North-south Climatic Gradient Research Articles

Responses of the reflectance indices PRI and NDVI to experimental warming and drought in European shrublands along a north–south climatic gradient

The aim of this study was to evaluate the use of ground-based canopy reflectance measurements to detect changes in physiology and structure of vegetation in response to experimental warming and drought treatment at six European shrublands located along a North–South climatic gradient. We measured canopy reflectance, effective green leaf area index (green LAIe) and chlorophyll fluorescence of dominant species. The treatment effects on green LAIe varied among sites. We calculated three reflectance indices: photochemical reflectance index PRI [531 nm; 570 nm], normalized difference vegetation index NDVI 680 [780 nm; 680 nm] using red spectral region, and NDVI 570 [780 nm; 570 nm] using the same green spectral region as PRI. All three reflectance indices were significantly related to green LAIe and were able to detect changes in shrubland vegetation among treatments. In general warming treatment increased PRI and drought treatment reduced NDVI values. The significant treatment effect on photochemical efficiency of plants detected with PRI could not be detected by fluorescence measurements. However, we found canopy level measured PRI to be very sensitive to soil reflectance properties especially in vegetation areas with low green LAIe. As both soil reflectance and LAI varied between northern and southern sites it is problematic to draw universal conclusions of climate-derived changes in all vegetation types based merely on PRI measurements. We propose that canopy level PRI measurements can be more useful in areas of dense vegetation and dark soils.

Experimental reconstruction of large-scale summer monsoon drought over India and the Tibetan Plateau using tree rings from 'High Asia'

We investigate the use of a network of 194 tree-ring chronologies from the “High Asia” region of the Himalaya, Karakoram, Tien Shan, and Tibetan Plateau for the reconstruction of large-scale summer monsoon drought variability over India and the Tibetan Plateau. This experiment was carried out to see if the High Asia tree-ring network contains large-scale information related to the north-south climate gradient over this region, which is known to be associated with the development of the Indian summer monsoon. The climate field chosen for experimental reconstruction was a 735-point grid of self-calibrated Palmer Drought Severity Indics (PDSI) calculated from CRU TS 2.1 gridded temperature and precipitation data. This PDSI grid was reconstructed for the summer monsoon season from a subset of tree-ring chronologies covering the period 1600-1989 using a reduced-space multivariate regression method, and both calibration and verification tests indicate that the reconstructions contain significant hindcast skill in substantial sub-regions of the grid, especially over India. Principal components analysis carried out on the almost 400 years of data over the complete grid indicates that the leading spatial mode or EOF of summer monsoon PDSI expresses a dominant north-south alternation in drought and wetness between India and the Tibetan Plateau, a leading mode also present in the instrumental data. Wavelet analysis of the time series expression of this leading EOF also reveals a very stable 40-60 year pattern of multidecadal variability in it. This time scale of variability appears to be related to slowly changing tropical ocean sea surface temperatures. Together, these results validate the use of a geographically restricted set of tree-ring chronologies over High Asia for the reconstruction of large-scale summer monsoon variability over India and the Tibetan Plateau.

Floristic and functional affiliations of woody plants with climate in western Amazonia

AbstractAim To test whether a direct relationship exists between the relative abundance of woody plant genera and precipitation regime along the north–south climate gradient of the western Amazon.Location Lowland rain forests in the western Amazon.Methods Floristic data on 91 woody plant genera, from 39 0.1‐ha plots across the western Amazon, and precipitation data from a 0.5° global data set were used to test for correlations between plant relative abundance (defined as percentage number of stems ≥ 2.5 cm diameter at breast height for each woody plant genus per plot) and derived dry‐season variables. Moisture preference was then assessed in terms of pioneer and shade‐tolerant life‐history strategy.Results There were significant associations between the distribution of plant relative abundances and seasonal precipitation variables in 34% of genera analysed. Significant differences were identified in size‐class distribution between dry affiliates and generalists. Dry affiliates were not dominant in any size class in any plot type, whereas climate generalists dominated most of the size classes in the dry plots and the mid‐range size classes in the wet plots. Dry‐affiliate genera were a minority, even in dry forests. Wet‐affiliate genera were correlated with shade tolerance, whereas genera with no rainfall affiliation were often pioneers.Main conclusions The results suggest that moisture variable seasonality influences community composition in a manner that can be related to the life‐history trade‐off between shade tolerance and pioneer ranking. One possible reason for higher diversity in wetter forests is that high rainfall amplifies the niche space available to shade‐tolerant plants. Determining which plant groups are constrained by which environmental variables can contribute to our understanding of how forest composition may be changing now, and how it may change under future climate: if shade‐tolerant trees are also drought‐intolerant, community structure in wet forests may be more vulnerable to future increases in moisture stress.

