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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.

Citation: 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):e02036. 10. 1002/ecs2.2036 Abstract. Community-level shifts in the distributions of plant functional traits associated with environmental change are expected to influence ecosystem functioning. However, few studies have identified traits that both respond to environmental change and affect ecosystem properties, thus limiting potential to scale the effects of environmental change through the community level. 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 mixeddeciduous 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 expected that understory traits would respond mainly to fire and canopy traits would respond mainly to soil resources. We further hypothesized that fire and resource conditions affect SOC stocks (1) through mass-ratio, by influencing trait abundance; (2) through non-additive effects, by influencing the FD of plant communities; or (3) directly, through either combustion or environmental controls on SOC stocks. 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 mass-ratio effects from canopy leaf traits and secondarily with non-additive effects 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-aggregated leaf traits and diversity in understory size traits can act as both response and effect traits in disturbed forests. Predicting forest SOC stocks using a response-effect trait framework will thus require knowledge of both canopy and understory trait distributions, as well as disturbance history.

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