Increases in understory plant cover and richness persist following restoration treatments in Pinus ponderosa forests

A combination of forest thinning followed by prescribed burning is widely applied in the western US to increase ecosystem resistance and resilience to disturbances. Understory plant community responses may be driven both by management treatments and climatic factors. Thus, responses to treatments during a 20-year megadrought have implications for the role of management in fostering adaptive capacity to climate change.

We used a network of five sites (600 plots) spanning an environmental gradient in ponderosa pine (Pinus ponderosa) forests of the American Southwest, an ecosystem that is broadly distributed and actively managed throughout the western US.  We used repeated long-term monitoring data to quantify plant community responses to treatment 1-5, 6-10, and >10 years post-implementation. Specifically, we focused on the effects of treatment and abiotic conditions on native and nonnative plant cover and species richness, and on the proportion of native species with northern (cool-mesic) biogeographic affinities.

Overall, thinning and prescribed burning nearly doubled native cover and increased native species richness by about 50% relative to untreated controls. These effects persisted for over a decade after treatment, even under the influence of significant and persistent drought. Cover and richness were also greater on intermediate to wet sites. Finally, native species with northern biogeographic affinities were reduced for up to five years after treatment relative to those with southern (warm-xeric) affinities, and in dry years, indicating that both management and interannual climate variability may foster shifts in plant communities that are more resilient to a warming climate.

Synthesis and applications: In ponderosa pine forests of the American Southwest, tree thinning followed by prescribed burning will generally promote restoration goals of increasing resilience to climate change by enhancing the diversity and abundance of native understory plant species, even during a persistent 20-year megadrought.

Methods
This archive includes long-term plant community data and forest monitoring data from 5 experimental sites (~ 600 plots) in Arizona, USA. These sites were established in the late 1990s to early 2000s to describe the effects of thinning and burning treatments on ecological processes in southwestern US ponderosa pine forests. Understory plant community data describe the presence and abundance of herbaceous and short-statured woody plants, with at least one survey before treatment, and multiple surveys after treatment at each site, as well as data from paired untreated control sites. Plants were identified in the field by trained botanists, or pressed and identified in the laboratory. Most individuals were identified to the species level, but occasioanlly could only be identified to genus. For descriptions of different fiels in the data arrchive, as well as methods of collection, see the "README.md" file