Does nitrogen transfer between plants confound 15N-based quantifications of N2 fixation?

Background and aims

Transfer of fixed N from legumes to non-legume reference plants may alter the ¹⁵N signature of the reference plant as compared to the soil N available to the legume. This study investigates how N transfer influences the result of ¹⁵N-based N₂ fixation measurements.

Methods

We labelled either legumes or non-legumes with ¹⁵N and performed detailed analyses of ¹⁵N enrichment in mixed plant communities in the field. The results were used in a conceptual model comparing how different N transfer scenarios influenced the ¹⁵N signatures of legumes and reference plants, and how the resulting N₂ fixation estimate was influenced by using reference plants in pure stand or in mixture with the legume.

Results

Based on isotopic signatures, N transfer was detected in all directions: from legume to legume, from legume to non-legume, from non-legume to legume, from non-legume to non-legume. In the scenario of multidirectional N transfer, N₂ fixation was overestimated by using a reference plant in pure stand.

Conclusions

Fixed N transferred to neighbouring reference plants modifies the ¹⁵N signature of the soil N available both to the reference plant and the N₂-fixing legume. This provides strong support for using reference plants growing in mixture with the legumes for reliable quantifications of N₂ fixation.     

Effects of water depth and livelihood activities on plant species composition and diversity in Nyando floodplain wetland,Kenya

The influence of water levels and livelihood activities on plant species composition, diversity and structuring of wetland ecosystem is a concern as wetlands undergo human exploitation and the increasing threat from climate change. To evaluate the effect of seasonal changes in water depth and human activities, plant density and species composition were assessed in wet and dry seasons and in natural and converted wetland zones of the Nyando papyrus wetland, Kenya. Three transects with different water regimes and livelihood activities were identified. Overall, 30 plant species were identified. In the less disturbed zone, differences between transects were small and 79 % (dry season) to 99 % (wet season) of the plant density consisted of obligate and facultative wetland plants. These groups were dominated by Cyperus papyrus and Vossia cuspidata. In the converted zone, facultative, facultative upland and upland plants became more important with 36 % of the plant density. The seasonally inundated zone had species diversity and species richness increased under dry conditions with more facultative upland species. In the converted zone, disturbance caused by vegetation removal, cropping and other livelihood activities lead to lower soil moisture and more colonization opportunity for facultative and upland species that are more adapted to dry conditions.     

Monsoonal influences on evapotranspiration of savanna vegetation of northern Australia

Data from savannas of northern Australia are presented for net radiation, latent and sensible heat, ecosystem surface conductance (G_s) and stand water use for sites covering a latitudinal range of 5° or 700 km. Measurements were made at three locations of increasing distance from the northern coastline and represent high- (1,750 mm), medium- (890 mm) and low- (520 mm) rainfall sites. This rainfall gradient arises from the weakened monsoonal influence with distance inland. Data were coupled to seasonal estimates of leaf area index (LAI) for the tree and understorey strata. All parameters were measured at the seasonal extremes of late wet and dry seasons. During the wet season, daily rates of evapotranspiration were 3.1-3.6 mm day^-1 and were similar for all sites along the rainfall gradient and did not reflect site differences in annual rainfall. During the dry season, site differences were very apparent with evapotranspiration 2-18 times lower than wet season rates, the seasonal differences increasing with distance from coast and reduced annual rainfall. Due to low overstorey LAI, more than 80% of water vapour flux was attributed to the understorey. Seasonal differences in evapotranspiration were mostly due to reductions in understorey leaf area during the dry season. Water use of individual trees did not differ between the wet and dry seasons at any of the sites and stand water use was a simple function of tree density. G_s declined markedly during the dry season at all sites, and we conclude that the savanna water (and carbon) balance is largely determined by G_s and its response to atmospheric and soil water content and by seasonal adjustments to canopy leaf area.     

