Factors Controlling Plant Distributions: Drought, Competition, and Fire in Montane Pines in Arizona

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Abstract
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 moisture and decreasing forest—floor light. With increasing elevation (1568—2296 m) in the Chiricahua Mountains in southeastern Arizona, soil moisture and plant cover increased and, as a result, mean forest—floor light levels decreased, in accordance with the models. The light—moisture 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 distributions, and (3) low soil moisture availability and lack of drought resistance should control lower elevational limits. With increasing elevation, however, fire frequency and litter depth also increased and soil temperature decreased. I tested the trade—off hypothesis and the role of these additional factors in controlling upper elevational limits of three pine species distributed along this gradient. Consistent with the trade—off hypothesis, results suggested that water stress controlled lower elevational limits of all three species. Seeds of each 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 stress. In contrast, seedlings were still alive in experimental plots within each species' range after 2 yr. Furthermore, with decreasing elevation, 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, seedling survival, and seedling abundance decreased but height growth increased. Thus, dry season water stress appeared to control lower elevational 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 were not controlled uniformly across species by light limitation. In Pinus leiophylla, the middle elevation species, low light and deep litter appeared to control the upper elevational limits. In a field experiment, P. leiophylla emergence and survival were significantly lower above its upper elevational limit than in plots within its range, removal of litter increased emergence, and removal of canopy increased seedling survival. In a greenhouse experiment, P. leiophylla was significantly less shade tolerant than higher elevation pine species. In contrast, P. discolor, the low elevation species, low light, deep litter, and low soil temperature appeared not to influence distribution. Emergence and survival were actually higher at high than middle elevations in the field experiment. Litter removal and canopy removal did not increase P. discolor emergence and survival, respectively, even at high elevation. In the highest elevation plots, P. discolor seedlings occurred in microsites slightly lower in light, higher in litter depth, and equivalent in soil temperature to random microsites, contrary to expectations if these variables were limiting. Finally, in greenhouse experiments, P. discolor was more shade tolerant than higher elevation species, including P. leiophylla. Two tests supported the hypothesis that the upper elevational limits of P. discolor were controlled by the high fire frequency found at higher elevation. First, P. discolor exhibited slow juvenile growth rates, thin bark, and other traits suggesting a lack of fire resistance compared with the two higher elevation pine species. Second, in two wild fires, survival of P. discolor stems was significantly lower than that for the other two species. This conclusion is corroborated by the observation that juvenile P. discolor occurred commonly at much higher elevations than did adults, into plots with very low light and soil temperature levels and very deep litter, a pattern likely resulting from fire suppression. Results for a third species, P. engelmannii, were equivocal, showing weak support for control of 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—associated regeneration of the other two species. Nevertheless, control of P. discolor upper elevational limits by fire may, in part, be a result of constraints imposed by drought resistance on maximum growth rate and height. These results suggest that fire, or other agents of selective mortality correlated with soil resource gradients, can exert strong control over plant distribution and community composition, and should be incorporated into the proposed general models relating plant strategies to community structure.

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