Browning Dawn M, Franklin Janet, Archer Steven R, Gillan Jeffrey K, Guertin D Phillip
Ecol Appl. 2014;24(6):1421-33. doi: 10.1890/13-2033.1.
Tree and shrub abundance has increased in many grasslands causing changes in ecosystem carbon and nitrogen pools that are related to patterns of woody plant distribution. However, with regard to spatial patterns of shrub proliferation, little is known about how they are influenced by grazing or the extent to which they are influenced by intraspecific interactions. We addressed these questions by quantifying changes in the spatial distribution of Prosopis velutina (mesquite) shrubs over 74 years on grazed and protected grasslands. Livestock are effective agents of mesquite dispersal and mesquite plants have lateral roots extending well beyond the canopy. We therefore hypothesized that mesquite distributions would be random on grazed areas mainly due to cattle dispersion and clustered on protected areas due to decreased dispersal and interspecific interference with grasses; and that clustered or random distributions at early stages of encroachment would give way to regular distributions as stands matured and density-dependent interactions intensified. Assessments in 1932, 1948, and 2006 supported the first hypothesis, but we found no support for the second. In fact, clustering intensified with time on the protected area and the pattern remained random on the grazed site. Although shrub density increased on both areas between 1932 and 2006, we saw no progression toward a regular distribution indicative of density-dependent interactions. We propose that processes related to seed dispersal, grass–shrub seedling interactions, and hydrological constraints on shrub size interact to determine vegetation structure in grassland-to-shrubland state changes with implications for ecosystem function and management.
许多草原上树木和灌木的数量有所增加，导致生态系统碳氮库发生变化，这与木本植物的分布模式有关。然而，关于灌木扩散的空间模式，我们对其受放牧影响的方式以及受种内相互作用影响的程度知之甚少。我们通过量化74年间放牧草原和保护草原上绒毛牧豆树（牧豆树属）灌木空间分布的变化，来解决这些问题。牲畜是牧豆树扩散的有效媒介，且牧豆树植物的侧根延伸范围远远超出树冠。因此，我们推测，在放牧区域，牧豆树的分布将主要由于牛群的分散而呈随机分布，而在保护区，由于扩散减少以及与草本植物的种间干扰减少，其分布将呈聚集分布；并且在侵占早期的聚集或随机分布，随着林分成熟和密度依赖相互作用增强，将让位于规则分布。1932年、1948年和2006年的评估结果支持了第一个假设，但我们没有找到支持第二个假设的证据。事实上，在保护区，聚集现象随时间加剧，而在放牧区，分布模式仍保持随机。虽然在1932年至2006年间，两个区域的灌木密度都有所增加，但我们并未看到向表明密度依赖相互作用的规则分布的演变。我们认为，与种子扩散、草本 - 灌木幼苗相互作用以及灌木大小的水文限制相关的过程相互作用，共同决定了从草原到灌木林状态变化中的植被结构，这对生态系统功能和管理具有重要意义。
