Physiological strategies of co-occurring oaks in a water- and nutrient-limited ecosystem

Abstract

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.

Decadal-Scale Reduction in Forest Net Ecosystem Production Following Insect Defoliation Contrasts with Short-Term Impacts of Prescribed Fires

“…Canopy foliar N concentrations and total N content were lower in the second flush of foliage following defoliation, and ecosystem WUE was reduced by approximately 42% during the growing season compared to pre-disturbance years [37,57]. During the growing seasons following insect defoliation, little difference in foliar N levels or assimilation rates of canopy oak foliage occurred, but overall canopy N content was lower following oak mortality compared to the pre-disturbance period [61,63]. However, foliar N content of sub-canopy and understory species increased following defoliation, and thus little net change in the total amount of N in foliage occurred and ecosystem WUE was not significantly different pre-and post-disturbance [37,57,63].…”

“…During the growing seasons following insect defoliation, little difference in foliar N levels or assimilation rates of canopy oak foliage occurred, but overall canopy N content was lower following oak mortality compared to the pre-disturbance period [61,63]. However, foliar N content of sub-canopy and understory species increased following defoliation, and thus little net change in the total amount of N in foliage occurred and ecosystem WUE was not significantly different pre-and post-disturbance [37,57,63]. Increased foliar N levels and enhanced maximum assimilation rates in the remaining pine needles were observed following relatively intense prescribed burns in the PNR, but not during low-intensity burns [25,64].…”

Biogeochemical Modelling vs. Tree-Ring Measurements - Comparison of Growth Dynamic Estimates at Two Distinct Oak Forests in Croatia

“…2005-2006, Figure 5), which is in contrast to the common negative response of oak trees' growth to temperature in spring and summer found in Čufar et al [47], and to the positive response of oaks to rainy, humid and cloudy conditions during the current year's summer [16]. Nevertheless, according to Renninger et al [48], it is highly important to account for groundwater table information when interpreting the response of oak ecosystems to dry conditions. Due to low vertical water conductivity of gleysol soil, forests at Pokupsko site are partly flooded with stagnating water during winter and early spring.…”

Nonlinear tree growth dynamics predict resilience to disturbance

“…For example, investigators have elucidated distinct physiological responses of different cooccurring tree genera (Breshears et al 2005, 2009, Eilmann and Rigling 2012, Mitchell et al 2013) and co-occurring oak species (Hu et al 2013, Renninger et al 2014 to drought. Such studies help investigators predict which tree species may be relatively resilient to this type of disturbance.…”

“…Testing this hypothesis cannot discern the mechanism driving tree survival or susceptibility to disturbance. However, unlike more mechanistic studies exploring trees' physiological response to a particular disturbance type (Breshears et al 2005, 2009, Eilmann and Rigling 2012, Hu et al 2013, Mitchell et al 2013, Renninger et al 2014), our approach is applicable to a multitude of disturbance types. We use the results of our work to begin to disentangle the mystery of why some trees appear susceptible to v www.esajournals.org disturbance, while conspecifics with apparently similar access to resources and co-dominance status in the canopy succumb.…”