DETERMINANTS OF PLANT-DISTRIBUTION - EVIDENCE FROM PINE INVASIONS

The question of which factors limit the occurrence of a plant species to a particular site is addressed by considering 53 cases in which the distribution of pines (Pinus species; Pinaceae) has changed in the last century. We consider expansions of pines in and adjacent to their natural ranges in the Northern Hemisphere and the spread from sites of introduction in the Southern Hemisphere well outside the contemporary range of pines. We first consider a neutral hypothesis (with respect to climate or biological interactions as determinants of invasion): invasion simply requires that a species is present in sufficient numbers, with sufficient propagules over sufficient time to invade. We then explore the relative importance of climatic changes. disturbance, competition (including competition between seedlings and herbaceous plants during early establishment), herbivory, pathogens, and other agents that might influence pine membership in communities. Determinants of susceptibility to invasion often interact in a complex fashion. Environmental stresses created by moisture and temperature appear to exercise primary control on invasibility at xeric and high-elevation sites, respectively, but play a smaller role at intermediate locations. At these sites, range limits are determined principally by interactions between pine seedlings and the resident biota in adjacent communities. This suggests that the effects of predicted global warming on the distribution of pines are unlikely to be simple functions of temperature or precipitation. except at climatic extremes. Pine invasions are most prevalent where there is limited competition in the regeneration niche and occur more easily in habitats where the dominant growth form is most different from that of pines, namely, in grasslands. The disturbance regime in the receiving habitat is important and interacts directly and indirectly with the "inherent" susceptibility to invasion. Severe disturbances may initiate pine invasions at any latitude, but are essential for reducing the cover of vigorous plant growth (and hence competition in the regeneration niche) in tropical and near-tropical regions. Contemporary practices such as deforestation and increased grazing pressure and those practices leading to accelerated erosion, modified fire regimes, and climate amelioration essentially duplicate ice-age stresses and disturbances that shaped plant communities in the Holocene. The fundamental role of biotic factors (either direct or indirect) in regulating the distribution of pines is discussed with reference to aspects of land husbandry including the management of biological invasions.