Above- and Belowground Net Primary Production in a Temperate Mixed Deciduous Forest

Wet events increase tree growth recovery after different drought intensities

Understanding the dynamics of tree recovery after drought is critical for predicting the state of tree growth in the context of future climate change. While there has been a great deal of researches showing that drought events can cause numerous significant negative effects on tree growth, the positive effects of post-drought wetting events on tree growth remain unclear. Therefore, we analyzed the effect of wet and dry events on the radial growth of trees in Central Asia using data on the width of tree rings. The results showed that 1) Drought is the main limiting factor for radial growth of trees in Central Asia, and that as the intensity and sensitivity of drought increases, tree resistance decreases and recovery rises, and more frequent droughts reduce tree resistance. 2) Tree radial growth varied significantly with wet and dry conditions, with wet events before and after drought events significantly enhancing tree radial growth. 3) When drought is followed by a wetting event, the relationship between tree resistance and recovery is closer to the "line of full resilience", with a significant increase in recovery, and compensatory growth is more likely to occur. Thus, wetting events have a significant positive effect on tree radial growth and are a key factor in rapid tree growth recovery after drought.

Adaptation mechanisms of leaf vein traits to drought in grassland plants

Leaf veins play an important role in water transport, and are closely associated with photosynthesis and transpiration. Resource heterogeneity in the environment, particularly in water resources, causes changes in leaf vein structure and function, thereby affecting plant growth and community assemblages. Therefore, it is necessary to explore the spatial variation and evolutionary mechanisms of leaf veins in natural communities. Natural communities are composed of dominant and non-dominant species. However, few studies to date have explored the trait variation of dominant and non-dominant species on a large scale. In this study, we set up 10 sampling sites along the water gradient (from east to west) in the Loess Plateau of China, and measured and calculated the vein density (vein length per unit area, VLA), vein diameter (VD), and vein volume ratio (VVR) of 173 species, including dominant and non-dominant species. The mean values of VLA, VD, and VVR were 10.95 mm mm-2, 22.24 μm, and 3%, respectively. VD and VVR of the dominant species were significantly higher than those of the non-dominant species. Unexpectedly, there was no significant change in the VLA with the water gradient, although the VD increased with drought. Leaf vein traits did not change significantly with evolution. There was a significant trade-off between VLA and VD. Our findings demonstrate that the response of veins to environmental changes is dependent on the degree of drought and provide new insights for further large-scale studies.

Litter quality and decomposition responses to drought in a northeastern US deciduous forest

Even though drought impacts on tree physiology have been identified, whether drought affects leaf litter chemistry that, in turn, influences litter decay rates is still poorly understood. We compared litter quality and decomposition for two cohorts of leaves from five co-occurring seasonally deciduous tree species: Acer saccharum, Tilia americana, Quercus rubra, Quercus alba, and Ostrya virginiana. One cohort experienced a growing-season drought, and the other cohort came from the same trees in the ensuing, post-drought growing season. Leaf litter production was greater for drought litter than post-drought litter for all five species. Specific leaf area and nitrogen concentrations were 20% greater for the drought cohort than the post-drought cohort. Concentrations of non-structural carbohydrates were about 14% greater for the drought cohort, except for greater values for post-drought A. saccharum litter. Pectin in the middle lamella of leaf litter was 31% lower for the drought cohort compared to post-drought cohort. We found few differences in litter decay rates between drought and post-drought cohorts, although Q. rubra litter had more decomposition for the post-drought cohort than the drought cohort, whereas A. saccharum litter had more decomposition for the drought cohort than the post-drought cohort. Leaf litter decay rates for the drought cohort were related to litter nitrogen and lignin concentrations, whereas decay rates for the post-drought cohort were related to litter carbohydrate concentrations. Our findings suggest that the role of drought events on seasonally deciduous forest ecosystems must recognize species-specific, idiosyncratic responses in leaf litter quality and decomposition.

