POTENTIAL IMPACTS OF TIMBER HARVESTING ON A RARE UNDERSTORY PLANT, CLEMATIS-HIRSUTISSIMA VAR ARIZONICA

Arizona leatherflower Clematis hirsutissiuma Pursh. var. arizonica (He ller) Erickson, a geographically rare species, is potentially threaten ed by timber harvest in its primary habitat. To determine the optimal conditions for reproduction and population growth and to assess the im pact 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 herb ivory and population growth. In 1994, we also assessed bud mortality, seed quality and insect seen predation. To determine whether physiolog ical or edaphic conditions could explain reproductive performance unde r varying light regimes, we also measured litter depth, relative cover , foliar gas exchange and water potential under field conditions. Resu lts suggest that intermediate light levels (50-65% of full sun) are op timal for Clematis reproduction and population growth. At natural ligh t levels lower than 40%, plants had significantly lower stem productio n, seed production, ann photosynthetic rates than plants growing in hi gher light levels. At light levels greater than 75%, although photosyn thetic rates were significantly higher than at lower light levels, pla nts suffered from (I) significantly higher bud mortality; (2) signific antly lower seed viability and seedling establishment; (3) significant ly higher mammalian herbivory in two of the four years, (4) significan tly higher exposure to competing grasses and other plant species; and (5) significantly lower mid-season water. potentials. Experimental rem oval of canopy cover had inconsistent impacts on Clematis reproduction . While stem and seed production decreased, seedling recruitment was g reater than controls by the fourth year of the study. Conversely, expe rimental shading consistently and significantly increased Clematis rep roduction. In comparison to all other environments, shade-addition env ironments had decreased bud mortality, increased seed set in 1992, and led to the greatest increases in seedling recruitment and total numbe rs 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 pote ntial and photosynthetic rate), but may have been the result of lower titter depth in shade-addition environments. This suggests that forest litter accumulation in closed canopies may be adversely affecting Cle matis population growth. (C) 1997 Published by Elsevier Science Ltd.