We investigated species-specific relationships among two species of vascular epiphytes and ten host tree species in a coastal plain forest in the southeastern United States. The epiphytes Tillandsia usneoides and Polypodium polypodioides were highly associated with particular host species in the field, but host traits that favored colonization were inadequate to fully explain the epiphyte-host associations for either epiphyte. Field transplant experiments that bypassed epiphyte colonization demonstrated that the growth of epiphytes was significantly higher on host tree species that naturally bore high epiphyte loads than on host species with few or no epiphytes. These species-specific relationships were highly correlated with the water-holding capacity of the host tree's bark. Positive and negative effects of throughfall, light attenuation by the canopy, and bark stability did not explain the overall patterns of host specificity, but did correlate with some epiphyte-host species relationships. The relative importance of particular host traits differed between the "atmospheric epiphyte" Tillandsia, and the fern Polypodium, which roots in the bark of its hosts. Species-specific interactions among plants, such as those described here, suggest that communities are more than individualistic assemblages of co-occurring species.

Circumstantial evidence suggests that Artemisia tridentata may out—compete Pinus ponderosa and P. jefferyi for water at ecotones between shrub steppe and montane forest vegetation in the Great Basin. Other studies indicate that within the shrub steppe Artemisia may act as a nurse plant for a third species of pine, P. monophylla. We used field experiments to study these contrasting effects of Artemisia on P. ponderosa and P. monophylla within the context of the distributional patterns in western Nevada of all three species on andesite, and on sites where hydrothermal activity has altered the andesite. At intermediate elevations in the Great Basin Artemisia and P. monophylla are restricted to unaltered desert soils, whereas P. ponderosa is restricted to acidic, nutrient—poor altered andesite. Although mature P. monophylla were virtually absent in our study plots on altered andesite, first— and second—year seedlings were common. On adjacent unaltered andesite, all size classes of P. monophylla occurred, and P. monophylla seedlings were associated with Artemisia shrubs. Pinus ponderosa and P. jefferyi adults and seedlings were rare on unaltered andesite, but a wide range of size classes was found on altered andesite. In experiments, all P. ponderosa seedlings on unaltered andesite were consumed by predators regardless of positive or negative spatial association with shrubs. Of the P. monophylla seedlings that germinated on unaltered andesite, all that were under shrubs survived, but only 6% of those that germinated in the intershrub spaces survived. On the open altered andesite the mortality of P. monophylla seedlings due to abiotic stress was high, with a final survival of only 3%, whereas 28% of P. ponderosa seedlings survived the first growing season on altered andesite. On unaltered andesite, survival and conductance of P. ponderosa saplings was enhanced by shrub removal, but P. monophylla survival was significantly higher under shrubs than in shrub—removal plots or in intershrub spaces. In Artemisia—removal experiments, we found that Artemisia competed with P. ponderosa seedlings and saplings for water. Removal of Artemisia decreased water use efficiency (WUE) of P. monophylla seedlings. The absence of Artemisia may restrict Pinus monophylla from outcrops of altered andesite in the Great Basin, but provide refuges for P. ponderosa.

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As forests age, their structure and productivity change, yet in some cases, annual rates of water loss remain unchanged. To identify mechanisms that might explain such observations, and to determine if widely different age classes of forests differ functionally, we examined young (Y, approximately 25 years), mature (M, approximately 90 years) and old (O, approximately 250 years) ponderosa pine (Pinus ponderosa Dougl. ex P. Laws.) stands growing in a drought-prone region of central Oregon. Although the stands differed in tree leaf area index (LAIT) (Y = 0.9, M = 2.8, O = 2.1), cumulative tree transpiration measured by sap flow did not differ substantially during the growing season (100-112 mm). Yet when water was readily available, transpiration per unit leaf area of the youngest trees was about three times that of M trees and five times that of O trees. These patterns resulted from a nearly sixfold difference in leaf specific conductance (KL) between the youngest and oldest trees. At the time of maximum transpiration in the Y stand in May-June, gross carbon uptake (gross ecosystem production, GEP) was similar for Y and O stands despite an almost twofold difference in stand leaf area index (LAIS). However, the higher rate of water use by Y trees was not sustainable in the drought-prone environment, and between spring and late summer, KL of Y trees declined fivefold compared with a nearly twofold decline for M trees and a < 30% reduction in O trees. Because the Y stand contained a significant shrub understory and more exposed soil, there was no appreciable difference in mean daily latent energy fluxes between the Y stand and the older stands as measured by the eddy-covariance technique. These patterns resulted in 60 to 85% higher seasonal GEP and 55 to 65% higher water-use efficiency at the M and O stands compared with the Y stand.