The Contribution of Roots, Mycorrhizal Hyphae, and Soil Free-Living Microbes to Soil Respiration and Its Temperature Sensitivity in a Larch Forest

Soil respiration plays a critical role in driving soil carbon (C) cycling in terrestrial forest ecosystems. However, evidence to demonstrate the response of roots, mycorrhizal hyphae, and soil free-living microbes of soil respiration and their temperature sensitivity (Q10) remains lacking. Here, we used a root exclusion method to assess the contribution and response of root respiration (Rroot), mycorrhizal respiration (Rmyc), and (soil organic matter) SOM respiration (Rsom) to soil temperature in a larch forest. During the growing period, the contributions of Rroot, Rmyc, and Rsom to soil respiration were 42%, 6%, and 52%, respectively. The respiration rates of all components increased exponentially with increasing temperature. Based on these constitutive respiration rates with soil temperature, the Q10 values for Rroot, Rmyc, and Rsom were 3.84, 5.18, and 1.86, respectively. The results showed that the response to temperature change was different among roots, mycorrhizal hyphae, and microbes in the soil, while the temperature sensitivity of autotrophic respiration was higher than that of heterotrophic respiration. Importantly, the Rmyc at this site was extremely sensitive to temperature, although its overall emission was small. Mycorrhizal associations were identified as the key drivers of soil respiration and temperature sensitivity. A good understanding of the different soil CO2 efflux components will provide useful information for determining soil C fluxes and predicting soil C dynamics under changing environments.

Litterfall Hg deposition to an oak forest soil from southwestern Europe

Litterfall constitutes one of the main vectors for mercury (Hg) transfer to forested ecosystems, so we studied the deposition of Hg through senescent vegetation (oak leaves, twigs and miscellaneous) in a deciduous forest plot of Southwest Europe dominated by Quercus robur in 2015 and 2016. Total Hg concentrations increased in the following order: bole wood (1.4 μg kg^-1) < bark (8.3 μg kg^-1) < twigs (12.2 μg kg^-1) < miscellaneous (36.0 μg kg^-1) < oak leaves (39.3 μg kg^-1) < mineral soil (42.4 μg kg^-1) < Oi horizons (48.7 μg kg^-1) < Oe + Oa horizons (71.6 μg kg^-1). Mercury accumulation rates in oak leaves during the growing season were 0.15-0.18 μg kg^-1 day^-1. Mercury deposition fluxes were 26 and 21 μg m^-2 yr^-1 for 2015 and 2016, respectively, with oak leaves being the fraction that contributed the most. Mercury determination in litterfall sorted biomass fractions lead to a more accurate estimation of the total annual Hg deposition fluxes through litterfall. Higher Hg content was obtained for organic horizons (average of 60.2 μg kg^-1) than for mineral soil (mean of 42.4 μg kg^-1), but the soil Hg pool was higher in the latter. The results confirmed the necessity of taking into account the Hg pool in the deeper mineral soil layers as they accumulate substantial quantities of Hg associated to organic C and Al compounds, preventing its mobilization to other compartments of the terrestrial ecosystems.

Global patterns and climatic controls of belowground net carbon fixation

Carbon allocated underground through belowground net primary production represents the main input to soil organic carbon. This is of significant importance, because soil organic carbon is the third-largest carbon stock after oceanic and geological pools. However, drivers and controls of belowground productivity and the fraction of total carbon fixation allocated belowground remain uncertain. Here we estimate global belowground net primary productivity as the difference between satellite-based total net primary productivity and field observations of aboveground net primary production and assess climatic controls among biomes. On average, belowground carbon productivity is estimated as 24.7 Pg y^-1, accounting for 46% of total terrestrial carbon fixation. Across biomes, belowground productivity increases with mean annual precipitation, although the rate of increase diminishes with increasing precipitation. The fraction of total net productivity allocated belowground exceeds 50% in a large fraction of terrestrial ecosystems and decreases from arid to humid ecosystems. This work adds to our understanding of the belowground carbon productivity response to climate change and provides a comprehensive global quantification of root/belowground productivity that will aid the budgeting and modeling of the global carbon cycle.

Coarse particulate organic matter dynamics in ephemeral tributaries of a Central Appalachian stream network

Headwater ephemeral tributaries are interfaces between uplands and downstream waters. Terrestrial coarse particulate organic matter (CPOM) is important in fueling aquatic ecosystems; however, the extent to which ephemeral tributaries are functionally connected to downstream waters through fluvial transport of CPOM has been little studied. Hydrology and deposition of leaf and wood, and surrogate transport (Ginkgo biloba leaves and wood dowels) were measured over month-long intervals through the winter and spring seasons (6 months) in 10 ephemeral tributaries (1.3–5.4 ha) in eastern Kentucky. Leaf deposition and surrogate transport varied over time, reflecting the seasonality of litterfall and runoff. Leaf deposition was higher in December than February and May but did not differ from January, March, and April. Mean percent of surrogate leaf transport from the ephemeral tributaries was highest in April (3.6% per day) and lowest in February (2.5%) and May (2%). Wood deposition and transport had similar patterns. No CPOM measures were related to flow frequency. Ephemeral tributaries were estimated to annually contribute 110.6 kg AFDM·km⁻¹·yr⁻¹ of leaves to the downstream mainstem. Ephemeral tributaries are functionally connected to downstream waters through CPOM storage and subsequent release that is timed when CPOM is often limited in downstream waters.

