Seasonal and successional dynamics of size-dependent plant demographic rates in a tropical dry forest

Abstract

Tropical forests are globally important for biodiversity conservation and climate change mitigation but are being converted to other land uses. Conversion of seasonally dry tropical forests (SDTF) is particularly high while their protection is low. Secondary succession allows forests to recover their structure, diversity and composition after conversion and subsequent abandonment and is influenced by demographic rates of the constituent species. However, how these rates vary between seasons for different plant sizes at different successional stages in SDTF is not known. The effect of seasonal drought may be more severe early in succession, when temperature and radiation are high, while competition and density-dependent processes may be more important at later stages, when vegetation is tall and dense. Besides, the effects of seasonality and successional stage may vary with plant size. Large plants can better compete with small plants for limiting resources and may also have a greater capacity to withstand stress. We asked how size-dependent density, species density, recruitment and mortality varied between seasons and successional stages in a SDTF. We monitored a chronosequence in Yucatan, Mexico, over six years in three 0.1 ha plots in each of three successional stages: early (3–5 years-old), intermediate (18–20 years-old) and advanced (>50 years-old). Recruitment, mortality and species gain and loss rates were calculated from wet and dry season censuses separately for large (diameter > 5 cm) and small (1–5 cm in diameter) plants. We used linear mixed-effects models to assess the effects of successional stage, seasonality and their changes through time on demographic rates and on plant and species density. Seasonality affected demographic rates and density of large plants, which exhibited high wet-season recruitment and species gain rates at the early stage and high wet-season mortality at the intermediate stage, resulting in an increase in plant and species density early in succession followed by a subsequent stabilization. Small plant density decreased steadily after only 5 years of land abandonment, whereas species density increased with successional stage. A decline in species dominance may be responsible for these contrasting patterns. Seasonality, successional stage and their changes through time had a stronger influence on large plants, likely because of large among-plot variation of small plants. Notwithstanding the short duration of our study, our results suggest that climate-change driven decreases in rainy season precipitation may have an influence on successional dynamics in our study forest as strong as, or even stronger than, prolonged or severe droughts during the dry season.

Autogenic regulation and resilience in tropical dry forest

“…Actually, rainfall and its seasonal distribution are likely the main factors influencing this process in tropical dry forests (Allen et al., 2017). This is consistent with studies in other tropical forests, where not only growth but also mortality is largely driven by inter‐annual changes in precipitation, particularly by severe droughts (Aleixo et al., 2019; Saenz‐Pedroza et al., 2020; Toledo et al., 2011). Moreover, growth is affected by other environmental factors such as temperature or solar radiation, which change continuously (high intra‐ and inter‐annual variability) but are unrelated to autogenic regulation (Lewis, Malhi, et al., 2004).…”

Successional shifts in tree demographic strategies in wet and dry Neotropical forests

“…and Neomillspaughia emarginata ; Dupuy et al, 2012, Schorn, 2021), in addition to understorey shrubs and treelets that are characteristic of OGFs (e.g., Piper spp., Psychotria spp. and Eugenia axillaris ; Dupuy et al, 2012; Rüger et al, 2018; Saenz‐Pedroza et al, 2020). The slow growth of SLB species could be explained, in part, by their shade intolerance, which could lead to slow growth once the forest canopy closes or their short stature (i.e., individuals were probably monitored close to their maximum size, where they already showed decreasing growth rates).…”

“…The dry forest in Nizanda (Oaxaca) has many arborescent cacti (Pérez‐García et al, 2010). Secondary forests in both dry forests have regenerated after shifting agriculture (Lebrija‐Trejos et al, 2008; Saenz‐Pedroza et al, 2020). The few plots classified as OGF in Yucatán have not suffered agricultural land use for ≥100 years.…”

“…Chazdon, 2014;Finegan, 1996). In Yucatán, SLB species were also most abundant during the first decades and then declined, possibly owing to their higher resprouting ability or their higher drought tolerance (Guillemot et al, 2022) Dupuy et al, 2012;Rüger et al, 2018;Saenz-Pedroza et al, 2020).…”

Diversidad arbórea y carbono almacenado en selvas bajo manejo forestal comunitario en Yucatán, México

“…3). La dominancia de estas especies, consideradas como generalistas de la sucesión, puede deberse a una combinación de atributos como alta capacidad de dispersión, colonización y rebrote y altas tasas de crecimiento (Dupuy et al, 2012;Sanaphre-Villanueva et al, 2017;Pineda-Herrera et al, 2019;Saenz-Pedroza et al, 2020).…”