In the Andean forests of northern Patagonia, Argentina, exotic conifers were introduced around the early 1940´s in experimental stations of the former Argentinean Forest Institute (IFONA) (Defossé, 2015). Up to the late 1980´s, part of these plantations replaced natural forests mainly composed of native mixed Austrocedrus chilensis (D. Don) Pic. Serm. & Bizarri (ciprés de la cordillera, hereafter ciprés) and Nothofagus dombeyi (Mirb.) Oerst. (coihue) (Sarasola et al., 2006). Although these plantations showed high growth rates, they changed the structure and function, including loss of connectivity and biodiversity, and the micro-environment of the replaced forests (Sarasola et al., 2006; Paritsis & Aizen, 2008). This land-use policy was gradually abandoned, and about 4,700 ha of planted conifers still remain within Patagonian forests (Ríos Campano, personal communication). One challenge of today´s global silviculture is to gradually reconvert monoculture plantations into compositional and structurally heterogeneous stands in which native plants prevail (Albanesi et al., 2008).

Canopy opening is one of the first steps for actively restoring natural mixed forests in dense and mono-specific plantations of exotic conifers. This practice mimics natural gap dynamics and creates conditions for re-establishment of native plants. In some cases, they remain in the understory as either seeds or seedlings that develop under favorable micro-environmental conditions whereas in others, active restoration for tree establishment is needed (Spies & Franklin, 1989; McIver & Starr, 2001).

These openings produce abrupt changes in soil temperature, water content, light, and nutrient cycling in the forest understory. These changes vary from site to site, and are also affected by the shape and size of the gaps and by the overstory tree species involved (Veblen et al., 1981; Carlson & Groot, 1997; Heinemann et al., 2000). Differences in openings structure, in turn, could affect future successional paths, favoring the recruitment of some species while delaying others (Gray et al., 2002; D’Antonio & Chambers, 2006). In conifer stands, small openings may frequently appear as circles, due to the death and fall of one or a few trees (Kuuluvainen, 1994). Rectangular gaps, by instance, are generally produced as a consequence of tree fall due to snow avalanches and/or wind disturbances (Nowacki & Kramer, 1998; Bava, 1999). Rectangular gaps allow light entrance parallel to the larger side, increasing the direct light reaching the gap (Álvarez Pacheco & Lara, 2008). Large disturbances (natural or artificial) favor the establishment of shade intolerant species, while small ones benefit the shade tolerant (Zhu et al., 2003; Gobbi, 2007). Small gaps made in adult stands of exotic species, can facilitate the reintroduction of native species by improving surrounding micro-environmental conditions, as compared to non vegetated areas (Engel & Parrotta, 2001).

The creation of canopy openings within plantations is one of the first steps to promote adequate environmental conditions for the establishment of target native plants (Carlson & Groot, 1997; Muscolo et al., 2014). The success of restoring previous native forests, however, depends on the knowledge of the former ecosystem functions (Whisenant, 1999; Baron et al,. 2002), the ecology, and particularly the safe site requirements (sensu Harper, 1977) of the target species, and the silvicultural treatment that allows meeting those requirements (Rhoades et al., 2009). In Patagonia, active restoration by sowing or seedling plantation accelerates restoration after natural or man-caused disturbances (i.e. fire, grazing, etc.) (Urretavizcaya et al. 2012; Pafundi et al., 2014).

In northwestern Patagonia-Argentina, mixed coihue-ciprés forests extend from the 36º 30’ S along the eastern slope of the Andes. Ciprés is the dominant species in xeric and low altitude areas, coihue dominates mesic and mid-altitude areas, while both species intermix and co-dominate in transitional zones between mesic and xeric areas, where they naturally coexist (Dezzotti, 1996). Within xeric sites, ciprés establishment is linked to the nurse protection effect that exerts a nearby shrub over a young ciprés seedling (Kitzberger et al., 2000; Urretavizcaya & Defossé, 2013). In mesic areas, instead, ciprés seedlings establish under high herbaceous and shrub cover, in small and mid-sized gaps (Gobbi, 2007). Coihue is a shade-intolerant, pioneer species which in mesic areas of northern Patagonia, regenerates at small-scale openings (from 400 to 600 m²) mainly produced by tree-falls (Veblen, 1989). However in Patagonia, there is a lack of knowledge about the effects of artificial openings in conifer plantations and their impact on seedling of native species planted for restoring former natural forests.

