Net precipitation in a semideciduous forest... NET PRECIPITATION IN A SEMIDECIDUOUS FOREST FRAGMENT IN VIÇOSA CITY, MG

1 Received on 23.09.2015 accepted for publication on 01.09.2016. 2 Universidade Federal de Viçosa, Programa de Pós-Graduação em Ciência Florestal, Viçosa, MG Brasil. E-mail: <jpfreitaz@yahoo.com.br>. 3 Universidade Federal de Viçosa, Departamento de Engenharia Florestal, Viçosa, MG Brasil. E-mail: <herlycarlostdias@gmail.com> and <eshamir@ufv.br>. 4 Universidade Federal de São Carlos, Departamento de Ciências Ambientais , Sorocaba, SP Brasil. E-mail: <kellytonello@yahoo.com>. *Corresponding author.


INTRODUCTION
The hydrologic cycle is defined as the movement of water and the changes in its state during such movement.Understanding the water dynamics on the planet is the starting point for hydrologic studies (QUEIROZ; OLIVEIRA, 2013).
Universally, the volume of water in each stage of the hydrologic cycle is relatively constant.However, when considering a limited area, the amounts of water in each part of the cycle vary continuously within a wide range.Overabundance and scarcity of rainfall represent the extremes of this variation in a given area (GARCEZ, 1976).
The rainfall that precipitates in a forest may follow two pathways: it may either return to the atmosphere through evapotranspiration (concomitant loss of soil water through evaporation and of plant water through transpiration) or reach the soil through the litter or trees stems.A portion of the water that reaches the soil generates surface runoff and reaches watercourses or surface water tanks.The other part is temporarily stored through soil infiltration and may be released to the atmosphere through evapotranspiration, be kept as soil water for a while longer, or get filtrated as underground water.Anyway, the soil-stored water that was not evapotranspirated flows slowly through the forest and originates runoff, which sustains water sources (FREITAS et al., 2013).
The hydrologic cycle has several components, but one of them is often overlooked in research: rainfall interception by vegetation.Rainfall interception has a major importance in water balance, especially in areas with large forests.The influence of vegetation on catching and redistributing rainwater is quite significant in the context of water balance of a given area (OLIVEIRA et al., 2008).
The amount of water precipitated above the canopy is frequently confused with the amount of water available to the soil.The lack of information on interception of the water that precipitates through the vegetation can lead to errors in the real measures of the amount sucessionais, faz com que a água siga caminhos diferentes, interceptação, escoamento pelo tronco, precipitação interna.Esta informação nos ajuda a compreender que este processo pode ser demorado e é o caminho natural das regenerações.
of water that will effectively contribute to maintaining soil available humidity, which may compromise water balance calculation.This is due to the fact that part of the precipitation in contact with vegetation accumulates on leaves and branches and then returns to the atmosphere through evaporation (losses by interception).Another part of the water flows through stems and drips from leaves, reaching the soil.This other part, added to the part of the rain that directly crosses the canopy, forms the net precipitation, which effectively contributes to recharging soil water (OLIVEIRA et al., 2008).
Stemflow is the rainwater which, after being retained by the canopy, runs off through branches and stems toward the soil.In order for stemflow to occur, canopy must first be saturated, i.e., the maximum water retention capacity must be reached.After the canopy is saturated, as the rain continues to fall, the process of stemflow takes place (SHINZATO et al., 2011).
In view of the importance of the Atlantic Forest biome and of the great devastation that its ecosystems have been going through, studies that characterize the water regime in its areas are needed.Such studies would enable a better understanding of how water gets distributed across the biome and how the relationship between water and ecosystems takes place at different successional stages of the Forest.Thus, we aimed to compare the throughfall, rainfall interception, stemflow, and net precipitation in areas of a semideciduous seasonal forest fragment at early and advanced stages of regeneration in Viçosa city, Minas Gerais state, southeastern Brazil.

