Collins Scott L, Smith Melinda D
Department of Biology, Castetter Hall, University of New Mexico, Albuquerque, New Mexico 87131, USA.
Ecology. 2006 Aug;87(8):2058-67. doi: 10.1890/0012-9658(2006)87[2058:siofag]2.0.co;2.
Natural disturbances affect spatial and temporal heterogeneity in plant communities, but effects vary depending on type of disturbance and scale of analysis. In this study, we examined the effects of fire frequency (1-, 4-, and 20-yr intervals) and grazing by bison on spatial and temporal heterogeneity in species composition in tallgrass prairie plant communities. Compositional heterogeneity was estimated at 10-, 50-, and 200-m2 scales. For each measurement scale, we used the average Euclidean Distance (ED) between samples within a year (2000) to measure spatial heterogeneity and between all time steps (1993-2000) for each sample to measure temporal heterogeneity. The main effects of fire and grazing were scale independent. Spatial and temporal heterogeneity were lowest on annually burned sites and highest on infrequently burned (20-yr) sites at all scales. Grazing reduced spatial heterogeneity and increased temporal heterogeneity at all scales. The rate of community change over time decreased as fire frequency increased at all scales, whereas grazing had no effect on rate of community change over time at any spatial scale. The interactive effects of fire and grazing on spatial and temporal heterogeneity differed with scale. At the 10-m2 scale, grazing increased spatial heterogeneity in annually burned grassland but decreased heterogeneity in less frequently burned areas. At the 50-m2 scale, grazing decreased spatial heterogeneity on 4-yr burns but had no effect at other fire frequencies. At the 10-m scale, grazing increased temporal heterogeneity only on 1- and 20-yr burn sites. Our results show that the individual effects of fire and grazing on spatial and temporal heterogeneity in mesic prairie are scale independent, but the interactive effects of these disturbances on community heterogeneity change with scale of measurement. These patterns reflect the homogenizing impact of fire at all spatial scales, and the different frequency, intensity, and scale of patch grazing by bison in frequently burned vs. infrequently burned areas.
自然干扰会影响植物群落的空间和时间异质性，但影响因干扰类型和分析尺度而异。在本研究中，我们研究了火灾频率（1年、4年和20年间隔）和野牛放牧对高草草原植物群落物种组成的空间和时间异质性的影响。在10平方米、50平方米和200平方米的尺度上估计组成异质性。对于每个测量尺度，我们使用一年内（2000年）样本之间的平均欧几里得距离（ED）来测量空间异质性，并使用每个样本在所有时间步长（1993 - 2000年）之间的平均欧几里得距离来测量时间异质性。火灾和放牧的主要影响与尺度无关。在所有尺度上，空间和时间异质性在每年火烧的地点最低，在火烧频率低（20年）的地点最高。放牧在所有尺度上降低了空间异质性并增加了时间异质性。随着火灾频率在所有尺度上增加，群落随时间变化的速率下降，而放牧在任何空间尺度上对群落随时间变化的速率没有影响。火灾和放牧对空间和时间异质性的交互作用随尺度而不同。在10平方米的尺度上，放牧增加了每年火烧草原的空间异质性，但降低了火烧频率较低地区的异质性。在50平方米的尺度上，放牧降低了4年火烧地的空间异质性，但在其他火灾频率下没有影响。在200平方米的尺度上，放牧仅在1年和20年火烧地增加了时间异质性。我们的结果表明，火灾和放牧对中生草原空间和时间异质性的个体影响与尺度无关，但这些干扰对群落异质性的交互作用随测量尺度而变化。这些模式反映了火灾在所有空间尺度上的同质化影响，以及野牛在频繁火烧与不频繁火烧地区斑块放牧的不同频率、强度和尺度。
