Microclimate–forage growth linkages across two strongly contrasting precipitation years in a Mediterranean catchment

Abstract

Given the complex topography of California rangelands, contrasting microclimates affect forage growth at catchment scales. However, documentation of microclimate-forage growth associations is limited, especially in Mediterranean regions experiencing pronounced climate change impacts. To better understand microclimate-forage growth linkages, we monitored forage productivity and rootzone soil temperature and moisture (0-15 and 15-30 cm) in 16 topographic positions in a 10-ha annual grassland catchment in California's Central Coast Range. Data were collected through two strongly contrasting growing seasons, a wet year (2016-17) with 287-mm precipitation and a dry year (2017-18) with 123-mm precipitation.Plant-available soil water storage (0-30 cm) was more than half full for most of the wet year; mean peak standing forage was 2790 kg ha −1 (range: 1597-4570 kg ha −1 ).The dry year had restricted plant-available water and mean peak standing forage was reduced to 970 kg ha −1 (range: 462-1496 kg ha −1 ). In the wet year, forage growth appeared energy limited (light and temperature): warmer sites produced more forage across a 3-4°C soil temperature gradient but late season growth was associated with moister sites spanning this energy gradient. In the dry year, the warmest topographic positions produced limited forage across a 10°C soil temperature gradient until late season rainfall in March. Linear models accounting for interactions between soil moisture and temperature explained about half of rapid, springtime forage growth variance. These findings reveal dynamic but clear microclimate-forage growth linkages in complex terrain, and thus, have implications for rangeland drought monitoring and dryland ecosystems modeling under climate change.

Terrain attributes and forage productivity predict catchment-scale soil organic carbon stocks

“…At the hillslope scale in a 2-ha grassland in Southern California, Gessler et al (2000) sampled soils on a SW-facing transect and reported that a Compound Topographic Index explained 78% of the variability in SOC stocks with more SOC in lower landscape positions. At our study site, additional forage production in a wet year was favored in concave locations, especially where soil moisture remained plentiful during a late season dry spell (Devine et al, 2019). This suggests that a wet year produces lateral redistribution of water and defines lower hillslope zones with surplus moisture where peak forage is produced and more SOC is accumulated over time, as indicated by higher NDVI (Fig.…”

“…Forage was oven dried at 60°C for 48 h before weighing. We recorded grass and forb height and species composition using the dry-weight rank method (Ratliff and Frost, 1990) as previously reported in Devine et al (2019). Gradients in annual forage production are diffuse across the study catchment; there are no shrubs or perennial grass patches creating islands of enhanced soil quality as found in several semi-arid and arid ecosystems such as Mediterranean oak woodlands (Dahlgren et al 1997b).…”

“…1 and 2; Table 2). Linkages among wet-year NDVI, elevation, curvature, and SOC suggest that hillslope position and vegetation appear to be more important drivers of SOC patterns than aspect in central California semiarid grasslands (Tables 2 and 4), even though insolation differences forced 3-10°C soil temperature gradients in the catchment and noticeable differences in plant available moisture during wet-up periods (Devine et al, 2019). This lack of a clear, direct relationship between SOC and aspect is in contrast to several other catchment-scale studies.…”

“…There are unresolved questions related to how climate change will impact annual range production and SOC stocks in complex topography. Warmer temperatures and greater insolation on south-facing slopes can enhance winter forage growth during wet years in California annual rangelands (Devine et al, 2019;Evans et al, 1975), and this may partly compensate for the negative effect that enhanced drying has on forage growth and its feedback to SOC. However, mid-winter droughts can also sharply impact growth on south-facing aspects (Devine et al, 2019).…”

“…Complex terrain has diverse microclimates and forage productivity that varies at tens of meters (Devine et al, 2019), and this is expected to impact SOC stock patterns at catchment scale. Thus, understanding SOC distribution across these variable microclimates and landscape positions is needed to better predict climate change implications for landscape C storage.…”