Visual rating and the use of image analysis for assessing canopy foliage density in a pecan provenance collection during leaf fall

Authors: Pisani, Cristina; Bock, Clive; RANDALL, JENNIFER - New Mexico State University

Submitted to: Journal of Forestry Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2023
Publication Date: N/A
Citation: N/A

Interpretive Summary: Pecan is native to North America and is a nut crop of considerable economic importance. Pecan’s native range extends from the far north of the U.S.A. in Iowa and Ohio, throughout the Mississippi River watershed and the rivers of eastern and central to west Texas and Louisiana, and south to Oaxaca, in southern Mexico. Due to the extent of its native range, pecan is adapted to a wide climatic range, from mild to harsh winters to very humid and semiarid temperatures, which suggests great genetic diversity within the species. A provenance collection of pecan is located at the USDA-ARS Southeastern Fruit and Tree Nut Research Station (SEFTNRS). The trees in the collection originated from nuts that were collected in 1986 and 1987 from wild pecan trees in 19 different locations or “provenances” from its native range. Only a few horticulturally important traits have been characterized in the pecan provenance collection, one important trait being tree canopy structure. In pecan orchards, canopy characteristics may be related to its genetics, and pest and disease effects, or other aspects of tree health. Such canopy characteristics can provide insight on the genetic control and environmental cues involved. The objectives of this study were to i) determine the ability of visual raters to estimate pecan canopy foliage density (as a measure of leaf fall) both accurately and reliably, ii) explore the impact of different rater data on the outcome of an analysis, and assess whether the raters are consistent in their estimations, and iii) determine the relationship between mid-autumn (approximately mid leaf fall) canopy foliage density of pecan trees from different provenances collected from different latitudes in North America. The preliminary results indicate that we can safely conclude that estimates by raters are both accurate and reliable for assessing canopy foliage density in pecan trees during the period of leaf fall. The same approach might be used to assess canopy foliage density after bud break. We emphasize that instructing and training raters is an important aspect to ensure accuracy and reliability of canopy foliage density assessments. Future studies should explore the value of using Standard Area Diagrams (SADs) of pecan canopy foliage density to improve accuracy and reliability of rater estimates. SADs will be useful for rating different pecan canopies, identifying the genetics involved, and helping guide subsequent breeding selection processes.

Technical Abstract: A native pecan collection planted at the USDA-ARS Southeastern Fruit and Tree Nut Research Station in Byron, GA represents trees from the native range of the species. The collection (867 trees) is a valuable genetic resource to characterize important horticulturally traits with the ultimate aim of developing improved pecan cultivars. One important trait is canopy foliage density during leaf fall. Understanding the seasonal canopy dynamics can provide insights to the species diversity, environmental cues and breeding potential of germplasm. In this study, we explored the ability of visual raters to determine canopy foliage density as an indicator of leaf shed during fall. The canopy foliage density was estimated by 4 visual raters on a subset of the provenance collection (76 trees) from 19 provenances in November. The estimates were compared to assumed actual values measured using image analysis. Mean canopy foliage density using the image analysis software was slightly less compared to the visual estimates (11.9% vs 18.4%, respectively). At higher canopy foliage densities, all four rates showed a greater tendency to overestimate compared to actual values, but overall agreement between raters and actual values was good ('c = 0.849 to 0.915), and inter-rater reliability was high (R2 = 0.910 to 0.953). The provenance from Missouri (MO-L), the northernmost provenance, had the lowest canopy foliage density as measured by image analysis or all four raters. Families from states in Mexico (MX), where the southernmost provenances originated, were mostly grouped together. However, the family from Oaxaca (MX-O) was not grouped with the other MX provenances based on estimates by raters 1 and 3. The results show that the higher the source latitude of the provenance trees, the lower the canopy foliage density. Based on regression, the source provenance latitude explained approximately 0.609 of the variation measured using image analysis, and 0.551 to 0.640 when based on the rater estimates of canopy foliage density. Visual assessment of pecan canopy foliage density due to late season leaf fall for comparing pecan genotypes appears to provide generally accurate and reliable estimates and could be used in future studies of the whole provenance collection, and for other pecan germplasm.