Climate-driven mortality and distributional shifts of a widespread tree species across North America
Among the many reports of forest mortality globally, the dieback of trembling aspen (Populus tremuloides) has been of particular concern, given that it is the most widespread tree species in North America and supports high levels of biodiversity and productivity. A variety of factors have been implicated in the decline (e.g., wildfires, disease, and insect outbreaks), but accumulating evidence suggests that drought stress is a primary driver.Using remote sensing data, my collaborators and I seek to (1) document aspen mortality rates and distributional shifts, and (2) determine the drivers that best predict aspen performance across its broad, North American range.
Fire-driven filtering of plant functional traits and consequences for forest drought vulnerability
An increase in both drought and fire frequency with climate change threaten forest productivity and function, including their ability to serve as a carbon sink. Predicting forest vulnerability to the dual stressors of drought and fire remains difficult, though the physiological traits of trees that confer tolerance to drought (e.g. embolism risk) and fire (e.g. resprouting ability) provide a promising framework. We are conducting a meta-analysis of pre- and post-fire forest communities to understand the degree of overlap between fire- and drought-adaptive traits in tree species.
Effects of increased fire frequency on the productivity and drought tolerance of temperate broadleaf forests
Minimizing forest vulnerability to climate change has emerged as a primary goal of forest management. Several recent studies suggest that forest thinning is a promising management tool. By reducing forest basal area, thinning effectively increases water availability to the remaining trees, thereby reducing forest vulnerability to drought. Another potential strategy to moderate drought-induced stress is through the repeated application of prescribed fire. Like thinning, repeat fires can reduce forest basal area, but fire can also affect nutrient cycling and cause cambium and root injury to remaining trees. Despite its widespread use, fire has been largely overlooked as a management tool to reduce forest drought vulnerability. To test the long-term effects of repeat fires on forest drought vulnerability, Dr. Refsland and collaborators measured the annual radial growth and isotopic composition (δ13C and δ18O) of adult oaks (Quercus stellata and Q. falcata) in the Missouri Ozarks that experienced either no fire, periodic (every 4 years) or annual prescribed fires from 1950 – 2015.
Importance of drought-fire interactions for tree recruitment
Fire activity may have opposing effects on the drought vulnerability of juvenile trees because of its impact on multiple ecological processes. For fire-tolerant tree species that survive to resprout, fire exposure directly impacts tree physiology by killing shoot tissue and thus increasing root-to-shoot ratios. This can positively affect tree water relations by reducing losses to transpiration. Fire also indirectly impact juvenile tree physiology through its effects on local microclimate and nutrient cycling. The consumption of standing vegetation by fire increases understory light availability, soil surface temperatures and local evaporative demand, promoting a more xeric microclimate that may intensify drought stress. Additionally, by reducing organic inputs to soil and driving losses of surface nitrogen (N) through combustion, increased fire activity may further constrain growth of juvenile trees during drought by intensifying N limitation of tree growth. Depending on whether resprouting or environmental effects predominate, the impacts of increased burning on forest regeneration could be positive, negative or neutral.
We tested how fire affects the drought vulnerability of juvenile trees by imposing summer drought on natural and planted populations of Quercus alba juveniles located in periodically burned and unburned sites in Shawnee National Forest, IL, USA. We then compared the drought-driven declines in leaf water potential, gas exchange and growth rates for juveniles in burned and unburned sites. We found that resprouting after fire temporarily improved leaf water relations for juveniles, but that fire ultimately exacerbated drought-driven declines in tree growth by promoting a warmer, drier microclimate and intensifying N limitation.
Collaborators: Jennifer Fraterrigo
Key publication: Refsland, T.K. and J.M. Fraterrigo. 2018. Fire increases drought vulnerability of Quercus alba seedlings by altering forest microclimate and nitrogen availability. Functional Ecology. https://doi.org/10.1111/1365-2435.13193
Functional traits and soil carbon
A primary goal of community ecology is to predict the consequences of changing biodiversity for ecosystem function. Plant functional traits are important to this goal as traits can determine species’ responses to resource gradients and disturbance (response traits) as well as their effects on ecosystem function (effect traits). Certain traits may be deemed dual response–effect traits if those traits favored by the environment are those that also influence ecosystem function. Because such linked traits provide a direct means of scaling community‐level changes to changes in ecosystem functioning, identifying them could enhance capacity to predict future ecosystem services.
Despite increasing adoption of the response–effect trait framework, few empirical studies have tested for the existence of response–effect traits in fire-impacted forests and it is unknown whether fire’s effects on plant trait composition are consistent across resource gradients where fire intensity and vegetation recovery can strongly vary. We measured canopy and understory plant functional traits, characterizing the most abundant functional trait value (community‐weighted mean; CWM) and the functional diversity (FD), across a soil resource gradient in fire‐managed mixed‐deciduous forests to determine how traits both respond to a disturbance‐resource gradient and affect stocks of active and stable soil organic carbon (SOC) fractions.
We found that understory traits responded to soil resource conditions and fire, whereas only canopy CWM leaf dry matter content (LDMC) varied with resource conditions; no canopy traits varied with fire. Among the response traits, canopy CWM LDMC and diversity in the maximum height of the understory were related to SOC stocks, suggesting they play dual roles as response and effect traits. SOC stocks were primarily associated with mean canopy leaf trait values and secondarily with the diversity of trait values from the canopy and understory. There were also strong, fraction‐dependent patterns in SOC stocks with fire disturbance. Repeatedly burned forests characterized by resource conservative traits (i.e., high canopy CWM LDMC) had a higher relative proportion of active SOC, whereas unburned forests characterized by resource acquisitive traits (i.e., high canopy CWM leaf nitrogen content) had a higher relative proportion of stable SOC. Our results suggest that canopy community-mean leaf traits and diversity in understory size traits can act as both response and effect traits in disturbed forests.
Collaborator: Jennifer Fraterrigo
Key publication: Refsland, T.K. and J. M. Fraterrigo. 2017. Both canopy and understory traits act as response-effect traits in fire-managed forests. Ecosphere 8(12). https://doi.org/10.1002/ecs2.2036