Plant and soil responses of an alpine steppe on the Tibetan Plateau to multi-level nitrogen addition

Background

Although plant growth in alpine steppes on the Tibetan Plateau has been suggested to be sensitive to nitrogen (N) addition, the N limitation conditions of alpine steppes remain uncertain.

Methods

After 2 years of fertilization with NH₄NO₃ at six rates (0, 10, 20, 40, 80 and 160 kg N ha^?1 yr^?1), the responses of plant and soil parameters as well as N₂O fluxes were measured.

Results

At the vegetation level, N addition resulted in an increase in the aboveground N pool from 0.5?±?0.1 g m^?2 in the control plots to 1.9?±?0.2 g m^?2 in the plots at the highest N input rate. The aboveground C pool, biomass N concentration, foliar δ¹⁵N, soil NO₃ ^?-N and N₂O flux were also increased by N addition. However, as the N fertilization rate increased from 10 kg N ha^?1 yr^?1 to 160 kg N ha^?1 yr^?1, the N-use efficiency decreased from 12.3?±?4.6 kg C kg N^?1 to 1.6?±?0.2 kg C kg N^?1, and the N-uptake efficiency decreased from 43.2?±?9.7 % to 9.1?±?1.1 %. Biomass N:P ratios increased from 14.4?±?2.6 in the control plots to 20.5?±?0.8 in the plots with the highest N input rate. Biomass N:P ratios, N-uptake efficiency and N-use efficiency flattened out at 40 kg N ha^?1 yr^?1. Above this level, soil NO₃ ^?-N began to accumulate. The seasonal average N₂O flux of growing season nonlinearly increased with increased N fertilization rate and linearly increased with the weighted average foliar δ¹⁵N. At the species level, N uptake responses to relative N availability were species-specific. Biomass N concentration of seven out of the eight non-legume species increased significantly with N fertilization rates, while Kobresia macrantha and the one legume species (Oxytropics glacialis) remained stable. Both the non-legume and the legume species showed significant ¹⁵N enrichment with increasing N fertilization rate. All non-legume species showed significant increased N:P ratios with increased N fertilization rate, but not the legume species.

Conclusions

Our findings suggest that the Tibetan alpine steppes might be N-saturated above a critical N load of 40 kg N ha^?1 yr^?1. For the entire Tibetan Plateau (ca. 2.57 million km²), a low N deposition rate (10 kg N ha^?1 yr^?1) could enhance plant growth, and stimulate aboveground N and C storage by at least 1.1?±?0.3 Tg N yr^?1 and 31.5?±?11.8 Tg C yr^?1, respectively. The non-legume species was N-limited, but the legume species was not limited by N.     

The effect of fertilization on sap flux and canopy conductance in a Eucalyptus saligna experimental forest

Land devoted to plantation forestry (50 million ha) has been increasing worldwide and the genus Eucalyptus is a popular plantation species (14 million ha) for its rapid growth and ability to grow well on a wide range of sites. Fertilization is a common silvicultural tool to improve tree growth with potential effects on stand water use, but the relationship between wood growth and water use in response to fertilization remains poorly quantified. Our objectives in this study were to determine the extent, timing and longevity of fertilization effects on water use and wood growth in a non‐water limited Eucalyptus saligna experimental forest near Hilo, HI. We evaluated the short‐ and long‐term effects of fertilization on water use by measuring sap flux per unit sapwood area, canopy conductance, transpiration per unit leaf area and water‐use efficiency in control and fertilized stands. Short‐term effects were assessed by comparing sap flux before and after fertilizer application. Long‐term effects were assessed by comparing control plots and plots that had received nutrient additions for 5 years. For the short‐term response, total water use in fertilized plots increased from 265 to 487 mm yr^?1 during the 5 months following fertilization. The increase was driven by an increase in stand leaf area accompanied by an increase in sap flux per unit sapwood area. Sap flux per unit leaf area and canopy conductance did not differ during the 5 months following fertilizer additions. For the last 2 months of our short‐term measurements, fertilized trees used less water per unit carbon gain (361 compared with 751 kg H₂O kg C^?1 in control stands). Trees with 5 years of fertilization also used significantly more water than controls (401 vs. 302 mm yr^?1) because of greater leaf area in the fertilized stands. Sap flux per unit sapwood area, sap flux per unit leaf area, and canopy conductance did not differ between control and fertilized trees in the long‐term plots. In contrast to the short‐term response, the long‐term response of water use per unit wood growth was not significant. Overall, fertilization of E. saligna at our site increased stand water use by increasing leaf area. Fertilized trees grew more wood and used more water, but fertilization did not change wood growth per unit water use.     

