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Ground-based canopy reflectance was measured perpendicular to the crop surface and in discrete wavebands over irrigated corn with several imposed N treatments for comparison to SPAD chlorophyll meter measurements and to plant tissue total N concentration . In dense stands , black oak often fills a " hole " in the canopy , sometimes leaning 15 to 20 degrees to do so . For this reason its holiness is acknowledged in the prayer , ' Blessed be the Lord who covers us , together with all his people Israel and Jerusalem , with the canopy of peace . Paper birch , however , requires larger canopy gaps than yellow birch to successfully regenerate ( Leak 1991 ) . Although these complexes are often quite large , they are easily concealable in the triple canopy jungles of Colombia .