Isotopic analysis in columnar cactus (Stenocereus griseus) spines in the Desierto de la Tatacoa: climatic and physiological approaches Juanita Moreno1, Eloisa Lasso de Paulis2, Catalina González1 1Laboratorio de Palinología y Paleoecología Tropical, Departamento de Ciencias Biológicas, Universidad de los Andes 2Laboratorio de Ecología y Fisiología Vegetal, Departamento de Ciencias Biológicas, Universidad de los Andes Abstract Tropical dry forests are one of the most endangered ecosystems due to human activity and climate change effects. However, there are few studies related to tropical dry forests dynamics and their responses to climate change, which is the case of the Desierto de la Tatacoa, a tropical dry forest located in the inter-Magdalena valley in Colombia. Although is not easy to find reliable climatic proxies in arid regions, isotopic analyses in columnar cactus spines have shown that these tissues record recent climate events. In this way, δ13C was analyzed in 74 spines of an individual Stenocereus griseus, from la Tatacoa, in order to find a suitable proxy for this region. An age-model for the cactus was reconstructed, with 18 spines dated using F14C. The age of the cactus was >17 years and its mean growth rate (± SD) was 12.6±5.5 cm/year, variating from 1.4 to 21 cm/year and affected by the significant decrease during branching process. Isotopic signal was compared with climatic variables (temperature, precipitation, relative humidity, vapour pressure deficit) for 1998-2004 time series. The most relevant correlation between δ13C signal and climatic variables was found for temperature and VPD, this correlation indicates how cactus respond to different environmental conditions and contributes in the understanding of climate change effects over S. griseus and the Desierto de la Tatacoa. Keywords: isotopic signal, Stenocereus griseus, columnar cactus, tropical dry forest, growth rate, ecophysiology, climate change, tropical dry forest, VPD, δ13C, F14C. Introduction conservation programs that have received more attention (Bullock et al., 1995, Dirzo et Tropical dry forests provide many ecosystem al., 2011). services for thousands of people around the world, and although tropical rainforests are In order to contribute with tropical dry more diverse, tropical dry forests count with forests conservation, it is important to a higher variety of species activities. Despite understand their dynamics and analyze how this, tropical dry forests are the most climate change can affect these ecosystems. threatened tropical ecosystems and the least One way to achieve this understanding is the studied (Janzen, 1988; Miles et al., 2006; reconstruction of past environments. Mooney, 2011). This lack of studies has not However, compared to other ecosystems allowed the development of conservation there are no many suitable proxies for recent policies that protect these ecosystems, climate changes in arid and semi-arid contrary to the tropical rainforests regions, as a consequence of erosion (English studies related to these cacti are limited, et al., 2007). Sutton et al. (1976) suggested which impedes the understanding of that δ13C variations in Opuntia bigelovii were physiological processes of columnar cacti responding to long-term environmental and consequently their habitat (Villalobos et changes and they proposed that these al., 2007) isotopic records were a useful tool for past environments reconstruction. For this reason, in an attempt to reconstruct recent climate changes in the Desierto de la Following this idea, English et al. (2007) Tatacoa we studied the δ13C composition in studied the isotopic composition in lignified spines of Stenocereus griseus. Furthermore, spines of a columnar cactus (Carnegiea spines were dated with F14C allowing the gigantea) in the Sonoran Desert. They used construction of an age-model for the cactus, these tissues because they are added and which finally contributed to the preserved sequentially in cacti stems, which understanding of cactus growth that is really useful for environments remained unknown. Besides the reconstruction. For this study, they analyzed reconstruction of recent climate changes, δ13C and δ18O composition in the spines of a stable isotopes in spines indicated the 4-m-tall saguaro. And they found a strong different physiological processes that cactus correlation between δ13C and δ18O signals have under different environmental and both of them showed near-annual conditions. oscillations over approximately 15 years. The reconstruction of past environments and After these analyses, they established that the understanding of physiological processes changes in isotopic signal of spines were in Stenocereus griseus help us to study the related to rainfall and vapor pressure deficit dynamics of this important tropical dry (VPD). The lowest δ13C and δ18O values forest, Desierto de la Tatacoa. Ultimately, were observed