How aridity variations affect Prosopis caldenia growth in transitional forests in the semiarid Argentinean Pampas

Abstract

The semiarid Pampas in central Argentina, occupied by caldén’s (Prosopis caldenia Burkart) woodlands (caldenales), had been affected by several wet and dry periods in the last century. Nowadays, the caldenales cover about 17 million hectares of central Argentina and their current state is critical due to massive deforestation rates. Caldén has an important dendrochronological potential since it produces sensitive tree rings linked to climate oscillations, fire events, competence, insect breakout, among others. The standard dendrochronological analysis can be improved with new advances in serial modeling and multivariate ordination techniques for handling problems related to the comparison of dendrochronological samples. We applied a statistical algorithm, BIOdry, which integrates conventional procedures for modeling patterns between annual diameter increments and drought. P. caldenia dendrochronological data were used accounting for multiple sources of variation from the sample design, and comparing patterns from contrasting climatic portions of the study site. The relation between diameter growth dynamics of caldén with temperature and precipitation fluctuations along the second half of the last century was analyzed. Populations at the Northern limit of its natural distribution area were selected. Monthly temperature and precipitation were evaluated in order to identify relative water surplus and deficit periods and an annual aridity index (AAI) was calculated. The objectives of the present work were: to analyze, describe and model response patterns between the P. caldenia diameter growth increment and the AAIs. A common growth response pattern and differences in tree and population level were also analysed. The algorithm BIOdry showed a good behavior, according to the considered statistical parameters (all the fixed effects were statistically significant). Several and common problems associated with the nature of the tree-ring data in modeling (pseudorreplication, autocorrelation, and nested random effects) were efficiently addressed. P. caldenia diameter growth, at tree level significantly responds to different AAI variation intensities along the studied period. A high frequency growth variability at population level was detected when trees establishment happened. These associations ended when trees become older. Low frequency growth variability at population level was significant throughout the analyzed period, presumably linked to age and management history. These results provide news insights in relation to the climate impact on growth dynamics of caldén in the Argentinean pampas and the value of new statistical tools in order to improve dendrochronological studies.

Introduction

Changes in the precipitation and temperature seasonal regimes during the 20^th century had affected ecosystem productivity across the world (Spiecker et al., 1996; Zhou et al., 2001; Leal et al., 2008). Nevertheless, complete understanding about long-term effects of such changes on diverse terrestrial ecosystems remains to be established. Some ecoregions might have been more affected by climate change than others, such as the case of transitional forests: forests growing at the edge of their geographical distributions, where the tree species are more affected by limiting factors (Hampe and Petit, 2005; Rozas and Lamas, 2009). Understanding alterations of these ecosystems are critically important to determine the responses of vegetation communities to the expected climate change (Thomas et al., 2001; Iverson et al., 2004; Travis and Dytham, 2004). These changes would be altering the ecophysiological activity in such ecosystems, and a better understanding of these alterations is mandatory to properly establish management policies (Hampe and Petit, 2005).

Argentinean semiarid pampas, located in the central portion of the country, are covered by extensive transitional forests which had suffered irregular alterations in water availability and other ecological changes during the last century (Viglizzo and Frank, 2006). Climatic alterations in these ecosystems have been accomplished by severe changes in human activities. The western portion of the region has changed from dry periods with active wind erosion processes to wet periods with severe floods events (Minetti and Vargas, 1997; Contreras et al., 2013). Mean temperatures have increased by about 0.3 to 0.6 °C from the late 19^th century, while thermo trends were constantly zero prior to 1910 (Trenberth et al., 2007). Two warming periods occurred in the area between 1910–1940 and 1970 onward (Villalba et al., 2003).

Starting in 1930, the average rainfall in the whole area diminished by 200 mm, with isohyets showing eastward displacements; at the same time, wind frequency and speed increased and together with an inadequate soil management practices triggered into soil erosion process (Bernardos et al., 2001). A couple of decades later, rainfall conditions improved with a westward displacement of the isohyets (Minetti and Vargas, 1983a, b; 1996; Minetti and Poblete, 1989; Minetti, 1991). And at the same time, many abandoned areas were converted into grazing lands or croplands. The dust-bowl, a long stagnation period that lasted until the 1960 s, was followed by a quick agro and ecological recovery (Viglizzo and Frank, 2006).

Important transitional forests in the Argentinean pampas are the locally known as caldenales, which are the forest ecosystems dominated by caldén trees (Prosopis caldenia Burkart); the caldenales grow between the pampas grasslands and the monte shrublands. The Caldenales support 931 plants species (59 of them have great potential in medical applications), and 333 species of vertebrate, many of them endemic to the central Argentina (SAyDS, 2007). Nevertheless, the high deforestation rate on the region has produced the removal of the 82% of the original cover, with a remaining area of approximately 17 million of hectares in central Argentina (SAyDS, 2007). As areas occupied by the caldenales have shrunken, the adjacent grasslands have been encroached by secondary forests (Distel and Bóo, 1995; Dussart et al., 1998).

Understanding the effects of the irregular alterations of climate and land use changes on the caldenales dynamics would require considering not only the water availability spatial variations (phreatic level) but also the special adaptations of individual trees. Several studies developed in the Northern limit of the caldenales reported a high intra-populational genetic variability. The allogamous behavior of the species could mean that individual trees in the same populations responds to limiting factors in a very different way (Saidman, 1985, 1986, 1990, 1993; Saidman and Vilardi, 1987; Vega and Hernández, 2005).

Caldén trees can act like recorders of past ecological conditions since this species produce annual growth rings (Villalba et al., 2000). Such anatomical feature has been traditionally used in dendrochronological analysis to reconstruct past climatic variations, evaluate tendencies in radial growth, biomass production, net primary productivity, etc, (Martinelli, 2004; Schöngart and Wittmann, 2010; Mbow et al., 2013). Caldén tissues have proved to be sensitive not only to climate (Villalba et al., 2000; Bogino and Jobbágy, 2011) but also to fires, competence, and soil conditions (Krebs and Fisher, 1931; Dussart et al., 1998; Medina et al., 2000; Bogino and Villalba, 2008) allowing us to understand historical climatic alterations and long-term behaviors in the tree growth.

The objective of this study was to understand the growth responses of the caldenales transitional forests to the registered variations in the climatic conditions over the second part of the last century. The analyzed forests have different levels of human intervention across a latitudinal gradient from South to North on the semiarid Argentinean pampas. We aim to study Caldén growth alterations not only among populations but also between individual trees. Considering the possible tree Caldén growth individual variations, we expect asynchronous responses in forest growth: different individual growth responses to observed historical changes in water availability. To achieve these objectives, the following tasks were developed: i) analyze, describe and model the response patterns between diameter growth of the caldenales at tree and population level, ii) compute historic annual aridity indexes representing the water availability conditions across the study site during the second part of the last century; and iii) identify common patterns in growth at population/individual levels.