California Chaparral and Its Global Significance

Fire-driven vegetation type conversion in Southern California

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.

Differences in root phenology and water depletion by an invasive grass explains persistence in a Mediterranean ecosystem

PREMISE

Flexible phenological responses of invasive plants under climate change may increase their ability to establish and persist. A key aspect of plant phenology is the timing of root production, how it coincides with canopy development and subsequent water-use. The timing of these events within species and across communities could influence the invasion process. We examined above- and belowground phenology of two species in southern California, the native shrub, Adenostoma fasciculatum, and the invasive perennial grass, Ehrharta calycina to investigate relative differences in phenology and water use.

METHODS

We used normalized difference vegetation index (NDVI) to track whole-canopy activity across the landscape and sap flux sensors on individual chaparral shrubs to assess differences in aboveground phenology of both species. To determine differences in belowground activity, we used soil moisture sensors, minirhizotron imagery, and stable isotopes.

RESULTS

The invasive grass depleted soil moisture earlier in the spring and produced longer roots at multiple depths earlier in the growing season than the native shrub. However, Adenostoma fasciculatum produced longer roots in the top 10 cm of soil profile in May. Aboveground activity of the two species peaked at the same time.

CONCLUSIONS

The fact that Ehrharta calycina possessed longer roots earlier in the season suggests that invasive plants may gain a competitive edge over native plants through early activity, while also depleting soil moisture earlier in the season. Depletion of soil moisture earlier by E. calycina suggests that invasive grasses could accelerate the onset of the summer drought in chaparral systems, assuring their persistence following invasion.