Effect of understory management on phenological responses of eastern black walnut on an alluvial Arkansas soil

Black walnut (Juglans nigra L.) is commonly grown in agroforestry practices for nuts and/or timber with little knowledge of how understory herbage management might affect tree phenology. We compared black walnut plant type (variety and wild-type) for phenological response in date of budburst, leaf area index (LAI), quantum yield of photosystem II (Fv/Fm), radial stem growth, and total chlorophyll concentration in response to understory herbage N fertilization and herbage suppression in an agroforestry practice on an alluvial soil. Herbage treatments generally had little impact on phenological responses, although N fertilization increased herbage yield that delayed budburst. Radial stem growth commenced at about budburst (mid-April) and virtually ceased in late October, about 30 days before leaf drop. Early-season radial stem growth was greater for the wild-type than the variety, while the variety had more rapid late-season growth. The wild-type had consistently greater LAI than the variety at any given day-of-year (DOY), and usually had greater LAI than the variety regardless of herbage treatment. Leaves of both plant types maintained a high Fv/Fm during most of the growing season, but total chlorophyll concentration decreased across DOY regardless of plant type. N fertilizer should be applied with understory herbage suppression to avoid delay of budburst. Further study is needed of phenological responses during longer growth periods, or as affected by latitude, soil water, and soil fertility.