Pushing limits: Altered temperature and precipitation differentially affect plant species inside and beyond their current ranges

Background/Question/Methods

There is increasing evidence that climate change is leading to shifts in plant species geographic ranges.  It is projected that many species will need to establish viable populations beyond their current ranges or risk extinction in coming decades.  However, current understanding of how range distributions may shift in response to climate change is based solely on observed historical shifts and model projections.  Although recently observed range shifts offer strong empirical evidence of climate change impacts, it is impossible to decouple climate effects from those of other environmental changes.  Because many biotic and abiotic factors at both local and regional scales control species’ range-limits, experimentation is essential to test the underlying mechanisms.  To this end, we embedded a fully factorial manipulation of temperature (+3°C) and precipitation intensity (+20% above ambient) into a natural 520-km latitudinal climate gradient in three upland prairies in the Pacific Northwest (PNW) to determine how future climate change will affect the range distribution of native plant species.  Treatments were initiated in 2010, and in the fall, twelve native forbs and grasses that have their northern range limits within the PNW were seeded into each plot.  Germination, survivorship, plant size, and seed set were measured in 2011.

Results/Conclusions

For species’ planted within their current range, increased temperature negatively impacted germination rates, but this negative heating effect disappeared when the species were moved poleward beyond their current range.  Germination also presented the most significant hurdle to species’ success.  Once species were able to germinate, a species’ current range no longer impacted survivorship, plant growth, or fecundity.  Instead, warming negatively impacted survivorship regardless of current range, but if individuals were able to survive, they grew bigger and produced more seeds in the heated treatment.  Increased plant growth in the heated treatments could be explained by an indirect effect of increased nutrient availability.  We found minimal effects of added precipitation on any life-history stages, but when significant, increased precipitation decreased germination and survivorship.  Our results are consistent with predictions that many species will need to expand their ranges poleward or higher in elevation to successfully maintain viable populations.  However, multiple years of data will be necessary to understand if, once established, these species’ will be able to maintain positive population growth.