Tree growth estimates based on diameter change in seasonally dry tropical forest must account for shrinkage related bias

Tree growth estimates based on diameter change in seasonally dry tropical forest must account for shrinkage related bias.

Presented by
Rutuja Chitra Tarak

Rutuja Chitra Tarak, R. Sukumar

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Introduction: What conservation problem or question does your study address?

At community level tape or dendrometer based diameter increment is the std. method for measuring tree wood growth. However, besides wood increment diameter change also results from difference in water storage of the tree at the time of measurement. Shrinkages are often observed in dry forests with large year to year variations. We ask how much is the extent of bias in tree growth estimates due to water related changes and if they could be easily estimated at community level.

Methods: What were the main research methods you used?

In the 50ha Permanent Forest Dynamics plot in the Seasonally Dry Tropical Forest of Mudumalai Wildlife Sanctuary, DBH of all stems above 1cm DBH are measured every four years from 1988 through 2012 with tape. We compared tape based total growth with ring based wood growth during each of the 6 census intervals for 9 trees of the two dominant canopy species – Tectona grandis and Terminalia crenulata. The difference between total growth and wood growth was termed as water growth. At the plot level, we considered late wet season (Sep-Nov) rainfall of the successive census years as a proxy for water growth and tested if it predicted total growth.

Results: What are your most important results?

Large shrinkages were observed in some trees, leading to large water storage variation (Fig. 1). For the selected 9 trees, we found large variations in total growth within and across censuses, whereas ring based wood growth was relatively stable (Fig. 2). Tape growth and water growth were tightly correlated. At the stand level, tape based total growth increased with Sep-Nov rainfall differences, but its variability was not substantially explained (Fig. 3).

Discussion: What are your important discussion points and what is the relevance of your results to conservation (if any)?

For the 9 trees, variability in water growth was large in all censuses, while bias was +ve,-ve or none in a given census. Water growth explained most of the tape growth. At the plot level total growth was related to Sep – Nov rainfall difference between censuses, but low correlation meant water growth variability cannot be predicted by this simple metrics. Our results have strong implications on the understanding of climate – growth relationships & C sequestration rates.

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