湖北九宫山成熟常绿落叶阔叶混交林3种优势种的邻域效应研究

以最近邻株距离统计研究了湖北九宫山成熟常绿落叶阔叶混交林3种优势种青冈栎(Cyclobalanopsis glauca)、甜槠(Castanopsis eyrei)和短柄枹栎(Quercus serrata var.brevipetiolata)的邻域效应,进而探讨了局域尺度上的竞争/帮促关系与物种共存格局.结果显示,青冈栎、甜槠和短柄枹栎的最近邻株距离不存在下限;青冈栎的胸径大小与其种内最近邻株距离存在正相关(p=0.029),甜槠的胸径大小与种内、种间最近邻株距离均无相关性(p≥0.360),而短柄枹栎的胸径大小与种间最近邻株距离为负相关(p=0.040);3树种中任意一个的种内与种间最近邻株距离都没有显著差异(p≥0.122).这些结果表明,九宫山成熟常绿落叶阔叶混交林内邻域尺度上的竞争排斥并未充分激化,但是青冈栎的种内竞争仍然在起作用,而甜槠的竞争效应不明显,短柄枹栎则依赖种间帮促甚于相互竞争.同种和异种之间的排斥效果没有差异表明局域尺度的物种共存格局可能出于随机过程而非负密度制约过程.     

Positive associations between the cushion plant Arenaria polytrichoides (Caryophyllaceae) and other alpine plant species increase with altitude in the Sino‐Himalayas

Question: Does the facilitative effect of cushion plants increase with elevation as a result of increases in environmental harshness? Does this hypothesis apply in the Sino‐Himalayan Mountains? Location: Lakaka Pass on the Baima Snow Mountains (28°20′N, 99°05′E), SW China. Methods: We evaluated the spatial association of several plant species with the cushion plant Arenaria polytrichoides (Caryophyllaceae) at two elevations (4500 m and 4700 m) in the study site and monitored temperature, moisture and nutritional status of soil beneath and outside the cushions. Results: While 14 species grow more frequently associated with the cushions at the higher elevation, at the lower site only three species were positively associated with cushions. Eleven of the species that occurred at both elevations changed their spatial association from neutral or negative with cushions at the lower site to positive at the higher elevation site. Substrate temperatures were rather similar between the cushions and areas of bare ground. Cushions maintained higher moisture than areas of bare ground at both elevations. Soils beneath cushions contained significantly more available nitrogen and potassium compared to open areas at the higher elevation. Conclusions: Our results show that facilitation by A. polytrichoides cushions increases with elevation in the Sino‐Himalayan region. This facilitation effect of A. polytrichoides cushions is probably due to the improved nutrient availability provided by cushion plants in the higher elevation, and these conditions probably permit increased plant recruitment, growth and survival.     

Spatial patterns of desert annuals in relation to shrub effects on soil moisture

Questions: What are the effects of a shrub (Haloxylon ammodendron) on spatial patterns of soil moisture in different seasons? How does productivity of understorey annuals respond to these effects? Are such effects always positive for annuals under shrubs? Location: South Gurbantunggut Desert, northwest China. Methods: Using geostatistics, we explored seasonal patterns of topsoil moisture in a 12 × 9‐m plot over the growing season. To determine spatial patterns of understorey annuals in response to H. ammodendron presence, biomass of annuals was recorded in four 0.2 × 5.0‐m transects from the centre of a shrub to the space between shrubs (interspace). We also investigated vertical distribution of root biomass for annuals and soil moisture dynamics across soil profiles in shrub‐canopied areas and interspaces. Results: Topsoil moisture changed from autocorrelation in the wet spring to random structure in the dry season, while soil moisture below 20 cm was higher in shrub‐canopied areas. Across all microhabitats, soil moisture in upper soil layers was higher than in deeper soil layers during the spring wet season, but lower during summer drought. Topsoil was close to air‐dry during the dry season and developed a ‘dry sand layer’ that reduced evaporative loss of soil water from deeper layers recharged by snowmelt in spring. Aboveground biomass of understorey annuals was lowest adjacent to shrub stems and peaked at the shrub margin, forming a ‘ring’ of high herbaceous productivity surrounding individual shrubs. To acclimate to drier conditions, annuals in interspaces invested more root biomass in deeper soil with a root/shoot ratio (R/S) twice that in canopied areas. Conclusions: Positive and negative effects of shrubs on understorey plants in arid ecosystems are commonly related to nature of the environmental stress and tested species. Our results suggest there is also microhabitat‐dependence in the Gurbantunggut Desert. Soil water under H. ammodendron is seasonally enriched in topsoil and deeper layers. Understorey annuals respond to the effect of shrubs on soil water availability with lower R/S and less root biomass in deeper soil layers and develop a ‘ring’ of high productivity at the shrub patch margin where positive and negative effects of shrubs are balanced.     

