Height growth of planted black spruce seedlings in response to interspecific vegetation competition: a comparison of four competition measures at twomeasuring positions

Four simple measures of interspecific competition (percent cover visually e stimated in the field, percent cover derived from hemispherical photographs , percent full sunlight measured by a ceptometer, and gap light index deriv ed from hemispherical photographs) obtained at two reference positions (the top and the middle of crop seedlings) were evaluated in relation to two gr owth variables (relative height growth rates in 1998 and during 1996 to 199 8) of black spruce (Picea mariana (Mill.) BSP) seedlings planted on boreal mixedwood sites in southeastern Manitoba. The four competition measures ass essed at the two measuring positions explained 57.2-68.0% of the total vari ation in black spruce height growth rate. Significant relationships were fo und among the four measures, and between the two measuring positions for ea ch measure. The measuring position was not critical for all competition mea sures except the percent full sunlight measured by the ceptometer, for whic h the middle position was much better. When assessed at their preferred pos itions, the four competition measures ranked as follows: (i) percent cover derived from hemispherical photographs or percent full sunlight measured by the ceptometer; (ii) gap light index derived from hemispherical photograph s; and (iii) visually estimated percent cover of vegetation.