Isotopic analysis in columnar ( 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. griseus was calculated as 12.6 the calibration its F14C age corresponded to ± 5.5 cm/year, without taking into account 1998, indicating that the age of this cactus the growth rate during branching process. was >17 years. Clam calibration was not functional after 2008.7 which corresponds to Every 2.8±0.9 cm an areole emerged from the spine located at 169.7 cm. the stem. In this way, each year the cactus produced approximately four areoles, which From 48.6 to 169.7cm an age-model was means that every three months a new areole constructed with 9 spines, and the growth was produced and had around seven spines. rate was estimated as 12.6±5.7 cm/year. Once this was determined, δ13C values were Growth rate is not lineal (Figure 1), as can be compared to four different climatic variables: seen during branching process, at 107.9 cm, temperature, precipitation, relative humidity in which the growth rate decreased to 1.4 and VPD. cm/year, growing 5 cm from 2001.5 to 2005. Growth rate was 12.6±4.2 cm/year before the Climatic variables ramification, from 2005 to 2006, the Related to climatic variables, from 1998 to ramification grew 20.9cm and then from 2014, monthly average temperature was 2006 to 2008.7 growth rate was 13.3±1.1 from 25.6 to 31.1 °C, precipitation varied cm/year. from 0.4 to 416.1 mm and relative humidity percentages were from 50 to 83%. VPD was The last spine, located at 252,6cm, was calculated using temperature and relative growing during April of 2015. Which means humidity (Murray, 1967), and values were that from 2008.7 to 2015 the ramification from 557.9 to 2133.23 Pa. grew 82.7cm. In this way, the average growth rate for these years was estimated as Applying Mann-Kendall trend test (α=0.05), 12.7 cm/year, and this allowed the age-model decreasing trends were shown in temperature Figure 2. Cactus age-model. Nine spines were calibrated (black dots) for the reconstruction. Cactus age is >17 years and its growth rate is not completely lineal. Growth rate for S. griseus has been established as 12.6 ± 5.5 cm/year, during branching process it decreased to 1.4 cm/year. The age of the other nine spines (gray dots) was established according to the age-model of calibrated spines.

and VPD series from 1998 to 2005 (p- rate did not vary significantly across cactus value=2.84E-04; p-value=3.54E-04) (Fig. 3- growth in comparison to the variations that 4), while relative humidity had an increasing Stenocereus thurberi have shown. In S. trend (p-value=2.7E-04) (Annex-Fig.5). thurberi growth rates in individual are Nonetheless, these tendencies are not 7 ± 6 cm/year when height is less than uniform in all the series, and increasing trend 1m and 62±3 cm/year when it is more than is exposed in both series from 2005, after the 5m (Parker, 1988). branching process (p-value=3.82E-05; p- value=1.47E-03). Finally, there was no Contrary to Parker (1988) growth rate in significant trend in precipitation series single stems or ramifications S. griseus (Annex-Fig. 6). growth rate did not decrease as stem size increased. Assuming that the growth rate Knowing that each areole is produced every established for S. griseus is invariable, it is three months, a moving average was possible to determine its age as ~21 years. calculated for the climatic variables (Fig. 2- Nevertheless, in other columnar cacti, such 5). The moving average grouped 3 months of as saguaro (C. gigantea), growth rates are each climatic series. And the comparisons slower at youth, which makes possible that were made with the moving average and each this S. griseus individual is older (Drezner, δ13C value that corresponded to a spine of 2005). Despite of this, there are no one areole. established patterns for S. griseus that contribute to an exact growth rate Discussion establishment for this individual before the 48.6 cm. Age-model The growth rate for this S. griseus has been Related to growth rate decrease during established as 12.6 ± 5.5 cm/year. Without branching process compared to C. gigantea, taking into account branching process, this there was no correlation between rainfall and ramification growth in S. griseus (Drezner, δ13C analyses and climatic variables 2003), as in the absence of an increasing During branching process only four spines trend in precipitation series before or during grew, which means that during 3.5 years branching. This leads to think that slow isotopic signal was not able to register growth rate can be addressed to low water climatic events, making the model not availability (Hastings and Alcom, 1961 and reliable during these years. In this way, for Steenbergh and Lowe, 1983). Furthermore, comparisons of δ13C values and climatic branching process has been determined as a series, this period was not taken into account. complex process in columnar cacti that requires time and multiple resources Congruent periods were found comparing (Schwager et al., 2013). In this case, S. temperature with δ13C values (Fig. 3). griseus have multiple ramifications which Specifically, during 1998 both series had increase even more the complexity and decreasing trends and increasing trends resources requirement, so the cactus have occurred from 2005 to 2014, same tendencies differential allocation of resources for other were seen in VPD series (Fig. 4). Two ramifications or sexual growth, which notable peaks were exposed in temperature ultimately contributes in growth rates and VPD series from 2005.7 to 2006.7, and decrease (Bowers, 1996). both were recorded by isotopic signal.

