Comparison of leaf gas exchange and stable isotope signature of water-soluble compounds along canopy gradients of co-occurring Douglas-fir and European beech

Combined δ(13) C and δ(18) O analyses of water-soluble leaf and twig phloem material were used to determine intrinsic water-use efficiency (iWUE) and variability of stomatal conductance at different crown positions in adult European beech (Fagus sylvatica) and Douglas-fir (Pseudotsuga menziesii) trees. Simultaneous gas exchange measurements allowed evaluation of the differences in calculating iWUE from leaf or phloem water-soluble compounds, and comparison with a semi-quantitative dual isotope model to infer variability of net photosynthesis (A(n) ) between the investigated crown positions. Estimates of iWUE from δ(13) C of leaf water-soluble organic matter (WSOM) outperformed the estimates from phloem compounds. In the beech crown, δ(13) C of leaf WSOM coincided clearly with gas exchange measurements. The relationship was not as reliable in the Douglas-fir. The differences in δ(18) O between leaf and phloem material were found to correlate with stomatal conductance. The semi-quantitative model approach was applicable for comparisons of daily average A(n) between different crown positions and trees. Intracanopy gradients were more pronounced in the beech than in the Douglas-fir, which reached higher values of iWUE at the respective positions, particularly under dry air conditions.     

Leaf δ18O of remaining trees is affected by thinning intensity in a semiarid pine forest

Silvicultural thinning usually improves the water status of remaining trees in water‐limited forests. We evaluated the usefulness of a dual stable isotope approach (δ¹³C, δ¹⁸O) for comparing the physiological performance of remaining trees between forest stands subjected to two different thinning intensities (moderate versus heavy) in a 60‐year‐old Pinus halepensis Mill. plantation in semiarid southeastern Spain. We measured bulk leaf δ¹³C and δ¹⁸O, foliar elemental concentrations, stem water content, stem water δ¹⁸O (δ¹⁸O_{stem water}), tree ring widths and leaf gas exchange rates to assess the influence of forest stand density on tree performance. Remaining trees in low‐density stands (heavily thinned) showed lower leaf δ¹⁸O, and higher stomatal conductance (g_s), photosynthetic rate and radial growth than those in moderate‐density stands (moderately thinned). By contrast, leaf δ¹³C, intrinsic water‐use efficiency, foliar elemental concentrations and δ¹⁸O_{stem water} were unaffected by stand density. Lower foliar δ¹⁸O in heavily thinned stands reflected higher g_s of remaining trees due to decreased inter‐tree competition for water, whereas higher photosynthetic rate was largely attributable to reduced stomatal limitation to CO₂ uptake. The dual isotope approach provided insight into the early (12 months) effects of stand density manipulation on the physiological performance of remaining trees.     

Technical note: some observations on the conversion of dental enamel δ18O(p) values to δ18O(w) to determine human mobility

It has become a widespread practice to convert δ(18)O(p) values measured in human and animal dental enamel to a corresponding value of δ(18)O(w) and compare these data with mapped δ(18)O(w) groundwater or meteoric water values to locate the region where the owner of the tooth lived during the formation of the enamel. Because this is a regression procedure, the errors associated with the predicted δ(18)O(w) values will depend critically on the correlation between the comparative data used to perform the regression. By comparing four widely used regression equations we demonstrate that the smallest 95% error is likely to be greater than ±1% in δ(18)O(w) , and could be as large as ±3.5%. These values are significantly higher than those quoted in some of the recent literature, and measurements with errors at the higher end of this range would render many of the published geographical attributions statistically unsupportable. We suggest that the simplest solution to this situation is to make geographical attributions based on the direct comparison of measured values of δ(18)O(p) rather than on predicted values of δ(18)O(w).     