Effects of Experimental Soil Warming and Water Addition on the Transpiration of Mature Sugar Maple

Sugar maple (Acer saccharum), an economically important timber and syrup species, is not expected to flourish under projected future climates. The objectives of our study were to (1) tease apart the effects of warming and soil moisture availability on transpiration rates of mature sugar maple trees with a full-factorial soil warming × water addition experiment and (2) determine the primary environmental driver(s) of sugar maple transpiration in the upper Midwestern United States. Over three growing seasons, we monitored sap flux of 33 trees in eight 100-m² plots, two replicates each of four treatments: (1) heat (soil warmed +4°C), (2) water (1.3 × ambient growing season precipitation, (3) heat + water, and (4) control. As expected, sugar maple transpiration decreased under the heat treatments in all years and increased in water treatments in years 1 and 2, all mediated primarily by soil moisture. However, under the heat + water treatment, supplemental water compensated for the warming-induced soil evaporation only in year 1 (2011), which was the driest year. Despite clear evidence of a soil moisture-mediated treatment effect, light was the dominant driver of seasonal variation in sap flux in this sugar maple-dominated ecosystem with relatively short growing seasons. However, sap flux was reduced with decreases in soil moisture, and therefore, net C gain likely was as well. Overall, our results suggest that even though this northern temperate species is primarily limited by light, sugar maple productivity may be reduced by a warming climate on drier sites within its current range, if warming is not accompanied by a sufficient increase in precipitation.     

Water relations of ponderosa pines in Patagonia Argentina: implications for local water resources and individual growth

Replacement of grasslands by forests may result in increased water consumption, and the magnitude of this will depend on stand density. To test this hypotheses and evaluate the impact of pine plantations on hydric resources in Patagonia Argentina, we measured over two seasons (1999–2000 and 2000–2001) and at two densities of ponderosa pine plantations (350 and 500 trees ha^?1) the following variables: soil water content, leaf water potential (ψ), individual tree growth, individual sap flow, and response of sap flow density (u) to vapor pressure deficit (VPD). Stand transpiration (T) and whole-plant liquid-phase hydraulic conductance (L) were also estimated. Pre-dawn ψ varied from about –0.5 to –1.0 MPa. No differences were found in midday maximum u (1100–1800 hours) on clear days between the 2 measurement years, throughout each season, or between different densities of plantation. Sapflow density was also not correlated with soil water storage up to 1.4 m soil depth. Sapflow increased until VPD of about 2.3 kPa, and decreased at VPD >4 kPa, describing a hysteresis in the afternoon. Values of L in Patagonian trees were similar to those recalculated from published data for pines of the same height in the USA. Average stand transpiration increased with increased density (2.07 and 3.08 mm day^-1 for 350 and 500 pines ha^-1, respectively) and size of the trees. We conclude that ponderosa pines in Patagonia Argentina use more water, in a magnitude depending on the density of the trees, than native grasslands. Accordingly, ponderosa pines use deep water to maintain high water potential and transpiration rates even during the dry season.     

