Tussock size and annual grass productivity
To determine tussock size of P. ligularis and P. speciosa ,
aboveground volume of the tussocks was calculated using the equation of
Derner et al. (2012):
V = 1/3 Πh (r 2 + rR +R 2)
where V is the volume, h is the distance from the soil
surface to the top of tussock (height), r is the radius of the
basal area occupied by tussock, and R is the radius of the area
at top of tussock. We used this equation because the circumference of
tussocks increases from the base to the top. Since a high number ofP. humilis tussocks was observed in the +N and +NW plots four
years after the start of the experiment, this species was also included
in this study. However, in C and +W plots we did not find an adequate
number of P. humilis tussocks to consider sampling and further
statistical analysis. Determinations were made on randomly selected
tussocks in December 2019.
To determine the annual grass productivity, a 50 x 50 cm quadrat was
randomly selected in each plot, taking into account that if the quadrat
fell on a shrub individual, it was selected again to avoid shrubs in the
samples. This is due to the difficulty of estimating annual shrub growth
using this method, as there are few shrub individuals per unit area,
which would overestimate the calculated ANPP. All aboveground plant
biomass was cut at the soil level and transferred to the laboratory,
where green biomass, dry biomass, and litter were separated. The plant
biomass was then oven-dried at 70 °C for 72 h. Cacti and herbaceous
dicotyledons were excluded from the analysis due to their low
representativeness in the samples (<1%). Subsequently,
biomass was weighted and ANPP was determined as the biomass of each type
divided by the quadrat area in each year and expressed as g
m-2 yr-1. Productivity was measured
once a year during peak green grass biomass (December) from 2013 to
2019. We chose to harvest in this month because both temperature and
soil moisture are favorable for growth, while later (summer) the
temperature is higher, but soil water availability decreases and
atmospheric water demand increases. Moreover, a study applying eddy
covariance techniques in the same Patagonian steppe shows the peak of
gross primary productivity and ecosystem net carbon exchange in December
(Burek et al. 2023, under revision). Sala & Austin (2000) determined
that the method for ANPP estimation that introduces the least error in
grasslands with marked seasonality is that consisting of a single annual
harvest.
Shrub size and annual shrub
productivity
Three individuals of each of the most abundant shrub species at the site
(A. prolifera , A. volckmannii , and S. filaginoides )
were selected and targeted per plot in 2015. In December of each year
from 2015 to 2019, the height and major and perpendicular diameter of
each selected individual were measured. To determine the major and
perpendicular diameter of each, photographs were taken of the top of the
canopy including a scale, and the images were analyzed with the ImageJ
program (Ferreira & Rasband, 2012). The biomass of each individual was
estimated by applying allometric relationships estimated by Oñatibia et
al. (2010) for the same species and in an area close to that of the
present study. The equations used were the following:
b = 0.000003 x dh 3.9321 for A.
prolifera
b = 0.0002 x dh 3.1402 for A.
volckmannii
b = 0.0002 x dh 3.1876 for S.
filaginoides
where b is the estimated aboveground biomass (g) and dh is
the average diameter between the major and perpendicular plus plant
height (cm). Annual growth was determined as the change in estimated
biomass between consecutive years and expressed per area. ANPP was
estimated as annual growth per area multiplied by the percent cover of
each species at the site. When dead individuals were observed among
those marked, this was recorded and the number of dead plants per year
was determined. In that case, new individuals were marked and measured.
If plants considered dead sprouted in subsequent years, they were
removed from the dead plant category.