The hypotheses upon which this study was based are: 1) Canopy gaps performed on a Pinus contorta Dougl. (lodgepole pine, ~25 yr) planted in a former mixed ciprés-coihue native forest, significantly change micro-environmental conditions (soil moisture and temperature, and PAR) as compared to untreated monoculture and, 2) Different gaps types affect the performance (survival and growth) of both ciprés and coihue planted seedlings.

In order to test these hypotheses, we analyzed during two growing seasons (2010-2011 and 2011-2012), how small rectangular and small and medium circular canopy openings in a Pinus contorta plantation affects the: i) photosynthetic active radiation (PAR) and soil temperature and moisture; and ii) survival and growth of planted ciprés and coihue seedlings. The results will provide basic information for early stages of restoration of former mixed ciprés-coihue forests now occupied by exotic conifer monocultures.

In our study, the measured micro-environmental conditions inside the Pinus plantation varied with respect to those in the different gap types. Photosynthetic active radiation and soil moisture were lower below Pinus canopy as compared to rectangular or circular gaps, although soil temperature at 10 cm depth was similar in all treatments. Within gap types, medium circular gaps presented significantly higher values of PAR as compared to small circular gaps, while rectangular gap presented intermediate values. In general within pine plantations, PAR reaching the soil surface depends on tree density and height, diminishing as canopy closes and approaches the lower stratum (Donoso Zegers, 1997; Holst & Mayer, 2005). Our study showed that higher amounts of PAR occurring in medium circular as compared to small circular gaps, may have been directly influenced by size opening and sun position. Similar results have been reported by Gálhidy et al. (2005) in Fagus spp. forests. These authors found that the border of big gaps received similar values of incoming light as the center of smaller gaps. In general, incoming light passing through tree canopies in radiation angles lower than 90° reached the soil surface as diffuse light in small circular gaps, and as direct light in medium circular gaps (Donoso Zegers, 1997).

In our study, the particular shape and orientation of rectangular gaps (the longest side from east to west direction) may have favored the occurrence of subtle changes in other macro- and micro-environmental conditions. In Patagonia, the winds blow most of the time from the west, increasing their speed during the growing season (Defossé, 1995). The long and narrow shape of rectangular gaps and their orientation may have facilitated wind movement (Gómez Sanz, 2004), thus producing that shorter and more frequent sunflecks reached the ground, producing a more dynamic light distribution (Pearcy, 1990). Also the combination of rectangular gap width, tree height, and vegetation density, could have interfered with the wind flow and created little swirls than could have affected the temperature of the area (Oke, 1988). Related studies in the region are needed to explore the real effects of wind in gaps on the environmental variables.

Besides the possible wind effects, the east to west location of rectangular openings made them receiving similar or even higher PAR values than circular gaps at the end of the growing seasons. In fact, these values were similar at the end of the first growing season, but were significantly higher in rectangular openings as compared to circular gaps at the end of the second season (see Fig. 2). These significantly higher PAR values may have been related to the rapid decrease of daylight occurring at this latitude (42° 43’ S) at the beginning of the fall (Donoso Zegers, 1997). Because of their east to west location, rectangular openings continued receiving sunlight after it has vanished in either medium or small circular gaps. Also, the light received may have also been affected by the tree density of the stand. To cast light on this issue, it is important therefore, to conduct further studies with different tree densities.

The association between soil temperature and light, as a response to canopy openings, has been described by several authors (Donoso Zegers 1997; Gray et al., 2002; Brang et al., 2005; Muscolo et al., 2010). In general, large openings present higher increases in soil temperature and incoming light as compared to small openings and/or in the unmodified control. Also, and because of canopy shadow effects and organic matter accumulation on the soil surface, soil temperatures are in general lower in closed areas as compared to canopy openings (Breshears et al., 1998; Muscolo et al., 2007). Along the two seasons analyzed, our data showed that all gap types presented similar soil temperatures (at 10 cm depth) than those inside the Pinus plantation (control). It should be remembered, however, that when the experiment was set after the cutting treatment, the soil was scarified. This uneven pine needle removal product of this scarification, could have produced dissimilar soil temperature values among gap types at the beginning and up to mid-summer of the first growing season. As the season advanced and succession progressed, the soil started to be covered by herbs and grasses; as a consequence, these differences disappeared (see Fig. 3).

Related to ambient temperatures, during the two growing seasons analyzed, mean temperatures were slightly above the long-term average (2.2 °C higher). The 10-cm depth soil temperatures registered during the summer of both seasons, suggest that top soil temperatures never reached elevated temperatures as to compromise survival of either ciprés or coihue seedlings. The lower seedling survival found during the first growing season for coihue in medium circular (88 %) as compared to rectangular openings (99 %) or small circular gaps (96 %), may have been caused by the unusual frost period occurred one month after plantation. In effect, mean minimum ambient temperatures remained below 0 °C during the whole month of August (average of –2 °C), as compared to the long-term mean of 0.1 °C. We assumed that this unusual frost may have increased coihue seedling mortality in the less “environmentally protected” medium circular gaps (more exposed to frost effects) than in the other two gap types.

