دانلود رایگان مقاله بقای تسهیل کاشت کلاستر بدون رشد در توسعه اولیه جنگل گرمسیری

Abstract

Interspecific interactions are considered to be important structuring forces in early successional vegetation. Whereas competition seems to prevail in less severe environments, facilitation tends to increase in importance in harsh environments. Hence, facilitation is expected to play an important role in degraded tropical areas with high irradiance, heat loads, and evapotranspiration, where conditions are far from optimum for most forest species. We planned a restoration project on the southeastern Brazilian coastal plain to compare growth and survival of tree seedlings planted at two densities (isolated or aggregated) in a factorial experiment with nutrient addition. We monitored survival, height, ground level diameter, and crown projection of 4132 saplings from 19 species that ranged along a successional gradient, over an 18-month period. We used mixed-effect models to analyze the relationship between species performance and treatments, and Akaike's information criterion (AIC) to select the models. The best model showed higher survival in aggregated plantations (indicating facilitation) for non-pioneer (slower growing) species. In contrast, we found lower diameter growth in aggregated plantation (indicating competition). Fertilizer addition did not affect survival in clusters, but, surprisingly, it had a negative effect on isolated plants of both pioneer and non-pioneer species. Fertilizer addition had a positive effect on diameter and crown projection growth in aggregated plantations (reducing competition), especially for pioneer species. Thus, whether facilitation or competition was the predominant interaction depended on the effect analyzed. As establishment of non-pioneer species in disturbed sites can be challenging, restoration designs could take advantage of higher survival rates in clusters and use resource addition to ameliorate growth and decrease competition for limited resources.

Discussion

Although competition and facilitation can occur together, the balance between them will result in net positive or negative interaction. Our results show that this balance varies within the same system (Callaway 1995), depending on the effect analyzed (Hastwell & Facelli 2008; Gómez-Aparicio 2009; Prévosto et al. 2012). In ourstudy, whereas growth was reduced, survival was enhanced in aggregated systems, especially for non-pioneers, which is consistent with previous studies showing more positive responses of shade-tolerant non-pioneer species to nurse plants (Gómez-Aparicio et al. 2004; Padilla & Pugnaire 2006; Yang et al. 2010). The results suggest that high irradiance stress and temperature may be major factors limiting seedling survival in open tropical degraded areas. Accordingly, Castanho and Prado (2014) found that shade, either artificial or from an adult plant, was the mechanism responsible for facilitating seedling survival (although not growth) in this system. Since high irradiance and temperatures can lead to photoinhibition (Loik & Holl 1999; Ehleringer & Sandquist 2006), pioneer species able to cope with higher light conditions may act as nurse plants (Padilla & Pugnaire 2006) by reducing irradiance and temperature, improving conditions for more non-pioneer species. In this sense, Pereira (2014),studying in the same area,reported seedlingsshowing signs of photoinhibition, but with lower levels of photosynthetic stress, soil and foliar temperature in an aggregated system.