MATERIAL AND METHODS
The study was conducted in a seasonal semideciduous forest fragment located within the Estação de Pesquisas, Treinamento e Educação Ambiental Mata do Paraíso (Figure 1), a conservation unit situated in Viçosa municipality, eastern Minas Gerais state, Southeastern Brazil.The site has an area of 194 ha and a mean altitude of 650 m.
Net precipitation in a semideciduous forest...The conservation unit is located in the Córrego Santa Catarina Watershed, which is an affluent of São Bartolomeu River, on the Rio Doce Watershed.Vegetation in the site is semideciduous seasonal tropical forest (VELOSO, 1991).
According to Köppen's classification, the region has a hot temperate climate, with rainy summers and dry, cold winters (Cwb).Mean annual precipitation, mean relative humidity, and mean annual temperature are 1268.2mm, 81%, and 20 ºC respectively, according to the data obtained in a local meteorological station for the 1968-2010 period (LORENZON et al., 2013).
Gross precipitation was measured using a rain gauge with a 167-cm 2 catchment area, which was installed in a tower above the forest canopy.
For throughfall quantification, six plots (20 x 20 m) were established, three in each regeneration stage area, with a 10-m spacing among them.Each plot consisted of 25 rain gauges distant 5 m from each other.Gauges had an individual catchment area of 75.4 cm 2 in the early regeneration area and 81.7 cm 2 in the advanced regeneration area.Throughfall was quantified using the equation: where Tf is the throughfall (mm), V is the volume of water collected in each rain gauge (ml), and A is the catchment area of each gauge (cm 2 ).
For stemflow calculation, a subplot (10 x 10 m) was established within each throughfall plot.In those subplots, polyurethane rainfall collectors were adapted to tree stems having circumference at breast height > 15 cm, totalizing 129 trees: 27 in plot 1, 19 in plot 2, 23 in plot 3, 26 in plot 4, and 17 in plots 5 and 6, each.A hose was affixed to the collectors and directed rainwater to individual plastic recipients.Stemflow was calculated using the equation: where Sf is the stemflow (mm), V is the volume of water gathered in each collector (L), and SA is the subplot area (100 m 2 ).
Net precipitation (NP) was obtained by the sum of throughfall and stemflow, according to the equation: Losses by interception (I) were measured by the difference between gross precipitation (GP) and net precipitation, according to the equation: Gross precipitation, stemflow, and net precipitation were calculated for each rainfall event collected in the plots, and then averages were taken for each plot.
Data was obtained between January 2012 and June 2013.Whenever possible, recordings were made soon after each rainfall event.Thus, each data collection consisted of one or more precipitations.Measurements were taken using measuring cylinders and measuring buckets.
Data was recorded in spreadsheets and subjected to correlation analysis, analysis of variance (ANOVA) at 5% probability using software SAS (SAS Institute, 2002), and linear regression analysis.

RESULTS
Gross precipitation in the period was 1934 mm, the rainiest months being January, February, and November 2012 and February, March, and April 2013.The driest months were April through September 2012 and May and June 2013.July 2012 was the only month with no precipitation.A striking difference of more than 200 mm could be noticed between January 2012 and January 2013 (TABLE 1).
Throughfall was 1526 mm in the area at early stage of regeneration, which corresponded to 79.3% of gross precipitation, and 1406 mm in the area at advanced stage of regeneration, the equivalent to 72.6% of gross precipitation.On average, throughfall was higher in plots of the area at early stage of regeneration (TABLE 2), except in May and August 2012.Throughfall in the early-regeneration area was lower than in the advancedregeneration one only at the rainfall class lower than 2.5 mm (TABLE 3).
Stemflow varied considerably between the areas at early and advanced stages of regeneration, by F test at 5% probability (F 1,100 = 31.705;P = 0.00).The values recorded for this parameter were 8.62 mm in the early-regeneration area and 29.42 mm in the advancedregeneration one (TABLE 2), corresponding to 0.44% and 1.52% of gross precipitation, respectively.Additionally, stemflow showed a high coefficient of correlation with gross precipitation in the earlyregeneration area, unlike what was observed the advanced-regeneration one (Figure 2).Net precipitation in a semideciduous forest...

DISCUSSION
Gross precipitation in the studied period differed from climate normals in only 252 mm, showing that the period was quite typical in terms of rainfall distribution.
In August 2012, throughfall in the advancedregeneration area was higher than gross precipitation, resulting in a negative interception.As throughfall is usually lower than gross precipitation due to rainwater interception by the forest canopy, this value can be explained by the rainfall spatial distribution, which  was not uniform across the entire watershed area.Besides, rain gauges installed within the forest can also collect an additional amount of water that is directed to the gauge by leaves located above it.Lorenzon et al. (2013), working in the same area, also observed throughfall values higher than gross precipitation ones.Rainfall interception did not vary significantly among plots of the areas at early and advanced stages of regeneration (F 1,100 = 1.702;P = 0.1950).The coefficient of determination was as low as 0.52 and 0.33 in the areas at early and advanced stages of regeneration, respectively.This can be explained by several factors that affect rainfall, such as: the last precipitation in the site, which can interfere with the volume of intercepted rainwater; canopy humidity, since an already humid canopy intercepts less rain than a dry one; canopy density, as a more closed canopy intercepts more water than an open one; rain intensity, since lighter rainfalls get intercepted to a higher extent; and wind occurrence, which, by shaking the tree crowns, drops the water that got temporarily intercepted.
Although Lorenzon et al. (2013) had already evaluated hydrological variables in the studied site, monitoring such variables across the years is important due to changes in the forest dynamics.For instance, forest ecosystems can change over time due to the death of trees, which alters the canopy structure, or to the birth of new individuals.Additionally, rainfall distribution may also change along the years, which corroborates the need for a continuing monitoring of the studied variables.

CONCLUSIONS
The amount of water that runs off through trees stems, i.e. the stemflow, increases along with forest age, as does rainfall interception by the forest canopy.However, rainfall interception showed a low correlation with gross precipitation.Throughfall and net precipitation showed a linear tendency in relation to gross precipitation, the former being the one that contributes the most to recharging soil water.
Stemflow also increases linearly along with gross precipitation in the forest area at early stage of regeneration.Such correlation is lower in the area at advanced stage of regeneration.
Our study showed that rainwater distribution in the forest changes as its regeneration progresses.The replacement of species along different successional stages renders water to follow different pathways, such as interception by the canopy, stemflow, and throughfall.This information helps us comprehend that this process can be slow, yet it is the natural way of forest regeneration.
** Significant by F test at 5% probability.