Measurement of net ecosystem exchange, productivity and respiration in three spruce forests in Sweden shows unexpectedly large soil carbon losses

Measurement of net ecosystem exchange was made using the eddy covariance method above three forests along a north-south climatic gradient in Sweden: Flakaliden in the north, Knottasen in central and Asa in south Sweden. Data were obtained for 2 years at Flakaliden and Knottasen and for one year at Asa. The net fluxes (Nep) were separated into their main components, total ecosystem respiration (Rt) and gross primary productivity (Pg). The maximum half-hourly net uptake during the heart of the growing season was highest in the southernmost site with −0.787 mg CO2 m−2 s−1 followed by Knottasen with −0.631 mg CO2 m−2 s−1 and Flakaliden with −0.429 mg CO2 m−2 s−1. The maximum respiration rates during the summer were highest in Knottasen with 0.245 mg CO2 m−2 s−1 while it was similar at the two other sites with 0.183 mg CO2 m−2 s−1. The annual Nep ranged between uptake of −304 g C m−2 year−1 (Asa) and emission of 84 g C m−2 year−1 (Knottasen). The annual Rt and Pg ranged between 793 to 1253 g C m−2 year−1 and −875 to −1317 g C m−2 year−1, respectively. Biomass increment measurements in the footprint area of the towers in combination with the measured net ecosystem productivity were used to estimate the changes in soil carbon and it was found that the soils were losing on average 96–125 g C m−2 year−1. The most plausible explanation for these losses was that the studied years were much warmer than normal causing larger respiratory losses. The comparison of net primary productivity and Pg showed that ca 60% of Pg was utilized for autotrophic respiration.

Pools and fluxes of carbon in three Norway spruce ecosystems along a climatic gradient in Sweden

This paper presents an integrated analysis of organic carbon (C) pools in soils and vegetation, within-ecosystem fluxes and net ecosystem exchange (NEE) in three 40-year old Norway spruce stands along a north-south climatic gradient in Sweden, measured 2001–2004. A process-orientated ecosystem model (CoupModel), previously parameterised on a regional dataset, was used for the analysis. Pools of soil organic carbon (SOC) and tree growth rates were highest at the southernmost site (1.6 and 2.0-fold, respectively). Tree litter production (litterfall and root litter) was also highest in the south, with about half coming from fine roots (<1 mm) at all sites. However, when the litter input from the forest floor vegetation was included, the difference in total litter input rate between the sites almost disappeared (190–233 g C m−2 year−1). We propose that a higher N deposition and N availability in the south result in a slower turnover of soil organic matter than in the north. This effect seems to overshadow the effect of temperature. At the southern site, 19% of the total litter input to the O horizon was leached to the mineral soil as dissolved organic carbon, while at the two northern sites the corresponding figure was approx. 9%. The CoupModel accurately described general C cycling behaviour in these ecosystems, reproducing the differences between north and south. The simulated changes in SOC pools during the measurement period were small, ranging from −8 g C m−2 year−1 in the north to +9 g C m−2 year−1 in the south. In contrast, NEE and tree growth measurements at the northernmost site suggest that the soil lost about 90 g C m−2 year−1.

