Potential impacts of timber harvesting on a rare understory plant, Clematis hirsutissima var. arizonica

Arizona leatherflower Clematis hirsutissiuma Pursh. var. arizonica (Heller) Erickson, a geographically rare species, is potentially threatened by timber harvest in its primary habitat. To determine the optimal conditions for reproduction and population growth and to assess the impact of canopy removal, we experimentally manipulated canopy cover by adding and reducing shade over plants growing in varying light levels. Over four years (1991-1994), we assessed reproduction, mammalian herbivory and population growth. In 1994, we also assessed bud mortality, seed quality and insect seed predation. To determine whether physiological or edaphic conditions could explain reproductive performance under varying light regimes, we also measured litter depth, relative cover, foliar gas exchange and water potential under field conditions. Results suggest that intermediate light levels (50-65% of full sun) are optimal for Clematis reproduction and population growth. At natural light levels lower than 40%, plants had significantly lower stem production, seed production, and photosynthetic rates than plants growing in higher light levels. At light levels' greater than 75%, although photosynthetic rates were significantly higher than at lower light levels, plants suffered from (1) significantly higher bud mortality; (2) significantly lower seed viability and seedling establishment; (3) significantly higher mammalian herbivory in two of the four years; (4) significantly higher exposure to competing grasses and other plant species; and (5) significantly lower mid-season water potentials. Experimental removal of canopy cover had inconsistent impacts on Clematis reproduction. While stem and seed production decreased, seedling recruitment was greater than controls by the fourth year of the study. Conversely, experimental shading consistently and significantly increased Clematis reproduction. In comparison to all other environments, shade-addition environments had decreased bud mortality, increased seed set in 1992, and led to the greatest increases in seedling recruitment and total numbers of individuals in 1993 and 1994. Improved Clematis reproduction in shade-addition environments in comparison to naturally closed canopies was not attributable to measured physiological parameters (water potential and photosynthetic rate), but may have been the result of lower litter depth in shade-addition environments. This suggests that forest litter accumulation in closed canopies may be adversely affecting Clematis population growth.