Xingquan Rao

Diurnal, daily, seasonal and annual patterns of sap-flux-scaled transpiration from an Acacia mangium plantation in South China

Abstract• In this study of a 19-year-old Acacia mangium plantation with a basal area of 26.6 m2 ha−1 in subtropical South China, whole tree sap flow was measured continuously over a two-year period and the dependence of stand sap flow (Et) on environmental factors was investigated at diurnal (hourly), daily, seasonal and annual scales.• Daytime hourly mean Et was linearly correlated to photosynthetically active radiation (Q) and vapor pressure deficit (D) in each season during the whole study period (R2 > 0.57, P < 0.001), whereas daily daytime mean Et was less tightly coupled to Q and D (R2 < 0.50).• Pronounced hysteresis was observed between Et and Q as well as D and the extent of the hysteresis varied seasonally.• Total annual stand transpiration was higher in the first year (244.5 mm) than in the second year (185.8 mm) although rainfall was lower in the first year (1 122.4 mm) than in the second year (1 342.5 mm), from which it can be concluded that annual stand transpiration did not simply increase with increased annual rainfall.Résumé• Le flux de sève a été suivi dans une plantation d’Acacia mangium âgée de 19 ans avec une surface terrière de 26,6 m2 ha−1 dans le Sud subtropical de la Chine. Les mesures ont été menées en continu pendant deux ans et la dépendance de la transpiration du peuplement (Et) par rapport aux facteurs environnementaux a été étudiée à des échelles diurnes (échelle horaire), journalières, saisonnières et annuelles.• Pendant la journée, la moyenne horaire de Et était linéairement corrélée au Rayonnement photosynthétiquement actif (Q) et au déficit de pression de vapeur d’eau (D) pendant toute la période d’étude (R2 > 0, 57, P < 0,001) alors que la moyenne journalière de Et était moins bien couplée à Q et D (R2 < 0, 50).• Une hystérésis marquée entre Et et Q en même temps que D a été observé; son amplitude variait avec la saison.• La transpiration annuelle totale du peuplement a été plus élevée la première année (244,5 mm) que la deuxième année (185,8 mm) bien que les précipitations étaient plus faibles la première (1 122,4 mm) que la seconde année (1 342,5 mm). Nous en concluons que la transpiration annuelle du peuplement ne s’accroît pas simplement avec l’augmentation des précipitations annuelles.

CAN Canopy Addition of Nitrogen Better Illustrate the Effect of Atmospheric Nitrogen Deposition on Forest Ecosystem

Increasing atmospheric nitrogen (N) deposition could profoundly impact community structure and ecosystem functions in forests. However, conventional experiments with understory addition of N (UAN) largely neglect canopy-associated biota and processes and therefore may not realistically simulate atmospheric N deposition to generate reliable impacts on forest ecosystems. Here we, for the first time, designed a novel experiment with canopy addition of N (CAN) vs. UAN and reviewed the merits and pitfalls of the two approaches. The following hypotheses will be tested: i) UAN overestimates the N addition effects on understory and soil processes but underestimates those on canopy-associated biota and processes, ii) with low-level N addition, CAN favors canopy tree species and canopy-dwelling biota and promotes the detritus food web, and iii) with high-level N addition, CAN suppresses canopy tree species and other biota and favors rhizosphere food web. As a long-term comprehensive program, this experiment will provide opportunities for multidisciplinary collaborations, including biogeochemistry, microbiology, zoology, and plant science to examine forest ecosystem responses to atmospheric N deposition.

Consistent effects of canopy vs. understory nitrogen addition on the soil exchangeable cations and microbial community in two contrasting forests.

Anthropogenic N deposition has been well documented to cause substantial impacts on the chemical and biological properties of forest soils. In most studies, however, atmospheric N deposition has been simulated by directly adding N to the forest floor. Such studies thus ignored the potentially significant effect of some key processes occurring in forest canopy (i.e., nitrogen retention) and may therefore have incorrectly assessed the effects of N deposition on soils. Here, we conducted an experiment that included both understory addition of N (UAN) and canopy addition of N (CAN) in two contrasting forests (temperate deciduous forest vs. subtropical evergreen forest). The goal was to determine whether the effects on soil exchangeable cations and microbial biomass differed between CAN and UAN. We found that N addition reduced pH, BS (base saturation) and exchangeable Ca and increased exchangeable Al significantly only at the temperate JGS site, and reduced the biomass of most soil microbial groups only at the subtropical SMT site. Except for soil exchangeable Mn, however, effects on soil chemical properties and soil microbial community did not significantly differ between CAN and UAN. Although biotic and abiotic soil characteristics differ significantly and the responses of both soil exchangeable cations and microbial biomass were different between the two study sites, we found no significant interactive effects between study site and N treatment approach on almost all soil properties involved in this study. In addition, N addition rate (25 vs. 50 kg N ha(-1) yr(-1)) did not show different effects on soil properties under both N addition approaches. These findings did not support previous prediction which expected that, by bypassing canopy effects (i.e., canopy retention and foliage fertilization), understory addition of N would overestimate the effects of N deposition on forest soil properties, at least for short time scale.