Ecophysiological responses of native invasive woody <Emphasis Type="Italic">Juniperus virginiana</Emphasis> L. to resource availability and stand characteristics in the semiarid grasslands of the Nebraska Sandhills

Vegetation in grasslands is changing at an unprecedented rate. In the Nebraska Sandhills, this shift is attributed in part to encroachment of the woody species Juniperus virginiana. We investigated changes in resource availability and their feedback on seasonal trends in photosynthetic characteristics of J. virginiana trees scattered in open grasslands vs. a dense 57-year-old stand. Dense stand exhibited lower volumetric soil water content, NH₄ ⁺, NO₃ ^–, and δ¹³C, as well as foliage δ¹³C, δ¹⁵N, and N content, compared to grasslands. Water potential was higher in trees in grasslands compared to dense stand. J. virginiana in dense stand exhibited similar trends to trees in grasslands for net photosynthetic rate (P _N), stomatal conductance, transpiration, maximum photochemical efficiency of PSII, maximum carboxylation velocity, and maximum rate of electron transport. P _N peaked early summer and declined in the fall, with trees in open grasslands lagging behind those in dense stand. Plasticity of this species may place it at a competitive advantage in the Sandhills, further altering grasslands vegetation and ecosystem processes.     

Relevance of using whole-ring stable isotopes of black spruce trees in the perspective of climate reconstruction

Studies in dendroisotope chemistry suggested that latewood cellulose contains better climatic records than whole-ring cellulose. However, this approach has never been tested on northeastern Canadian spruce trees. This study compares dendroisotopic series of cellulose from late and whole ring, and analyses their statistical relationships with hydro-climatic variables with the aim of selecting the best suited protocol for future hydro-climatic reconstruction in the downstream sector of Churchill River basin of Labrador, Canada. To this end, δ¹³C and δ¹⁸O series from latewood (LW) and whole ring (WR) α-cellulose of black spruce trees (Picea mariana [Mill.] B.S.P.) were produced for the 1940–2010 period. The results show strong correlations between LW and WR isotopic series suggesting that there are no important variation in the isotopic ratios during the growing year and that black spruce trees use photosynthates of the current growing season to form their earlywood. Moreover, LW and WR δ¹³C and δ¹⁸O show similar relationships with both maximum temperature (T_max) and Churchill River discharge. Correlations are higher when combining δ¹³C and δ¹⁸O for LW and WR. Overall, those correlations support the indirect relationship between tree-ring isotopic series and river discharge, as they are integrators of several climatic variables and derived parameters (T_max, relative humidity, evapotranspiration, etc.). The LW and WR isotopic series give similar statistical relationships with hydro-climatic variables, and the WR treatment is faster (separation easier compared to LW). Thus, for black spruce the use of combined isotopic series in WR can be favored over LW for hydro-climatic reconstruction in the study region.     

The relationship between rainfall,water source and growth for an endangered tree

Abstract It is now reasonably well understood that the human impact on the environment since industrialization has led to significant changes in climate. Here we attempt to develop a predictive understanding of the effects that future changes in climate may have on vegetation structure and species diversity. We do this through a determination of the relationship between radial growth and water source for Widdringtonia cedarbergensis Marsh. Our results show that there was no significant relationship between monthly radial growth, as determined using dendrometer bands, and rainfall. There is, however, a significant relationship between the δ¹⁸O composition of the water extracted from the trees and the rain δ¹⁸O values. We speculate that W. cedarbergensis exploits water derived from rain that flows off the rocky substrate of the study area into sumps between the bedding planes of the rocks on which they grow. This runoff occurs rapidly during rain events resulting in δ¹⁸O values for the trees sourcing this water not to be significantly different from that of the rain. Rainfall therefore has to be sufficient to refill these sumps on which the trees are dependent. The dendrometer bands reflect a slow but steady growth of the trees at the study site. While this growth is not dependent on rainfall, it is dependent on reliable access to available water. If climate change predictions for the region are realized and rainfall is reduced then this species will be affected. W. cedarbergensis is endemic to only a very small area within the Cedarberg Mountains in South Africa and is also one of the few trees growing in the fynbos. The extinction of this species in the wild will fundamentally affect both the vegetation structure and species composition of the region.     

Unthinned slow-growing ponderosa pine (Pinus ponderosa) trees contain muted isotopic signals in tree rings as compared to thinned trees

Key message

The muted wood isotopic signal in slow-growing trees of unthinned stands indicates lower responsiveness to changing environmental conditions compared to fast-growing trees in thinned stands.

