Characteristics Of Vegetation Communities And Nitrogen And Phosphorus Availability In Degraded Ecosystems In Karst Region In Guizhou, China

Rocky desertification, mainly induced by rapid decline in vegetation cover, has resulted in severe ecological and environmental problems such as erosion of biodiversity, and destruction of habitat, soil degradation in the karst land dominantly in central and southwestern Guizhou in southwest China. Guizhou is located in the center of a widespreaded karst region in southweatern China. Due to the continuous and intensive anthropogenic disturbances (e.g. overgrazing, tree-cut, cultivation), forest stands have been extensively transformed into grasslands or deserts in present study region in the past decades. However, karst ecosystem is an ecosystem controlled by karst environment, effects of irregular deforestation both on biodiversity conservation and on ecosystem functioning has not received sufficient and synthetic analysis.The changes of vegetation quantitive properties and functionings of ecosystems are weak nowadays in the study of degradation and recovery of karst ecosystem. To address the relations between vegetation variables, nutrient availability and ecosystem fucntioning, the plant diversity, partitioning of biomass, major nutrients among the different vegetation types were studied by a pilot study using a degradation sequence of three karst ecosystems with different degree of vegetation decline from Puding County, Central Guizhou, South West China. The three ecosystems, namely forest stand (FO), shrubland (SH) and shrub-and-grassland (SHG), were identified on the canopy cover, community height and tree density. The purposes of the present study are to set bases for further research on biogeiochemical cycling of bioelements in soil-plant systems in karst regions, to guide ecological and envitonmental protection and restoration. The results obtained are as follows:1 Plant diversity and community structureThere was a decreasing trend both in community structure and biodiversity with the degradation trend of the vegetations. The species quantity was significantly greater in the forest stand (FO) and in the shrubland stand (SH) than in the shrub-and-grassland stand (SHG), but not difference between the FO and the SH, while the woody species quantity was in order of FO> SH> SHG. The diversity indices (n) presented as SH> FO> SHG, and diversity indices (Ⅳ) as FO> SH> SHG. The Shannon indices on shrub layers were higher in the FO and in the SH than that in the SHG (P<0.01), not difference between the FO and the SH, but it was larger on herb layers in the SH than in the FO (P<0.01), without differences between the SHG and either the FO or the SH. The populations of main shrubs on shrub layers both in the FO and in the SHG fell into clumped distribution patterns, but they were randomly distributed in the SH.2 Partitioning of vegetation biomass and nutrient distributionThe results showed that:(1) there was a decreasing trend both in vegetation biomass above ground and in soil nutrient availability as well as nutrient returning through littering, an increasing trend of fine root biomass and herbaceous biomass with the degradation trend of the ecosystems. (2) The nutrient accumulations in ecosystem components varied with the vegetation biomass pattern. The N and P accumulations in forest floor organic matter were significantly higher than that in herbaceous layer and in fine roots in the FO, while the N and P accumulations in herbaceous layer and in fine roots were higher than or similar to that in floor organic matter in the SH and the SHG. (3) There was a sharp decline in nutrient uptake and decomposition percentage, with an increase in percentage of nutrient translocated and returned to soil with the increasing degradation in vegetation from forest to grassland. In particular, the bio-migration and bio-return ratios of N was in the order of SHG> SH> FO, but its uptake and decomposition ratio in order of FO> SHG> SH and FO> SH> SHG respectively. Similarly, the migration and decomposition ratios of P in FO > SHG> SH and FO> SH> SHG, and bio-absorption ratio as SH> FO> SHG, and its bio-return ratio in order of SH> SHG> FO.The results showed that the nutrient return to soil through litterfall was gradually decreased, whereas the biomass of fine roots increased gradually with the increasing trend of vegetation decline. This suggested that major nutrients such as N have undergone a relatively intense translocation and been exhausted in rhizospheric zone for their use in vegetable parts due to loss under vegetation decline in degraded ecosystems. Compared with the forest stand, the shrubland and the shrub-and-grassland have been vulnerable due to lower capability in self-sustaining and regulating under serious disturbances. Finally, it is suggested that the biogeochemical characterization of major nutrients would be an important sector for the functional stability of karst ecosystems.3 Nutrient limitation in soils on the growth of dominant woody speciesTo assess the patterns of nutrient limitation in the subtropical karst ecosystems in Guizhou province, southwestern China, we quantified soil nutrient concentrations and leaf chemicals of overstorey plants such as nutrient concentrations, stoichiometry (N/P ratios) across the three sites. The results showed:(1) that community N/P ratios increased from the FO, through the SH to the SHG. The lowest N/P ratio occurred at the FO (16.1±5.1) with species combined, intermediate at the SH (25.1±7.3), and highest at the SHG (33.5±9.6); (2) that the indirect measures of this study, e.g. N/P ratios and nutrient resorption proficiency, consistently indicated that P is cycled much more efficiently than N across this karst environments. This study demonstrated that the karst vegetations were generally at P-limited or N-and P-co-limited relative to N/P ratios and nutrient resorption proficiency.4 Nutrient use efficiency and ecological characteristics of fine rootsThe nutrient resorption proficiency (RP) and fine root properties have been used to assess the adaptation strategy of plants under nutrient stresses. The nutrient reuse effieicecies increased with the decline of N and P availability in soils. The nitrogen and phosphorus resorption proficiencies (NRP and PRP) ranged from 6.5 to 25.2 mg g-1 and from 0.3 to 1.2 mg g-1 respectively, the average value of which was lowest at the SHG (mean=10.4 mg g-1 and 0.5 mg g-1 for NRP and PRP, respectively) and highest at the FO (mean=22.4 mg g-1 and 0.9 mg g-1 for NRP and PRP, respectively). Although there was not difference in phosphorus resorption efficiencies (PRE) among species and across the sites, the N and P concentrations in senesced leaves were positively correlated respectively with the N and P concentrations in mature leaves for all the species pulled togetherGenerally, fine root biomass was dominantly distributed in soil depth of 0-10 cm, sharply decreasing with soil depth. Living fine root biomass in 0-10 cm possessed 83.36%, 86.91% and 93.79% of the total fine root biomass within depth of 0-10 cm while it possessed 42.78%,56.75% and 53.38% of the total root biomass of the same depth respectively under the forest, the shrub and the shrub-and-grass stands. Aboveground biomass of the three stands was found correlated with the living fine root biomass in 0-10 cm and mean N and P contents of plant leaves was in a significant correlation with specific length of living fine root for the three stands investigated. Therefore, nutrient uptake and retention by the living fine roots may be of particular importance in aboveground biomass establishment and ecosystem functioning.