Rainfall-Controlled Tree Growth in High-Elevation Subtropical Treelines

Abstract

It is generally assumed that tree growth in the upper limit of a forest is mainly controlled by summer temperature. This general statement is mostly based on studies from extra‐tropical mountains and has been rarely evaluated in subtropical latitudes frequently characterized by drier climates. In the subtropical mountains from Northwestern Argentina (∼23° S), annual precipitation decreases with elevation from >1500 mm at 1200– 1500 m, to <200 mm above 4000 m. In consequence, tree growth at high elevations in the region may be seriously limited by water supply. In order to assess the influence of precipitation on tree growth, we evaluated the relationships between climatic variations and radial growth in four species growing at different altitudinal zones: Juglans australis from the montane cloud forest at 1800 m; Alnus acuminata from the montane savanna‐like woodland at 2700 m; Prosopis ferox from the subalpine dryland at 3500 m; and Polylepis tarapacana from the high‐elevation alpine dryland at 4750 m. Dendrochronological techniques were used to relate variations in annual ring width with instrumental climatic records. Growth rings were correctly dated to the year of formation, and the cross‐dated series standardized using autoregressive models to reduce non‐climatic signals present in the records. Tree‐ring chronologies, ranging from 117 to 341 years, were compared, during the common period, with instrumental climatic records using correlation‐function analysis. In spite of the remarkable differences in elevation and environmental conditions among the sampling sites, correlation functions with climate indicated that the radial growth of the four species is largely controlled by precipitation. In most cases, increased precipitation during the previous and current growing seasons favors tree growth, while temperatures are negatively correlated with radial growth, likely due to the negative effect on water availability. These results indicate that the generalized idea of upper‐treeline growth limited by summer temperatures should be carefully evaluated in low‐latitude environments and does not apply to subtropical areas with severe water deficits or strong moisture seasonalities.

Lake Fluctuations, Plant Productivity, and Long-Term Variability in High-Elevation Tropical Andean Ecosystems

“…5, parts A and B) and the fact that both variables were correlated with instrumental data of the previous year. Our results support previous observations about the positive relationship between P. tarapacana radial growth and rainfall of the previous growing season to the tree ring formation (Argollo et al, 2004;Morales et al, 2004;Soliz et al, 2009). The same pattern is also reflected in lake level fluctuations where size variation depends on precipitations from the previously rainy season.…”

“…In this study, three tree-ring chronologies from P. tarapacana were developed by incorporating two new chronologies from Ramada and Cerro Negro sites as well as updating and extending back in time the previously published chronology from Granada Volcano (1659-1999) (Morales et al, 2004) (Fig. 1, part A; Table A1 in online Appendix).…”

“…For example, in subtropical mountains, water balance variability seems to play a key role in ecosystem functioning (Morales et al, 2004;Lupo et al, 2007). Quantifying the relationship between climate variability and plant productivity is essential to understand the functioning of these ecosystems and the potential consequences of future climate scenarios.…”

“…In general, low-latitude Andes are expected to be severely affected by global warming, particularly at higher elevations Urrutia and Vuille, 2009). Related changes in precipitation patterns are also expected to have strong ecological effects, particularly in the southern tropical Andes, where plant growth is largely limited by rainfall (Morales et al, 2004;Soliz et al, 2009).…”

“…The study of tree rings for ecological purposes (dendroecology) is a useful tool to assess trends in plant growth for decades to millennia (Schweingruber, 1996;D'arrigo et al, 2000;Wang et al, 2004;Srur et al, 2011). During the past decade, our group developed dendrochronological methods for tropical mountains, based on samples of the long living treelet P. tarapacana, which allow multi-century reconstructions of radial growth patterns and their relationship with climate variability (Argollo et al, 2004;Morales et al, 2004Morales et al, , 2012Soliz et al, 2009). We hypothesized that inter-annual lake fluctuations are an indicator of water balance and ecosystem productivity that should also be reflected in tree radial growth.…”