Saguaro spines record rhythms of growth and moisture

Successive spines form an ordered record

Successive areoles and spines form near the apex of Carnegiea gigantea. As the stem elongates, older areoles lie progressively further from the growing region. Spines sampled along one rib can therefore be arranged in order of formation. Assigning actual dates requires an additional clock because growth rate changes with age, season and water availability [English et al. 2010].

English and colleagues combined time-lapse photography, marking of growing spines and isotope analyses. They studied naturally established plants near Tucson, Arizona. Some experiments involved a single spine or a single individual. Separating these scales allowed them to compare growth within a day with records accumulated over successive seasons [English et al. 2010].

A daily clock within spine structure

Coloured marks were applied to growing spines on thirteen plants from August to October 2006. Band counts were then compared with the time between marks. Analysis of 122 spines showed that one light-and-dark band pair corresponded approximately to one day of growth. The contrasting appearance arose from the arrangement of longer and shorter fibres and differences in tissue density [English et al. 2010].

This clock makes it possible to cut sections representing successive days and analyse their isotope composition. A growing spine elongates at its base, making its tip older than the tissue next to the areole. Combining records from successively produced spines allowed the authors to reconstruct about seventy days of one season. Overlapping intervals helped them check the chronology [English et al. 2010].

Oxygen links rainfall to stem water reserves

Isotopes are forms of the same element that differ in mass. The oxygen analysis measured the ratio of heavier ¹⁸O to lighter ¹⁶O. This ratio changes in plant water during rainwater uptake, mixing with existing reserves and evaporation. Forming spine tissue retains part of that signal. During the study period, δ¹⁸O values fell after larger rain events and rose during dry intervals [English et al. 2010].

The route from root to apex matters. Newly absorbed water mixes with the large reservoir already present in the cactus, and water composition differs among stem regions. The oxygen record can therefore smooth brief rainfall pulses and respond with a delay. The authors related the observed pattern to the balance between water uptake and loss, while leaving the exact contributions of individual processes for further experiments [English et al. 2010].

Carbon also reflects physiology

The ratio of ¹³C to ¹²C depends on processes associated with carbon dioxide assimilation. In CAM plants, carbon enters metabolism at night, while favourable conditions can also permit some direct daytime uptake. Changes in the contribution of these pathways can alter δ¹³C in new tissue. Carbon dioxide diffusion within photosynthetic tissues may also matter [English et al. 2010].

In the study, δ¹³C variability was associated with vapour pressure deficit, a measure of the drying effect of air. Proposed mechanisms included changes in the contribution of daytime and night-time assimilation and diffusion limitations within the photosynthetic stem. The authors did not directly validate these explanations with gas-exchange measurements. They retained them as physiological hypotheses consistent with the isotope record [English et al. 2010].

Twenty-two years and the limits of interpretation

A series collected along one rib of a roughly four-metre saguaro represented at least 22 years of growth. Its chronology used seasonal δ¹³C changes and independent radiocarbon dating. The researchers used changes in atmospheric ¹⁴C following nuclear weapons testing as a time marker. This series allowed them to estimate annual stem elongation and areole production [English et al. 2010].

In this single plant, year-to-year δ¹³C variability showed little correspondence with rainfall totals. It grew in a small drainage that collected water, which may have reduced its sensitivity to drier years. The winter pause in growth also creates a gap in the direct record of new tissue. Spines thus provide a valuable archive of growth and plant responses whose interpretation requires site information, dating and physiological understanding [English et al. 2010].

A later study by Hultine and colleagues compared six sites in the northern Sonoran Desert in 2014. At Cave Creek and Papago Park, observations additionally covered four growing seasons from 2013 to 2016. Coloured spine markings delimited successive growth increments. Spine-tissue δ¹³C analyses were combined with stem elongation and areole measurements and, at the two intensively studied sites, changes in stem geometry as a proxy for water storage [Hultine et al. 2018].

Across sites, faster growth was associated with higher, or less negative, δ¹³C values. The authors related precipitation to vapour pressure deficit by calculating a site moisture index as their ratio. Including atmospheric drying demand helped describe growth differences. The relationships were consistent with proposed roles for nocturnal stomatal conductance and carbon-fixation conditions in shaping the isotope signal [Hultine et al. 2018].

Year-to-year comparison gave a different picture. Despite changes in growth, no significant interannual δ¹³C variation was detected at either intensively studied site. Differences between sites remained clearer than the isotope response to an individual wetter season. Spine tissue can therefore reflect more persistent conditions of plant functioning while responding less strongly to brief rainfall changes. The authors considered buffering by reserves and the contribution of previously assimilated carbon to new tissue [Hultine et al. 2018].

The rainfall window also mattered. Growth was better related to conditions spanning the preceding summer and winter than to the current growing season alone. Stem-volume measurements supported an explanation in which earlier reserve replenishment affects later elongation. Comparison with English’s record shows why spine dating, individual location and comparison scale jointly determine how its archive is read [Hultine et al. 2018] [English et al. 2010].