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Competitive effects of a dominant palm on sapling performance in a Neotropical rainforest

Citation

Martínez-Ramos, Miguel; Noriega-Piña, Karen; Piñero, Daniel; Valverde, Teresa (2021), Competitive effects of a dominant palm on sapling performance in a Neotropical rainforest, Dryad, Dataset, https://doi.org/10.5061/dryad.pvmcvdnm8

Abstract

Eradication of herbivores, due to human disturbances, produces a demographic outburst of highly competitive prey species, which in turn reduces plant species diversity. This happens at Los Tuxtlas tropical rainforest, Mexico, where a population outburst of the understory palm Astrocaryum mexicanum is ostensibly excluding tree species, but how this is occurring is still unknown. We used a neighborhood approach to explore the effects of palm shading and palm crowding on the survival and growth (RGR) of saplings of six common tree species. Sixteen to 32 saplings (1.5-2.5 m height) per species were used as focal individuals of circular neighborhoods (4 m radius), which included palms ≥ 1cm stem length potentially competing for light or soil resources. Shading was estimated using hemispherical photographs. Overall, survivorship was high combining all species (93.8%/yr). In most species shading produced a displacement of the crown, which increased with specific leaf area of species. In three species shading had a negative effect on RGR without any effect of crowding, the contrary occurred in two species, and in one species no effects were found. No effects due to trees (DBH ≥ 1cm) crowding were detected. The shading effect increased with species leaf dry matter content (LDMC), while the effect of crowding declined with LDMC and increased with sapling total leaf area. We argue that the species-dependent palm shading/crowding effects were related to the shade tolerance of sapling species. In the long-term such species-specific responses could have consequences for forest structure and composition, as saplings develop to mature stages.

Methods

We selected six common tree species to test the hypothesized competitive effects of Astrocaryum on saplings. Three were understory species (maximum heights of 9–15 m): Faramea occidentalis (Rubiaceae; Faramea hereafter), Psychotria faxlucens (Rubiaceae; Psychotria), and Trophis mexicana (Moraceae; Trophis). The other three were canopy species (maximum height: 25–35 m): Brosimum alicastrum (Brosimum), Poulsenia armata (Poulsenia), and Pseudolmedia glabrata (all Moraceae; Pseudolmeldia).

We used 16–32 focal saplings per species, each 1.5–2.5 m in height. To have spatially independent samples, a minimum distance of 10 m separated saplings of the same species. For each sapling, we measured the diameter at breast height (DBH), height (from the ground to the tallest vertical point of the crown), the maximum (Dmax), and minimum diameter (Dmin) of the crown, and counted the number of leaves. We used a caliper (0.1 mm resolution) to take two perpendicular DBH records. We averaged the two readings to have a single DBH value per sapling. We marked the stem where the DBH readings were taken with indelible paint. This enabled us to minimize possible errors in the measurement of DBH one year after and to have good growth rate estimates. Additionally, we measured the distance between the stem base of a focal sapling and the vertical projection of the central point of the sapling's crown (D1), and the distance between the stem base of the sapling and the stem base of the nearest Astrocaryum palm (D2). With this data, we constructed a crown displacement index (CDI) as D1/D2.

At the beginning of our study, we randomly collected 3–5 branches and 3–5 leaves from three to eight different saplings per species to characterize functional profiles of the species, considering five traits. 

We quantified light limitation produced by the nearest Astrocaryum on focal saplings using hemispherical photographs taken above the crown of each focal sapling and above the crown of the nearest palm shading the sapling. The height and crown cover of the nearest Astrocaryum palm were registered. The photographs were analyzed with the software Gap Light Analyzer (GLA) to estimate the percent of forest canopy openness and the daily mean values of transmitted total light radiation.

To quantify palm crowding effects on sapling growth, we constructed a neighborhood index as. Here Si was the crown cover (m2) of a sapling I, and dij the distance between the stem base of the focal sapling (j) and the stem base of the nearest palm i. We calculated a NCIP value for each 4‐m radius neighborhood, where all Astrocaryum palms with a stem length ≥ 1 cm were included. The palms with crowns 2 m above the ground and within a distance <2 m from the stem base of the focal saplings were shading the saplings, while those smaller than 2 m and within a distance < 4 m were assumed to be competing for soil resources. In the NCIP, we used palm crown cover as a measure of Si2, as this attribute is a good indicator of Astrocaryum vigor.

All these data were introduced to Excel databases. 

Usage Notes

Due to DBH measurement errors (detected as outliers) and overexposed fish-eye photographs, some data were not taking into account in the provided databases.