Abstract

To examine the physiological processes associated with higher growth rates after thinning, we analyzed the oxygen isotopic values in wood (δ¹⁸O_w) of 12 ponderosa pine (Pinus ponderosa) trees from control, moderately, and heavily thinned stands and compared them with wood-based estimates of carbon isotope discrimination (?¹³C), basal area increment (BAI), and gas exchange. We found that (heavy) thinning led to shifts and increased inter-annual variability of both stable carbon and oxygen isotope ratios relative to the control throughout the first post-thinning decade. Results of a sensitivity analysis suggested that both an increase in stomatal conductance (g _s) and differences in source water among treatments are equally probable causes of the δ¹⁸O_w shift in heavily thinned stands. We modeled inter-annual changes in δ¹⁸O_w of trees from all treatments using environmental and physiological data and found that the significant increase in δ¹⁸O_w inter-annual variance was related to greater δ¹⁸O_w responsiveness to changing environmental conditions for trees in thinned stands when compared to control stands. Based on model results, the more muted climatic response of wood isotopes in slow-growing control trees is likely to be the consequence of reduced carbon sink strength causing a higher degree of mixing of previously stored and fresh assimilates when compared to faster-growing trees in thinned stands. Alternatively, the muted response of δ¹⁸O_w to climatic variation of trees in the control stand may result from little variation in the control stand in physiological processes (photosynthesis, transpiration) that are known to affect δ¹⁸O_w.     

Population life-cycle and stand structure in dense and open stands of the introduced tall herb <Emphasis Type="Italic">Heracleum mantegazzianum</Emphasis>

Populations of the introduced Heracleum mantegazzianum consist of dense central stands, which gradually give way to open stands towards the margins. To analyse whether open stands are due to unsuitable conditions or represent the invading front for further spread, we studied life-cycle, population dynamics, stand structure and soil conditions of open and dense stands over two transition periods. Populations decreased during the first interval but increased after the extremely dry and warm summer of 2003 during the second interval. Open stands had shorter generation times, lower height, smaller proportions of small individuals and were less in equilibrium with the environment than dense stands. In open stands, growth to higher stages was most important, while in dense stands delayed development (self-loops) had a strong effect on population growth; stasis and fecundity contributed most to the difference in λ between stand types. By petiole extension H. mantegazzianum may raise its leaves just above the resident vegetation. Therefore, younger stages develop faster in open stands, whereas strong competition by conspecific adults leads to longer generation times and a higher proportion of small individuals in dense stands. Disturbance due to extreme climatic conditions in summer 2003 equalised population dynamics of both stand types. Life-cycle variation between stand types makes it difficult to infer simple management rules. However, our data suggest that small and/or open stands of H. mantegazzianum may eventually serve as initials for further spread after land-use changes, whereas dense stands are stable and may represent sources of propagules.     

Changes in environmental controls on the growth of Abies alba Mill. in the Vosges Mountains, north-eastern France, during the 20th century

Aim  To analyse the radial growth of silver fir ( Abies alba Mill.) in north-eastern France in relation to soil, climate and interspecific competition factors and to check the temporal stability of their effects during the 20th century.
Location  The Vosges Mountains, located in north-eastern France.
Methods  Data were collected from 143 plots regularly distributed over three altitude ranges (from 330 to 1100 m), three humus forms (moder to mull) and seven types of stands, including a pure stand of silver fir and six mixed stands containing this species. For each plot, an index of mean radial growth of silver fir was calculated, independent of tree age and calendar year of ring formation. Relationships between radial growth and ecological factors were analysed using linear mixed-effects models.
Results  Radial growth of silver fir is low in the presence of Picea abies or of a high density of other dominant trees . Low aluminium concentration and good nitrogen supply, measured by the soil C:N ratio, are correlated with high radial growth. Water reserves, related to available water storage capacity and to lateral run-off, are also correlated with high radial growth. Analysis of environmental effects during the 20th century showed that growth was strongly correlated with nitrogen supply at the start of the century, and with aluminium toxicity and climate at the end of the century. The radial growth of trees located on sites with low nitrogen supply was lower before 1970 than that of trees located on nitrogen-rich soils. At the end of the century, radial growth of silver fir was the same for all levels of nitrogen nutrition.
Main conclusions  Our results indicate that nitrogen supply was a limiting factor for the mean radial growth of silver fir before the 1970s. Eutrophication during the 20th century may then have eliminated limitation by nitrogen.     

