Many forest ecosystems have experienced recent declines in productivity; however, in some alpine regions, tree growth and forest expansion are increasing at marked rates. Dendrochronological analyses at the upper limit of alpine forests in the Tibetan Plateau show a steady increase in tree growth since the early 1900s, which intensified during the 1930s and 1960s, and have reached unprecedented levels since 1760. This recent growth acceleration was observed in small/young and large/old trees and coincided with the establishment of trees outside the forest range, reflecting a connection between the physiological performance of dominant species and shifts in forest distribution. Measurements of stable isotopes (carbon, oxygen, and nitrogen) in tree rings indicate that tree growth has been stimulated by the synergistic effect of rising atmospheric CO2 and a warming-induced increase in water and nutrient availability from thawing permafrost. These findings illustrate the importance of considering soil-plant-atmosphere interactions to understand current and anticipate future changes in productivity and distribution of forest ecosystems.
Ongoing climate variability strongly affects high-elevation forests, influencing the wood formation process (e.g., xylogenesis). Furthermore, spatio-temporal studies to establish links of wood properties and tree performance are needed. Using linear mixed-effects models, empirical cumulative distribution functions, and spatial analysis, we explore time trends and space connections of wood density of Pinus hartwegii Lindl. to remotely sensed variables (Moderate Resolution Imaging Spectro-radiometer MODIS-derived) in two high-elevation forests in México, Tláloc (TLA) and Jocotitlán (JOC) Mountains. Results indicated that elevation and cambial age effects are important factors explaining wood density variation. Minimum earlywood—MID, average—AVE, and maximum latewood density—MXD were statistically similar between mountains (p > 0.05), but TLA showed a significant increase in MID over time with higher values after 1950. Wood density values and spatial correlations were site-dependent with TLA exhibiting the highest correlations between MXD and the Normalized Difference Vegetation Index (NDVI) of the spring season (r = 0.59, p < 0.05). Overall, correlations to remotely sensed information were positive with MXD, negative for MID and divergent for AVE. Historical temperature defines MID along the elevation gradient, while MXD was related to soil moisture only at low-elevation sites where soils are deeper. We found that two high-elevation forests, 115 km away from each other, with similar climate, soil, and vegetation, behaved differently regarding their xylogenesis, indicating the potential of using the link between wood micro-density and remotely sensed information to understand forest response to climate change effects.
In spite of potential benefits and positive assessments of reducing primary tillage operations, only a small part of irrigated row crops is currently managed using reduced tillage, for reasons that include concerns about its agronomic suitability for certain crop rotations. Three years of a tomato/corn rotation under standard and no-tillage management were used to understand the fate of a fertilizer and cover crop nitrogen (N) application. Uptake of both inputs was reduced under no-tillage during the year of application, in this case a tomato crop. As a result, more input N was retained in the soil in this system. The initial challenge of reduced tomato yields diminished as no-tillage management remained in place and the soil N reservoir developed. Corn production was not affected by tillage treatment. Inclusion of a legume cover crop increased the amount of fertilizer N retained in the soil over time, more so under no-tillage than under standard tillage, emphasizing the benefit of cover crops in reducing the amount of fertilizer required to maintain productivity. While acceptance of reduced tillage ultimately depends on economic performance, the results of this study support its agronomic viability for irrigated row crops.
espanolEl incremento de CO 2 atmosferico puede influir en el crecimiento de la vegetacion provocando cierre estomatico, cambios en la eficiencia de uso de agua o un efecto de fertilizacion que lleva a mayor crecimiento. Sin embargo, la respuesta de la vegetacion depende de las condiciones ambientales y de la fisiologia de cada especie. El objetivo del presente estudio fue evaluar composicion de isotopos de carbono y oxigeno ( 13 C y 18 O), discriminacion neta de 13 C ( 13 C), tasa de crecimiento en area basal (IAB) y eficiencia de uso de agua intrinseca (iWUE) de Taxodium mucronatum Ten. en los ultimos 100 anos en tres regiones del centro de Mexico. Las hipotesis fueron: 1) la respuesta fisiologica de T. mucronatum Ten. a la variacion climatica es diferente entre regiones; 2) los cambios en la tasa de crecimiento en area basal y variables fisiologicas de ahuehuete sugieren efectos de fertilizacion en el ultimo siglo. Las regiones estudiadas fueron: Estado de Mexico (MEX), Queretaro (QRO) y Morelos (MOR) con precipitacion de 600, 750 y 1038 mm. En cada sitio se muestrearon nueve arboles para construir una cronologia comun. En los anillos de crecimiento se analizo 13 C y 18 O para estimar la discriminacion ( 13 C), concentracion interna de CO 2 ( C i ) y la iWUE en el ultimo siglo. Los datos se analizaron como muestras repetidas en el tiempo con espaciado variable. La diferencia entre sitios se probo con un analisis multivariado de la varianza (MANOVA) y pruebas de contrastes. El 13 C en la madera y C i variaron con el regimen de humedad en MEX y MOR. En las ultimas cinco decadas, 18 O y la temperatura media anual (TMA) aumentaron 4 ‰ y de 1.2 a 2.4 °C, respectivamente. Desde 1950, 13 C decrecio en 1.1 ‰ y la iWUE aumento 50 %. El incremento en Ca, iWUE y TMA se relaciono con la disminucion de IAB en el sito MEX, y aumento pequeno en IAB en QRO y MOR, lo que indica un efecto diferente de fertilizacion de CO 2 de acuerdo con el gradiente de humedad. EnglishThe increase of atmospheric CO 2 can influence the growth of vegetation causing stomatal closure, changes in the water- use efficiency, or a fertilization effect that leads to greater growth. However, the response of vegetation depends on the environmental conditions and physiology of each species. The objective of this study was to evaluate the composition of carbon and oxygen isotopes ( 13 C and 18 O), the net discrimination of 13 C ( 13 C), the growth rate in basal area (IAB) and the intrinsic water-use efficiency (iWUE) of Taxodium mucronatum Ten. (Montezuma baldcypress) over the last 100 years in three regions of central Mexico. The hypotheses were: 1) the physiological response of T. mucronatum Ten. to climatic variation is different among regions; 2) changes in the growth rate in basal area and physiological variables of the Montezuma baldcypress tree suggest that fertilization effects took place in the last century. The regions under study were: Estado de Mexico (MEX), Queretaro (QRO), and Morelos (MOR), with 600, 750, and 1038 mm rainfall. Nine trees per site were sampled to build a common chronology. In the growth rings, 13 C and 18 O were analyzed to estimate discrimination ( 13 C), internal CO 2 concentration ( C i ), and iWUE in the last century. The data were analyzed as samples repeated over time with variable spacing. The difference between sites was tested with a multivariate analysis of variance (MANOVA) and contrasts tests. The 13 C on wood and C i varied according to the moisture regime in MEX and MOR. In the past five decades, 18 O increased by 4 ‰ and the average annual temperature (AAT) increased from 1.2 to 1.4 ° C. Since 1950, 13 C decreased by 1.1 ‰ and iWUE increased by 50 %. The increase in Ca, iWUE, and AAT was related to the IAB decrease in MEX site, and a slight IAB increase in QRO and MOR, indicating a different CO 2 fertilization effect depending on the moisture gradient.