Ecophysiological responses of two closely related Magnoliaceae genera to seasonal changes in subtropical China

Aims Plants use a variety of hydraulic strategies to adapt to seasonal drought that differ by species and environmental conditions. The early-diverging Magnoliaceae family includes two closely related genera with contrasting leaf habits, Yulania (deciduous) and Michelia (evergreen), which naturally inhabit temperate and tropical regions, respectively. Here, we evaluate the hydraulic strategy of species from both genera that have been ex situ conserved in a subtropical region to determine how they respond to the novel cool–dry season climatic pattern.Methods We measured ecophysiological traits in five Michelia and five Yulania species conserved in the South China Botanical Garden in both wet and dry season conditions and monitored the whole-year sap flow for four of these species.Important findings We found that Magnoliaceae species that have been ex situ conserved in a subtropical climate did not suffer from excessive water stress due to the mild drought conditions of the dry season and the ecophysiological adjustments the species made to avoid this stress, which differed by leaf habit. Specifically, deciduous species completely shed their leaves during the dry season, while evergreen species decreased their turgor loss points, dry mass based photosynthetic rates, stomatal conductance and specific leaf areas (SLAs) compared to wet season measurements. In comparing the two distinct leaf habits during the wet season, the leathery-leaved evergreen species had higher leaf hydraulic conductance and leaf to sapwood area ratios than the papery-leaved deciduous species, while the deciduous species had greater hydraulic conductivity calculated on both a stem and leaf area basis, dry mass based photosynthetic rates, leaf nutrients, SLAs and stomatal sizes than the evergreen species. Interestingly, species from both genera maintained similar sap flow in the wet season. Both photosynthetically active radiation and vapour pressure deficit affected the diurnal patterns of sap flow in the wet season, while only vapour pressure deficit played a dominant role in the dry season. This study reveals contrasting hydraulic strategies in Yulania and Michelia species under subtropical seasonal conditions, and suggests that these ecophysiological adjustments might be affected more by leaf habit than seasonality, thus reflecting the divergent evolution of the two closely related genera. Furthermore, we show that Magnoliaceae species that are ex situ conserved in a subtropical climate are hydraulically sound, a finding that will inform future conservation efforts of this ancient family under the threat of climatic change.     

树木胸径大小对树干液流变化格局的偏度和时滞效应

通过分析具不同水力结构的马占相思、荷木和粉单竹液流变化格局的偏度和时滞,探讨液流的空间分布特征及对冠层蒸腾的影响。结果表明:荷木的液流格局偏度和时滞随树木胸径的增加呈减小的趋势,但马占相思由于冠层开阔和林分分化程度高而规律不明显,粉单竹液流偏度随胸径减少,由于冠幅较小,接受的光照较均匀,个体间的时滞差异不明显,但时滞值比胸径近似的荷木小。树干水分传输过程中存在液流再分配的现象,边材的导水效率可能是影响时滞的重要原因。冠层蒸腾的空间异质性与树木储存水有关,大树储存水较多,冠层蒸腾的异质性小;小树储存水较少,液流被优先分配到光照充足的东南方位,导致冠层蒸腾较高的异质性。旱季受土壤水分的限制,大树储存水对蒸腾的贡献大于湿季,而小树蒸腾由于受到储水容量的制约,储存水对蒸腾的贡献小于湿季。冠层接受光照的迟或早以及辐射量的大小是引起蒸腾时间变化格局和树干不同方位液流格局差异的重要原因,但液流的横向交换弱化了这种现象,往往是个体间的差异掩盖了方位的差异。湿季较小胸径的树木比偏值(枝下高与胸高处液流偏度的比值)大于旱季,而较大胸径的树木比偏值恰好相反,总体而言,比偏值随着胸径的增加而逐渐下降。     

Inorganic soil nitrogen under grassland plant communities of different species composition and diversity