Does resin tapping affect the tree-ring growth and climate sensitivity of the Chinese pine (Pinus tabuliformis) in the Loess Plateau,China?

Resin tapping could affect water and nutrient transport processes in Chinese pine trees, rendering them more vulnerable to extreme climatic events, such as drought, and affecting the ecological function of forests in semi-arid regions. This study evaluated how resin tapping affects the tree-ring growth and climate sensitivity of Chinese pine in the Loess Plateau. We compared tree-ring growth patterns between the tapped and untapped faces of tapped trees, and investigated tree-ring growth and its response to climate between tapped and untapped trees in a forest stand during the 1967–2017 period. Tapped trees showed asymmetrical growth patterns after resin tapping, with narrower rings near the tapped face and wider ones near the untapped face. Furthermore, tapped trees had inter-annual variations consistent with those of untapped trees except for the years 2000 and 2001, with significantly lower values following resin tapping, and tree-ring growth then returning to normal. The climate response analysis indicated that the tree-ring growth of both tapped and untapped trees was negatively affected by monthly mean temperatures during the early growing season (May to July) in the post-resin-tapping period. Furthermore, tree-ring growth in tapped trees also revealed significant correlation with water vapour deficit and the Palmer drought index, which indicates that tapped trees are more vulnerable to drought. Further studies based on stable isotopes (i.e. δ¹³C, δ¹⁸O, and δ¹⁵N) could improve our understanding of the physiological mechanisms that regulate the effects of resin tapping on tree-ring growth.     

Isotopes reveal contrasting water use strategies among coexisting plant species in a Mediterranean ecosystem

Variation in the stable carbon and oxygen isotope composition (δ(13) C, Δ(18) O) of co-occurring plant species may reflect the functional diversity of water use strategies present in natural plant communities. We investigated the patterns of water use among 10 coexisting plant species representing diverse taxonomic groups and life forms in semiarid southeast Spain by measuring their leaf δ(13) C and Δ(18) O, the oxygen isotope ratio of stem water and leaf gas exchange rates. Across species, Δ(18) O was tightly negatively correlated with stomatal conductance (g(s) ), whereas δ(13) C was positively correlated with intrinsic water use efficiency (WUE(i) ). Broad interspecific variation in Δ(18) O, δ(13) C and WUE(i) was largely determined by differences in g(s) , as indicated by a strong positive correlation between leaf δ(13) C and Δ(18) O across species The 10 co-occurring species segregated along a continuous ecophysiological gradient defined by their leaf δ(13) C and Δ(18) O, thus revealing a wide spectrum of stomatal regulation intensity and contrasting water use strategies ranging from 'profligate/opportunistic' (high g(s) , low WUE(i) ) to 'conservative' (low g(s) , high WUE(i) ). Coexisting species maintained their relative isotopic rankings in 2?yr with contrasting rainfall, suggesting the existence of species-specific 'isotopic niches' that reflect ecophysiological niche segregation in dryland plant communities.     

Carbon (δ13C) and nitrogen (δ15N) stable isotope composition in plant and soil in Southern Patagonia's native forests

Stable isotope natural abundance measurements integrate across several biogeochemical processes in ecosystem N and C dynamics. Here, we report trends in natural isotope abundance (δ¹³C and δ¹⁵N in plant and soil) along a climosequence of 33 Nothofagus forest stands located within Patagonia, Southern Argentina. We measured 28 different abiotic variables (both climatic variables and soil properties) to characterize environmental conditions at each of the 33 sites. Foliar δ¹³C values ranged from ?35.4‰ to ?27.7‰, and correlated positively with foliar δ¹⁵N values, ranging from ?3.7‰ to 5.2‰. Soil δ¹³C and δ¹⁵N values reflected the isotopic trends of the foliar tissues and ranged from ?29.8‰ to ?25.3‰, and ?4.8‰ to 6.4‰, respectively, with no significant differences between Nothofagus species (Nothofagus pumilio, Nothofagus antarctica, Nothofagus betuloides). Principal component analysis and multiple regressions suggested that mainly water availability variables (mean annual precipitation), but not soil properties, explained between 42% and 79% of the variations in foliar and soil δ¹³C and δ¹⁵N natural abundance, which declined with increased moisture supply. We conclude that a decline in water use efficiency at wetter sites promotes both the depletion of heavy C and N isotopes in soil and plant biomass. Soil δ¹³C values were higher than those of the plant tissues and this difference increased as annual precipitation increased. No such differences were apparent when δ¹⁵N values in soil and plant were compared, which indicates that climatic differences contributed more to the overall C balance than to the overall N balance in these forest ecosystems.     