Chloroform fumigation-incubation (CFI) has made possible the extensive characterization of soil microbial biomass carbon (C) (MBC). Defining the non-microbial C mineralized in soils following fumigation remains the major limitation of CFI. The mineralization of non-microbial C during CFI was examined by adding 14 C-maize to soil before incubation. The decomposition of the 14 C-maize during a 10-d incubation after fumigation was 22.5% that in non-fumigated control soils. Re-inoculation of the fumigated soil raised 14 C-maize decomposition to 77% that of the unfumigated control. A method was developed which varies the proportion of mineralized C from the unfumigated soil (UF C ) that is subtracted in calculating CFI biomasss C. The proportion subtracted (P) varies according to a linear function of the ratio of C mineralized in the fumigated (F C ) and unfumigated samples (F C /UF C ) with two parameters K 1 and K 2 (P = K 1 F C /UF C ) + K 2 ). These parameters were estimated by regression of CFI biomass C, calculated according to the equation MBC = (F C − PUF C )/0.41, against that derived by direct microscopy in a series of California soils. Parameter values which gave the best estimate of microscopic biomass from the fumigation data were K 1 = 0.29 and K 2 = 0.23 (R 2 = 0.87). Substituting these parameter values, the equation can be simplified to MBC = 1.73F C − 0.56UF C . The equation was applied to other CFI data to determine its effect on the measurement of MBC. The use of this approach corrected data that were previously difficult to interpret and helped to reveal temporal trends and changes in MBC associated with soil depth. Key words: Chloroform fumigation-incubation, soil microbial biomass, microscopically estimated biomass, carbon, control, 14 C
La investigacion sobre la capacidad de los suelos tropicales de favorecer el crecimiento rapido de especies forestales es necesaria para optimizar, preservar y seleccionar tierras para la produccion de madera. En este trabajo el indice de sitio (SI) de las plantaciones de Eucalyptus grandis y E. urophylla en el sureste de Mexico se asocio a propiedades quimicas y fisicas. A partir de un modelo conjunto de SI vs. contenido de arena, el ajuste indica que SI mejora mientras el contenido de arena aumenta (p<0.01). El contenido de arena de 28% en la superficie del suelo (0-5 cm) y de 24% en la capa subyacente del suelo (5-30 cm) es el minimo deseado para establecer rodales de alta productividad. Las mejores condiciones de SI se encontraron en suelos arenosos con un contenido de arena de 65%. Se midieron tambien otras variables, tales como densidad aparente del suelo, mineralizacion de N y materia organica, pero no se correlacionaron con el crecimiento rodal o SI. La capacidad de intercambio cationico estuvo inversamente relacionada al crecimiento del arbol, lo cual se explico como una relacion indirecta ya que los suelos con mayor contenido de particulas finas del suelo (arcilla+limo) fueron menos productivos
Soil mineral assemblage influences the abundance and mean residence time of soil organic matter both directly, through sorption reactions, and indirectly, through influences on microbial communities. Though organo-mineral interactions are at the heart of soil organic matter cycling, current models mostly lack parameters describing specific mineral assemblages or phases, and treat the mineral-bound pool as a single homogenous entity with a uniform response to changes in climatic conditions. We used pyrolysis GC/MS in combination with stable isotopes and radiocarbon abundance to examine mineral-bound soil organic matter fractions from a lithosequence of forest soils. Results suggest that different mineral assemblages tend to be associated with soil organics of specific molecular composition, and that these unique suites of organo-mineral complexes differ in mean residence time. We propose that mineralogy influences the composition of the mineral-bound soil organic matter pool through the direct influence of mineral surface chemistry on organo-mineral bond type and strength in combination with the indirect influence of soil acidity on microbial community composition. The composition of the mineral-bound pool of soil organic matter is therefore partially dictated by a combination of compound availability and sorption affinity, with compound availability controlled in part by microbial community composition. Furthermore, results are suggestive of a preferential sorption of N-containing moieties in Fe-rich soils. These bonds appear to be highly stable and confer extended mean residence times.
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