Tree invasion of a montane meadow complex: temporal trends,spatial patterns,and biotic interactions

Questions: Do spatial and temporal patterns of encroachment of Pinus contorta and Abies grandis in a montane meadow suggest strong biotic controls on the invasion process? Location: Forest–meadow mosaic, 1350 m a.s.l., Cascade Range, Oregon, US. Methods: We combined spatial point pattern analysis, population age structures, and a time‐series of stem maps to quantify spatial and temporal patterns of conifer invasion over a 200‐yr period in three plots totaling 4 ha. Results: Trees established during two broad, but distinct periods (late 1800s, then at much greater density in the mid‐1900s). Recent invasion was not correlated with climatic variation. Abies grandis dominated both periods; P. contorta established at lower density, peaking before A. grandis. Spatially, older (≥90 yr) P. contorta were randomly distributed, but older A. grandis were strongly clumped (0.2‐20 m). Younger (<90 yr) stems were positively associated at small distances (both within and between species), but were spatially displaced from older A. grandis, suggesting a temporal shift from facilitation to competition. Establishment during the 1800s resulted in widely scattered P. contorta and clumps of A. grandis that placed most areas of meadow close to seed sources permitting more rapid invasion during the mid‐1900s. Rapid conversion to forest occurred via colonization of larger meadow openings – first by shade‐intolerant P. contorta, then by shade‐tolerant A. grandis– and by direct infilling of smaller openings by A. grandis. Conclusions: In combination, spatial and temporal patterns of establishment suggest an invasion process shaped by biotic interactions, with facilitation promoting expansion of trees into meadows and competition influencing subsequent forest development. Once invasion is initiated, tree species with different life histories and functional traits can interact synergistically to promote rapid conversion of meadow to forest under a broad range of climatic conditions.     

The role of nurse functional types in seedling recruitment dynamics of alternative states in rangelands

In arid ecosystems, recruitment dynamics are limited by harsh environmental conditions and greatly depend on the net outcome of the balance between facilitation and competition. This outcome can change as a consequence of degradation caused by livestock overgrazing. Also, distinct plant species may show a differential response to a common neighbour under the same environmental conditions. Therefore, ecosystem degradation could affect the net balance of plant-plant interactions, which can also depend on the functional traits of potential nurse species. The aim of this study is to assess the influence of alternative degradation states on (i) the density of seedlings of perennial species emerging in four microsite types, and on (ii) the relative interaction intensity (RII) between seedlings and potential nurses belonging to three functional types (deep- and shallow-rooted shrubs, and tussock grasses). During three years, we recorded seedling density of perennial species in four alternative degradation states in grass-shrubby steppes from northwestern Patagonia. The density of emerged seedlings of perennial species decreased sharply as degradation increased, showing non-linear responses in most microsites. Seedling density underneath deep-rooted shrubs was higher than underneath shallow-rooted shrubs and tussock grasses. Also, deep-rooted shrubs were the only functional type that recorded seedling emergence in highly degraded states. Deep-rooted shrubs had facilitative effects on the seedlings emerging and surviving underneath them, independently of ecosystem degradation. In contrast, RII between shallow-rooted shrubs and recently emerged seedlings, switched from positive effects in the less degraded states, to negative effects in the most degraded state. Tussock grasses recorded the weakest intensity of facilitative interactions with recently emerged seedlings, switching to competitive interactions as degradation increased. Our results suggest that species with key functional traits should be considered in management and restoration plans for rangelands with different degradation levels, since they have a strong influence in the net outcome of plant-plant interactions and in the recruitment dynamics of arid ecosystems.     

Safe sites for tree regeneration in wooded pastures: A case of associational resistance?

Abstract. Question: Are tree saplings in wooded pastures spatially associated with specific nurse structures or plants that facilitate tree sapling survival? Location: Wooded pastures in the Jura Mountains, Switzerland. Methods: In two sites, 73 km apart, we sampled 294 plots of 4 m², systematically distributed on 1 ha. We recorded number and height of all established Picea abies saplings (> 1 a of age and up to 40 cm in height), and visually estimated cover of rocks, shrubs, tree stumps, overhanging tree branches and unpalatable plant species. Results: Despite differences in site characteristics, we found overall positive effects of cover of unpalatable plants, rocky outcrops and tree stumps on the density of Picea saplings. Plots with tree stumps and higher cover of rocky outcrops and unpalatable plants were more likely to contain Picea saplings. Conclusions: Unpalatable plants, rocky outcrops and tree stumps seem to form safe sites for Picea saplings in this grazed system, improving their establishment and survival. Our findings support the idea that associational resistance drives the dynamics of wooded pastures, but experimental evidence for this hypothesis is still required.