The vegetation, natural regeneration and tree growth were studied in the mangrove forest of the Ejido de San Blas, state of Nayarit, Pacific coast of Mexico (21° 34' N), as the ecological basis to estimate yields of wood, to assess the influence of the current community-based harvesting system on forest structure and regeneration, and to formulate a new management plan for the area with silviculture for commercial production of mangrove wood as the major aim.
It was detected, from the review of past and contemporary global literature, that mangrove research, mainly in the second half of this century, has been mostly focused on areas such as biology, ecology and physiology. Aspects of the use and management of mangrove forests by humans, however, are of relatively recent consideration. In Mexico, the production of wood from mangrove forests is, and has been, intensive. Most of this production is from forests without management plans, and though some of these plans exist in the state of Nayarit, they lack basic data on regeneration, diameter increment, volume estimation and harvesting system applied.
The above mentioned situation, together with a relative large amount of mangrove wood extracted and the increasing public awareness about the fragility of these coastal wetlands, eventually caused the cessation of logging operations in the 13 Ejidos of the state of Nayarit, arguing that the commercial production of this mangrove species has been practiced under non-sustainable basis. Taking into account these circumstances, the present thesis was directed to describe and measure natural ecosystem processes in the mangrove forest of the Ejido de San Blas, and to review the effects on this forest of the local woodcutting procedure. With these data, and based on the analysis of biophysical and socioeconomic conditions of the study area, a joint management plan was also prepared including both professional and local forest-related knowledge.
Forest structure. Three major mangrove tree species (sensu Tomlinson 1986, 1994) were found in the study area: Rhizophora mangle L. (Rhizophoraceae), Laguncularia racemosa L. Gaertn f. (Combretaceae) and Avicennia germinans (L.) Stearn (Avicenniaceae). They were forming monospecific stands parallel to the river shore with Rhizophora situated in the lowest position, Laguncularia in the middle position and Avicennia in the highest position. The non-mangrove flora associated to the mangrove vegetation (49 species) was considered unusually rich for a mangrove environment not far from its northernmost range of distribution (31° N). Continuous provision of freshwater in the study area, both from runoff and seepage, was suggested as the cause to maintain low soil water salinities (10-12 ppt) and the high species richness of mangrove associates.
Canopy height ranged from 7 to 25 m with emergent trees taller than 30 m, especially in old-growth stands of Laguncularia. Basal area varied from 12 to 51 m² ha·¹ for all trees ≥ 2.5 cm DBH, and stand density ranged from 898 to 32 800 stems ha·¹ for all individuals >1.3 m high. Compared to mangroves in other places of Mexico and of the world, the mangrove forest in the study area had a higher number of individuals per hectare, a lower canopy height, and a similar basal area.
Natural regeneration. This occurred both in undisturbed stands and canopy openings (created by lightning strikes) of the mangrove forest of the Ejido de San Blas, and was originated exclusively from sexual reproduction in Rhizophora and from seeds and vegetative shoots in Avicennia and Laguncularia.
In undisturbed stands, the regeneration stock was evaluated as satisfactory for Laguncularia, sufficient for Rhizophora, and irregular for Avicennia. In canopy gaps, natural regeneration of Laguncularia was nine (height size class 2: 30 cm < plants <1.3 m high) and almost 20 (height size class 3: 1.3 m < plants <2.5 cm DBH) times more abundant (p <0.0359) than that occurring in undisturbed forest stands of this species.
Survival in undisturbed stands of the Ejido de San Blas was high (>67 %) in the three mangrove tree species for size classes 2 and 3, but low (<32 %) for Laguncularia and Avicennia seedlings of size class 1 (plants <30 cm high). Desiccation was the most important cause of mortality, but damage by crabs was also found in Rhizophora propagules of some plots.
Although no statistically significant (p >0.05), growth in stem length was greater in vegetative shoots (11-27 cm year·¹) than in seed-derived individuals (5-25 cm year·¹) of the size classes 2 and 3. Considering the three mangrove tree species and both sexual and vegetative reproduction, vegetative shoots of Laguncularia size class 3 had the largest mean annual stem growth recorded in the study (27 cm year·¹). In comparison to other mangrove areas, seedlings of Rhizophora and Avicennia from the Ejido de San Blas had higher survival rates and grew faster in stem length than their counterpart in either the same or in different species of the genera. ..................................................................