Effects of light on herbaceous layer productivity across a forest-savanna continuum

Background/Question/Methods

The herbaceous vegetation across forest-savanna ecosystems is very important for biodiversity, carbon and nutrient cycling, and wildlife habitat.  Variation in canopy structure across a gradient from oak-pine forest to oak-grassland savanna affects the light transmittance to the understory and may influence the productivity and richness in this stratum. To better understand the effects of light environment on the herbaceous layer, we conducted a study in southeastern Oklahoma, USA where overstory vegetation structure was controlled using thinning and prescribed fire. We periodically measured overstory canopy structure using hemispherical photographs and light interception by herbaceous layer using a ceptometer and related these variables to understory productivity during the 2013 growing season.

Results/Conclusions

Herbaceous biomass production increased as canopy openness (fraction of canopy not obscured be the overstory vegetation) increased with the relationship log(herbaceous biomass) in g m^-2 = -0.35 + 0.061*overstory openness; r²=0.62. The association between herbaceous biomass vegetation and canopy openness varied by burning regime. Within the herbaceous vegetation, annual production was positively correlated with light intercepted by the understory (log herbaceous biomass = 0.88 + 4.12* light intercepted by herbaceous layer).  These results indicate that light reaching the understory stratum is a primary driver of productivity for herbaceous vegetation and the radiation captured by this layer influences its productivity.