Seasonal courses of key parameters of nitrogen, carbon and water balance in European beech (Fagus sylvatica L.) grown on four different study sites along a European North–South climate gradient during the 2003 drought

During the growing season of the exceptionally dry and warm year 2003, we assessed seasonal changes in nitrogen, carbon and water balance related parameters of mature naturally grown European beech (Fagus sylvatica L.) along a North–South transect in Europe that included a beech forest stand in central Germany, two in southern Germany and one in southern France. Indicators for N balance assessed at all four sites were foliar N contents and total soluble non-protein nitrogen compounds (TSNN) in xylem sap, leaves and phloem exudates; C and water balance related parameters determined were foliar C contents, δ13C and δ18O signatures. Tissue sampling was performed in May, July and September. The N related parameters displayed seasonal courses with highest concentrations during N remobilization in May. Decreased total foliar N contents as well as higher C/N ratios in the stands in central Germany and southern France compared to the other study sites point to an impaired N nutrition status due to lower soil N contents and precipitation perception. TSNN concentrations in leaves and phloem exudates of all study sites were in ranges previously reported, but xylem sap content of amino compounds in July was lower at all study sites when compared to literature data (c. 1 μmol N mL−1). In September, TSNN concentrations increased again at the two study sites in southern Germany after a rain event, whereas they remained constant at sites in central Germany and southern France which hardly perceived precipitation during that time. Thus, TSNN concentrations in the xylem sap might be indicative for water balance related N supply in the beech trees. TSNN profiles at all study sites, however, did not indicate drought stress. Foliar δ13C, but not foliar C and δ18O followed a seasonal trend at all study sites with highest values in May. Differences in foliar δ13C and δ18O did not reflect climatic differences between the sites, and are attributed to differences in altitude, photosynthesis and δ18O signatures of the water sources. Except of low TSNN concentrations in the xylem sap, no physiological indications of drought stress were detected in the trees analysed. We suppose that the other parameters assessed might not have been sensitive to the drought events because of efficient regulation mechanisms that provide a suitable physiological setting even under conditions of prolonged water limitation. The uniform performance of the trees from southern France and central Germany under comparably dry climate conditions denotes that the metabolic plasticity of mature beech from the different sites studied might be similar.

Genetic Variation in Echinacea angustifolia Along a Climatic Gradient

Echinacea angustifolia is a widespread species distributed throughout the Great Plains region of North America. Genetic differentiation among populations was investigated along a 1500 km north-south climatic gradient in North America, a region with no major geographical barriers. The objective of the study was to determine if genetic differentiation of populations could be explained by an isolation-by-distance model or by associations with climatic parameters known to affect plant growth and survival. Historical climatic data were used to define the nature of the climatic gradient and AFLP markers were used to establish patterns of population genetic differentiation among ten Echinacea populations collected from North Dakota to Oklahoma. A total of 1290 fragments were scored using six EcoRI/MseI and three PstI/MseI primer combinations. Assessment of the correlation between climatic, genetic and geographic distances was assessed by Mantel and partial Mantel tests. PstI/MseI combinations produced significantly fewer fragments, but a larger percentage was unique compared with EcoRI/MseI markers. Using estimates of F(ST), populations in Oklahoma and southern Kansas were identified as the most divergent from the other populations. Both the neighbour-joining tree and principal co-ordinate analysis clustered the populations in a north-south spatial orientation. About 60% of the genetic variation was found within populations, 20% among populations and the remaining 20% was partitioned among groups that were defined by the topology of the neighbour-joining tree. Significant support was found for the isolation-by-distance model independent of the effects of annual mean precipitation, but not from annual mean temperature and freeze-free days. Echinacea angustifolia populations exhibit genetic divergence along a north-south climatic gradient. The data support an isolation-by-distance restriction in gene flow that is independent of annual mean precipitation.

Diet of the nominate Southern Grey Shrike Lanius meridionalis meridionalis in the north of its range (Mediterranean France)

Capsule In this region the diet is mainly cold-blooded prey, mostly insects such as beetles. Aims To describe the diet of this newly separated, poorly documented and endangered species. Method Diet was inferred from pellet analysis, collected during a single breeding and winter period in the steppe of the Crau. Results A total of 5409 prey were identified from 257 pellets. Vertebrates were seldom taken, except by adults (small passerines) during the fledgling period. High seasonal differences were found. Hymenoptera were largely consumed in autumn, Arachnida in autumn and winter, Orthoptera in summer and autumn and Lepidoptera larvae in winter and spring and by fledglings. Nevertheless, Coleoptera were ingested in large proportions all year round. Carabidae were the main prey in winter and Melolonthidae were especially important for adults during the nestling period, as were Cetoniidae for the fledglings. Conclusion Small mammals and small birds were less exploited in France and Spain (L. m. meridionalis) than in Israel (L. m. elegans or L. m. aucheri), whereas the opposite might be expected, following a north–south climatic gradient. Thus, the nominate subspecies L. m. meridionalis differed in diet from L. m. elegans or L. m. aucheri.

