Extent and drivers of vegetation type conversion in Southern California chaparral

Abstract

The native chaparral shrublands of Southern California support exceptional biodiversity and provide critical ecological services, but increased fire frequency threatens to extirpate much of the chaparral due to long regeneration times needed between fires for many species. When short fire intervals inhibit shrub recovery, this favors invasion of exotic herbaceous species, and vegetation type conversion from woody shrubs to grassland is therefore a serious ecological concern in this biodiversity hotspot. Despite a history of field studies documenting the detrimental effect of short‐interval fire, the extent of vegetation type conversion and the conditions under which it occurs have not been documented at a landscape scale. Our objective was thus to provide an unbiased assessment of how and how much vegetation type conversion is occurring in Southern California chaparral. We used a chronosequence of aerial photographs to quantify percentage woody and herbaceous cover change from 1953 to 2016 across randomly sampled plots in San Diego County, then related conversion and decline to a range of explanatory variables including fire, proximity to human disturbance, and biophysical landscape characteristics. Within the 63‐yr study period, there was substantial net woody cover loss, and in the plots that were initially more than 75% woody cover in 1953, 59% experienced a decline, with a mean woody cover loss of 22.5%. Of these, 28% heavily type‐converted to the point that herbaceous vegetation covered more than 50% of the plot. The top drivers for woody conversion and decline included a fire interval shorter than 15 yr and total number of fires, actual evapotranspiration, and elevation. Although human land use variables were not strong independent contributors to either chaparral conversion or decline, 26% of the initial vegetated plots were directly converted to development or other human disturbance types. The combination of direct habitat loss and unintentional vegetation type conversion represents a substantial ecological impact in Southern California that can have far‐reaching impacts via loss of ecological services and by increasing the flammability of the landscape in general. Efforts to reduce fire frequency will be key to preventing further losses.

Short-interval fires and vegetation change in southern California

“…This could be due to vegetation trends being more strongly correlated with moisture gradients driven by elevation than by distance from the coast. Indeed, Meng et al (2014) and Syphard et al (2019b) found elevation to be a strong driver of vegetation change with more chaparral conversion occurring at lower elevations. Analysis by Storey et al (2019) included a more extensive range of chaparral stands in southern California and found vegetation change was greatest in the eastern portion of their study region and at higher elevations and was a result of drought and fire interactions.…”

“…A single short-interval fire, however, does not always lead to chaparral conversion. Previous work with remotely sensed data suggest that water availability (Park et al, 2018, Syphard et al, 2019a), elevation (Meng et al, 2014), and mean annual temperature (Storey et al, 2021) explain more variation among chaparral stand recovery (although also see Syphard et al (2019b)). These studies utilized imagery with 30-m spatial resolution and/or datasets with a 30-m scale.…”

“…A shortened fire interval has been recognized as one of the main causes of chaparral conversion to non-native annual grasses (Keeley, 2000, Syphard et al, 2019b). This is because many chaparral species require five-to-ten years to reach reproductive maturity (Zammit and Zedler, 1993) and at least 15 years to establish a robust seedbank (Syphard et al, 2018b, Park et al, 2018, Keeley, 2000).…”

New Biomass Estimates for Chaparral-Dominated Southern California Landscapes

“…Where the fire return interval of shrublands approximates the historic fire return interval (average 55 years for low-elevation shrubland) [8,14,15], shrub cover recovers after 10-14 years, while biomass keeps accumulating [16,17]. However, in many areas, the fire return interval has decreased, often in conjunction with an increase in non-native plant species, drought, and nitrogen deposition [18][19][20]. Under these conditions, post-fire biomass recovery can be impeded and, in some cases, may result in type conversion from native shrubland to non-native grassland [20].…”

“…However, in many areas, the fire return interval has decreased, often in conjunction with an increase in non-native plant species, drought, and nitrogen deposition [18][19][20]. Under these conditions, post-fire biomass recovery can be impeded and, in some cases, may result in type conversion from native shrubland to non-native grassland [20].…”