Herbáceas anuais e perenes da caatinga respondem de forma similar às variações na disponibilidade hídrica?

Abstract

The consequences of predicted climate changes are highly worrying, considering that the dynamics of the herbaceous component of the caatinga strongly reflects the dynamics of rainfall. Thus, we question how these dry forests will tolerate future climate changes, as rainfall reductions of more than 30% are expected, increasing consecutive days of drought. Therefore, our work supports the general hypothesis that the simulated changes in the precipitation patterns of a caatinga fragment modify the herbaceous trade-off relationships, altering the resource allocation in the vegetative functional characteristics and in the time in which they occur. We selected annual herbaceous plants (Portulaca oleracea L. and Desmodium glabrum (Mill.) DC) and caatinga perennials (Talinum triangulare (Jacq.) Will. and Commelina benghalensis L.). The seeds of these herbaceous plants were collected at the end of the rainy season in a fragment of caatinga forest, located at the Experimental Station of the Instituto Agronômico de Pernambuco (IPA) in Caruaru, Pernambuco. After 10 days of seed germination in a BOD chamber, the seedlings were transferred to plastic bags measuring 250 cm2 in circumference, containing 3 kg of autoclaved soil from the same location. After 20 days of acclimatization in 100% of CC (field capacity) the rainfall simulation experiment (SC) was built with 4 species x 3 treatments (T) x 30 repetitions each, lasting one year, as follows: T1- SC of rainy years; T2- SC of years close to the historical average; T3-SC of dry years. For this, we used the historical record of the last 60 years of rainfall at the IPA, which underwent descriptive and interquartile statistical analysis to determine the rainfall intervals that allowed for grouping the years for the construction of these treatments. For the first chapter of this thesis we assume the hypothesis that extreme changes in precipitation modify the vegetative responses below and above ground, differently, between annual and perennial herbaceous species. Specifically, we answered the following questions: (i) What happens to vegetative growth and biomass allocation in annual and perennial grasses in the wet, medium and dry year simulation? (ii) Does the above-ground and below-ground biomass allocation, when water resource is limited, follow an “ideal partition” pattern? Annual species were more sensitive to rainfall variations, with drastic reductions in aboveground growth in the dry year simulation, while perennials continued to grow or showed a moderate reduction. In leaves, species reduce biomass, production and expansion in dry years, but invest in water content and longevity. Below ground, annuals exploit the water resources of the surface layer, and perennials, depending on the species, invest in root growth, accessing the resource in the underground layers during the dry years. If predicted reductions are confirmed, the direction of allocation of herbaceous biomass may follow a "functional balance". In the second chapter of the Thesis, we hypothesize that if the years become drier, herbaceous plants will reflect such variations with changes in the beginning and duration of their phenophases. We verified that leaf emission is seasonal, but in perennial species the event is close to uniformity, while it is highly seasonal in annual species. Dry years induce less sprouting and in a more concentrated form, in a few months in annual and perennial species, in addition to making the sprouting of a perennial species seasonal. Within the rainy season, leaf changes in annuals and perennials can be anticipated and intensified in the simulation of rainy years, contrary to what was observed in dry years, being late and less intense. Such temporal and structural responses may or may not be a result of efficient strategies for greater tolerance in vegetative growth responses in extremely dry or rainy years.