Linking light attenuation, sunflecks, and canopy architecture in mesic shrub thickets

Abstract

Expansion of shrubs into grasslands is often accompanied by a reduction in understory light and an associated reduction of shade-intolerant species. However, effects of specific canopy architectural characteristics on the light environment under shrub thickets are unknown. Our objective was to determine what characteristics of canopy architecture most influence understory light in monospecific shrub thickets. We quantified understory light and canopy architecture for five shrub species in the eastern United States that have a history of expansion, and we used multiple regression to determine which canopy characteristics best predicted light attenuation and relative contribution of sunflecks. Measurements included leaf angle, leaf azimuth, branch bifurcation ratio, leaf area index (LAI), canopy depth (the vertical distance from the bottommost leaf to the top of the canopy), and leaf area density (LAD) as well as understory photosynthetic photon flux density (PPFD). The best predictor of light attenuation and the occurrence of sunflecks for all species was canopy depth. Multiple leaf and plant-level traits were correlated with canopy depth but not with LAI or LAD. The invasive shrub Elaeagnus umbellata had the lowest understory light levels of the species examined although LAI values for Morella cerifera and Rhododendron maximum were higher. Branch bifurcation ratios for E. umbellata were significantly higher than for other species and this likely contributed to the differences in light attenuation and suppression of sunflecks. The potential of shrubs to intercept light is primarily dependent on vertical distribution of leaves in the canopy which is itself correlated with fine-scale, species-specific variations in leaf display.

Application of hyperspectral vegetation indices to detect variations in high leaf area index temperate shrub thicket canopies

“…fruits and flower parts) components, and weighed to the nearest 0.1 g. Leaf litter mass was converted to LAI using values for specific leaf area (SLA). SLA was determined for each species using leaf tracings on a paper of known density (Brantley & Young, 2010b). In 2004, peak LAI values (midAugust) in M. cerifera thickets were estimated with a Li-Cor LAI-2000 portable integrating radiometer (Li-Cor Biosciences, Lincoln, Nebraska, USA) above each litter trap and these data were reported in Brantley and Young (2007).…”

“…Elaeagnus umbellata has lower leaf angles than M. cerifera (i.e. leaves are horizontally rather than vertically inclined) and E. umbellata foliage is biased towards the south where as M. cerifera foliage is randomly oriented with no directional bias (Brantley & Young, 2010b). Additionally, leaves of E. umbellata are more evenly distributed within the canopy which reduces leaf clumping and increases efficiency of light interception in relation to biomass (Brantley & Young, 2010b).…”

“…leaves are horizontally rather than vertically inclined) and E. umbellata foliage is biased towards the south where as M. cerifera foliage is randomly oriented with no directional bias (Brantley & Young, 2010b). Additionally, leaves of E. umbellata are more evenly distributed within the canopy which reduces leaf clumping and increases efficiency of light interception in relation to biomass (Brantley & Young, 2010b). Highly clumped foliage and high leaf angles are likely the reason M. cerifera thickets can maintain relatively high LAI values, and these characteristics combine to make both optical methods of LAI estimation and remote-sensing methods difficult.…”

Diurnal patterns of photosynthesis, chlorophyll fluorescence, and PRI to evaluate water stress in the invasive species, Elaeagnus umbellata Thunb.

“…The structural architecture of E. umbellata is very similar to plants in shaded environments. Leaves are positioned facing south and leaf angles are horizontal to maximize sunlight capture (Brantley and Young 2009).…”

High‐elevation ground‐layer plant community composition across environmental gradients in spruce‐fir forests

“…L. Dobbs and Parker 2004). Members of this genus, especially Rhododendron maximum L. (rosebay rhododendron), thrive in acidic soils and have been shown to reduce the recruitment of other woody species and decrease the cover and diversity of the herbaceous layer as a result of direct competition, modification of understory microclimate, and changes to upper soil horizons resulting from heavy accumulation of slow decaying acidic litter (Monk et al 1985;Phillips and Murdy 1985;Plocher and Carvell 1987;Clinton et al 1994;Nowacki and Abrams 1994;Clinton 2003;Brantley and Young 2010). Rhododendron thickets have also been shown to alter N cycling and soil extracellular enzyme activity (Wurzburger and Hendrick 2007).…”