Spatial distributions of understory light along the grassland/forest continuum: effects of cover, height, and spatial pattern of tree canopies
Introduction
The understory light environment is a key determinant of vegetation pattern and ecosystem processes, and varies spatially perhaps more than any resource used by plants (Bazzaz, 1996). Spatial patterns of understory light are determined by several overstory characteristics, particularly spatial pattern, height, and cover of woody plants. These overstory characteristics vary concurrently along vegetation gradients and may produce complex patterns of understory light across vegetation gradients because the spatial pattern and height of trees can change with increases in woody-plant cover (Padien and Lajtha, 1992). These gradients of vegetation structure can be viewed as a continuum that ranges from grassland with no woody canopy cover to forest with nearly complete woody canopy cover (Belsky and Canham, 1994, Breshears and Barnes, 1999). Many extensive semiarid shrublands and woodlands span substantial portions of the grassland/forest continuum. For example, piñon-juniper woodlands, which are the most extensive ecosystem type in the western USA, can span a large proportion of this continuum (West, 1988, Padien and Lajtha, 1992, Milne et al., 1996). The understory light environment is important in these woodlands because of its effects on microclimate (e.g. solar radiation, soil temperature, leaf temperature, soil evaporation), which is strikingly different in canopy patches (i.e. directly below tree crowns) than in intercanopy patches (Breshears et al., 1997b, Breshears et al., 1998). The center portions of canopy patches receive 40% less near-ground solar radiation than intercanopies through the year, and >50% less during much of the growing season (Breshears et al., 1997b). Consequently, soil temperatures in intercanopy patches can exceed those in canopy patches by >10°C in summer, which in turn results in increased soil evaporation rates in intercanopy patches (Breshears et al., 1998). Further, microclimate affects plant processes such as seedling establishment, germination, facilitation, and growth (Floyd, 1983, Padien and Lajtha, 1992, Martens et al., 1997), as evident in the differences in distributions of understory plants in canopy and intercanopy patches (Arnold, 1964, Lohstroh and Van Auken, 1987, Armentrout and Pieper, 1988, Van Auken and Lohstroh, 1990, Ludwig et al., 1997). Hence, the spatial distribution of understory light—and in particular the difference between canopy and intercanopy patches — can have a large influence on vegetation dynamics for sites along the grassland/forest continuum.
Several studies have quantified heterogeneity of understory light at an individual site along the grassland/forest continuum — in piñon-juniper woodlands (Lin et al., 1992, Breshears et al., 1997b) as well as other systems (see reviews in Scholes and Archer (1997) and McPherson (1997)). However, a systematic analysis is lacking of how understory light patterns vary along the grassland/forest continuum with changing stand characteristics (spatial pattern, height, and cover). Our first objective was to evaluate the relative importance of changes in canopy structural characteristics (cover, spatial pattern, and height) in the understory light environment for sites along the grassland/forest continuum. To address this objective we systematically varied these three structural characteristics independently to generate plots with differing overstories. Along the grassland/forest continuum, however, spatial pattern, height, and cover vary concurrently and may be not be independent (Padien and Lajtha, 1992). Consequently, our second objective was to quantify the spatial pattern in the understory light environment in three actual woodland sites along an elevational gradient for which cover, spatial pattern, and height of overstory trees changed simultaneously. The sites were piñon-juniper woodlands in northern New Mexico, USA. We addressed these objectives using a ray-tracing model to estimate the understory light. Our results quantify how changes in canopy structural characteristics produce complex patterns of understory light. The trends we found are broadly applicable to other sites along the grassland/forest continuum.
Section snippets
Understory light modeling
We constructed a computer simulation model that depicts tree crowns as three-dimensional ellipsoids and calculates transmitted direct beam photosynthetically active radiation (PAR) after attenuation by the crowns. It is similar in concept to other models (Norman and Welles, 1983, Kuuluvainen and Pukkala, 1989, Oker-Blom et al., 1989, Wang and Jarvis, 1990, Pukkala et al., 1991, Pukkala et al., 1993, Cescatti, 1997a, Cescatti, 1997b, Brunner, 1998 de Castro and Fetcher, 1998, Lappi and Stenberg,
Systematic variation of canopy structural characteristics
Simulations based on the generated plots enabled us to evaluate the independent effects of cover, spatial pattern, and height on the understory light along the grassland/forest continuum. The percent canopy cover for the grassland/forest continuum ranged from 0 to 81%, over which mean understory light for a plot decreased with increasing canopy cover in a nearly linear fashion (Fig. 1A). Furthermore, different spatial patterns (regular, random, aggregated) produced nearly equivalent mean
Understory light along the grassland/forest continuum
Our results represent a landscape analysis of how concurrent changes in cover, spatial pattern, and height of trees affect understory light. We found that the mean understory light at a plot was most influenced by cover and was modified by height; spatial pattern had a negligible effect on the mean. Consequently, in situations where the mean for a plot is the issue of concern, cover and height may be sufficient parameters for predicting mean understory light for a plot; spatial pattern, which
Acknowledgements
We thank Susan R. Johnson, Katherine E. Dayem, and Robert J. Lucero for assistance with field data collection, George Fenton for compiling the solar radiation data, and Orrin B. Myers for comments. This project was supported by the Los Alamos National Laboratory through the Laboratory-Directed Research and Development Office and utilized data previously collected through support of the Environmental Restoration Project.
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- 1
Present address: Department of Land, Air, and Water Resources, University of California, Davis, Davis, CA 95616, USA.
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Present address: Atmosphere and Climate Group, Mail Stop D407, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.