Unexpected spatial patterns of natural regeneration in typical uneven-aged mixed pine-oak forests in the Sierra Madre Occidental, Mexico

Natural forest regeneration, i.e. self-renewal of forest stands, involves the replacement of old trees by the next generation and is influenced by environmental factors. The spatial structure of tree regeneration depends on and also influences the properties of the stands themselves. Few studies have investigated spatial patterns of naturally regenerated areas in Mexican pine–oak forests, which are considered one of the world's top 34 biodiversity hotspots. In this study, we analyzed the spatial patterns, particularly the spatial structure, in clusters of naturally regenerated trees in seven 100 × 100 m plots in the Sierra Madre Occidental (northern Mexico), in relation to three factors: slope, geographical aspect and distance between each sapling to the edge of the nearest gap in the canopy. Three indices were used to describe spatial structure and the data were analyzed by bivariate Ripley's K(t)-functions and three-parameter and six-parameter Weibull models. The results indicate that sapling regeneration was marginal in canopy gaps. Sapling density was ten times higher under the canopy cover, close to the edge than in the gaps. On average, the first maximum number of saplings was detected inside the canopy at about 81 cm from the gap edge, forming ring-type spatial patterns around the canopy gaps. These results contrast with the gap dynamics described in many other studies. We attribute these findings to the nurse effect of trees, which ameliorate abiotic effects, such as the extreme drought that occurred in 2011 and 2012 in the study region. In covered zones close to canopy edges, the lowest sapling regeneration occurred on gently sloping areas and on south and south-east facing slopes. This can probably be attributed to adverse effects caused by the abundant leaf litter remaining in such areas and by the low level of shading associated with the slope orientation. We recommend adapting or changing the current forest management system to improve continuous forest regeneration (including adaptive silviculture). We also strongly support i) research on the effects of cattle grazing on natural regeneration in the region and ii) reduction of livestock pressure, which is essential to support forest renewal. Complementary reforestation, in addition to maintenance of the few saplings growing within the gaps, may help enhance forest regeneration.