Conservation Biology Institute, Corvallis, Oregon, USA.
Department of Geography, San Diego State University, San Diego, California, USA.
Ecol Appl. 2022 Sep;32(6):e2626. doi: 10.1002/eap.2626. Epub 2022 Jun 2.
One consequence of global change causing widespread concern is the possibility of ecosystem conversions from one type to another. A classic example of this is vegetation type conversion (VTC) from native woody shrublands to invasive annual grasslands in the biodiversity hotspot of Southern California. Although the significance of this problem is well recognized, understanding where, how much, and why this change is occurring remains elusive owing to differences in results from studies conducted using different methods, spatial extents, and scales. Disagreement has arisen particularly over the relative importance of short-interval fires in driving these changes. Chronosequence approaches that use space for time to estimate changes have produced different results than studies of changes at a site over time. Here we calculated the percentage woody and herbaceous cover across Southern California using air photos from ~1950 to 2019. We assessed the extent of woody cover change and the relative importance of fire history, topography, soil moisture, and distance to human infrastructure in explaining change across a hierarchy of spatial extents and regions. We found substantial net decline in woody cover and expansion of herbaceous vegetation across all regions, but the most dramatic changes occurred in the northern interior and southern coastal areas. Variables related to frequent, short-interval fire were consistently top ranked as the explanation for shrub to grassland type conversion, but low soil moisture and topographic complexity were also strong correlates. Despite the consistent importance of fire, there was substantial geographical variation in the relative importance of drivers, and these differences resulted in different mapped predictions of VTC. This geographical variation is important to recognize for management decision-making and, in addition to differences in methodological design, may also partly explain differences in previous study results. The overwhelming importance of short-interval fire has management implications. It suggests that actions should be directed away from imposing fires to preventing fires. Prevention can be controlled through management actions that limit ignitions, fire spread, and the damage sustained in areas that do burn. This study also demonstrates significant potential for changing fire regimes to drive large-scale, abrupt ecological change.
全球变化引起广泛关注的后果之一是生态系统从一种类型向另一种类型转换的可能性。南加利福尼亚生物多样性热点地区从本地木本灌丛到入侵一年生草本的植被类型转换 (VTC) 就是一个经典的例子。尽管这个问题的重要性已得到广泛认可，但由于使用不同方法、不同空间范围和不同尺度进行的研究结果存在差异，理解这种变化发生的地点、程度和原因仍然难以捉摸。在驱动这些变化的短间隔火灾的相对重要性方面存在特别大的分歧。使用空间代替时间来估计变化的时间序列方法得出的结果与随着时间在一个地点进行的变化研究结果不同。在这里，我们使用 1950 年至 2019 年的航空照片计算了南加利福尼亚的木本和草本覆盖的百分比。我们评估了木本覆盖物变化的程度以及火灾历史、地形、土壤湿度和距人类基础设施的距离在解释跨空间范围和区域层次变化中的相对重要性。我们发现，所有地区的木本覆盖物都有大量净减少，草本植被有大量扩展，但最显著的变化发生在北部内陆和南部沿海地区。与频繁、短间隔火灾有关的变量一直被列为灌木到草原类型转换的解释的最高排名，但低土壤湿度和地形复杂性也是强烈的相关因素。尽管火灾的重要性一致，但驱动因素的相对重要性在地理上存在很大差异，这些差异导致了 VTC 的不同映射预测。这种地理差异对于管理决策很重要，除了方法设计上的差异外，这也可能部分解释了以前研究结果的差异。短间隔火灾的压倒性重要性对管理具有影响。这表明，应将行动方向从实施火灾转变为预防火灾。可以通过限制点火、火势蔓延和在燃烧区域内减少损失的管理措施来控制预防。本研究还表明，改变火灾发生频率可能会导致大规模、突然的生态变化。