Stand basal area and solar radiation amplify white spruce climate sensitivity in interior Alaska: Evidence from carbon isotopes and tree rings

The negative growth response of North American boreal forest trees to warm summers is well documented and the constraint of competition on tree growth widely reported, but the potential interaction between climate and competition in the boreal forest is not well studied. Because competition may amplify or mute tree climate‐growth responses, understanding the role current forest structure plays in tree growth responses to climate is critical in assessing and managing future forest productivity in a warming climate. Using white spruce tree ring and carbon isotope data from a long‐term vegetation monitoring program in Denali National Park and Preserve, we investigated the hypotheses that (a) competition and site moisture characteristics mediate white spruce radial growth response to climate and (b) moisture limitation is the mechanism for reduced growth. We further examined the impact of large reproductive events (mast years) on white spruce radial growth and stomatal regulation. We found that competition and site moisture characteristics mediated white spruce climate‐growth response. The negative radial growth response to warm and dry early‐ to mid‐summer and dry late summer conditions intensified in high competition stands and in areas receiving high potential solar radiation. Discrimination against ¹³C was reduced in warm, dry summers and further diminished on south‐facing hillslopes and in high competition stands, but was unaffected by climate in open floodplain stands, supporting the hypothesis that competition for moisture limits growth. Finally, during mast years, we found a shift in current year's carbon resources from radial growth to reproduction, reduced ¹³C discrimination, and increased intrinsic water‐use efficiency. Our findings highlight the importance of temporally variable and confounded factors, such as forest structure and climate, on the observed climate‐growth response of white spruce. Thus, white spruce growth trends and productivity in a warming climate will likely depend on landscape position and current forest structure.     

Exploring climate forcing of growth depression in subfossil South Swedish bog pines using stable isotopes

Comparison between growth variability, based on ring-width (RW) analysis, and moisture-sensitive signals in tree-ring carbon and oxygen stable-isotope composition provides increased understanding of how climate and hydrology influenced bog pines (Pinus sylvestris L.) at two sites in southern Sweden during the mid- and late Holocene. Tree-ring sequences from two subfossil trees collected at raised bogs having different hydrology and catchment size were analyzed to probe the stable-isotope signals associated with two bog-wide episodes of growth depression, one during the Holocene Thermal Maximum and the other during the Neoglacial Transition. The occurrence of lower whole-wood δ¹³C and cellulose δ¹³C and δ¹⁸O values immediately prior to the onset of growth depression in both trees, suggesting increased atmospheric relative humidity, is consistent with the notion that excessive effective moisture impeded tree growth. Correlation analysis indicates that the growth response lagged about three years behind the decline in δ¹³C and δ¹⁸O values in each tree, possibly reflecting relatively slow rise in the local water table in response to wetter climate.     

Climatic impacts and drought control of radial growth and seasonal wood formation in Pinus halepensis

Short- and long-term growth responses to drought and climatic influences still remain poorly understood. In this study, we investigated the impact of climatic drivers (temperature, precipitation) and drought, using the Standardized Precipitation Index (SPI) calculated at different time scales (1–48?months), on earlywood (EW) and latewood (LW) widths in Pinus halepensis. Nine forests subjected to dry summer conditions were sampled in Mediterranean semi-arid areas from north-eastern Spain. In addition, we explored the seasonal dynamics of cambial activity and wood formation in relation to short-term climate variability. We found two peaks of tracheid cell production corresponding to EW (May–June) and LW (mid-July–August) growth phases, associated with a sharp decrease in enlarging cells in early July in response to low water availability. In the period of analysis (1970–2005), EW growth was positively correlated with precipitation in previous December and current January, April, May and June, while it was negatively correlated with temperature in June and July. LW was correlated positively with minimum temperatures in January. Probably this was an indirect relationship as a consequence of increased EW width at higher January temperatures. Drought affected more negatively EW than LW formation as evidenced the higher SPI-EW correlation (r?=?0.72) than the SPI-LW one (r?=?0.54). The strongest EW response to drought was observed in July, whereas the highest LW response to drought occurred in September; and this seasonal pattern matched the phases of lowest EW and LW tracheid production. Under a future reduction of winter and spring precipitation, the studied forests may show a decrease in tracheid cell production, causing a decline of radial growth, a reduction in hydraulic conductivity and, indirectly, a hampered carbon uptake in such semi-arid woodlands.     