Drought negates growth stimulation due to root herbivory in pasture grasses

Predicted increases in extreme weather are likely to alter the interactions between organisms within ecosystems. Whilst many studies have investigated the impacts of climate change on aboveground plant-insect interactions, those belowground remain relatively unexplored. Root herbivores can be the dominant taxa in grasslands, potentially altering plant community dynamics. To better predict the impact of climate change on grasslands, we subjected four Australian pasture grasses (Cynodon dactylon, Paspalum dilatatum, Microlaena stipoides and Lolium perenne) to contrasting rainfall regimes [a press drought (i.e. sustained, moderate water stress), a pulse drought (water stress followed by periodic, infrequent deluge event) and a well-watered control], with and without root herbivores; a manual root cutting treatment was also included for comparison. Plant growth, rooting strategy, phenology and biochemistry were measured to evaluate above and belowground treatment responses. Watering treatments had a larger effect on plant productivity than root damage treatments: press drought and pulse drought treatments reduced biomass by 58% and 47%, respectively. Root herbivore damage effects were species dependent and were not always equivalent to root cutting. The combination of pulse drought and root herbivory resulted in increased root:shoot ratios for both P. dilatatum and L. perenne, as well as decreased biomass and delayed flowering time for P. dilatatum. Plant biomass responses to root damage were greatest under well-watered conditions; however, root damage also delayed or prevented investment in reproduction in at least one species. Our findings highlight the important role of soil-dwelling invertebrates for forecasting growth responses of grassland communities to future rainfall regime changes.

Atmospheric mercury deposition to forests in the eastern USA

Atmospheric mercury (Hg) deposition to forests is important because half of the land cover in the eastern USA is forest. Mercury was measured in autumn litterfall and weekly precipitation samples at a total of 27 National Atmospheric Deposition Program (NADP) monitoring sites in deciduous and mixed deciduous-coniferous forests in 16 states in the eastern USA during 2007-2014. These simultaneous, uniform, repeated, annual measurements of forest Hg include the broadest area and longest time frame to date. The autumn litterfall-Hg concentrations and litterfall mass at the study sites each year were combined with annual precipitation-Hg data. Rates of litterfall-Hg deposition were higher than or equal to precipitation-Hg deposition rates in 70% of the annual data, which indicates a substantial contribution from litterfall to total atmospheric-Hg deposition. Annual litterfall-Hg deposition in this study had a median of 11.7 μg per square meter per year (μg/m²/yr) and ranged from 2.2 to 23.4 μg/m²/yr. It closely matched modeled dry-Hg deposition, based on land cover at selected NADP Hg-monitoring sites. Mean annual atmospheric-Hg deposition at forest study sites exhibited a spatial pattern partly explained by statistical differences among five forest-cover types and related to the mapped density of Hg emissions. Forest canopies apparently recorded changes in atmospheric-Hg concentrations over time because litterfall-Hg concentrations decreased year to year and litterfall-Hg concentrations were significantly higher in 2007-2009 than in 2012-2014. These findings reinforce reported decreases in Hg emissions and atmospheric elemental-Hg concentrations during this same time period. Methylmercury (MeHg) was detected in all litterfall samples at all sites, compared with MeHg detections in less than half the precipitation samples at selected sites during the study. These results indicate MeHg in litterfall is a pathway into the terrestrial food web where it can accumulate in the prey of songbirds, bats, and raptors.

Litterfall production and fine root dynamics in cool-temperate forests

Current understanding of litterfall and fine root dynamics in temperate forests is limited, even though these are the major contributors to carbon and nutrient cycling in the ecosystems. In this study, we investigated litterfall and fine root biomass and production in five deciduous and four coniferous forests at the Gwangneung Experimental Forest in Korea. We used ingrowth cores to measure fine root production and root turnover rate. The litterfall was separated into leaves, twigs, and others, and then leaves were further separated according to species. Annual litterfall mass was not significantly different between the years, 360 to 651 g m^-2 in 2011 and 300 to 656 g m^-2 in 2012. Annual fine root (<5 mm) production was significantly higher in 2012 (421 to 1342 g m^-2) than in 2011 (99 to 872 g m^-2). Annual litterfall mass was significantly different among the stands, while fine root production did not statistically differ among the stands. The average fine root turnover rate, calculated by dividing the annual fine root production by the maximum standing fine root biomass, was 1.65 for deciduous forests and 1.97 for coniferous forests. Fine root production constituted 18–44% of NPP, where NPP was the sum of woody biomass production, litterfall production, and fine root production. Belowground production was a greater fraction of NPP in more productive forests suggesting their greater carbon allocation belowground.