We measured aboveground plant biomass and soil inorganic nitrogen pools in a biodiversity experiment in northern Sweden, with plant species richness ranging from 1 to 12 species. In general, biomass increased and nitrate pools decreased with increasing species richness. Transgressive overyielding of mixed plant communities compared to the most productive of the corresponding monocultures occurred in communities with and without legumes. N₂-fixing legumes had a fertilizing function, while non-legumes had a N retaining function. Plant communities with only legumes had a positive correlation between biomass and soil nitrate content, whereas in plant communities without legumes they were negatively correlated. Both nitrate and ammonium soil pools in mixed non-legume communities were approximately equal to the lowest observed in the corresponding monocultures. In mixed legume/non-legume communities, no correlation was found for soil nitrate with either biomass or legume biomass as percentage of total biomass. The idea of complementarity among species in nitrogen acquisition was supported in both pure non-legume and mixed non-legume/legume communities. In the latter, however, facilitation through increased nitrogen availability and retention, was probably dominating. Our results suggest that diversity effects on biomass and soil N pools through resource use complementarity depend on the functional traits of species, especially N₂ fixation or high productivity.     

Nighttime sap flow of Acacia mangium and its implications for nighttime transpiration and stem water storage

Aims Nighttime sap flow of trees may indicate transpiration and/or recharge of stem water storage at night. This paper deals with the water use of Acacia mangium at night in the hilly lands of subtropical South China. Our primary goal was to reveal and understand the nature of nighttime sap flow and its functional significance.Methods Granier's thermal dissipation method was used to determine the nighttime sap flux of A. mangium. Gas exchange system was used to estimate nighttime leaf transpiration and stomatal conductance of studied trees.Important findings Nighttime sap flow was substantial and showed seasonal variation similar to the patterns of daytime sap flow in A. mangium. Mean nighttime sap flow was higher in the less precipitation year of 2004 (1122.4 mm) than in the more precipitation year of 2005 (1342.5 mm) since more daytime transpiration and low soil water availability in the relatively dry 2004 can be the cause of more nighttime sap flow. Although vapor pressure deficit and air temperature were significantly correlated with nighttime sap flow, they could only explain a small fraction of the variance in nighttime sap flow. The total accumulated water loss (E L) by transpiration of canopy leaves was only ~2.6–8.5% of the total nighttime sap flow (E t) during the nights of July 17–18 and 18–19, 2006. Therefore, it is likely that the nighttime sap flow was mainly used for refilling water in the trunk. The stem diameter at breast height, basal area and sapwood area explained much more variance of nighttime water recharge than environmental factors and other tree form features, such as tree height, stem length below the branch, and canopy size. The contribution of nighttime water recharge to the total transpiration ranged from 14.7 to 30.3% depending on different DBH class and was considerably higher in the dry season compared to the wet season.     

Nocturnal sap flow in the C<Subscript>3</Subscript>-CAM species, <Emphasis Type="Italic">Clusia minor</Emphasis>

Clusia minor L. is a C₃-CAM species in which Crassulacean acid Metabolism (CAM) is induced, among other factors, by water deficit. We propose that CAM induction by natural drought in C. minor shifts the sap flow pattern from daytime to a night-time one, and that the decreased osmotic potential due to increased malate content in droughted plants aids in the increase in nocturnal sap flow. In order to test these hypotheses, we followed for 2 years the seasonal changes in parameters of water relationships and sap flow velocity in one single, freestanding tree growing in Caracas. Leaf water and osmotic potential were measured psychrometrically, nocturnal proton accumulation by titration of aqueous leaf extracts and sap flow density with thermal dissipation probes. Leaf water, osmotic and turgor potential remained relatively high throughout the seasons. Nocturnal proton accumulation was nil under extreme drought or after frequent and heavy rains, and high after moderate rainfall. Estimated malate and citrate concentrations contributed up to 80 and 60%, respectively, of the value of osmotic potential. The shape of the daily courses of sap flow velocity varied seasonally, from mostly diurnal during the dry season to mostly nocturnal after a short dry spell during the rainy season, when nocturnal acid accumulation attained high values. There was a strong positive relationship between the proportion of the integrated sap flow courses corresponding to the night and dawn [H⁺] (r ² = 0.88). Increased nocturnal sap flow in the CAM stage of the tree of C. minor may be explained by a lower osmotic potential due to an increased acid concentration, together with increased stomatal aperture, as suggested by increased nocturnal acid accumulation probably due to nocturnal CO₂ fixation.     

