Fire increases drought vulnerability of Quercus alba juveniles by altering forest microclimate and nitrogen availability

Abstract

Shifts in rainfall patterns due to climate change are expected to increase drought stress and mortality in forests. Natural and anthropogenic fire regimes are also changing, highlighting the need to understand the interactive effects of fire and drought on tree ecophysiological response and growth. Using rainout shelters, we imposed summer drought on natural and planted populations of Quercus alba juveniles located in periodically burned and unburned sites in Shawnee National Forest, IL, USA. A subset of planted juveniles was clipped to simulate topkill. We measured predawn leaf water potential (Ψpd), leaf gas exchange (Amax, gmax) and relative growth rate (RGR) across treatments to test two hypotheses: (H1) Fire reduces juvenile drought stress by improving water relations through increased root‐to‐shoot ratios after topkill, or (H2) fire exacerbates juvenile drought stress by promoting a warmer, drier microclimate or amplifying drought‐induced nitrogen (N) limitation. Burned stands had higher vapour pressure deficits and 13% lower soil inorganic N availability than unburned stands. Rainout shelters reduced soil moisture (0–45 cm) by 10%–24% relative to ambient conditions. Consistent with H2, small, natural resprouts in burned stands experienced greater drought stress than unburned juveniles, with a 7% decrease in leaf nitrogen content (LNC), a 21%–29% reduction in Amax and gmax and a 41% reduction in RGR under drought. Drought effects on unburned juveniles, in contrast, were limited to a 5% reduction in LNC and a neutral to positive effect on leaf gas exchange and RGR. Large natural juveniles were largely unaffected by drought. Recent resprouts (i.e., clipped, planted juveniles) experienced less drought stress than unclipped juveniles, providing partial support for H1. Collectively, these results suggest that resprouting after fire can temporarily improve leaf water relations until root‐to‐shoot ratios re‐equilibrate. In contrast, fire can exacerbate drought‐driven declines in the growth of small juveniles by both promoting a warmer, drier microclimate and intensifying N limitation. Our results suggest that despite the high drought tolerance of Quercus spp., fire‐driven changes to local microclimate and resource conditions could limit tree recruitment under future scenarios of rainfall variability. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13193/suppinfo is available for this article.

Can restoration of fire‐dependent ecosystems reduce ticks and tick‐borne disease prevalence in the eastern United States?

“…Prescribed fire shifts forest microclimate toward a greater frequency of temperature and humidity conditions known to limit tick success and survival. Mechanistically, reductions in canopy and understory density and the creation of gap space from prescribed burning increase solar exposure (Stevens, 2017), reduce evapotranspiration (Clark et al, 2012), and increase understory wind speed (Ma et al, 2010), promoting hotter and drier understory conditions during the daytime (Iverson & Hutchinson, 2002; Refsland & Fraterrigo, 2018) and colder temperatures at night. This enhances diurnal temperature fluctuations and enables a higher frequency of moisture and temperature extremes relative to tick tolerances for behavior, development time, molting success, and overall survival (Schulze et al, 2001; Vail & Smith, 1998).…”

Quantifying fire severity: a brief review and recommendations for improvement

“…The sensitivity of different species to wildfires may be very different, and secondary hazards may occur after a fire, thus it is often difficult to determine if the death of certain organism is caused by wildfire or the result of ecosystem response. The cause of tree death can be complex, and many factors can lead to misjudgment; for example, bark beetles that are not easy to detect spread quickly after forest fire (Ray et al 2019;Stevens-Rumann et al 2015), changes in soil physical and chemical properties after wildfires (Holden et al 2016;Ludwig et al 2018;Quintero-Gradilla et al 2015), and changes of microclimate after forest fires (Refsland and Fraterrigo 2018;Watts and Kobziar 2015).…”

Different ways to die in a changing world: Consequences of climate change for tree species performance and survival through an ecophysiological perspective

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