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1.
We analysed the relationship between plant species richness and productivity on first-year-old fields at two similar sites in central Europe. At both sites, a wide range of productivity levels was available resulting from different long-term fertilisation. In order to identify underlying mechanisms of the species richness–productivity relationship we included the seasonal dynamics and the number of individuals of each species in our analysis. We sampled 10 and 21 plots, respectively, at the two sites in May, June and July by harvesting all aboveground parts of vascular plants in 0.25 m2 subplots. Species richness, number of individuals of each species and community biomass as a surrogate of productivity were recorded in each sample.At one site, the relationship between species richness and biomass was significantly positive in the May and June harvest. This relationship disappeared in the July harvest due to a reduction in species richness at high productivity levels. The relations between species richness and number of individuals and between number of individuals and biomass paralleled the species richness–productivity relation but the individual number–biomass relationship remained positive until the last harvest. Between-species differences in individual number–community biomass relationships and their seasonal dynamics revealed “interspecific competitive exclusion” even though the species richness–biomass relationships were not negative or hump-shaped. At the second site, species richness was not related to productivity or to number of individuals. Our study demonstrated the importance of temporal dynamics and regional processes in understanding species richness–productivity patterns. 相似文献
2.
We have studied the underlying factors responsible for the heterogeneous ecophysiological status of a semiarid Stipa tenacissima L. steppe in a subcatchment of SE Spain by assessing population composition and habitat structure of S. tenacissima stands. To do this, we measured and estimated 18 variables (11 biotic and seven abiotic) in 15 plots randomly distributed
in the subcatchment, and then zoned this area by plot affinity using PCA. This analysis produced three sectors determined
mainly by S. tenacissima cover and soil depth variables. The linear relationship fitted between S. tenacissima tussock biomass and tussock density in monospecific stands (both logarithmic) indicated a curve close to −1, suggesting that
the system is close to the maximum constant yield state. Ecophysiological measurements (gas exchange, fluorescence and individual
leaf area index) were taken in two periods with different water availability in a representative plot in each sector. The
intraspecific competition (inferred from the density dependence of green biomass) and rock outcrops were the main factors
influencing the ecophysiological status in the study area. While, in the wet season, intraspecific competition regulated water
consumption in zones where S. tenacissima tussocks (monospecific stands) are dominant, during the dry season, stands in zones with extensive rock outcrops and stone
cover (tussocks in “soil pockets”) had no access to non-rainfall water gains because of the adjacent bare soil, and so in
these stands, gas exchange was lower and photoinhibition higher. This article stresses the importance of considering the connection
between tussocks and bare-ground interspace in the functional and structural analysis of semiarid steppes. 相似文献
3.
We simulated the self-thinning of Rhizophora mangle mangrove forests with the spatially explicit simulation model KiWi. This model is an application of the field-of-neighbourhood (FON) approach, which describes an individual tree by a competition function defined on the zone of influence (ZOI) around the stem. The FON causes growth depression of the trees involved. Sustained growth depression results in tree death. We propose a subdivision of the biomass density trajectories (bdt), obtained during the thinning process, into four segments related to characteristic shapes of the stem diameter distribution of the cohort. A positive skewness of the stem diameter distribution, indicating that the majority of the individuals are small and hindered in growth, is necessary for the occurrence of a linear segment within the bdt, the so-called 'self-thinning line'. This segment is the third bdt segment according to our classification. It is reached when the positive skewness of the stem diameter distribution is maximal and ends when the skewness reaches its second zero transition. The thinning line is therefore linked to the homogenisation process, which forces the symmetry of the stem distribution. We show that the ongoing search for a universal slope for the linear segment of the biomass-density trajectory (bdt) cannot succeed, since it is species-specific and may also be site-dependent. The slope increases with increasing competition strength of the individuals. Nevertheless, the lower limit of the slope is pre-defined by geometrical constraints and modified by the actual strength of the neighbourhood competition. Although the simulations were all carried out with growth parameters of the mangrove species Rhizophora mangle, our results should be qualitatively valid and form a plausible theoretical framework to account for different biomass-density trajectories. 相似文献
4.
杉木林自疏过程密度调节规律的研究 总被引:18,自引:0,他引:18
提出了森林自然稀疏过程中密度调节规律新模型,应用收缩扩张算法以山杨、云南松等树种森林自疏过程中密度资料对新模型进行了验证。验证结果表明:所提出的森林自疏规律模型能很好拟合实际的观测资料,具有良好的使用价值;所采用的非线性方程最优拟合方法是科学的,从而丰富了该领域的研究方法。交提出的森林自然稀疏过程密度调节规律模型应用于杉木林自疏过程密度变化规律研究,效果很理想,可为杉木林经营管理提供参考。 相似文献
5.
