Improving gap light index (GLI) to quickly calculate gap coordinates

Abstract

Understory light is essential to the establishment and growth of understory plants and varies temporally and spatially within gaps. The previously defined gap light index (GLI) is a good model for assessing understory light levels, but it is time-consuming to determine gap coordinates, which are crucial to GLI, for numerous points within a gap. This paper introduces the geometric calculation (GeoCalc) of gap coordinates. GeoCalc quickly obtains gap coordinates for any specified point within a canopy gap and takes into account the tridimensional profile of the gap and the slope and aspect of the ground. The GeoCalc-based GLI was validated by the GLI derived from hemispherical photographs taken at 93 sampling points within seven natural gaps. Our results demonstrate that GeoCalc-based GLI was strongly positively correlated and not significantly differed from the GLI derived from hemispherical photographs. Next, to analyze gap light regimes and the effects of gap size, canopy height, and topography, three natural gaps of various size were selected and simulated as nine gaps with 1 and 1.5 times canopy height or on the opposite slope. Finally, we have summarized characteristics of GeoCalc-based GLI and its application.

Estimation of canopy gap size and gap shape using a hemispherical photograph

“…The annual precipitation is 700-850 mm, most of which occurs during June and August. The mean annual temperature is 4.7°C, with annual extremes of 36.5 and -37.5°C (Hu and Zhu 2008).…”

“…Gap size, including mainly canopy gap size and expanded gap size, and gap shape are two of the most important characteristics of forest gap (Runkle 1992;Diaci 2002). Gap size is critical in determining the recruitment and establishment of different tree species (Brokaw 1985) and is a good index to characterize light penetration and other resource availability within gaps (Denslow 1987;Hu and Zhu 2008). Moreover, gap size can be used to calculate the forest turnover rate as well (Chandrashekara and Ramakrishnan 1994;Valverde and Silvertown 1997).…”

A new calculation method to estimate forest gap size

“…The environmental conditions of forest gap microsite are affected by the characteristics of the canopy gap, including gap size, gap shape, height of border trees, and the duration of gap existence (gap age) (Denslow, 1980;Runkle, 1981;Gagnon et al, 2004;Hu and Zhu, 2008). Of the gap characteristics, gap size is considered to be the most important as it plays a significant role in characterizing light penetration and resource availability within a forest gap (Brokaw, 1985;Denslow, 1987;Veblen, 1989;Whitmore, 1989;Ida, 2000;Zhu et al, 2003;Hu et al, 2007;Hu and Zhu, 2008).…”

“…Of the gap characteristics, gap size is considered to be the most important as it plays a significant role in characterizing light penetration and resource availability within a forest gap (Brokaw, 1985;Denslow, 1987;Veblen, 1989;Whitmore, 1989;Ida, 2000;Zhu et al, 2003;Hu et al, 2007;Hu and Zhu, 2008). Furthermore, gap size strongly affects forest natural regeneration, such as seedling emergence, survival, establishment, and growth (Denslow, 1980;Brokaw, 1982;Veblen, 1989;Canham et al, 1990;Pacala et al, 1994;Palik et al, 1997;McGuire et al, 2001;Zhu et al, 2003;Lima, 2005;.…”

“…Gap size, therefore, has received much attention in the research of forest science (Brokaw, 1985;Denslow, 1987;Lawton, 1988;Ida, 2000;Zhu et al, 2003;Hill et al, 2005). However, the methods for accurate estimation of gap size have been the important focus of attention for foresters and ecologists because accurate gap size is the basis in calculating the forest turnover rate (Veblen, 1989;Chandrashekara and Ramakrishnan, 1994;Valverde and Silvertown, 1997;Lima, 2005;Hu and Zhu, 2008).…”