Short-term dynamics of the carbon isotope composition of CO2 emitted from a wheat agroecosystem--physiological and environmental controls

Understanding environmental and physiological controls of the variations in δ(13) C of CO(2) respired (δ(13) C(R)) from different compartments of an ecosystem is important for separation of CO(2) fluxes and to assess coupling between assimilation and respiration. In a wheat field, over 3 days we characterised the temporal dynamics of δ(13) C(R) from shoots and roots, from the soil and from the whole agroecosystem. To evaluate the basis of potential variations in δ(13) C(R), we also measured δ(13) C in different organic matter pools, as well as meteorological and gas exchange parameters. We observed strong diel variations up to ca. 6% in shoot, root and soil δ(13) C(R), but not in δ(13) C of the putative organic substrates for respiration, which varied by not more than ca. 1% within 24 h. Whole ecosystem-respired CO(2) was least depleted in (13) C in the afternoon and most negative in the early morning. We assume that temporally variable respiratory carbon isotope fractionation and changes in fluxes through metabolic pathways, rather than photosynthetic carbon isotope fractionation, governs the δ(13) C of respired CO(2) at the diel scale, and thus provides insights into the metabolic processes related to respiration under field conditions.     

Can stable carbon isotopes (δ13C) in soil carbon be used to describe the dynamics of Eucalyptus savanna–rainforest boundaries in the Australian monsoon tropics?

The history of isolated patches of monsoon rainforest within large tracts of Eucalyptus savanna is poorly understood because of the scarcity of reliable palaeoecological records in the Australian monsoon tropics. Elsewhere in the world, the ratio of the stable isotopes ¹³C to ¹²C (δ¹³C) in soil organic matter has shed light on the dynamics of rainforest–savanna boundaries because tropical grasses with the C4 photosynthetic pathway have a distinct δ¹³C signature (–17 to –9‰) compared with that of woody plants with the C3 photosynthetic pathway (–32 to –22‰). In order to determine the magnitude of the variation in δ¹³C, unreplicated soil profiles were sampled beneath different vegetation types on three boundaries between Eucalyptus savanna and rainforest that were both growing on Tertiary age laterite parent material. Replicated (n = 3) soil profiles, which were also derived from Tertiary age laterite, were sampled from beneath: (i) dense stands of African grasses within a frequently burnt Eucalyptus savanna; and within the same long unburnt Eucalyptus savanna, (ii) patches of African and natives grasses and (iii) clumps of Acacia trees. The strongly negative δ¹³C values of soil organic matter derived from the frequently burnt and long unburnt grassy understoreys in the Eucalyptus savannas showed that a considerable amount of the soil carbon was derived from C3 (woody) species despite the presence of a ground layer dominated by C4 grasses. However, a feature of these data was the considerable variability among the three ‘replicate’ profiles. The surface soil samples from beneath three clumps of Acacia trees in the unburnt Eucalyptus savanna had much less variable δ¹³C values and were similar to two of the three monsoon rainforests sampled. The pattern of δ¹³C values from unreplicated soil profiles from different vegetation types across three rainforest boundaries was also very variable and not always obviously related the known disturbance history of the extant vegetation. Given the considerable variability within and between vegetation types with contrasting disturbance histories, it is concluded that the use of carbon stable isotopes to advance understanding of the dynamics of rainforest and Eucalyptus savanna boundaries will require further development, such as determination of the ¹⁴C age and δ¹³C values of different soil carbon fractions.     

