Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Resumen:

Anthropogenic activities have produced changes in natural ecosystems worldwide. In tropical regions in South America, industrialization of cities and forest clearance via burning are the main activities releasing pollutants into the atmosphere and inducing changes in nutrient deposition patterns and climate of primary forests. Nitrogen (N) and phosphorus (P) are considered the main limiting nutrients of plant growth since their availability is vital for net primary productivity. Therefore, any change in N or P availability in soils would likely alter important mechanisms of forests dynamics such as growth and survival. Changes in soil pH (acidity), nutrient cycles and altered nutrient stocks affect N and P availability and affect various physiological processes of trees. Although low levels of nutrient deposition have been reported for montane forests in southern Ecuador (ca. 5 kg ha–1 for N, 0.49 kg ha–1 for P), even these levels are expected to lead to changes in forest structure and dynamics over the long term. The responsiveness of forest to changes in resource availability varies with forest age and successional status, as well as life stage of the plant. Young plants (tree seedlings) should be more responsive to changes in nutrient availability than mature forest trees. Further, any demographic response is more likely to be visible in seedlings before mature trees because of the more rapid dynamics of seedlings. Therefore, I studied the regeneration dynamics of montane forest to understand which nutrient-related processes are involved in the growth and establishment of seedlings at both the individual and community levels. The Ecuadorian NUtrient Manipulation EXperiment (NUMEX) has been designed to study the response of montane forest to moderate fertilization. The experiment has been set up over an elevation range across three main study sites (1000 m. a.s.l: Bombuscaro; 2000 m. a.s.l: San Francisco; 3000 m. a.s.l: Cajanuma) within the Podocarpus National Park and San Francisco Reserve. The factorial experiment consists of four blocks containing four experimental plots (N, P, NP and control) in every study site (16 plots per elevation). Fertilization has been done since 2008, adding moderate quantities of fertilizer (50 kg ha–1 y–1 of N and/or 10 kg ha–1 y–1 of P). Three different approaches were used to assess the seedling community and common species responses to fertilization. First, the seedling community was monitored in natural forest at 2000 m over three consecutive years (2011 – 2013). In 192 monitoring subplots (1m2 each) in San Francisco, all seedlings and saplings were mapped and tagged. Measurements of height, diameter, herbivory and leaf production were recorded for all individuals, and the number of recruited and dead seedlings was counted every year. Second, allocation patterns and stoichiometry of seedlings of the six most common species were determined across the elevation gradient. Naturally occurring seedlings of the following species were harvested: Clarisia racemosa and Pouteria torta in Bombuscaro, Graffenrieda emarginata and Palicourea angustifolia in San Francisco and Grafferieda harlingii and Hedyosmum purpuracens in Cajanuma. Morphology (biomass allocation, herbivory and foliar areas such as SLA, LAR and LA) and foliar stoichiometry (nutrient contents and N:P ratios) were recorded and analyzed. Third, a reciprocal transplantation experiment (STE) within the NUMEX experimental plots monitored seedlings of the most common species in Bombuscaro, (Pouteria torta), to assess specific responses in seedling performance over one year. These three levels of analysis showed differential responses of the species community and common species to fertilization. Contrary to lowland forests, the community seedlings in this montane forest showed a moderate response. The density of seedling individuals decreased following nutrient addition, as a result of lower recruitment in treatments plots, but mortality was unaffected. Consequently, density-dependent mechanisms activated by additional N uptake did not show evidence of being a main driver to changes in species density. The community of seedlings did not show significant growth in response to fertilization although seedlings were higher in the N treatment. However, N addition increased herbivory over all seedlings and plots, which could have masked other seedlings growth responses. Common species seem to be well adapted to the relatively poor soils since these species were not favored by nutrient addition showing no change in growth or leaf traits. Over the long-term, common species might lose their dominance by being less competitive than faster growing species. However, it would need a longer period of monitoring under continued fertilization to produce visible shifts in community composition. Species-specific responses are hard to determine in such species rich communities. Therefore, analyses of the six common species were conducted to complement the community-level study. Leaf morphology and foliar stoichiometry were assessed from harvested seedling from the experimental plots. Both N and P had effects on seedling traits, suggesting co-limitation of N and P in montane tree species in all elevations. However, foliar N:P ratios and the more frequently response to P addition indicated this nutrient might be more limiting than N along the gradient. Responsiveness of the studied species varied between the six species. Stronger foliar P concentration compared with N in all species but Pouteria torta suggested higher P consumption in montane forest species, similar to the responses of several other tropical plant species after P addition. Increased herbivory was only evident in the opposite responses of G. emarginata (N and N+P addition) and P. angustifolia (P addition) at 2000 m suggesting that plant selection by herbivores is driven by resource quality. Most of the species had higher belowground biomass in root fractions following fertilization, except for G. harlingii at 3000 m, the only species that showed significant increase in aboveground biomass after nutrient addition. Pouteria torta seedlings showed no significant changes in species performance after fertilization one year after establishment of the STE. Pouteria seems to be well-adapted to poor soils, since N and P addition did not alter foliar nutrient concentrations. Nevertheless, N and N+P addition significantly increased mortality and diameter growth rates. P addition resulted in higher leaf area loss and shifted carbon allocation to root growth. These responses indicated consequences in the competitive strength in the understory and recruitment success of Pouteria torta. However, the ambiguous response in some attributes (growth and herbivory) not related with mortality made it difficult to predict the future abundance of this species in long term. In conclusion, complementary studies demonstrated that specific nutrient limitation for N or P in montane species seem not to be a rule in rich species ecosystems although nutrient addition did alter some pattern of growth and survival. Thus, nutrient fertilization might affect various mechanisms and dynamics of plant communities, the balance of which will only play out over long time scales.