Short dry spells in the wet season increase mortality of tropical pioneer seedlings

Abstract

Variation in plant species performance in response to water availability offers a potential axis for temporal and spatial habitat partitioning and may therefore affect community composition in tropical forests. We hypothesized that short dry spells during the wet season are a significant source of mortality for the newly emerging seedlings of pioneer species that recruit in treefall gaps in tropical forests. An analysis of a 49-year rainfall record for three forests across a rainfall gradient in central Panama confirmed that dry spells of ‡10 days during the wet season occur on average once a year in a deciduous forest, and once every other year in a semi-deciduous moist and an evergreen wet forest. The effect of wet season dry spells on the recruitment of pioneers was investigated by comparing seedling survival in rain-protected dry plots and irrigated control plots in four large artificially created treefall gaps in a semi-deciduous tropical forest. In rain-protected plots surface soil layers dried rapidly, leading to a strong gradient in water potential within the upper 10 cm of soil. Seedling survival for six pioneer species was significantly lower in rain-protected than in irrigated control plots after only 4 days. The strength of the irrigation effect differed among species, and first became apparent 3-10 days after treatments started. Root allocation patterns were significantly, or marginally significantly, different between species and between two groups of larger and smaller seeded species. However, they were not correlated with seedling drought sensitivity, suggesting allocation is not a key trait for drought sensitivity in pioneer seedlings. Our data provide strong evidence that short dry spells in the wet season differentially affect seedling survivorship of pioneer species, and may therefore have important implications to seedling demography and community dynamics.

Allometric relationships between seed mass and seedling characteristics reveal trade-offs for neotropical gap-dependent species

“…However, the important point is that both the experimental observations and our modelling indicate that bare soil in a large gap can experience sufficient drying to kill shallow rooted seedlings. This was confirmed by Engelbrecht et al (2006) who observed up to 20% mortality of 3.5-month-old seedlings of pioneer species after 4 days of rainfall exclusion, rising to up to 40% after 11 days, in comparison to a control treatment.…”

“…This is mainly because in the dry-season, the soil is much drier at depth so that the zone wetted by rainfall dries both by evapotranspiration and downward movement, whereas during the wet-season capillary recharge slows the rate of surface drying. Engelbrecht et al (2006) observed 4Q Springer the drying of bare soil in large gaps during the middle of the wet-season on the Barro Colorado Nature Monument adjacent to BCI. After only 4 days of rainfall exclusion by a plastic roof, the matric potential of soil from a depth of 5-10 mm had fallen to -4.5 MPa.…”

“…In the tropical semi-deciduous forest on Barro Colorado Island, Panama, short dry spells of a few days duration are common during the wet-season (Rand and Rand 1982), and they can result in rapid soil drying in canopy gaps. For example, on bare soil in large canopy gaps, Engelbrecht et al (2006) reported that the soil water potential close to the soil surface can drop to -4.5 MPa after 4 days without rain. We have used tropical gap-dependent tree species for this study because their seed masses cover almost five orders of magnitude and because the constraint of only being able to regenerate in gaps makes them especially vulnerable to soil drying.…”

Seasonal variation in soil and plant water potentials in a Bolivian tropical moist and dry forest

“…However this comes at the cost of an increased transpirational water loss from the leaf. Especially in dry forests, where evaporative demand is greater than in moist forests and the dry season forms a bottleneck for survival of juvenile trees (Engelbrecht et al 2006), pioneer species will be filtered out of the landscape and persist in relatively wet habitats. In a recent study we addressed such hydraulic habitat partitioning among 40 different dry-forest tree species and our findings suggested that, similar to S. riparium, other pioneer species also had greater moisture requirements, and were thus less drought-tolerant, than shade-tolerant species (Markesteijn 2010).…”

Alvaradoa amorphoides germination at low water potential and the role of the antioxidant system

“…Water availability in these ecosystems affects the germination patterns (Garwood,983) and is considered one of the most important factors in modifying the structure and dynamics of this community (Murphy and Lugo,986). In a tropical dry forest, the soil water potential in the dry season can achieve up to -.5 MPa (Engelbrecht et al, 2006). The majority of species produces and disperses seeds in the dry season before reliable rains begin, which then allows them to germinate when the soil humidity is optimal (Baskin and Baskin, 998;Ceccon et al, 2006).…”