surface area. More effective even than any given rain event, “overbank flood events have generally been regarded as the main hydrologic mechanism for replenishing groundwater and soil water in riparian areas” [Westbrook 2006: 8].

The significance of this study is that beaver dams can maintain the water table in forests, creating resilience to drought. Beaver dams do this by causing water to overflow the banks of the river and spread over a greater surface area.

Modeling intrinsic potential for beaver (Castor canadensis) habitat to inform restoration and climate change adaptation, Dittbrenner et al. 2018

Beavers are recognized for their ability to restore floodplain hydrology and biological function, yet finding suitable places for their reintroduction remains a conservation challenge. The goal of this study was to identify places in the Snohomish River basin of Washington state suitable for beaver reintroduction.

Because of their abilities to modify streams and floodplains, beavers have the potential to play a critical role in shaping how riparian and stream ecosystems respond to climate change. The Pacific Northwest of the United States is experiencing increases in annual air temperature and decreases in snow pack and summer precipitation, resulting in lower base flows, particularly in streams that rely on late season snowmelt. Climate shifts have altered stream-temperature regimes to the detriment of cold-water fishes, including Pacific salmon. Recent increases in winter precipitation and storm magnitude have increased the potential for stream scour, channel incision, and floodplain disconnection, thereby promoting the drying of adjacent riparian areas [Dittbrenner 2018: 2].

By damming streams, beavers create pond and wetland complexes that increase spatial heterogeneity and geomorphic complexity, species and habitat diversity, and therefore ecosystem resilience to climate-induced environmental change. Beaver impoundments slow stream velocity allowing sediment suspended in the water column to settle, aggrading incised stream systems, and reconnecting streams with their floodplains. The increase in surface water promotes groundwater recharge, storage, and supplementation during base flows. The increased geomorphic complexity also promotes higher thermal variability and coldwater refugia in deeper waters and in areas of downstream upwelling [Dittbrenner 2018: 2].

To qualify as a suitable site for beaver reintroduction, a site needs to be intrinsically suitable beaver habitat and clear of competing human interests.

Of 5,019 stream km assessed in this study, just 33% had moderate or high intrinsic potential for beaver habitat. “Of the riparian areas around streams with high intrinsic potential for beaver, 38% are on public lands and 17% are on large tracts of privately-owned timber land” [Dittbrenner 2018: 1], while the rest was on human-dominated landscapes (agricultural, industrial, residential, etc.). Thus, the areas available for beaver reintroduction are limited. Even so, the authors argue that beavers can play a critical role in adapting to climate change, and they propose that watersheds dominated by public ownership, “provide ample opportunities to test how beavers can be reintroduced into landscapes where they are absent or at low population levels” [Dittbrenner 2018: 11].

Beaver restoration would reduce wildfires, Maughan 2013

Politicians often call for logging and fuel reduction to prevent future wildfires. However, it’s not good logging trees that are burning in such fires so much as cheatgrass, annual weed, dry brush and dead weeds. Reintroducing beaver to create ponds could raise the water table, increase humidity in the drainage area (thus reducing burn intensity) and provide a refuge for animals during a fire.

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

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.