Global climate change is projected to produce warmer, longer, and more frequent droughts, which have the potential to trigger widespread tree die-off, as exemplified by recent regional-scale die-off of pinyon pine in the southwestern US.  Despite the importance of such potentially rapid and extensive changes in vegetation, drought-induced tree mortality cannot be predicted with confidence because experimental studies focusing on drought-induced tree mortality are lacking.  To address this issue, we are initiating a pair of experimental studies in which water will be excluded from trees under ambient and increased temperatures.  Trees in the first study have smaller root balls (~0.5 m diameter x ~0.5 m deep) and have been placed within different temperature regions of the Biosphere 2 facility.  Trees in the second study will have larger root balls (~3 m diameter x ~2 m deep) and will be transplanted on an elevation gradient, as a surrogate for changing temperatures.  Here we provide an overview of these two studies and provide an initial assessment of a key issue relevant to their comparison: what proportion of roots is likely to accompany the two types of transplants and to what degree will this influence the overall results.

Results/Conclusions

Based on existing published literature values, we evaluated how depth and lateral spread distributions for pinyons and for other woody plants relate to the root ball volumes of interest.  The synthesis indicated that shoot height to root depth ratio for pinyon is small in comparison with most tree species.  Our estimates indicate that a root ball of 3 m diameter x 2 m deep will likely capture the overwhelming majority of both tap and lateral roots of pinyon trees under 2 m in height for landscapes with well-developed soils, whereas a 0.5 m diameter x 0.5 m deep root ball on a < 2 m tall tree will include less than half of the maximum rooting depth and less than a quarter of the maximum lateral spread.  These assessments provide a basis for considering differential responses associated with root ball size in the two studies, which can enable the broader and more important issue of improving predictive capability for drought-induced tree mortality.