## 3 Vegetation Phenology
### Phenological Patterns Observed on the Ground  
Evergreen, deciduous and semi-deciduous tropical tree species have leaves with different life spans. Some (not mutually exclusive) hypotheses that could explain how this cycle relates to environmental factors could be [76]: (a) competition for nutrients controls the senescence of individual leaves; (b) deciduous leaves evolved as a drought-avoidance mechanism; (c) the trade- off between leaf costs and benefits controls the life span; and (d) long-lived leaves have a high nutrient use efficiency (defined as total plant dry mass divided by foliar nitrogen or phosphorus mass). [PERSON] [77] suggested that at a broader scale, some biotic factors could act on the leaf phenological cycle, and demonstrated that herbivores could influence leaf phenology of understory plants in a tropical forest in Panama, as 34% of the annual leaf production was observed during the dry season when insect abundance and damage to expanding leaves were lower. The vegetation in the floodplains along the rivers in Amazonia is exposed to flood pulse hydrological regimes and experiences periodical inundation of large areas of forest. In these areas, there are two main vegetation formations: _Varzeas_ and _Igapos_. _Varzeas_ can be defined as forests flooded by white-water rivers (sediment rich rivers originating in the Andes), while _Igapos_ are nutrient-poor areas flooded by black water Rivers (sediment poor, organic matter rich rivers originated in lowland forests) [78]. Despite the variability of phenological traits in these physiognomies, studies in central Amazonia suggest that the flush of new leaves in _Varzea_ occurs in August in most species, while in the _Igapo_ vegetation type, new leaves are flushed in July and August, and also in October, December and February [79, 80, 81, 82, 83, 84]. Generalizations for temporal pattern of leaf production in _Terra Firme_ forests are more difficult to perceive due to the lack of one strong event to mark or homogenize a signal to correlate with the vegetation response, such as the flood pulses in the forests along the rivers. Regular periods of low rainfall could act as such an event but unfortunately there is not a standardized way to define dry season in the literature, making more difficult an integrative analyses. For instance, the dry season definition can vary from the period of lower rainfall without a defined threshold ([73] and many others) to a very clear defined threshold, such rainfall less than 50 mm\(\cdot\)month\({}^{-1}\)[85] or less than 100 mm rain per month [86]. It has been suggested that dry season can be characterized by the periods during which evapotranspiration exceeds precipitation and the soil water available to plants declines [87] and 100 mm\(\cdot\)month\({}^{-1}\) rainfall could be used as a mean modeled threshold for Amazon evergreen forests [88]. In addition to temporal patterns of rainfall, seasonal variation in irradiance also occurs because of changes in cloud cover between dry and wet seasons and solar angle throughout the year and in different latitudes, affecting tropical forest canopies [13, 89]. In a review of the literature, [90] suggested that tropical tree species could take advantage of maximum irradiance to produce new organs, conferring on them two main benefits (although co-varying environmental factors cannot be excluded). First, studies have shown that there is an increase of phenological activity and net primary production during the months of solar radiation peak when water is available. Secondly, leaf flush during the dry season would represent an advantage as insect activity and fungal pathogens are reduced during this period. More recent studies also support these hypotheses [14, 91, 92], although bimodal distributions of flushing of new leaves have been reported [91, 93, 94] as well as regions with negligible relationships between leaf production and leaf mortality with climate seasonality [95] and species with new leaves production during the wet season [96].