Potential Resilience of Low-Latitude Ecosystems to Climate Change - How Much & How Long?

Climate change experiments including temperature and drought manipulations have long been conducted in northern and mid-latitude biomes. Here we report on soil and plant responses to climate change from two lower-latitude systems: deciduous forests of the southeastern U.S. Piedmont (33°53'17.87"N, 83°21'40.92"W) and Costa Rican montane forests (8o47'7"N, 82o57'32"W). In the Whitehall Forest Warming Facility of Athens, GA after 3-years of continuous +3oC and +5oC soil warming, soil carbon fluxes, pools and chemistries exhibited negligible responses to warming in both drought (2011, 2012) and non-drought (2013) years. Similarly, warming had little effect on soil net nitrogen mineralization or on tree growth and survivorship. These data suggest highly-weathered soils, such as the Ultisols of the Piedmont and potentially Ultisol and Oxisol soils of tropical systems, will not exhibit dramatic increases in soil respiration in response to climate warming. Although temperature, soil moisture and season had important direct effects, warming treatments did not impact temperature sensitivities of soil respiration. In another 5-year field study at the Las Cruces Biological Station, Costa Rican montane tree species from high-elevations did not exhibit differences in growth or survivorship when planted at 2oC warmer mid-elevation plots. However, when grown at 4oC warmer low-elevation sites, these individuals did not survive for more than a few months. In a controlled temperature and drought study, growth of high-elevation trees either did not respond to warmer and/or drier conditions (Quercus insignis), or actually grew better under drier conditions (Viburnum costaricanum), at least over the initial 3-month establishment period. These and other results suggest that while high-elevation tropical montane species may be resilient to moderate warming (+2oC), they will not survive much hotter conditions of +4oC. Further, while drier soil moisture (20% versus 30% moisture) in our controlled pot study did not hinder the growth of these trees, in actual field settings drought is accompanied by reductions in cloud cover, reduced relative humidity and increased UV exposure, highlighting complex effects of drought on real-world environmental conditions.