TEMPORAL VARIATION IN TEMPERATURE AND RAINFALL DIFFERENTIALLY AFFECTSECTOMYCORRHIZAL COLONIZATION AT 2 CONTRASTING SITES

We examined the roles that seasonal shifts in precipitation and temper ature played in the ectomycorrhizal (ECM) colonization of pinyon pine (Pinus edulis Engelm.) at two contrasting sites in northern Arizona. P inyons growing in ash and cinder soils experienced much greater water and nutrient stress than pinyons growing nearby in sandy-loam soils. O ver a one year period, we obtained monthly measurements of ECM coloniz ation, root zone soil moisture and temperature, and air temperature an d precipitation. Four major patterns emerged. Firstly, although climat e as measured by ambient temperature and precipitation did not vary be tween the two sites, soil temperature was significantly higher and soi l moisture significantly lower at the cinder site than at the sandy-lo am site. Secondly, ECM colonization was significantly higher at the ci nder site for 5 of 12 months. Thirdly, although nearly 70 % of the var iation in ECM colonization of pinyons growing in cinder soil was predi cted by a combination of soil moisture and soil temperature, these sam e variables had little predictive power for pinyons growing in sandy-l oam soils. Air temperature and precipitation were also significantly c orrelated with ECM colonization at the cinder site but not the sandy-l oam site. Fourthly, a watering experiment showed that ECM colonization significantly increased with supplemental water at the cinder site, b ut not at the sandy-loam site. Thus, in two sites that did not differ in plant community or climate, ectomycorrhizas in cinder soils were fa r more sensitive to changes in moisture and temperature than ectomycor rhizas in sandy-loam soils.