Effects of canopy gaps in Douglas -fir forests and resource gradients on fecundity and growth of understory herbs

Effects of small-scale disturbances on fecundity and growth rates of Linnaea borealis L., Tiarella trifoliata L. var. unifoliata, and Trillium ovatum Pursh. were studied in old-growth forests in the Tsuga heterophylla Zone on the west slope of the Cascade Mountain Range of Oregon. Performance of each species was compared in experimental gaps of different sizes (created in 1990) and in relation to gradients of light and soil moisture. Effects of plant age and size and rhizome depth were additionally evaluated for Trillium . Locations of seedlings were surveyed for all three species relative to positions within and around gap and control treatments.;Growth and fecundity of all three species were higher in or around gaps than under closed canopies. Growth and fecundity of Linnaea and Tiarella were greater within gaps than in controls and adjacent borders of gaps. Fecundity of Linnaea was greater in large gaps than in small gaps and controls. Tiarella seedlings were present in gaps and on gap borders but not in controls. Fecundity and leaf area of Trillium were greatest in positions north of canopy gaps. Trillium seedlings were present on gap borders but no seedlings were found within gaps. Within gaps, Trillium also had lower rates of seed production (per m2 cover), horizontal growth, and relative increases in abundance than did the other two species.;Performance of all three species varied more with gap position and resources than with gap size. Within gaps, fecundity was lowest in central positions for Linnaea and in northern positions for Tiarella . Unlike Trillium, growth rates of these species were greater in southern than in positions north of gaps. Effects of gap position were modified in some cases by debris, shrubs, and topographical structures. Fecundity increased with light (at low soil moisture) for Linnaea and soil moisture for Tiarella; growth rates increased with soil moisture for both of these species. Fecundity of Trillium was a function of plant development and resources and was most directly related to leaf area. Seed production and leaf area of Trillium peaked at low light intensity. Canopy heterogeneity therefore influences life cycles of these forest herbs.