Drought tolerance in Pinus ponderosa and Juniperus virginiana seedlings under controlled environment

Populations of Pinus ponderosa and Juniperus virginiana are expanding into the semiarid Sandhills grassland in Nebraska (USA). While P. ponderosa is expanding from historical grassland-woodland ecotones, J. virginiana is spreading throughout the Sandhills. One of the main factors contributing to the structure of ecosystem vegetation in semi-arid grasslands is soil water availability. The objective of this study is to examine the ecophysiological basis of this expansion by determining the responses of two year-old seedlings of P. ponderosa and J. virginiana to a 40-day dry down period under controlled environment. Net photosynthesis (A), stomatal conductance (gs), maximum carboxylation rate (Vcmax) and maximum electron transport (Jmax) were higher, and water potential (Ψw) was more negative in J. virginiana than P. ponderosa throughout the study. PSII maximum efficiency (Fv/Fm) did not differ among species. Net photosynthesis and gs in P. ponderosa and J. virginiana declined gradually in the first 15 and 18 days of water deprivation respectively, before significantly dropping to minimal rates. Vcmax and Jmax declined linearly with the increase in drought stress. This decline was steeper for P. ponderosa than J. virginiana. The decrease in Ψw and Fv/Fm lagged behind the other measured parameters. Pinus ponderosa and J. virginiana maintained relatively high Ψw for 27 and 30 days of water deprivation respectively before exhibiting significant declines. Negative net photosynthetic rates were recorded in P. ponderosa beyond day 33 and in J. virginiana at day 40 (while Ψw = -2.7 and -4.3 MPa, respectively).  We conclude that P. ponderosa used a drought avoidance strategy, with less capacity to maintain assimilation through drought stress than J. virginiana. This helps explain the widespread expansion of J. virginiana relative to P. ponderosa in the Sandhills.