Spatial distribution of biomass and links to reported disturbances in tropical lowland forests of southern Mexico

Figures & data

Figure 1.  Spatial distribution of the ecoregions in Mexico.

Figure 2.  Relative importance of the disturbance events (in % of plots with disturbances) and average severity reported (scale: 1 = very light to 4 = severe).

Figure 3.  Spatial distribution of natural disturbances in Mexico.

(A) Pests and diseases; (B) wind. Scale: 1 = very light to 4 = severe.EYP: East Yucatan Peninsula; GM-EC: The Gulf of Mexico and eastern Chiapas; SMF: Sierra Madre Foothills; SPC: South Pacific Coast; SYP: South Yucatan Peninsula; WPC: West Pacific Coast; WYP: West Yucatan Peninsula.

Figure 4.  Spatial distribution of human disturbances in Mexico.

(A) Fire; (B) grazing; (C) land-use change; (D) forest management; (E) roads; and (F) settlements. Scale: 1 = very light to 4 = severe.

EYP: East Yucatan Peninsula; GM-EC: The Gulf of Mexico and eastern Chiapas; LUC: Land-use change; Mgmt: Forest management; SMF: Sierra Madre Foothills; SPC: South Pacific Coast; SYP: South Yucatan Peninsula; WPC: West Pacific Coast; WYP: West Yucatan Peninsula.

Figure 5.  Spatial distribution of human and natural disturbances, and aboveground living biomass.

(A) Human disturbances; and (B) natural disturbances. Human disturbances scale: 0 = no disturbance to 16 = severe; natural disturbances scale: 0 = no disturbance to 6 = severe.

EYP: East Yucatan Peninsula; GM-EC: The Gulf of Mexico and eastern Chiapas; SMF: Sierra Madre Foothills; SPC: South Pacific Coast; SYP: South Yucatan Peninsula; WPC: West Pacific Coast; WYP: West Yucatan Peninsula.

Figure 6.  Spatial distributions in Mexico.

(A) Biomass clusters and outliers. (B) Aboveground living biomass, derived from the inverse weighted distance interpolation.

EYP: East Yucatan Peninsula; GM-EC: The Gulf of Mexico and eastern Chiapas; H-H: Clusters of high biomass plots; H-L: Outlier of a high biomass plot within a cluster of low biomass plots; L-H: Low biomass plot within a cluster of high biomass plots; L-L: Clusters of low biomass plots; SMF: Sierra Madre Foothills; SPC: South Pacific Coast; SYP: South Yucatan Peninsula; WPC: West Pacific Coast; WYP: West Yucatan Peninsula.

Table 1.  Brief outline of human and natural disturbances.

Human disturbances	Description	Effect on forest biomass and structure
Fire	Forest fires in tropical Mexico originate from burning of grassland or shifting cultivation that gets out of control, as such they are considered as human induced. Approximately 95% are surface fires	Depending on the severity of the fire, it affects the organic matter, understory vegetation and lower leaves and branches of the canopy trees. Severe canopy fires may kill the trees
Roads	Roads in forested areas are either established temporarily to extract timber or permanently to facilitate displacement of people and goods	Short- or long-term elimination of the forests on the space occupied by the road. The effect on forest depends on the role of the road, low and short-term effect in temporal timber extraction roads to severe and permanent in the case of paved roads
Forest management	Forest management is mostly concentrated in high biomass areas of south and east Yucatan Peninsula and eastern Chiapas. Implies selective cutting of a few commercial trees per hectare on a rotational scheme (25–35 years)	During and after harvesting damage of the remaining stand that may kill the affected trees. Additional damage due to forest roads. Due to low-intensity harvesting, the effect on forest structure is usually low
Land-use change	This disturbance refers to forests that are in the fallow stage of the slash-and-burn system	Converts the intact forests to secondary forests in different stages of recovery. Biomass can be recovered after 40–80 years of fallow
Grazing	Cattle grazing in tropical forests are mainly restricted to secondary forests (forests that are part of the land-use change system)	The effect of grazing on forest structure is usually low, but is an indirect indicator of the state of the forest
Settlements	Clearance of intact forest to establish settlements. Around the houses original forests are replaced by so-called home gardens	The effect of settlements on forest structure depends on the characteristics of the population, such as number of inhabitants and economic activity, among others
Natural disturbances	Description	Effect on forest biomass and structure
Storm	Hurricanes are frequent both on the Caribbean and Pacific coasts, affecting forests particularly close to the coast as the wind force decreases when hurricanes move inland	Windthrow results when wind loads exceed the strength of tree stems or root systems. Damage to forests due to hurricanes can vary between the partially uprooting and stem-breaking of a few canopy trees to complete blow down of all trees. The changes in stand structure, addition of large woody material and modification of soil profiles during windthrow events have long-lasting ecological consequences. Particularly when damage is severe, subsequent forest fires are more likely to occur and are prone to be more severe as more forest debris is available
Pests and diseases	Pests and diseases are usually limited to certain trees species, thus the intensity depends on the status of the species within the forest community. Particularly, shoot-boring insects are important pests in tropical forests	Particularly, shoot-boring insects in tree plantation may affect large parts of the stands; in natural vegetation the impact of pests and diseases is generally low. The highest intensity recorded in the study plots is medium

Table 2.  Number of plots used in the analysis with human disturbances, with natural disturbances, without disturbances and all plots in each ecoregion.