Invariant community structure of soil bacteria in subtropical coniferous and broadleaved forests

Soil bacteria may be influenced by vegetation and play important roles in global carbon efflux and nutrient cycling under global changes. Coniferous and broadleaved forests are two phyletically distinct vegetation types. Soil microbial communities in these forests have been extensively investigated but few studies have presented comparable data regarding the characteristics of bacterial communities in subtropical forests. We investigated soil bacterial biomass and community composition in three pairs of coniferous and broadleaved forests across a subtropical climatic gradient. We found that bacterial biomass differed between the coniferous and broadleaved forests across the subtropical climate gradient; however, this difference disappeared at some individual sites. In contrast, the same 90 bacterial genera were found in both forest types, and their relative abundances didn’t differ between the forest types, with the exception of one genus that was more abundant in broadleaved forests. Soil nitrogen or moisture was associated with bacterial groups in the coniferous and broadleaved forests, respectively. Thus, we inferred that these forests can respond differently to future changes in nitrogen deposition or precipitation. This study highlights soil bacterial invariant community composition in contrasting subtropical forests and provides a new perspective on the potential response and feedback of forests to global changes.

Nocturnal sap flow characteristics and stem water recharge of Acacia mangium

In this paper, we studied the nocturnal stem water recharge of Acacia mangium. It is helpful to improve the precision of canopy transpiration estimation and canopy stomatal conductance, and to further understand the lag time of canopy transpiration to stem sap flow. In this study, the whole-tree sap flow in an A. mangium forest was measured by using Granier’s thermal dissipation probe for over two years in the hilly land of South China. The environmental factors, including relative humidity (RH), precipitation, vapor pressure deficit (VPD), photosynthetically active radiation (PAR), and air temperature (Ta) were recorded simultaneously. The stem water recharge of A. mangium was analyzed on both daily and monthly scales. Sap flux density was lower at night than during the day. The time range of nighttime sap flux density was longer in the dry season than in the wet season. The water recharging mainly occurred from sunset to midnight. No significant differences were observed among inter-annual nighttime water recharges. Nighttime water recharge had no significant correlation with environmental factors, but was well correlated with the diameter at breast height, tree height, and crown size. In the dry season the contribution of nighttime water recharge to total transpiration had significant correlations with daytime transpiration, total transpiration, VPD, PAR and Ta, while in the wet season it was significantly correlated with daily transpiration and total transpiration.

Combining sap flow measurement-based canopy stomatal conductance and ~(13)C discrimination to estimate forest carbon assimilation

The available methods for studying C uptake of forest and their problems in practices are reviewed, and a new approach to combining sap flow and 13C techniques is proposed in this paper. This approach, obtained through strict mathematic derivation, combines sap flow measurement-based canopy stomatal conductance and 13C discrimination to estimate instantaneous carbon assimilation rate of a forest. Namely the mean canopy stomatal conductance (gc) acquired from accurate measurement of sap flux density is integrated with the relationship between 13C discrimination (Δ) and Ci/Ca (intercellular/ambient CO2 concentrations) and with that between Anet (net photosynthetic rate) and gco2 (stomatal conductance for CO2) so that a new relation between forest C uptake and Δ as well as gc is established. It is a new method of such kind for studying the C exchange between forest and atmosphere based on experimental ecology.

Faster carbon accumulation in global forest soils

Comparing soil organic carbon (SOC) stocks across space and time is a fundamental issue in global ecology. However, the conventional approach fails to determine SOC stock in an equivalent volume of mineral-soil, and therefore, SOC stock changes can be under- or overestimates if soils swell or shrink during forest development or degradation. Here, we propose to estimate SOC stock as the product of mineral-soil mass in an equivalent mineral-soil volume and SOC concentration expressed as g C Kg-1 mineral-soil. This method enables researchers to compare SOC stocks across space and time. Our results show an unaccounted SOC accumulation of 2.4 - 10.1 g C m-2 year-1 in the 1m surface mineral-soils in global forests. This unaccounted SOC amounts to an additional C sink of 0.12 – 0.25 Pg C year-1, which equals 30 – 62% of the previously estimated annual SOC accumulation in global forests. This finding suggests that forest soils are stronger C sinks than previously recognized.