Contrasting above- and belowground sensitivity of three Great Plains grasslands to altered rainfall regimes

Intensification of the global hydrological cycle with atmospheric warming is expected to increase interannual variation in precipitation amount and the frequency of extreme precipitation events. Although studies in grasslands have shown sensitivity of aboveground net primary productivity (ANPP) to both precipitation amount and event size, we lack equivalent knowledge for responses of belowground net primary productivity (BNPP) and NPP. We conducted a 2-year experiment in three US Great Plains grasslands--the C4-dominated shortgrass prairie (SGP; low ANPP) and tallgrass prairie (TGP; high ANPP), and the C3-dominated northern mixed grass prairie (NMP; intermediate ANPP)--to test three predictions: (i) both ANPP and BNPP responses to increased precipitation amount would vary inversely with mean annual precipitation (MAP) and site productivity; (ii) increased numbers of extreme rainfall events during high-rainfall years would affect high and low MAP sites differently; and (iii) responses belowground would mirror those aboveground. We increased growing season precipitation by as much as 50% by augmenting natural rainfall via (i) many (11-13) small or (ii) fewer (3-5) large watering events, with the latter coinciding with naturally occurring large storms. Both ANPP and BNPP increased with water addition in the two C4 grasslands, with greater ANPP sensitivity in TGP, but greater BNPP and NPP sensitivity in SGP. ANPP and BNPP did not respond to any rainfall manipulations in the C3 -dominated NMP. Consistent with previous studies, fewer larger (extreme) rainfall events increased ANPP relative to many small events in SGP, but event size had no effect in TGP. Neither system responded consistently above- and belowground to event size; consequently, total NPP was insensitive to event size. The diversity of responses observed in these three grassland types underscores the challenge of predicting responses relevant to C cycling to forecast changes in precipitation regimes even within relatively homogeneous biomes such as grasslands.

Factors controlling carbon distribution on reforested minelands and regenerating clearcuts in Appalachia, USA

Increasing carbon (C) storage in soils of degraded lands, such as surface coal mines, is of interest because of its potential role in mitigating increases in atmospheric CO2. While it has been shown that reforesting degraded lands can significantly increase C storage in soils, there are limited studies addressing what processes control soil C in these systems. A study was initiated with the following objectives: 1) quantify the amount of soil C accumulating on reforested mine lands; and 2) examine several biological processes that govern the amount of C sequestered into soil (decomposition, soil respiration and microbial dynamics). A chronosequence approach was used to examine C changes with time in reforested mine lands (years 1, 3, and 8) and unmined regenerating clear-cuts (years 4, 12 and 20). From a C perspective, our results indicated that the young reforested mines (ages 1 and 3) differed significantly from the older mines (age 8) and all regenerating clear-cuts for all parameters examined. However, after 8 years litterfall, microbial biomass C and nitrogen (N), microbial activity, litter decomposition and CO2 efflux were similar on the mine as that found on the 12-year-old naturally regenerating clear-cut. Although soil organic C (SOC) content was lower on the reforested mines than the regenerating forests, rates of SOC accumulation were greater on the mine sites, likely because the young mine lands were initially devoid of SOC and conditions were suitable for rapid sequestration.

The effect of limited availability of N or water on C allocation to fine roots and annual fine root turnover in Alnus incana and Salix viminalis