基于液流格型特征值和标准化方法分析胸径和土壤水分对荷木液流的影响

基于液流格型特征值-偏度和峰度分析了不同胸径荷木在水分利用方面的差异,并利用标准化的方法消除强影响因子(光合有效辐射,PAR)对液流的影响,研究了弱影响因子(土壤湿度)与液流的关系.结果表明：荷木胸径越大,偏度越小,液流密度峰值越靠后,相应的液流峰值越大,蒸腾量也越大.与旱季相比,荷木大树在湿季偏度较小,液流密度到达峰值时间靠后,峰值大,蒸腾量也大;而小树偏度在旱、湿季的差异不明显,蒸腾量差异也不大.用PAR峰值对荷木个体蒸腾和土壤湿度进行标准化后,荷木个体蒸腾与土壤湿度呈更明显的正相关关系.在土壤湿度较大的季节,荷木大树的蒸腾量随土壤湿度增加而上升的速率基本稳定;而中等木和小树的某些个体则明显下降,说明这些树木的蒸腾和吸收土壤水分的能力可能接近极限.     

Late-stage canopy tree species with extremely low δ13C and high stomatal sensitivity to seasonal soil drought in the tropical rainforest of French Guiana

We assessed the daily time‐courses of CO₂ assimilation rate (A), leaf transpiration rate (E), stomatal conductance for water vapour (g_s), leaf water potential ( Ψ _w) and tree transpiration in a wet and a dry season for three late‐stage canopy rainforest tree species in French Guiana differing in leaf carbon isotope composition ( δ ¹³C). The lower sunlit leaf δ ¹³C values found in Virola surinamensis ( ? 29·9‰) and in Diplotropis purpurea ( ? 30·9‰), two light‐demanding species, as compared to Eperua falcata ( ? 28·6‰), a shade‐semi‐tolerant species, were clearly associated with higher maximum g_s values of sunlit leaves in the two former species. These two species were also characterized by a high sensitivity of g_s, sap flow density (Ju) and canopy conductance (g_c) to seasonal soil drought, allowing maintenance of high midday Ψ _w values in the dry season. The data for Diplotropis provided an original picture of increasing midday Ψ _w with increasing soil drought. In Virola, stomata were extremely sensitive to seasonal soil drought, leading to a dramatic decrease in leaf and tree transpiration in the dry season, whereas midday Ψ _w remained close to ? 0·3 MPa. The mechanisms underlying such an extremely high sensitivity of stomata to soil drought remain unknown. In Eperua, g_s of sunlit leaves was non‐responsive to seasonal drought, whereas Ju and g_c were lower in the dry season. This suggests a higher stomatal sensitivity to seasonal drought in shaded leaves than in sunlit ones in this species.     

Variation in seedling growth of 11 perennial legumes in response to phosphorus supply

Phosphorus (P) deficiency is a major problem for Australian agriculture. Development of new perennial pasture legumes that acquire or use P more efficiently than the current major perennial pasture legume, lucerne (Medicago sativa L.), is urgent. A glasshouse experiment compared the response of ten perennial herbaceous legume species to a series of P supplies ranging from 0 to 384 µg g^?1 soil, with lucerne as the control. Under low-P conditions, several legumes produced more biomass than lucerne. Four species (Lotononis bainesii Baker, Kennedia prorepens F.Muell, K. prostrata R.Br, Bituminaria bituminosa (L.) C.H.Stirt) achieved maximum growth at 12 µg P g^?1 soil, while other species required 24 µg P g^?1. In most tested legumes, biomass production was reduced when P supply was ≥192 µg g^?1, due to P toxicity, while L. bainesii and K. prorepens showed reduced biomass when P was ≥24 µg g^?1 and K. prostrata at ≥48 µg P g^?1 soil. B. bituminosa and Glycine canescens F.J.Herm required less soil P to achieve 0.5 g dry mass than the other species did. Lucerne performed poorly with low P supply and our results suggest that some novel perennial legumes may perform better on low-P soils.     

