Differential responses of soil respiration to warming highlight the sensitivity of tropical Andean tree species to environmental change

Abstract

ABSTRACT : Tropical forests are the largest terrestrial Carbon sink, they occupy 12% to 15% of the Earth's surface but contain about 25% of the world's Carbon biomass, with soils storing ~1500-2400 PgC. Recent studies suggest that forest soils can go from Carbon sinks to sources in the next decades. In the tropics, most information has been collected in lowlands, leaving tropical highland ecosystems, important Carbon regulators that are particularly vulnerable to warming, relatively understudied. In this study, we explored the temperature sensitivity of soil respiration (𝑅𝑠) and its autotrophic and heterotrophic components (𝑅𝑎and 𝑅ℎ, respectively) among 14 dominant plant species (inter and intra-specific variation) in an experimental thermosequence in the Colombian Andes. This thermosequence includes a control site at an elevation of 2486 meters above sea level (mean temperature 14ºC) where seedlings were germinated, and two lower sites that represented an average warming of 8ºC (1326 m.a.s.l) and 12ºC (645 m.a.s.l) where some seedlings were transplanted. Soil and water conditions were controlled such that temperature and meteorological conditions were the only source of variation among treatments. At each site, in all the trees that survived after an acclimation period of one year, we measured 𝑅𝑠 and its components in four measurement campaigns, spanning an overall measurement period of one year. With this information, we calculated 𝑄10 values (the factor by which 𝑅𝑎 and 𝑅ℎ increases for every 10-degree rise in temperature). Our results show, as expected, that 𝑅𝑎 and 𝑅ℎ both increase with the warming, being highest (and more variable) in the warmest site. 𝑄10 values for 𝑅ℎ (𝑄10=2 and 𝑄10=1.71 for the +8°C and +12°C, respectively) in general were much higher than those for 𝑅𝑎 (the species with the highest sensitivity was Quercus humboldtii (𝑄10=3.92) and the lowest was Tibouchina lepidota (𝑄10=0.03)), suggesting that the increase in 𝑅𝑠with warming was mostly driven by increased 𝑅ℎ. Importantly, this behavior in 𝑄10 highlights the differential effects of temperature in dominant Andean species, which impacts forest composition and function. More importantly, the non-linear sensitivity of 𝑅𝑠 to increased temperature, suggests that warming affects the capacity of Andean forests to regulate Carbon fluxes. Overall, our results provide useful information for adapting the management of Andean forests to the impacts of climate change, as well as for the refinement of ecological models that support projections of global environmental change and the Carbon cycle.