Management and climate variability effects on understory productivity of forest and savanna ecosystems in Oklahoma, USA

The productivity of herbaceous and understory woody vegetation is critical for wildlife habitat, livestock forage, and biodiversity, and is influenced by both annual weather patterns and tree dominance. With the goals to inform management and understand climate change implications, we determined the effects of tree harvest, prescribed fire, and 31 yr of climate variability on understory aboveground net primary productivity (ANPP) for ecosystems ranging from mature forest to grassland in a long‐term study in southeastern Oklahoma, USA. In 1984, starting with a mature forest dominated by Pinus echinata and Quercus stellata, replicated experimental units were created by various combinations of pine harvest, hardwood thinning, and subsequent fire return intervals (1–4 yr and none). Understory ANPP (forbs, grasses, and woody plants) was measured by clip plots at the end of each growing season. Stepwise regression models were developed between understory ANPP and tree basal area, litter accumulation, fire return interval, and monthly, seasonal, and annual weather variables. Understory ANPP was dominated by grasses and ranged from 27 g·m⁻²·yr⁻¹ for the mature forest to 374 g·m⁻²·yr⁻¹ for an annually burned grassland/savanna. In general, herbaceous ANPP was inversely related to tree dominance (basal area), litter accumulation, and early and late growing season temperatures, and positively related to June precipitation. Understory woody ANPP was influenced by tree basal area, positively influenced by precipitation, and negatively influenced by summer soil moisture deficits. Our results indicate that prescribed fire, through its negative influence on tree basal area and litter accumulation, is critical to maintaining highly productive understories and that a three‐year return interval is a threshold to stall redevelopment of forest. For herbaceous ANPP, timing of precipitation, especially mid‐growing season, appears more important than total precipitation, and higher temperatures within the range our site experienced did not have a large negative effect. In contrast, understory woody ANPP was negatively influenced by drought indicating climate change may have variable effects on different functional groups.