Decomposition of 13C-labelled plant material in a European 65–40° latitudinal transect of coniferous forest soils: simulation of climate change by translocation of soils

Standard 13C-labelled plant material was exposed over 2–3 yr at 8 sites in a north–south climatic gradient of coniferous forest soils, developed on acid and calcareous parent materials in Western Europe. In addition to soils exposed in their sites of origin, replicate units containing labelled material were translocated in a cascade sequence southwards along the transect, to simulate the effects of climate warming on decomposition processes. The current Atlantic climate represented the most favourable soil temperature and moisture conditions for decomposition. Northward this climatic zone, where the soil processes are essentially temperature-limited, the prediction for a temperature increase of 3°C estimated a probable increase of C mineralisation by 20–25% for the boreal zone and 10% for the cool temperate zone. Southward the cool Atlantic climate zone, (the Mediterranean climate), where the processes are seasonally moisture-limited, the predicted increase of temperature by 1–2°C little affected the soil organic matter dynamics, because of the higher water deficit. A significant decrease of C mineralisation rates was observed only in the superficial layers recognised in Mediterranean forest soils as ‘xeromoder’ and subject to frequent dry conditions. In the deeper Mediterranean soil organic horizons (the mull humus types), representing the major C storage in this zone, C mineralisation was not affected by a simulated 2°C temperature increase. The temperature effect is probably counteracted by a higher water deficit.

Nutrient conditions in drained peatlands along a north-south climatic gradient in Sweden

Soil nutrient conditions were investigated in a series of five peatland afforestation trials (established 1971) along a north-south gradient in Sweden. All areas were drained low-sedge mires and planted with Scots pine ( Pinus sylvestris (L.)). Soil samples from the topsoil (0–20 cm) were collected 22 years after drainage with two ditch spacings (7.5 and 60 m) and PK-fertilization with three fertilization regimes. Total and available nutrient concentrations were determined. Results indicated that pH and total concentrations of N, P and Ca in the topsoil of extensively drained areas increased with decreasing temperature sum in similar site types. Concentrations of K and Mg were not correlated to temperature sum. Tree height and volume were positively correlated to concentrations of P, K and Ca, but not to N, and only for temperature sums ≥ 950 d.d. (day degrees). Hence, it appeared that P and/or K directly or through their effects on N mineralization, limited tree growth in the south, whereas another growth factor was limiting in the north. We examined the available/total concentration ratios for the soil nutrients, with the intention to reveal possible climate-dependent trends in those ratios. The available/total ratios of P and Ca were found to increase with increasing temperature sum. For Ca in southern areas the ratios were higher in intensively drained and PK-fertilized plots than in the extensively drained and unfertilized treatment, possibly indicating that peat mineralization had been more intensive. The Ca-ratios for the two northernmost areas, which showed little or no tree growth reaction, did not increase even in the intensively drained and fertilized treatments. We hypothesise that the treatment-induced changes in available/total Ca ratio along the climate gradient also reflect post-drainage intensity of N-mineralization. Hence, the absence of increased Ca-ratios at temperature sums below 950 d.d. may indirectly indicate that tree growth is limited by the microbial release of organically bound N.

Comparison of Conventional and Targeted Insecticide Application for Control of Smokybrown Cockroaches (Dictyoptera: Blattidae) in Three Urban Areas of Alabama

Targeted and conventional (perimeter spray) insecticide application strategies were compared in 3 geographically distinct urban areas of Alabama. Cockroach abundance in sticky traps did not correspond to the known north-south climate gradient in Alabama. Greatest mean catch was in Mobile, and least in Auburn. Despite varying environmental and ecological conditions across Alabama, similar reductions and recovery rates of smokybrown cockroach abundance were observed in all 3 urban areas for 3 different treatments (perimeter spray, targeted spray, and 2-bait combination). A 2-bait combination was more effective in reducing smokybrown cockroach catch than a conventional perimeter spray, whereas a 3-liter targeted spray performed as well as perimeter sprays. Less insecticide was applied using targeted applications than perimeter sprays. Targeted applications of either baits or sprays did not require significantly more time to apply than a perimeter spray.