RESUMEN
Shrub encroachment has emerged as a global phenomenon over the past century. Multiple drivers have been put forward to explain the increased shrub dominance in various ecosystems around the world. However, the potential role of phenology in regulating shrub encroachment is not well understood. We address this issue using 3-year continuous monitoring of the phenology of coexisting shrubs and grasses combined with observations of ecohydrological processes (water uptake) and soil conditions (root zone soil moisture, soil texture, and soil temperature) at four study sites in Inner Mongolia, China, with shrub coverage of Caragana microphylla ranging from 0%, to 6.8%, 26.8% and 34.2%. Along such an encroachment gradient, shrubs exhibited progressively earlier onsets and later ends of the growing season, with an overall extension in growing season length by 15â¯days to 22â¯days in the later stages of shrub encroachment. Conversely, the coexisting grasses showed earlier occurrences both in spring and autumn phenological phases, which resulted in a phenological gap between shrubs and grasses. Thus, a positive feedback could exist between these phenological changes and shrub encroachment. In shrub patches, soils were wetter, with finer texture, and with more suitable temperatures for plant survival and development, which favored the lengthening of growing season of shrubs. The longer growing seasons are associated with longer periods of water use and photosynthesis for shrubs, and better opportunities for water uptake, with the overall effect of facilitating shrub growth and further expansion.
Asunto(s)
Caragana/fisiología , Ecosistema , Poaceae , China , Monitoreo del Ambiente , Retroalimentación Fisiológica , Hidrología/métodos , Isótopos de Oxígeno/análisis , Estaciones del Año , Suelo/química , Temperatura , Agua/análisisRESUMEN
Revegetation facilitated by legume species introduction has been used for soil erosion control on the Loess Plateau, China. However, it is still unclear how vegetation and soil resources develop during this restoration process, especially over the longer term. In this study, we investigated the changes of plant aboveground biomass, vegetation cover, species richness and density of all individuals, and soil total nitrogen, mineral nitrogen, total phosphorus and available phosphorus over 11 years from 2003 to 2013 in three treatments (natural revegetation, Medicago sativa L. introduction and Melilotus suaveolens L. introduction) on the semi-arid Loess Plateau. Medicago significantly increased aboveground biomass and vegetation cover, and soil total nitrogen and mineral nitrogen contents. The Medicago treatment had lower species richness and density of all individuals, lower soil moisture in the deep soil (i.e., 1.4-5m), and lower soil available phosphorus. Melilotus introduction significantly increased aboveground biomass in only the first two years, and it was not an effective approach to improve vegetation biomass and cover, and soil nutrients, especially in later stages of revegetation. Overall, our study suggests that M. sativa can be the preferred plant species for revegetation of degraded ecosystems on the Loess Plateau, although phosphorus fertilizer should be applied for the sustainability of the revegetation.
Asunto(s)
Conservación de los Recursos Naturales , Fabaceae/fisiología , Especies Introducidas , Suelo/química , Agricultura , China , Medicago sativa , Nitrógeno/análisis , Fósforo/análisisRESUMEN
Past studies have widely documented the decrease in species diversity in response to addition of nutrients, however functional diversity is often independent from species diversity. In this study, we conducted a field experiment to examine the effect of nitrogen and phosphorus fertilization ((NH4)2 HPO4) at 0, 15, 30 and 60 g m-2 yr-1 (F0, F15, F30 and F60) after 4 years of continuous fertilization on functional diversity and species diversity, and its relationship with productivity in an alpine meadow community on the Tibetan Plateau. To this purpose, three community-weighted mean trait values (specific leaf area, SLA; mature plant height, MPH; and seed size, SS) for 30 common species in each fertilization level were determined; three components of functional diversity (functional richness, FRic; functional evenness, FEve; and Rao's index of quadratic entropy, FRao) were quantified. Our results showed that: (i) species diversity sharply decreased, but functional diversity remained stable with fertilization; (ii) community-weighted mean traits (SLA and MPH) had a significant increase along the fertilization level; (iii) aboveground biomass was not correlated with functional diversity, but it was significantly correlated with species diversity and MPH. Our results suggest that decreases in species diversity due to fertilization do not result in corresponding changes in functional diversity. Functional identity of species may be more important than functional diversity in influencing aboveground productivity in this alpine meadow community, and our results also support the mass ratio hypothesis; that is, the traits of the dominant species influenced the community biomass production.
Asunto(s)
Cyperaceae/efectos de los fármacos , Nitrógeno/farmacología , Fósforo/farmacología , Hojas de la Planta/efectos de los fármacos , Poaceae/efectos de los fármacos , Ranunculaceae/efectos de los fármacos , Biodiversidad , Biomasa , Cyperaceae/clasificación , Cyperaceae/crecimiento & desarrollo , Fertilizantes , Pradera , Humanos , Filogenia , Hojas de la Planta/crecimiento & desarrollo , Poaceae/clasificación , Poaceae/crecimiento & desarrollo , Ranunculaceae/clasificación , Ranunculaceae/crecimiento & desarrollo , TibetRESUMEN
Fertilization via nutrient deposition and agricultural inputs is one of the most important factors driving decreases in plant diversity. However, we still do not fully understand which processes (niche process or neutral process) are more important in leading to decreases in plant diversity caused by fertilization. A hypothesis-based approach was used to test the relative importance of niche versus neutral processes along a fertilization gradient in an alpine meadow community on the eastern Tibetan plateau, China. Niche overlap values were calculated for species biomass, and the null model was used to generate the values of niche overlap expected at random. A linear regression modeling was used to evaluate the relationship between functional traits (specific leaf area, leaf dry matter content, and leaf total nitrogen concentration) and species relative abundance. Our results demonstrated that observed niche overlap for species biomass was significantly higher than expected at lower fertilization gradients. Moreover, we also found a significantly negative correlation between species relative abundance and specific leaf area and leaf dry matter content, but a significantly positive correlation between relative abundance and leaf nitrogen concentration at lower fertilization gradients. However, these relationships were not significant at higher fertilization gradients. We concluded that community assembly is dynamic progression along the environmental gradients, and niche and neutral processes may together determine species diversity loss in response to fertilization.
Asunto(s)
Nitrógeno/farmacología , Plantas/clasificación , Plantas/efectos de los fármacos , Biodiversidad , Biomasa , China , Ecosistema , Fertilizantes/análisis , Modelos Lineales , Hojas de la Planta/efectos de los fármacos , Hojas de la Planta/crecimiento & desarrolloRESUMEN
Understanding the relationships between vegetative and environmental variables is important for revegetation and ecosystem management on the Loess Plateau, China. Lucerne (Medicago sativa L.) has been widely used in the region to improve revegetation, soil and water conservation, and to enhance livestock production. However, there is little information on how environmental factors influence long-term succession in lucerne-rich vegetation. Our objective was to identify the main environmental variables controlling the succession process in lucerne-rich vegetation such that native species are not suppressed after sowing on the Loess Plateau. Vegetation and soil surveys were performed in 31 lucerne fields (three lucerne fields without any management from 2003-2013 and 28 fields containing 11-year-old lucerne with one cutting each year). Time after planting was the most important factor affecting plant species succession. Cutting significantly affected revegetation characteristics, such as aboveground biomass, plant density and diversity. Soil moisture content, soil organic carbon, soil available phosphorus and slope aspect were key environmental factors affecting plant species composition and aboveground biomass, density and diversity. Long-term cutting can cause self-thinning in lucerne, maintain the stability of lucerne production and slow its degradation. For effective management of lucerne fields, phosphate fertilizer should be applied and cutting performed.