FACTORS CONTROLLING PLANT-DISTRIBUTIONS - DROUGHT, COMPETITION, AND FIRE IN MONTANE PINES IN ARIZONA

Recent models suggest that a trade-off in plants between tolerance of water limitation vs. tolerance of light limitation results in changes in dominant species over productivity gradients of increasing soil moi sture and decreasing forest-floor light. With increasing elevation (15 68-2296 m) in the Chiricahua Mountains in southeastern Arizona, soil m oisture and plant cover increased and, as a result, mean forest-floor light levels decreased, in accordance with the models. The light-moist ure trade-off hypothesis predicts that, over this gradient, (1) shade tolerance and drought resistance should be negatively correlated, (2) decreasing light and lack of shade tolerance (i.e., tolerance of light competition) should control upper elevational limits of species distr ibutions, and (3) low soil moisture availability and lack of drought r esistance should control lower elevational limits. With increasing ele vation, however, fire frequency and litter depth also increased and so il temperature decreased. I tested the trade-off hypothesis and the ro le of these three additional factors in controlling upper elevational limits of three pine species distributed along this gradient. Consiste nt with the trade-off hypothesis, results suggested that water stress controlled lower elevational limits of all three species. Seeds of eac h species germinated with the summer rains in experimental plots below their respective lower elevational limits, but all seedlings died by the end of the following May-June drought, apparently from water stres s. In contrast, seedlings were still alive in experimental plots withi n each species' range after 2 yr. Furthermore, with decreasing elevati on, seedlings of the three species increasingly occurred in microsites with relatively low light, low soil temperature, and deep litter, all reflecting high soil moisture compared to random microsites. From the middle to the lower portion of each species' range, recruitment, seed ling survival, and seedling abundance decreased but height growth incr eased. Thus, dry season water stress appeared to control lower elevati onal limits by causing high mortality of young seedlings, rather than by curtailing seed germination or the performance of older seedlings. Inconsistent with the trade-off hypothesis, upper elevational limits w ere not controlled uniformly across species by light limitation. In Pi nus leiophylla, the middle elevation species, low light and deep litte r appeared to control the upper elevational limits. In a field experim ent, P. leiophylla emergence and survival were significantly lower abo ve its upper elevational limit than in plots within its range, removal of litter increased emergence, and removal of canopy increased seedli ng survival. In a greenhouse experiment, P. leiophylla was significant ly less shade tolerant than higher elevation pine species. In contrast , in P. discolor, the low elevation species, low light, deep litter, a nd low soil temperature appeared not to influence distribution. Emerge nce and survival were actually higher at high than middle elevations i n the field experiment. Litter removal and canopy removal did not incr ease P. discolor emergence and survival, respectively, even at high el evation. In the highest elevation plots, P. discolor seedlings occurre d in microsites slightly lower in light, higher in litter depth, and e quivalent in soil temperature to random microsites, contrary to expect ations if these variables were limiting. Finally, in greenhouse experi ments, P. discolor was more shade tolerant than higher elevation speci es, including P. leiophylla. Two tests supported the hypothesis that t he upper elevational limits of P. discolor were controlled by the high fire frequency found at higher elevation. First, P. discolor exhibite d slow juvenile growth rates, thin bark, and other traits suggesting a lack of fire resistance compared with the two higher elevation pine s pecies. Second, in two wild fires, survival of P. discolor stems was s ignificantly lower than that for the other two species. This conclusio n is corroborated by the observation that juvenile P. discolor occurre d commonly at much higher elevations than did adults, into plots with very low light and soil temperature levels and very deep litter, a pat tern likely resulting from fire suppression. Results for a third speci es, P. engelmannii, were equivocal, showing weak support for control o f upper elevational limits by light. The lack of a light-soil moisture trade-off in these species may result from P. discolor's strategy of exploiting nurse tree sites at low elevation and the apparent fire-ass ociated regeneration of the other two species. Nevertheless, control o f P. discolor upper elevational limits by fire may, in part, be a resu lt of constraints imposed by drought resistance on maximum growth rate and height. These results suggest that fire, or other agents of selec tive mortality correlated with soil resource gradients, can exert stro ng control over plant distribution and community composition, and shou ld be incorporated into the proposed general models relating plant str ategies to community structure.