Climate–fire interactions constrain potential woody plant cover and stature in North American Great Plains grasslands

Aim: Disturbances such as fire operate against a backdrop of constraints imposed by climate and soils to influence grass–woody plant abundance. However, little is known of how these factors interact to determine the upper limits of woody cover and stature in grasslands, in which shrub/tree abundance has been increasing globally. Location: Kansas, Oklahoma, Texas, USA. Time period: 2004–2014. Major taxa studied: Angiosperms and gymnosperms. Methods: Using a database of 1,466 sites and quantile regression to derive precipitation-based upper limits to woody cover and height within grasslands of the central/southern Great Plains, USA, we assessed how soil texture and climate-related fire probabilities [two groups; low fire probability, P(F_low), versus high fire probability, P(F_hi)] might influence realization of the climate potential. Results: Soil texture had no substantive influence on regional-scale woody cover, but taller plants were predicted on sandy soils. Woody plant height potential increased linearly with increasing annual precipitation, becoming asymptotic at c. 800 mm for both the P(F_low) and the P(F_hi) fire groups, after which P(F_low) areas were predicted to support taller plants. Potential woody cover also increased linearly with annual precipitation until c. 800 mm, after which predictions of maximum % cover were similar under both fire groups. Main conclusions: Precipitation was the overriding factor constraining potential woody cover and height, particularly in drier regions, with fire playing a minor role at these regional scales. In contrast to height potential, cover potential remained similar for both P(F_low) and P(F_hi) sites. Dynamic adjustments in woody plant architecture and allocation to foliage and stems, wherein areal cover is maintained when height is suppressed has implications for remote sensing, primary production and biogeochemical processes. Our analyses indicate drier grasslands [< 800 mm mean annual precipitation (MAP)] undergoing woody plant encroachment have the potential to become shrublands (e.g. short woody plants, low cover), whereas wetter areas have the potential to become woodland or forest (taller woody plants, high cover).