The effect of stand age and microhabitat on soil seed banks in Mediterranean Aleppo pine forests after fire

Soil samples from three microhabitats (gaps, beneath shrubs and beneath trees) in five stands of various post-fire ages (6–55 years) were collected in an east Mediterranean Aleppo pine Pinus halepensis forest. Total germinable seed bank densities varied between 300 and 1300 seeds per m². Herbaceous taxa were the major constituents of the germinable seed bank in gaps, regardless of stand age. Perennials were the major components beneath shrubs in all stands except the youngest stand where herbaceous species were the major components in all microhabitats. Important tree and shrub species (e.g., Pinus halepensis, Quercus calliprinos, Pistacia lentiscus, Phillyrea latifolia) of the mature pine forest were not an important component of the soil seed bank and therefore, little resemblance was observed between the above-ground plant species composition and soil seed bank composition. This is consistent with the fact that these species regenerate by resprouting rather than by germination from the seed bank. Both microhabitats and forest-stands, which were of different ages, contributed to the variation in taxa richness, germinable seed density and diversity among samples. The effect of small-scale spatial heterogeneity (among microhabitats) was much more pronounced. In contrast to other studies, species richness, species diversity, and density of seed banks did not decrease with post-fire age. Moreover, stand age was a poor predictor for these attributes of the soil seed bank in an Aleppo pine forest. The heterogeneity plays an important role in conservation and management of this ecosystem.     

Drought timing and local climate determine the sensitivity of eastern temperate forests to drought

Projected changes in temperature and drought regime are likely to reduce carbon (C) storage in forests, thereby amplifying rates of climate change. While such reductions are often presumed to be greatest in semi‐arid forests that experience widespread tree mortality, the consequences of drought may also be important in temperate mesic forests of Eastern North America (ENA) if tree growth is significantly curtailed by drought. Investigations of the environmental conditions that determine drought sensitivity are critically needed to accurately predict ecosystem feedbacks to climate change. We matched site factors with the growth responses to drought of 10,753 trees across mesic forests of ENA, representing 24 species and 346 stands, to determine the broad‐scale drivers of drought sensitivity for the dominant trees in ENA. Here we show that two factors—the timing of drought, and the atmospheric demand for water (i.e., local potential evapotranspiration; PET)—are stronger drivers of drought sensitivity than soil and stand characteristics. Drought‐induced reductions in tree growth were greatest when the droughts occurred during early‐season peaks in radial growth, especially for trees growing in the warmest, driest regions (i.e., highest PET). Further, mean species trait values (rooting depth and ψ₅₀) were poor predictors of drought sensitivity, as intraspecific variation in sensitivity was equal to or greater than interspecific variation in 17 of 24 species. From a general circulation model ensemble, we find that future increases in early‐season PET may exacerbate these effects, and potentially offset gains in C uptake and storage in ENA owing to other global change factors.     

The impact of agricultural land use changes on soil organic carbon dynamics in the Danjiangkou Reservoir area of China

Aims

Over recent decades, a large uncultivated area has been converted to woodland and shrubland plantations to protect and restore riparian ecosystems in the Danjiangkou Reservoir area, a water source area of China’s Middle Route of the South-to-North Water Transfer Project. Besides water quality, afforestation may alter soil organic carbon (SOC) dynamics and stock in terrestrial ecosystems, but its effects remain poorly quantified and understood.

Methods

We investigated soil organic C and nitrogen (N) content, and δ ¹³C and δ ¹⁵N values of organic soil in plant root-spheres and open areas in an afforested, shrubland and adjacent cropped soil. Soil C and N recalcitrance indexes (RIC and RIN) were calculated as the ratio of unhydrolyzable C and N to total C and N.

Results

Afforestation significantly increased SOC levels in plant root-spheres with the largest accumulation of C in the afforested soil. Afforestation also increased belowground biomass. The C:N ratios in organic soil changed from low to high in the order the cropped, the shrubland and the afforested soil. The RIC in the afforested and shrubland were higher than that in cropped soil, but the RIN increased from the afforested to shrubland to cropped soil. The δ¹⁵N values of the organic soil was enriched from the afforested to shrubland to cropped soil, indicating an increased N loss from the cropped soil compared to afforested or shrubland soil. Changes in the δ¹³C ratio further revealed that the decay rate of C in the three land use types was the highest in the cropped soil.

Conclusions

Afforestation increased the SOC stocks resulted from a combination of large C input from belowground and low C losses because of decreasing soil C decomposition. Shifts in vegetation due to land use change could alter both the quantity and quality of the soil C and thus, have potential effects on ecosystem function and recovery.