Regrowth after clipping and the effect of local competition were studied in a natural population of Erica multiflora in a Mediterranean shrubland, by removing neighbours at 1 and 2 m around the target plants during four growing seasons. Removal of surrounding natural vegetation increased the number, the density (number of sprouts per stump area) and the biomass of the sprouts growing from clipped plants. Target plants ònly interacted with their near neighbours. Target plants had a negative relative increment in the number of sprouts per stump during the 18 months immediately following treatment, but a positive increment thereafter, which suggests that there was a constant or episodic recruitment of sprouts within the stump after clipping. Competition treatment had a non-significant effect on the negative increment of sprouts per stump. The self-thinning trajectory was different for the different competition treatments: there was an allometric negative relationship between density of sprouts and mean biomass of survivors during all sampling periods in genets without neighbours in a 1-m radius; the self-thinning trajectory of sprouts in genets without neighbours in a 2-m radius was short, a net increase in sprouts per stump area was accompanied by an increase in mean sprout biomass 30 months after clipping. During the same period, however, plants with neighbours showed a decline in both the sprout biomass and density. 相似文献
6.
Structure, above-ground biomass and dynamics of mangrove ecosystems: new data from French Guiana 总被引:5,自引:0,他引:5
The article presents new results on the structure and the above-ground biomass of the various population types of mangroves
in French Guiana. Nine mangrove stands were studied, each composed of three to ten adjoining plots with areas that varied
depending on the density of the populations. Structural parameters and indices were calculated. Individuals representative
of the three groups of taxa present were felled:Avicennia germinans (L) Stearn, Rhizophora spp., and Laguncularia racemosa (L) Gaertn. The trunks, branches and leaves were sorted and weighed separately. The biomass was obtained by determining the
allometric relationships, the general equation selected being of the type y = a
o
x
a1, where the diameter (x) is the predictive variable. The total above-ground biomass varied from 31 t ha−1 for the pioneer stages to 315 t ha−1 for mature coastal mangroves, but with large variations depending on the structural characteristics at each site. The results
place the Guianese mangroves among those with high biomass, although lower than those reported for Asia. Based on the relationships
between structural parameters and standing biomass, in particular with the use of the “self-thinning rule”, population dynamics
models are proposed.
Received: 16 August 1996 / Accepted 17 January 1998 相似文献
7.
格氏栲种群调节模型的研究 总被引:22,自引:1,他引:21
在前人对植物种群自然稀疏过程的研究基础上,结合自适应通用增长模型,推导出一种能描述密度变化全过程的理论模型,lnN=aln^2S+blnS+c,其中N和S分别为种群密度和单株基面积,a、b、c为参数。 相似文献
8.
Markku Larjavaara 《Journal of theoretical biology》2010,265(1):63-124
Wind routinely topples trees during storms, and the likelihood that a tree is toppled depends critically on its allometry. Yet none of the existing theories to explain tree allometry consider wind drag on tree canopies. Since leaf area index in crowded, self-thinning stands is independent of stand density, the drag force per unit land can also be assumed to be independent of stand density, with only canopy height influencing the total toppling moment. Tree stem dimensions and the self-thinning biomass can then be computed by further assuming that the risk of toppling over and stem maintenance per unit land area are independent of stand density, and that stem maintenance cost is a linear function of stem surface area and sapwood volume. These assumptions provide a novel way to understand tree allometry and lead to a self-thinning line relating tree biomass and stand density with a power between −3/2 and −2/3 depending on the ratio of maintenance of sapwood and stem surface. 相似文献
9.
10.
The self-thinning process was monitored in crowded Kandelia obovata Sheue, Liu & Yong stands over four years. The frequency distribution of tree phytomass was an L-shape, which was kept over
the experimental period. Spearman’s rank correlation coefficient for phytomass decreased as the time span of the comparison
became longer, a result which indicates that the rank of phytomass changes as stands grow. Death of trees resulted from one-sided
competition, i.e., death occurred in lower-rank trees. Surviving trees continued to grow. Whatever the current spatial distribution
of the trees, death occurred randomly and the spatial distribution gradually became close to random as stands grew. The self-thinning
exponent was 1.46, which can be regarded as evidence in favor of the 3/2 power law of self-thinning. Relative growth rate,
RGR, decreased in proportion to decreasing relative mortality rate, RMR, with a proportionality constant of 1.57, which was
not significantly different from the slope of the self-thinning exponent. This experimental result probably justifies the
assumption that the ratio of RGR to RMR in the mean phytomass-density trajectory for any self-thinning population with different
densities becomes constant as the growth stage progresses. 相似文献