Browning Dawn M, Archer Steven R, Asner Gregory P, McClaran Mitchel P, Wessman Carol A
School of Natural Resources, University of Arizona, P.O. Box 210043, Tucson, Arizona 85721, USA.
Ecol Appl. 2008 Jun;18(4):928-44. doi: 10.1890/07-1559.1.
Woody plant abundance is widely recognized to have increased in savannas and grasslands worldwide. The lack of information on the rates, dynamics, and extent of increases in shrub abundance is a major source of uncertainty in assessing how this vegetation change has influenced biogeochemical cycles. Projecting future consequences of woody cover change on ecosystem function will require knowledge of where shrub cover in present-day stands lies relative to the realizable maximum for a given soil type within a bioclimatic region. We used time-series aerial photography (1936, 1966, and 1996) and field studies to quantify cover and biomass of velvet mesquite (Prosopis velutina Woot.) following its proliferation in a semidesert grassland of Arizona. Mapping of individual shrubs indicated an encroachment phase characterized by high rates of bare patch colonization. Upon entering a stabilization phase, shrub cover increases associated with recruitment and canopy expansion were largely offset by contractions in canopy area of other shrub patches. Instances of shrub disappearance coincided with a period of below-average rainfall (1936-1966). Overall, shrub cover (mean +/- SE) on sandy uplands with few and widely scattered shrubs in 1902 was dynamically stable over the 1936-1996 period averaging approximately 35% +/- 5%. Shrub cover on clayey uplands in 1936 was 17% +/- 2% but subsequently increased twofold to levels comparable to those on sandy uplands by 1966 (36% +/- 7%). Cover on both soils then decreased slightly between 1966 and 1996 to 28% +/- 3%. Thus, soil properties influenced the rate at which landscapes reached a dynamic equilibrium, but not the apparent endpoint. Although sandy and clayey landscapes appear to have stabilized at comparable levels of cover, shrub biomass was 1.4 times greater on clayey soils. Declines in shrub cover between 1966 and 1996 were accompanied by a shift to smaller patch sizes on both sandy and clayey landscapes. Dynamics observed during the stabilization phase suggest that density-dependent regulation may be in play. If woody cover has transitioned from directional increases to a dynamic equilibrium, biomass projections will require monitoring and modeling patch dynamics and stand structure rather than simply changes in total cover.
木本植物丰度在全球稀树草原和草原中呈增加趋势，这一点已得到广泛认可。缺乏有关灌木丰度增加速率、动态变化及范围的信息，是评估这种植被变化如何影响生物地球化学循环时不确定性的主要来源。预测木本植被覆盖变化对生态系统功能的未来影响，需要了解当今林分中的灌木覆盖相对于生物气候区域内给定土壤类型的可实现最大值所处的位置。我们利用时间序列航空摄影（1936年、1966年和1996年）和实地研究，对绒毛牧豆树（Prosopis velutina Woot.）在亚利桑那州半荒漠草原扩散后的覆盖度和生物量进行了量化。对单株灌木的测绘表明，存在一个以裸地高定殖率为特征的侵占阶段。进入稳定阶段后，与新植株和冠层扩展相关的灌木覆盖度增加，在很大程度上被其他灌木斑块冠层面积的收缩所抵消。灌木消失的情况与一段降雨量低于平均水平的时期（1936 - 1966年）相吻合。总体而言，1902年在沙质高地上分布稀疏的灌木覆盖度（平均值±标准误）在1936 - 1996年期间动态稳定，平均约为35%±5%。1936年粘质高地上的灌木覆盖度为17%±2%，但随后增加了两倍，到1966年达到与沙质高地相当的水平（36%±7%）。两种土壤上灌木覆盖度在1966年至1996年间均略有下降，降至28%±3%。因此，土壤性质影响了景观达到动态平衡的速率，但不影响明显的终点。尽管沙质和粘质景观似乎在相当的覆盖度水平上达到了稳定，但粘质土壤上的灌木生物量是沙质土壤的1.4倍。1966年至1996年间灌木覆盖度的下降，伴随着沙质和粘质景观上斑块尺寸向更小的转变。在稳定阶段观察到的动态变化表明，密度依赖调节可能在起作用。如果木本植被覆盖已从定向增加转变为动态平衡，那么生物量预测将需要监测和模拟斑块动态及林分结构变化，而不仅仅是总覆盖度的变化。
