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Pyrophytic oak landscapes across the central and eastern United States are losing dominance as shade-tolerant, fire-sensitive, or opportunistic tree species encroach into these ecosystems in the absence of periodic, low-intensity surface fires. Mesophication, a hypothesized process initiated by intentional fire exclusion by which these encroaching species progressively create conditions favorable for their own persistence at the expense of pyrophytic species, is commonly cited as causing this structural and compositional transition. However, many questions remain regarding mesophication and its role in declining oak dominance. In the present article, we review support and key knowledge gaps for the mesophication hypothesis. We then pose avenues for future research that consider which tree species and tree traits create self-perpetuating conditions and under what conditions tree-level processes might affect forest flammability at broader scales. Our goal is to promote research that can better inform restoration and conservation of oak ecosystems experiencing structural and compositional shifts across the region.

Although snags and coarse woody debris are a small component of ecosystem respiration, disturbances can significantly increase the mass and respiration from these carbon (C) pools. The objectives of this study were to (1) measure respiration rates of snags and coarse woody debris throughout the year in a forest previously defoliated by gypsy moths, (2) develop models for dead stem respiration rates, (3) model stand-level respiration rates of dead stems using forest inventory and analysis data sets and environmental variables predisturbance and postdisturbance, and (4) compare total dead stem respiration rates with total ecosystem respiration and net ecosystem exchange. Respiration rates were measured on selected Pinus and Quercus snags and coarse woody debris each month for 1 year in a northeastern U.S. temperate forest. Multiple linear regression using environmental and biometric variables including wood temperature, diameter, density, species, and decay class was used to model respiration rates of dead stems. The mass of snags and coarse woody debris increased more than fivefold after disturbance and respiration rates increased more than threefold. The contribution of dead stems to total ecosystem respiration more than tripled from 0.85% to almost 3% and respiration from dead stems alone was approximately equal to the net ecosystem exchange of the forest in 2011 (fourth year postdisturbance). This study highlights the importance of dead stem C pools and fluxes particularly during disturbance and recovery cycles. With climate change increasing the ranges of many forest pests and pathogens, these data become particularly important for accurately modeling future C cycling.

Correlation analyses were carried out for the dynamics of leaf water potential in two broad-leaf deciduous tree species in a sandy site under a range of air vapor pressure deficits and a relatively dry range of soil conditions. During nights when the soil is dry, the diffuse-porous, isohydric and shallow-rooted Acer rubrum does not recharge its xylem and leaf water storage to the same capacity that is observed during nights when the soil is moist. The ring-porous, deep-rooted Quercus rubra displays a more anisohydric behavior and appears to be capable of recharging to capacity at night-time even when soil moisture at the top 1 m is near wilting point, probably by accessing deeper soil layers than A. rubrum. Compared to A. rubrum, Q. rubra displays only a minimal level of down-regulation of stomatal conductance, which leads to a reduction of leaf water potential during times when vapor pressure deficit is high and soil moisture is limiting. We determine that the two species, despite typically being categorized by ecosystem models under the same plant functional type-mid-successional, temperate broadleaf-display OPEN ACCESSForests 2013, 4 1107 different hydraulic strategies. These differences may lead to large differences between the species in water relations, transpiration and productivity under different precipitation and humidity regimes.

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Oak species are well suited to water-limited conditions by either avoiding water stress through deep rooting or tolerating water stress through tight stomatal control. In co-occurring species where resources are limited, species may either partition resources in space and/or time or exhibit differing efficiencies in the use of limited resources. Therefore, this study seeks to determine whether two co-occurring oak species (Quercus prinus L. and Quercus velutina Lam.) differ in physiological parameters including photosynthesis, stomatal conductance, water-use (WUE) and nitrogen-use efficiency (NUE), as well as to characterize transpiration and average canopy stomatal responses to climatic variables in a sandy, well-drained and nutrient-limited ecosystem. The study was conducted in the New Jersey Pinelands and we measured sap flux over a 3-year period, as well as leaf gas exchange, leaf nitrogen and carbon isotope concentrations. Both oak species showed relatively steep increases in leaf-specific transpiration at low vapor pressure deficit (VPD) values before maximum transpiration rates were achieved, which were sustained over a broad range in VPD. This suggests tight stomatal control over transpiration in both species, although Q. velutina showed significantly higher leaf-level and canopy-level stomatal conductance than Q. prinus. Average daytime stomatal conductance was positively correlated with soil moisture and both oak species maintained at least 75% of their maximum canopy stomatal conductance at soil moistures in the upper soil layer (0-0.3 m) as low as 0.03 m(3) m(3)(-3). Quercus velutina had significantly higher photosynthetic rates, maximum Rubisco-limited and electron-transport-limited carboxylation rates, dark respiration rates and nitrogen concentration per unit leaf area than Q. prinus. However, both species exhibited similar WUEs and NUEs. Therefore, Q. prinus has a more conservative resource-use strategy, while Q. velutina may need to exploit niches that are locally higher in nutrients and water. Likewise, both species appear to tap deep, stable water sources, highlighting the importance of rooting depth in modeling transpiration and stomatal conductance in many oak ecosystems.