Ecoregion	Human disturbance	Natural disturbance	Without disturbance	All plots
South Yucatan Peninsula	572	175	382	1035
West Yucatan Peninsula	408	347	326	929
East Yucatan Peninsula	372	335	284	807
South Pacific Coast (including Soconusco)	231	53	59	303
Gulf of Mexico and eastern Chiapas	469	98	350	836
West Pacific Coast	118	66	10	146
Sierra Madre Foothills	62	9	16	80
Total	2232	1083	1427	4136

Table 3.  Average biomass densities in high, low and outlier categories in all plots and in all ecoregions.

Ecoregions	H-H			H-L			L-H			L-L			No group
	N	AGLB		N	AGLB		N	AGLB		N	AGLB		N	AGLB
All plots	945	169.1		136	180.7		321	32.1		874	26.0		1642	75.9
South Yucatan Peninsula	524	158.6		4	208.7		172	33.5		23	22.2		308	70.0
West Yucatan Peninsula	74	155.8		43	140.9		47	28.0		333	27.8		431	73.0
East Yucatan Peninsula	317	179.7		20	160.4		90	32.5		72	22.5		307	83.3
South Pacific Coast	5	283.4		9	158.1		2	25.8		74	26.6		132	75.4
Gulf of Mexico and eastern Chiapas	25	271.6		46	217.3		10	24.3		326	25.2		322	75.6
West Pacific Coast	NP	NP		11	219.6		NP	NP		24	28.7		101	86.0
Sierra Madre Foothills	NP	NP		3	214.4		NP	NP		22	22.8		41	73.7

AGLB: Aboveground living biomass in Mg DM/ha; H-H: High biomass clusters; H-L: High biomass outlier surrounded by low biomass plots; L-H: Low biomass outlier surrounded by high biomass plots; L-L: Low biomass clusters; N: Number of plots; NP: No plots.

Table 4.  Multiple variable ANOVA with biomass clusters, human and natural disturbance presence/absence as independent variables and a square root conversion of the aboveground biomass data as dependent variable.

Ecoregion	Model			Residual			Independent variables
	SS	df		SS	df		F	r^2	Cluster	Human	Natural
All plots	36373.1	5		9206.4	2270		1793.7	0.80	***	***	***
South Yucatan Peninsula	7461.0	5		2920.3	717		366.4	0.72	***	*	NS
West Yucatan Peninsula	4872.3	5		1434.3	491		333.6	0.77	***	***	***
East Yucatan Peninsula	7557.8	5		1609.3	493		463.1	0.82	***	*	**
Gulf of Mexico and eastern Chiapas	6386.8	5		1715.0	401		298.7	0.79	***	***	NS

The significance of the independent variables is indicated separately.

***p < 0.01; **p < 0.05; *p <0.1.

df: Degrees of freedom; F: Fisher’s test statistic; NS: Not significant; r²: Coefficient of determination; SS: Sum of squares.

Table 5.  ANOVA and Tukey´s test for homogeneous groups of aboveground living biomass, according to the intensity of the specific disturbance.

Disturbance	Intensity category (groups in Mg ha^-1)
	Severe	Medium	Light	Very light	Without
Fire	32.0 a	44.0 ab	59.0 b	92.9 c	95.2 c
Roads	54.3 ab	65.0 ab	69.1 a	136.3 b	95.2 ab
Mgmt	142.7 b	90.7 a	110.1 ab	133.7 ab	95.2 a
LUC	23.2 a	26.9 a	42.0 a	38.5 a	95.2 b
Grazing	29.5 a	40.5 a	53.2 a	49.1 a	95.2 b
Settlements	NA	NA	57.2 a	NA	95.2 a

The letters indicate statistically significant differences between intensity classes of each disturbance. For example, for plots with fire, those with severe disturbance recorded (a) have less biomass than those with light disturbance (b) and both have less biomass than plots with very light or without disturbances (c), whereas plots with medium disturbance (ab) have similar density as plots with severe or light disturbances.

LUC: Land-use change; Mgmt: Forest management.

Table 6.  Statistically significant coefficients of the multiple linear regressions between a square root conversion of the aboveground biomass data and specific human disturbances for each ecoregion (p < 0.01).

Ecoregion	Intercept	Fire	Road	Mgmt	LUC	Grazing	Mult R
South Yucatan Peninsula	10.67	-0.90	NA	0.28	-0.75	-1.17	0.57
West Yucatan Peninsula	8.28	-1.11	NA	NA	NA	-0.63	0.45
East Yucatan Peninsula	11.45	-1.59	NA	0.88	NA	NA	0.49
Gulf of Mexico and eastern Chiapas	8.00	NA	NA	NA	-0.56	-0.55	0.28
South Pacific Coast	8.18	NA	NA	NA	-0.85	NA	0.33
All plots	9.43	-0.23	-0.05	0.01	-0.19	-0.20	0.43

Plots with natural disturbances were excluded.

LUC: Land-use change; Mgmt: Forest management; Mult R: Coefficient of multiple determination.

Table 7.  Level of biomass reduction in plots with human disturbances in percentage of a square root conversion of the aboveground biomass data of undisturbed plots, applying the regression equations of Table 6.

Ecoregion	Fires (%)	Roads (%)	Mgmt (%)	LUC (%)	Grazing (%)
South Yucatan Peninsula	-24	NA	8^†	-29	-45
West Yucatan Peninsula	-35	NA	NA	NA	-28
East Yucatan Peninsula	-27	NA	16^†	NA	NA
Gulf of Mexico and eastern Chiapas	NA	NA	NA	-24	-21
South Pacific Coast	NA	NA	NA	-40	NA
All plots	-1	-9	12^†	-26	-21

^†Plots with higher biomass within a disturbance class than undisturbed plots.

LUC: Land-use change; Mgmt: Forest management.