Even each cactus age-model is site-specific Amplitudes size in δ13C values were more (Drezner, 2006), this first age-model for related to VPD series than temperature did. Stenocereus griseus in the Desierto de la δ13C series registered congruently these Tatacoa, contributes to growth rates and amplitudes in three periods: from 1998 to branching processes understanding, using a 2000, from 2009.6 to 2010 and from 2011.6 recently established methodology for spines to 2014 (Fig. 3). dating using F14C (English et al., 2007)

Figure 3. Temperature and δ13C series. Shaded region ( ) represents the branching process, a 3.5-period in which isotopic signal of S. griseus spines is not a reliable temperature proxy. There is a ddecreasing trend in temperature series from 1998 to 2000 that is congruent to δ13C trend. Arrow is indicating an increasing trend in both series observed from 2005 to 2014.

As no-congruent signal was exposed for again to a lower discrimination in the cactus precipitation series and δ13C values, S. (Shirke and Pathre, 2004). griseus is being more sensitive to temperature and VPD. More positive δ13C RH series was closer to δ13C series, values are being registered from 2005, which overlapping in some occasions, and the is attributed to cactus longevity but also to whole RH series showed an increased trend, higher temperatures (Ehleringer et al. 1992). in comparison to the other three variables This demonstrates that S. griseus spines (Annexes-Fig. 5). However, amplitudes were could be useful for the determination of not related and signals were not clear warmer periods in past environments enough. The clearest congruence was reconstruction, as saguaro did (English et al., presented in 1999, 2010.4 and 2011.2 peaks. 2007) Despite the unclear signals, close RH signal contributed to find δ13C amplitudes similar to In fact, δ13C seems to be recording ENSO VPD. events in 1998 and 2009-2010, registered in VPD series (Fig. 4). This may be occurring Even trends for climatic series were due to temperature increasing during ENSO increasing from 2005, there is a period in that stresses S. griseus and consequently which δ13C values are not congruent at all reduce δ13C, because it’s discrimination will with temperature or VPD series. This period be lower (Hamerlynck, 2004; English et. al, started after the two peaks in 2005.7 and 2007; GGW, 2016). Also, lower values of 2006.7 and finished in 2010. Which VPD occur during ENSO events as a reinforces the idea that δ13C values in S. consequence of stomatal conductance griseus spines are more sensitive to decrease which alters C1/C2, contributing significant environmental changes, which

Figure 4. VPD and δ13C series. Gray shaded region ( ) represents the branching process, a 3.5-period in which isotopic signal of S. griseus spines is not a reliable proxy. Blue shaded regions ( ) represent ENSO events and its congruence with δ13C signal. There is a ddecreasing trend in temperature series from 1998 to 2000 that is congruent to δ13C trend. Arrow is indicating an increasing trend in both series observed from 2005 to 2014. helps in the reconstruction of recent past Bullock, S.H., Mooney, H.A., Medina, E., 1995. climate events. Seasonally Dry Tropical Forests. Cambridge University Press.

Besides the absence of trend in precipitation, Dirzo, R., Young, H.S., Mooney, H.A., Ceballos, G., this series present the least congruence 2011. In: Seasonally Dry Tropical Forests: between the four climatic variables (Annexes Ecology and Conservation. Island Press, - Fig. 6). However, this does not mean that Washington, DC. 18 S. griseus spines are not useful because δ O Drezner T.D., 2003. A test of the relationship signals may be recording precipitation between seasonal rainfall and saguaro cacti variability (English et al., 2010b) branching patterns. Ecography 26:393–404

The use of S. griseus spines to construct an Drezner, T.D., 2005. Saguaro (Carnegiea gigantea, age-model and reconstruct recent climate Cactaceae) Growth Rate over Its American Range and the Link to Summer Precipitation. The changes help in the understanding of growth Southwestern Naturalist, 50(1): 65-68. and physiological features of this species under climate change scenarios. Moreover, Drezner, T.D., 2006. Regeneration of Carnegiea this study contributes to understand the gigantea (Cactaceae) since 1850 in three dynamics of the Desierto de la Tatacoa under populations in the northern Sonoran Desert. Acta Oecologica 29, pp. 178–186. climate variations and develops a novel method in Colombia that improves tropical Ehleringer, J.R., Phillips, S.L., Comstock, J.P., 1992. dry forests and S. griseus knowledge and, at Seas