Pine-oak ecosystems are globally distributed even though differences in anatomy and leaf habit between many co-occurring oaks and pines suggest different strategies for resource use, efficiency and stomatal behavior. The New Jersey Pinelands contain sandy soils with low water- and nutrient-holding capacity providing an opportunity to examine trade-offs in resource uptake and efficiency. Therefore, we compared resource use in terms of transpiration rates and leaf nitrogen content and resource-use efficiency including water-use efficiency (WUE) via gas exchange and leaf carbon isotopes and photosynthetic nitrogen-use efficiency (PNUE) between oaks (Quercus alba, Q. prinus, Q. velutina) and pines (Pinus rigida, P. echinata). We also determined environmental drivers [vapor pressure deficit (VPD), soil moisture, solar radiation] of canopy stomatal conductance (GS) estimated via sap flow and stomatal sensitivity to light and soil moisture. Net assimilation rates were similar between genera, but oak leaves used about 10% more water and pine foliage contained about 20% more N per unit leaf area. Therefore, oaks exhibited greater PNUE while pines had higher WUE based on gas exchange, although WUE from carbon isotopes was not significantly different. For the environmental drivers of GS, oaks had about 10% lower stomatal sensitivity to VPD normalized by reference stomatal conductance compared with pines. Pines exhibited a significant positive relationship between shallow soil moisture and GS, but only GS in Q. velutina was positively related to soil moisture. In contrast, stomatal sensitivity to VPD was significantly related to solar radiation in all oak species but only pines at one site. Therefore, oaks rely more heavily on groundwater resources but have lower WUE, while pines have larger leaf areas and nitrogen acquisition but lower PNUE demonstrating a trade-off between using water and nitrogen efficiently in a resource-limited ecosystem.

Abstract. We used eddy covariance and meteorological measurements to estimate net ecosystem exchange of CO 2 (NEE), gross ecosystem production (GEP), evapotranspiration (Et), and ecosystem water use efficiency (WUE e ; calculated as GEP / Et during dry canopy conditions) in three upland forests in the New Jersey Pinelands, USA, that were defoliated by gypsy moth (Lymantria dispar L.) or burned using prescribed fire. Before disturbance, half-hourly daytime NEE during full sunlight conditions, daily GEP, and daily WUE e during the summer months were greater at the oakdominated stand compared to the mixed or pine-dominated stands. Both defoliation by gypsy moth and prescribed burning reduced stand leaf area and nitrogen mass in foliage. During complete defoliation in 2007 at the oak stand, NEE during full sunlight conditions and daily GEP during the summer averaged only 14 and 35 % of pre-disturbance values. Midday NEE and daily GEP then averaged 58 and 85 %, and 71 and 78 % of pre-defoliation values 1 and 2 years following complete defoliation, respectively. Prescribed fires conducted in the dormant season at the mixed and pinedominated stands reduced NEE during full sunlight conditions and daily GEP during the following summer to 57 and 68 %, and 79 and 82 % of pre-disturbance values, respectively. Daily GEP during the summer was a strong function of N mass in foliage at the oak and mixed stands, but a weaker function of N in foliage at the pine-dominated stand. Ecosystem WUE e during the summer at the oak and mixed stands during defoliation by gypsy moth averaged 1.6 and 1.1 g C kg H 2 O −1 , representing 60 and 46 % of predisturbance values. In contrast, prescribed fires at the mixed and pine-dominated stands had little effect on WUE e . Two years following complete defoliation by gypsy moth, WUE e during the summer averaged 2.1 g C kg H 2 O −1 , 80 % of predisturbance values. WUE e was correlated with canopy N content only at the oak-dominated stand. Overall, our results indicate that WUE e during and following non-stand replacing disturbance is dependent on both the type and time since disturbance.