during El Niño years, this research shows the importance of suggesting that spines of columnar cacti can conservation for this cactus and the tropical be used as proxies for past climate events, as dry forest that inhabits. tree rings do. Ultimately, these records also contribute to understand the columnar cacti Methods ecophysiology and metabolic processes Spine sampling under different environmental conditions For isotopic analyses, we collected spines in (English et al., 2010a) April 2015 from a Stenocereus griseus individual (3° 13’ 55.7’’N, 75° 09’28.2” W) In this way, columnar cacti located in which height was 3.5 m and had eight tropical dry forest could help to reconstruct ramifications. The sampling began at 48 cm past environments of tropical dry forest. For of the main stem, due to the absence of the current project, Stenocereus griseus, a preserved spines before this height, and multi-stemmed cactus located in the Desierto continued along the first ramification, 2.52 m de la Tatacoa, one of the most important tall, that emerges at 107 cm of the main stem tropical dry forests in Colombia, seems to be of the cactus. a great tool for past environments reconstruction (IGAC,1977). Additionally, For the sampling, we pulled out with pliers although S. griseus represents an important the longest spine of each areole in the same resource of food for humans and animals, rib of the ramification, this spine collection was done from the top of the ramification (2.52 m) to the bottom of preserved spines of Comparisons between climatic variables and the main stem (48 cm). For the highest isotopic signals areoles we used a 3-m-tall ladder. Once we Once we obtained the δ13C values and gathered a spine, it was saved inside a plastic assigned age for each spine we proceeded to bag labeled with the height of the areole, compare the isotopic signal with different which was measured with a folding ruler that climatic variables. We obtained information we also used to measure the heights of the from Villavieja, a weather station from the cactus and the ramification. In this way, we Instituto de Hidrología, Meteorología y sampled 74 spines for δ13C analyses and we Estudios Ambientales de Colombia also used 18 to analyze F14C and calculate (IDEAM), located at xx km from the cactus spines age and generate an age-model for the that we sampled. From Villavieja we cactus growth. obtained data related to temperature, precipitation and relative humidity (RH). We Isotopic analyses used temperature and RH from Villavieja to We took the 74 spines to the Laboratory of calculate environmental vapour pressure Tropical Palinology and Paleoecology deficit (VPD), (Murray, 1967). All the (PALEO), at the Universidad de los Andes, climatic data that we obtained was from Bogotá, Colombia, and dried them overnight January 1990 to December 2014. at 70°C. Then, we cut the tip of each spine (~3mm) and saved them again in labeled In order to make the comparisons, we plotted plastic bags. After this, we took these spine the isotopic signal for each one of the 74 tips to the Max Planck Institute (Max- spines and at the same time we plotted the Planck-Gesellschaft) in Germany for δ13C climatic variable. After this, we determined analyses using isotope ratios mass the periods in which the signal was spectrometer. We standardized the values convergent and trends in isotopic signal and based on internal calibrations with Peedee climatic variables were analyzed with the belemnite (PDB) and the values that we Mann-Kendall trend test in PAST (Hammer 0 obtained are reported as per mil ( /00) relative et al., 2001) PDB. Results After we dried and cut all the tips of the spines, we rinsed and soaked fragments δ13C analyses below the tip of 18 spines with milli-Q water Within the δ13C values in 74 spines at during 30 minutes and then we saved them different heights of the S. griseus sampled, 14 0 for the F C analyses in the mass the maximum value was -10.17 /00 and the 0 spectrophotometer to calculate the age of minimum was -13.46 /00 (Fig. 1). Mann- each spine. Once we obtained these F14C Kendall trend test, δ13C data showed a values we made a calibration to generate an statistically significant increasing trend age-model for the cactus growth with Clam. during cactus growth (p-value=4.57E-07). We calibrated 9 spines from 48 to 202 cm Once δ13C analyses were carried out, an age because for the most recent spines the model was assigned to each spine, according to the was not reliable. In order to determine the reconstruction of a F14C age-model ages of the other 8 spines we established growth rates and calculated the age for each spine. Figure 1. δ13C for 74 spines from 48.6 to 252.4 cm of the columnar cactus S. griseus. There is an increasing trend in δ13C series along the 203.8 cm of growth. F14C Age-model reconstruction for this cactus. The growth The first spine dated was at 48.6cm and after rate for this S.