Cool-dry season depression in gas exchange of canopy leaves and water flux of tropical trees at the northern limit of Asian tropics

Trees on the northern boundary of Asian tropics experience hot-humid and cool-dry seasons, but little is known about their seasonal dynamics in canopy physiology. We used a canopy crane to reach the canopy of nine tropical tree species and measured canopy leaf gas exchange, water status, and trunk sap flux during the hot-humid and cool-dry seasons in Xishuangbanna, China. We found that most tree species exhibited significant reductions in maximum photosynthetic rate (A_max), stomatal conductance (g_smax), predawn and midday leaf water potentials, and maximum sap flux density in the cool-dry season. Compared to the hot-humid season, A_max declined by 19–60%, and maximum water flux declined by ?14% (an increase) to 42%. The cool-dry season decline in A_max of four species can be partly explained by an increased stomatal limitation (decreased g_smax and intercellular CO₂ concentrations). Therefore, a predicted increase in drought in this region may decrease the carbon sequestration and productivity of these forests. We did not find a tradeoff between performance (A_max in the hot-humid season) and persistence through the cool-dry season; species with higher A_max in the hot-humid season did not show higher percent seasonal declines in the cool-dry season. A_max was significantly and positively associated with the trunk sap flux for both seasons, but the association was weaker in the cool-dry season. Thus, our results suggest that some tradeoffs and trait associations are environment dependent. Our results are important for understanding carbon and water fluxes of seasonal tropical forests and their responses to environmental changes.

     

Metallophytes—a view from the rhizosphere

Abundance of Fabaceae declines in representation through post-fire-succession in fynbos vegetation of the Cape Floristic Region (CFR). This reduction in legume occurrence coincides with a known decline in post-fire soil P availability. It was hypothesized that the disappearance of legume species during post-fire succession is due to an inability to acquire P effectively from sparingly soluble sources. P-acquisition strategies and response to P supply were compared between legume (Aspalathus, Cyclopia, Indigofera, Podalyria) and non-legume (Elegia, Leucadendron, Protea) genera when supplied with 1 or 10 mg P kg^?1 dry sand. Each genus consisted of a seeder (non-persistent) and resprouter (persistent) species. Non-legumes showed a greater investment in below-ground biomass, more root clusters, with higher concentrations of carboxylates exuded by cluster roots and carboxylates that were better suited to the mobilization of sparingly soluble P compared to legumes. The growth response to increased P supply was 53% higher in legumes than in non-legumes. The lack of a growth response to an elevated P supply in the non-legumes was attributed to N-limitation. Legume resprouters had a higher investment in cluster-root biomass and a lower capacity to down-regulate P-uptake than the seeders. Therefore the inability to acquire sufficient P from low concentration and sparingly soluble soil P-sources may contribute to the lack of indigenous legume persistence in fynbos vegetation of the CFR.     

Sap flux of five co-occurring tree species in a temperate broad-leaved forest during seasonal soil drought

In an old-growth forest in Central Germany, sap flux was studied in five broad-leaved tree species that were assumed to differ in drought sensitivity. Under moist soil conditions, average daily sap flux density (J _s) in the outermost xylem varied by a factor of 2.3 among the species (67–152 g cm⁻² per day, n=5 trees per species), and declined in the sequence Fagus sylvatica > Acer pseudoplatanus > Tilia cordata > Carpinus betulus > Fraxinus excelsior. Decreasing soil moisture content (Θ) resulted in linearly reduced J _s in four of the species. During a dry period, J _s was reduced by 44% in T. cordata, 39% in F. sylvatica, 37% in A. pseudoplatanus and 31% in C. betulus compared to sap flux at equal vapour pressure deficit (D) in the wet period. F. excelsior, the only ring-porous species studied, lacked a significant response in J _s to D and Θ. The relative reduction in water use during the dry period was not related to the assumed drought sensitivity of the species as inferred from their abundance in natural woodlands. J _s was positively correlated with tree diameter at breast height (DBH) in three species but decreased with DBH in two species. Dyeing experiments revealed that DBH accounted for 94% of the variation in sapwood area found in a bulk sample of all diffuse-porous trees. This suggests that DBH is a reliable estimator of sapwood area of temperate diffuse-porous species irrespective of species identity. In contrast, sap flux density was found to be greatly dependent on tree species. The estimated whole-plant water use for diffuse-porous trees of a given diameter (49 cm) ranged between 74 and 168 kg per day per species under moist soil conditions. Thus, in temperate mixed forests, species-specific differences in water use can result in a considerable spatial heterogeneity of canopy transpiration.     