The effect of limited nitrogen (N) or water availability on fine root growth and turnover was examined in two deciduous species, Alnus incana L. and Salix viminalis L., grown under three different regimes: (i) supply of N and water in amounts which would not hamper growth, (ii) limited N supply and (iii) limited water supply. Plants were grown outdoors during three seasons in covered and buried lysimeters placed in a stand structure and filled with quartz sand. Computer-controlled irrigation and fertilization were supplied through drip tubes. Production and turnover of fine roots were estimated by combining minirhizotron observations and core sampling, or by sequential core sampling. Annual turnover rates of fine roots <1 mm (5-6 year(-1)) and 1-2 mm (0.9-2.8 year(-1)) were not affected by changes in N or water availability. Fine root production (<1 mm) differed between Alnus and Salix, and between treatments in Salix; i.e., absolute length and biomass production increased in the order: water limited < unlimited < N limited. Few treatment effects were detected for fine roots 1-2 mm. Proportionally more C was allocated to fine roots (≤2 mm) in N or water-limited Salix; 2.7 and 2.3 times the allocation to fine roots in the unlimited regime, respectively. Estimated input to soil organic carbon increased by ca. 20% at N limitation in Salix. However, future studies on fine root decomposition under various environmental conditions are required. Fine root growth responses to N or water limitation were less pronounced in Alnus, thus indicating species differences caused by N-fixing capacity and slower initial growth in Alnus, or higher fine root plasticity in Salix. A similar seasonal growth pattern across species and treatments suggested the influence of outer stimuli, such as temperature and light.

Litterfall mercury dry deposition in the eastern USA

Mercury (Hg) in autumn litterfall from predominately deciduous forests was measured in 3 years of samples from 23 Mercury Deposition Network sites in 15 states across the eastern USA. Annual litterfall Hg dry deposition was significantly higher (median 12.3 micrograms per square meter (μg/m(2)), range 3.5-23.4 μg/m(2)) than annual Hg wet deposition (median 9.6 μg/m(2), range 4.4-19.7 μg/m(2)). The mean ratio of dry to wet Hg deposition was 1.3-1. The sum of dry and wet Hg deposition averaged 21 μg/m(2) per year and 55% was litterfall dry deposition. Methylmercury was a median 0.8% of Hg in litterfall and ranged from 0.6 to 1.5%. Annual litterfall Hg and wet Hg deposition rates differed significantly and were weakly correlated. Litterfall Hg dry deposition differed among forest-cover types. This study demonstrated how annual litterfall Hg dry deposition rates approximate the lower bound of annual Hg dry fluxes.

Productivity of ephemeral headwater riparian forests impacted by sedimentation in the southeastern United States coastal plain

Riparian forests serve an essential function in improving water quality through the filtering of sediments and nutrients from surface runoff. However, little is known about the impact of sediment deposition on productivity of riparian forests. Sediment inputs may act as a subsidy to forest productivity by providing additional nutrients for plant uptake or may act as a stress by creating anoxic soil conditions. This study determined how sediment deposition affected riparian forests along ephemeral headwater streams at Ft. Benning, Georgia, USA. Above- and belowground productivity, leaf-area index (LAI), and standing crop biomass for fine roots, shrubs, and trees were compared along a gradient of present sedimentation rates in 17 riparian forests. Annual litterfall production was determined from monthly collections using 0.25- m(2) traps; woody biomass was determined from annual diameter at breast height (DBH) measurements using species-specific allometric equations; fine root productivity was determined using sequential coring; LAI was measured by expanding specific leaf area by annual litterfall production; and shrub biomass was determined using species-specific biomass equations based on height and root collar diameter. Significant declines in litterfall, woody biomass production, fine root production, LAI, and shrub biomass were found with as little as 0.1 to 0.4 cm yr(-2) sedimentation. We conclude that the levels of sedimentation in this study do not subsidize growth in ephemeral headwater riparian forests but instead create a stress similar to that found under flooded conditions.

Effects of seasonality, litter removal and dry-season irrigation on litterfall quantity and quality in eastern Amazonian forest regrowth, Brazil

Litterfall quantity and quality may respond to alterations in resource availability expected with ongoing land-use and climate changes. Here, we quantify the effects of altered resource availability on non-woody litterfall quantity and quality (nitrogen and phosphorus concentrations) in eastern Amazonian forest regrowth (Brazil) through two multi-year experimental manipulations: (1) daily irrigation (5 mm d−1) during the dry season; and (2) fortnightly litter removal. Consistent with other tropical forest data litterfall exhibited seasonal patterns, increasing with the onset of the dry season and declining with the onset of the rainy season. Irrigation did not affect litterfall mass and had little impact on nitrogen (N) or phosphorus (P) concentrations and return, except for decreasing litter P concentration at the end of two irrigation periods. Litter removal did not alter litterfall mass or P concentration, but progressively reduced litterfall N during the course of the experiment. Overall, these results suggest significant resistance to altered resource availability within the bounds of our experimental treatments; our findings may help to constrain carbon and nutrient cycling predictions for tropical forests in response to land-use and climate changes.