Soil moisture was similar in the three different gap types, being significantly higher than the one registered within the Pinus plantation. These results are in accordance with those reported by Albanesi et al. (2008) when comparing soil moisture in the center of circular gaps with those found within the surrounding Abies stand plantation. In both cases, soil water diminution within plantations as compared to open gaps could be clearly explained by the tree canopy effects. In fact, precipitation is intercepted by the canopy, following then different paths (stem flow, evaporation, etc.), and reducing water amounts that reaches the open ground (Kimmins, 1997).

Considering the combined effects of PAR, soil temperature and soil moisture on seedling survival and growth, results showed that maximum soil temperatures were achieved when soil water was at a minimum, and PAR declined to medium values. In forest ecosystems with high luminosity, direct sunlight during many hours reach different points of the understory, and may generate photo-inhibition in plants adapted to low radiation such as ciprés (Pearcy, 1990). Ciprés is a shade–tolerant specie (Gyenge et al., 2007), whose transpiration rate is restricted through stomatal closure, so the capacity of cooling is limited and therefore susceptible to heat stress (Gyenge et al., 2007). For this reason, in their first years as seedlings, ciprés grows better under the protection of a nurse plant (Gobbi, 2007).

Related to collar diameter growth, significantly higher values were shown during the first season by both ciprés and coihue seedlings grown in rectangular as compared to circular gaps. This higher relative growth was later compensated, showing all gap types similar values at the end of the second growing season. A larger seedling collar diameter may help improve root system function, by increasing the absorption capacity of water and/or nutrients (Mexal & Landis, 1990; Letourneau, 2006). Related to water availability, mean precipitation in the first growing season was 42% higher than the mean for the second growing season. This higher water availability may have produced favorable conditions for growth of root collar and hence for root biomass (Ericsson, 1995). In the second season, coihues´ height in rectangular openings reached 40 cm, while in the circular gaps it only reached from 20 to 30 cm. These differences, however, were not significant, because of the high variance in growth recorded in the medium circular gaps. This tendency to grow higher in height and diameter in rectangular and medium circular openings as compared to small circular gaps, may be explained by the fact that small circular gaps received lower amounts of PAR during the early growing season. This coincides with the study by Donoso et al. (2013), who found that coihue seedlings grew more in diameter and height in medium circular gaps as compared to small ones. Confirming these findings, Lusk & del Pozo (2002) found that survival and growth of seedlings of coihue and other 11 Chilean rainforest trees increase as light in the gaps increase. Coihue presents a great efficiency in photochemical and CO₂ assimilation in sunny days with highly radiation values. Besides coihue present higher accumulation of photoprotective pigments, which contribute to the mechanism for thermal dissipation in case of excess of energy under high light (Zuñiga et al., 2006). However and for the species grown in our region, it seems that more studies involving the addition of more frequent measurements of light quality (direct and diffuse light) during the growing season, may be needed to help elucidate these uncertainties.

With regard to which one of the three opening types may be recommended for future silvicultural treatments, results showed that two growing seasons after planting, both ciprés and coihue seedlings presented similar survival and growth rates regardless of the shape and size considered. Other studies in Patagonia involving seedling plantations in natural gaps of native forests indicate that their size varied, related to dominant height, from ratios of 0.5 to 1.5, or more (Dezzotti et al., 2003; Donoso et al., 2013). In our study, and irrespective of the shapes, these artificially created gaps varied in their height:width ratio from 0.47 to 0.90 (min-max) in the Pinus contorta stand analyzed. Through meta-data analyses, Zhu et al. (2014) showed that for plant establishment (as compared to nearby controls), the effect of gap size is more important in artificially created gaps than on natural gaps.

Although this study was conducted in only one plantation (with 1000 tree ha^–1 in a plain area) and on a small range of opening sizes (small and medium), it represents a valid approach to recover the natural forest based on a silvicultural technique applied to an exotic pine plantation. It seems then that other openings sizes, emphasizing on rectangular shapes, should be experimented in the future in other Patagonian plantations. Also other tree densities should be evaluated for better understanding its effects on the micro-environmental conditions such as radiation and soil moisture. This aspect may help define silvicultural treatments in restoration plans in Patagonia or elsewhere, involving the re-conversion of plantations into former native forests.