Modelling Seasonal and Inter-annual Variations in Carbon and Water Fluxes in an Arid-Zone <Emphasis Type="Italic">Acacia</Emphasis> Savanna Woodland, 1981–2012

Changes in climatic characteristics such as seasonal and inter-annual variability may affect ecosystem structure and function, hence alter carbon and water budgets of ecosystems. Studies of modelling combined with field experiments can provide essential information to investigate interactions between carbon and water cycles and climate. Here we present a first attempt to investigate the long-term climate controls on seasonal patterns and inter-annual variations in water and carbon exchanges in an arid-zone savanna-woodland ecosystem using a detailed mechanistic soil–plant–atmosphere model (SPA), driven by leaf area index (LAI) simulated by an ecohydrological model (WAVES) and observed climate data during 1981–2012. The SPA was tested against almost 3 years of eddy covariance flux measurements in terms of gross primary productivity (GPP) and evapotranspiration (ET). The model was able to explain 80 and 71% of the variability of observed daily GPP and ET, respectively. Long-term simulations showed that carbon accumulation rates and ET ranged from 20.6 g C m^?2 mon^?1 in the late dry season to 45.8 g C m^?2 mon^?1 in the late wet season, respectively, primarily driven by seasonal variations in LAI and soil moisture. Large climate variations resulted in large seasonal variation in ecosystem water-use efficiency (eWUE). Simulated annual GPP varied between 146.4 and 604.7 g C m^?2 y^?1. Variations in annual ET coincided with that of GPP, ranging from 110.2 to 625.8 mm y^?1. Annual variations in GPP and ET were driven by the annual variations in precipitation and vapour pressure deficit (VPD) but not temperature. The linear coupling of simulated annual GPP and ET resulted in eWUE having relatively small year-to-year variation.     

Up-scaling to stand transpiration of an Asian temperate mixed-deciduous forest from single tree sapflow measurements

Species diversity in mixed forest stands is one of the factors that complicate up-scaling of transpiration from individual trees to stand level, since tree species are architecturally and functionally different. In this study, thermal dissipation probes were used to measure sap flow in five different tree species in a mixed-deciduous mountain forest in South Korea. Easily measurable tree characteristics that could serve to define individual tree water use among the different species were employed to scale up transpiration from single trees to stand level. Tree water use (TWU) was derived from sap flux density (SFD) and sapwood area (SA). Canopy transpiration E was scaled from TWU while canopy conductance (g _c) was computed from E and VPD. SFD, TWU and g _c were correlated with tree diameter at breast height (DBH) for all the five measured species (SFD: R ² = 0.21, P = 0.036; TWU: R ² = 0.83, P < 0.001; g _c: R ² = 0.63, P < 0.001). Maximum stand transpiration (E) during June, before the onset of the Asian monsoon rains, was estimated at 0.97 ± 0.12 mm per day. There was a good (R ² = 0.94, P < 0.0001) agreement between measured and estimated E using the relationship between TWU and DBH. Our study shows that using functional models that employ converging traits among species could help in estimating water use in mixed forest stands. Compared to SA, DBH is a better scalar for water use of mixed forest stands since it is non-destructive and easily obtainable.