Extreme environments in the forests of Ushuaia, Argentina

Abstract

[1] A survey over two mountain slopes (Glaciar Martial and Cerro Guanaco) in the vicinity of Ushuaia (Tierra del Fuego, Argentina) showed normal results for the region in terms of chlorophyll concentration in the leaves of the dominant tree species Nothofagus antarctica, N. pumilio and N. betuloides, and soil variables such as temperature, moisture, pH, and concentration of nitrogen, sodium and potassium. Solar radiation, on the other hand, showed high values of ultraviolet over the 200 -400 nm range, suggesting that the environment is extreme in terms of incoming solar radiation. The forest canopy absorbs and/or reflects a significant amount of that radiation. In separate analyses we showed that these tree species contain UVabsorbing pigments (cyanidin, delphidin, and flavonol glycosides). We submit that the rippled and glossy surface of leaves serves as a reflection/backscattering mechanism that protects their inner structure and function. The presence of krummholz (= twisted, dwarf trees) in the upper end of the forest shows the effects of an extreme environment.

Comment on “Extreme environments in the forests of Ushuaia, Argentina” by Hector D'Antoni et al.

“…Data of D'Antoni et al [2007] show typical ground‐level solar UV spectra except for dramatic increases in UV‐C radiation, especially as λ approaches 200 nm. These UV‐C fluxes far exceeded those they reported in the UV‐A and UV‐B and were high even in the forest where UV‐A and UV‐B fluxes were low.…”

“…Even within the instrument's range of maximum sensitivity, significant irradiance near 250 nm was reported. We present calculated irradiance (“tuv” model of Madronich and Flocke [1995]) in Figure 1, along with the 250 nm values of D'Antoni et al [2007] Even using a much lower ozone level in the model than would have occurred on the date of their UV measurements (100 DU instead of 300 DU), the 250 nm values they report exceed the calculated values by more than 15 orders of magnitude. They even considerably exceed the irradiance calculated for zero ozone, and in two cases (CG1 and CG3 at 8 and 12 μ W cm −2 nm −1 , respectively) they surpass the extraterrestrial flux.…”

“…Clear‐sky spectral irradiance (8‐stream version of the tuv radiative transfer model [ Madronich and Flocke , 1995]) and 250 nm values from D'Antoni et al [2007]. Model values calculated for 100 DU ozone (close to the lowest value ever recorded), and for 300 DU (a reasonable value for the date of D'Antoni's measurements).…”

“…An investigation of influences on treeline near Ushuaia, Tierra del Fuego, Argentina, by D'Antoni et al [2007] reported high ultraviolet‐C radiation (UV‐C, λ < 280 nm) in terrestrial sunlight. Because of well‐known atmospheric absorption of UV, any portion of the ground‐level UV‐C fluxes they reported would be impossible.…”

Comment on “Record solar UV irradiance in the tropical Andes, by Cabrol et al.”

“…Indeed, the very method to measure atmospheric ozone content from satellites or from the ground is based on this understanding. (2) Measurements of very high ambient UV radiation have been reported for this region before (D'Antoni et al, 2007(D'Antoni et al, , 2008 but have been shown to be instrument artifacts from stray light (Flint et al, 2008); the extreme UV index values reported by C14 must therefore be taken with additional caution lest a new round of incorrect measurement becomes validation of previously reported mistakes.…”